Liquid Redistribution during Liquid-Phase Sintering

A simple two-dimensional packing model, consisting of arrays of circular particles, was used to calculate the free energy changes associated with the filling of pores of different coordinations with liquid. The calculations were used to determine the equilibrium distributions of a liquid in different packing arrangements of particles. The effect of both the volume fraction of liquid and shrinkage on liquid distribution was examined. It was found that, when liquid redistribution can easily take place, the volume fraction of liquid phase, the pore size distribution of a powder compact, and the amount of densification that has occurred all influence the homogeneity of the distribution of the liquid phase. In addition, the model predicts that, as shrinkage occurs or as the volume fraction of liquid phase increases, the pores will try to fill sequentially in order of increasing size. A consequence of this sequential filling of the pores is that the radius of curvature of the liquid meniscus, and hence the driving force for liquid-phase sintering, changes systematically as shrinkage occurs. The modeling suggests that the driving force for sintering changes in a way that depends on the initial overall pore size distribution of the particle arrangement and the way the pore size distribution changes during sintering.     

Morphological Changes in Process-Related Large Pores of Granular Compacted and Sintered Alumina

Large pore defects clearly develop in Al₂O₃ ceramics during sintering. These large pores originate from voids caused by the incomplete deformation and adhesion of powder particles in collapsed dimples at the centers and boundaries of granules in the green compacts. The coalescence of pores, with limited shrinkage, during densification and grain growth in the late intermediate to final stages of sintering, is considered responsible for the development of the large pores. The mechanism of pore coalescence is explained by thermodynamic arguments, which demonstrate that the largest pores result in a stable system.     

Effect of Interface Structure on the Microstructural Evolution of Ceramics

The interface atomic structure was proposed to have a critical effect on microstructure evolution during sintering of ceramic materials. In liquid-phase sintering, spherical grains show normal grain growth behavior without exception, while angular grains often grow abnormally. The coarsening process of spherical grains with a disordered or rough interface atomic structure is diffusion-controlled, because there is little energy barrier for atomic attachments. On the other hand, kink-generating sources such as screw dislocations or two-dimensional (2-D) nuclei are required for angular grains having an ordered or singular interface structure. Coarsening of angular grains based on a 2-D nucleation mechanism could explain the abnormal grain growth behavior. It was also proposed that a densification process is closely related to the interface atomic structure. Enhanced densification by carefully chosen additives during solid state sintering was explained in terms of the grain-boundary structural transition from an ordered to a disordered open structure.     

Liquid Phase Sintering of Alumina, I. Microstructure Evolution and Densification

The microstructure evolution and densification of alumina containing 10 vol% calcium aluminosilicate glass and 0.5 wt% magnesium oxide sintered at 1600°C were quantified by measuring the evolution of pore-size distribution, the redistribution of liquid phase, and the fraction of closed and open pores. The densification stopped at a limiting relative density during the final stage of sintering, and the small and large pores were filled simultaneously by glass during sintering. In addition, the results indicate that the pressure build-up of the trapped gases in pores causes a significantly negative contribution to the driving force, and consequently the observed reduction in densification during the final stage of liquid phase sintering.     

A two-dimensional Monte Carlo model for pore densification in a bi-crystal via grain boundary diffusion: Effect of diffusion rate,initial pore distance,temperature, boundary energy and number of pores

A two-dimensional Monte Carlo (MC) model is introduced for simulating the evolution of the pore on a bi-crystal grain boundary via grain boundary diffusion. Simulated pore shrinkage kinetics is found to be consistent with previously reported results over variable grain boundary diffusion rates and initial pore distances while the essential characteristics of the microstructural evolution are simultaneously realized. The influence on the pore densification kinetics of grain boundary motion, boundary energy ratio, simulation temperature and pore interactions in an array is found such that pore shrinkage rate increases as the grain boundary motion, the simulation temperature and the grain boundary energy increase. The interactions of the pores are found to hinder the pore densification. The body of results signify that the more elongated the pore shape and the shaper the pore tip region are favored for the faster pore shrinkage kinetics during the simulated densification process via grain boundary diffusion.     

Microstructural evolution and densification behavior of porous kaolin-based mullite ceramic added with MoO3

Microstructural evolution and densification behavior of porous kaolin-based mullite ceramic added with MoO₃ were investigated. The results indicated that MoO₃ addition not only lowered the secondary mullitization temperature to below 950?°C, but also facilitated effectively the anisotropic growth of mullite grains. Fine mullite whiskers grew and interlocked with one another in the pre-existing pore regions, in-situ forming a stiff 3D skeleton structure of mullite whiskers, which arrested further densification of the sample. On the other hand, due to the great capillary attraction of small pores, the liquid phase tended to spread over small grains, which favored the growth from small mullite grains into whiskers at the expense of the liquid phase. Consequently, competitive mechanisms of sintering and crystal growth of mullite functioned, which further limited the sample densification. As a result, the total linear shrinkage of the sample added with MoO₃ after firing at 1400?°C was only ??2.75%, and its porosity was retained at as high as 67%.     

Sintering of High-Purity Nickel Oxide

The results of an investigation of densification and grain growth during the sintering of high-purity NiO in various atmospheres are presented. The sintering of NiO powders is divided into three stages. Grain growth during the third stage follows the empirical equation D ²= K t, where D is the average grain diameter, K a rate constant, t the sintering time. The activation energy of grain growth was found to be 55 kcal. per mole. The shrinkage and weight loss during the sintering process in vacuum were larger than in argon, air, and oxygen. Grain growth during sintering also differed with the sintering atmosphere. It was concluded that the sintering process is not explained reasonably by the theory of a semiconductor.     

Sintering Behavior and Electrical Properties of Nanosized Doped-ZnO Powders Produced by Metallorganic Polymeric Processing

Homogeneous and nanosized (28 nm crystallite size) doped-ZnO ceramic powders were obtained by a metallorganic polymeric method. Calcining and granulating resulted in green compacts with uniform powder packing and a narrow pore-size distribution (pore size 19 nm). Dense ceramic bodies (>99% of theoretical) were fabricated by normal liquid-phase sintering at 850° and 940°C for 1–5 h. Apparently, the low pore-coordination number allowed a uniform filling of the small pores by the liquid formed in the early stages of sintering, and, consequently, high shrinkage and rapid densification occurred in a short temperature interval (825°–850°C). At these sintering temperatures, limited grain growth occurred, and the grain size was maintained at <1 μm. Ceramics so-fabricated showed a nonlinear coefficient, α, of ≥70, and a breakdown voltage, V _b (1 mA/cm²), of ≥1500 V/mm. The high electrical performance of the doped-ZnO dense ceramics was attributed to liquid-phase recession on cooling, which enhanced the ZnO-ZnO direct contacts and the potential barrier effect.     

Effect of Sintering Temperature on the Densification of Al2O3

Alumina powder compacts sintered at various temperatures were isostatically hot-pressed. The specimens sintered to the closed-pore state can be densified further by hot isostatic pressing. If the open pores are eliminated during sintering, sintering at a low temperature is desirable to achieve a full densification after hot-pressing. Sintering at high temperatures causes pores to be trapped inside the grains; these pores are difficult to eliminate by subsequent hot-pressing.     

Grain Growth During Gas-Pressure Sintering of β-Silicon Nitride

The microstructures of gas-pressure-sintered materials from β-Si₃N₄ powder were characterized in terms of the diameter and aspect ratio of the grains. The size distributions of diameters in materials fabricated by heating for 1 h at 1850° to 2000°C were nearly constant when they were normalized by average diameters because of normal grain growth. The rate-determining step in the densification and grain growth was expected to be the diffusion of materials through the liquid phase. The activation energy for grain growth was 372 kJ/mol. The average aspect ratio of the grains was 3 to 4, whereas that of large grains was smaller because of shape accommodation. The fracture toughness was about the same as that of material from α-Si₃N₄ powder despite the smaller aspect ratio of the grains     

菱镁矿煅烧过程中氧化镁烧结与晶粒生长动力学的研究

用XRD、BET和TEM研究了菱镁矿煅烧过程中氧化镁烧结与晶粒长大动力学。用表面积降低动力学模型和颗粒长大动力学方程对结果进行了分析,得出结果认为菱镁矿煅烧过程中氧化镁微晶的烧结可分为三个阶段:(1)微晶氧化镁结构的调整,伴随着晶粒迅速长大、比表面积急剧下降;(2)团聚体内孔容积保持不变,团聚体也不收缩,这时烧结过程为表面扩散和蒸发-凝聚所控制;(3)烧结过程为体积或晶界扩散所控制,团聚体出现显著收缩。     

液相烧结氧化铝陶瓷及其烧结动力学分析

研究了CuO TiO2复相添加剂对Al2O3陶瓷烧结性能、显微结构的影响以及添加剂形成液相时Al2O3陶瓷的烧结动力学.结果显示:添加剂的加入明显地促进了Al2O3陶瓷的烧结致密度.添加剂含量对致密有明显影响,含量越高,烧结速率越快.当添加剂(CuO TiO2)为2%(质量分数),CuO/TiO2质量比为1/2时,Al2O3样品致密度最高.添加剂的存在使Al2O3晶粒发生较快生长,晶粒形貌为等轴状.通过等温烧结动力学,确定掺杂Al2O3陶瓷烧结激活能为25.2kJ/mol,表明可能是氧离子和铝离子在液相中的扩散作用控制了烧结过程.     

CuO+TiO2复相添加剂对氧化铝陶瓷烧结性能和显微结构的影响及其烧结动力学分析

研究了CuO＋TiO2复相添加剂对氧化铝陶瓷烧结性能，显微结构的影响以及形成液相时氧化铝陶瓷烧结动力学。添加剂的加入极大的促进了氧化铝陶瓷的烧结。当CuO与TiO2质量比为1：2的时候，氧化铝样品致密度最高。液相含量对致密度有明显影响，液相含量越高，烧结速率越快。添加剂的存在使氧化铝晶粒细化，晶粒形貌为等轴状。利用等温烧结的实验方法研究了烧结动力学，结果表明，是由氧离子和铝离子的扩散作用控制了烧结过程。     

Two-Step Sintering of Nanocrystalline ZnO Compacts: Effect of Temperature on Densification and Grain Growth

Two-step sintering (TSS) was applied on nanocrystalline zinc oxide (ZnO) to control the accelerated grain growth occurring during the final stage of sintering. The grain size of a high-density (>98%) ZnO compact produced by the TSS was smaller than 1 μm, while the grain size of those formed by the conventional sintering method was ∼4 μm. The results showed that the temperature of both sintering steps plays a significant role in densification and grain growth of the nanocrystalline ZnO compacts. Several TSS regimes were analyzed. Based on the results obtained, the optimum regime consisted of heating at 800°C (step 1) and 750°C (step 2), resulting in the formation of a structure containing submicrometer grains (0.68 μm). Heating at 850°C (step 1) and then at 750°C (step 2) resulted in densification and grain growth similar to the conventional sintering process. Lower temperatures, e.g., 800°C (step 1) and 700°C (step 2), resulted in exhaustion of the densification at a relative density of 86%, above which the grains continued to grow. Thermogravimetric analysis results were used to propose a mechanism for sintering of the samples with transmission electron micrographs showing the junctions that pin the boundaries of growing grains and the triple-point drags that result in the grain-boundary curvature.     

Theoretical analysis of experimental densification kinetics in final sintering stage of nano-sized zirconia

The experimental densification kinetics of 7.8 mol% Y₂O₃-stabilized zirconia was analyzed theoretically during isothermal sintering in the final stage. By taking concurrent grain growth into account, a possible value of the grain-size exponent n was examined. The Coble’s corner-pore model recognized widely was found not to be applicable for explaining the densification kinetics. The corner-pore model of n = 4 shows a significant divergence in the kinetics at different temperatures. Microstructural observation shows that most pores are not located at grain corners and have a size comparable to the surrounding grains. The observed pore structure is similar to the diffusive model where single pore is surrounded by dense body. The diffusive model combined with theoretical sintering stress predicts n = 1 or n = 2, which shows a good consistence to the measured densification kinetics. During sintering of nano-sized powder, it is found that the densification kinetics can be explained distinctively by the diffusive single-pore model.     

The Effect of Particle Size Distributions on the Microstructural Evolution During Sintering

Microstructural evolution and sintering behavior of powder compacts composed of spherical particles with different particle size distributions (PSDs) were simulated using a kinetic Monte Carlo model of solid‐state sintering. Compacts of monosized particles, normal PSDs with fixed mean particle radii and a range of standard deviations, and log‐normal PSDs with fixed mode and a range of skewness values were studied. Densification rate and final relative density were found to be inversely proportional to initial PSD width. Grain growth was faster during the early stages of sintering for broad PSDs, but the final grain sizes were smaller. These behaviors are explained by the smallest grains in the broader PSDs being consumed very quickly by larger neighboring grains. The elimination of the small grains reduces both the total number of necks and the neck area between particles, which in turn reduces the regions where vacancies can be annihilated, leading to slower densification rates. The loss of neck area causes grain growth by surface diffusion to become the dominant microstructural evolution mechanism, leading to poor densification. Finally, pore size was shown to increase with the width of PSDs, which also contributes to the lower densification rates.     

Sintering Behavior of Fully Agglomerated Zirconia Compacts

Fully agglomerated superfine zirconia powders were prepared with the coprecipitation and spray-drying method. The compaction of such powders shows no fragmentation of the agglomerates. The sintering behavior of the compacts was studied and two sintering stages were identified: densification within agglomerates at temperatures not higher than 1250°C and the removal of interagglomerate pores at temperatures above 1600°C. The interagglomerate pores are difficult to remove, and sintering between agglomerates even at 1600°C is still insignificant. Heating of the compacts at temperatures above 1600°C leads only to grain growth and the entrapping of pores in large grains.     

Effect of Pore Distribution on Microstructure Development: I, Matrix Pores

A model has been developed to describe the effect of the matrix (first-generation) pore distribution on microstructure development in the final stages of sintering. A model of simultaneous densification and grain growth was used to predict the effects of the number of pores per grain and the pore size distribution on microstructure evolution. Increasing the number of pores per grain was predicted to increase the densification rate, the grain growth rate, and the relative densification rate/grain growth rate ratio. Narrowing the pore size distribution was predicted to inhibit grain growth initially and to increase the densification rate indirectly. Overall, the pore distribution was predicted to have a strong influence on microstructure development and sintering kinetics.     

Numerical simulation and experimental verification of dry pressed MgTiO3 ceramic body during pressureless sintering

To clarify the densification law of dry pressed MgTiO₃ ceramic body during pressureless sintering, SOVS model (Skorohod-Olevsky Viscous Sintering model) modified with creep characteristics was embedded into finite element software Abaqus. The selected model can effectively express the grain boundary characteristics and densification mechanism. The change law of relative density, shrinkage rate, sintering stress, and grain size of MgTiO₃ cylindrical specimens was investigated by the above numerical simulation method. It showed that the average relative density of ceramic body rose from 60% to 97%, and the shrinkage rate respectively reached 17.28% and 11.99% in axial and radial direction. The average grain size increased from 1 to 6 μm. In order to verify the accuracy of the simulation results, corresponding sintering experiments on cylindrical specimens were carried out to obtain actual sintering densities and shrinkage rates. It showed that the errors of relative density and shrinkage were below 5% and 2%. Grain growth trend was also basically consistent with the simulation results. After that, the above numerical simulation method was applied into the prediction of fabricating MgTiO₃ filter with complex structure. Therefore, the present work provided a reliable numerical simulation method to predict the densification behavior of MgTiO₃ ceramics during the pressureless sintering process, which was helpful to design and fabricate microwave dielectric products.     

Role of Grain Boundaries in Sintering.

During sintering, grain boundaries act either as sinks or as diffusion paths for lattice vacancies. Thus the configuration of the grain boundaries will have an important effect on the rate of sintering. Grain growth during sintering changes the configuration of grain boundaries relative to pores and thus may markedly influence the shrinkage rate; for example, for uniformly distributed pores the shrinkage rate will increase as the grain size decreases. Impurity additions will increase the sintering rate if they increase diffusion rates, but they may also increase sintering rates by impeding grain-boundary movement. Some experimental evidence is presented to support these conclusions.