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  • porous silicon  (61)
  • stability  (58)
  • Springer  (119)
  • American Physical Society
  • Blackwell Publishers Ltd
  • 2000-2004  (119)
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  • Springer  (119)
  • American Physical Society
  • Blackwell Publishers Ltd
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  • 1
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    Circuits, systems and signal processing 19 (2000), S. 423-435 
    ISSN: 1531-5878
    Keywords: Time-varying autoregressive models ; stability ; smoothness priors ; Tihkonov regularization ; constrained optimization
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract The stability of time-varying autoregressive (AR) models is an important issue in such applications as time-varying spectrum estimation and electroencephalography simulation and estimation. In some cases, such as time-varying spectrum estimation, the models that exhibit roots near unit moduli are difficult to use. Thus a tighter stability condition such as stability with a positive margin is needed. A time-varying AR model is stable with a positive margin if the moduli of the roots of the time-varying characteristic polynomial are somewhat less than unity for every time instant. Recently, a new method for the estimation of the time-varying AR models was introduced. This method is based on the interpretation of the underdetermined time-varying prediction equations as an ill-posed inverse problem that is solved by Tikhonov regularization. The method is referred to as the deterministic regression smoothness priors (DRSP) scheme. In this paper, a stabilization method in which the DRSP scheme is augmented with nonlinear stability constrainst is proposed. The problem is formulated so that stability with a positive margin can also be achieved. The problem is solved iteratively with an exterior point algorithm. The performance of the algorithm is studied with a simulation. It is shown that the proposed approach is well suited to stable modeling of signals containing narrowband transitions.
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  • 2
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    Circuits, systems and signal processing 19 (2000), S. 13-25 
    ISSN: 1531-5878
    Keywords: Singular systems ; delay ; consistency condition ; stability ; instability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract In this paper, the general class of singular systems with delay and linear constant coefficient singular systems with delay are discussed. First, several definitions of stability are presented for singular systems with delay, and general sufficient stability conditions and instability conditions are obtained. Second, stability and instability are analyzed for linear constant coefficient singular systems with delay.
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  • 3
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    Acta biotheoretica 48 (2000), S. 207-218 
    ISSN: 1572-8358
    Keywords: Dynamical population ; fishing efforts ; metapopulation ; time scales ; aggregation method ; equilibrium ; stability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract This work presents a specific stock-effort dynamical model. The stocks correspond to two populations of fish moving and growing between two fishery zones. They are harvested by two different fleets. The effort represents the number of fishing boats of the two fleets that operate in the two fishing zones. The bioeconomical model is a set of four ODE's governing the fishing efforts and the stocks in the two fishing areas. Furthermore, the migration of the fish between the two patches is assumed to be faster than the growth of the harvested stock. The displacement of the fleets is also faster than the variation in the number of fishing boats resulting from the investment of the fishing income. So, there are two time scales: a fast one corresponding to the migration between the two patches, and a slow time scale corresponding to growth. We use aggregation methods that allow us to reduce the dimension of the model and to obtain an aggregated model for the total fishing effort and fish stock of the two fishing zones. The mathematical analysis of the model is shown. Under some conditions, we obtain a stable equilibrium, which is a desired situation, as it leads to a sustainable harvesting equilibrium, keeping the stock at exploitable densities.
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  • 4
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    Queueing systems 34 (2000), S. 1-35 
    ISSN: 1572-9443
    Keywords: multiple access ; CDMA ; rates of convergence ; stability ; functional limit theorems ; transient analysis ; Markov-modulated capture channel
    Source: Springer Online Journal Archives 1860-2000
    Topics: Computer Science
    Notes: Abstract We consider the slotted ALOHA protocol on a channel with a capture effect. There are M 〈 ∞ users each with an infinite buffer. If in a slot, i packets are transmitted, then the probability of a successful reception of a packet is q i. This model contains the CDMA protocols as special cases. We obtain sufficient rate conditions, which are close to necessary for stability of the system, when the arrival streams are stationary ergodic. Under the same rate conditions, for general regenerative arrival streams, we obtain the rates of convergence to stationarity, finiteness of stationary moments and various functional limit theorems. Our arrival streams contain all the traffic models suggested in the recent literature, including the ones which display long range dependence. We also obtain bounds on the stationary moments of waiting times which can be tight under realistic conditions. Finally, we obtain several results on the transient performance of the system, e.g., first time to overflow and the limits of the overflow process. We also extend the above results to the case of a capture channel exhibiting Markov modulated fading. Most of our results and proofs will be shown to hold also for the slotted ALOHA protocol without capture.
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  • 5
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    Celestial mechanics and dynamical astronomy 78 (2000), S. 227-241 
    ISSN: 1572-9478
    Keywords: stability ; normal form ; spin-orbit resonance
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We consider a model of spin-orbit interaction, describing the motion of an oblate satellite rotating about an internal spin-axis and orbiting about a central planet. The resulting second order differential equation depends upon the parameters provided by the equatorial oblateness of the satellite and its orbital eccentricity. Normal form transformations around the main spin-orbit resonances are carried out explicitly. As an outcome, one can compute some invariants; the fact that these quantities are not identically zero is a necessary condition to prove the existence of nearby periodic orbits (Birkhoff fixed point theorem). Moreover, the nonvanishing of the invariants provides also the stability of the spin-orbit resonances, since it guarantees the existence of invariant curves surrounding the periodic orbit.
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  • 6
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    Journal of computational analysis and applications 2 (2000), S. 293-308 
    ISSN: 1572-9206
    Keywords: parabolic equations ; ADI scheme ; stability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract An ADI scheme for solving three-dimensional parabolic equations withfirst-order derivatives and variable coefficients has been developed basedon our previous papers and the idea of the modified upwind differencescheme. This ADI scheme is second-order accurate and unconditionallystable. Further, a small parameter can be chosen which makes it suitablefor simulating fast-transient phenomena or for computations on fine spatialmeshes. The method is illustrated with numerical examples.
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  • 7
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    Queueing systems 36 (2000), S. 327-349 
    ISSN: 1572-9443
    Keywords: multiclass networks ; networks with feedback ; Skorokhod Problem ; Skorokhod Mapping ; Lipschitz continuity ; stability ; load conditions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Computer Science
    Notes: Abstract We consider a four-class two-station network with feedback, with fluid inputs and a head-of-the-line generalized processor sharing discipline at each station. We derive the Skorokhod Problem associated with the network and obtain algebraic sufficient conditions for Lipschitz continuity of the associated Skorokhod Map. This provides the first example of a multiclass network with feedback for which the associated Skorokhod Problem has been proved to be regular. As an elementary application, we show that under the conditions which guarantee Lipschitz continuity the network is stable if and only if the usual load conditions apply.
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  • 8
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    Journal of dynamics and differential equations 12 (2000), S. 117-167 
    ISSN: 1572-9222
    Keywords: singular perturbation ; standing pulses ; stability ; Hopf bifurcation ; reaction-diffusion system
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract Bifurcation phenomena from standing pulse solutions of the problem $$\varepsilon \tau u_t = \varepsilon ^2 u_{xx} + f(u,v),{\text{ }}v_t = v_{xx} + g(u,v)$$ is considered. ε(〉0) is a sufficiently small parameter and τ is a positive one. It is shown that there exist two types of destabilization of standing pulse solutions when τ decreases. One is the appearance of travelling pulse solutions via the static bifurcation and the other is that of in-phase breathers via the Hopf bifurcation. Furthermore which type of destabilization occurs first with decreasing τ is discussed for the piecewise linear nonlinearities f and g.
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  • 9
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    Catalysis letters 68 (2000), S. 55-58 
    ISSN: 1572-879X
    Keywords: promoting effect ; B2O3 ; Cu/ZnO/Al2O3 catalyst ; methanol synthesis ; stability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The addition of B2O3 to a Cu/ZnO/Al2O3 catalyst increased the activity of the catalyst for methanol synthesis after an induction period during the reaction. The stability of the B2O3-containing Cu/ZnO/Al2O3 catalyst was greatly improved by the addition of a small amount of colloidal silica to the catalyst.
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  • 10
    ISSN: 1572-879X
    Keywords: potassium desorption ; stability ; excitation ; iron catalyst ; Rydberg atoms
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Well‐characterized catalyst model compounds of KAlO2 and KFeO2 are investigated by thermal desorption of potassium from the material. The desorbing fluxes of ions, atoms and highly excited states (field ionizable Rydberg states) were studied with surface and field ionization detectors in a vacuum apparatus. From the Arrhenius plots the activation energies for desorption of K and K+ were determined. The chemical state of potassium at the surfaces is concluded to be: ionic on KAlO2 (with the K desorption barrier of 1.76 eV) and covalent on KFeO2 (barrier of 2.73 eV). These results agree with the data obtained earlier for industrial catalysts for ammonia and styrene production. They are interpreted in terms of the Schottky cycle, which is completed for KAlO2 and fails for KFeO2. This failure indicates a non‐equilibrium desorption process. K Rydberg states are only found to desorb from KFeO2, in agreement with the suggestion that such states in some way are responsible for the catalytic activity.
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  • 11
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    Set-valued analysis 8 (2000), S. 253-266 
    ISSN: 1572-932X
    Keywords: Hausdorff metric ; linear inequality systems ; stability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract In this paper, we propose a Hausdorff metric to measure the “distance” between two linear inequality systems on a real normed space X. For this topology, which comes through a pseudo-metric in the set Σ of linear inequality systems, the closedness of the feasible set mapping is studied, and at the same time a characterization of the stability of the subset Σ c of consistent sytems is given.
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  • 12
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    Annals of operations research 99 (2000), S. 251-265 
    ISSN: 1572-9338
    Keywords: stochastic programming ; bond portfolio management ; interest ratescenarios ; stability ; sensitivity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics , Economics
    Notes: Abstract The bond portfolio management problem is formulated as a multiperiod two-stage or multistage stochastic program based on interest rate scenarios. These scenarios depend on the available market data, on the applied estimation and sampling techniques, etc., and are used to evaluate coefficients of the resulting large scale mathematical program. The aim of the contribution is to analyze stability and sensitivity of this program on small changes of the coefficients – the (scenario dependent) values of future interest rates and prices. We shall prove that under sensible assumptions, the scenario subproblems are stable linear programs and that also the optimal first-stage decisions and the optimal value of the considered stochastic program possess acceptable continuity properties.
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  • 13
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    Applications of mathematics 45 (2000), S. 161-176 
    ISSN: 1572-9109
    Keywords: reaction-diffusion system ; unilateral conditions ; quasivariational inequality ; Leray-Schauder degree ; eigenvalue ; stability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract We consider a reaction-diffusion system of the activator-inhibitor type with unilateral boundary conditions leading to a quasivariational inequality. We show that there exists a positive eigenvalue of the problem and we obtain an instability of the trivial solution also in some area of parameters where the trivial solution of the same system with Dirichlet and Neumann boundary conditions is stable. Theorems are proved using the method of a jump in the Leray-Schauder degree.
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  • 14
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    Advances in computational mathematics 12 (2000), S. 229-250 
    ISSN: 1572-9044
    Keywords: numerical analysis ; shallow water problems ; DIRK methods ; stability ; 65L06 ; 65L20 ; 65M12 ; 65M20
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract We construct A‐stable and L‐stable diagonally implicit Runge–Kutta methods of which the diagonal vector in the Butcher matrix has a minimal maximum norm. If the implicit Runge–Kutta relations are iteratively solved by means of the approximately factorized Newton process, then such iterated Runge–Kutta methods are suitable methods for integrating shallow water problems in the sense that the stability boundary is relatively large and that the usually quite fine vertical resolution of the discretized spatial domain is not involved in the stability condition.
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  • 15
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    BIT 40 (2000), S. 62-73 
    ISSN: 1572-9125
    Keywords: Gaussian elimination ; stability ; pivoting
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract It has been recently shown that large growth factors might occur in Gaussian Elimination with Partial Pivoting (GEPP) also when solving some plausibly natural systems. In this note we argue that this potential problem could be easily solved, with much smaller risk of failure, by very small (and low cost) modifications of the basic algorithm, thus confirming its inherent robustness. To this end, we first propose an informal model with the goal of providing further support to the comprehension of the stability properties of GEPP. We then report the results of numerical experiments that confirm the viewpoint embedded in the model. Basing on the previous observations, we finally propose a simple scheme that could be turned into (even more) accurate software for the solution of linear systems.
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  • 16
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    BIT 40 (2000), S. 611-639 
    ISSN: 1572-9125
    Keywords: Runge-Kutta methods ; stability ; convergence ; stiff problems
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract This paper studies the stability and convergence properties of general Runge-Kutta methods when they are applied to stiff semilinear systems y′(t) = J(t)y(t) + g(t, y(t)) with the stiffness contained in the variable coefficient linear part. We consider two assumptions on the relative variation of the matrix J(t) and show that for each of them there is a family of implicit Runge-Kutta methods that is suitable for the numerical integration of the corresponding stiff semilinear systems, i.e. the methods of the family are stable, convergent and the stage equations possess a unique solution. The conditions on the coefficients of a method to belong to these families turn out to be essentially weaker than the usual algebraic stability condition which appears in connection with the B-stability and convergence for stiff nonlinear systems. Thus there are important RK methods which are not algebraically stable but, according to our theory, they are suitable for the numerical integration of semilinear problems. This paper also extends previous results of Burrage, Hundsdorfer and Verwer on the optimal convergence of implicit Runge-Kutta methods for stiff semilinear systems with a constant coefficients linear part.
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  • 17
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    BIT 40 (2000), S. 226-240 
    ISSN: 1572-9125
    Keywords: Stochastic differential equations ; regularisation ; stability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract This paper is devoted to the numerical analysis of ill-posed problems of evolution equations in Banach spaces using certain classes of stochastic one-step methods. The linear stability properties of these methods are studied. Regularisation is given by the choice of the regularisation parameter as α = $$\sqrt {\tau _n }$$ , where τ n is the stepsize and provides the convergence on smooth initial data. The case of the approximation of well-posed problems is also considered.
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  • 18
    ISSN: 1572-8781
    Keywords: porous silicon ; cis-platin ; drug delivery ; calcium phosphate ; carbo-platin
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine , Technology
    Notes: Abstract In this work, the incorporation and characterization of cis-platin (cis-diammine dichloroplatinum(II)), carbo-platin [cis-diammine(cyclobutane-1,1-dicarboxylato] platinum(II)), and Pt(en)Cl2 (ethylenediamminedichloro platinum(II)) within layers of calcium phosphate on porous Si/Si substrates are described. These materials have been characterized by scanning electron microscopy, secondary ion mass spectrometry, and X-ray energy dispersive spectroscopy. The diffusion of platinum species from the doped calcium phosphate layers has also been investigated by UV-visible absorption spectrometry and inductively-coupled plasma spectroscopy. The influence of initial platinum concentration, the impact of thermal annealing of the calcium phosphate/porous Si/Si matrix, as well as the effect of varying the ligand coordination sphere of the Pt complex on its ability to be delivered to the surroundings have also been analyzed. For the case of cis-platin, it is found that increasing the concentration of platinum complex in the electrolyte during cathodic growth of calcium phosphate results in a relatively greater concentration of Pt incorporated into the calcium phosphate layers and a larger amount of Pt which subsequently can be delivered to the surrounding medium upon exposure to solvent.
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  • 19
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    Archive of applied mechanics 70 (2000), S. 17-29 
    ISSN: 1432-0681
    Keywords: Key words free-surface flow ; solidification ; strip casting ; steady state ; nonuniqueness ; stability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Summary The paper is concerned with a one-dimensional analysis of plane open-channel flow with continuous solidification. The process is of relevance for recent developments in the casting of steel and other metals. The bottom of the channel consists of a rotating casting roll and a horizontal cooling table, where the solidified material is withdrawn with given velocity. The study is restricted to the region downstream of the top of the casting roll. Surface tension is neglected. In the main part of the analysis inviscid fluid flow is considered since the Reynolds number is very large in the applications. It is found that the steady-state solutions are nonunique in a certain parameter range. In addition to a continuous solution, there are two solutions including hydraulic jumps, with one hydraulic jump being located on the casting roll, the other one on the cooling table. Regarding the stability of the non unique solutions, the evolution of disturbances is investigated numerically as an initial-value problem. It is concluded that the hydraulic jump on the cooling table is unstable, while the other discontinuous solution as well as the continuous solution are stable for sufficiently small disturbances. Which stable solution is attained in the steady state, depends on the history of the process. Friction at the liquid/solid interface is taken into account in the last part of the analysis. A constant friction coefficient is assumed. It is found that the history of the process determines the steady-state solution if, and only if, the friction coefficient is sufficiently small. For larger values of the friction coefficient, the steady-state solution is unique and independent of the history of the transient process. Furthermore, for sufficiently large friction coefficients, stable hydraulic jumps are found, in contrast to the inviscid case, also on the cooling table.
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  • 20
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    Journal of intelligent and robotic systems 27 (2000), S. 195-209 
    ISSN: 1573-0409
    Keywords: wall-climbing robot ; electromagnetic grippers ; stability ; additional support element ; sliding and turning over conditions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Computer Science , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Legged-climbing robot is considered. Each foot of the robot has an electromagnet system for robot"s holding on a metal surface. This surface can be both vertical and inclined, including negative slope. Analytical calculation of robot stability under turn over or sliding conditions has been made. Critical slopes have been determined. One of these slopes corresponds to minimal reserve of robot stability towards sliding and another to minimal reserve of robot stability towards turning-over. As total reserve of stability of a robot is always equal to the minimal one of these reserves. Additional support elements of elastic material with high coefficient of friction, along with electromagnet, allows to increase minimal reserve of robot stability towards sliding. The use of such support elements leads to redistributing force of normal support reaction between electromagnet (which surface has low coefficient of friction) and additional support element (which surface has high coefficient of friction). It is just what leads to increasing the total friction force and as a consequence to increasing of minimal reserve of robot stability towards sliding.
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  • 21
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    Letters in mathematical physics 53 (2000), S. 313-320 
    ISSN: 1573-0530
    Keywords: partial differential equations ; nonlinearities ; symmetries ; stability ; minimization
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics , Physics
    Notes: Abstract We suggest a simple but general method of establishing symmetry properties of stable solutions of nonlinear elliptic equations. The method relies on characterization of symmetry breaking with a help of zero modes and on a generalization of the Perron–Frobenius theory.
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  • 22
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    Journal of statistical physics 101 (2000), S. 731-746 
    ISSN: 1572-9613
    Keywords: attractive Bose–Einstein condensates ; nonlinear Schrödinger equation ; stability ; ground state ; variational arguments
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We propose the critical nonlinear Schrödinger equation with a harmonic potential as a model of attractive Bose–Einstein condensates. By an elaborate mathematical analysis we show that a sharp stability threshold exists with respect to the number of condensate particles. The value of the threshold agrees with the existing experimental data. Moreover with this threshold we prove that a ground state of the condensate exists and is orbital stable. We also evaluate the minimum of the condensate energy.
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  • 23
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    Acta applicandae mathematicae 62 (2000), S. 23-130 
    ISSN: 1572-9036
    Keywords: stability ; functional equations ; Cauchy difference ; semigroup ; inequalities ; approximate
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract In this paper, we study the stability of functional equations that has its origins with S. M. Ulam, who posed the fundamental problem 60 years ago and with D. H. Hyers, who gave the first significant partial solution in 1941. In particular, during the last two decades, the notion of stability of functional equations has evolved into an area of continuing research from both pure and applied viewpoints. Both classical results and current research are presented in a unified and self-contained fashion. In addition, related problems are investigated. Some of the applications deal with nonlinear equations in Banach spaces and complementarity theory.
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    International journal of fracture 105 (2000), S. 57-79 
    ISSN: 1573-2673
    Keywords: Interface toughness ; interface debonding ; stability ; adhesive interface ; bimaterial.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract In this paper, we studied the interface debonding when a crack perpendicularly approaches an interface between two dissimilar elastic materials. An interface toughness law was first defined according to an adhesive model governing the interface fracture. By analysing the interaction between the normally approaching crack and the interface crack and by tacking account of the adhesive forces at ends of the interfacial crack, a model for studying the interface debonding and the debonding stability was established. It is observed that the interface debonding toughness depends strongly on the mixed mode locally produced over the plastic adhesive zone of the interface. Moreover, the interface debonding may be unstable, i.e. the interface debonding length may jump from an initial value to a certain final value under critical remote loading. This jump may be surprisedly important in certain cases. These results agree with the experimental works gathered so far and can be used to explain the mechanism of 'crack arrestor' formed by an interface.
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  • 25
    ISSN: 1573-4943
    Keywords: Methanol dehydrogenase ; Ca2+ ; binding ; activity ; stability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The effects of exogenously added Ca2+ on the enzymatic activity and structural stability of methanol dehydrogenase were studied for various Ca2+ concentrations. Methanol dehydrogenase activity increased significantly with increasing concentration of Ca2+, approaching saturation at 200 mM Ca2+. The effect of Ca2+ on the activation of MDH was time dependent and Ca2+ specific and was due to binding of the metal ions to the enzyme. Addition of increasing concentration of Ca2+ caused a decrease of the intrinsic tryptophan fluorescence intensity in a concentration-dependent manner to a minimum at 200 mM, but with no change in the fluorescence emission maximum wavelength or the CD spectra. The results revealed that the activation of methanol dehydrogenase by Ca2+ occurred concurrently with the conformational change. In addition, exogenously bound Ca2+ destabilized MDH. The potential biological significance of these results is discussed.
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    Acta mechanica Sinica 16 (2000), S. 264-272 
    ISSN: 1614-3116
    Keywords: nonlinear dynamics ; bifurcation ; stability ; fluid-solid interaction
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract This paper studies interactions of pipe and fluid and deals with bifurcations of a cantilevered pipe conveying a steady fluid, clamped at one end and having a nozzle subjected to nonlinear constraints at the free end. Either the nozzle parameter or the flow velocity is taken as a variable parameter. The discrete equations of the system are obtained by the Ritz-Galerkin method. The static stability is studied by the Routh criteria. The method of averaging is employed to examine the analytical results and the chaotic motions. Three critical values are given. The first one makes the system lose the static stability by pitchfork bifurcation. The second one makes the system lose the dynamical stability by Hopf bifurcation. The third one makes the periodic motions of the system lose the stability by doubling-period bifurcation.
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  • 27
    ISSN: 1573-5125
    Keywords: discharge effects on lotic invertebrates ; disturbance ; persistence ; stability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Temporal and spatial trends were examined in benthic macroinvertebrate and physical-chemical data collected for at least ten years at ten sites along the plains reach of the Cache la Poudre River, Colorado, USA. A distinct longitudinal environment gradient was found as many of the water chemistry parameter levels changed downstream from the reference site. Seasonal Kendall analyses on individual sites indicated that several chemical parameters, including conductivity, un-ionized NH3-N and NO2-N have increased since the beginning of the study within most sites. Levels of some parameters (e.g., dissolved oxygen, un-ionized NH3-N) violated aquatic life standards a few times during the study. Over 175 taxa of macroinvertebrates (primarily insects) were collected in the study reach from 1981–1996. Results from detrended correspondence analyses (DCA) on macroinvertebrate data indicated that this stretch of the river exhibited little longitudinal change beyond the two farthest upstream sites. There was a decline in macroinvertebrate density and total number of taxa within most individual sites during 1983–1984, corresponding with the highest recorded discharge in 75 years (1983) and a prolonged, heavy spring runoff in 1984. Taxa richness and density recovered to pre-1983 levels within a few months to a year following the high flows at most sites. These findings suggested that the macroinvertebrate assemblages had low resistance to disturbance, but high resilience. However, the results from DCAs and Kendall's Coefficient of Concordance (W) on individual sites for the entire study period suggested a similar macroinvertebrate community structure through time. It would appear that the composition and abundance of the lotic macroinvertebrate assemblages in the Poudre River has remained relatively constant over the long-term. This has occurred even with some potentially negative changes in water chemistry and increased urban development.
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    Journal of porous materials 7 (2000), S. 295-298 
    ISSN: 1573-4854
    Keywords: porous silicon ; photo-voltage ; d.c. conductivity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract In this study n-type and p-type PV devices have been fabricated by anodising Si wafers with various resistivities in aqueous ethanoic HF solution followed by deposition of semitransparent contacts. semitransparent deposition onto the porous layers. Various methods have been used for the optimisation of problematic contact to nanoporous layers. The measurements of importance are current-voltage (I-V) characteristics, the photovoltaic (PV) decay time spectrum, and the PV response as a function of excitation energy.
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    Journal of porous materials 7 (2000), S. 307-310 
    ISSN: 1573-4854
    Keywords: porous silicon ; photoluminescence ; degradation
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    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The evolution, under vacuum, of the photoluminescence (PL) intensity of porous silicon (PS) has been studied as function of anodisation conditions, laser line and post-anodisation treatments. It was shown that the degradation of the PL intensity depends on the internal structure of PS. In particular, the degradation is important for PS layers formed essentially by crystallites having small size or where amorphous phase exists. The experimental results have been interpreted using a theoretical model, which takes into account the variation with time of the local concentration of the luminescent centers.
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  • 30
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    Journal of porous materials 7 (2000), S. 319-321 
    ISSN: 1573-4854
    Keywords: porous silicon ; stress measurements ; thermal annealing
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Macroscopic stress measurements are used to monitor Porous Silicon processing. Silicon wafer of 1Ω cm resistivity, n-type and 〈1 0 0〉 orientation were used as starting material. Porous Silicon layers with a porosity of 57% and a thickness of 85 μm, fabricated by electrochemical anodisation, were differently dried, then the evolution of the wafer deflection has been followed with storage time in air. Thermal treatments both in inert and oxidant atmosphere have been performed up to 1000°C. The stress behaviour vs. temperature allows to estimate the hydrogen desorption activation energy.
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  • 31
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    Journal of porous materials 7 (2000), S. 373-376 
    ISSN: 1573-4854
    Keywords: porous silicon ; superlattices ; oxidation
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    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Initial stage of porous silicon (PS) formation has been studied in an original way. Multilayer structures constituting of very thin layers of low porosity and thick layers of high porosity have been fabricated and characterised by optical tools and electron microscopy. The non linear behaviour resulting in a change in the dissolution velocity has been quantified by using a stack layer structure. Finally using thermal oxidation it has been shown that, due to the selective oxidation as a function of the porosity, porous silicon can be used to produce a Si/SiO2 like structure.
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  • 32
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    Journal of porous materials 7 (2000), S. 345-348 
    ISSN: 1573-4854
    Keywords: porous silicon ; interface growth ; Laplacian growth ; linear stability analysis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract We develop a Laplacian model of interface growth which includes basic features of the anodisation of silicon in hydrofluoric acid. Our aim is to find mechanisms for the characteristic properties of porous silicon formation, such as the transition from electropolishing to pore formation and the typical pore distance. The local etching rate of the interface between the semiconductor and the electrolyte is determined by the local current density. We model the diffusive transport of charge carriers in the semiconductor and of reactants in the electrolyte including the basic features of the electrochemical reaction at the interface. A linear stability analysis of a flat and planar interface is performed in order to study the initial state of pore formation.
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  • 33
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    Journal of porous materials 7 (2000), S. 357-361 
    ISSN: 1573-4854
    Keywords: porous silicon ; X-ray diffraction ; differential scanning calorimetry
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The structural properties of (111) oriented p+ type Porous Silicon (PS) samples are investigated using various X-ray diffraction techniques and compared to (001) p+ type PS layer structure. High resolution X-ray diffractometry was used to record rocking curves and reciprocal space maps, giving indications about the crystalline quality of the PS samples as well as about the pore orientation. X-ray diffraction and reflectivity performed on thin PS layers allow to estimate the layer thickness, porosity and roughness of the PS/substrate interface.
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  • 34
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    Journal of mathematical chemistry 28 (2000), S. 325-340 
    ISSN: 1572-8897
    Keywords: numerical method ; stability ; Hopf bifurcation ; coupled oscillator
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mathematics
    Notes: Abstract A second-order accurate numerical method has been proposed for the solution of a coupled non-linear oscillator featuring in chemical kinetics. Although implicit by construction, the method enables the solution of the model initial-value problem (IVP) to be computed explicitly. The second-order method is constructed by taking a linear combination of first-order methods. The stability analysis of the system suggests the existence of a Hopf bifurcation, which is confirmed by the numerical method. Both the critical point of the continuous system and the fixed point of the numerical method will be seen to have the same stability properties. The second-order method is more competitive in terms of numerical stability than some well-known standard methods (such as the Runge–Kutta methods of order two and four).
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  • 35
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    Journal of porous materials 7 (2000), S. 107-110 
    ISSN: 1573-4854
    Keywords: porous silicon ; conductivity ; percolation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The AC conductivity of a percolation model with local energetical disorder for porous Silicon in three dimensions, σ(ω), is studied by Monte Carlo simulations. The model includes both diffusion and recombination processes and σ(ω) is obtained by a Fourier transform of the mean-square displacement of the carriers, where hopping diffusion of a single type of carrier (either an electron or an exciton) and two types of carriers (an electron and a hole) are considered. It is found that at low temperatures, the behavior of σ(ω) depends sensitively on the type of carrier considered.
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  • 36
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    Journal of porous materials 7 (2000), S. 233-237 
    ISSN: 1573-4854
    Keywords: porous silicon ; integrated optics ; wave guides
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract In this work we report a principle that allows one to write visible light emitting silicon patterns of arbitrary shape down to the sub-micrometer scale. We demonstrate that porous Si growth can electrochemically be initiated preferentially at surface defects created in an n-type Si substrate by Si++ ion bombardment. Using a focused ion beam (FIB) as a source of ions, arbitrary defect patterns can be written into a substrate. The growth of light emitting porous silicon is then selectively achieved by an electrochemical treatment which triggers Si dissolution only at these defect sites. The selectivity of the electrochemical dissolution reaction can be attributed to a facilitated Schottky barrier breakdown at the implanted surface defects which leads to the desired pore formation in confined surface areas.
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  • 37
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    Journal of porous materials 7 (2000), S. 267-270 
    ISSN: 1573-4854
    Keywords: porous silicon ; photoluminescence ; electroluminescence ; band structure ; experimental
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Electroluminescence, photoluminescence, and current-voltage measurements have been carried out on a series of samples consisting of porous silicon on top of crystalline silicon. The electroluminescence spectral distributions are markedly shifted towards long wavelengths in comparison to those seen with photoluminescence. The results are discussed and explained in terms of the energy-band structure of porous silicon. The problem of injecting electrons as well as holes into the porous silicon layer is pointed out and discussed.
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  • 38
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    Journal of porous materials 7 (2000), S. 315-318 
    ISSN: 1573-4854
    Keywords: porous silicon ; nonradiative ; radiative recombination
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    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract An experimental investigation of the general characteristics of nonradiative and radiative recombination of charge carriers in strongly excited porous silicon is presented. It is shown that photoconductivity, photomagnetoelectric effect, quantum yield, and intensity of visible radiation of porous silicon demonstrates strong nonlinearities against laser excitation intensity. It is suggested that the band-to-band Auger recombination is dominant similar to that in crystalline silicon, whereas the visible luminescence is determined by the bimolecular process. The nonequilibrium density of charge carriers Δn ≈ 1019 cm−3, and the bimolecular radiative recombination coefficient Brad ≤ 9 × 10−14 cm3/s have been found.
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  • 39
    ISSN: 1573-4854
    Keywords: porous silicon ; thermal annealing ; nuclear magnetic resonance spectroscopy ; infrared spectroscopy
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract 29Si nuclear magnetic resonance (NMR) spectra and infrared spectra were measured for as-prepared and annealed porous silicon (PS) samples to characterize the change of PS structure. Annealing changed the infrared spectra remarkably: after 4-h annealing, the signals due to SiH2 disappeared and the intensity of the signals due to SiH decreased. On the other hand, the 29Si NMR spectra with magic-angle spinning (MAS) were not much affected by the annealing. The linewidth of spectra without MAS, however, increased with annealing time with the peak location unchanged. Annealing caused hydrogen on the PS surface to be desorbed, especially in the case of SiH2 species, and (SiH)2 dimer structure was produced during the annealing.
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  • 40
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    Journal of porous materials 7 (2000), S. 335-338 
    ISSN: 1573-4854
    Keywords: porous silicon ; oxidation ; calorimeter ; desorption ; activation energy
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The oxidation of porous silicon has been studied using differential scanning calorimeter. The oxidation was found to consist of two parts with different activation energies. This indicates the existence of two different reaction mechanism. The results from the hydrogen desorption measurements have been used to study the different oxidation behaviour of the n- and p+-type porous silicon. The results show that the dihydride structure dominates on the surface of the n-type porous silicon, contrary to p+-type porous silicon, where the monohydride is the major structure. Explanations of these features are discussed. Using the activation energy, the surface termination effects are investigated. The best improvement in the activation energy was observed in the sample, whose surface was partially stabilized by ammonium groups.
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  • 41
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    Journal of porous materials 7 (2000), S. 381-383 
    ISSN: 1573-4854
    Keywords: porous silicon ; microstructure ; infrared spectroscopy
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    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract We have investigated the effect of light exposure and ultrasonic (US) treatment on the formation of porous Si layers grown by electroless stain-etching technique. It was shown that; the He-Ne laser exposure resultedin a considerable increase in both the hydrogenation and the oxidation amounts in n-type Si, but a decrease in p-type wafers. The effect is attributable to effective change in the concentration of free hole carriers. The UV light exposure has led to the shift at the peak positions, indicating probably a change in bonding configuration, and increase in oxidation. Also, a correlation was established between the ultrasonic treatment and the microstructure. The US treated samples exhibit a decrease in hydrogenation and oxidation. UV exposure together with the US has led to a further decrease in both hydrogen and oxygen amounts, which was rather indicative of an excessive surface etching.
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  • 42
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    Journal of porous materials 7 (2000), S. 385-388 
    ISSN: 1573-4854
    Keywords: hydrogen desorption ; photoluminescence decay ; porous silicon
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    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The decay under illumination of the photoluminescence (PL) intensity ofn-type porous silicon (PS) samples prepared by electrochemical etching has been investigated. We have found that the PL evolution with illumination time presents two different stages: an initial very fast decay which lasts ∼300 s, followedby a second one, much slower, which extends for times longer than 104 s. Thisevolution suggests that two different mechanisms could be responsible for the PLintensity decay. Samples subjected to different illumination times were studied byThermal Desorption Spectroscopy (TDS). The desorption rate of H2 and SiHx species was monitored during linear heating of the samples. A qualitative correlationbetween the decay of the PL intensity under illumination and the amount of H2 and SiHx species evolved from the illuminated samples has been observed. Experimentaldata suggest that H2 could be desorbed from the sample during the illuminationtime through a photoinduced H2 desorption process, inducing the decrease of the PLintensity.
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    Journal of porous materials 7 (2000), S. 323-326 
    ISSN: 1573-4854
    Keywords: positron annihilation ; porous silicon
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Positron lifetime spectroscopy has been used to investigate a porous silicon film subjected to heat treatments up to 1170°C. Annealings between 300 and 500°C resulted in a 17% mass increase of the film due to oxygen uptake following the effusion of hydrogen. The positron data also indicate that vacancy clusters are formed in the silicon oxide layer or the silicon oxide—silicon interface surrounding the nanocrystallites as oxygen replaces the effusing hydrogen. The vacancy cluster concentration, which may have a bearing on the photoluminescent properties, increased by a factor of three with heating to 500°C and then decreased to one-third the original value at higher temperatures. Above 900°C vacancy migration and clustering occurred, accompanied by visible deterioration of the film.
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    Journal of porous materials 7 (2000), S. 339-343 
    ISSN: 1573-4854
    Keywords: porous silicon ; infrared spectroscopy ; oxidation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The Si–Si vibrational states near the surface region of porous silicon has been characterized using Fourier Transform Infrared Spectroscopy (FTIR) due to its enlarged surface area. By means of anodic etch and oxidization experiments, two Si–Si vibration modes of porous silicon have been identified as near the surface regions and in the bulk, respectively. The intensity of absorption peak at 620 cm−1, which originates from the Si–Si bonds vibrations on the surface and near surface regions of porous silicon, is found to vary depending on the length of etch and degree of oxidation of porous silicon, which exists before etching and is recovered again after fully oxidation. The peak of 610 cm−1 doesn't change throughout the oxidation experiment, and to be assigned for Si–Si bond vibrations in the bulk. With an extra irradiation of Nd:Yag laser on the PS sample the Raman and FTIR spectra reveal a red shift. These results can give an interpretation to explain the different phenomenon of Si–Si vibrations of Raman and FTIR spectroscopy.
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  • 45
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    Journal of porous materials 7 (2000), S. 367-371 
    ISSN: 1573-4854
    Keywords: porous silicon ; luminescence ; light assisted chemical etching
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract A new approach to the problem of preparation of laterally structured luminescent porous silicon is proposed. The effect is based on the photosensitivity of chemical etching of silicon. Contrary to the other technique recently reported where the porous layer was modified with light assisted chemical dissolution, a one stage anodisation-free process is used. Any desired lateral structure can be produced, depending on the illumination pattern, which is defined by optical imaging. A 2D micro-array was prepared as an example of this fast mask-free technology with an accuracy of 5 μm. Samples have a broadband visible photoluminescence centred at 680 nm when illuminated with UV or blue light. The results are analysed to determine the possible achievable accuracy of the technique and to improve our understanding of the mechanism of light assisted etching.
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  • 46
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    Journal of porous materials 7 (2000), S. 397-400 
    ISSN: 1573-4854
    Keywords: porous silicon ; Raman spectroscopy ; immersion plating ; silver
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    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Raman scattering from porous silicon layer into which silver is immersion-plated was studied. Ag-deposited samples show extra Raman bands. Heat treatment of the Ag-deposited samples results in a great decrease in such Raman bands. Also dipping in hydrofluoric acid solution causes a spectral change. Some comments on the assignment of the Raman peaks of the Ag-deposited porous silicon are given, and the structure of porous silicon on which metal is immersion-plated is discussed.
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  • 47
    ISSN: 1573-4854
    Keywords: porous silicon ; photoluminescence ; synchrotron radiation
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    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Using synchrotron as a tunable excitation source, we have carried out a study on the photoluminescence systematics from a series of porous silicon samples prepared under different conditions, Luminescence spectra were recorded with excitation photon energies tuned to the Si L3,2 absorption edge (∼100 eV). The luminescence yield was in turn used to monitor the Si L3,2-edge absorption characteristics of porous silicon. A trend of luminescence wavelength and intensity as a function of preparation conditions emerges. Other related observations are also noted.
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  • 48
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    Journal of porous materials 7 (2000), S. 191-195 
    ISSN: 1573-4854
    Keywords: cell growth ; porous silicon ; cytotoxicity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The paper reports the results of the study of cell culture growth at the surface of porous silicon. They show that porous and poly(nano)crystalline Si offer significant advantages over bulk Si surfaces for cell adherence and viability: these materials do not require coating with substances such as polylysine to support cell growth; porous Si is light-addressable because of photoluminescence and photovoltaic effects noted [Unal and Bayliss, J. Appl. Phys. 80, 3532 (1996)], allowing the potential for optical data transfer and less susceptibility to interference from external electronic equipment; finally nanostructured coatings can be applied to most object shapes, giving flexibility in their application.
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  • 49
    ISSN: 1573-4854
    Keywords: porous silicon ; superlattices ; photoluminescence
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Porous silicon photoluminescence and electroluminescence can be controlled by periodically modulating the material porosity to form high quality multilayer stacks and microcavities. Important issues not yet fully addressed are (a) the precise role played by this microstructuring, given that the luminescence is distributed throughout the entire structure and that the low porosity layers are highly absorbing at short wavelengths, and (b) whether the quality of such microcavities could be sufficient to support lasing. Using both experimental and theoretical techniques, the emission and reflection properties of different porous silicon single and multilayer structures have been investigated in order to understand further and exploit the nature of light propagation within them.
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  • 50
    ISSN: 1573-4854
    Keywords: porous silicon ; patterning ; photolythography dielectric filters ; reactive ion etching (RIE) ; microoptics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Porous silicon (PS) layer systems have a broad range of possible applications. An advantage is the good control of the refractive index and the etch rate of the layers by the applied current density and the time respectively. For micro-optical devices you need patterned PS. For some optical devices it is not sufficient to have only one filter but it is necessary to form filters with different properties on a small area. We applied a method (M. Frank, U.B. Schallenberg, N. Kaiser, and W. Buß, in Conference on Miniaturized Systems with Microoptics and Micromechanics, edited by M.E. Moamedi, L.J. Hornbeck, and K.S.J. Pister (SPIE, San Jose, 1997), SPIE Proceedings Series 3008, p. 265) to PS which fits this goal by the following steps: fabrication of the desired reflectors below each other and partial removal of upper reflectors with reactive ion etching (RIE). The technological aspects of patterning PS after the fabrication are an important topic of this work. Problems are discussed in detail and solutions are given.
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    Journal of porous materials 7 (2000), S. 115-118 
    ISSN: 1573-4854
    Keywords: porous silicon ; synchrotron radiation ; coulomb blocade
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract In this paper we demonstrate that photo-electron emission excited by X-UV synchrotron radiation can be used as a “contactless” probe of the gross conduction processes in porous silicon. Moreover we demonstrate that this approach reveals the underlying conduction geometry. We show that conduction in porous silicon is to some degree controlled by percolation phenomena and finally present data which support the notion that the fundamental blocking process may be Coulomb Blockade [P.A. Lee, Physica B 189, 1–5 (1993); D. Ali and H. Ahmed, Appl. Phys. Lett. 64, 2119–2120 (1994)].
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    Journal of porous materials 7 (2000), S. 263-266 
    ISSN: 1573-4854
    Keywords: porous silicon ; time-resolved photoluminescence ; decays
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Porous silicon has been studied with time-resolved photoluminescence, and growth as well as decay curves have been measured at several detection energies, with sample temperatures between 10 and 300 K. In the decay curves, three components are mainly observed, a small one which is very fast, with time scales of the order of nanoseconds or faster, the main component having time scales of the order of milliseconds, and a very small, very slow component, with time scales of the order of seconds. The main components can in most—but not all—cases be fitted well with stretched exponentials containing two fitting parameters. Of these, it comes out that the parameter accounting for disorder or the like depends only little upon detection energy and temperature, whereas the parameter accouting for the development in time decreases substantially for increasing temperature. The results are discussed.
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  • 53
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    Journal of porous materials 7 (2000), S. 279-282 
    ISSN: 1573-4854
    Keywords: porous silicon ; optical absorption ; theoretical modelling
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The optical properties of porous silicon (p-Si) are calculated from the electronic band structure obtained by means of an sp3s* tight-binding Hamiltonian and a supercell model, in which the pores are columns detched in crystalline silicon (c-Si). The disorder in the pore sizes and the undulation of the silicon wires are considered by the existence of arandom perturbative potential, which produces non-vertical interband transitions, otherwise forbidden. A typical interval around each k-vector (optical window), where non-vertical transitions make an important contribution, depends on the value of the disorder and its order of magnitude is given by l−1, where l is the localization length. The calculated absorption spectra are compared with experiments, showing good agreement.
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  • 54
    ISSN: 1573-4854
    Keywords: nanocrystalline silicon ; porous silicon ; cathodoluminescence
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Luminescence emission of LPCVD polycrystalline silicon films has been studied by cathodoluminescence (CL) in the scanning electron microscope. As-deposited films show visible luminescence with dominant blue band. The relative intensity of blue emission is enhanced by implantation and by slight anodization treatments. Our investigations are consistent with previous PL results and indicate that the origin of blue emission is related to quantum confinement effects. On the other hand, the effect of annealing in these samples is a reduction of the CL signal that could be related to the increase of the nanocrystals size.
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  • 55
    ISSN: 1573-4854
    Keywords: porous silicon ; photoacoustic spectroscopy ; energy band
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Porous silicon has been studied with photoluminescence, photoluminescence excitation, and photoacoustic spectroscopy. From the luminescence data, an energy-level diagram related to the luminescence is constructed. The diagram is confirmed in detail by the photoacoustic spectra. The results are discussed with the conclusion that they are in good agreement with the surface-band oxyhydride-like emitter, which recently has been established as the source for the photoluminescence from porous silicon.
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  • 56
    ISSN: 1573-4854
    Keywords: porous silicon ; Raman spectroscopy ; morphology
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Raman-light scattering in porous silicon samples with oriented quantum wires was studied. It was shown, that the experimental data depends on the type of organization of wire system. The explanation of observed effect is discussed.
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    Journal of porous materials 7 (2000), S. 287-290 
    ISSN: 1573-4854
    Keywords: porous silicon ; sensors ; gas absorption
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract In this paper we report the sensitivity of porous silicon photoluminescence (PL) to diluted mixtures of methane and carbon monoxide in synthetic air. We also investigate the separate effect of synthetic air, purified nitrogen and relative humidity on both photoluminescence and conductance (G). Porous silicon samples have been prepared from n-type silicon substrates. We find that PL intensity and G decrease in synthetic air with respect to their values in N2. Presence of carbon monoxide reduces the PL intensity while methane provokes the opposite behaviour. The dependence of the PL spectra on methane and carbon monoxide concentrations has been investigated. The observed effects can be related to gas induced modifications in porous surface and suggest that porous silicon can be employed in gas sensor technology.
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    Journal of porous materials 7 (2000), S. 299-301 
    ISSN: 1573-4854
    Keywords: porous silicon ; Schottky emission ; photoluminescence
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Electrical transport in Gold/porous silicon/crystalline silicon junctions has been studied. The junctions are found to improve when the porous silicon is exposed to a hydrogen plasma before depositing the top metal. The hydrogen passivated junctions exhibited higher current levels and emitted light at lower voltages as compared to the unhydrogenated ones. Internal photoemission measurements were carried out to investigate the gold/porous silicon barrier. The barrier height determined from the Fowler plot is independent of the top material. The temperature dependence of the barrier height is similar to that of the crystalline silicon energy gap.
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    Journal of porous materials 7 (2000), S. 11-16 
    ISSN: 1573-4854
    Keywords: porous silicon ; kinetics electrochemistry ; etching ; surface analysis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The electrochemistry of porous silicon formation has been investigated by different electrochemical as well as surface analytical methods. The kinetics of pore nucleation was observed as small steps in fast current and potential pulse transients. Oxidic intermediates were identified by ex-situ XPS. Cyclic voltammetry in solutions of different HF concentration was correlated with the etching rate of silicon dioxide. On the basis of these experimental data, an electrochemical model for the porous silicon formation is presented.
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  • 60
    ISSN: 1573-4854
    Keywords: porous silicon ; four-wave mixing ; microwave harmonics generation
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    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract We apply optical contactless techniques, namely the four-wave-mixing and microwave harmonics generation for the characterization of nanocrystalline free-standing films and platelets of microcrystalline porous silicon. We observe (i) full carrier localization and significant lifetime shortening in free-standing films, which is thought to be a manifestation of their low-dimensional confinement, (ii) increased carrier lifetime in microcrystalline porous silicon, presumably originating from passivated surface states at the surface of pores.
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    Journal of porous materials 7 (2000), S. 407-410 
    ISSN: 1573-4854
    Keywords: porous silicon ; stain etching ; morphology ; HREM ; reflectance spectrometry
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Porous silicon films, nearly 100 nm thick, were produced by stain etching of n+-type silicon substrates. The films were studied by a non-destructive technique: dielectric function profiles were deduced by spectral reflectance via a finite difference model, and porosity was computed by the Effective Medium Approximation. The obtained information, combined with High Resolution Electron Microscopy observations,provided a deeper insight on the relations among technological process, morphology and reflective properties. Our preliminary results outline the possibility to control the porosity profile as well the reflectance of the porous films via the oxidising species concentration in the stain etching solution.
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  • 62
    ISSN: 1573-4854
    Keywords: stain etching ; porous silicon ; neutron reflection
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    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract We have monitored the stain-etching of silicon in real time using neutron reflection. The etching process does not occur uniformly with time and possible dynamics of the process is under consideration. Distinct irregularities are produced on the PS surface and some gas bubbling from the cell is observed.
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    Journal of porous materials 7 (2000), S. 77-80 
    ISSN: 1573-4854
    Keywords: porous silicon ; ZnSe ; electrochemical deposition
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Electroplating of II-VI semiconductors like ZnSe into porous silicon can be an efficient and low cost method to fill the porous volume with a transparent and conductive material. With n-type porous layers, ZnSe impregnation is more effective near the sample surface because of reaction rate limitations due to diffusion in the electrolyte. In this paper, it is shown that the deposition of ZnSe into p-type porous silicon can be localized in the lower part of the porous layer if the reduction reaction rate is monitored by limiting the charge carrier supply. This can be done by controlling the power of the laser beam which photo-generates the carriers at the bottom of the pores. Studying the porous layer chemical composition by Auger electron spectroscopy confirms that the deposit is localized at the pore bottom, whereas the changes in the chemical composition of the porous silicon surface are analyzed by infra-red spectroscopy.
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    Journal of porous materials 7 (2000), S. 97-101 
    ISSN: 1573-4854
    Keywords: AC impedance ; porous silicon ; residual moisture ; aging effects
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The present work aims to study the AC impedance of porous silicon films left in the contact with the anodizing bath after their growth is over. It is shown that the layers grown at p-type silicon experience noticeable changes of their impedance behavior resulting in the gradual growth of electrical resistance of the p-PS layer. This change is assumed to be related with the deposition of a passive layer at the pore bottoms associated with reabsorption of dissolved Si-containing material from the electrolyte. The layers formed at n-Si also show changes of the electrical impedance, although they are much more complicated than in the previous case (dependence of the kinetics of changes on the polarization current and polarization time, presence of the inductive loops at the impedance spectra, etc.
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    Journal of porous materials 7 (2000), S. 169-172 
    ISSN: 1573-4854
    Keywords: EXAFS ; XEOL ; porous silicon
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Results of an EXAFS investigation on porous Silicon carried out by X-ray Excited Optical Luminescence (XEOL) and Total Electron Yield (TEY) techniques, at the Si K absorption edge, are reported. For the first time XEOL spectra of porous silicon have been recorded in a wide energy range (1800–2500 eV) and EXAFS signals have been singled out from them. Simultaneous TEY and XEOL measurements yield to different results: in particular TEY-EXAFS is sensitive up to the third coordination shell of Si, while XEOL-EXAFS reveals only the contributions of the first two coordination shells; moreover they show a different dependence on changes of the etching parameters. This evidences the sensitivity of XEOL technique to the local structure of the quantum confined luminescent sites. The dependence of the light emission properties on the main preparation parameters and their influence on the short-range structure of red and yellow porous silicon samples are also investigated.
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  • 66
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    Journal of porous materials 7 (2000), S. 187-190 
    ISSN: 1573-4854
    Keywords: porous silicon ; temperature programmed desorption ; FTIR ; silane
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The gaseous species desorbed from porous silicon (PS) were investigated using the method of temperature programmed desorption (TPD) and fourier transform infrared spectroscopy (FTIR). Silicon wafers (25–50 Ω cm, p−, FZ) were anodised in 40% HF and HF/C2H5OH electrolytes. The PS samples were linearly heated at 1.5 K s−1 using a custom built heating unit in a oil-free pump backed vacuum chamber at a base pressure of 〈10−8 torr. A quadrupole mass spectrometer, which was used as the detector, was fitted in line of sight of the sample at a distance of about 6 mm. It was observed that silane was liberated during the heating of porous silicon samples produced from both electrolytes. The peak temperature at which this occurred was at 570 ± 10 K. This temperature coincides with the temperature of silicon-silicon bond breakage in ≡Si–SiH3 groups on the pore walls, as shown by the FTIR results. It is proposed that silane formation involves the reaction of the Si-silyl group with moisture: ≡Si–SiH3 + H2O → ≡Si–OH + SiH4.
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  • 67
    ISSN: 1573-4854
    Keywords: porous silicon ; oxidation ; liquid crystal
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract In this paper we demonstrate the filling of porous silicon (PS) layers with liquid crystals (LC's) in order to control the reflectance electrically. The preparation of PS and the choice of the right group of LC's will be presented. Especially an oxidation of PS is necessary so that the methods and parameters of oxidation will also be discussed. As a first result the increasing and decreasing of the thickness oscillations in the reflectance as a function of the applied voltage can be observed.
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    Journal of porous materials 7 (2000), S. 227-231 
    ISSN: 1573-4854
    Keywords: porous silicon ; micromachining ; localised anodisation ; fibre positioning
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Low-loss optical fibre connections require deep grooves etched in silicon substrate for accurate fibre positioning. As shown in this paper these grooves can be obtained by using localised formation of porous silicon on patterned substrates. Cr-Au masking layer with a duration in HF solution longer than 30 min is used to fabricate grooves with a depth higher than 75 μm. N+-type silicon provides grooves with a pseudo-V shape which is compatible with accurate fibre alignment. By using this technology, arrays of optical fibres are positioned with an accuracy higher than 1 μm.
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    Journal of porous materials 7 (2000), S. 243-246 
    ISSN: 1573-4854
    Keywords: porous silicon ; oxidation ; refractive index ; optical waveguide ; optical losses
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Further results on the optical properties of oxidised porous silicon as a function of porosity and oxidation conditions are presented in this paper. The refractive index varies logarithmically with the oxidation time irrespective of the porosity of the sample and the oxidation temperature. Burried waveguides have been realised from as-prepared and oxidised porous silicon. Optical losses of 18 dB cm−1 at λ = 1.3 μm have been obtained after oxidation at 800°C for 35 min.
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    Journal of porous materials 7 (2000), S. 119-123 
    ISSN: 1573-4854
    Keywords: porous silicon ; light-emitting diode ; series resistance ; external quantum efficiency
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract In this paper, two novel structures of porous silicon (PS) light-emitting diodes (LEDs) are proposed aiming at the reduction of series resistance, Rs. The basic idea of the novel structures is to suppress the excessive growth of nanoporous silicon (nano-PS) layer that is electroluminescence- (EL-) active but highly resistive. The initial wafer of the first structure consists of a lightly-doped layer stacked on a highly-doped substrate. As a consequence of anodization, nano-PS layer is formed only in the lightly-doped layer, while meso-PS layer with moderate resistivity is formed in the highly-doped substrate. The second structure consists of alternately stacked nano- and meso-PS layers, since it is expected that multiple thin nano-PS layers connected in series are less resistive than a single thick nano-PS layer. Preliminary experimental results proved the effectiveness of these novel structures on the reduction of Rs.
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    Journal of porous materials 7 (2000), S. 271-273 
    ISSN: 1573-4854
    Keywords: porous silicon ; photoconductivity ; energy band gap
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Photocurrent measurements have been carried out on a series of samples consisting of porous silicon on top of crystalline silicon, in the temperature range 10–300 K. From the experimental data set, the electrical band-gap energy of porous silicon is deduced to be (1.80 ± 0.01) eV, independent of sample temperature. The results are discussed with the conclusion that for the samples studied here, the electrical bandgap in porous silicon is of molecular nature and cannot be related to quantum-confinement properties of nanocrystals of elemental silicon.
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    Chemistry of heterocyclic compounds 36 (2000), S. 115-133 
    ISSN: 1573-8353
    Keywords: thiophthalylium salts ; methods of preparation ; structure ; stability ; reactivity ; electrophilic properties
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Methods of preparation, chemical reactions, the structures and reactivity of thiophthalylium ions are reviewed.
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    Journal of scientific computing 15 (2000), S. 441-456 
    ISSN: 1573-7691
    Keywords: modified conjugate gradient method ; conjugate gradient method ; Krylov space ; convergence rate ; stability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Computer Science
    Notes: Abstract We consider the modified conjugate gradient procedure for solving A $$\underline x $$ = $$\underline b $$ in which the approximation space is based upon the Krylov space associated with A 1/p and $$\underline b $$ , for any integer p. For the square-root MCG (p=2) we establish a sharpened bound for the error at each iteration via Chebyshev polynomials in $$\sqrt A$$ . We discuss the implications of the quickly accumulating effect of an error in $$\sqrt A$$ $$\underline b $$ in the initial stage, and find an error bound even in the presence of such accumulating errors. Although this accumulation of errors may limit the usefulness of this method when $$\sqrt A$$ $$\underline b $$ is unknown, it may still be successfully applied to a variety of small, “almost-SPD” problems, and can be used to jump-start the conjugate gradient method. Finally, we verify these theoretical results with numerical tests.
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    Neural processing letters 12 (2000), S. 129-144 
    ISSN: 1573-773X
    Keywords: associative memory ; dynamical systems ; Glauber dynamics ; Hopfield model ; infinite dimensional state space ; stability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Computer Science
    Notes: Abstract A generalization of the Little–Hopfield neural network model for associative memories is presented that considers the case of a continuum of processing units. The state space corresponds to an infinite dimensional euclidean space. A dynamics is proposed that minimizes an energy functional that is a natural extension of the discrete case. The case in which the synaptic weight operator is defined through the autocorrelation rule (Hebb rule) with orthogonal memories is analyzed. We also consider the case of memories that are not orthogonal. Finally, we discuss the generalization of the non deterministic, finite temperature dynamics.
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    Dynamics and control 10 (2000), S. 47-61 
    ISSN: 1573-8450
    Keywords: stability ; robust control ; Lyapunov approach ; discrete system
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract Control design of nonlinear discrete-time uncertain systems with (possibly fast) uncertain parameters is considered. We study the effect of only partially compensating the uncertainty. The optimal choice of a design parameter, which indicates the amount of compensation, is recommended.
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    Dynamics and control 10 (2000), S. 255-276 
    ISSN: 1573-8450
    Keywords: continuous-time systems ; frequency weighted model reduction ; gradient flow ; optimization ; stability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract Inthis paper, a model reduction technique based on optimizationis presented. The objective function minimized is the impulseenergy of the overall system. An extension of the technique tothe frequency weighted case is also presented, where single-sidedor double-sided weightings can be incorporated in the reductionprocedure. The paper proposes an alternative to find an optimizationsolution by solving ordinary differential equations which aregradient flow associated with the objective function to be minimized.Two examples are presented to illustrate the effectiveness ofthe method.
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    Aquatic geochemistry 6 (2000), S. 1-17 
    ISSN: 1573-1421
    Keywords: lakes ; density ; compressibility ; expansibility ; conductivity ; stability ; pvt properties
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences
    Notes: Abstract In recent years, a number of workers have studied the stability of deep lakes such as Lake Tanganyika, Lake Baikal and Lake Malawi. In this paper, the methods that can be used to determine the effect that the components of lakes have on the equation of state are examined. The PVT properties of Lakes have been determined by using apparent molal volume data for the major ionic components of the lake. The estimated PVT properties (densities, expansibility and compressibilities) of the lakes are found to be in good agreement with the PVT properties (P) of seawater diluted to the same salinity. This is similar to earlier work that showed that the PVT properties of rivers and estuarine waters could also be estimated from the properties of seawater. The measured densities of Lake Tanganyika were found to be in good agreement (± 2 × 10-6 g cm-3) with the values estimated from partial molal properties and the values of seawater at the same total salinity (ST = 0.568‰). The increase in the densities of Lake Tanganyika waters increased due to changes in the composition of the waters. The measured increase in the measured density (45 × 10-6 g cm-3) is in good agreement (46 × 10-6 g cm-3) with the values calculated for the increase in Na+, HCO3 -, Mg2+, Ca2+ and Si(OH)4. Methods are described that can be used to determine the conductivity salinity of lakes using the equations developed for seawater. By combining these relationships with apparent molal volume data, one can relate the PVT properties of the lake to those of seawater.
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    Mechanics of composite materials 36 (2000), S. 475-480 
    ISSN: 1573-8922
    Keywords: stability ; three-dimensional theory ; tribotechnics ; elastoplastic deformations ; viscoelasticity ; surface ; tracking and dead loads ; laminated coating ; piecewise-homogeneous model ; active loading
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The problem of surface instability of laminated coatings with inelastic properties is considered within the framework of a model of piecewise-homogeneous media on the basis of the three-dimensional linearized theory of stability. A general statement of the problem is formulated and the basic characteristic equations are derived. The solutions of particular problems are obtained for elastoplastic and viscoelastic models of solids.
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    Journal of applied spectroscopy 67 (2000), S. 852-856 
    ISSN: 1573-8647
    Keywords: porous silicon ; photoluminescence spectrum ; photoexcitation spectrum
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract It is found that the films of n +-type porous silicon of low (10–50%) porosity exhibit photoluminescence in the region 400–500 nm after a 5-month storage in an air atmosphere. The spectrum of blue photoluminescence of the least porous but strongly oxidized films has maxima at 417, 435, and 465 nm. The same spectrum structure manifests itself upon the introduction of an Er3+- and Yb3+-containing complex. The mechanisms of blue photoluminesence are discussed.
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    Journal of dynamical and control systems 6 (2000), S. 503-510 
    ISSN: 1573-8698
    Keywords: nonconservative mechanical systems ; stability ; Lyapunov functions ; attraction domain
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract Mechanical systems subjected to dissipative, gyroscopic, conservative, and also nonconservative positional forces are considered. The question of the effect of dissipative, gyroscopic, and conservative forces on the motion stability of a mechanical systems is determined by classical Kelvin–Chetaev theorems [1]. The presence of nonconservative positional forces considerably complicates the situation and excludes direct application of these theorems. Applying Lyapunov's functions method the condition of asymptotic stability of a mechanical system under the action of all listed above forces is obtained. Moreover, the estimation of the attraction domain in phase space is found. The precession system which is used in the solution of some problems in the applied theory of the gyroscopic systems is also examined. The connection between the stability of origin and precession systems is detected. Theoretical results are applied to the stabilization problem of stationary motion of the balanced gimbal suspension gyro by means of external moments.
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  • 81
    ISSN: 1573-5060
    Keywords: Avena ; genetic correlation ; genotype-environmentinteraction ; stability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Abstract In order to test if selection can improve a population's adaptation to diverse environments simultaneously, three cycles of recurrent selection based on grain yield in Iowa, Idaho, and Norway were practiced in an oat (Avena sativaL.) population developed from North American, Scandinavian, and wild species (A. sterilis L.) germplasm sources. Specific objectives were to determine if selection: increased mean yields across environments and within all environments; changed the genetic correlation of yields in different environments; and changed genetic variation for yield within the population. We evaluated 100 to 210 randomly-chosen families from each cycle of selection at three Iowa locations, in Idaho, and in Norway for two years. Grain yield within each location and mean yields across locations increased significantly over cycles of selection. Mean yields across locations expressed as a percent of the original population mean increased at a rate of 2.6% per year. Several families from the third cycle population exhibited both high mean yields across locations and consistently high yields within all locations. Average genetic correlations of yield in different environments were higher in the second cycle than in the original population. A trend of reduced genetic variation and heritability was observed in Iowa only. These results suggest that we successfully improved mean population yield both within and across locations, and yield stability across environments, and in developing families with outstanding adaptation to diverse environments.
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  • 82
    ISSN: 1573-5168
    Keywords: amberjack ; myosin heavy chain ; cDNA ; α-helix ; coiled coils ; stability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The structural stability of fish myosin depends upon species and temperatures of water in which fish live. Primary, secondary, and quaternary structures of myosin heavy chain (MyHC) from three species of fish living at different temperature ranges have been compared with those of rabbit MyHC in order to investigate the differences in stability. Primary structure of MyHC, although being accessible for warm-water and cold-water fish (carp and walleye pollack), was not available in previous for tropical-water fish literature; so in this study primary structure of MyHC of the tropical-water fish amberjack has been determined by cloning and sequencing its cDNA. The MyHC has 1938 amino acid residues (AA), which are almost as much as as those of carp and walleye pollack. The amberjack MyHC is 91–95% homologous with other fish and rabbit MyHCs. There is a discernible difference between animal species with stable myosin rod (amberjack, carp, and rabbit) and walleye pollack with unstable rod. Stable rod species have a high probability of forming coiled-coil around the COOH-terminal end of the rod, while the pollack has a low coiled-coil formation probability. In addition, the average scores of the coiled-coil for myosin rod were rabbit (1.738) 〉 amberjack (1.691) 〉 carp (1.680) 〉 walleye pollack (1.674) which correlated exactly with the observed stability. The results suggest that coiled-coil forming ability, particularly around the COOH-terminal end, directs structural stability of fish myosin rod.
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    Multidimensional systems and signal processing 11 (2000), S. 109-124 
    ISSN: 1573-0824
    Keywords: 2-D linear systems ; optimal control ; stability ; unit memory linear repetitive processes ; numerical methods
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract Because many optimal control problems require solution using iterative procedures they fall naturally in the realm of 2-D systems where the two dimensions are response time horizon and iteration index, respectively. The paper uses this observation to employ 2-D systems theory, in the form of unit memory repetitive process techniques, to analyse local stability and convergence behaviour of a continuous optimal control algorithm based on dynamic system optimisation and parameter estimation. Existing work is extended to incorporate unmatched terminal constraints. Necessary and sufficient conditions for stability are obtained whose evaluation require the solution of a difficult eigenvalue problem. The paper shows how solutions can be achieved using numerical and graphical facilities of MATLAB.
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    Multidimensional systems and signal processing 11 (2000), S. 339-358 
    ISSN: 1573-0824
    Keywords: 2-D systems ; descriptor systems ; singular systems ; stability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Abstract In this paper, we discussthe jump behavior and stability problems for 2-D linear shift-invariantsingular systems under the standard boundary conditions. It isshown that once a boundary condition or the input is inadmissiblein the classical sense, a group of non-causal or backward jumpsof the system states will be incited. This interpretation releasesthe conventional admissibility constraints on the boundary conditionsand inputs. Based on this observation, a systematic stabilitytheory is developed for 2-D singular systems. The well-knownbasic stability theorem for the 1-D singular systems or 2-D regularsystems is thus extended to the 2-D singular case.
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  • 85
    ISSN: 1573-5060
    Keywords: chile ; double haploid ; pepper ; pungency ; stability ; within-genotype variance
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Abstract The response, in terms of capsaicinoid content, of chile (Capsicum annuum L.) genotypes to different environments was studied. Double haploidlines, an F1 hybrid, and an open-pollinated cultivar estimated the genotype, environment, and genotype-by-environment interaction effect on the total capsaicinoids and on individual capsaicinoids. Significant differences were observed among the genotypes and among genotype-by-environment interactions over the environments. Among the genotypes in an environment, the within-genotype variances were also significantly different. The double haploid line, HDA 207, had low within-genotype variance for individual and total capsaicinoids, with the exception of the isomer of dihydrocapsaicin. Also for HDA 270, the genotype-by-environment interaction was negligible for individual and total capsaicinoids, Indicating stability across environments.
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  • 86
    ISSN: 1573-0778
    Keywords: CHO cells ; gel microdrops ; human antibody ; population parameters ; productivity ; stability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Abstract The long-term stability of high-level expression is the mostimportant factor to consider when choosing cell lines for the expression of recombinant proteins. Declining volumetricyields in large-scale fermentation can be caused by changes affecting the cell population as a whole such as loss in viability, depletion of nutrients or accumulation of metabolites affecting cell growth. Alternatively, geneticinstability may lead to the outgrowth of a less productive,metabolically favored sub-population. Currently a variety ofparameters are measured to monitor the condition of cells infermenters including glucose uptake, lactate accumulation andoxygen consumption; in addition, periodic viable cell countsallow the determination of the growth rate and viability of the population. All of these methods measure the condition ofthe cell population as a whole and changes must involve a significantly large proportion of the total culture in orderto be detectable. Here we report on a method that allows theevaluation of the productivity of individual cells. Using the gel microdrop secretion assay, we detected the appearance ofa sub-population of cells with lower productivity. Subsequentanalysis of the culture confirmed the existence of lower productivity cells with a lower vector copy number. Therefore,the single cell secretion assay proved to be a rapid method todetect and isolate a low productivity variant of the producer cell line.
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  • 87
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    Applied mathematics and mechanics 21 (2000), S. 987-994 
    ISSN: 1573-2754
    Keywords: stability ; chaos ; averaging method ; Galerkin method ; viscoelastic column ; O322
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Mathematics , Physics
    Notes: Abstract The dynamical stability of a homogeneous, simple supported column, subjected to a periodic axial force, is investigated. The viscoelastic material is assumed to obey the Leaderman nonlinear constitutive relation. The equation of motion was derived as a nonlinear integro-partial-differential equation, and was simplified into a nonlinear integro-differential equation by the Galerkin method. The averaging method was employed to carry out the stability analysis. Numerical results are presented to compare with the analytical ones. Numerical results also indicate that chaotic motion appears.
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  • 88
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    Applied mathematics and mechanics 21 (2000), S. 1177-1186 
    ISSN: 1573-2754
    Keywords: elastic foundation ; pipe conveying fluid ; coupled-mode flutter ; stability ; power series method ; 0353
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Mathematics , Physics
    Notes: Abstract The governing equation of solid-liquid couple vibration of pipe conveying fluid on the elastic foundation was derived. The critical velocity and complex frequency of pipe conveying fluid on Winkler elastic foundation and two-parameter foundation were calculated by power series method. Compared with pipe without considering elastic foundation, the numerical results show that elastic foundation can increase the critical flow velocity of static instability and dynamic instability of pipe. And the increase of foundation parameters may increase the critical flow velocity of static instability and dynamic instability of pipe, thereby delays the occurrence of divergence and flutter instability of pipe. For higher mass ratio β, in the combination of certain foundation parameters, pipe behaves the phenomenon of restabilization and redivergence after the occurrence of static instability, and then coupled-mode flutter takes place.
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  • 89
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    Applied mathematics and mechanics 21 (2000), S. 1390-1400 
    ISSN: 1573-2754
    Keywords: suspended solid particles ; continuum phase-coupled model ; stability ; moving jet ; numerical computation ; O359
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Mathematics , Physics
    Notes: Abstract The spatial stability equation of moving jet containing dense suspended solid particles was derived out by means of the continuum phase-coupled model. The stability curves of moving jet for different downstream distances, Reynolds number of flow-field, particle properties and velocities of jetting device are got by the finite difference method based on the asymptotic method and the Eulerian conservative difference scheme. Founded on the analysis of the obtained stability curves it is found that the positive velocity of jetting device widens the unstable frequency range of flow-field but the effect of the negative one is contrary. In addition, particles existing in the flow-field curb the instability of flow-field and the effect enhances with the decrease of Reynolds number of flow-field. These conclusions benefit learning the development of moving two-phase jet.
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  • 90
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    Applied mathematics and mechanics 21 (2000), S. 209-216 
    ISSN: 1573-2754
    Keywords: composite material ; rotational shell ; stability ; nonlinear ; O347.3
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Mathematics , Physics
    Notes: Abstract By adopting the energy method, a new method to calculate the stability of the composite shell of revolution is presented. This method takes the influence of nonlinear prebuckling deformations and stresses on the buckling of the shell into account. The relationships between the prebuckling deformations and strains are calculated by nonlinear Kármán equations. The numerical method is used to calculate the energy of the total system. The nonlinear equations are solved by combining gradient method and amendatory Newton iterative method. The computer program is also developed. An example is given to demonstrate the accuracy of the method presented.
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  • 91
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    Applied mathematics and mechanics 21 (2000), S. 237-242 
    ISSN: 1573-2754
    Keywords: system identification ; damped least square ; recursive algorithm ; convergence ; stability ; O231 ; O241
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Mathematics , Physics
    Notes: Abstract The recursive least square is widely used in parameter identification. But it is easy to bring about the phenomena of parameters burst-off. A convergence analysis of a more stable identification algorithm-recursive damped least square is proposed. This is done by normalizing the measurement vector entering into the identification algorithm. It is shown that the parametric distance converges to a zero mean random variable. It is also shown that under persistent excitation condition, the condition number of the adaptation gain matrix is bounded, and the variance of the parametric distance is bounded.
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  • 92
    ISSN: 1573-4854
    Keywords: porous silicon ; passivation ; photoluminescence intensity stability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract We have investigated the effects of the porous Si surface passivation by oxygen or nitrogen ion processing on the photoluminescence (PL) intensity and its stability using an electron cyclotron resonance ion source. The results indicate that an anneal before exposure to ion beam causes a PL intensity enhancement upon exposure to oxygen or nitrogen ion beam. A combination of an anneal and a passivation by exposure to nitrogen ion beam enhances the PL intensity by a factor of ∼2.5 compared to the intensity of the initial as-anodized PS and the enhanced intensity is stabilized for more than 180 min under Ar+-laser illumination while oxygen-ion-exposed PS exhibits an enhancement in PL intensity only by ∼1.4 and a decay in PL intensity by ∼20% after 180 min Ar+-laser illumination. The results suggest that the reaction of nitrogen with a PS surface plays a key role for the PL intensity enhancement and stability.
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  • 93
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    Journal of porous materials 7 (2000), S. 159-163 
    ISSN: 1573-4854
    Keywords: porous silicon ; EELS ; NFEELS ; SiO2 layer
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract First results of an Electron Energy Loss Spectroscopy in the Near Field (NFEELS) mode of n+ porous silicon are described here. Sequences of EELS spectra in the low loss energy range (0–30 eV) were recorded, using a scanning transmission electron microscope, as the e-beam was scanned across a nano-hole surrounded by Si platelets. This technique is shown to be very sensitive to spectral and spatial changes in the electromagnetic field distribution outside the surface of nanoparticles, governed by their local nature and shape.
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  • 94
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    Journal of porous materials 7 (2000), S. 239-242 
    ISSN: 1573-4854
    Keywords: porous silicon ; interference fringes ; infrared-spectroscoy
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Here we present the software utility “ProSpect” which allows to process and interpret the data of infrared spectrophotometry studies of thin porous films. These spectra are often complicated by the presence of interference fringes. The program allows to remove fringes and thus enhance the intensity of useful IR bands as well as calculate optical parameters of the films (medium thickness value, refractive index, deviation of the thickness from medium value). Application of ProSpect in analysis of the FT-IR spectra of non-uniform porous silicon films is illustrated.
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  • 95
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    Journal of porous materials 7 (2000), S. 125-130 
    ISSN: 1573-4854
    Keywords: porous silicon ; photoluminescence ; electroluminescence ; scanning tunneling microscopy
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Porous silicon films as used in efficient blue-green electroluminescent devices (internal efficiency about 0.1%) were studied by scanning tunneling microscopy light emission spectroscopy (STMLES) as well as photoluminescence (PL) and electroluminescence (EL) spectroscopy. Areas of the n-type porous Si surfaces with small particles of about 5 nm dimensions gave STMLE, but areas with larger structures gave no emission. Clear STMLE spectra gave a peak at 630 nm, quite different from the EL peak at 500 nm. Whereas the PL peak at 700 nm was consistent with the STM indication of quantised entities, the EL seemed more readily explicable in terms of defects at the metal contact barrier.
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  • 96
    ISSN: 1573-4854
    Keywords: porous silicon ; erbium containing films ; electrochemical cathode treatment
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Electrochemical treatment of porous silicon (PS) in ethanol solution of Er(NO3)3 was investigated to obtain material suitable for optoelectronic application. The voltammograms of n+-type and p-type PS vs. an Ag/AgCl reference electrode were examined and compared with these of a Pt electrode. The basic cathode reactions were marked out the voltammograms: (i) the formation and the adsorption of atomic hydrogen; (ii) the formation of molecular hydrogen; (iii) the electrolysis of water and ethanol. No zones relating to on electrochemical transitions of Er ions were revealed on the voltammograms. Nevertheless, with the cathode polarization, the formation of an Er-containing deposit was observed at the surface of the cathode. The IR and SIMS analysis were used to study the composition of the deposits. The scheme of the electrochemical and chemical reactions at the cathode is discussed.
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  • 97
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    Journal of porous materials 7 (2000), S. 63-66 
    ISSN: 1573-4854
    Keywords: porous silicon ; oxidation ; IR spectroscopy ; vibrational analysis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Oxidation behavior of porous silicon under various environments of dry and wet air, and solution with and without appropriate oxidant at mild temperatures has been investigated. The progress of oxidation was followed by infrared spectroscopy. The presence of water vapor greatly accelerates the oxidation rate in comparison with the rate in dry air. The oxidized states are clarified with the help of oxidation experiments of partially hydrogen-desorbed porous silicon, which does not contain SiH2 and SiH3 as the hydride species. An oxidation mechanism is proposed to explain that oxidation is accelerated in the presence of water vapor and at the partially hydrogen-desorbed porous silicon. Further, oxidation behavior of porous silicon in solution containing appropriate oxidant is also investigated. The rate is very rapid and the oxidation does not produce the back-bond oxidized state of OySiHx in contrast to the oxidation in air.
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  • 98
    ISSN: 1573-4854
    Keywords: porous silicon ; pore propagation ; orientation dependence ; RBS ; EPR
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract A comparative study is presented on the pore propagation directions of porous silicon layers (PSL) formed on p+-type substrates of different orientations. PSLs were formed on plain (0 0 1) and (1 1 1) silicon wafers as well as on structured (0 0 1) wafers containing facets of various orientations. During anodization, regular pores follow the 〈0 0 1〉 direction on the (0 0 1) planes. While on the (1 1 1) planes fewer regular pores develop and seemingly propagate closely to the 〈1 1 1〉 direction. These results indicate that the pores propagate perpendicular to the surface i.e. along the field lines when the surface orientation is either (0 0 1) or (1 1 1). When the silicon surface provided (1 1 0) orientation (Chuang, Collins, and Smith, 1989), or its position is in between the (0 0 1) and (1 1 1) planes then the pores do not propagate perpendicular to the surface but along the 〈0 0 1〉 direction. All the phenomena exhibited might be explained by presuming that during formation, the pores propagate along the 〈1 0 0〉 directions, and that those 〈1 0 0〉 directions are preferred which are closely to the field lines. In PSLs formed on (0 0 1) surfaces the field lines and the 〈0 0 1〉 crystallographic direction are coincident. However, in the (1 1 1) oriented wafer where three equally probable 〈1 0 0〉 directions exist around the field lines, more irregular structure of PSLs will develop.
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  • 99
    ISSN: 1573-4854
    Keywords: porous silicon ; photoluminescence ; bandgap
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract We have measured the crystallite sizes, the bandgap energies, and the photoluminescence (PL) energies in porous silicon (PSi) samples having a wide range of porosities and kept in different ambient conditions. The dependence of the bandgap energy on the crystallite size agrees with theory. For PSi samples exposed to air and containing crystallites smaller than 5 nm, the PL intensity increases by several orders of magnitude and the PL peak energy shifts from the near infrared to the red, in agreement with the quantum confinement model for the PL. For crystallites smaller than 3 nm, there is a Stokes shift between the excitonic bandgap and PL energies, which increases to several hundreds of meV for sizes ∼2 nm, indicating that, in PSi exposed to air, the PL is not due to free excitons. Before exposure to air, very high porosity PSi samples emit at shorter wavelengths than after exposure to air, suggesting that the Stokes shift depends on the surface chemistry.
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  • 100
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    Journal of porous materials 7 (2000), S. 93-96 
    ISSN: 1573-4854
    Keywords: porous silicon ; diffusion-limited model ; electronic structure ; photoluminescence
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract We present the results of theoretical calculations for electronic structures and photoluminescence (PL) spectrum of porous silicon whose morphology is generated through the diffusion limited aggregation process of pores in a two-dimensional honeycomb lattice. We have found that due to irregularity of the structure most of its eigenstates near band gap are localized while some of them are relatively delocalized. The localization of the eigenstates near band gap causes band-gap narrowing analogous to the quantum confinement effect. Solving the time-dependent equations for the occupation numbers of the eigenstates, we show that the present model reproduces the stretched exponential decay of PL intensity observed in the experiments.
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