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Thermal Diffusion (thermal + diffusion)
Selected AbstractsImpact of Thermal Diffusion on Densification During SPSJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 2009Eugene A. Olevsky Spark-plasma sintering (SPS) has the potential for rapid (with heating rates reaching several hundred K/min) and efficient consolidation of a broad spectrum of powder materials. Possible mechanisms of the enhancement of consolidation in SPS versus conventional techniques of powder processing are categorized with respect to their thermal and athermal nature. This paper analyzes the influence of thermal diffusion, which is an SPS consolidation enhancement factor of a thermal nature. The Ludwig,Soret effect of thermal diffusion causes concentration gradients in two-component systems subjected to a temperature gradient. The thermal diffusion-based constitutive mechanism of sintering results from the additional driving force instigated by spatial temperature gradients, which cause vacancy diffusion. This mechanism is a commonly omitted addition to the free-surface curvature-driven diffusion considered in conventional sintering theories. The interplay of three mechanisms of material transport during SPS is considered: surface tension- and external stress-driven grain-boundary diffusion, surface tension- and external stress-driven power-law creep, and temperature gradient-driven thermal diffusion. It is shown that the effect of thermal diffusion can be significant for ceramic powder systems. Besides SPS, the results obtained are applicable to the ample range of powder consolidation techniques, which involve high local temperature gradients. The case study conducted on the alumina powder SPS demonstrates the correlation between the modeling and experimental data. It is noted that this study considers only one of many possible mechanisms of the consolidation enhancement during SPS. Further efforts on the modeling of field-assisted powder processing are necessary. [source] Diffusion of Adhesion Layer Metals Controls Nanoscale Memristive SwitchingADVANCED MATERIALS, Issue 36 2010J. Joshua Yang Thermal diffusion of Ti through Pt electrode forms Ti atom channels of 1 nm diameter along Pt grain boundaries, seeding switching centers and controlling nanoscale memristive switching. The image shows EFTEM maps of Ti overlaid on HRTEM images for a Si/SiO2 100 nm/Ti 5nm/Pt 15 nm sample in-situ annealed in ultrahigh vacuum at 250 °C for 1 hour. [source] Effect of Aluminum Doping on Microwave Permittivity of Silicon Carbide PowdersJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2008Fa Luo The permittivity and structure of commercial SiC powders before and after thermal diffusion of aluminum were explored in this study. The X-ray diffraction analysis and permittivity results show that heat treatment at 2000°C for 1 h has no influence not only on the structure of the SiC powders but also on the permittivity. Thermal diffusion of aluminum at 1800°, 1900°, and 2000°C was carried out for 1 h at 5 Pa in Ar atmospheres, respectively. A graphite die with two chambers was used during the experiment to separate the aluminum powders from the SiC powders. Aluminum vapor at high temperatures passes to the upper chamber through holes and diffuses into the SiC powders. The permittivities of the as-received SiC powders at 8.2,12.4 GHz were measured, and the results indicate that the real and imaginary parts of permittivity are improved much more than those of the original SiC powder. Both the real and the imaginary parts of the doped SiC increase with the diffusion temperatures, which could be attributed to defects from thermal diffusion of aluminum. [source] Loss of Chaotic Trabecular Structure in OPG-Deficient Juvenile Paget's Disease Patients Indicates a Chaogenic Role for OPG in Nonlinear Pattern Formation of Trabecular BoneJOURNAL OF BONE AND MINERAL RESEARCH, Issue 5 2004Phil Salmon PhD Abstract The RANK-RANKL-OPG system of osteoclast regulation may play a key role in determining chaotic structure in trabecular bone. Iliac trabecular bone from juvenile Paget's disease patients deficient in functional OPG shows parallel, anisotropic structure instead of normal chaotic structure. Evidence from experimental systems suggests that RANK-RANKL-OPG controls key nonlinear "chaogenic" parameters, such as friction, forcing frequency, feedback, and boundary forcing. The RANK-RANKL-osteoprotegerin (OPG) system of osteoclast regulation may play a key role in determining chaotic structure in trabecular bone. Iliac trabecular bone from juvenile Paget's disease (JPD) patients deficient in functional OPG shows parallel, anisotropic structure instead of normal chaotic structure. Evidence from experimental systems suggests that RANK-RANKL-OPG controls key nonlinear "chaogenic" parameters, such as friction, forcing frequency, feedback, and boundary forcing. The Belousov-Zhabotinsky reaction-diffusion system, the catalytic oxidation of CO on platinum surfaces, and thermal diffusion in liquid helium allow visualization of nonlinear emergent patterns such as labyrinthine structures, turbulence, and cellular structures, all of which bear some resemblance to trabecular bone. In JPD, the gene for OPG (TNFRSF11B) is subject to an inactivating mutation, leading to increased resorption and accelerated remodeling. Histomorphometric images of iliac crest trabecular bone from teenagers suffering from JPD show a highly unusual array of parallel, regular trabecular plates, instead of the typical chaotic, fractal patterns of normal trabecular bone. Loss of OPG function is associated with a change from chaotic to regular structure, suggesting that the RANK-RANKL-OPG system is controlling key nonlinear "chaogenic" parameters. Looking at trabecular bone from the perspective of nonlinear pattern formation may help understand other phenomena, such as the marked dependence of trabecular bone's architectural and mechanical quality on remodeling rate independent of the trabecular bone mass. [source] Impact of Thermal Diffusion on Densification During SPSJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 2009Eugene A. Olevsky Spark-plasma sintering (SPS) has the potential for rapid (with heating rates reaching several hundred K/min) and efficient consolidation of a broad spectrum of powder materials. Possible mechanisms of the enhancement of consolidation in SPS versus conventional techniques of powder processing are categorized with respect to their thermal and athermal nature. This paper analyzes the influence of thermal diffusion, which is an SPS consolidation enhancement factor of a thermal nature. The Ludwig,Soret effect of thermal diffusion causes concentration gradients in two-component systems subjected to a temperature gradient. The thermal diffusion-based constitutive mechanism of sintering results from the additional driving force instigated by spatial temperature gradients, which cause vacancy diffusion. This mechanism is a commonly omitted addition to the free-surface curvature-driven diffusion considered in conventional sintering theories. The interplay of three mechanisms of material transport during SPS is considered: surface tension- and external stress-driven grain-boundary diffusion, surface tension- and external stress-driven power-law creep, and temperature gradient-driven thermal diffusion. It is shown that the effect of thermal diffusion can be significant for ceramic powder systems. Besides SPS, the results obtained are applicable to the ample range of powder consolidation techniques, which involve high local temperature gradients. The case study conducted on the alumina powder SPS demonstrates the correlation between the modeling and experimental data. It is noted that this study considers only one of many possible mechanisms of the consolidation enhancement during SPS. Further efforts on the modeling of field-assisted powder processing are necessary. [source] Effect of Aluminum Doping on Microwave Permittivity of Silicon Carbide PowdersJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2008Fa Luo The permittivity and structure of commercial SiC powders before and after thermal diffusion of aluminum were explored in this study. The X-ray diffraction analysis and permittivity results show that heat treatment at 2000°C for 1 h has no influence not only on the structure of the SiC powders but also on the permittivity. Thermal diffusion of aluminum at 1800°, 1900°, and 2000°C was carried out for 1 h at 5 Pa in Ar atmospheres, respectively. A graphite die with two chambers was used during the experiment to separate the aluminum powders from the SiC powders. Aluminum vapor at high temperatures passes to the upper chamber through holes and diffuses into the SiC powders. The permittivities of the as-received SiC powders at 8.2,12.4 GHz were measured, and the results indicate that the real and imaginary parts of permittivity are improved much more than those of the original SiC powder. Both the real and the imaginary parts of the doped SiC increase with the diffusion temperatures, which could be attributed to defects from thermal diffusion of aluminum. [source] The Ludwig-Soret Effect on the Thermally Induced Phase Separation Process in Polymer Solutions: A Computational StudyMACROMOLECULAR THEORY AND SIMULATIONS, Issue 2 2009Sureshkumar B. Kukadiya Abstract The Ludwig-Soret effect was investigated in the thermally induced phase separation process via SD in polymer solutions under an externally imposed spatial linear temperature gradient using mathematical modeling and computer simulation. The mathematical model incorporated non-linear Cahn-Hilliard theory for SD, Flory-Huggins theory for thermodynamics, and the Ludwig-Soret effect for thermal diffusion. 2D simulation results revealed that the Ludwig-Soret effect had negligible impact on the phase separation mechanism in binary polymer solutions under a non-uniform temperature field, as reflected by the time evolution of the dimensionless structure factor and the transition time from the early to the intermediate stages of SD. [source] Full evolution of low-mass white dwarfs with helium and oxygen coresMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2007J. A. Panei ABSTRACT We study the full evolution of low-mass white dwarfs with helium and oxygen cores. We revisit the age dichotomy observed in many white dwarf companions to millisecond pulsar on the basis of white dwarf configurations derived from binary evolution computations. We evolve 11 dwarf sequences for helium cores with final masses of 0.1604, 0.1869, 0.2026, 0.2495, 0.3056, 0.3333, 0.3515, 0.3844, 0.3986, 0.4160 and 0.4481 M,. In addition, we compute the evolution of five sequences for oxygen cores with final masses of 0.3515, 0.3844, 0.3986, 0.4160 and 0.4481 M,. A metallicity of Z= 0.02 is assumed. Gravitational settling, chemical and thermal diffusion are accounted for during the white dwarf regime. Our study reinforces the result that diffusion processes are a key ingredient in explaining the observed age and envelope dichotomy in low-mass helium-core white dwarfs, a conclusion we arrived at earlier on the basis of a simplified treatment for the binary evolution of progenitor stars. We determine the mass threshold where the age dichotomy occurs. For the oxygen white dwarf sequences, we report the occurrence of diffusion-induced, hydrogen-shell flashes, which, as in the case of their helium counterparts, strongly influence the late stages of white dwarf cooling. Finally, we present our results as a set of white dwarf mass,radius relations for helium and oxygen cores. [source] Magnetotunneling into Fock,Darwin-like quantum dot states: Lateral matrix elements and the role of selection rulesPHYSICA STATUS SOLIDI - RAPID RESEARCH LETTERS, Issue 7 2010Gerold Kießlich Abstract We study theoretically the magnetotunneling transport through quantum dots formed by thermal diffusion of charged manganese interstitials in the vicinity of a GaAs quantum well [Phys. Rev. Lett. 101, 226807 (2008)]. In particular, we examine the lateral matrix elements between Landau subbands in the contact and Fock,Darwin-like states of an individual dot at high magnetic fields. We explicitly demonstrate the effect of spatial deformation of the dot on the wave function's overlap. The comparison with measured data suggests a selection rule similar to angular momentum conservation for tunneling into perfect Fock,Darwin states. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Polycrystalline Si films with unique microstructures formed from amorphous Si films by non-thermal equilibrium flash lamp annealingPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3-4 2010Keisuke Ohdaira Abstract Flash lamp annealing (FLA), with millisecond-order duration, can crystallize amorphous silicon (a-Si) films a few ,m thick on glass substrates, resulting in formation of polycrystalline Si (poly-Si) films with unprecedented periodic microstructures. The characteristic microstructure, formed spontaneously during crystallization, consists of large-grain regions, containing relatively large grains more than 100 nm in size, and fine-grain regions, including only 10-nm-sized fine grains. The microstructures results from explosive crystallization (EC), driven by heat generation corresponding to the difference of the enthalpies of meta-stable a-Si and stable crystalline Si (c-Si) states, which realizes lateral crystallization velocity on the order of m/s. The lateral crystallization may stop when the temperature of a-Si in the vicinity of c-Si, which is decided by both homogeneous heating from flash irradiation and thermal diffusion from c-Si, falls below a crystallization temperature. This idea is supported by the experimental fact that a lateral crystallization length decreases with decreasing pulse duration. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Soret Diffusion and Non-Ideal Dufour Conduction in Macroporous Catalysts with Exothermic Chemical Reaction at Large Intrapellet Damköhler NumbersTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 3 2007Laurence A. Belfiore Abstract The adiabatic temperature rise in catalytic pellets is predicted from a modified version of the Prater equation. Onsager reciprocal relations for coupled heat and mass transfer are violated in an analysis of thermal diffusion in macroporous catalysts with exothermic chemical reaction when Dufour conduction (i.e., the diffusion-thermo effect) is neglected. In this contribution, Dufour conduction is analyzed for both ideal and non-ideal pseudo-binary gas mixtures that simulate the production of methanol from carbon monoxide and hydrogen. In the diffusion-controlled regime at large intrapellet Damköhler numbers where intermolecular collisions provide the dominant resistance to mass transfer within the catalytic pores, temperatures in the catalytic core could be much greater than predictions based on the original Prater equation when the Prater number exceeds 0.30. The molecular flux of thermal energy includes Fourier's law, the interdiffusional flux, and Dufour conduction. Diffusional mass flux includes Fick's law and the Soret effect. All physicochemical properties of the reactive gas mixture exhibit temperature dependence. There is essentially no difference between maximum intrapellet temperature predictions that include or neglect ideal Dufour conduction when external catalytic surface temperatures range from 300-400 K and thermal diffusion enhances the flux of "smaller" reactants toward the centre of the catalyst. For "large-molecule reactants" that participate in exothermic reactions, thermal diffusion opposes Fick's law and Dufour conduction opposes Fourier's law. Under these conditions, it is demonstrated that core temperatures are overestimated by neglecting both off-diagonal coupling mechanisms (i.e., Soret diffusion and Dufour conduction). Prater numbers greater than unity and unrealistically high gas pressures are required to distinguish between maximum intrapellet temperatures for ideal and real gas simulations, where the latter consider two-body interactions for Lennard-Jones molecules in the virial equation of state. On prédit l'augmentation de la température adiabatique dans les pastilles catalytiques à partir d'une version modifiée de l'équation de Prater. Les relations réciproques d'Onsager pour le transfert de chaleur et de matière couplé sont violées dans une analyse de la diffusion thermique dans les catalyseurs macroporeux avec réaction chimique exothermique lorsque la conduction de Dufour (p.ex., l'effet de thermo-diffusion) est négligée. Dans cet article, on analyse la conduction de Dufour pour des mélanges de gaz pseudo-binaires idéaux et non idéaux qui simulent la production de méthanol à partir d'oxyde de carbone et d'hydrogène. Dans le régime à diffusion contrôlée à grand nombre de Damköhler entre les pastilles pour lesquels les collisions entre les molécules fournit la résistance dominante au transfert de matière à l'intérieur des pores catalytiques, les températures dans le noyau catalytique pourraient être bien plus grandes que les prédictions basées sur l'équation de Prater originale lorsque le nombre de Prater excède 0,30. Le flux moléculaire de l'énergie thermique inclut la loi de Fourier, le flux interdiffusionnel, et la conduction de Dufour. Le flux massique diffusionnel inclut la loi de Fick et l'effet Soret. Toutes les propriétés physicochimiques du mélange de gaz réactif montre une dépendance thermique. Il n'y a essentiellement pas de différence entre les prédictions des températures maximales entre les pastilles qui incluent ou négligent la conduction de Dufour idéale quand les températures de surface catalytiques externes sont comprises entre 300 et 400 K; la diffusion thermique améliore le flux des réactifs «plus petits» vers le centre du catalyseur. Pour les «réactifs composés de grandes molécules» qui participent aux réactions exothermiques, la diffusion thermique s'oppose à la loi de Fick et la conduction de Dufour à la loi de Fourier. Dans ces conditions, il est démontré que les températures de noyau sont surestimées en négligeant les deux mécanismes de couplage hors-diagonales (c.à-d. la diffusion de Soret et la conduction de Dufour). Des nombres de Prater plus grands que l'unité et des pressions de gaz élevées peu réalistes sont nécessaires pour distinguer les températures maximales entre les pastilles entre les simulations de gaz idéales et réelles, en considérant pour ces dernières les interactions à deux corps pour les molécules de Lennard-Jones dans l'équation d'état du viriel. [source] Surface Electromigration Patterns in a Confined Adsorbed Metal Film: Ga on GaNCHEMPHYSCHEM, Issue 12 2002Alexei Barinov Dr. Abstract The mass transport of gallium adatoms in a confined gallium bilayer on GaN(0001) is studied with photoelectron spectromicroscopy with the goal to identify the diffusing species and their lateral distribution during directional surface electromigration and/or "random" thermal diffusion. It has been found that only the gallium atoms from the second layer undergo biased diffusion involving formation of three-dimensional islands. The development of different gallium concentration patterns is described by means of a general model, considering the presence of vacancies and trapping centres for the diffusing atoms. [source] | ||||||||||||||||