ZrO_2-Y_2O_3超细粉烧结动力学的研究

通过对ZrO_2-Y_2O_3纳米级粉末烧结动力学过程的研究,分析了陶瓷的致密化机理。实验结果表明:超细粉末在烧结初期,晶界扩散起主要作用。在存在团聚体的粉末中,由于团聚体与基体间界面应力的相互作用,致使烧结体的密度降低,影响了陶瓷的显微结构,从而限制了在烧结后期获得细晶粒的陶瓷体。对于无团聚体的粉末,通过烧结过程中晶粒生长过程的控制,可获得相对密度达98.5%(1250℃保温2h),且晶粒尺寸仅为160nm的Y-TZP陶瓷体。     

两步烧结法制备纳米氧化钇稳定的四方氧化锆陶瓷

采用共沉淀法制备纳米氧化钇稳定的四方氧化锆（yttria stabilized tetragonal zirconia,3Y-TZP）粉体。利用X射线衍射、N2吸附–脱附等温线,透射电子显微镜对3Y-TZP粉体的物理性能和化学性能进行表征。研究了纳米3Y-TZP粉体的烧结曲线,分析了3Y-TZP素坯在烧结过程中的致密化行为和显微结构,探讨了两步烧结工艺对3Y-TZP纳米陶瓷微观结构的影响。结果表明：采用共沉淀法,在600℃煅烧2h后,可获得晶粒尺寸为13nm、晶型发育良好、团聚较少的纳米3Y-TZP粉体;采用两步烧结法,将素坯升温至1200℃保温1min后,再降温到1050℃保温35h,可获得相对密度大于98%,晶粒尺寸约为100nm的3Y-TZP陶瓷。两步烧结法通过控制煅烧温度和保温时间,利用晶界扩散及其迁移动力学之间的差异,使晶粒生长受到抑制,样品烧结致密化得以维持,实现在晶粒无显著生长前提下完成致密化。     

固相烧结--Ⅲ实验:超细氧化锆素坯烧结过程中的晶粒与气孔生长及致密化行为

研究了超细Y-TZP和YSZ粉料成型体在烧结中期的晶粒生长、气孔生长和致密化行为.根据作者前文[1,2]提出的致密化方程,可以满意地解释粉体及其成型体的性质,如初始颗粒尺寸、成型密度和气孔尺寸分布等对烧结的影响. 实验发现:成型体中的晶粒生长基本不受成型体性质的影响,但气孔生长同时受晶粒生长和致密化的影响,前者使气孔尺寸与晶粒尺寸同步生长,后者导致气孔收缩. 晶粒生长和致密化虽受不同的机制驱动,但通过同样扩散途径完成,使烧结中期的晶粒尺寸与密度呈线性关系. 理论分析和实验结果表明,成型体性质不改变这一线性关系,但可以改变直线的斜率,而升温速率对直线斜率的影响不大. 较大的二面角、较高的素坯密度、较窄的颗粒和气孔尺寸分布有利于获得较小的晶粒生长和较高的烧结密度.     

固相烧结——Ⅲ 实验：超细氧化锆素坯烧结过程中的 晶粒与气孔生长及致密化行为

研究了超细Y-TZP和YSZ粉料成型体在烧结中期的晶粒生长、气孔生长和致密化行为.根据作者前文     

陶瓷“家族”新成员——红外陶瓷

红外陶瓷是选择对红外辐射吸收很小的高纯原料，并采用真空热压（或烧结）的新工艺制备出来的。在真空热压条件下，有利于抑制陶瓷晶粒的生长，促使晶粒发生塑性流变；高温则有利于晶粒间的相互扩散。这样，通过真空热压或高温烧结，使陶瓷小晶粒迅速扩散、熔合而成一体，最大限度地降低了晶粒的表面自由能，从而产生出高密度几乎完全消除了微气孔的透明的红外陶瓷。     

纳米3Y-TZP粉体的制备及烧结曲线研究

采用共沉淀法制备纳米3Y-TZP粉体,并利用DSC-TG,XRD和TEM等测试方法对3Y-TZP的物理化学性能进行表征。通过研究纳米3Y-TZP粉体的烧结曲线,分析3Y-TZP素坯在烧结过程的致密化和显微结构。结果表明:在600℃下煅烧2h后,可获得晶粒尺寸为13nm,晶形发育良好、团聚较少的纳米3Y-TZP粉体。在烧结过程中,不同烧结阶段中的扩散机制不同,在烧结后期晶粒尺寸发生显著长大。     

不同烧结法对3Y-TZP陶瓷力学性能的影响

本文研究了低温烧结含3mol%氧化钇的四方多晶氧化锆(3Y-TZP)的烧结性能和力学性能,以及进行热等静压(HIP)后其力学性能的变化。成形后的3Y-TZP在常压、1300～1450℃温度下进行烧结。由于该粉料有很高的烧结活性,在1300℃低温烧成下就可获得相对密度大于94%的烧结体;在1350℃烧成温度下3Y-TZP获得了最佳的力学性能。其断裂韧性(KIC)和维氏硬度(HV)分别达到18.7MPa.m1/2和13.7GPa,其中应力诱导相变是其主要的增韧机理。对低温烧成的3Y-TZP陶瓷进行热等静压烧结后发现,HIP增大3Y-TZP陶瓷HV的作用显著,可使其增至14.3GPa。     

直接干压成型与真空烧结技术制备Nd:YAG透明陶瓷

以高纯氧化物粉体为原料,采用直接干压成型与固相反应烧结技术制备Nd:YAG透明陶瓷,并对其光学透过率和烧结致密化行为进行了研究。结果表明,经1 760℃烧结10 h后,250 MPa成型压力下的样品透过率最高,在1 064 nm处达到83.8%。其素坯的气孔率为40.5%,气孔平均孔径74.5 nm。烧结后陶瓷的显微结构致密,晶界干净清晰,断裂方式为沿晶断裂。将250 MPa成型压力下获得的陶瓷素坯在不同温度和时间下烧结,得到致密化轨迹、晶粒生长曲线以及显微结构演变等信息。通过理论拟合,得出低温下陶瓷致密化和晶粒生长的控制机制为晶界扩散。     

氧化锆陶瓷超塑性变形行为及机理

研究了不同晶粒尺寸的3Y-TZP在高温下的变形行为,探究了压头速度和变形温度对材料超塑性变形的影响。结果表明,3Y-TZP陶瓷的超塑性变形抗力随变形温度的提高和压头速度的降低而降低。变形后晶粒尺寸增大,但仍保持等轴晶。在1250℃采用应力突变法对不同晶粒尺寸的材料进行压缩试验,实验结果与经典的蠕变方程相吻合。在一定的晶粒尺寸范围内,3Y-TZP陶瓷的应力指数随着晶粒尺寸的增大而减小。在温度1250℃时,3Y-TZP的主要变形机制为伴随扩散的晶界滑动。     

鳞片石墨/3Y-TZP复合陶瓷烧结及其力学性能

以十二烷基苯磺酸钠(SDBS)为分散剂制备FG/3Y-TZP复合粉体,并且把复合粉体进行二次工艺酒精浮选处理,采用氩气气氛常压高温烧结,制备系列FG/3Y-TZP复合陶瓷。研究鳞片石墨(FG)含量和浮选工艺对复合陶瓷材料的物相、显微结构的影响;建立了材料显微结构与其体密和断裂韧性的相互关系,讨论了浮选处理的FG/3Y-TZP复合陶瓷的增强、增韧机理。结果表明:制备的FG/3Y-TZP在高温烧结后均为四方相;浮选工艺对所制备样品的显微结构和性能有显著影响。随着浮选时间的提高,样品致密度也提高,晶粒进一步长大;在FG含量为1.5%左右时,致密度和抗折性能均达到峰值。而浮选工艺的引入,则进一步使氧化锆表面附着的鳞片石墨变小变薄,一方面填充气孔,另外还有助于提高鳞片石墨活性,使复合材料的晶界处阻碍裂纹产生或者扩展;但是由于分散性和含量问题,随着鳞片石墨含量的提高,致密度和断裂韧性在急剧下降。     

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。     

Grain Size Associated Genes and the Molecular Regulatory Mechanism in Rice

Grain size is a quantitative trait that is controlled by multiple genes. It is not only a yield trait, but also an important appearance quality of rice. In addition, grain size is easy to be selected in evolution, which is also a significant trait for studying rice evolution. In recent years, many quantitative trait loci (QTL)/genes for rice grain size were isolated by map-based cloning or genome-wide association studies, which revealed the genetic and molecular mechanism of grain size regulation in part. Here, we summarized the QTL/genes cloned for grain size and the regulation mechanism with a view to provide the theoretical basis for improving rice yield and breeding superior varieties.     

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.