BN-YAION复合陶瓷的烧结行为

对ＢＮ－ＹＡｌＯＮ复合陶瓷进行了热压和无压烧结试验,对烧结体的密度变化和显微结构进行了研究,分析了影响ＢＮ基复合陶瓷致密化的主要因素．认为卡片房式结构是妨碍ＢＮ基复合陶瓷致密化的主要原因．热压过程中施加的压力足够大时,可以破坏这种卡片房式结构,使片状ＢＮ定向排列,因而能获得高致密度的ＢＮ基复合陶瓷．热压过程中若有液相出现,有利于片状ＢＮ定向排列,因而能促进ＢＮ基复合陶瓷的致密化．无压烧结时因不能消除原有的卡片房式结构,故虽有液相出现,也难以获得高致密度的ＢＮ基复合陶瓷     

BN—YAION复合陶瓷的烧结行为

对ＢＮ－ＹＡ１ＯＮ复便陶瓷进行了热压和无压烧结试验，对烧结体的密度变化和显微结构进行了研究，分析了影响ＢＮ基复合陶瓷致密化的主要因素。认为卡片房式结构是妨碍ＢＮ基复合陶瓷致密化的主要原因。热夺过程中施加的压力足够大时，可以破坏这种卡片房式结构，使片状ＢＮ定向排列，因而能获得高致密度的ＢＮ基复合陶瓷。热压过程中若有液相出现。，有利于片片ＢＮ定向排列，因而能促进ＢＮ基复合陶瓷的致密化。无压烧结时因不能     

Al2O3/h-BN自润滑复相陶瓷烧成工艺的探讨

通过探讨烧成方法和烧成温度对Al2O3/h-BN自润滑复相陶瓷物相组成、显微结构的影响,结果表明：常压烧成时,材料很不致密;热压烧成时由于压力和液相的共同作用,可以破坏片状h-BN长大时形成的卡片房式结构,促使片状h-BN定向排列,缩小材料的孔隙率,提高了材料的致密度。由于h-BN的卡片房式结构的阻碍,Al2O3/h-BN自润滑复相陶瓷材料较难烧结致密,需要高温热压烧成,烧成温度初步确定在1700～1800℃之间。     

六方氮化硼陶瓷的烧结及其结构与性能EI北大核心CSCD

以亚微米级h-BN粉体为原料,在不添加任何烧结助剂的情况下,分别采用无压烧结、热压烧结和放电等离子烧结（SPS）制备h-BN陶瓷,采用X射线衍射和扫描电子显微镜对烧结后样品的物相组成和显微结构进行测试和观察,研究不同烧结方法对h-BN陶瓷的致密度、晶粒取向、显微形貌及力学性能的影响,对比分析了不同烧结方法下坯体初始致密度对h-BN陶瓷性能的影响。结果表明：无压烧结无法实现h-BN陶瓷烧结致密化,力学性能较差,而通过热压和放电等离子烧结的方法均能得到结构致密、力学性能较好的h-BN陶瓷。相比于传统的无压和热压烧结,放电等离子烧结方法制备的h-BN陶瓷具有更高的致密度和更好的力学性能,而且晶粒更均匀细小,烧结温度可降低200℃以上。此外,坯体初始致密度的提高能显著提高h-BN陶瓷的抗弯强度和断裂韧性,但对热压和放电等离子烧结制备的h-BN陶瓷致密化的影响较小。     

六方氮化硼陶瓷的烧结及其结构与性能

以亚微米级h-BN粉体为原料,在不添加任何烧结助剂的情况下,分别采用无压烧结、热压烧结和放电等离子烧结(SPS)制备h-BN陶瓷,采用X射线衍射和扫描电子显微镜对烧结后样品的物相组成和显微结构进行测试和观察,研究不同烧结方法对h-BN陶瓷的致密度、晶粒取向、显微形貌及力学性能的影响,对比分析了不同烧结方法下坯体初始致密度对h-BN陶瓷性能的影响。结果表明:无压烧结无法实现h-BN陶瓷烧结致密化,力学性能较差,而通过热压和放电等离子烧结的方法均能得到结构致密、力学性能较好的h-BN陶瓷。相比于传统的无压和热压烧结,放电等离子烧结方法制备的h-BN陶瓷具有更高的致密度和更好的力学性能,而且晶粒更均匀细小,烧结温度可降低200℃以上。此外,坯体初始致密度的提高能显著提高h-BN陶瓷的抗弯强度和断裂韧性,但对热压和放电等离子烧结制备的h-BN陶瓷致密化的影响较小。     

LiTaO3压电陶瓷烧结工艺的优化

采用氮气保护热压烧结工艺制备LiTaO3压电陶瓷材料,研究了不同烧结方法和热压烧结压力对LiTaO3压电陶瓷致密度和力学性能的影响。无压烧结不能得到烧结致密的LiTaO3基压电陶瓷。1300℃/25MPa热压烧结制备的纯LiTaO3陶瓷的烧结致密度很低,只有91.5%,其各项力学性能很差。1300℃/35MPa热压烧结制备的纯LiTaO3陶瓷材料的致密度较高,已经达到了97%,材料烧结较致密,气孔较少,但由于LiTaO3本身性质所限其各项力学性能变化不大。     

AIN—BN复合陶瓷的结构与性能

以碳热还原法合成的AlN粉末和市售BN粉末为原料, 利用无压烧结工艺制备AlN－BN复合陶瓷, 研究了AlN－BN复合陶瓷结构和性能的关系.结果表明: 随着BN含量的增加, 在复合陶瓷中逐渐形成卡片房式结构, 阻碍材料的收缩和致密, 复合材料的致密度下降, 热导率和硬度也随之下降, 综合考虑热导率和硬度因素, 认为利用常压烧结工艺制备可加工AlN－BN复合陶瓷时, BN质量分数在10%～15%之间是合适的, 可以制备出热导率在100～140 W·m-1·K-1, 硬度HRA在55～75之间的AlN－BN复合陶瓷.     

AIN-BN复合陶瓷的结构与性能

以碳热还原法合成的AIN粉末和市售BN粉末为原料,利用无压烧结工艺制备AlN-BN复合陶瓷,研究了AlN-BN复合陶瓷结构和性能的关系。结果表明:随着BN含量的增加,在复合陶瓷中逐渐形成卡片房式结构,阻碍材料的收缩和致密,复合材料的致密度下降,热导率和硬度也随之下降,综合考虑热导率和硬度因素,认为利用常压烧结工艺制备可加工AlN-BN复合陶瓷时,BN质量分数在10％～15％之间是合适的,可以制备出热导率在100～140W·m~(-1)·K~(-1),硬度H_(R_A)在55～75之间的AlN-BN复合陶瓷。     

AlN-BN复合陶瓷的结构与性能

以碳热还原法合成的AlN粉末和市售BN粉末为原料, 利用无压烧结工艺制备AlN－BN复合陶瓷, 研究了AlN－BN复合陶瓷结构和性能的关系.结果表明: 随着BN含量的增加, 在复合陶瓷中逐渐形成卡片房式结构, 阻碍材料的收缩和致密, 复合材料的致密度下降, 热导率和硬度也随之下降, 综合考虑热导率和硬度因素, 认为利用常压烧结工艺制备可加工AlN－BN复合陶瓷时, BN质量分数在10%～15%之间是合适的, 可以制备出热导率在100～140 W·m-1·K-1, 硬度HRA在55～75之间的AlN－BN复合陶瓷.     

AlN-Pr2O3液相烧结SiC陶瓷

以AlN、Pr2O3做为SiC陶瓷液相烧结的复合助剂,选定不同的助剂含量(5wt％～ 20wt％)和不同的助剂摩尔比例(Pr2O3/AlN=1/3、1/1、3/1),在1800～2000℃温度下,采用热压和无压烧结的方法制备SiC陶瓷样品,并对这些陶瓷样品的性能进行了研究.实验结果表明,助剂比1/3组的样品显示出更有效地促进SiC陶瓷致密化,该组样品无压烧结最大相对密度为87％,热压烧结具有最高的相对密度96.1％、维氏硬度23.4 GPa、抗弯强度549.7MPa、断裂韧性5.36 MPa·m1/2,显微结构中可观察到晶粒拔出现象,断裂模式为沿晶断裂.     

氟化钙对常压烧结Si3N4/BN复相陶瓷结构与性能的影响

以微米级Si3N4和h-BN粉末为原料,CaF2–Al2O3–Y2O3为烧结助剂,采用常压烧结工艺制备了BN体积含量为25%的Si3N4/BN复相陶瓷。研究了CaF2添加量对Si3N4/BN复相陶瓷材料力学性能的影响,并通过X射线衍射和场发射扫描电镜分析了复相陶瓷的物相组成和显微组织。结果表明：随着CaF2添加量增加,制备的Si3N4/BN复相陶瓷材料气孔率逐渐增大,收缩率变小,相对密度减小。添加量为2%（质量分数）时,Si3N4/BN复相陶瓷的室温抗弯强度达145.5MPa。添加适量的CaF2可在Si3N4/BN复相陶瓷材料常压烧结过程中较大程度地破坏h-BN的卡片房式结构,将微米级的h-BN颗粒变成纳米级颗粒。     

Colloidal Processing of Zirconium Diboride Ultra‐High Temperature Ceramics

Colloidal processing of the Ultra‐High Temperature Ceramic (UHTC) zirconium diboride (ZrB₂) to develop near?net‐shaping techniques has been investigated. The use of the colloidal processing technique produces higher particle packing that ultimately enables achieving greater densification at lower temperatures and pressures, even pressureless sintering. ZrB₂ suspension formulations have been optimized in terms of rheological behavior. Suspensions were shaped into green bodies (63% relative density) using slip casting. The densification was carried out at 1900°C, 2000°C, and 2100°C, using both hot pressing at 40 MPa and pressureless sintering. The colloidally processed materials were compared with materials prepared by a conventional dry processing route (cold pressed at 50 MPa) and subjected to the same densification procedures. Sintered densities for samples produced by the colloidal route are higher than produced by the dry route (up to 99.5% relative density by hot pressing), even when pressureless sintering is performed (more than 90% relative density). The promising results are considered as a starting point for the fabrication of complex‐shaped components that can be densified at lower sintering temperatures without pressure.     

Modelling of elimination of strength-limiting defects by pressure-assisted sintering at low stress levels

The structural reliability of sintered products depends on large defects introduced during powder processing, which cannot be removed by pressureless sintering. Here, we present a model how a large single ellipsoidal void is deformed, and finally disappears by pressure-assisted sintering. Taya-Seidel’s model is applied to predict the shrinkage of a large void in a compressible linear viscous material by using bulk viscosity, shear viscosity, and sintering stress that are determined experimentally for sintering of alumina powder at low stress levels. The application of mechanical stress promotes the densification rate. Its effect is maximum for hot isostatic pressing (HIP) and minimum for sinter forging. The effect is intermediate for hot pressing (HP) and spark plasma sintering (SPS), because the hydrostatic component of stress varies with densification. While a crack-like defect can be removed during densification, a spherical void must be eliminated by shear deformation in the final stage during dwell time.     

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.     

Sintering and densification of nanocrystalline ceramic oxide powders: a review

Abstract

Observation of the unconventional properties and material behaviour expected in the nanometre grain size range necessitates the fabrication of fully dense bulk nanostructured ceramics. This is achieved by the application of ceramic nanoparticles and suitable densification conditions, both for the green and sintered compacts. Various sintering and densification strategies were adopted, including pressureless sintering, hot pressing, hot isostatic pressing, microwave sintering, sinter forging, and spark plasma sintering. The theoretical aspects and characteristics of these processing techniques, in conjunction with densification mechanisms in the nanocrystalline oxides, were discussed. Spherical nanoparticles with narrow size distribution are crucial to obtain homogeneous density and low pore-to-particle-size ratio in the green compacts, and to preserve the nanograin size at full densification. High applied pressure is beneficial via the densification mechanisms of nanoparticle rearrangement and sliding, plastic deformation, and pore shrinkage. Low temperature mass transport by surface diffusion during the spark plasma sintering of nanoparticles can lead to rapid densification kinetics with negligible grain growth.     

Processing and properties of biphasic calcium phosphates bioceramics obtained by pressureless sintering and hot isostatic pressing

Calcium phosphate powders were obtained from coprecipitation method using diammonium phosphate and calcium nitrate solutions. The adjustment of experimental parameters and Ca/P initial ratio allowed synthesizing stoichiometric HA and β-TCP powders and four biphasic calcium phosphates (BCP) with compositions equal to 20/80, 40/60, 60/40, 80/20, of HA and TCP respective in mass. Two sintering techniques, pressureless sintering and hot isostatic pressing (HIP) were performed to compare the sinterability of the stoichiometric and biphasic compounds. By pressureless sintering, densities higher than 98% were obtained. Results illustrate the negative impact of hydroxyapatite on the densification of BCP but its benefit effect on grain growth inhibition. After HIP treatment, transparent ceramics were obtained. The mechanical behavior of these fully densified compounds was examined in order to determine the role of the chemical composition on mechanical properties.     

α-SiAlON Ceramics Obtained by Slip Casting and Pressureless Sintering

Dense α-SiAlON ceramics were obtained by pressureless sintering of green compacts prepared using slip casting. The rheological properties of the reaction SiAlON suspension were optimized to achieve a high degree of dispersion with a high solids volume fraction, which resulted in homogeneous and relatively dense green bodies with high sintering ability, which could be densified by pressureless sintering at 1750°C for 2 h. The sintered samples revealed a high degree of uniformity and almost fully dense microstructures that consisted of many small, elongated grains homogeneously dispersed in the fracture surfaces, which had properties comparable with those of other SiAlONs obtained using hot pressing.     

纳米复相结构陶瓷的原位反应合成

简要介绍了原位反应合成纳米复相陶瓷的原理及其可行性，认为此方法是制备纳米复相结构陶瓷的有效方法. 同时讨论了氮化硼(h-BN)复相陶瓷的特点及其性能改善的关键因素，认为采用传统方法难以获得高性能的BN复相陶瓷. 提出了一系列原位化学反应，并采用热压或无压烧结获得了细小而均匀分散的非氧化物?氮化硼(Nobn)复相陶瓷，纳米级的BN片晶主要分布在基体晶粒的晶界处，当BN体积含量适当时即可获得一种全新的部分弱晶界陶瓷复合材料(PWICs)，这种材料具有很好的综合力学性能.     

Asynchronous densification of zirconia ceramics formed by stereolithographic additive manufacturing

Stereolithography has been proven as a feasible approach to make crack-free ceramic macrostructure with customized designs, but the microstructure, especially pore structure remains to be tailored more precisely for better performance, where the sintering protocol and related densification characteristics could play a vital role as the slurry preparation and debinding protocol do. Herein we report a phenomenon named “asynchronous densification”, that is, the surface region of zirconia ceramics formed by stereolithographic additive manufacturing would be densified prior to the bulk at 1200°C during the conventional pressureless sintering in air. The cause of this asynchronism is unclear but supposed to be correlated with low packing density, high sintering activity, poor thermal conduction of ceramics and impurities. Early densification of the surface may have negative effects towards ceramic components with more homogeneous microstructure, suppressed pore coalescence and limited grain growth, and therefore needs to be better controlled through optimization in sintering protocol.     

等离子活化烧结制备SiC/h-BN复相陶瓷

采用等离子活化烧结(plasma activated sintering,PAS)制备SiC/20%(体积分数)h-BN复相陶瓷,研究了烧结工艺对复相陶瓷密度、抗弯强度、硬度,以及显微结构的影响,并对比分析了PAS与热压(hot-pressing,HP)烧结工艺不同烧结机理。结果表明:在1600℃保温3min PAS烧结与在1850℃保温1h HP烧结制备出的SiC/20% h-BN复相陶瓷具有相近的性能和微观结构,PAS烧结效率远高于HP。当引入20%微米级h-BN在烧结过程中抑制SiC晶粒长大,PAS快速烧结细化晶粒的效应在烧结SiC/20% h-BN复相陶瓷时被抑制。