SrTiO3复合功能陶瓷的AC阻抗谱研究

本文采用一次烧成法制备出SrTiO3电容-压敏复合功能陶瓷,测试了样品的交流阻抗谱,确定了样品的等效电路,分析了不同氧化热处理条件下的晶粒和晶界电学特性.结果表明,SrTiO3复合功能陶瓷晶界存在多种受主态形态,氧化热处理过程中晶粒保持其半导体特性,晶界绝缘程度显著提高.     

卷曲温度对无取向硅钢组织结构及其变化的影响

通过工业试验和金相显微镜、扫描电镜、背散射衍射技术研究了卷曲温度对50W600无取向硅钢热轧板、冷轧板和退火冷轧板的组织结构及其变化的影响. 结果表明,无取向硅钢热轧板→冷轧板→退火冷轧板的织构演变为高斯织构{110}<001>→{111}和{100}面织构→{111}和少量{100}面织构. 卷曲温度从680℃提高到720℃,有利于无取向硅钢热轧板再结晶晶粒形核和长大,退火冷轧板的晶粒尺寸从40.88 mm增加到48.58 mm. 卷曲温度为680℃时,无取向硅钢退火冷轧板中{111}, {100}面织构分别为32.9%和6.1%(j);720℃时,其退火冷轧板中{111}和{100}面织构分别为25.1%和7.9%(j). 卷曲温度从680℃提高到720℃,无取向硅钢退火冷轧板的铁芯损耗从3.94 W/kg降到3.85 W/kg,磁感应强度从1.690 T提高到1.701 T.     

PTC型BaTiO3陶瓷的孪晶和孪晶界

用透射电镜和高分辨电镜观察研究了PTC型BaTiO_3陶瓷的孪晶和孪晶界结构,并提出了可能的{111}孪晶及孪晶界的原子结构模型。结果表明,{111}孪晶形成氧八面体共面的共格倾斜孪晶界,在孪晶界处可能存在一定量的A空位且原子密度要低于孪晶体内。同时还发现两种其它特征的孪晶现象。     

织构化（Na_（1/2）Bi_（1/2））TiO_3-BaTiO_3无铅压电陶瓷晶粒定向生长机制

以熔盐法合成的片状SrTiO3晶粒为模板,利用模板晶粒生长（TGG）技术制备晶粒沿[001]方向为取向的0.94（Na1/2Bi1/2）TiO3-0.06BaTiO3（简写为BNBT6）无铅压电陶瓷,采用X线衍射仪（XRD）、扫描电子显微镜（SEM）对陶瓷试样进行表征,采用透射电子显微镜（TEM）观察SrTiO3与BNBT6基体界面的微观结构.结果表明,BNBT6陶瓷晶粒定向生长过程分为2个阶段：首先是异质外延生长阶段,即在片状模板晶粒的诱导下,BNBT6基体粉体在SrTiO3模板晶粒表面外延生长,形成与模板取向完全一致的单晶生长层的过程;其次是同质外延生长阶段,即单晶生长层生成后吞噬BNBT6基体粉体逐步生长得到各向异性的高取向BNBT6陶瓷的过程.     

用高分辨透射电子显微技术研究0.94(Na1/2Bi1/2)TiO3-0.06BaTiO3的有序结构

用高分辨透射电子显微技术(high resolution transmission electron microscopy, HRTEM)和选区电子衍射(selected electron diffraction, SAED)技术对具有A位复合钙钛矿结构的0.94(Na1/2Bi1/2) TiO3(BNT)-0.06BaTiO3(BNBT6)陶瓷的显微结构进行了深入研究.沿[001],[011]和[111]3个晶带轴方向的SAED花样分析结果表明:A位Bi3 和Na 可以形成2种不同的有序结构──1/2{110}和1/2{111}超结构.据此,建立了BNT的1/2{110}和1/2{111}有序结构模型,其中1/2{110}型有序是由Bi3 层和Na 层沿[110]方向交替排列而形成,1/2{111}型有序是由Bi3 层和Na 层沿[111]方向交替排列而形成.利用多层法,通过计算机模拟了1/2{111}有序结构沿[001],[011]和[111]3个方向的HRTEM像,与实验拍摄的HRTEM像对照,验证了所建立的1/2{111}有序结构模型的正确性.同时,采用快速Fourier变换(fast Fourier transformation, FFT),由1/2(330)和1/2(330)超晶格反射斑点获得的一维晶格像,揭示了沿[110]方向局部Bi3 层和Na 层扭曲导致的位错和反相畴界(anti-phase boundary, APB), APB的存在表明结构中存在着Bi3 层和Na 层沿[110]方向的交替排列,证实BNBT6中存在着1/2{110}有序结构.BNBT6有序结构的HRTEM研究表明:A位Bi3 和Na 的化学有序是导致超晶格反射的主要原因.     

热水溶液中钛酸锶晶粒的结晶过程

研究了以TiCl4水解得到的H4TiO4胶体作为钛源,在热水溶液中制备SrTiO3晶粒的结晶过程,TEM和XRD结果表明,其结晶过程主要包括3个阶段,第1阶段主要是SrTiO3晶核的形成过程;第2阶段主要是SrTiO3晶粒的配向聚集生长过程;第3阶段主要是SrTiO3晶粒的溶解结晶过程,其中第1阶段的控制步骤为H4TiO4胶体的溶解,第2阶段中晶粒的配向聚集生长在短时间内即可完成,很快进入第3阶段小晶粒的溶解和大晶粒的重结晶过程,该阶段反应速度慢,需要较长时间才能得到结晶完好的晶粒,在热水溶液中以H4TiO4胶体作为钛源制备SrTiO3晶粒关键在于控制结晶过程的第1阶段和第2阶段。     

MgB2等镁化物添加剂对氧化铝陶瓷晶粒生长形貌的影响

研究了MgB2、MgF2和MgCl2添加到Al2O3陶瓷中时对晶粒形状的影响。添加5％MgB2时，在800℃左右的空气气氛中烧结．氧化铝陶瓷中生长出竹节状MgO晶须，在1450℃烧结时，Al2O3陶瓷晶粒生长成长柱状形状。添加5％MgF2时，在1450℃烧结时，Al2O3陶瓷晶粒可长成多棱体形状。而添加MgCl2时，Al2O3陶瓷晶粒的生长没有出现异常情况。讨论了添加不同镁化物对Al2O3陶瓷晶粒生长影响的原因及力学性能影响。     

Y—TZP陶瓷晶粒生长的控制

为了获得超细晶粒的Y-TZP陶瓷,制备了无团聚体的素坯,研究了在烧结过程中的晶粒生长和气孔变化的规律。在烧结初期,晶粒与气孔同时增大;在烧结中期,气孔的表面扩散是晶粒生长的主要机理;烧结后期,随着烧结温度的提高,晶界扩散是晶粒生长的主要机理。实验结果进一步表明:在1250℃,2h的烧结条件下,可获得晶粒尺寸仅为100～150nm的Y-TZP陶瓷。     

用高分辨透射电子显微技术研究0.94（Na（1/2）Bi（1/2））TiO3–0.06BaTiO3的有序结构

用高分辨透射电子显微技术（high resolution transmission electron microscopy,HRTEM）和选区电子衍射（selected area electron diffraction,SAED）技术对具有A位复合钙钛矿结构的0.94（Na1/2Bi1/2）TiO3（BNT）–0.06BaTiO3（BNBT6）陶瓷的显微结构进行了深入研究。沿[001],[011]和[111]3个晶带轴方向的SAED花样分析结果表明：A位Bi3＋和Na＋可以形成2种不同的有序结构——1/2{110}和1/2{111}超结构。据此,建立了BNT的1/2{110}和1/2{111}有序结构模型,其中1/2{110}型有序是由Bi^3＋层和Na^＋层沿[110]方向交替排列而形成,1/2{111}型有序是由Bi^3＋层和Na^＋层沿[111]方向交替排列而形成。利用多层法,通过计算机模拟了1/2{111}有序结构沿[001],[011]和[111]3个方向的HRTEM像,与实验拍摄的HRTEM像对照,验证了所建立的1/2{111}有序结构模型的正确性。同时,采用快速Fourier变换（fast Fourier transformation,FFT）技术,由1/2（330）和1/2（330）超晶格反射斑点获得的一维晶格像,揭示了沿[110]方向局部Bi3＋层和Na＋层扭曲导致的位错和反相畴界（anti-phase boundary,APB）,APB的存在表明结构中存在着Bi3＋层和Na＋层沿[110]方向的交替排列,证实BNBT6中存在着1/2{110}有序结构。BNBT6有序结构的HRTEM研究表明：A位Bi^3＋和Na^＋的化学有序是导致超晶格反射的主要原因。     

SrTiO3陶瓷晶界层电容器材料的晶界研究Ⅰ.晶界结构

利用高分辨电镜研究了低温一次烧结SrTiO_3陶瓷晶界层电容器材料的晶界结构,发现在西晶粒间存在着4种典型的晶界类型。通过构筑晶界形成的结构模型,揭示了两晶粒间晶界形成的特征,认为两晶粒间晶界相的形成既与两晶粒间的液相成份和杂质组成有关,亦与两晶粒的相对取向有关。此外,还与两次烧结SrTiO_3陶瓷晶界层电容器材料的晶界结构作了比较。     

银盐照相乳剂制备最新动向

通过分析《美国化学文摘》中关于银盐照相材料的乳剂制备专利,了解到近几年柯达、富士等公司申请的专利主要涉及富氯卤化银 T－颗粒乳剂的制备、在 T－颗粒上引入线性位错和碘化银补加方法。     

Habits of Grains in Dense Polycrystalline Solids

We show here that the boundaries of individual grains in dense polycrystals prefer certain crystallographic habit planes, almost as if they were independent of the neighboring crystals. In MgO, SrTiO₃, MgAl₂O₄, TiO₂, and aluminum, the specific habit planes within the polycrystal correspond to the same planes that dominate the external growth forms and equilibrium shapes of isolated crystals of the same phase. The observations decrease the apparent complexity of interfacial networks and suggest that the mechanisms of solid-state grain growth may be analogous to conventional crystal growth. The results also indicate that a model for grain-boundary energy and structure based on grain surface relationships is more appropriate than the widely accepted models based on lattice orientation relationships.     

Frozen Water Content in Maize at Low Temperatures And Its Effect on Dryer Performance

ABSTRACT

In some parts of the world, e. g. China, maize reaches a grain dryer at low temperatures, frequently in the range of -10°C to -20°C. The effect of a low grain temperature, and an equally low ambient temperature, was determined, partially by experimentation and partially by simulation. The percentage of moisture Frozen in maize depends on the grain moisture content and temperature. The low grain and ambient temperatures surprisingly have only a small effect on dryer capacity; however. the influence on the energy consumption is large.     

Designing nanomaterials with desired mechanical properties by constraining the evolution of their grain shapes

Grain shapes are acknowledged to impact nanomaterials' overall properties. Research works on this issue include grain-elongation and grain-strain measurements and their impacts on nanomaterials' mechanical properties. This paper proposes a stochastic model for grain strain undergoing severe plastic deformation. Most models deal with equivalent radii assuming that nanomaterials' grains are spherical. These models neglect true grain shapes. This paper also proposes a theoretical approach of extending existing models by considering grain shape distribution during stochastic design and modelling of nanomaterials' constituent structures and mechanical properties. This is achieved by introducing grain 'form'. Example 'forms' for 2-D and 3-D grains are proposed. From the definitions of form, strain and Hall-Petch-Relationship to Reversed-Hall-Petch-Relationship, data obtained for nanomaterials' grain size and conventional materials' properties are sufficient for analysis. Proposed extended models are solved simultaneously and tested with grain growth data. It is shown that the nature of form evolution depends on form choice and dimensional space. Long-run results reveal that grain boundary migration process causes grains to become spherical, grain rotation coalescence makes them deviate away from becoming spherical and they initially deviate away from becoming spherical before converging into spherical ones due to the TOTAL process. Percentage deviations from spherical grains depend on dimensional space and form: 0% minimum and 100% maximum deviations were observed. It is shown that the plots for grain shape functions lie above the spherical (control) value of 1 in 2-D grains for all considered grain growth mechanisms. Some plots lie above the spherical value, and others approach the spherical value before deviating below it when dealing with 3-D grains. The physical interpretations of these variations are explained from elementary principles about the different grain growth mechanisms. It is observed that materials whose grains deviate further away from the spherical ones have more enhanced properties, while materials with spherical grains have lesser properties. It is observed that there exist critical states beyond which Hall-Petch Relationship changes to Reversed Hall-Petch Relationship. It can be concluded that if grain shapes in nanomaterials are constrained in the way they evolve, then nanomaterials with desired properties can be designed.     

Mobility transition at grain boundaries in two‐step sintered 8 mol% yttria‐stabilized zirconia

Stagnation of grain growth is often attributed to impurity segregation, which becomes more severe as the grain size grows. In this respect, there is no evidence for segregation‐induced slowdown in the grain growth of yttria‐stabilized cubic zirconia, which obeys the parabolic law when the size increases by more than ten times. However, lowering the temperature below 1300°C triggers an abrupt slowdown, constraining the average grains to grow by less than 0.5 μm in 1000 hours despite a relatively large driving force imparted in the fine grains of ~0.5 μm. Yet isolated pockets of abnormally large grains, and even most remarkably, pockets of abnormally small grains, emerge in the same latter sample. Such extreme bifurcation of microstructure has never been observed before, and can be explained by an inhomogeneous distribution of immobile four‐grain junctions. The implications of these findings for two‐step sintering are discussed.     

The temperature dependence of the relative grain‐boundary energy of yttria‐doped alumina

Atomic force microscopy was used to measure the dimensions of grain‐boundary thermal grooves on the surfaces of Al₂O₃, 100 ppm Y‐doped Al₂O₃, and 500 ppm Y‐doped Al₂O₃ ceramics heated at temperatures between 1350°C and 1650°C. The measurements were used to estimate the relative grain‐boundary energies as a function of temperature. The relative grain‐boundary energies of Al₂O₃ decrease slightly with increased temperature. When the doped samples were heated, there was an overall increase in the grain‐boundary energy, attributed to a reduction in the grain boundary excess at higher temperature. The overall trend of increasing grain‐boundary energy was interrupted by abrupt reductions in grain‐boundary energy between 1450°C and 1550°C. In the same temperature range, there is an abrupt increase in the grain‐boundary mobility that is associated with a complexion transition. When the 100 ppm Y‐doped sample was cooled, there was a corresponding increase in the relative grain‐boundary energy at the same complexion transition temperature, indicating that the transition is reversible.     

Predicting the Grain-Size Distributions in High-Density, High-Purity Alumina Ceramics

This paper discusses the image-analyzer-based grain-size distributions (GSDs) of fully densified ceramics obtained from pressure casting a high-purity alumina powder, develops an algorithm for predicting the GSDs as a function of sintering time and temperature, and compares of the GSDs thus predicted with those experimentally observed. The GSD data for all sintered specimens, when nondimensionalized in terms of the median grain size, reduced to a single self-preserving GSD curve. The median grain was predicted as a function of sintering time and temperature using the classical kinetics equation. The GSDs predicted using the algorithm developed tallied well with those that were experimentally obtained.     

Quantification of Grain Boundary-Pore Contact During Sintering

A stereological method has been used to determine the degree of grain boundary-pore contact during sintering of Al₂O₃. Al₂O₃ doped with 200 ppm MgO exhibits a degree of contact of 5.7 times that expected from random intersections with pores, while pure Al₂O₃ shows a pore contact factor of 4.8. These data are larger than the values of 2.8 for sintered or hot-pressed UO₂, computed from published data, and values of 1.7 and 1.8 for sintered W and Cu powders, respectively. The degree of grain boundary-pore contact for each material remains constant throughout densification from pressed powder to near full density.     

Effect of porous structure on the performance of bidispersive catalysts for model reforming reactions

Making use of experimentally determined structural and kinetic parameters of the catalyst, a method was proposed, which makes it possible to assess the specific activity and the performance of the catalyst in the model reaction of reforming. Three reforming catalysts differing in activity were tested. The contribution of catalytic activity, porous structure and process temperature to the efficiency factors defined in this paper was analysed for each catalyst. The advantage of using bidispersive catalysts was substantiated, and the usefulness of the presence of macropores in the catalyst structure was determined numerically. Of the structural parameters affecting the performance of the porous bidispersive catalysts, macropore volume and macropore radius were found to be the most important. In the course of the study, the parameters of the porous structure were optimised for a catalyst of choice, which was used in the dehydrocyclisation of n-heptane and in the dehydrogenation of methylcyclohexane at different temperatures.     

Origin and Growth Kinetics of Platelike Abnormal Grains in Liquid-Phase-Sintered Alumina

The grain-growth behavior of Al₂O₃ compacts with small contents (≤10 wt%) of various liquid-forming dopants was studied. Equiaxed and/or elongated grains were observed for the following dopants: MgO, CaO, SiO₂, or CaO + TiO₂. The platelike grains, defined as the abnormal grains larger than 100 μm with an aspect ratio ≥5 and with flat boundaries along the long axis, were observed when the boundaries were wet with the liquid phase and the codoping satisfied two conditions of size and valence. These dopings were Na₂O + SiO₂, CaO + SiO₂, SrO + SiO₂, or BaO + SiO₂. However, an addition of MgO to the Al₂O₃ doped with CaO + SiO₂ resulted in the change of grain shape from platelike to equiaxial. Equiaxed grains were also observed for the MgO + SiO₂ doping, indicating that two conditions were necessary but not sufficient to develop the platelike grains. The fast growth rate of the platelike grains was explained by an increased interfacial reaction rate due to the codopants. AT the same time the codopants made the basal plane, which appeared as the flat boundaries, the lowest energy plane. The appearance of the platelike grains was favored in compacts with a small grain size and with a narrow size distribution at the onset of abnormal grain growth. Accordingly, the use of starting powders with a small particle size and narrow size distribution, smaller amounts of dopings, and high sintering temperature resulted in an increased number of the platelike grains.