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  • Articles  (4,629)
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  • Articles: DFG German National Licenses  (4,629)
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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Journal of superconductivity 12 (1999), S. 231-232 
    ISSN: 1572-9605
    Keywords: Superconductivity ; CDW ; supersolid ; coexistence ; Hubbard model
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract We study a possible coexistence of superconducting state and charge density waves which, in a broad sense, might be called a supersolid phase. We investigate the infinite dimensional (d=∞) attractive Hubbard model by applying a sublattice dependent Gutzwiller wave function $$g_{\text{A}}^{D_{\text{A}} } g_{\text{B}}^{D_{\text{B}} } |{\text{BCS}}$$ |BCS〉 as a variational wave function describing the ground state. One may naively expect that the BCS superconducting state evolves continuously to the Bose–Einstein condensed state of bipolarons as the attractive interaction increases, as far as the system is dilute. However, we show that our variational wave function has lower energy than the simple BCS wave function for all electron densities and the interaction strengths. Our variational parameters increase (g A,B→∞) as we increase the interaction strength (U→∞). The energy gap turns out to be a mixture of s and extended-s waves. In the vicinity of half-filling, we find a quantum transition from a simple superconducting phase to a supersolid phase with increase of the electron density and/or the interaction strength.
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Journal of superconductivity 12 (1999), S. 403-408 
    ISSN: 1572-9605
    Keywords: Superconductivity ; crystal structure ; Hg-based cuprate
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract The single-phased series of Sr-bearing Hg-1212 superconducting cuprate,(Hg0.7Mo0.3)Sr2(Sr1−x La x )Cu2O z has been prepared. X-ray diffraction showed that, the obtained samples belong to the 1212-structure with tetragonal space group P4/mmm, similar to that of (Hg, Mo)Sr2(Ca, Y)Cu2Oz, and stabilized in a wide compositional range of 0.25≤x≤0.75. Refinements of the structure are carried out in which the oxygen atoms at the (Hg, Mo) layer is shifted from high-symmetry position (0.5, 0.5, 0) to (x, x, 0). Magnetization and electrical resistivity measurements show that the as-prepared samples exhibit evidence for superconductivity and their superconducting properties were improved after O2 annealing with T onset c as high as 92 K.
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Journal of superconductivity 11 (1998), S. 641-648 
    ISSN: 1572-9605
    Keywords: Superconductivity ; Ag composites ; percolation ; granularity ; weak-link resistivity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract Temperature-dependent electrical resistivity in a set of YBa2Cu3O7−x /Ag composites, all prepared under an identical sintering schedule, is analyzed to extract granularity information. The weak-link resistivity ρ wl across the grain boundaries and the percolation factor α arising due to current frustration caused by misalignment of anisotropic grains and sample defects such as voids and cracks are estimated from the residual resistivity ρ0 and the temperature coefficient of resistivity dρ/dT. Variation of these parameters with Ag vol.% quantifying the extent of granularity indicates that granularity in the composites decreases and their electrical characteristics tend to be identical to that of Ag-free YBa2Cu3O7−x single crystals and epitaxial thin films as Ag vol.% approaches a value where onset of current percolation occurs through Ag channels. The increased α and decreased ρ wl observed at higher Ag vol.% explains the larger grains in the composites with narrow size distribution.
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  • 4
    ISSN: 1572-9605
    Keywords: Superconductivity ; La2.5Y0.5CaBa3(Cu0.88Fe0.12)7O z ; compensated oxide ; magnetic pair breaking
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract The structural and superconducting properties of single-phase La2.5−y Y0.5Ca1+y Ba3 (Cu0.88Fe0.12)7O z (LYCaBCuFe) (y= 0.0−1.0) compounds with triple perovskite structure are investigated using X-ray diffraction, resistivity, a.c. susceptibility, and oxygen content measurements. Increasing Ca substitution for La resulted in a decrease in unit cell axes and volume. T c R=0 shows a marginal increase from 31 K to 37 K for y = 0.0−0.21 and thereafter it decreases with increasing y leading to zero T c R=0 at y ≥ 0.84. This shows that the suppression of T c from 80 K to 31 K by Fe doping at x = 0.12 La2.5Y0.5CaBa3(Cu1−x Fe x )7O z cannot be compensated by appropriate hole doping with Ca in LaYCaBCuFe.
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  • 5
    Electronic Resource
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    Springer
    Journal of superconductivity 11 (1998), S. 707-712 
    ISSN: 1572-9605
    Keywords: Superconductivity ; antiferromagnetic order ; ErNi2Bi2C
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract Upper critical field and penetration depth of superconducting ErNi2B2C borocarbide thin films have been investigated and compared with the same properties of SmRh4B4 films. The coexistence of superconductivity and antiferromagnetic order is described through Machida's theory. A fairly satisfactory description of the experimental results is obtained.
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  • 6
    Electronic Resource
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    Springer
    Journal of superconductivity 11 (1998), S. 615-639 
    ISSN: 1572-9605
    Keywords: Superconductivity ; rare-earths
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract Using only two principles: (i) high-temperature superconductivity requires hypercharged oxygen, and (ii) the superconducting condensates are located in those parts of the crystal structures where they are unaffected by magnetic pair breaking, we are able to explain why certain rare-earth ions R are compatible with superconductivity and others are not, in the compounds RBa2Cu3O7, RBa2Cu4O8, RBa2Cu2NbO8, R2 − z Ce z CuO4, and R2 − z Ce z Sr2Cu2NbO10. Various defects are proposed as having central roles in the superconductivity or the suppression of superconductivity in these compounds. Many experiments for testing this physical picture are suggested.
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  • 7
    Electronic Resource
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    Springer
    Journal of superconductivity 11 (1998), S. 1-4 
    ISSN: 1572-9605
    Keywords: Superconductivity ; magnets ; cryogenics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract A review of U.S. Navy efforts in superconducting technology is given. Programs include development of superconducting magnets for motors in ship propulsion systems and to generate magnetic moments in mine sweeping systems. A program to develop superconducting quantum interference devices (SQUIDs) for detection of mines and buried ordnance has successfully been demonstrated. In electronic applications, superconducting filters, resonators, and antennae are being developed for radar, communication, and space systems. Concurrent with these superconductivity programs are efforts to improve the efficiency, reliability, and affordability of refrigeration systems.
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  • 8
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    Journal of superconductivity 11 (1998), S. 339-345 
    ISSN: 1572-9605
    Keywords: Superconductivity ; non-Fermi liquid
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract The critical temperature of a superconductor with a non-Fermi liquid ground state has been calculated. The density of states has two different forms and is energy dependent. The critical values of the coupling factor λ were calculated. The constant density of states results are obtained as particular cases. These results are an extension of the work done by Grosu et al. [Phys. Rev. B 56, 8298 (1997)].
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  • 9
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    Journal of superconductivity 11 (1998), S. 123-124 
    ISSN: 1572-9605
    Keywords: Superconductivity ; Nitrates ; Oxycarbonates
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract We have synthesised a series of compounds in both the (PbTl)Sr4Cu2O7(CO3)1−y(NO3)y (0.1≤y≤0.5) and TlSr2Ba2Cu2O7(CO3)1−y(NO3)y (0.1≤y≤0.2) systems. We report the synthesis conditions of these materials, an X-ray diffraction analysis of their structure and the changes induced in superconducting behaviour produced by the introduction of nitrate groups into the starting composition.
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  • 10
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    Journal of superconductivity 11 (1998), S. 129-132 
    ISSN: 1572-9605
    Keywords: Superconductivity ; TIPbSrCaCuO ; thin films
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract Thin films of (Tl,Pb)Sr2Ca0.8Y0.2Cu2O7, and (Tl,Pb)Sr2CuO5 can be grown in a single step process which involves sputter deposition from a mixed oxide target and simultaneous thermal evaporation of Tl2O. The use of a radiant heater has allowed extension of this in situ deposition process to full LaAlO3 and NdGaO3 wafers. Variations in the composition of the deposited film is 〈 4% across a 50 mm wafer while the thickness uniformity is ≍ 8%. The highest transition temperature for a (Tl,Pb)Sr2Ca0.8Y0.2 Cu2O7 film thus far is 83 K. The RMS surface roughness of (Tl,Pb)Sr2CuO5 films is uniform across the wafer and approximately 1% of the film thickness for films 20 to 100 nm thick.
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  • 11
    ISSN: 1572-9605
    Keywords: Superconductivity ; mercury ; chromium ; synthesis ; structure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract We report preliminary results concerning the synthesis and structural characterisation of the chromium stabilised 1201 phases: Hg1−xCrxSr2CuO4+δ. A systematic study of the formation of phases in this system has been undertaken, together with a combined powder neutron and synchrotron x-ray diffraction study, to address the issue of chromium clustering and ordering in this system.
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  • 12
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    Journal of superconductivity 11 (1998), S. 169-171 
    ISSN: 1572-9605
    Keywords: Superconductivity ; Alkali-doping ; Hg-based Cuprates
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract The effect of Na- and Li-doping in Hg-based cuprates has been studied. It has been found that superconductivity can be achieved in samples containing small amount of Na or Li. While little change is found in the crystalline structure, the formed superconducting phase have the onset Tc typically above 140 K and zero-resistance Tc up to 136K. More interestingly, the processing temperature for the doped Hg-1223 can be reduced significantly to near 700 °C which is promising for thin films growth on many technologically compatible substrates.
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  • 13
    ISSN: 1572-9605
    Keywords: Superconductivity ; thin film ; columnar defect ; pinning
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract The mixed state Hall effect has been studied in high quality HgBa2CaCu2O6+δ thin films while the pinning strength is modified by the irraddition of columnar defects. It has been shown that the pinning strength can be significantly improved by columnar defects. The sign reversal in Hall resistivity is found to be nearly independent of pinning while the scaling behavior between Hall resistivity (ρxy) and longitudinal resistivity (ρxx) is influenced by pinning. The exponent β in the scaling law β xy ∼ β xy β decrease from 1.8 ± 0.1 to 1.5 ± 0.1 as pinning is enhanced. This results suggest that the sign reversal and the scaling are unrelated, and pinning is not the mechanism of the sign reversal.
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  • 14
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    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 14 (1998), S. 229-240 
    ISSN: 1069-8299
    Keywords: filling of thin section ; finite element method ; surface tension ; interface element ; Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: An interface element to model the pressure discontinuity due to surface tension when applied to the filling of a thin section cavity is presented. The equations used to form the element matrix for the interface element are the line integral form of the continuity and momentum equations. During the development of the finite element model, the pressure difference across the free surface due to surface tension is treated as an additional traction and is applied to all element sides which form the free surface. Simple numerical examples are then presented to illustrate the technique on the filling of a rectangular thin section cavity. © 1998 John Wiley & Sons, Ltd.
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  • 15
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    Communications in Numerical Methods in Engineering 14 (1998), S. 253-269 
    ISSN: 1069-8299
    Keywords: potential flow ; optimization approach ; sensitivity analysis ; adjoint variable method ; finite elements ; Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: Potential flow problems around immersed bodies have been treated by an optimization approach. When the stream function is used as the field variable, the boundary values may not be known a priori and may be taken as the decision parameters to minimize integral objective functionals. The circulation integrals around the immersed bodies or the Kutta condition at the trailing edges of the bodies may be used to construct the objective function of optimization. The sensitivity analysis needed for the minimization process is performed by the adjoint variable method, while the numerical solutions of the primary (flow) and adjoint equations have been obtained by the finite element method. Having checked the present method with exact solutions and the classical superposition method, several flow problems involving one or more immersed bodies with or without circulation are investigated numerically. © 1998 John Wiley & Sons, Ltd.
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  • 16
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    Communications in Numerical Methods in Engineering 14 (1998), S. 289-304 
    ISSN: 1069-8299
    Keywords: stress intensity factors (SIF) ; finite element method (FEM) ; reciprocal work contour integral (RWCI) ; path-independent integrals (PII) ; displacement correlation technique (DCT) ; quarter-point displacement technique (QPDT) ; Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: A new method for improving the approximations of stress intensity factors computed from path-independent integrals is developed. The method uses Richardson's extrapolation. Numerical results are given to show the efficiency and the stability of the present method. © 1998 John Wiley & Sons, Ltd.
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  • 17
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    Communications in Numerical Methods in Engineering 14 (1998), S. 321-333 
    ISSN: 1069-8299
    Keywords: eigenvalue analysis ; sensitivity evaluation ; large-scale systems ; Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: This paper presents a general rank-1 matrix formula which allows for proper rearrangement of individual terms in multiproduct forms involving vectors and matrices. A far-reaching application of the new matrix formula to eigenvalue sensitivity evaluation is presented in the paper. Such an application reduces the sensitivity expressions to elegant, very fast and recursive forms with substantial savings in computer resources. The formula is applicable to rank-1 matrices of special structures which may constitute derivatives of the system state matrix, which is widely used in control system applications, with respect to various parameters of interest. In such cases, the use of the rank-1 formula yields exact non-approximate solutions which are identical to those obtained by other conventional formulas. The applicability of the rank-1 formula is believed to cover a wide variety of practical engineering systems pertaining to control and stability. © 1998 John Wiley & Sons, Ltd.
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  • 18
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    Communications in Numerical Methods in Engineering 14 (1998), S. 719-730 
    ISSN: 1069-8299
    Keywords: phase-change problems ; conduction-advection equation ; upwind weight function ; Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: A finite element formulation for solving transient multidimensional phase-change problems considering advective effects is presented. This temperature-based formulation includes the definition of a phase-change function able to deal with classical isothermal and non-isothermal phase-change cases. Moreover, a new upwind weight function is defined in order to avoid numerical oscillations in problems with dominant advective effects. Further, some important aspects related to its numerical implementation are also addressed. The ability of this methodology is illustrated, firstly, in the solution of a one-dimensional test example. Finally, the numerical simulation of a direct-chill continuous casting process is performed. © 1998 John Wiley & Sons, Ltd.
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  • 19
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    Communications in Numerical Methods in Engineering 14 (1998), S. 763-772 
    ISSN: 1069-8299
    Keywords: coupled vibrations ; Timoshenko beam ; boundary integral equation method ; symbolic computation ; Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: Symbolic computer algebra systems relieve one from the tedious task of different mathematical operations which are essential to obtain solutions. Due to their highly advanced features they have come to be used widely in computational mechanics. This paper describes an application of the modern computer algebra system Mathematica to the derivation of fundamental solutions necessary for the application of the boundary integral equation method. The problem treated is an asymmetric cross-section Timoshenko beam in free vibration. For this problem, the derivation of fundamental solutions involves lengthy mathematical operations which are very tedious if performed explicitly by hand. Therefore, using Mathematica we derive the fundamental solutions and generate the influence matrices from which the natural frequencies can be found. © 1998 John Wiley & Sons, Ltd.
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  • 20
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    Communications in Numerical Methods in Engineering 14 (1998), S. 751-761 
    ISSN: 1069-8299
    Keywords: acoustic ; electromagnetic ; integral equations ; scattering ; time domain ; radar cross-section ; Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: Analysis of high frequency scattering using pulsed illumination generates surface fields which are small over most of the scatterer most of the time. A reformulation of the usual integral equation time domain approach which exploits this is presented. It is shown that cost scaling can be reduced, with costs reduced by an order of magnitude for the examples presented, with negligible accuracy loss. © 1998 John Wiley & Sons, Ltd.
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  • 21
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    Communications in Numerical Methods in Engineering 14 (1998), S. 797-808 
    ISSN: 1069-8299
    Keywords: mesh equidistribution ; area preserving map ; singular BVP ; Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: In this paper, an adaptive mesh method is employed to solve a class of singular boundary value problems. The approach is based on an area-preserving map and some mesh shape control in two-dimensional space. Two benchmark problems, which both involve singularities in physical domains, are tested. © 1998 John Wiley & Sons, Ltd.
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  • 22
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    Communications in Numerical Methods in Engineering 14 (1998), S. 195-208 
    ISSN: 1069-8299
    Keywords: differential quadrature method ; elastic torsion ; numerical solution ; Poisson equation ; Laplace equation ; geometric mapping ; Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: The governing equation of an elastic prismatic shaft is the two-dimensional Poisson equation defined on the cross-sectional area of the shaft. In this paper, the differential quadrature method (DQM) is employed to solve the Poisson equation on some non-rectangular domains. Singularities, which may appear in the expression of stress components or boundary conditions at a degenerated point of the grid, are removed by means of the Taylor expansion. The results of three examples are compared with the exact solutions. It is shown that accurate results can be achieved by the DQM. In addition, three geometric transformations are conducted in the third example so that the effect of mapping on the convergence and accuracy of results is investigated. It is found that rapid convergence can be fulfilled if the degenerated point of the mesh falls on a Dirichlet boundary. The approach addressed in the paper can be extended to other potential problems governed by either the Poisson equation or the Laplace equation. © 1998 John Wiley & Sons, Ltd.
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  • 23
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    Communications in Numerical Methods in Engineering 14 (1998), S. 209-218 
    ISSN: 1069-8299
    Keywords: three-dimensional convection-diffusion equation ; fourth-order compact scheme ; iterative methods ; Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: We present an explicit fourth-order compact finite difference scheme for approximating the three-dimensional convection-diffusion equation with variable coefficients. This 19-point formula is defined on a uniform cubic grid. We compare the advantages and implementation costs of the new scheme with the standard 7-point scheme in the context of basic iterative methods. Numerical examples are used to verify the fourth-order convergence rate of the scheme and to show that the Gauss-Seidel iterative method converges for large values of the convection coefficients. Some algebraic properties of the coefficient matrices arising from different discretization schemes are compared. We also comment on the potential use of the fourth-order compact scheme with multilevel iterative methods. © 1998 John Wiley & Sons, Ltd.
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  • 24
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    Communications in Numerical Methods in Engineering 14 (1998), S. 241-251 
    ISSN: 1069-8299
    Keywords: finite elements ; pollutant ; saturated porous medium ; semi-implicit method ; velocity correction ; mass transfer ; Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: A practical problem of pollutant migration has been studied for different concentration differences and mass diffusivities using the finite element method. The results indicate that the pollutant takes years to travel 10 m into the water-saturated soil when the mass diffusivity and concentration differences are less. © 1998 John Wiley & Sons, Ltd.
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  • 25
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    Communications in Numerical Methods in Engineering 14 (1998), S. 277-285 
    ISSN: 1069-8299
    Keywords: numerical simulation ; steel ; quenching ; finite volume method ; Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: On the basis of the control volume method the algorithm and computer program for prediction of the hardness distribution in quenched steel specimens with complex geometries have been developed. The algorithm and computer program are designed to solve 2D situation problems such as the quenching of complex cylinders, cones, spheres, etc. The computer program consists of three parts: automatic computation of domain and grid generation, computation of cooling curve in grid-points, and computation of hardness in grid-points. The mathematical model has been tested experimentally. The test showed that the model describes the hardness distribution in a quenched steel specimen of a complex form, with quite satisfactory accuracy. © 1998 John Wiley & Sons, Ltd.
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  • 26
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    Communications in Numerical Methods in Engineering 14 (1998), S. 271-275 
    ISSN: 1069-8299
    Keywords: basis transformation ; interpolations ; finite elements ; thin plates ; Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: The 9 df thin plate element of Mohr and Mohr uses cubic interpolation to obtain values of w at the third points of the element sides, in turn interpolating from these and the vertex values within the element. Recently this element has been modified and successfully applied to ‘potential’ problems. Subsequently it was found that the interpolations of the element of Bazeley et al. (1965, 1968) gave identical results for potential problems. In the present paper it is shown that this is because the interpolations of the two elements are exactly equivalent. © 1998 John Wiley & Sons, Ltd.
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  • 27
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    Communications in Numerical Methods in Engineering 14 (1998), S. 313-319 
    ISSN: 1069-8299
    Keywords: fluid mechanics ; vortex dynamics ; viscous flow ; Navier-Stokes equation ; vortex methods ; splitting procedure ; Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: A numerical scheme has been obtained rigorously from the initial-boundary value problem for the Navier-Stokes (NS) equations in the context of the vortex method. The technique is based on a transformation of the NS equation into a parabolic equation which has an exact solution. The numerical scheme is derived by expanding the exact solution in Taylor series in powers of a small time interval. Numerical implementation is developed with use of vortex particles to represent the vortex flow domain. The method is used to solve practical engineering problems. The technique can also incorporate turbulence modelling. © 1998 John Wiley & Sons, Ltd.
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  • 28
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    Communications in Numerical Methods in Engineering 14 (1998), S. 347-354 
    ISSN: 1069-8299
    Keywords: thin wire loop antenna ; integro-differential equation ; frequency domain ; current distribution ; the weak formulation ; finite element technique ; Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: The circular loop antenna is analysed by using the electric field integro-differential equation in the frequency domain. The weak form of the integro-differential is derived and then the current distribution along the circular loop antenna is calculated by solving the resulting equation via the finite element technique. Accurate results are obtained using the linear shape and test functions. © 1998 John Wiley & Sons, Ltd.
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  • 29
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    Communications in Numerical Methods in Engineering 14 (1998), S. 355-365 
    ISSN: 1069-8299
    Keywords: transient dynamics ; interaction of the crack sides ; crack propagation ; time domain formulation ; boundary element method ; Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: For two-dimensional solids the multiregion concept and the time-domain approach in the boundary element method are employed to model cracks in transient dynamics. The propagation of cracks and the closing and opening of crack sides are simulated by disconnecting and connecting degrees of freedom of a stationary mesh, as was also demonstrated recently in statics by Beer. An iterative technique is developed to determine the changes of the extent of the area where there is connection, contact or no contact at the interface between dynamically loaded regions. © 1998 John Wiley & Sons, Ltd.
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    Communications in Numerical Methods in Engineering 14 (1998), S. 941-957 
    ISSN: 1069-8299
    Keywords: geometrical non-linear ; UL formulation ; drilling degrees of freedom ; generalized conforming ; arc-length method ; Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: An updated Lagrangian formulation of the generalized conforming flat shell element with drilling degrees of freedom is derived based on the incremental equation of virtual work of a three-dimensional (3D) continuum for a purely geometric non-linear analysis of the space structure. While solving the non-linear equations, the Euler-Newton method and modified Euler-Newton method are used in static analyses, and the modified arc-length method and Newton arc-length method for post-buckling problems. A number of numerical examples are given to demonstrate the effectiveness of the proposed approach and programs. © 1998 John Wiley & Sons, Ltd.
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    Communications in Numerical Methods in Engineering 14 (1998), S. 473-491 
    ISSN: 1069-8299
    Keywords: temporal acceleration ; viscoelastic ; recovery ; Taylor-Galerkin ; finite elements ; Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: This paper presents a finite element study based on a technique associated with time extrapolation to accelerate the convergence rate to the steady state for viscoelastic flows. The approach adopted is a local extrapolation method attributed to Neville. Temporal extrapolation is embedded within a time-marching Taylor-Galerkin/pressure-correction scheme as applied to the solution of model channel flow, 4:1 plane contraction flow and flow past a circular cylinder. In particular, consideration is given to obtaining steady-state solutions for an Oldroyd-B model. When extrapolation is performed for stress and velocity or pressure, then stress and velocity overshoot, which consequently leads to divergence. In contrast, a stable numerical scheme emerges when only the stress is extrapolated. © 1998 John Wiley & Sons, Ltd.
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    Communications in Numerical Methods in Engineering 14 (1998), S. 505-517 
    ISSN: 1069-8299
    Keywords: hypersingular integral equation ; continuous elements ; stress analysis ; Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: In this paper, a method for the evaluation of boundary stresses directly from the BEM solution of elastostatic problems using the static boundary integral equation is presented. The technique is valid for corners and edges as well as smooth points on the boundary, and involves defining a new interpolation function for continuous elements which incorporate certain continuity conditions arising from the hypersingular nature of the integrals involved. An integration technique based on the singularity subtraction method using series expansions is adopted for the hypersingular intergrals. Results are shown to be more accurate than those obtained with conventional techniques. © 1998 John Wiley & Sons, Ltd.
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    Communications in Numerical Methods in Engineering 14 (1998), S. 539-547 
    ISSN: 1069-8299
    Keywords: boundary-only element analysis ; thermal cracking ; crack trajectory ; singular elements ; traction formula ; Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: A boundary element procedure is developed concerning the prediction of the quasistatic crack growth in uniformly heated bimaterials. This procedure assumes the existence of an initial small crack in one of the two phases, and further cracking progress from this point due to thermal loading. The resulting mixed boundary value problem is solved by applying an incremental boundary-only method in conjunction with the multidomain technique. Fracture characterizing parameters are evaluated utilizing special crack tip singular elements and appropriate formulas. The crack path is predicted using the strain energy release rate criterion, and the mesh is updated at the end of each increment. The presented results are in good agreement with previously reported experimental results and those obtained by the finite element method. Various numerical studies were conducted and interpreted concerning crack-path dependence on individual material property mismatch. © 1998 John Wiley & Sons, Ltd.
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    Communications in Numerical Methods in Engineering 14 (1998), S. 559-567 
    ISSN: 1069-8299
    Keywords: gust ; buffet ; random ; vibration ; Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: A convenient and effective finite-element-based method for buffeting analysis of complex structures is presented. This method is very efficient for structures with the natural frequencies closely spaced because their corresponding modes and the cross-correlation terms between them should be included in the mode-superposition analysis. The imperfect correlation between gust excitations can also be dealt with conveniently. A numerical example is given which shows that by using this method the buffeting analysis of complex structures with thousands of degrees of freedom, dozens of imperfectly related excitations and a number of participating modes can be executed quite easily on an ordinary IBM/486 (or 586) personal computer. © 1998 John Wiley & Sons, Ltd.
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    Communications in Numerical Methods in Engineering 14 (1998), S. 519-527 
    ISSN: 1069-8299
    Keywords: shell element ; large strain ; sheet metal stamping ; Engineering ; Numerical Methods and Modeling
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    Topics: Mathematics , Technology
    Notes: The paper presents a general method of large strain determination over the deformed surface of a sheet metal stamping. It is demonstrated that the conventional degenerated shell element with two normal rotation degrees of freedom is not suitable for large deformation, especially when large element rotation is present. This inaccuracy is primarily caused by the fact that the displacement field description used in the degenerated shell element is only a first-order approximation with respect to the two rotation degrees of freedom, and is therefore suitable only for small rotation angles. The new method presented in this paper replaces the two rotation DOFs with three new degrees of freedom to describe the rotation of the surface normal so that the element deformation can be accurately described with no limitation on the amount of deformation and rotation involved. The advantages of this new method are: (i) a linear and accurate expression of the displacement field in terms of nodal DOFs is obtained; (ii) the formulation is easily incorporated into any existing degenerated shell elements; (iii) the strain calculation is accurate for any amount of element rigid body rotation; (iv) if the method is used in surface grid analysis, the algorithm will not only provide correct surface strains, but also their variation through the thickness direction, i.e. the bending deformation. © 1998 John Wiley & Sons, Ltd.
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    Communications in Numerical Methods in Engineering 14 (1998), S. 597-608 
    ISSN: 1069-8299
    Keywords: inverse problems ; dual systems ; vibrating rod ; Engineering ; Numerical Methods and Modeling
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    Topics: Mathematics , Technology
    Notes: The discretization of the differential equation governing the axial vibration of a rod with varying cross-section leads to a specially structured matrix pencil. This paper deals with the reconstruction of this pencil from its spectrum. An iterative algorithm for this problem and an analytic characterization of complementary solutions are given. The method is demonstrated on some examples. © 1998 John Wiley & Sons, Ltd.
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    Communications in Numerical Methods in Engineering 14 (1998), S. 621-632 
    ISSN: 1069-8299
    Keywords: segmentation ; finite elements ; adaptive ; image ; Engineering ; Numerical Methods and Modeling
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    Topics: Mathematics , Technology
    Notes: The image segmentation problem in computer vision is considered. Given a two-dimensional domain D and a function defined on it (the original image), the problem is to obtain a ‘cartoon’ associated with this function, namely to find a set of inner boundaries which divide D into subdomains (objects) in an optimal way. The optimality criterion used here is given by the Mumford-Shah (MS) and Blake-Zisserman model, which leads to a strongly non-linear problem. Related problems appear in multiphase continuum mechanics. An iterative procedure based on an h-adaptive finite element method is proposed for the solution of this problem. The mesh adaptivity enables an efficient solution technique, with the use of basic coarse discretization and a few local regions of high resolution where needed. © 1998 John Wiley & Sons, Ltd.
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    Communications in Numerical Methods in Engineering 14 (1998), S. 633-645 
    ISSN: 1069-8299
    Keywords: error estimation ; boundary elements ; adaptive mesh ; Engineering ; Numerical Methods and Modeling
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    Topics: Mathematics , Technology
    Notes: A posteriori error estimation for the boundary element method is developed. The nodal error is estimated from the difference of two solutions - one from the original BEM solution, and the other from interpolation of the original solution. Based on nodal errors, a matrix analysis is carried out to calculate the corresponding errors at source points. Two formulae for estimating global percentage errors are proposed in the paper. The first formula uses nodal errors directly to estimate element error, and the second uses an integral form to calculate element error to eliminate extremely high mesh concentration near areas with singular solutions. An h-version adaptive mesh refinement is implemented to study the accuracy of the proposed error estimation. Numerical examples show that the error estimator can correctly guide mesh refinement, and a final mesh can be obtained in a few iterations. © 1998 John Wiley & Sons, Ltd.
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    Communications in Numerical Methods in Engineering 14 (1998), S. 647-655 
    ISSN: 1069-8299
    Keywords: finite difference methods ; stability and convergence of numerical methods ; advection schemes ; Engineering ; Numerical Methods and Modeling
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    Topics: Mathematics , Technology
    Notes: This paper discusses implementation strategies for second-order finite difference discretizations of advection. Purely explicit and implicit methods both have disadvantages, and we consider semi-implicit schemes in which the flux is split into a primary implicit part and a secondary explicit correction. One-dimensional scalar advection is used as a model problem to analyse the leading order error terms and the stability of the schemes. Some of the splittings turn out to be unconditionally stable, but accuracy or monotonicity may deteriorate for large time steps. © 1998 John Wiley & Sons, Ltd.
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    Communications in Numerical Methods in Engineering 14 (1998), S. 697-708 
    ISSN: 1069-8299
    Keywords: incompressible flow ; generalized streamline operator ; upwinding tensor ; Engineering ; Numerical Methods and Modeling
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    Topics: Mathematics , Technology
    Notes: In the present work the backward-facing step problem is analysed in the framework of the finite element method. It is a very well-known benchmark problem for the numerical resolution of the incompressible Navier-Stokes equations. In particular, a generalized streamline operator technique (GSO) is used in the numerical approach to these equations. The results show very good agreement with those reported by other authors using different methodologies. © 1998 John Wiley & Sons, Ltd.
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    Communications in Numerical Methods in Engineering 14 (1998), S. 773-781 
    ISSN: 1069-8299
    Keywords: indirect address lists ; unstructured grids ; shared-memory parallel machines ; Engineering ; Numerical Methods and Modeling
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    Topics: Mathematics , Technology
    Notes: A way has been found to form indirect addressing lists in parallel on shared-memory parallel machines. The maximum possible speed-up for typical tetrahedral grids is approximately 1:23. The algorithm requires an additional scratch array to shift from the serial ‘elements surrounding points’ to the parallel ‘elements surrounding processors surrounding points’ paradigm. The algorithm developed is general in nature, i.e. applicable to all indirect addressing lists. All numerical methods requiring the construction of indirect data structures, such as sparse matrix linear algebra procedures, field and particle solvers operating on unstructured grids, and network flow applications should see a benefit from this algorithm when running on shared-memory parallel machines. © 1998 John Wiley & Sons, Ltd.
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    Communications in Numerical Methods in Engineering 14 (1998), S. 783-792 
    ISSN: 1069-8299
    Keywords: substructure ; frontal technique ; heat transfer ; moisture transfer ; displacement ; Engineering ; Numerical Methods and Modeling
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    Topics: Mathematics , Technology
    Notes: This paper presents a substructuring-frontal combined algorithm for the analysis of fully coupled heat, moisture and displacement problems in unsaturated soil. The method adopted for solving the assembled matrix equations has a significant bearing on the computer storage requirement and execution time. Numerical examples are given to validate the proposed model. © 1998 John Wiley & Sons, Ltd.
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    Communications in Numerical Methods in Engineering 14 (1998), S. 809-819 
    ISSN: 1069-8299
    Keywords: finite element analysis ; diffusion problems ; minimum time step size ; Engineering ; Numerical Methods and Modeling
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    Topics: Mathematics , Technology
    Notes: The solution of the equation system of the finite element method for diffusion problems is presented in the format of a sequence. The characteristics of the numerical results can thus be known through an analysis of this ‘theoretical expression’ of the solution. The characteristics of the sequence are analysed to yield expressions for the minimum time step size. © 1998 John Wiley & Sons, Ltd.
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    Communications in Numerical Methods in Engineering 14 (1998), S. 849-861 
    ISSN: 1069-8299
    Keywords: outgoing boundary condition ; Berkhoff ; discrete ; non-local ; surface waves ; scattering ; Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: A discrete non-local (DNL) boundary condition is used to solve the water waves propagation problem over variable depth. This condition is obtained by means of full solution of the discrete Helmholtz operator in a structured network. We consider a simulation of wave propagation around a circular island located on either a paraboloidal shoal or constant depth bathymetry. Such examples confirm the important improvement in accuracy for the DNL method over standard conditions in the near field. © 1998 John Wiley & Sons, Ltd.
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 153-166 
    ISSN: 0894-3370
    Keywords: Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Most methods for the numerical calculation of inverse Laplace transformations f(t) = L-1[F(s)] have serious limitations concerning the class of functions F(s) that can be inverted or the achievable accuracy. The procedures described in the paper can be used to invert rational as well as irrational or transcendental functions of the complex variable s. The required accuracy of the results can be enhanced without changing the algorithm, only at the cost of a longer computation time. The described methods were verified with many examples including transients in lumped/distributed systems with sections of lossy multiconductor transmission lines or with distributed RC elements. © 1998 John Wiley & Sons, Ltd.
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    Keywords: Engineering ; Numerical Methods and Modeling
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    Topics: Electrical Engineering, Measurement and Control Technology
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 167-181 
    ISSN: 0894-3370
    Keywords: Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: In this study, a software package is described which has been developed for the simulation of detection and tracking in surface wave high frequency (SW-HF) radars. The aim is to investigate the problems related to detection and tracking of surface targets at beyond the horizon ranges. In SW-HF radars target detection and tracking involves stochastic features such as target RCS fluctuations, atmospheric, galactic and/or man-made ambient interference components, and the sea clutter with the dominant resonant Bragg returns which affect target detection are all incorporated in the package. In the model, first, terrain data are fed into the simulator by means of a specially designed graphical user interface. Then, a scenario is prepared where the radar's location, coverage and operational parameters can be defined, together with different targets and their sailing routes. The radial propagation paths for angular resolution cells are extracted from the terrain data with the lengths of (possible) sea-island transitions. The surface wave path losses are calculated over the smooth spherical earth's surface with finite conductivity. Surface roughness and mixed-path propagation effects are also included in these calculations. The target detection is performed in the frequency domain after calculation of the noise floor, signal to noise (SNR) and clutter to noise (CNR) ratios. Since SW-HF radars yield coarse range and azimuth but accurate velocity measurements, different Kalman filter techniques are applied for the target tracking, and algorithms are added to improve the track-measurement data correlation. Various simulation tests are performed and the results are presented. © 1998 John Wiley & Sons, Ltd.
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 187-187 
    ISSN: 0894-3370
    Keywords: Engineering ; Numerical Methods and Modeling
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    Topics: Electrical Engineering, Measurement and Control Technology
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 273-274 
    ISSN: 0894-3370
    Keywords: Engineering ; Numerical Methods and Modeling
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 255-257 
    ISSN: 0894-3370
    Keywords: Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: We re-examine the potential drop method to give a direct estimation of the absolute error in transmission-line matrix (TLM) diffusion modelling. In particular, the open-circuit boundary is taken into account. Promising results are presented with a structure relevant to thermal analysis of semiconductor devices. © 1998 John Wiley & Sons, Ltd.
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 259-271 
    ISSN: 0894-3370
    Keywords: Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: There is a class of material processing simulators which require solution of the diffusion equation over a surface which is evolving due to some physical or chemical process related to the concentration of the diffusing species. Because of its explicit and unconditionally stable nature, the transmission line matrix (TLM) method is well suited to efficiently solve the diffusion equation in these instances. However, the methodology for using TLM on a dynamic problem space is not well established.This paper describes the development of algorithms to handle the insertion, deletion and motion of TLM nodes along the one-dimensional surface of a two-dimensional thin film process simulator called GROFILMS. These routines are completely compatible with concurrent time scaling to increase computational efficiency. Central to this development is the use of an asymmetric TLM cell which provides more flexibility for representation and minimizes the disturbance effects of these nodal operations. The scaling routines apply conservation of charge and continuity of current into the affected region to recompute the scattered or the incident pulses as a result of the evolving network. Verification of these results through comparison with known analytic solutions is achieved where possible. © 1998 John Wiley & Sons, Ltd.
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 277-287 
    ISSN: 0894-3370
    Keywords: Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: The solution of Maxwell's equations is considered using the finite difference time domain (FDTD) method and the client-server software paradigm on a heterogeneous network of distributed computers. The performance of compilers is evaluated and the PVM 3.3 message-passing harness, in both its conventional and TCP mode of operation, is compared with direct calls to the lesser known TCP/IP routines on the operating system. An example is given showing the impact of using different floating-point representations, in each subdomain, when computing field updates. As far as we are aware to date, there has been no discussion of the actual content of the communication between concurrent FDTD processes; accordingly we have defined one such protocol. © 1998 John Wiley & Sons, Ltd.
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 307-316 
    ISSN: 0894-3370
    Keywords: Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: This paper presents an FE modelling approach for the calculation of transient eddy currents in thin conductive layers, where the complexity of the geometry prohibits both a detailed FE modelling and the use of analytical tools. The method allows an estimation of the maximum values of eddy currents at a highly reduced modelling effort and the use of a commercial FE software package. A prerequisite is a slowly varying magnetic field that can be assumed to be unaffected by the eddy currents. It is shown that neither the source of the magnetic field nor the insulating environment has to be modelled. The model is built up exclusively from 2D elements and is excited via the magnetic vector potential. The spatial distribution of the latter is calculated separately in a magnetostatic calculation based on Biot-Savart's law. It is then applied in time-varying form as a dynamic boundary condition at every node of the model. The method was applied to a simple problem for which the results of a detailed FE calculation were available, to document its validity. Further numerical results are presented for the plasma vessel and the heat radiation shield of the Wendelstein 7-X fusion experiment in the case of an emergency discharge. © 1998 John Wiley & Sons, Ltd.
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 55-68 
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    Keywords: Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Multiresolution time domain (MRTD) analysis is applied directly to Maxwell's equations to model inhomogeneous dielectric material. In our approach, scaling and wavelet functions are used as a complete basis for the method of moments. The MRTD scheme is used to analyze different types of resonant cavity structures with varying dielectric perturbations in one, two and three dimensions. The results presented here agree very well with those obtained by FDTD, FEM and integral equation methods. MRTD allows for considerable savings in memory and computation time in comparison to FDTD, while maintaining the same accuracy of the results. © 1998 John Wiley & Sons, Ltd.
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 1-1 
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    Keywords: Engineering ; Numerical Methods and Modeling
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 105-121 
    ISSN: 0894-3370
    Keywords: Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: This paper describes a computationally efficient diakoptic algorithm for the solution of closed-mesh electrical network equations. The algorithm is designed for use with time-varying and/or discontinuous equations describing limited size power supply systems, allowing them to be torn into smaller subdivisions in a manner that is independent of their structure. The efficiency of the algorithm is demonstrated by comparing its performance with that obtained when using a more conventional sparse matrix technique, on the basis of the number of floating point operations required for the solution of a range of typical network problems. Mention is made of the parallel processes inherent in the resultant equation structure. © 1998 John Wiley & Sons, Ltd.
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 131-131 
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    Keywords: Engineering ; Numerical Methods and Modeling
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    Topics: Electrical Engineering, Measurement and Control Technology
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 221-229 
    ISSN: 0894-3370
    Keywords: Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: This paper presents a physical-mathematical model for abrupt heterojunction transistors and its solution using numerical methods with application to InP/InGaAs HBTs. The physical model is based on the combination of the drift-diffusion transport model in the bulk with thermionic emission and tunnelling transmission through the emitter-base interface. Fermi-Dirac statistics and bandgap narrowing distribution between the valence and conduction bands are considered in the model. A compact formulation is used that makes it easy to take into account other effects such as the non-parabolic nature of the bands or the presence of various subbands in the conduction process. The simulator has been implemented for distributed memory multicomputers, making use of the MPI message-passing standard library. In order to accelerate the solution process of the linear system, iterative methods with parallel incomplete factorization-based preconditioners have been used. © 1998 John Wiley & Sons, Ltd.
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 273-273 
    ISSN: 0894-3370
    Keywords: Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 233-242 
    ISSN: 0894-3370
    Keywords: Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: The propagation factors of buried microstrip lines are considered. The associated eigenvalue problems are solved by the edge-based vectorial finite element method (VFEM). To improve the higher-order VFEM solutions, the problem infinite domain is truncated by second-order absorbing boundary conditions (ABCs). This reduces the number of unknowns and, consequently, the numerical effort. Owing to the many advantages of both the edge-based VFEM and the second-order ABCs, this full-wave method is implemented by a general computer program for solving unbounded domain problems involving complex geometries and inhomogeneous media. The examples presented show the validity of this technique. © 1998 John Wiley & Sons, Ltd.
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 243-254 
    ISSN: 0894-3370
    Keywords: Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Various local absorbing boundary conditions are implemented in the TLM method and their corresponding reflections are compared as a waveguide termination. The instability in different ABCs is discussed and reflections from the terminating walls are computed. An improvement for the matched termination wall is proposed and reflection of this absorbing condition is compared with ordinary matched termination. © 1998 John Wiley & Sons, Ltd.
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 317-318 
    ISSN: 0894-3370
    Keywords: Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 289-298 
    ISSN: 0894-3370
    Keywords: Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Oscillatory flow present in the melt during InSb single crystal growth using an RF-heating Czochralski method has been numerically investigated by means of the finite difference method using the HSMAC algorithm. The thermal boundary conditions required for the numerical simulation model were obtained experimentally by measuring the temperature profile along the crucible of a Czochralski system by means of thermocouples mounted in the crucible. Results of numerical simulations showed that the use of a third-order upwind discretization scheme was necessary to catch the oscillatory behaviour of the fluid flow in the melt. It was shown that this oscillatory behaviour strongly depends on the crystal rotation rate. Indeed, the oscillation period increases when the crystal rotation rate is above a critical rotation rate. In order to avoid such oscillations, crystal rotation rates lower than this critical value of crystal rotation rate must be selected for the growth of high quality crystals free of striations. © 1998 John Wiley & Sons, Ltd.
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 299-306 
    ISSN: 0894-3370
    Keywords: Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: The application of the FDTD algorithm on generalized non-orthogonal meshes, following the basic ideas of Holland (1983), has been investigated by many authors for several years now, and detailed dispersion analysis as well as convergence studies have been published. Already in 1992 also a general stability criterion was given for the time integration using the standard leap-frog scheme (Lee et al.). Many authors, however, still propose some damped time stepping algorithms to work around unexpected instabilities in the discretization method. In this paper the origin of this type of instability is revealed, and a technique to obtain a stable discretization of Maxwell's equations on non-orthogonal grids is proposed. To obtain more insight into the stability properties of the method, it is reformulated according to the matrix-vector notation of the Finite Integration Technique. © 1998 John Wiley & Sons, Ltd.
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 123-130 
    ISSN: 0894-3370
    Keywords: Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: This paper presents the electrode separation method for the boundary condition of a-Si TFT mixed-level simulation. The Poisson equation and the continuity equation are formulated into equivalent circuits. So, a circuit simulator can be used to handle the two-dimensional numerical simulation of a-Si TFT. The boundary condition problem between a semiconductor and an external circuit is solved by the electrode separation method. An electrode is separated into two nodes to fit Kirchhoff's current law and the semiconductor equations, respectively. A simple a-Si TFT/LCD circuit is taken as an example for the electrode separation method. For mixed-level simulation this technique is very useful. © 1998 John Wiley & Sons, Ltd.
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 133-151 
    ISSN: 0894-3370
    Keywords: Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: Most traditional theories of speech production are currently based on plane waves. However, it is well known that, for acoustic waveguides, higher acoustical modes start to propagate and can become predominant above cut-on frequencies. This paper thus presents the transmission line matrix method, a numerical method initially designed for electromagnetic waves, and its adaptation to acoustic waveguides. The method, and in particular the representation of boundary conditions, is validated by comparison with known analytical theories. It is then used to show the dramatic effect of higher order modes upon the radiation characteristics of uniform ducts, as well as the importance of source location. Finally, first applications to bent and bifurcating rectangular ducts are presented, and the transfer function of a vowel [a] is shown to display frequency patterns typical of those measured on human subjects and that cannot be explained by one-dimensional propagation only. © 1998 John Wiley & Sons, Ltd.
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 189-205 
    ISSN: 0894-3370
    Keywords: Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: The atomic mixing model that forms the basis of the IMPETUS software is described in detail. The model simulates the mixing and particle emission that occurs when a solid is bombarded with energetic particles, such as in SIMS or SNMS. The methods employed for computing the deposition of the bombarding particles and their energies along with the modelling of the particle yield and the surface recession speed are described.The material volume concentrations are governed by a set of partial differential equations. A description of the finite element method that is employed for their solution is given. Results from the application of IMPETUS II to a number of typical structures are given. © 1998 John Wiley & Sons, Ltd.
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 207-219 
    ISSN: 0894-3370
    Keywords: Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: The IMPETUS II code simulates the atomic mixing and particle emission that occurs when a solid is bombarded by energetic particles (as in SIMS or SNMS). The underlying model consists of a system of partial differential equations that are solved by a finite difference method (FDM). Special techniques are also employed to model thin layers and sharp interfaces, to deal efficiently with wide homogeneous layers (when the solution is tending to a steady state), to model linear diffusion in order to smooth the sharp interfaces before they enter to domain of the FDM.In this paper the special techniques are described in detail. Results from test problems, demonstrating these techniques, are shown. An algorithm that describes the way the IMPETUS II code is structured is given. © 1998 John Wiley & Sons, Ltd.
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 3-19 
    ISSN: 0894-3370
    Keywords: Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: This paper presents an efficient space domain integral formulation of planar dielectric structures based on the concepts of multiresolution analysis (MRA) theory. Battle-Lemarie multiresolution expansions are utilized in the moment method solution of a volume integral equation for the unknown electric field. The generation of sparse moment matrices is explained in view of the cancellation property of wavelet basis functions. The formulation is then applied to two- and three-dimensional dielectric structures including waveguides and resonators, and the numerical results and consequences of matrix sparsity are discussed in detail. © 1998 John Wiley & Sons, Ltd.
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 21-40 
    ISSN: 0894-3370
    Keywords: Engineering ; Numerical Methods and Modeling
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    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: An efficient wavelet-packet-based time-frequency technique is proposed in this paper. This technique has found applications in inverse synthetic aperture radar (ISAR). It combines the Wigner-Ville distribution (WVD) of each sub-band signal partitioned from an input signal by the wavelet-packet transform (WPT). These distributions are used to estimate the parameters in Doppler processing. Similar parametric compensation is carried out in range processing so that a specific scatterer is focused in both range and cross-range. The end result uses the Fourier imaging method to obtain a high-resolution radar image. This technique is primarily based on estimation and compensation of the Doppler scattering parameters of the dominant scatterers for improved focus. It can be of great value to the scattering-oriented ISAR imaging. © 1998 John Wiley & Sons, Ltd.
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 41-54 
    ISSN: 0894-3370
    Keywords: Engineering ; Numerical Methods and Modeling
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    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: The local support and vanishing moment property of wavelet bases have been recently used to obtain a sparse matrix representation of integral equations in the spatial domain. In this paper, an application of the cubic spline and the corresponding semi-orthogonal wavelets in the spectral domain is proposed for the evaluation of the reflection coefficient for open/short transmission lines. Because of the nearly optimal time (space)-frequency (wavenumber)-window product of the cubic spline and wavelet, the double spectral integrals appearing in the formulation can be computed more efficiently than with the commonly used piecewise sinusoidal (PWS) or triangular basis functions. It is shown that the time-frequency-window product of the triangular and PWS function are close to each other, whereas those of the cubic spline/wavelet are close to 0·5, the lowest possible value corresponding to functions of Gaussian class. Both the PWS and wavelet bases are applied to microstrip and coplanar waveguides with isotropic and anisotropic substrates, and the results are compared with published theoretical and experimental data. It is observed that even though the number of splines/wavelets required for an accurate representation of the current distribution in the transmission line is almost twice as high as the number of PWS functions, the overall computation time decreases significantly in the former case. © 1998 John Wiley & Sons, Ltd.
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 69-84 
    ISSN: 0894-3370
    Keywords: Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: The use of wavelet expansions in numerical solutions of electromagnetic frequency-domain integral equation formulations is steadily growing. In this paper we review the recently suggested impedance matrix compression (IMC) method for a more effective integration of wavelet-based transforms into existing numerical solvers. The difference between the IMC method and the previous approaches to applying wavelets in computational electromagnetics is twofold. Firstly, the transformation is effected by means of a digital filtering approach. This approach renders the transform algorithm adaptive and facilitates the derivation of a basis which best suits the problem at hand. Secondly, the conventional thresholding procedure applied to the impedance matrix is substituted for by a compression process in which only the significant terms in the expansion of the (yet-unknown) current are retained and hence a substantially smaller number of coefficients has to be determined. A few numerical results are included to demonstrate the advantages of the presented method over the currently used ones. The feasibility of ensuring a slow growth in the number of unknowns even when there is a rapid increase in the problem complexity is shown by an illustrative example. © 1998 John Wiley & Sons, Ltd.
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 87-103 
    ISSN: 0894-3370
    Keywords: Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology
    Notes: An efficient novel algorithm is introduced for ground wave propagation problems. First, ground wave propagation characteristics for a vertically polarized short electric dipole over a smooth spherical earth are reviewed, reducing the vector electromagnetic problem for the three-dimensional spherical geometry to an equivalent two-dimensional rectilinear scalar potential problem which is solved by spectral analysis and synthesis. Alternative evaluations of the spectral integral yield ray optical and normal mode solutions, which are conventionally referred to as the Norton and Wait formulations, respectively. Combining these formulations in an efficient manner yields a hybrid algorithm which is constructed so as to account adaptively for the characteristics of ground wave propagation in interference, intermediate and diffraction regions (including mixed paths) for various source and/or receiver heights. Numerical comparisons are made with reference results obtained via the parabolic equation (PE) method, in parametric ranges where PE is reliable; this permits assessment of the effectiveness of the hybrid approach. © 1998 John Wiley & Sons, Ltd.
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    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 11 (1998), S. 183-184 
    ISSN: 0894-3370
    Keywords: Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology
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    Advanced Materials for Optics and Electronics 8 (1998), S. 101-105 
    ISSN: 1057-9257
    Keywords: biosensor ; protein immobilisation ; protein modification ; electrochemistry ; nitrotyrosine ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Use of electrosynthetic methodology allows the production of hen egg-white lysozyme (HEWL) either mononitrated at tyrosine 23 or bisnitrated at tyrosines 20 and 23, but never nitrated at tyrosine 53. This is a different sequence from that obtained by the chemical nitrating agent tetranitromethane, and when reduced by dithionite, the selectively modified enzyme can be anchored at pH 5 via the unique aromatic amino group to magnetic beads or other suitable matrices. HEWL so immobilised loses less than 10% of cell-wall lytic activity compared with the approximately 50% loss of activity when immobilised by conventional methodology at pH 9 via essentially random reaction at lysine residues and other functionalities which are nucleophilic at this pH. This result offers promise as a general method for selective protein immobilisation in biosensors and similar applications. © 1998 John Wiley & Sons, Ltd.
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    Advanced Materials for Optics and Electronics 8 (1998), S. 157-160 
    ISSN: 1057-9257
    Keywords: Chemistry ; Polymer and Materials Science
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    Advanced Materials for Optics and Electronics 8 (1998), S. 187-193 
    ISSN: 1057-9257
    Keywords: antifouling ; biofouling ; hydrogel ; benzalkonium chloride ; ultraviolet-visible spectroscopy ; underwater optics ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: A hydrogel based on poly(2-hydroxyethyl methacrylate) (PHEMA) containing benzalkonium chloride (BAK) can be used as an environmentally acceptable, fouling-resistant material in the marine environment. The loaded hydrogel system is transparent and has the potential to be used in the protection of optical ports in underwater instruments. Ultraviolet-visible (UV-vis) spectroscopy was used to study the optical properties of the material after a marine exposure period. The optical transmittance of PHEMA/ BAK was higher for 10 weeks than that detected for poly(methyl methacrylate) (PMMA), a material currently used in commercial instruments, which confirmed the superior fouling resistance of the PHEMA/ BAK combination. The UV-vis spectroscopic method was quick, relatively cheap and accurate enough to allow the effects of the development of marine fouling on transparent surfaces for use in marine underwater optical applications to be monitored. © 1998 John Wiley & Sons, Ltd.
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    Advanced Materials for Optics and Electronics 8 (1998), S. 247-262 
    ISSN: 1057-9257
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: The synthesis of a number of side chain liquid crystalline polymers for quadratic non-linear optical (NLO) applications is reported. The active chromophore is a charge transfer (CT) biphenyl derivative which possesses mesogenic properties itself. The liquid crystalline behaviour of these polymers was established by differential scanning calorimetry (DSC), optical microscopy and X-ray diffraction. The active chromophores were oriented by the standard corona poling technique and the degree of axial ordering was determined as a function of poling conditions by linear optical absorption. Growth of the second-harmonic generation (SHG) signal was used to probe the induction of polar order. These experiments clearly indicated that liquid crystallinity results in an enhancement of the polar order over that of isotropic materials. The second-order NLO susceptibility tensor components d31 and d33 were measured by the SHG technique. The ratio d33/ d31 was found to be much larger than 3, in agreement with molecular statistical models. Values of d33 up to 30-35 pm V-1 were obtained at 1064 nm fundamental wavelength. These values are essentially not resonance-enhanced, since the chromophore absorption occurs below 350 nm. © 1998 John Wiley & Sons, Ltd.
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    Advanced Materials for Optics and Electronics 8 (1998), S. 317-324 
    ISSN: 1057-9257
    Keywords: gas sensing ; nitrogen dioxide ; stilbene ; biphenyl ; UV-visible spectroscopy ; thin films ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Thin films of a selection of stilbene and biphenyl side-chain substituted liquid crystal polymers based on polysiloxanes were deposited using an automated dip-coating technique and exposed to either 100 ppm NO2 gas and/or concentrated nitric acid vapour, the consequent effect being monitored by changes in the UV-visible spectra of the material. No effective response to NO2 was observed from the biphenylene analogue, but the stilbene derivatives showed spectral changes to suggest that an interaction occurs between the vapour and the bridge position of the stilbene side-chain. The stilbenes also show a marked pre-conditioning phenomenon upon exposure to nitric acid vapour prior to exposure to nitrogen dioxide. This procedure gives a material that has a more reversible response on exposure to NO2 gas than an anagolous film that has not been pre-treated. Copyright © 1998 John Wiley & Sons, Ltd.
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    Keywords: conductive polymers ; polyaniline ; palladium dopants ; platinum dopants ; catalytic hydrogenation ; active centres ; catalyst aging ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: The chemical and structural surface-aging effects brought about by the presence of water in emeraldine base (EB) polyaniline (PANI) doped with Pd or Pt protonic acids were studied. IR spectroscopy, XRD, XPS and heterogeneous catalytic hydrogenation (alkyne→alkene→alkane) were applied to characterise the PANI-Pd and PANI-Pt. Interpretation of the results gave the surface characteristics, structure, chemical catalytic activity and stability mainly of PANI-Pd specimens. The unique form of catalytically active centres therein was the surface complex [PdCl4]2- with Pd 3d5/2 BE=337·7 eV. The most promising among the PANI-Pd catalysts studied were those dried in a slow, long procedure (3 months, zeolite 5A). Copyright © 1998 John Wiley & Sons, Ltd.
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    Advanced Materials for Optics and Electronics 8 (1998), S. 121-128 
    ISSN: 1057-9257
    Keywords: self-assembly ; charge transfer ; electron-conducting monolayers ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Suitably modified mono- and bimolecular films, including bilayer lipid membranes (BLMs), offer exceptionally good opportunities for probing electric field effects on charge transfer and redox reactions in biosensor and molecular electronics research and development.This work presents the redox reactions of tetracyano-p-quinodimethane (TCNQ) molecules incorporated in a self-assembled octadecanethiol monolayer (SAM) on polycrystalline gold electrodes, depending upon the type of supporting electrolyte cations and their concentration. Our results show that TCNQ-modified Au-SAM electrodes exhibit selectivity versus alkali metal cations in aqueous supporting electrolyte (∽10 kJ mol-1 difference between K+ and Li+ and between Cs+ and K+). The slope of the ‘calibration curves’ for Li+ and K+ is about 59 mV per decade of concentration of the analyte. The explanation of this behaviour is based on the Donnan potential model; however, an ion-pairing effect can also be involved. Our preliminary results show also that the TCNQ molecules within the octadecanethiol monolayer may act as a molecular redox device. © 1998 John Wiley & Sons, Ltd.
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    Advanced Materials for Optics and Electronics 8 (1998), S. 147-155 
    ISSN: 1057-9257
    Keywords: synthesization ; chalcopyrite ; CuGaSE2 single phase ; X-ray diffraction ; stoichiometric deviations ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: The dependence of the structural parameters on compositional deviations of CuGaSe2 has been studied. These deviations have been induced along an ingot by a single fusion of the components at 1150 °C and subsequent slow cooling in a stationary ampoule in a vertical furnace. All along the sample a single chalcopyrite phase is present and a compositional gradient along the ingot was found by energy-dispersive analysis of X-rays (EDAX) measurements, the upper part being rich in Ga (series B) and the lower part in Cu (series A), with Cu/Ga ratios of 0·95 and 1·1 respectively. A hypothesis of the existence of two phases in the melt is proposed to explain these facts. The unit cell parameters, anion displacement and Cu and Ga occupation numbers in their sublattices were analysed by X-ray powder diffraction and Rietveld refinement methods. In series A the occupation numbers are near stoichiometry, while in series B a Cu defect appears. In both series, changes in unit cell parameter are related to changes in Cu content, suggesting the presence of a fraction of Cu ions either as interstitials or at Ga sites when Cu is in excess, or of Cu vacancies in its sublattice when there is a Cu deficiency. © 1998 John Wiley & Sons, Ltd.
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    Advanced Materials for Optics and Electronics 8 (1998), S. 195-199 
    ISSN: 1057-9257
    Keywords: chiral waveguides ; preparation of modified ATR method ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Chiral properties of planar waveguide structures with a core layer formed by a poly(methyl methacrylate) (PMMA) host matrix doped with chiral santonin were investigated by means of a modified attenuated total reflection (ATR) method. Distinct modification of the observed ATR spectra was revealed for right and left circularly polarised incident beams. From comparison of the experimental spectra with theoretical curves, the following specific rotation was obtained: [α]20633=11,200 deg cm2 g-1 (santonin/ PMMA mass ratio 2:1). The origin of the observed optical activity and its influence on the waveguide dispersion characteristics are briefly discussed. © 1998 John Wiley & Sons, Ltd.
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    Advanced Materials for Optics and Electronics 8 (1998), S. 213-213 
    ISSN: 1057-9257
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: No abstract
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    Advanced Materials for Optics and Electronics 8 (1998), S. 263-267 
    ISSN: 1057-9257
    Keywords: semiconductors ; low-dimensional systems ; crystal structure ; optical properties ; photoluminescence ; Raman spectra ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: The structural, optical and related properties (i.e. photoluminescence and resonance Raman spectra) of some synthetic (i.e. unconventional) low-dimensional semiconductor systems such as K2Cd3S4, [CH3SC(NH2)=NH2]3PbI5 and [H3N(CH2)6NH3]BiI5 are reported. They are compared with the properties of the corresponding higher-dimensionality systems. A blue shift of the excitonic bands and an enhancement of their binding energy and intensity were observed by decreasing the dimensionality or the size of the materials active part. The results are similar to those obtained from conventional semiconductors by decreasing the dimensionality or the size and are attributed to quantum confinement of excitons. © 1998 John Wiley & Sons, Ltd.
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    Advanced Materials for Optics and Electronics 8 (1998), S. 309-316 
    ISSN: 1057-9257
    Keywords: optical gas sensing ; polysiloxane ; azobenzene ; NO2 ; molecular modelling ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Siloxane copolymers having as side-chains azobenzene derivatives bearing different electron-withdrawing and donating substituents were deposited as thin films by dip coating, and their behaviour upon exposure to 100 pm NO2 was studied by UV/visible spectroscopy. Electron-donating substituents at the ortho positions on the aromatic rings in the azo unit have significant influence on the absorbance changes produced by exposure to NO2, and this is explained by modelling, which suggests that interaction between the electrophilic nitrogen atom in NO2 and the electron clouds of the azobenzene skeleton is responsible for the sensing process, rather than the formation of a Wheland intermediate or other product of chemical reaction. Especially effective substituents are methoxy groups in either aromatic ring ortho or the azo linkage, which also produce a significant increase in intensity of the long-wavelength n-π* transition. This moves the optical interrogation signal to a wavelength range of particular benefit for potential applications. Copyright © 1998 John Wiley & Sons, Ltd.
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    Advanced Materials for Optics and Electronics 8 (1998), S. 303-308 
    ISSN: 1057-9257
    Keywords: polyaniline ; emeraldine base ; carbon fluoride ; lithium cells ; aprotic solvent ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Polyaniline (PANI) has been used for modification of the electrochemical behaviour of a carbon fluoride ((CFx)n) cathode in a lithium cell. PANI and (CFx)n powders were carefully mixed and the electrochemical properties and kinetic parameters of the composite (CFx)n-PANI cathode were evaluated by galvanostatic and potentiodynamic techniques. An increase in exchange current has been found for electrodes with addition of polyaniline in the form of emeraldine base (EB). During the reduction process of carbon fluoride using 1 M lithium perchlorate solution in organic solvent, the ternary intercalation compound CLixF is formed and then irreversibly decomposed into carbon and lithium fluoride. The insertion of lithium cations into the (CFx)n layered structure is diffusionally controlled, hence improvement in electronic properties of this fluoride material by the presence of conducting PANI chains and enhancement of discharge performance were found for 25% addition of PANI. Copyright © 1998 John Wiley & Sons, Ltd.
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  • 88
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    Advanced Materials for Optics and Electronics 8 (1998), S. 285-294 
    ISSN: 1057-9257
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Field effect transistors with an organic material as active layer are at present essentially used to determine the mobilities in these materials. Until now, in analysing the measured current characteristics, only the simplest (Shockley) model has been used which accounts neither for this type of thin film transistor (TFT), which operates in depletion and accumulation, nor for the nature of the carriers. Starting from two-dimensional simulations for the analogous silicon TFT, we have developed an analytical model for the TFT that accounts for several peculiarities of the current characteristics of this type of transistor. In addition, a first modification has been developed which describes the situation when the charged states are polarons and bipolarons, as is the case in organic materials. Applications to published experimental current characteristics show that a general reanalysis is needed. Copyright © 1998 John Wiley & Sons, Ltd.
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  • 89
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    Advanced Materials for Optics and Electronics 8 (1998), S. 111-119 
    ISSN: 1057-9257
    Keywords: PPV ; conjugated polymers ; quasi-particles ; ab initio calculation ; infrared spectra ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: We investigate the quasi-particle conformational defects (excitons, polarons and bipolarons) in phenylene vinylene oligomers (PVOs) consisting of up to 12 repeat units. The conformations are determined by minimisation of the total Hartree-Fock energy calculated at 3-21G level. The Hartree-Fock calculations of the excited states are followed by the CI-Singles calculation. The bond length alternation along the oligomer chain is discussed. On the basis of these results the vibrational transitions are calculated. To correct the basis set truncation error, the vibrational frequencies are multiplied by a uniform scaling factor. The types of vibrations are assigned. The theoretical spectra are in good agreement with the experimental infrared vibrational spectrum of poly(p-phenylene vinylene) (PPV). © 1998 John Wiley & Sons, Ltd.
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  • 90
    ISSN: 1057-9257
    Keywords: gallium nitride ; nanocrystallites ; detonation ; gallium azide ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: High quality nanoscale, phase-pure hexagonal gallium nitride (GaN) crystallites have been synthesized by the thermal induced detonation of molecular precursors of the type (R3N)Ga(N3)3 (R=CH3, C2H5, etc.). The method allows the control of the particle size regime from 2 to about 1000 nm. X-ray diffraction and Rietveld simulations revealed an anisotropic platelet-like shape of the particles. The obtained GaN material was as well characterized by transmission electron microscopy and electron diffraction, photoluminescence spectroscopy, SEM, IR, RAMAN, thermal gas effusion/mass spectrometry, thermal analysis, elemental analysis. Gas absorption measurements (BET method) showed a specific surface area of about 90 m2 · g-1. © 1998 John Wiley & Sons, Ltd.
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  • 91
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    Advanced Materials for Optics and Electronics 8 (1998), S. 181-186 
    ISSN: 1057-9257
    Keywords: poly(4,7-benzothiophene vinylene) ; polymer interfaces ; conjugated polymers ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: In this paper we determine the LUMO level position of poly(4,7-benzothiophene vinylene) (PBTV), a heterocyclic poly(phenylene vinylene) (PPV) analogue, using tunnelling current measurements. We compare the energy level positions with those measured for PPV and determine their shifts. The experimental results are compared with theoretical predictions and good agreement is observed. © 1998 John Wiley & Sons, Ltd.
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  • 92
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    Advanced Materials for Optics and Electronics 8 (1998), S. 215-228 
    ISSN: 1057-9257
    Keywords: fullerene ; fullerides ; charge transfer complexes ; spectroscopy ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Some possible uses of vibrational and electronic spectroscopies in characterising the basic interactions in C60 and C70 complexes are pointed out. We report on the wealth of infrared spectra of (usually) single crystals of C60 and C70 clathrates and complexes with organic donors. The changes in the spectral parameters of the complexes in comparison with those of the neutral fullerene molecules are attributed to redistribution of the charge on the spherical C60 or elongated C70 molecules in their compounds. Some general considerations on the origin of the IR, NIR, VIS and UV spectral variations are also given. © 1998 John Wiley & Sons, Ltd.
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  • 93
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    Advanced Materials for Optics and Electronics 8 (1998), S. 269-276 
    ISSN: 1057-9257
    Keywords: organic charge transfer salts ; conducting polymers ; electronic properties ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: We are exploring the electronic properties of novel materials such as conducting polymer-coated polyester fibres and charge transfer complexes configured in resistive, gated (FET) and diode configurations. In this paper we will consider several areas where we are attempting to make sensors and/ or primitive devices from molecular materials. Our most successful result so far is the temperature-dependent resistivity of polypyrrole-coated polyester fibres. Here we find that the conductivity is thermally activated and produces a reliable, highly sensitive indicator of temperature in a cryogenic environment. Using parallel configurations of fibres, sensors with different limiting resistances can be fabricated. We have also placed insulated electrical gates on single crystals of various metallic and insulating charge transfer salts and have attempted to alter the charge transfer character by the application of high electric fields. With polymethylmethacrylate (PMMA) insulating layers, gate voltages up to 150 V have been acquired with no leakage or heating. Similar measurements on diode devices fabricated from charge transfer complexes with different electronic structures are also in progress, and our preliminary results will be presented. Prospects and plans for developing these materials into viable sensors and devices are discussed. © 1998 John Wiley & Sons, Ltd.
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  • 94
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    Advanced Materials for Optics and Electronics 8 (1998), S. 277-284 
    ISSN: 1057-9257
    Keywords: calix[4]resorcinarenes ; charge transfer (CT) complexes ; tetracyanoquinodimethane (TCNQ) ; tetracyanoethylene (TCNE) ; non-linear optical (NLO) materials ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Lipophilic calix[4]resorcinarenes - condensation products of resorcinol and lauryl aldehyde comprising four resorcinol units - form electron donor-acceptor complexes with electron acceptors tetracyanoquinodimethane (TCNQ) and tetracyanoethylene (TCNE) in organic solvents. Their formation was confirmed by EPR and UV-vis spectroscopy, except for compounds c1 and c3 which did not form charge transfer (CT) complexes with TCNE. Titration experiments involving calixarenes and electron acceptors indicated the presence of species of different stoichiometries. Complexes with ligands c1-c4 were characterised spectroscopically. CT bands appeared in most cases. © 1998 John Wiley & Sons, Ltd.
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  • 95
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    Advanced Materials for Optics and Electronics 8 (1998), S. 229-245 
    ISSN: 1057-9257
    Keywords: unimolecular rectifier ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Asymmetries in the macroscopic and the nanoscopic DC electrical conductivity through Langmuir-Blodgett multilayers and monolayers of γ-(n-hexadecyl)quinolinium tricyanoquinodimethanide (HDQ-3CNQ, 1) are due to a transition from the ground-state zwitterion to a probably neutral excited-state conformer, thus confirming without a shadow of a doubt the rectification of electrical current by a single molecule. © 1998 John Wiley & Sons, Ltd.
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  • 96
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    Advanced Materials for Optics and Electronics 8 (1998), S. 1-8 
    ISSN: 1057-9257
    Keywords: electrodeposition ; copper-indium-selenide ; thin films ; layer-by-layer deposition ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: The electrochemical bath used for growing device-quality CIS (CuInSe2) thin films by co-deposition as well as layer-by-layer (LBL) deposition was characterised and optimised with respect to the film properties. The bath composition was varied by changing the Cu, In and Se ion concentrations in specific ratios in both co-deposition and LBL deposition. The film properties were analysed using techniques such as SEM (scanning electron microscopy), EPMA (electron probe microanalysis), AES (Auger electron spectroscopy) and XRD (X-ray diffraction). The structural, morphological and compositional properties of the films were characterised and their variation is attributed to the bath composition and growth conditions. © 1998 John Wiley & Sons, Ltd.
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  • 97
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    Advanced Materials for Optics and Electronics 8 (1998), S. 13-22 
    ISSN: 1057-9257
    Keywords: remote PECVD ; silane ; nitrogen ; silicon nitride ; argon dilution ; emission spectroscopy ; metastable states ; atomic nitrogen ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: In a series of two papers we describe the effect of argon dilution of the nitrogen passed through the RF discharge region on the plasma composition, growth rate and some characteristics of silicon nitride films deposited by remove PECVD. In this part we report the results of an emission spectroscopic study of the plasma obtained in an SiH4-N2-Ar mixture. It is shown that argon in metastable electronic excited states plays an important role during the RPECVD of silicon nitride films by providing a high concentration of atomic nitrogen which is necessary for the promotion of film growth. In Part II the influence of argon dilution on the growth rate, composition and some properties of silicon nitride films deposited by capacitively and inductively coupled remote PECVD is discussed. © 1998 John Wiley & Sons, Ltd.
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  • 98
    ISSN: 1057-9257
    Keywords: remote PECVD ; silane ; nitrogen ; silicon nitride ; argon dilution ; hydrogen content ; stoichiometry ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: In Part I we reported the results of an emission spectroscopic study of the plasma obtained in an SiH4-N2-Ar mixture. It was shown that argon in metastable electronic excited states provides a high concentration of atomic nitrogen. In this part we report the results of a study of the influence of argon dilution on the growth rate, composition and properties of silicon nitride films. The exact influence of nitrogen dilution with argon depends on the process parameters and on the method of coupling of the RF power, but it is found in general that a high concentration of atomic nitrogen leads to changes in the relative amounts of Si-Hj and N-Hi bonds and in the Si/N ratio of deposited films. In particular, it is shown that hydrogen incorporation can be reduced and improved stoichiometry can be obtained. © 1998 John Wiley & Sons, Ltd.
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  • 99
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    Advanced Materials for Optics and Electronics 8 (1998), S. 61-76 
    ISSN: 1057-9257
    Keywords: magnetic molecular materials ; conducting molecular materials ; polyoxometalates ; TTF ; ET ; conducting polymers ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: The different approaches used by the authors for the synthesis of new molecular materials exhibiting simultaneously magnetic and conducting properties are presented here. The aim is to prepare materials where a magnetic and a conducting sublattice coexist and/or interact in order to obtain either coexistence of properties or coupling between them. The strategy described is a hybrid one that combines various types of inorganic metal complexes with planar organic π-electron donors of the TTF family or with organic matrices made of conducting polymers. There are four main combinations: (i) magnetic polyoxometalates with organic donors - this strategy has already produced more than 10 radical salts where a magnetic character coexists with a conducting or semiconducting one; (ii) small magnetic anions with organic donors - in this case one of the most promising results has been the synthesis of one of the very few known examples of magnetic molecular metals; (iii) ferro- and ferrimagnetic oxalate-bridged bimetallic layers with organic donors; (iv) magnetic polyoxometalates incorporated into electrodeposited films of conducting polymers. © 1998 John Wiley & Sons, Ltd.
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    Advanced Materials for Optics and Electronics 8 (1998), S. 87-91 
    ISSN: 1057-9257
    Keywords: friction coefficient ; wear coefficient ; tribology ; conducting polymers ; 2,6-naphthalenedisulphonate (2,6-NDS) ; 1,5-naphthalenedisulphonate (1,5-NDS) and 2-naphthalenesulphonate (NS) polypyrroles ; sliding test ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: It is known that, with careful control of conditions, polypyrrole films with counter-ions of toluene sulphonic acid sodium salt and methane phosphonic acid sodium salt can be produced with friction coefficients comparable with or even better than PTFE. Here we now report a systematic study of polypyrrole with various planar anions for tribological bearing applications. Thus naphthalene disulphonate-doped polypyrrole has a kind of laminar structure with very good adhesion to the surface. Polymer films were electrodeposited on glass lenses and tested in a friction apparatus for friction and wear measurements. The film orientation was measured by low-angle X-ray diffraction and the surface structure was evaluated by both AFM and SEM for different film thicknesses. The friction coefficient and wear rate of such bearings were measured under loads up to 5 N and at speeds up to 30 mm s-1 and were found to be 0·06 and 0·04 nm mm-1 respectively. © 1998 John Wiley & Sons, Ltd.
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