Densification and Shrinkage During Liquid-Phase Sintering

The process of densification and shrinkage during the final stage of liquid-phase sintering is described. The densification occurs by the liquid filling of pores during grain growth. The pore filling results in an instantaneous drop of liquid pressure in the compact and causes gradual accommodation of grain shape. The grain shape accommodation by the growth causes the specimen shrinkage. At the same time, the grains tend to restore their spherical shape, resulting in microstructure homogenization around filled pores. The process of densification and shrinkage appears to be determined by the growth of grains during sintering.     

掺杂Al的纳米ZrO2无压烧结致密化过程的研究

采用无压烧结对平均粒径为30～50nm的ZrO2纳米粉,及掺Al的ZrO2纳米粉的烧结致密化进行研究。实验结果表明ZrO2坯休的致密化主要发生在1150～1250℃范围内。理论分析表明,通过掺杂Al改善其显微结蛄构．提高生坯密度,能降低该粉体的烧结致密化温度。     

微晶玻璃烧结过程中晶化和致密化的关系

以钢渣和粉煤灰为主要原料,采用烧结工艺,制得以透辉石为主晶相的微晶玻璃;通过热分析、X射线衍射、收缩率、扫描电镜等分析方法,详细说明了在烧结过程中晶化和致密化的关系;试验结果表明晶化主要发生在烧结的前期,后期以致密化为主,晶化会影响烧结体的致密化。     

Very Rapid Densification of Nanometer Silicon Carbide Powder by Pulse Electric Current Sintering

Rapid densification of a nanometer SiC powder doped with 2.04 wt% Al₄C₃ and 0.4 wt% B₄C was conducted by using a nonconventional sintering technique called pulse electric current sintering (PECS). In all experiments, the sintering temperature and applied pressure were kept to be 1600^oC and 47 MPa, respectively, while heating rates varied between 100^oC/min and 400^oC/min and the holding time was either 2 or 5 min. All of the specimens which were PECS-sintered under various conditions reached near-theoretical density. The microstructures of the rapidly densified SiC ceramics consisted of large elongated grains, and the grain size increased with the increase of heating rate. Polytype transformation of SiC occurred during the PECS process, where faster heating favored the formation of 6H polytype while slower heating favored 4H polytype.     

常压烧结制备氧化钨陶瓷靶材的致密化研究

采用常压固相烧结工艺,制备出高纯度、高致密度的氧化钨靶材。考察了粉体粒度、成型压强、烧结温度和保温时间等对靶材致密度的影响。测试结果表明,以粒度0.27μm的粉体为原料,成型压强为60MPa,烧结温度为1200℃,保温时间为1h的条件下,可以制备出高致密度的氧化钨靶材,其组成为高纯的单斜晶相。     

Creep and Densification During Sintering of Glass Powder Compacts

The simultaneous creep and densification of glass powder compacts was studied as a function of low applied uniaxial stress, temperature, and particle size. The creep rate can be expressed as the sum of the contribution from the applied stress that varies linearly with stress, and a contribution due to anisotropic densification that varies linearly with the densification rate. For a constant applied stress, the ratio of the creep rate to the densification rate is almost independent of both termperature and density. While these observations are consistent with the model of Scherer for the viscous sintering of glass, other observations show significant deviations from the model. Both the densification rate and the creep viscosity, which has an exponential dependence on porosity, show much stronger dependence on density compared with theoretical predictions.     

Densification Behaviors of Fine-Alumina and Coarse-Alumina Compacts during Liquid-Phase Sintering with the Addition of Talc

The densification behavior of fine alumina (mean particle size of ∼0.31 μm) and coarse alumina (mean particle size of ∼4.49 μm) during liquid-phase sintering with additions of talc have been studied, as well as the microstructural evolution. Small amounts (0, 5, and 10 wt%) of talc were added to the fine alumina and coarse alumina, which were sintered at various temperatures for 2 h. When 5 wt% of talc was added to the coarse alumina, densification proceeded rapidly above the liquid-formation temperature in alumina–talc compacts, because of the promotion of a rearrangement process of the solid grains by the liquid phase. The addition of 10 wt% of talc greatly accelerated densification by increasing the volume fraction of liquid. On the other hand, in the fine alumina, which has a higher activity and a greater driving force for sintering, appreciable densification started below the liquid-formation temperature, which prevented further densification after liquid formation. Moreover, the densification was suppressed as the talc content increased. The rigid skeleton of solid grains that was formed by densification below the liquid-formation temperature is believed to have suppressed the rearrangement process of the solid grains, and further densification of the compacts was retarded, even after the formation of a liquid phase above the liquid-formation temperature.     

热压烧结制备高致密度B4C陶瓷及其致密化

以B4C粉末为原料,以酚醛树脂热解碳为烧结助剂,采用真空热压烧结工艺制备出高致密度的B4C陶瓷材料,探讨了酚醛树脂含量和烧结温度对材料致密化的影响,对比分析了纯B4C和掺碳B4C的热压烧结致密化过程.研究显示,烧结温度的升高有利于材料的致密化,酚醛树脂的添加则显著活化了B4C的烧结.实验条件下酚醛树脂的最优添加量为3％（质量分数）,烧结过程中热解碳固溶于B4C晶格,显著加速了其在1600℃至1800℃区间的致密化速率.     

Si3N4-MgO-CeO2陶瓷烧结中的致密化与自动析晶

用等离子放电烧结的方法制备了Si3N4-MgO-CeO2陶瓷,用排水法测定了密度,用X射线衍射的方法测定了物相变化。发现在烧结过程中,高于l450℃时,MgO-CeO2就会与氮化硅粉末表面的SiO2反应形成硅酸盐液相,促进烧结致密化,冷却后形成玻璃相留在晶界,氮化硅的致密化在l500℃接近完成。但高于l550℃烧结,MgO反而会析晶,提高氮化硅陶瓷的高温性能。     

Effect of Heating Rate on Sintering and Coarsening

The sintering of zinc oxide powder compacts has been investigated at constant rates of heating of 0.5° to 15°C/min. For samples with the same initial relative density (0.50), the temperature derivative of the densification strain versus density fits within a single, relatively narrow band. At low temperatures the densification rate as a function of temperature increases almost linearly with the heating rate. The data, covering a wide density range of 0.5 to 0.98, are consistent with an analysis that accounts for the coarsening (defined as an increase in the mean pore separation) in terms of two classes of microstructural coarsening processes: those associated with densifying and with nondensifying mechanisms.     

HDPE蠕变行为的研究

研究HDPE的蠕变行为不仅有助于从分子运动机理上揭示聚合物的黏弹性行为,还能够预测材料使用过程中的尺寸稳定性及长期承载能力,减小工程设计误差,确保材料使用的安全性,因而具有重要的理论意义和实用价值。回顾了蠕变理论的发展,综合了近年来有关HDPE蠕变行为及形变机理的研究进展,并以HDPE单向拉伸格栅为例简要分析了此项研究的发展及应用前景。     

Thermodynamics of Densification: I, Sintering of Simple Particle Arrays, Equilibrium Configurations, Pore Stability, and Shrinkage

Equilibrium configurations for linear and closed arrays (rings and regular polyhedra containing a single pore) of identical particles (cylinders or spheres) were determined by minimizing the array's surface and grain-boundary energies with the assumption that each particle conserves its mass. The change in free energy between the initial and equilibrium configuration increases with dihedral angle (i.e., the equilibrium angle). More significantly, it is shown that pores will shrink to an equilibrium size if the number ( n ) of coordinating particles is greater than a critical value. The critical pore coordination number ( n _c) increases with the dihedral angle. Only pores with n n _c are thermodynamically unstable during sintering. It is also shown that any mass-transport mechanism can lead to pore shrinkage while a connecting path to the pore surface remains open. The effective sintering "stress" (i.e., driving force) increases with the dihedral angle and decreases to zero as the equilibrium configuration is reached. Sintering stresses increase with decreasing coordination number. It is also shown that the shrinkage strain for closed arrays increases with the pore coordination number. Rearrangement phenomena within a powder compact are discussed with regard to resultant sintering forces on nonsymmetrically coordinated particles.     

Bending Creep Test to Measure the Viscosity of Porous Materials during Sintering

A bending creep test is proposed for measuring the change in viscosity of a porous material during densification. Equations, based on simple beam deflection theory, were derived to obtain the viscosity from a series of loading experiments using rectangular samples of different densities. By measuring the deflection in the center of the specimen between the spans, the viscosity of a porous material during densification can be measured. Experiments with porous Y₂O₃-stabilized ZrO₂ beams were used to illustrate the bending creep test. Consistent with theory the viscosity increased from 50 to 400 GPa·s as the sample densified from 87% to 98% density. The rapid increase in viscosity was considered to be a result of both densification and grain growth.     

氟化物对低温烧结氧化镁陶瓷的影响

研究了添加不同种类氟化物的氧化镁陶瓷的各种烧结条件和显微结构.实验发现:添加MgF2可以在相对低的温度下显著提升制品的致密度并促进晶粒的生长,说明采用MgF2可以实现低温烧结MgO.这说明MgO-MgF2体系中烧结驱动力即总界面自由能随着粒子总表面积的不断降低而降低.     

Effect of Aspect Ratio and Liquid-Phase Content on Densification of Alumina-Silicon Carbide Whisker Composites

Densification of alumina-SiC whisker composites by pressureless sintering is inhibited as a result of whisker interference with particle rearrangement and composite shrinkage. Reduction of the aspect ratio improves densification by improving particle/whisker packing for increased green densities and enhances the ability of the whiskers to rearrange themselves during sintering. Increasing the amount of liquid phases present during sintering also improves densification by aiding whisker rearrangement.     

Elimination of large Pores During Gas-Pressure Sintering of β'-Sialon

The effect of N₂-gas pressure on the liquid filling of large pores (20 to 120 μm in diameter) is studied in sintered β'-sialon ( z = 1). The initially sintered sialon with large pores is sintered again and infitrated by a liquid (41A1₂O₃-41Y₂O₃-14Si₃Y₄-4AIN (wt%)) at 17800°C for various times under 0.1-, 0.3-, and 0.5-MPa (1-, 3-, and 5-atm) N₂. The liquid fills large interconnected pores; the size of the pores filled with liquid increases with N₂-gas pressure and time. In some liquid pockets, gas bubbles are formed and subsequently disappear during prolonged sintering treatment. The liquid-filling be havior with sintering pressure and time is explained by gas pressure in the pore and thermal decomposition of the material. The benefit of gas-pressure sintering for the elimination of large pores is assessed.     

Sintering of Ceramic Particulate Composites: Effect of Matrix Density

Composites consisting of a fine-grained, polycrystalline zinc oxide matrix and <10 vol% coarse, rigid silicon carbide inclusions were prepared by the same mixing procedure and then compacted to produce samples with matrix densities of 0.45 and 0.68 of the theoretical. The samples were sintered under identical temperature profiles in separate experiments that employed either a constant rate of heating of 4°C/min or near isothermal heating at 735°C. The ratio of the densification rate of the composite matrix to the densification rate of the unreinforced zinc oxide was found to be independent of the initial matrix density. This ratio increased significantly with temperature in the constant-heating-rate experiments but was relatively constant in the isothermal experiments. The results indicate that microstructural coarsening may be an important mechanism for explaining the reduced sinterability of polycrystalline matrix composites.     

压实密度与烧成收缩率的探讨

探讨了压制成型后产品压实密度与烧成收缩率的关系，提出了在各种原材料烧失量稳定的情况下，用控制压实密度代替用控制压力来保证坯件收缩的新理念。     

Effect of Sintering Atmosphere on the Densification and Electrical Properties of Alumina

The effect of sintering atmosphere on the final density and electrical properties of alumina compacts has been investigated using two different oxygen pressures: air and CO/CO₂. Measuring of electrical behavior has been considered a tool for determining the mechanism responsible for densification. Finally, the importance of a reducing atmoshphere on the electrical behavior of polycrystalline alumina is pointed out.