两段式无压烧结制备纳米二氧化锆(3Y)材料

研究了两段式无压烧结制备纳米二氧化锆(ZrO_2)(3Y)陶瓷材料过程中晶粒生长与致密化的协同控制。将粒度为30nm的商品纳米ZrO_2(3Y)粉体进行冷等压成型制成相对密度为49%的素坯,然后将坯体加热至1 250℃以获得94%的相对密度,后降温至1 050℃保温20 h,制得相对密度大于99%晶粒尺寸为100 nm左右的ZrO_2(3Y)。研究表明,在烧结过程中利用晶粒生长与气孔收缩的活化能的差异以协调晶粒生长与致密化的关系,找出晶界迁移与晶界扩散、晶格扩散协同作用区域,在该区域晶粒生长受到抑制,而致密化得以维持,从而实现晶粒生长的抑制和坯体的完全致密化是两段式无压烧结的关键所在。     

含有烧结助剂的复相陶瓷材料烧结过程的元胞自动机模拟

基于晶界能和晶界曲率的晶粒生长驱动力理论,建立了含有烧结助剂的复相陶瓷晶粒生长的元胞自动机模型并进行了模拟。结果表明,烧结助剂对晶界有着强烈的钉扎作用,其晶粒生长指数小于未含烧结助剂时的生长指数。模拟结果与制备的含有烧结助剂的Al2O3／TiN复相陶瓷材料微观形貌组织吻合,表明所建立的模型适用于含有烧结助剂的陶瓷材料烧...     

先进陶瓷材料固相烧结理论研究进展

系统介绍了陶瓷材料固相烧结理论的历史和研究进展,综述了用来描述烧结前期、中期和后期的烧结理论和烧结模型.目前烧结理论大多局限于烧结全程的某一阶段,且只研究某一种扩散机制起主导作用,多数理论烧结模型不能完全反映真实烧结参数,烧结单元模型的定量描述不够完善,缺乏描述烧结全程的烧结模型,且大多研究局限于基础研究,如物质的传输机制、致密化过程、气孔和晶粒生长机制.因此,建立多种扩散机制耦合作用的全期烧结模型,进一步研究烧结动力学,用计算机模拟烧结的真实条件,建立能定量描述的烧结模型,是未来烧结理论研究的方向.     

Si-Al-Zr-O系超微细晶复相陶瓷研究

回顾了目前Si-Al-Zr-O系超微细晶复相陶瓷的主要研究方法并分析了这些方法的特点.针对应用超微细粉致密化制备Si-Al-Zr-O系陶瓷材料存在的烧结温度高(一般高于1500℃),致密化难,增韧相种类有限且成本高等问题,提出了一种新的制备方法——非晶原位受控晶化法,在较低温度1100~1200℃下制得均匀、致密、高可靠性的Si-Al-Zr-O系超微细晶复相陶瓷.该方法能有效避免高温烧结以及烧结过程中纳米粉末的团聚和晶粒异常长大,实现显微结构的有效控制,是一种具有发展前景的制备技术.     

固相烧结后期晶粒和气孔拓扑生长演化的二维相场模拟

本文采用二维相场模型模拟固相陶瓷烧结后期，晶粒和气孔耦合生长演化过程。以连续变化的组分参量和长程序参量（LRO)表征微观烧结体的组分相和晶相扩散拓扑结构，由Cahn-Hillard(CH)方程和Ginzburg-Laudau(TDGL)方程分别控制组分场和取向场时间相关的扩散演化，并且利用半隐傅立叶频域法高效地模拟了烧结后期晶粒和气孔拓扑形貌的扩散演化，并且利用半隐傅立叶频域法高效地模拟了烧结后晶粒和气孔拓扑形貌的演化过程和统计分析生长参数，并通过与完全致密体晶粒生长速率比较，量化分析残余气孔对昌粒生长的影响。     

先进陶瓷材料烧结新技术研究进展

先进陶瓷材料作为工程材料和功能材料的重要组成部分,在新能源、通信电子、半导体、航空航天等工业领域具有广阔的应用前景。但是由于陶瓷粉体多为离子键或共价键化合物,采用传统烧结工艺制备致密陶瓷材料所需的烧结温度较高,保温时间较长,不可避免地会导致晶粒粗化及气孔残留,进而影响陶瓷材料的各项性能。为了降低烧结温度、缩短烧结时间、提高烧结致密度与材料性能,各国研究人员先后开发了多种新型烧结技术,包括放电等离子烧结(spark plasma sintering,SPS)、闪烧(flash sintering,FS)、冷烧结(cold sintering,CS)以及振荡压力烧结(oscillatory pressure sintering,OPS)等。利用上述烧结技术,可显著降低陶瓷材料的烧结温度和烧结时间,提升材料的各项性能,从而使陶瓷材料的应用范围得以扩展。从理论及应用两方面综述了先进陶瓷材料烧结新技术的研究进展,阐述了烧结新技术在高性能陶瓷材料制备过程中的技术优势和应用前景,期望为陶瓷烧结新技术的研究、开发及应用提供参考。     

钛酸钡系PTCR半导体陶瓷烧结过程的计算机模拟 Ⅱ.烧结中后期及奥氏熟化过程中的晶粒生长

对Monte-Carlo计算机模拟法的Q－States Potts模型进行了修正,用于模拟BaTiO3系PTCR半导体陶瓷烧结中后期的晶粒生长和奥氏熟以及液相性质对晶粒生长的影响。模拟结果表明：在含有液相的二相系统,钛酸钡半导瓷样品在1240℃附近体积收缩率基本上达到最大值,以后的保温阶段是在液相控制下的晶粒生长和致密化过程,这和大多数实验结果是一致的。     

掺CeO2的MgO-CaO材料缺陷研究

以分析纯Ca(OH)2和Mg(OH)2为原料,CeO2为掺杂剂,利用行星球磨机混合后压制成型,经1650℃烧成,保温3h,制备了MgO-CaO材料。通过XRD和SEM对试样进行了分析表征。结果表明,随CeO2掺入量的增大,CaO的晶格常数先增大后减小,试样的体积密度不断增大,显气孔率减小,致密化程度增强;在烧结致密化过程中,MgO晶粒长大比CaO更为明显,试样晶粒间的气孔逐渐减少,但MgO晶粒中的气孔逐渐增多,而CaO晶粒中未有气孔出现;CeO2促进材料致密的实质是与CaO发生置换固溶,Ce4+进入CaO晶格内,增大了Ca2+空位浓度,有利于Ca2+的扩散。     

两种通电快速烧结方法制备超细晶粒纯钨

采用在超高压力下通电快速烧结新方法在不添加任何烧结助剂的条件下制备出相对密度为97.9%、晶粒尺寸小于1 μm的超细晶粒纯钨块体材料,研究了细钨粉块体的致密化行为.在超高压力下通电烧结过程中,超高压力使烧结样品具有高密度,而样品的力学性能则主要得益于通电烧结.与"放电等离子体烧结"方法相比,超高压力下通电烧结不但能更好的保持材料的原始晶粒尺寸,还能进一步细化晶粒.     

微波烧结氧化锌压敏电阻的致密化和晶粒生长

研究了微波烧结的ZnO压敏电阻的致密化和生长动力学, 微波烧结温度从900～1200℃, 保温时间从20min～2h. 研究表明, 微波烧结ZnO压敏电阻的物相组成和传统烧结的样品没有区别; 微波烧结有助于样品的致密化, 并降低致密化温度. 随着烧结温度的升高, 致密化和反致密化作用共同影响样品的密度, 其中Bi的挥发是主要影响因素. 微波烧结ZnO压敏电阻的晶粒生长动力学指数为2.9～3.4, 生长激活能为225kJ/mol, 传统烧结的ZnO压敏电阻的晶粒生长动力学指数为3.6～4.2, 生长激活能为363kJ/mol. 液相Bi₂O₃、尖晶石相和微波的“非热效应”是影响微波烧结ZnO压敏电阻陶瓷晶粒生长的主要因素.     

Three dimensional simulation of microstructure evolution for ceramic tool materials

The observation and scientific quantitative characterization of three dimensional microstructure evolution during sintering process of ceramic tool materials is important to investigate the influence of nano-particles on mechanical properties. The relationship between microstructure and mechanical properties of ceramic tool materials can be established to direct the development of nano-composite ceramic tool materials by the research of the grain growth, grain boundary migration, distribution of nano-particles and microstructure densification at the different sintering temperature and pressure. In this paper, a 3D Monte Carlo model of three-phase nano-composite ceramic tool material is built and applied to simulate the microstructure evolution during sintering process. In this model, the grain boundary energy of each phase and interfacial energy between two phases are taken into consideration as the driving forces for grain growth. The sintering temperature and pressure are successfully coupled into the Monte Carlo simulation model. The microstructure evolution of defect free three-phase nano-composite ceramic tool materials is successfully simulated at different sintering temperature and pressure. The simulation results show that the higher the sintering temperature is, the faster the grain growth. However, the sintering pressure has little effect on the grain growth.     

Reaction sintering fabrication of (AlN, TiN)-Al2O3 composite

The (AlN, TiN)-Al₂O₃ composites were fabricated by reaction sintering powder mixtures containing 10-30 wt.% (Al, Ti)-Al₂O₃ at 1420-1520°C in nitrogen. It was found that the densification and mechanical properties of the sintered composites depended strongly on the Al, Ti contents of the starting powder and hot pressing parameters. Reaction sintering 20 wt.% (Al, Ti)-Al₂O₃ powder in nitrogen in 1520°C for 30 min yields (AlN, TiN)-Al₂O₃ composites with the best mechanical properties, with a hardness HRA of 94.1, bending strength of 687 MPa, and fracture toughness of 6.5 MPa m^1/2. Microstructure analysis indicated that TiN is present as well dispersed particulates within a matrix of Al₂O₃. The AlN identified by XRD was not directly observed, but probably resides at the Al₂O₃ grain boundary. The fracture mode of these composites was observed to be transgranular.     

The effect of polysiloxane on the properties of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-NbC composite material produced by pyrolysis process

Recently, studies have been developed in order to obtain Al₂O₃-NbC composite materials. The reinforced materials have shown good potential to be used as cutting tool materials at high-speed cutting and high temperature as a substitute to WC-Co material. The main disadvantage to produce these alumina-reinforced materials is the necessity to use pressure assisted sintering or high sintering temperatures to produce dense bodies. Manufacturing of composite ceramic materials derived from polymer reactive filler has been intensively investigated. Polymer pyrolysis is a relatively new and very promising method for obtaining ceramic material in complex shapes and lower sintering temperatures. This work investigated a ceramic composite matrix based in SiC_xO_y and Al₂O₃ and reinforced with NbC obtained by means of the active fillers pyrolysis process. The results obtained in this work demonstrate that using a mixture of polysiloxanes produces a composite material with better properties when compared to others polymer materials.     

Mit TiN-Partikeln Lasermodifizierte Al2O3-Keramik im reversierenden Gleitkontakt unter Variation der Luftfeuchte und Temperatur

With TiN particles lasermodified Al₂O₃ ceramic under oscillating sliding contact at different humidities and temperatures A slightly porous, commercially available alumina ceramic was surface modified up to 170 μm thickness by adding TiN particles. The multiphase surface structure of this laser treated ceramic consisted of about 12 vol.% TiN, 16 vol.% grain boundary phase and 72 vol.% Al₂O₃. Tribological tests on the modified ceramic and for reference also on a highly dense alumina and titania were carried out in oscillating sliding contact against Al₂O₃ balls. In these tests, the temperature of the specimens was varied between 28°C and 500°C. At room temperature the relative humidity of the surrounding air was changed between 3% and 70% and additional tests were run by using distilled water as interfacial medium. The resulting multiphase microstructure showed substantially reduced friction and wear at different temperatures and also above relative humidities of about 35% at room temperature compared with the highly dense, commercially available alumina.     

Electron microscopy characterization of hot-pressed Al substituted Li7La3Zr2O12

Hot-pressing was used to prepare a dense (97% relative density) cubic Al substituted Li₇La₃Zr₂O₁₂ material at temperatures lower than typically used for solid-state and/or liquid phase sintering. Electron microscopy analysis revealed equiaxed grains, grain boundaries, and triple junctions free of amorphous and second phases and no Al segregation at grain boundaries. These results suggest that Al₂O₃ and/or Al cannot act as a sintering aid by reducing grain boundary mobility. If Al₂O₃ acts as a sintering aid its main function is to enter the lattice as Al to increase the point defect concentration of the slowest moving species.     

Pressureless-sintering behavior of nanocrystalline ZrO2–Y2O3–Al2O3 system

ZrO₂–Y₂O₃–Al₂O₃ nanocrystalline powders have been synthesized using chemical coprecipitation method. Nano-powders were compacted uniaxially and densified in a muffle furnace. Densification studies showed that a fully dense pellet of ZrO₂(3Y) and a 99% relative density for 5 mol% Al₂O₃ doped ZrO₂(3Y) were obtained after sintering at 1200 °C. The presence of Al₂O₃ inhibits grain growth and suppresses the densification process. Full densification and the maximum microhardness of 17.8 GPa were achieved for the ZrO₂(3Y)/5 mol% Al₂O₃ composites sintered at 1250 °C.     

Sintering mechanism of fine zirconia powders with alumina added by powder mixing and chemical processes

The present study examined the sintering behavior of fine zirconia powders (containing 2.9 mol% Y₂O₃) with and without small amounts of Al₂O₃ added by different ways: powder mixing (PM), homogeneous precipitation (HP), and hydrolysis of alkoxide (HA). The initial sintering behavior was examined by both measurement methods of a constant rate of heating and isothermal shrinkage. In the PM process, Al₂O₃ particles pinned the shrinkage of zirconia powder compact at the initial stage and diffuse toward zirconia surface to enhance the sintering. This initial sintering mechanism was explained by the grain-boundary diffusion for the Al₂O₃-free powder and the volume diffusion for Al₂O₃-added powder. When Al₂O₃ was added to zirconia powder by HP and HA processes, the densification rate was more stimulated compared to the PM process. The initial sintering mechanism did not change by the way for Al₂O₃ addition. The Al₂O₃ addition by chemical processes of HP and HA tended to enhance the grain growth of zirconia, while the uniform microstructure was achieved because of homogeneous addition of Al₂O₃ by those processes.     

Fabrication of Al2O3–ZrB2 in situ composite by SHS dynamic compaction: A novel approach

Al₂O₃–ZrB₂ in situ composites of 97% of theoretical density were successfully fabricated by a novel self-propagating high temperature synthesis (SHS) dynamic compaction, using less expensive raw materials zirconium oxide, boron oxide, and aluminium. The process is fast, energy efficient, where no furnace sintering is required. The process inhibits and controls the grain growth and microstructure. The densification behaviour and correlation with microstructure of the SHS dynamic compacts were compared with the furnace sintered composite samples where the composite powder was prepared by SHS process. The furnace sintered samples showed coarser grain growth and maximum density of 94.5% of theoretical density was achieved. The SHS dynamic compacted in situ composite had much finer grains in the range of 0.5–3 μm with density 95.5% of the theoretical value. The average grain size was found to decrease from 10 μm to 1.4 μm for alumina and from 5.4 μm to 1.0 μm for zirconium diboride from furnace sintering to SHS dynamic compaction, respectively. Addition of Al₂O₃ as a diluent during SHS reaction enhanced the density to 97%. During SHS dynamic compaction, the amount of liquid and the time interval at which the sample stays at high temperature are the controlling factor of the final microstructure and the densification of the composite.     

Fabrication and microstructure of in situ toughened Al2O3/Fe3Al

Fe₃Al nano-particles and commercial purity Al₂O₃ powders were used as raw materials to fabricate in situ reinforced Al₂O₃/Fe₃Al nano/micro-composites. Densification and microstructure were studied. The Al₂O₃ matrix grains were characterized by platelet grains. The Fe₃Al particles inhibited the grain growth of Al₂O₃ grains and limited the densification of the composites. In Al₂O₃/Fe₃Al composites, the Fe₃Al particles were uniformly dispersed in the Al₂O₃ matrix. The major Fe₃Al micro-particles, about 1 μm in average size, existed at Al₂O₃ grain boundaries, and the Fe₃Al nano-particles were found embedded in the matrix grains. The grain size of the intragranular particles ranged from several to several hundred nanometers. The grain size and aspect ratio of Al₂O₃ platelet grains and distribution of intragranular Fe₃Al could be optimized by controlling the Fe₃Al contents and sintering process. The in situ formed Al₂O₃ platelet grains, as well as Fe₃Al dispersoids, were beneficial to the increase of the mechanical properties of alumina.