Thermodynamic Studies on the AlN Sintering Powders Treated With Phosphate Species

The processing of aluminum nitride (AlN) ceramics in aqueous media requires the use of a surface layer to protect the surface of the particles against hydrolysis. This surface layer might influence the densification, affecting the reactions between AlN and sintering additives. The present paper describes a thermodynamic and experimental approach to evaluate the effects of a phosphate-based protecting surface layer on the densification of AlN in the presence of YF₃–CaF₂ as sintering aids, and to predict the densification behavior during sintering using thermodynamic assessments. Based on thermodynamic calculations and the measured weight loss of the samples during heating to sintering temperature, the chemical reactions occurring during firing were proposed. The proposed reactions were related to the experimental results as well as the final properties of the AlN samples, namely, thermal conductivity, microstructure, secondary phases, and density.     

Thermodynamic Benefit of Abnormal Grain Growth in Pore Elimination During Sintering

Free-energy analyses performed on closed particle-pore arrays show that the presence of an abnormal grain thermodynamically favors the shrinkage of large pores to which is is adjacent. This gives an explanation to the experimentally observed phenomenon of abnormal-grain-growth-promoted densification in barium titanate.     

Thermodynamic Behavior of Copper-Coated Silicon Carbide Particles during Conventional Heating and Spark Plasma Sintering

75Cu·25SiC (vol%) compacts were prepared using copper-coated SiC particles and spark plasma sintering (SPS). The preliminary thermal performance of the coated particles was determined using simultaneous DSC-TG-MS measurement. Characterization of compacts using XRD and SEM techniques was conducted to investigate the physical and chemical changes during the SPS operation. It was found that CuO decomposed at 850° and 500°C during conventional heating and SPS, respectively. Cu₂O facilitated the densification of Cu/SiC composites. The optimized sintering temperature of Cu/SiC composites using SPS was ∼730°C. The compacts showed improved hardness because of the SiC reinforcement.     

氧化铝陶瓷及其烧结

氧化铝陶瓷具有良好的性能,故应用广泛。本文介绍了氧化铝变体、氧化铝陶瓷的性质和用途。论述了氧化铝陶瓷烧结动力学并分析了影响烧结的主要因素。     

Master Sintering Curve: A Practical Approach to Sintering

One of the ultimate objectives for sintering studies is to be able to predict densiflcation results under different thermal histories for a given processing method. It has been reported that the geometric parameters related to sintering often are functions only of density for a given powder and green-body process, provided that one diffusion mechanism dominates in the sintering process. Based on this report, the concept of a master sintering curve has been developed that characterizes the sintering behavior for a given powder and green-body process regardless of the heating profiles. The formulation and construction of the master sintering curve are given in this paper. A model experiment on sintering of alumina is used and analyzed to demonstrate this new concept. Examples of the master sintering curves obtained from other powder systems (ZnO, nickel, A1₂O₃(5 vol% TiO₂), and A1₂O₃(5 vol% ZrO₂)) are presented. When this new method is used, densification behavior can be predicted under arbitrary temperature-time excursions following a minimal set of preliminary experiments, and these predictions can be used in planning sintering strategies. Moreover, deviations from the assumption of a single mechanism can be observed readily.     

Sintering of Fine Oxide Powders: II, Sintering Mechanisms

Conventional and new sintering mechanisms have been investigated using fine powders of CeO₂ and Y₂O₃ of excellent sinterability. We have verified the validity of Herrings scaling law for 60%–84% relative density and found that it is consistent with grain-boundary-diffusion control. At lower densities, we have found that pores larger than the critical size, in the sense of Kingery and Francois, can still be sintered readily. This is rationalized by a new sintering mechanism based on particle repacking concurrent with particle coarsening, resulting in a higher packing factor. Very fine, surface-active powders that coarsen rapidly are uniquely capable of taking advantage of this new sintering mechanism, which along with their propensity to homogenization, accounts for their remarkable sinterability even at very low green densities.     

Diffusion Sintering: II, Initial Sintering Kinetics of Alumina

The initial sintering kinetics of alumina have been studied by measuring the isothermal shrinkage of compacts of several alumina powders in air. The shrinkage of these compacts can best be described by a grain-boundary vacancy diffusion model for the temperature range 1200° to 1600°C. The behavior of the compacts is consistent with the model after an initial shrinkage has occurred. The magnitude of this initial shrinkage is constant for identical specimens and is independent of the sintering temperature.     

GPLE失活动力学和烧结动力学

介绍了Funtes和Bartholomew提出的GPLE失活动力学和烧结动力学的研究进展。     

Effect of Atmosphere on Sintering of High Cr_2O_3 Bearing Refractories Considered from Thermodynamic Calculation

Pure Cr2O3 refractories and high Cr2O3 bearing refractories are difficult to sinter in oxidizing atmosphere,but they can be sintered easily in carbon embedded conditions.In the latter case it is attributed to the formation of liquid CrO in reducing atmosphere.The thermodynamic calculation shows that the Cr2O3 is reduced by CO at 1 550 ℃ to chromium carbides,which disproves the existence of liquid CrO.This is confirmed by further sintering experiment of the Cr2O3 refractories.Effect of atmosphere on sinterin...     

The Zirconia–Hafnia System: DTA Measurements and Thermodynamic Calculations

The equilibrium temperature ( T ₀) at which the tetragonal and monoclinic phases of either ZrO₂ or HfO₂ coexist is generally defined by the middle temperature of A _s (the onset transformation temperature on heating) and M _s (the onset transformation temperature on cooling). It cannot be directly determined due to the athermal nature of the martensitic transformation. Practically, the determination of T ₀ is important for the prediction of A _s and M _s in ZrO₂ or HfO₂-based materials. In this work, the ZrO₂–HfO₂ system was studied experimentally by differential thermal analysis (DTA) to obtain the martensitic tetragonal ⇔ monoclinic transformation temperatures in the temperature range of 1273–1973 K. The T ₀ temperatures obtained for ZrO₂ and HfO₂ are 1367±5 and 2052±5 K, respectively. They are adopted for the assessments of the Gibbs energy parameters of these two oxides. A reasonably calculated ZrO₂−HfO₂ phase diagram is presented.     

NZP族陶瓷材料的合成与烧结

本文回顾了磷酸锆钠族(NZP)陶瓷材料的合成和烧结,并进一步指出了合成NZP粉体中存在的问题和促进NZP陶瓷致密化的万法。     

烧结微晶玻璃的生产及其着色

介绍了烧结微晶玻璃的工艺原理、生产工艺，结合实践重点分析和讨论结微晶玻璃典型的一次着色工艺及着色剂各类和着色机理。     

聚苯乙烯粉选择性激光烧结的支撑烧结研究

为了解决聚苯乙烯(PS)粉选择性激光烧结(SLS)成型工艺中制件成型缺陷问题,通过一系列烧结实验,分析了缺陷的种类及原因,利用"二次烧结"现象,以试样Z向尺寸高度和尺寸方差为评价指标,在最优烧结工艺参数支撑扫描速度4 500 mm/s,支撑扫描间隔为3 mm下对不同层数和类型的支撑扫描进行了对比实验。结果表明,支撑扫描可有效改善制件翘曲问题,当支撑层数为8层,支撑类型为网格型时,制件质量较好。     

Numerical Simulation of Solid-State Sintering: I, Sintering of Three Particles

A kinetic, Monte Carlo model, capable of simulating microstructural evolution sintering in a two-dimensional system of three particles, has been presented. The model can simulate several mechanisms simultaneously. It can simulate curvature-driven grain growth, pore migration and coarsening by surface diffusion, and densification by diffusion of vacancies to grain boundaries and annihilation of these vacancies. Morphologic changes and densification kinetics are used to verify the model.     

The Sintering Process of Quartz Ceramics

It is shown, based on the available evidence, that the heating and cooling rates in the heat-treatment regimes of quartz ceramics and its commercial products are of minor importance for the properties of the material and are limited only by the process equipment potentialities. Still, the use of higher heating and cooling rates makes the production of quartz ceramics more cost-effective.     

The Sintering of Supported Metal Catalysts

Metal catalysts are commonly employed in the form of metal dispersed as small crystallites on high surface area supports. The use of these supported metal catalysts increases the utilization of the metal as a catalyst since a large fraction of the metal atoms are at the surface of the small metal crystallites. Another important function of the support is to physically separate the small metal crystallites and thereby hinder the agglomeration of the small metal crystallites into larger crystallites. This agglomeration would decrease the number of surface metal atoms per unit mass of metal, and thereby decrease the utilization of the metal and the activity of the catalyst.     

碳化硅泡沫陶瓷烧结温度和烧结机理的研究

将制备碳化硅泡沫陶瓷的浆料通过烘干、制粉、干压成型、烧结来探讨烧结温度对制品性能的影响。试验结果表明样品的最高抗弯强度出现在1400℃、保温2h的工艺条件下，而不是更高的烧结温度1450℃。主要原因在于过高的烧结温度导致碳化硅氧化严重，生成了大量的方石英，方石英在随后的冷却过程中出现微裂纹所致。而碳化硅泡沫陶瓷的烧结机理主要是玻璃相对碳化硅颗粒的包覆、连接作用和新相莫来石的生成。     

Densification of Yttria Transparent Ceramics: The Utilization of Activated Sintering

Highly transparent 0.5 at.% Tm:Y₂O₃ ceramics were prepared by using solid‐state reaction combined with vacuum sintering method, with ZrO₂ and Al₂O₃ as sintering aids. Doping amount of ZrO₂ was fixed at 1 at.%, while the effect of Al₂O₃ on densification, microstructure evolution, and transmittance of the Y₂O₃ ceramics was carefully studied. It was found that the addition of Al₂O₃ was very effective in improving densification of Y₂O₃, due to the formation of an Al‐rich eutectic phase Y₄Al₂O₉ (YAM) during the sintering process. As the content of Al₂O₃ was increased from 0 to 81.8 wt ppm, porosity of the ceramics was decreased and transmittance was increased. However, when the content of Al₂O₃ was increased to 137 wt ppm, a secondary phase began to segregate at grain junctions. Further increase in the amount of Al₂O₃ led to an increase in both amount and size of the secondary phase. At the optimized content of Al₂O₃ with 81.8 wt ppm, the Tm:Y₂O₃ ceramics sintered at 1860°C for 13 h exhibited an in‐line transmittance of 83.0% at 2000 nm and 76.5% at 600 nm. It is expected that this finding can be readily applied to other transparent ceramics.