陶瓷烧结新技术—微波烧结

本文综述了微波烧结陶瓷材料这一新技术在国内外的研究，应用和进展，并介绍和指出了微波烧结的原理，特点及其未来展望。     

反应烧结氮化硅的重烧结

本文采用添加 Y_2O_3 和 Al_2O_3 制备的反应烧结氮化硅(RBSN)作为前驱材料,在 1 atm 氮气中进行高温重烧结,可制得致密的氮化硅工程陶瓷。研究结果如下:(1)根据重烧结过程中的线收缩,X 射线衍射分析和微观结构的研究结果,认为反应烧结氮化硅的高温重烧结受液相烧结机理控制,同时在烧结的开始阶段有明显的诱导期存在;(2)由填料所产生的 SiO 气氛对控制氮化硅的热分解是必不可少的;(3)由于重烧结工艺使制品收缩小(<6.5％)、抗弯强度高(550～668MN/m~2)、Weibull模数大(28),因此它是一种制造氮化硅工程陶瓷的好方法。     

铝土矿的烧结与均化烧结

采用荧光-能谱分析和场发射扫描电镜研究了4种D-K型铝土矿及其烧结块料的化学-矿物组成和显微结构,并对比分析了4种市售均化料的显微结构。结果表明:致密的水铝石质原料可获得致密化烧结,少量结构疏松的水铝石质原料不易获得致密化烧结;由水铝石和高岭石两矿物构成的铝土矿结构复杂,其中的致密、均匀者可获得均匀的刚玉-莫来石组合;有些烧结料的相分布不均匀,或疏松或玻璃相胶结刚玉。均化烧结料中局部富集的少量SiO2和杂质被均化并生成液相,致使刚玉晶体被液相胶结,从而降低了固-固相结合率,反映在宏观性质上却是显气孔率和吸水率极低而体积密度并不高,这是因为形成了较多的封闭式气孔。相比之下,A l2O3含量较低的刚玉-莫来石两相组合的均化料,既可实现均化又能获得含玻璃相很少的致密料。     

烧结氮化硼

本发明涉及烧结氮化硼制品氮化硼粉末可被冷压,但冷压材体无论在真空或是保护气体条件下都不能在烧结炉中高温固化成抗高温的材料。这是由于超过1500℃,B_2O_3或Ca_3(PO_4)_2结合剂(加量直至30％)都不能有效的使冷压烧结材料具有可为的机械强度,材料变     

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.     

烧结助剂对碳化硅烧结性能的影响

1　前言碳化硅具有稳定、耐腐蚀、强度高、导热性能好、热膨胀系数小的特点 ,作为高温结构陶瓷可广泛应用于耐热、耐磨以及使用环境苛刻的部位。通常碳化硅材料需要在很高的温度下 ,外加一定的压力 ,具有保护气氛的条件才能达到烧结。这不仅使产品成本偏高 ,同时限制了其应用的范围。为此在一系列显微结构设计的基础上 ,通过向碳化硅粉料中加入适宜的烧结助剂 ( 1 0 % )和防氧化剂 ,使ＳｉＣ在常压、无保护气氛的条件下经1 40 0℃ ,3ｈ即可达到烧结。2　实验过程本实验所用主要原料为纯度 99%的碳化硅粉料 (唐山碳化硅厂生产 )、纯度 99%Ａ…     

玻璃陶瓷烧结体的烧结性质研究

通过差热分析和扫描电镜分析了不同玻璃组分的烧结和结晶过程，发现不同的氧化物添加剂对玻璃性质影响很大。添加剂种类、升温速率、玻璃粘度、颗粒大小、压力条件等都影响玻璃陶瓷烧结体的性质。     

金属催化剂烧结机制及抗烧结策略

高温反应环境下金属催化剂易发生烧结，从而导致其活性降低甚至失活。因此，提高其热稳定性是多相催化的重大挑战。本文综述了金属催化剂以颗粒迁移和Ostwald熟化为主的两种烧结机制，整理了通过颗粒粒径分布、颗粒生长动力学、原位透射电镜观测、实验与计算预测四种判断烧结机制的方法；指出温度、化学势、催化剂自身物性是影响烧结的主要因素。其中，温度影响金属颗粒的动能，是引起烧结的主要物理因素；化学势大小受金属与载体间相互作用影响，是影响烧结的化学因素之一。同时围绕金属-载体相互作用、空间限域及其他新颖的抗烧结策略，总结了近年来在提高催化剂抗烧结性能方面的研究进展。最后从催化剂制备、结构分析和性能测试方面，提出了基于抗烧结金属催化剂研究及构建的发展方向。     

反应烧结碳化硅陶瓷的制备及烧结机理

用溶胶-凝胶法合成的无机／有机杂化材料结合SiC+C混合粉料制成了反应烧结碳化硅陶瓷素坯,并对由这种素坯制成的碳化硅陶瓷进行了物相鉴定和显微结构观察;借助Si-C相图对反应烧结碳化硅的烧结机理进行了研究,分析表明其主要烧结机理为溶解-再沉淀型。     

烧结复合式烧结金属丝网颗粒移动床过滤器研究

A new gas clean-up process called “integrated sintered metal screen moving granular bed“ (ISMSMGB) for the integrated gasification combined cycle (IGCC) and pressured fluidized bed combustion (PFBC) was developed on the basis of a sintered metal candle filter and a cross-flow moving granular bed filter. This is a combination of the surface and deep bed filtering processes. A set of facilities was established and a series of cold model tests were carried out. The dust removal efficiency and the pressure drop of the filter were measured and analyzed. The results show that this process features the advantages of the moving bed for high capacity as well as high inlet dust load and the surface filter for high efficiency. Meanwhile, the granules moving downward cleans the cake on the screen surface, so that the system is operated at steady state.     

Sintering of Particulate Composites under a Uniaxial Stress

The sintering of particulate composites consisting of a finegrained zinc oxide matrix and <10 vol% of coarse silicon carbide inclusions was investigated at 725°C under a uniaxial stress of ⋍250 kPa. Data for the densification and creep rates of the composite were compared with those for the unreinforced matrix. The inclusions cause a drastic reduction in the measured densification and creep rates. They have a greater effect on the densification process, however; the ratio of the densification rate to the creep rate of the composite is approximately half that of the unreinforced compact. The factors that cause the reduction in this ratio are present from the very early stage of sintering.     

Pressureless Sintering of a Ceramic Matrix with Multiple Rigid Inclusions: Finite Element Model

Interaction among the dispersed second phase of rigid inclusions in a ceramic matrix undergoing densification is investigated by a viscoelastic finite element method with a two-dimensional multiple-inclusion model. The interaction is negligible when the volume fraction of inclusions is low (<20%) and/or when the inclusions are uniformly distributed. The interaction becomes significant when the volume fraction of inclusions is high or the inclusions are agglomerated. The densification rates in the region between the inclusions are enhanced to some degree when the inclusions are closely packed. This enhancement will, however, cease when the inclusions get much closer, eventually coming in contact with each other. In general, the agglomeration or close packing of inclusions results in retardation in the overall densification of the sintering matrix.     

Effects of Shape and Size of Inclusions on the Sintering of ZnO—ZrO2 Composites

The densification behavior and microstructure development of ZnO matrices containing rigid Zro₂ inclusions were studied, with special emphasis on the effect of inclusion shape and size. The inclusions retarded the densification of the matrix, regardless of the inclusion shape and size. For large inclusions with diameters of 10 μm, dense regions developed between inclusion particles. The inclusion particles and dense regions formed a continuous network, which constrained the densification of the composites. The inclusion shape had a small effect on the development of dense regions. Severe retardation in densification was observed for compacts containing inclusions with diameters of 10 μm. In these cases, dense regions between inclusion particles did not develop. The formation of the continuous network cannot be applicable to small inclusions as an origin of retardation of densification.     

Reaction behavior during Spark Plasma Sintering of cBN-TiCN-Ni powders

Polycrystalline cubic boron nitride (PcBN) is the designation given to composites constituted by cBN hard particles within a ceramic/metallic matrix. These composites are normally processed in severe pressure and temperature conditions to achieve full densification and prevent hexagonal BN formation, which would decrease the composite hardness. In this work, the Spark Plasma Sintering (SPS) technique was investigated as an alternative to sinter cBN-TiCN based composites, with and without addition of metallic Ni. The initial compositions were selected according with calculated phase diagrams, using the Thermo-calc software. The thermal behavior during SPS was studied up to 2000°C, namely the densification, reactivity and phase transformations. A larger densification was achieved with Ni addition, but full removal of open porosity was only possible at 1700 °C, where the cBN phase transformation to hBN completely occurred. In agreement with the thermodynamic calculations, other matrix phases, as TiB₂ and Ni₃B, were formed during sintering.     

无压烧结硼化锆基ZrB2-SiC复相陶瓷的结构与性能

以钇铝石榴石-YAG为烧结助剂,通过无压烧结制备了ZrB2-SiC复相陶瓷。研究了烧结助剂含量对烧结材料力学性能和显微结构的影响,材料的显微结构由扫描电镜SEM及其能谱分析EDS测定。研究结果表明,烧结助剂(YAG)和原料中的杂质形成玻璃相填充在晶界上,显著促进了硼化锆基ZrB2-SiC复相陶瓷的致密化。     

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.     

Sintering with Rigid Inclusions

Rigid inclusions retard the densification of a sintering body by creating a hydrostatic tensile stress in the matrix. Two models of this process are presented and compared with others from the literature. The composite can be represented by a composite sphere, with the core representing the inclusion. Alternatively, a self-consistent (s-c) calculation can be performed in which the sintering material is regarded as being surrounded by a composite matrix with a slower densification rate. The results differ only in that the shear modulus of the matrix is replaced by the shear modulus of the composite in the s-c calculation. The stress in the inclusion cannot exceed twice the sintering pressure, unless the Poisson's ratio of the matrix is negative. Predictions of higher stresses by previous authors carry the erroneous implication that Poisson's ratio is negative. Since the predicted stresses are not large, models of this type cannot account for the experimentally observed retardation of densification of polycrystalline matrices by inclusions.     

Effect of Inclusions on Densification: I, Microstructural Development in an Al2O3 Matrix Containing a High Volume Fraction of ZrO2 Inclusions

The microstructural changes produced by large (38 to 53 μ m), single-crystal ZrO₂ inclusions (0, 0.09, 0.30 volume fractions, based on solid volume) within an Al₂O₃ powder matrix were detailed as a function of constrained densification. Composite powder compacts were produced by pressure filtration for conditions where the Al₂O₃ slurry was either flocced or dispersed. For both conditions, the ZrO₂ inclusions constrained densification. Microstructural observations for all composites revealed (1) the presence of cracks with large opening displacements between inclusions and (2) large density variations within the matrix. The cracks were most frequent at high volume fraction of inclusions in composites produced from flocced slurries and apparently originated during specimen preparation. Their large opening displacment was a result of matrix densification. Fewer cracks were observed in composites produced from dispersed slurries. Instead, these microstructures were dominated by large variations in matrix density, viz., dense regions surrounding low-density regions, not consitent with the initial packing density of the matrix powder. The denser regions were formed early in the densification schedule. The lower-density regions eventually developed into regions containing large, elongated voids as the Al₂O₃ matrix grains became larger with heat-treatment time. This pore enlargement process was shown to result from the disappearance of necks between originally sintered grains and appeared similar to the thermodynamic instability observed in thin films and constrained fibers.     

Sintering behavior and mechanical properties of nano-sized Cr3C2/Al2O3 composites prepared by MOCVI process

A process using metal-organic chemical vapor infiltration (MOCVI) conducted in fluidized bed was employed for the preparation of nano-sized ceramic composites. The Cr-species was infiltrated into Al₂O₃ granules by the pyrolysis of chromium carbonyl (Cr(CO)₆) at 300–450 °C. The granulated powder was pressureless sintered or hot-pressed to achieve high density. The results showed that the dominant factors influencing the Cr-carbide phases formation, either Cr₃C₂ or Cr₇C₃, in the composite powders during the sintering process were the temperature and oxygen partial pressure in the furnace. The coated Cr-phase either in agglomerated or dispersive condition was controlled by the use of colloidal dispersion. The microstructures showed that fine (20 –600 nm) Cr_xC_y grains (≤8 vol.%) located at Al₂O₃ grain boundaries hardly retarded the densification of Al₂O₃ matrix in sintering process. The tests on hardness, strength and toughness appeared that the composites with the inclusions (Cr₃C₂) had gained the advantages over those by the rule of mixture. Even 8 vol.% ultrafine inclusions have greatly improved the mechanical properties. The strengthening and toughening mechanisms of the composites were due to grain-size reduction, homogenous dispersion of hard inclusions, and crack deflection.     

Factors Controlling the Sintering of Ceramic Particulate Composites: II, Coated Inclusion Particles

Composite powders were synthesized by coating coarse ZrO₂ inclusion particles with a cladding of fine-grained, crystalline ZnO powder using a chemical precipitation technique. Three different inclusion sizes (1, 3, and 14 μm) were used by selecting the size of the starting ZrO₂ powder, and the volume fraction of the inclusions was controlled by the amount of ZnO precipitated. The powders were compacted by uniaxial pressing in a die and then sintered at a constant heating rate of 4°C/min to 1500°C. The sintering kinetics were almost independent of the inclusion volume fraction, and of the inclusion size, for inclusion contents up to ∼40 vol%. Furthermore, composites containing up to ∼40 vol% inclusions were sintered to almost full density under the same conditions used for the unreinforced matrix. This is considerably better than the densities obtained for conventionally mixed powders, where a modest inclusion content (< ∼ 10 vol%) has been observed to cause a severe reduction in the sintered density of the composite matrix. The kinetic data and microstructural observations are a further indication that the main factors which oppose the free sintering of ceramic particulate composites are processing-related; these factors are (i) inclusion-inclusion interactions which constrain the matrix and (ii) the packing of the matrix phase in regions immediately surrounding the inclusions.