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  • Articles  (38)
  • Other Sources
  • nanoparticles  (23)
  • permeability  (15)
  • Springer  (38)
  • American Geophysical Union (AGU)
  • 1995-1999  (38)
  • Technology  (38)
  • 1
    Electronic Resource
    Electronic Resource
    The Effect of Substrate Temperature on the Properties of Nanostructured Silicon Carbide Films Deposited by Hypersonic Plasma Particle Deposition (1999)
    Springer
    Journal of nanoparticle research 1 (1999), S. 31-42 
    Details
    ISSN: 1572-896X
    Keywords: nanoparticles ; thermal plasma ; nanostructural film ; particle deposition ; silicon carbide ; film hardness
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics , Technology
    Notes: Abstract Nanostructured silicon carbide films have been deposited on molybdenum substrates by hypersonic plasma particle deposition. In this process a thermal plasma with injected reactants (SiCl4 and CH4) is expanded through a nozzle leading to the nucleation of ultrafine particles. Particles entrained in the supersonic flow are then inertially deposited in vacuum onto a temperature-controlled substrate, leading to the formation of a consolidated film. In the experiments reported, the deposition substrate temperature Ts has ranged from 250°C to 700°C, and the effect of Ts on film morphology, composition, and mechanical properties has been studied. Examination of the films by scanning electron microscopy has shown that the grain sizes in the films did not vary significantly with Ts. Micro-X-ray diffraction analysis of the deposits has shown that amorphous films are deposited at low Ts, while crystalline films are formed at high Ts. Rutherford backscattering spectrometry has indicated that the films are largely stoichiometric silicon carbide with small amounts of chlorine. The chlorine content decreases from 8% to 1.5% when the deposition temperature is raised from 450°C to 700°C. Nanoindentation and microindentation tests have been performed on as-deposited films to measure hardness, Young's modulus and to evaluate adhesion strength. The tests show that film adhesion, hardness and Young's modulus increase with increasing Ts. These results taken together demonstrate that in HPPD, as in vapor deposition processes, the substrate temperature may be used to control film properties, and that better quality films are obtained at higher substrate temperatures, i.e. Ts≈700°C.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1010033413357
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  • 2
    Electronic Resource
    Electronic Resource
    Preparation and Characterization of Nano-structured Ceramic Powders Synthesized by Emulsion Combustion Method (1999)
    Springer
    Journal of nanoparticle research 1 (1999), S. 197-204 
    Details
    ISSN: 1572-896X
    Keywords: emulsion ; combustion ; ceramic ; nanoparticles ; synthesis ; barium titanate powder
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics , Technology
    Notes: Abstract The emulsion combustion method (ECM), a novel powder production process, was originally developed to synthesize nano-structured metal-oxide powders. Metal ions in the aqueous droplets were rapidly oxidized by the combustion of the surrounding flammable liquid. The ECM achieved a small reaction field and a short reaction period to fabricate the submicron-sized hollow ceramic particles with extremely thin wall and chemically homogeneous ceramic powder. Alumina, zirconia, zirconia–ceria solid solutions and barium titanate were synthesized by the ECM process. Alumina and zirconia powders were characterized to be metastable in crystalline phase and hollow structure. The wall thickness of alumina was about 10 nm. The zirconia–ceria powders were found to be single-phase solid solutions for a wide composition range. These powders were characterized as equiaxed-shape, submicron-sized chemically homogeneous materials. The powder formation mechanism was investigated through the synthesis of barium titanate powder with different metal sources.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1010096214711
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  • 3
    Electronic Resource
    Electronic Resource
    Engineering Aspects of Preparation of Nanocrystalline Particles in Microemulsions (1999)
    Springer
    Journal of nanoparticle research 1 (1999), S. 267-276 
    Details
    ISSN: 1572-896X
    Keywords: nanoparticles ; microemulsions ; reaction engineering ; formation model ; simulation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics , Technology
    Notes: Abstract Engineering aspects of the preparation of palladium nanoparticles in non-ionic w/o-microemulsions are examined. In order to achieve reproducible synthesis conditions a semi-batch reactor with a standardized design is used. Influences of the stirring rate and of different ways of concentration control on the product properties are observed. For reproducible synthesis it is important to establish appropriate and defined preparation conditions. Monodisperse palladium particles of around 5 nm size are obtained by adding the microemulsion containing the palladium salt at a constant feed rate to the precharged microemulsion containing the reducing agent. A quantitative kinetic model is proposed to describe particle formation in microemulsions. Unknown parameters of the model have been estimated by independent examinations or can be achieved by fitting to the experimental data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1010068903324
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  • 4
    Electronic Resource
    Electronic Resource
    Influence of Some Parameters on the Synthesis of ZrO2 Nanoparticles by Heating of Alcohol–aqueous Salt Solutions (1999)
    Springer
    Journal of nanoparticle research 1 (1999), S. 349-352 
    Details
    ISSN: 1572-896X
    Keywords: alcohol–aqueous salt solutions ; ZrO2 ; nanoparticles ; aging
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics , Technology
    Notes: Abstract Parameters that influence ZrO2 (3-mol% Y2O3 stabilized) nanoparticles prepared by heating of alcohol–aqueous salt solutions were investigated. It revealed that the kind of alcohol used significantly affected the particle size and morphology of the as synthesized nano-ZrO2 powders. The ratio of alcohol to water (R/H) was also important to conduct the gelation process. The dispersion and sintering behavior of the powder could be optimized via aging. By carefully controlling the process, weakly agglomerated ZrO2 nanoparticles with an average particle size of 13-nm (TEM) were achieved. The classical DLVO theory was employed to clarify the effect of solvent on powder morphology, an aging mechanism was proposed as well.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1010056830551
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  • 5
    Electronic Resource
    Electronic Resource
    Measurement of the Density of Polymeric Nanoparticulate Drug Carriers by Isopycnic Centrifugation (1999)
    Springer
    Journal of nanoparticle research 1 (1999), S. 411-418 
    Details
    ISSN: 1572-896X
    Keywords: isopycnic centrifugation ; buoyant density ; drug delivery ; nanoparticles ; sucrose linear gradient
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics , Technology
    Notes: Abstract The development of polymer nanoparticles as drug carriers requires numerous steps including several in vitro evaluations in cell cultures and biocompatibility. To perform these experiments, it is crucial to express the particle concentration as the number of particles per volume unit or as the particle surface area. Calculation of these suspension characteristics can be perfomed knowing the size and the density of the nanoparticles as well as the polymer concentration. While particle size and polymer concentration are parameters being determined routinely, this study proposes to measure the density of the nanoparticle drug carriers by isopycnic centrifugation using linear sucrose gradients. The method was found to be very reproducible and it presents the advantage of being applied on a small sample of nanoparticles.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1010031605547
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  • 6
    Electronic Resource
    Electronic Resource
    A High Efficiency, High Throughput Unipolar Aerosol Charger for Nanoparticles (1999)
    Springer
    Journal of nanoparticle research 1 (1999), S. 115-126 
    Details
    ISSN: 1572-896X
    Keywords: nanoparticles ; aerosol charger ; unipolar charging ; Fuchs theory ; experiments
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics , Technology
    Notes: Abstract A novel aerosol charger has been developed, which has high efficiency and high throughput especially for nanometer particles in the size range of 3–50 nm. Unipolar charging with high ion concentration and long charging time is used to obtain the high charging efficiency. High throughput is achieved by reducing particle loss within the charger. This is accomplished by directing ion flow and aerosol flow in the same direction and by the use of sheath air flow. The charger configuration is of a longitudinal design – the direction of aerosol stream and ion stream are flowing parallel along the longitudinal axis of the charger. The charger consists of four sections: the inlet zone, the ion production zone, the unipolar charging zone, and the exit zone. In the inlet and ion production zones, unipolar ions are generated using Po210 radioactive sources with an electric field designed to separate the positive and negative ions, and to focus the selected unipolar ions into the core region of the charger. The ions with the selected polarity is then attracted to the charging zone by an uniform electric field created by a series of ring electrodes applied with a linear ramped voltage. Aerosol entering the charger is sheathed with clean gas flow in order to keep the aerosol in the core region. A novel exit design with a reversed electric field is incorporated in order to minimize the charged particles loss. The performance of the charger is first evaluated using computer simulation and then constructed for experimental validation. Experiment data have demonstrated that the charger achieves 90% and 95% charged-particles penetration efficiency and with 22% and 48% extrinsic charging efficiency at 3 and 5 nm particle sizes, respectively. These performance data represent significant improvement, over a factor of 10, compared with the existing chargers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1010087311616
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  • 7
    Electronic Resource
    Electronic Resource
    Nanomaterials and Nanotechnology Websites (1999)
    Springer
    Journal of nanoparticle research 1 (1999), S. 317-317 
    Details
    ISSN: 1572-896X
    Keywords: nanomaterials ; nanotechnology ; nanoparticles ; World Wide Web
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics , Technology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1010073520628
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  • 8
    Electronic Resource
    Electronic Resource
    Measurements of Silica Aggregate Particle Growth Using Light Scattering and Thermophoretic Sampling in a Coflow Diffusion Flame (1999)
    Springer
    Journal of nanoparticle research 1 (1999), S. 169-183 
    Details
    ISSN: 1572-896X
    Keywords: coflow diffusion flame ; light scattering ; thermophoretic sampling ; flame hydrolysis deposition (FHD) ; fractal aggregate ; nanoparticles
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics , Technology
    Notes: Abstract The evolution of silica aggregate particles in a coflow diffusion flame has been studied experimentally using light scattering and thermophoretic sampling techniques. An attempt has been made to calculate the aggregate number density and volume fraction using the measurements of scattering cross section from 90° light scattering with combination of measuring the particle size and morphology from the localized sampling and a TEM image analysis. Aggregate or particle number densities and volume fractions were calculated using Rayleigh–Debye–Gans and Mie theory for fractal aggregates and spherical particles, respectively. Using this technique, the effects of H2 flow rates on the evolution of silica aggregate particles have been studied in a coflow diffusion flame burner. As the flow rate of H2 increases, the primary particle diameters of silica aggregates have been first decreased, but, further increase of H2 flow rate causes the diameter of primary particles to increase and for sufficiently larger flow rates, the fractal aggregates finally become spherical particles. For the cases of high flame temperatures, the particle sizes become larger and the number densities decrease by coagulation as the particles move up within the flame. For cases of low flame temperatures, the primary particle diameters of aggregates vary a little following the centerline of burner and for the case of the lowest flame temperature in the present experiments, the sizes of primary particles even decrease as particles move upward.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1010092113802
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  • 9
    Electronic Resource
    Electronic Resource
    Characteristics of Fe2O3 Nanoparticles from Doped Low-pressure H2/O2/Ar Flames (1999)
    Springer
    Journal of nanoparticle research 1 (1999), S. 163-167 
    Details
    ISSN: 1572-896X
    Keywords: Fe2O3 ; nanoparticles ; low-pressure flame ; Fe(CO)5 ; flame synthesis approach
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics , Technology
    Notes: Abstract A burner stabilized premixed low-pressure flame has been used to generate iron-oxide (Fe2O3) nanoparticles with sizes in the range 7–20 nm. The H2/O2/Ar flames were doped with different amounts of iron-pentacarbonyl (Fe(CO)5) with concentrations in the range 524–2096 ppm. The influence of precursor concentration on composition, structure, morphology, and size have been studied utilizing transmission electron microscopy (TEM), X-ray powder diffraction (XRD), measurements of the specific surface area (BET), and infrared spectroscopy (FT-IR). The product particles consist of both, the γ- and the α-phase of Fe2O3. Average particle sizes were measured in the range 7.4–16 nm depending on precursor concentration and flame conditions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1010056617437
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  • 10
    Electronic Resource
    Electronic Resource
    Preparation of Nanoparticles of Barium Ferrite from Precipitation in Microemulsions (1999)
    Springer
    Journal of nanoparticle research 1 (1999), S. 215-221 
    Details
    ISSN: 1572-896X
    Keywords: nanoparticles ; magnetic materials ; microemulsions ; barium ferrite
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics , Technology
    Notes: Abstract Magnetic nanoparticles of barium ferrite (BaFe12O19) have been synthesized using a microemulsion mediated process. The aqueous cores of water-in-oil microemulsions were used as constrained microreactors for the precipitation of precursor carbonate and hydroxide particles. These precursors were then calcined at 925°C for 12 h, during which time they were transformed to the hexagonal ferrite. The pH of reaction was varied between 5 and 12, and it was found that the fraction of non-magnetic hematite (α-Fe2O3) in the particles varied with the pH of reaction, thus affecting the magnetic properties of the particles. The same precursor particles were also prepared by bulk co-precipitation reaction for comparison. It was found that the microemulsion derived nanoparticles of barium ferrite had both higher intrinsic coercivity (Hc) and saturation magnetization (σs) than the particles derived from bulk co-precipitation. Particles were analyzed by electron microscopy, X-ray diffraction, differential thermal analysis (DTA), thermogravimetric analysis (TGA) and vibrating sample magnetometry (VSM). The best barium ferrite particles produced by the microemulsion synthesis method yielded an intrinsic coercivity of 4310 Oe and a saturation magnetization of 60.48 emu/g.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1010004616528
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  • 11
    Electronic Resource
    Electronic Resource
    Aerosol Catalysis on Nickel Nanoparticles (1999)
    Springer
    Journal of nanoparticle research 1 (1999), S. 253-265 
    Details
    ISSN: 1572-896X
    Keywords: nanoparticles ; surface state ; reactivity ; aerosol catalysis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics , Technology
    Notes: Abstract Nickel nanoparticles produced by spark discharges were used as aerosol catalyst for the formation of methane. The available surface area of the particles was determined using different methods. It was found that the surface area available for nitrogen adsorption and, therefore, for the methanation reaction remained virtually constant during restructuring of the agglomerates while the surface area based on the mobility was significantly reduced. In general, the reaction parameters such as activation energy and reaction rates agree well with the values for single nickel crystals and foils. At temperatures above 350°C the activation energy and the photoelectric activity of the particles decrease indicating the formation of graphite on the particle surface. Also the change of the work function points to the build up of multiple layers of graphite on the particle surface. The surprisingly low temperature for the surface deactivation may indicate an enhanced formation of carbon atoms at the surface.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1010016919254
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  • 12
    Electronic Resource
    Electronic Resource
    Formation of Silver and Gold Dendrimer Nanocomposites (1999)
    Springer
    Journal of nanoparticle research 1 (1999), S. 353-368 
    Details
    ISSN: 1572-896X
    Keywords: dendrimers ; nanoparticles ; polymer-inorganic nanocomposites ; templates
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics , Technology
    Notes: Abstract Structural types of dendrimer nanocomposites have been studied and the respective formation mechanisms have been described, with illustration of nanocomposites formed from poly(amidoamine) PAMAM dendrimers and zerovalent metals, such as gold and silver. Structure of {(Au(0))n−PAMAM} and {(Ag(0))n−PAMAM} gold and silver dendrimer nanocomposites was found to be the function of the dendrimer structure and surface groups as well as the formation mechanism and the chemistry involved. Three different types of single nanocomposite architectures have been identified, such as internal (‘I’), external (‘E’) and mixed (‘M’) type nanocomposites. Both the organic and inorganic phase could form nanosized pseudo-continuous phases while the other components are dispersed at the molecular or atomic level either in the interior or on the surface of the template/container. Single units of these nanocomposites may be used as building blocks in the synthesis of nanostructured materials.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1010060404024
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  • 13
    Electronic Resource
    Electronic Resource
    Strain Directed Assembly of Nanoparticle Arrays Within a Semiconductor (1999)
    Springer
    Journal of nanoparticle research 1 (1999), S. 329-347 
    Details
    ISSN: 1572-896X
    Keywords: nanoparticles ; self assembly ; nanotechnology ; precipitation ; elastic properties
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics , Technology
    Notes: Abstract The use of strain to direct the assembly of nanoparticle arrays in a semiconductor is investigated experimentally and theoretically. The process uses crystal strain produced by a surface structure and variations in layer composition to guide the formation of arsenic precipitates in a GaAs-based structure grown at low temperature by molecular beam epitaxy. Remarkable patterning effects, including the formation of single and double one-dimensional arrays with completely clear fields are achieved for particles in the 10-nm size regime at a depth of about 50-nm from the semiconductor surface. Experimental results on the time dependence of the strain patterning indicates that strain controls the late stage of the coarsening process, rather than the precipitate nucleation. Comparison of the observed particle distributions with theoretical calculations of the stress and strain distributions reveals that the precipitates form in regions of maximum strain energy, rather than near extremum points of hydrostatic stress or dilatation strain. It is therefore concluded that the patterning results from modulus differences between the particle and matrix materials rather than from other strain related effects. The results presented here should be useful for extending strain directed assembly to other materials systems and to other configurations of particles.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1010052731395
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  • 14
    Electronic Resource
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    Probing Single Nanometer-scale Particles with Scanning Tunneling Microscopy and Spectroscopies (1999)
    Springer
    Journal of nanoparticle research 1 (1999), S. 459-466 
    Details
    ISSN: 1572-896X
    Keywords: STM ; nanoparticles ; colloids ; Met-Cars ; photon emission ; spectroscopy
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics , Technology
    Notes: Abstract Scanning tunneling microscopy can be used to isolate single particles on surfaces for further study. Local optical and electronic properties coupled with topographic information collected by the scanning tunneling microscope (STM) give insight into the intrinsic properties of the species under study. Since each spectroscopic measurement is done on a single particle, each sample is ‘monodisperse’, regardless of the degree of heterogeneity of the original preparation. We illustrate this with three example systems – a metal cluster of known atomic structure, metal nanoparticles dispersed from colloid suspensions, and metallocarbohedrenes (Met-Cars) deposited with other reaction products. Au and Ag nanoparticles were imaged using a photon emission STM. The threshold voltage, the lowest bias voltage at which photons are produced, was determined for Au nanoparticles. Electronic spectra of small clusters of Ni atoms on MoS2 were recorded. Preliminary images of Zr-based Met-Car-containing soot were obtained on Au and MoS2 substrates and partial electronic spectra were recorded of these possible Met-Car particles.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1010048032304
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  • 15
    Electronic Resource
    Electronic Resource
    A New Probe for Mechanical Testing of Nanostructures in Soft Materials (1999)
    Springer
    Journal of nanoparticle research 1 (1999), S. 495-499 
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    ISSN: 1572-896X
    Keywords: optical tweezers ; nanoparticles ; particle-medium interaction ; dynamic mechanical testing ; dynamic moduli ; viscoelasticity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics , Technology
    Notes: Abstract We report a new application of the optical tweezers, where a harmonically driven oscillating tweezer is combined with the forward light scattering and lock-in amplification techniques, for probing the mechanics of nanostructures in soft materials in a broad frequency range. Model independent dynamic moduli G′ and G″ of the material at a localized, sub-micron area can be measured directly from the displacement and the phase shift of the particle in the oscillating trap. The probe particles can be as small as 200 nm and the displacement of the particle was in the range of a few nanometers. To illustrate the new methodology, we show the microscopic viscoelastic properties of a transient polymer network in the vicinity of a silica bead.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1010075223642
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  • 16
    Electronic Resource
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    Polymer-like Behavior of Inorganic Nanoparticle Chain Aggregates (1999)
    Springer
    Journal of nanoparticle research 1 (1999), S. 9-15 
    Details
    ISSN: 1572-896X
    Keywords: chain aggregates ; aerosol aggregates ; nanoparticles ; titania nanoparticles
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics , Technology
    Notes: Abstract Studies of the behavior of nanoparticle chain aggregates (NCA) have shown properties similar to those of molecular polymers. Like polymer chains, NCA tend to gather up and become more compact when heated. Under tensile stress, folded chain segments pull out and the NCA elongates. When the tension is relaxed, the chains contract. The stretching of NCA may contribute to the ductility of compacts made from nanoparticles, a subject of current research interest. In a well established technological application, carbon black and pyrogenic silica NCA produce remarkable increases in elastic modulus and tensile strength when added to commercial rubber. This may be due to the mechanical interaction between the polymer chains and NCA. However, basic mechanisms of NCA elasticity differ from those of molecular polymers. The alignment of chain segments when the NCA are subjected to tension probably results from rotation and translation at grain boundaries between neighboring nanocrystals. The elastic properties depend on the van der Waals forces between segments of the chain that fold to minimize surface free energy. Under tension, these segments pull out, but tend to reform when the tension is relaxed. The processes that lead to NCA formation and control the strength of interparticle bonds are briefly reviewed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1010017830037
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  • 17
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    From Discoveries to Novel Nanodevices (1999)
    Springer
    Journal of nanoparticle research 1 (1999), S. 137-139 
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    ISSN: 1572-896X
    Keywords: molecular self-assembly ; nanoelectronics ; nanoparticles ; biostructures
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics , Technology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1010053206861
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  • 18
    Electronic Resource
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    Coagulation of Charged Aerosols (1999)
    Springer
    Journal of nanoparticle research 1 (1999), S. 185-195 
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    ISSN: 1572-896X
    Keywords: charged aerosols ; coagulation ; charge distribution ; population balance ; nanoparticles
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics , Technology
    Notes: Abstract We consider the coagulation of an aerosol embedded in a stationary atmosphere of bipolar ions. Particles respond to the ionic environment by developing an instantaneous charge the fluctuations of which may produce attraction or repulsion between the particles. The governing parameter is the charge asymmetry factor which quantifies the relative charging efficiency of positive and negative ions. We use a Monte Carlo method to solve the coagulation equation in the free-molecule regime. We perform simulations for conditions ranging from symmetric and nearly symmetric environments (e.g. flames, ionizers), which result in particles that are on the average neutral to highly asymmetric conditions (low-pressure plasmas), which produce a substantial non-zero net charge. In symmetric ionic atmospheres we find that electrostatic interactions are unimportant and particles grow as if in the absence of charging ions. In asymmetric bipolar atmospheres, electrostatic interactions between particles are repulsive, the mean particle size grows logarithmically in time and the resulting size distributions are significantly narrower than the classical self preserving distributions.
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    URL: http://dx.doi.org/10.1023/A:1010044230640
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  • 19
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    Gold Nanoparticles: Production, Reshaping, and Thermal Charging (1999)
    Springer
    Journal of nanoparticle research 1 (1999), S. 243-251 
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    ISSN: 1572-896X
    Keywords: gold ; nanoparticles ; size selection ; thermal charging ; particle synthesis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics , Technology
    Notes: Abstract Gold nanoparticles are of great interest for various nanoelectronic applications, e.g., for making single electron transistors or very fine leads to molecular size entities. For this and other applications, it is important that all particles have controllable size and shape. In this paper, we describe the production of size-selected gold aerosol particles in the 20 nm range made by evaporation in a high-temperature tube furnace and subsequent size selection. To obtain spherical particles, it was necessary to reshape the particles at high temperature, which was investigated for temperatures between 25°C and 1200°C. High-resolution transmission electron microscopy showed that the degree of crystallinity became higher for higher reshaping temperature. During reshaping at high temperature, an anomalous charging behavior was discovered, whereby negatively as well as positively charged particles became multiply negatively charged. Possible mechanisms for explaining this thermally activated phenomenon are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1010012802415
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  • 20
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    Prediction of Spherule Size in Gas Phase Nanoparticle Synthesis (1999)
    Springer
    Journal of nanoparticle research 1 (1999), S. 277-291 
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    ISSN: 1572-896X
    Keywords: nanoparticles ; specific surface area ; surface melting ; coalescence ; surface diffusion ; flame particle synthesis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics , Technology
    Notes: Abstract Surprisingly, there is still no rational yet practical method to reliably predict absolute ‘primary’ nanospherule sizes and, hence, specific surface areas, in gas phase flame nanoparticle synthesis. The present paper summarizes our approach to this important problem, using a plausible and tractable coagulation–coalescence (two-rate process) model, but with an important modification to the rate of nanoparticle coalescence. The Smoluchowski equation is used to describe the particle Brownian coagulation rate process (free-molecule regime), together with the assumption that the particle population follows a self-preserving size distribution. The decisive coalescence process, driven by the minimization of surface energy of the coalescing nanoparticles, is presumed to occur via the mechanism of surface diffusion. However, a curvature-dependent energy barrier for surface-diffusion is proposed, taking into account the extended ‘surface-melting’ behavior of nanoparticles. This is shown here to have the effect of accelerating the coalescence rate of touching nanoparticles, leading to absolute sizes (at the predicted onset of aggregate formation) in encouraging agreement with available experiments. It was found that the coalescence rate, especially with a curvature-augmented surface diffusivity, is far more sensitive to particle size than is the Brownian coagulation rate. As a result, when cast in terms of characteristic process times, a distinct crossover generally exists, allowing the determination of observed ‘primary’ spherule sizes within larger aggregates. This approach is successfully applied here to several published synthesis examples of vapor-derived nanosized alumina and titania. Its broader implications for nanoparticle synthesis in non-isothermal reactors, including our own counterflow diffusion flame reactor, are also briefly summarized.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1010021004233
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  • 21
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    Synthesis of Magnetite Nanoparticles by Precipitation with Forced Mixing (1999)
    Springer
    Journal of nanoparticle research 1 (1999), S. 393-396 
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    ISSN: 1572-896X
    Keywords: magnetite ; nanoparticles ; forced mixing ; wet synthesis approach ; superparamagnetism
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics , Technology
    Notes: Abstract Synthesis of magnetite nanoparticles by precipitation with forced mixing is presented. Using this method it is easy to obtain a high product saturation degree and the constant pH value of the reaction system. The TEM and XRD measurments show that the average size of the product magnetite particles is less than 6.0-nm in the condition of [Fe2+]/[Fe3+]=0.5. The magnetic properties of the samples are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1010091625981
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    Aero-sol-gel Reactor for Nano-powder Synthesis (1999)
    Springer
    Journal of nanoparticle research 1 (1999), S. 379-392 
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    ISSN: 1572-896X
    Keywords: nanoparticles ; aero-sol-gel ; aerosol ; sol-gel ; hydrolysis/condensation ; silica
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics , Technology
    Notes: Abstract This paper discusses a new approach to the synthesis of nano-structured oxides where sol-gel reactions are carried out in aerosol droplets. This aero-sol-gel (ASG) reactor allows for manipulation of the structure, chemical composition and surface area of silica powders through variation of process parameters. ASG powders differ in nanostructure from other continuous process powders such as pyrolytic and solution-route powders. ASG powders contain mesopores (〉2-nm) and micropores (〈2-nm), the mesopores being responsible for high surface areas measured by nitrogen adsorption using BET theory. Primary particles of close to molecular scale are believed to lead to exceedingly large specific surface areas on the order of 600-m2/g. These primary particles aggregate into nanometer scale mass-fractal aggregates that cluster in micron scale agglomerates. Under controlled reaction conditions the powder structure is reproducible as measured by small-angle X-ray scattering, SAXS, analysis. The ASG reactor displays transport effects similar to those previously seen in laminar flame reactors as evidenced by the effect of reactor geometry and reactant concentration on product structure.
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    URL: http://dx.doi.org/10.1023/A:1010066622968
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    Role of Nanoparticles in Photocatalysis (1999)
    Springer
    Journal of nanoparticle research 1 (1999), S. 439-458 
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    ISSN: 1572-896X
    Keywords: nanoparticles ; quantum size ; photocatalysis ; dopants ; sensitization ; nanocrystalline films
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics , Technology
    Notes: Abstract The aim of this review paper is to give an overview of the development and implications of nanotechnology in photocatalysis. The topics covered include a detailed look at the unique properties of nanoparticles and their relation to photocatalytic properties. Current applications of and research into the use of nanoparticles as photocatalysts has also been reviewed. Also covered is the utilization of nanoparticles in doped, coupled, capped, sensitized and organic–inorganic nanocomposite semiconductor systems, with an effort to enhance photocatalytic and/or optical properties of commonly used semiconductor materials. The use of nanocrystalline thin films in electrochemically assisted photocatalytic processes has been included. Finally, the use of nanoparticles has made a significant contribution in providing definitive mechanistic information regarding the photocatalytic process.
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    URL: http://dx.doi.org/10.1023/A:1010044830871
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    A Simple Model for the Variation of Permeability due to Partial Saturation in Dual Scale Porous Media (1997)
    Springer
    Transport in porous media 27 (1997), S. 243-264 
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    ISSN: 1573-1634
    Keywords: permeability ; Darcy's law ; unsaturated flow ; dual scale ; resin transfer molding ; liquid injection molding.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract The main focus of this work is to model macroscopically the effects of partial saturation upon the permeability of dual scale fibrous media made of fiber bundles when a Newtonian viscous fluid impregnates it. A new phenomenological model is proposed to explain the discrepancies between experimental pressure results and analytical predictions based on Darcy's law. This model incorporates the essential features of relative permeability but without the necessity of measuring saturation of the liquid for its prediction. The model is very relevant for the small scale industrial systems where a liquid is forced to flow through a fibrous porous medium. It requires four parameters. Two of them are the two permeability values based on the two length scales. One length scale is of the order of magnitude of the individual fiber radius and corresponds to the permeability of the completely staurated medium, the other is of the order of magnitude of the distance between the fiber bundles and corresponds to the permeability of the partially saturated medium. The other two parameters are the lengths of the two partially saturated regions of the flow domain. The two lengths of the partially saturated region and the permeability of the fully saturated flow domain can be directly measured from the experiments. The excellent agreement between the model and the experimental results of inlet pressure profile with respect to time suggests that this model may be used to describe the variation of the permeability behind a moving front in such porous media for correct pressure prediction. It may also be used to characterize the fibrous medium by determining the two different permeabilities and the relative importance of the unsaturated portion of the flow domain for a given architecture.
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    URL: http://dx.doi.org/10.1023/A:1006544107324
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    Wavelet upscaling based on piecewise bilinear approximation of the permeability field (1996)
    Springer
    Transport in porous media 23 (1996), S. 125-134 
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    ISSN: 1573-1634
    Keywords: permeability ; upscaling ; flow prediction
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract A method for upscaling of permeability in heterogeneous porous media is presented. The upscaled field takes the form K = e Y , where Y, in two dimensions, is a piecewise bilinear function. The method is tested on a number of random permeability fields, with different integral scale/correlation length and variance. The numerical results show that this method conserves much more of the heterogeneous fingering than classical block-based upscaling methods, e.g., geometric mean.
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    URL: http://dx.doi.org/10.1007/BF00178122
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    Permeability evolution induced by the precipitation of an advected solute: Effect of the microscopic and the macroscopic scales competition on the clogging mechanism (1996)
    Springer
    Transport in porous media 24 (1996), S. 341-375 
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    ISSN: 1573-1634
    Keywords: precipitation ; porous medium ; clogging ; feedback mechanism ; permeability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract A model is proposed for coupling the one-dimensional transport of solute with surface precipitation kinetics which induces the clogging of an initially homogeneous porous medium. The aim is to focus the non-linear feedback effect between the transport and the chemical reaction through the permeability of the medium. A Lagrangian formulation, used to solve the coupled differential equations, gives semi-analytical expressions of the hydrodynamic quantities. A detailed analysis reveals that the competition between the microscopic and macroscopic scales controls the clogging mechanism, which differs depends on whether short or long times are considered. In order to illustrate this analysis, more quantitative results were obtained in the case of a second and zeroth order kinetic. It was necessary to circumvent the semi-analytic character of the solutions problem by successive approximation. A comparison with results obtained by simulations displays a good agreement during the most part of the clogging time.
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    URL: http://dx.doi.org/10.1007/BF00154097
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    The effect of anisotropic permeability on free convective boundary layer flow in porous media (1995)
    Springer
    Transport in porous media 19 (1995), S. 79-92 
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    ISSN: 1573-1634
    Keywords: Porous medium ; convection ; boundary layer ; anisotropy ; permeability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract The effect of an anisotropic permeability on thermal boundary layer flow in porous media is studied. The convective flow is induced by a vertical, uniformly heated surface embedded in a fluid-saturated medium. A leading-order boundary layer theory is presented. It is shown that the thickness of the resulting boundary layer flow is different from that obtained in an isotropic porous medium. In general, an anisotropic permeability induces a fluid drift in the spanwise direction, the strength of which depends on the precise nature of the anisotropy. Conditions are found which determine whether or not the boundary layer flow is three-dimensional.
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    URL: http://dx.doi.org/10.1007/BF00716050
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    Effective properties for flow calculations (1995)
    Springer
    Transport in porous media 20 (1995), S. 169-196 
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    ISSN: 1573-1634
    Keywords: effective flow properties ; reservoir geology ; permeability ; transmissibility
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract In this paper we discuss the background to the problems of finding effective flow properties when moving from a detailed representation of reservoir geology to a coarse gridded model required for reservoir performance simulation. In so doing we synthesize the pictures of permeability and transmissibility and show how they may be used to capture the effects of the boundary conditions on the upscaling. These same concepts are applied to the renormalization method of calculating permeability, to show its promise as an accurate, yet fast method.
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    URL: http://dx.doi.org/10.1007/BF00616929
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    Numerical and analytical study to estimate the effect of two length scales upon the permeability of a fibrous porous medium (1995)
    Springer
    Transport in porous media 21 (1995), S. 1-17 
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    ISSN: 1573-1634
    Keywords: Resin transfer molding ; permeability ; fibrous porous medium ; porous cylinders ; creeping flow ; length scales
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract A fibrous porous medium with two length scales is modeled as a bed of porous cylinders aligned perpendicular to the flow of viscous fluid. The flow behavior is described using Stokes and Darcy flow equations in the regions around (higher length scale) and within the cylinders (lower length scale) respectively. The typical ratio of higher and lower length-scale regions enable us to invoke lubrication approximation and simplify the equations to develop a closed form solution for the overall permeability of this dual-scale porous medium. A parametric analysis is performed to explore the dependence of permeability on factors such as the volumetric ratio of higher and lower length-scale regions, permeability and size of inclusions in the smaller length-scale region. The analytical model is compared with the numerical results and the trend is compared with the experiments.
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    URL: http://dx.doi.org/10.1007/BF00615332
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    A stochastic model for the permeability characteristics of saturated cemented porous media undergoing freezing (1996)
    Springer
    Transport in porous media 22 (1996), S. 345-357 
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    ISSN: 1573-1634
    Keywords: permeability ; stochastic model ; freezing ; porous cemented materials
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract As the temperature of a saturated porous medium drops, the water in the pores starts to freeze. Since the temperature at which the phase change takes place is dependent on the pore size, the permeability of the medium changes continuously. Simultaneously, due to the expansion of water on freezing, it is forced to migrate through the pore body thus inducing stresses in material matrix. The stresses developed and the consequent frost damage are therefore dependent on the change in the permeability characteristics of the medium on freezing. This paper deals with the numerical prediction of permeability characteristics of porous cemented media saturated with water undergoing progressive freezing. A bond percolation model is used to generate the pore structure according to an assumed poresize distribution. Permeability of the medium at various temperatures is computed by solving the network problem. The computed results are compared with other analytical and experimental results. The proposed model predicts a threshold temperature below which permeability drops to zero. This phenomenon is crucial in developing a deeper understanding of the mechanism of frost damage to cemented porous materials such as bricks, stone, concrete, etc.
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    URL: http://dx.doi.org/10.1007/BF00161631
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    A Simple Model of a Two-Layered High-Temperature Liquid-Dominated Geothermal Reservoir as a Part of a Large-Scale Hydrothermal Convection System (1998)
    Springer
    Transport in porous media 33 (1998), S. 3-27 
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    ISSN: 1573-1634
    Keywords: geothermal reservoir ; high temperature ; Kakkonda ; natural convection ; numerical modeling ; permeability ; super-critical fluid
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract The Kakkonda geothermal reservoir, Japan, is a typical high-temperature liquid-dominated geothermal reservoir, except for its distinctive two-layered temperature structure. It has a shallow permeable reservoir of 230–260°, and a deep less permeable reservoir of 350–360°. Geology and hydrology indicate that the shallow reservoir is one to two orders of magnitude more permeable than the deep reservoir, but that the two reservoirs communicate. It has been widely assumed in engineering and scientific circles that the connection between the two reservoirs is a zero or low permeability barrier to fluid flow. We show that this hypothesis is untenable, based on both physical evidence and numerical simulation. We numerically model the evolution of the geothermal system as it heats after emplacement of an intrusion. The two-layered temperature structure is found to be a consequence of the permeability difference, i.e. the two-layered permeability structure.
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    URL: http://dx.doi.org/10.1023/A:1006533307354
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    Macroscale Properties of Porous Media from a Network Model of Biofilm Processes (1998)
    Springer
    Transport in porous media 31 (1998), S. 39-66 
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    ISSN: 1573-1634
    Keywords: network model ; biofilm ; biobarrier ; permeability ; Monod kinetics ; adsorption
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract We demonstrate how a network model can predict porosity and permeability changes in a porous medium as a result of biofilm buildup in the pore spaces. A biofilm consists of bacteria and extracellular polymeric substances (EPS) bonded together and attached to a surface. In this case, the surface consists of the walls of the porous medium, which we model as a random network of pipes. Our model contains five species. Four of these are bacteria and EPS in both fluid and adsorbed phases. The fifth species is nutrient, which we assume to reside in the fluid phase only. Bacteria and EPS transfer between the adsorbed and fluid phases through adsorption and erosion or sloughing. The adsorbed species influence the effective radii of the pipes in the network, which affect the porosity and permeability. We develop a technique for integrating the coupled system of ordinary and partial differential equations that govern transport of these species in the network. We examine ensemble averages of simulations using different arrays of pipe radii having identical statistics. These averages show how different rate parameters in the biofilm transport processes affect the concentration and permeability profiles.
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    URL: http://dx.doi.org/10.1023/A:1006506104835
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    An equation of motion for an incompressible Newtonian fluid in a packed bed (1995)
    Springer
    Transport in porous media 18 (1995), S. 185-198 
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    ISSN: 1573-1634
    Keywords: Finite element ; permeability ; Navier-Stokes ; packed bed ; spherical particles
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract The application of a volume average Navier-Stokes equation for the prediction of pressure drop in packed beds consisting of uniform spherical particles is presented. The development of the bed permeability from an assumed porous microstructure model is given. The final model is quasi-empirical in nature, and is able to correlate a wide variety of literature data over a large Reynolds number range. In beds with wall effects present the model correlates experimental data with an error of less than 10%. Numerical solutions of the volume averaged equation are obtained using a penalty finite element method.
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    URL: http://dx.doi.org/10.1007/BF01064677
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    Network Model of Flow, Transport and Biofilm Effects in Porous Media (1998)
    Springer
    Transport in porous media 30 (1998), S. 1-23 
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    ISSN: 1573-1634
    Keywords: biofilm ; network model ; permeability ; transport ; numerical diffusion
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract In this paper, we develop a network model to determine porosity and permeability changes in a porous medium as a result of changes in the amount of biomass. The biomass is in the form of biofilms. Biofilms form when certain types of bacteria reproduce, bond to surfaces, and produce extracellular polymer (EPS) filaments that link together the bacteria. The pore spaces are modeled as a system of interconnected pipes in two and three dimensions. The radii of the pipes are given by a lognormal probability distribution. Volumetric flow rates through each of the pipes, and through the medium, are determined by solving a linear system of equations, with a symmetric and positive definite matrix. Transport through the medium is modeled by upwind, explicit finite difference approximations in the individual pipes. Methods for handling the boundary conditions between pipes and for visualizing the results of numerical simulations are developed. Increases in biomass, as a result of transport and reaction, decrease the pipe radii, which decreases the permeability of the medium. Relationships between biomass accumulation and permeability and porosity reduction are presented.
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    URL: http://dx.doi.org/10.1023/A:1006560705680
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    Effects of serial and parallel pore nonuniformities: Results from two models of the porous structure (1996)
    Springer
    Transport in porous media 25 (1996), S. 335-350 
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    ISSN: 1573-1634
    Keywords: Cauchy-Schwarz-Bunjakovskij inequality ; computational fluid dynamics ; effective diffusivity ; permeability ; pore-size distribution ; specific surface area
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract The effects of parallel-type and serial-type pore nonuniformities on the effective diffusivity and the permeability of a porous material were evaluated, constant porosity and constant specific surface area being assumed. Two structural models were considered. In the first model, the porous structure was described as a bundle of cylindrical capillaries penetrating the whole thickness of the material and in the other it was described instead as a collection of randomly distributed obstacles hindering transport. Both models predicted that parallel-type pore nonuniformities produce an increase in permeability compared with uniform structures having the same porosity and specific surface area. Both models also predicted that the increase in permeability due to parallel-type pore nonuniformities would be larger than the increase in effective diffusivity. Regarding serial-type pore nonuniformities, both models predicted a decrease in permeability and that this decrease would be greater than the decrease in effective diffusivity. The predicted changes in effective diffusivity due to nonuniformities of the sample differed for the two structural models.
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    URL: http://dx.doi.org/10.1007/BF00140987
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    Preferential Flow-Paths Detection for Heterogeneous Reservoirs Using a New Renormalization Technique (1997)
    Springer
    Transport in porous media 26 (1997), S. 1-23 
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    ISSN: 1573-1634
    Keywords: upscaling ; renormalization ; permeability ; local flux ; heterogeneity.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract We have devised a renormalization scheme which allows very fast determination of preferential flow-paths and of up-scaled permeabilities of 2D heterogeneous porous media. In the case of 2D log-normal and isotropically distributed permeability-fields, the resulting equivalent permeabilities are very close to the geometric mean, which is in good agreement with a rigorous result of Matheron. It is also found to work well for geostatistically anisotropic media when comparing the resulting equivalent permeabilities with a direct solution of the finite-difference equations. The method works exactly as King's does, although the renormalization scheme was modified to obtain tensorial equivalent permeabilities using periodic boundary conditions for the pressure gradient. To obtain an estimation of the local fluxes, the basic idea is that if at each renormalization iteration all the intermediate renormalized permeabilities are stored in memory, we are able to compute -- ad reversum -- an approximation of the small-scale flux map under a given macroscopic pressure gradient. The method is very rapid as it involves a number of calculations that vary linearly with the number of elementary grid blocks. In this sense, the renormalization algorithm can be viewed as a rapid approximate pressure solver. The ‘exact’ reference flow-rate map (for the finite-difference algorithm) was computed using a classical linear system inversion. It can be shown that the preferential flow paths are well detected by the approximate method, although errors may occur in the local flow direction.
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    URL: http://dx.doi.org/10.1023/A:1006515616347
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    Infiltration of Liquid Metals in Porous Compacts: Modeling of Permeabilities During Reactive Melt Infiltration (1999)
    Springer
    Transport in porous media 36 (1999), S. 43-68 
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    ISSN: 1573-1634
    Keywords: reactive melt infiltration ; ceramic ; composites ; porous compacts ; permeability ; modeling.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract Reactive infiltration is a fast and cost-effective technique for manufacturing ceramic-matrix composites (CMCs). CMCs are used in elevated temperature applications like rocket engine casings, jet nozzles, gas turbine blades and nuclear cladding. There is an urgent need for minimizing experimental costs as well as optimizing process parameters during manufacture, so that we have minimized manufacturing costs and reduced infiltration times. Towards this end, the objective of this research was to develop an integrated micro-macro model of reactive flow of molten silicon in a porous preform consisting of carbon-coated silicon carbide fibers and then optimize process parameters computationally. The overall objective of the research was to arrive at a modified equation of Darcy's law for flow through a porous medium with the help of numerical/computational modeling. This paper deals with the flow of silicon through porous carbon at the macro level. The macro flow of silicon was integrated with an available micro model by determining the transient porosity from the micro model and using it in Darcy's law written for the macro flow of silicon. From the results of this study, we recommend suitable process parameters such as initial temperature of the solid reactant and the specific kind of reactants to be used for achieving complete infiltration. These conclusions are drawn after observation of the rate of decrease of permeability with more reaction.
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    URL: http://dx.doi.org/10.1023/A:1006574912165
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    Validation of the Slow Compressional Wave in Porous Media: Comparison of Experiments and Numerical Simulations (1999)
    Springer
    Transport in porous media 36 (1999), S. 149-160 
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    ISSN: 1573-1634
    Keywords: poroelasticity ; Biot's theory ; slow wave ; permeability ; acoustics.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract We perform numerical simulation of ultrasonic experiments on poroelastic samples, in which Biot's slow compressional wave had been observed. The simulation is performed using OASES modeling code, which allows to compute elastic wave fields in layered poroelastic media. Modeled were the experiments of Plona (1980), Rasolofosaon (1988), and our own measurements. In all the three situations, a good agreement between experiment and simulations has been observed. This further confirms the fact that Biot's theory of poroelasticity, on which the simulations were based, adequately describes the behavior of the porous materials under investigations at ultrasonic frequencies.
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    URL: http://dx.doi.org/10.1023/A:1006676801197
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