Time Evolution of Demixing in Oxides under an Oxygen Potential Gradient

Time evolution of concentration profiles leading to steady-state demixing in oxides under oxygen partial pressure gradient is treated based on a microscopic model by applying the path probability method of irreversible statistical mechanics. The redistribution of vacancies and cations toward the steady-state distribution has essentially been reduced to the diffusion of vacancies and the interdiffusion of cations along the specimen. Atomistic features of the problem, such as a vast difference in the relaxation times for the redistribution of vacancies and cations, are emphasized. Time required for the system to reach the steady state is then estimated in terms of fundamental material properties.     

Demixing of Materials under Chemical Potential Gradients

A steady-state demixing of initially homogeneous solid solution which occurs in binary oxides under oxygen partial pressure gradient is treated by the path probability method of irreversible statistical mechanics from an atomistic point of view. The larger the difference between the diffusion coefficients of constituent species is, the larger is the degree of demixing. Essential features of demixing by diffusion, however, do not depend on whether the driving force is the chemical potential gradient, the electric field, or others.     

Mass Transport and Chemical Demixing of Y-Ba-Cu-O Compounds During Sintering in Temperature Gradients

A thin cylindrical powder compact (3-mm diameter by 20-mm) with a nominal composition of YBa₂Cu₃O_x was sintered in a static temperature gradient of 25°C/mm in air for 12 h. The temperature at the hot end of the sample was maintained at 940°±5°C. Considerable mass transport and chemical demixing occurred along the length of the sample. The results are consistent with the formation of a liquid phase at the hot end of the sample, transport of the liquid down the temperature gradient, and the formation of different eutectic compositions along the length of the sample. The implications for processing high-T_c superconductors in the system Y-Ba-Cu-O are discussed.     

Elasticity and Anelasticity of Uranium Oxides at Room Temperature: II, Hyperstoichiometric Oxides

The Young's modulus and internal friction of hyperstoichiometric uranium oxide at room temperature were determined by a sonic technique. Young's modulus decreased, whereas internal friction increased, with increasing O/U ratios. These effects are attributed to the precipitation of U₄O₉. A complete microstructural characterization is needed.     

Transport of Impurities out of a Two-Phase Crystal Layer into a Melt under the Effect of a Temperature Gradient: Mechanisms and Kinetics

Possible mechanisms and kinetics of removal of eutectic-forming impurities from a two-phase crystal layers with a transverse temperature gradient are considered. Mathematical models are suggested for various mechanisms of impurity transport out of a crystal layer into the surrounding melt. Equations for estimating the purification rate of layers with open and closed pores are derived and analyzed. These equations are intercompared and are checked against the results of numerical solution. It is shown that, in the case of the migration of closed inclusions, the purification rate is higher than in the case of the diffusion of impurities out of open pores.     

勾形磁场下重力水平和温度梯度对CdZnTe分离结晶生长的影响

为了了解勾形磁场下相关参数对CdZnTe晶体生长的影响,利用有限元法对坩埚内的热量和动量传递过程进行了全局数值模拟。分析了不同的磁场强度下重力水平、壁面温度梯度对CdZnTe晶体生长过程的影响。结果表明：重力水平存在1个临界值,此时CdZnTe熔体内最大流函数最小,流动最弱。随着温度梯度逐渐增大,熔体内最大流函数也逐渐增大,熔体的流动越来越强,不利于晶体稳定生长。通过施加勾形磁场,能有效抑制熔体内的流动,有利于晶体的稳定生长,为地面条件下制备大尺寸CdZnTe晶体创造了条件。     

Stationary Spatial Temperature Gradient in Gas Chromatography

Abstract

Imposing an increasing stationary temperature gradient along the length of a gas chromatographic column is proposed as a technique for improving separation. Side outlet ports with control valves would allow the process to have the same advantages of programmed temperature gas chromatography without the disadvantages of temperature transients. For a simple model of gas chromatography the analysis provides expressions for temporal moments at any point along the column as a function of the temperature gradient. Reduced retention times, sharpening of peaks, and higher symmetry are predicted due to increasing the temperature gradient. The relationship to chromathermography is discussed.     

Migration of Small Pores in Potassium Chloride Due to a Temperature Gradient

Pore migration in KCl crystals subjected to a temperature gradient occurs by a vaporization-condensation mechanism. The behavior of small (<100 μm) pores was observed using hot-stage microscopy. A size-dependent probability of pore motion was noted in which the fraction of mobile pores in a given size range steadily increased with size. Pores larger than =50 μm were always mobile. These observations are explained in terms of the probabilities of pore-dislocation intersections. Screw-type dislocations are found to be essential for providing molecular height surface ledges needed for vaporization and pore motion. The observation that some small pores moved and then stopped is explained by the fact that the vaporization rate becomes very low when the pore-dislocation intersection occurs at the edge of a faceted pore.     

Surface Tension of Some Liquid Oxides and Their Temperature Coefficients

Surface-tension data are reported for liquid Al₂ O₃, B₂ O₃, GeO₂, P₂ O₅, and SiO₂. Abnormal positive temperature coefficients for B₂ O₃, GeO₂, and SiO₂ are shown to be due mainly to changes in the liquid structure (dissociation) with temperature.     

Migration of Large Gas-Filled Pores in Potassium Chloride Due to a Temperature Gradient

The migration of gas-filled pores in KCl subjected to a temperature gradient occurs by a vaporization-condensation mechanism. In many cases the vaporization step, rather than gas-phase diffusion, can limit the rate of migration. Movement of large (>100 μ m ), argon-filled pores in pure and Ba-doped KCl was observed using hot-stage microscopy. Velocities of large pores in pure KCl were limited by the diffusion rates of KCl monomers and dimers in argon. This observation is consistent with a model in which the vaporization rate is moderately reduced due to widely spaced surface ledges, but is still rapid with respect to gas-phase diffusion. The velocities of large pores in 10 and 40 ppm Ba-doped samples were significantly lower than those in pure samples. A model was developed which explains these decreased migration rates in terms of impurity poisoning of surface kink sites. The model couples detailed expressions for vaporization kinetics with those for gas-phase diffusion rates.     

铁钙混合氧化物脱硫剂的硫化与再生过程研究

煤炭是世界上最丰富的化石燃料,有效提高煤炭利用效率和控制环境污染是目前面临的重要研究课题。先进的整体煤气联合循环发电（IGCC）和燃料电池（MCFC）技术被认为是煤炭利用的最佳途径。高温脱除燃料 气中的H2S是其中的关键技术,高温脱硫目前存在的问题是脱硫剂使用过程的粉化。本文研制出铁钙混合氧化物脱硫剂,采用固定床反应器,考察了不同配比铁混合氧化物的硫化及再生性能。结果表明,当Fe2O3与CaO等摩尔比时,脱硫剂有良好的硫化和再生性能。用H2O（g) O2进行再生时,先后有H2S和SO2逸出,并伴有单质硫形成。该脱硫剂连续硫化－再生循环稳定,且硫容逐渐增加,脱硫效率和机械强度亦逐步提高。     

Influence of Coherency Stress on Kinetic Demixing

The influence of coherency stress, arising from differences in the partial molar volumes of the cations or an imposed displacement boundary condition, on composition profiles during kinetic demixing in ternary oxides subjected to a gradient in oxygen chemical potential is investigated. Coherency stresses can either enhance or diminish the magnitude of the kinetic demixing and shifts in interfacial composition owing to coherency stress on the order of several atomic percent, measured with respect to the case where stress is neglected, can be expected. For (Co, Mg)O subjected to displacement boundary conditions, steady-state and time-dependent demixing profiles are calculated and compared with experimental measurements. Although the composition profiles in the demixed solids are modified because of coherency stress, the main features of kinetic demixing remain unchanged.     

添加剂对负压-温度梯度镀镍层结构与性能的影响

研究了十二烷基硫酸钠对负压-温度梯度镀镍层结构与性能的影响。结果表明:向镀液中加入适量的十二烷基硫酸钠后,所得镀层的平均晶粒尺寸由7μm减小至5μm,组织结构更加致密;显微硬度明显提高且不受阴极电流密度的影响;在质量分数为10%的HCl溶液和质量分数为10%的H2SO4溶液中的腐蚀率分别降低了7.7%和5.7%。     

Temperature Gradient Method for Determining Firing Range of Ceramic Bodies

An improved version of Steger's temperature gradient method of studying the firing behavior of ceramic bodies, clays, and glazes is given. The test specimen (8 in. long) is subjected, from one end to the other, to a temperature gradient of several hundred degrees. Such properties as shrinkage, hardness, bloating, crazing, and color are observed for the entire range of temperature. Furnace design and method of plotting results are given.     

浮法玻璃退火温度梯度控制与生产实践

通过应力控制过程,分析纵向应力效应,结合生产实践阐述了退火窑的纵向温度控制的重要性。     

氧化物、复合氧化物的制备

氧化物、复合氧化物作为无机功能陶瓷材料已引起世人注目。系统地介绍了固相法、气相法、共沉淀法、冷冻干燥法、溶胶－凝胶法、水溶液电解法、高压法、水热法等合成氧化物、复合氧化物的方法及其优缺点。     

稳态下玻璃电熔炉中锡电极的温度分布

根据STANNEX电极的热导率曲线,以平壁导热为模型,建立了锡电极中心轴向温度分布的简化数学模型,根据锡电极使用的实际情况建立实验炉,测试了锡电极在稳态下的边界温度以及锡电极中心在稳态下的实际温度分布,根据实验测得的边界条件对锡电极的温度分布作推导,得出了锡电极的轴向温度分布,对电极导电的牵引(如采用哪种联结材料,选择合适的连接方式)具有指导意义,同时对玻璃电熔炉的散热与功率配置以及电极使用过程中的冷却提供了参考,根据实际温度情况采取相应的冷却措施,从而可以延长电极的使用寿命.     

Demixing of Aqueous Polymer Two-Phase Systems in Low Gravity

Abstract

When polymers such as dextran and poly(ethy1ene glycol) are mixed in aqueous solution, biphasic systems often form. On Earth the emulsion formed by mixing the phases rapidly demixes because of phase density differences. Biological materials can be purified by selcctive partitioning between the phases. In the case of cells and other particulates thc efficiency of these separations appears to be compromised by the demixing process. To modify this process and to evaluate the potential of two-phase partitioning in space, experiments on the effects of gravity on phase emulsion demixing were undertaken. The behavior of phase systems possessing phase density differences and systems with essentially identical phase densities was studied at one-g and during low-g parabolic aircraft maneuvers. Results indicate that demixing can occur rather rapidly in space, although more slowly than on Earth. We have examined the demixing process from a theoretical standpoint by applying the theory of Ostwald ripening. This theory predicts demixing rates many ordcrs of magnitude lower than observed. Other possible demixing mechanisms are considered.     

Nonequilibrium Phase Formation in Oxides Prepared at Low Temperature: Fergusonite-Related Phases

Sol–gel methods have been developed to prepare YNbO₄, YTaO₄, and other rare-earth niobates and tantalates with fergusonite-related crystal structures. At low temperatures, all of the fergusonites, with the exception of SmTaO₄, crystallize in a metastable tetragonal (T') structure similar to that of tetragonal zirconia. Although all of the equilibrium forms of these oxides adopt a crystal structure containing an ordered distribution of the trivalent and pentavalent cations, a random cation distribution is obtained in the metastable T'phase. Metastable phase formation is often ascribed solely to kinetically limited topotactic crystallization. However, the changes in the grain size and unit-cell volumes that accompany the metastable-to-equilibrium fergusonite conversions imply that other physical phenomena induced by small-particle synthesis, namely the Gibbs-Thompson pressure effect and the increased contribution of surface energy, cannot be ignored.