Microwave Sintering of Partially Stabilized Zirconia

Partially stabilized zirconia has been rapidly sintered in a microwave apparatus to good final densities (95% to 98% of theoretical density). The grain size was not noticeably different from that of conventionally sintered material. Properties of the sintered material are given.     

Nanosize Powder of Zirconia. Explosive Method of Production and Properties

A powder of zirconia with a typical particle size of 5–6 nm is synthesized by an explosive method. It is shown that the stabilization of the hightemperature cubic modification under standard conditions is due to the small particle size. The sequence of phase transformations is studied. The phasetransition temperatures are found to be twice those for the standard powders. It is suggested that release of a considerable amount of heat during transition of the amorphous phase to the crystalline state plays a determining role in the stabilization of the size of the primary gel particles in the explosive synthesis.     

氧化锆活性对烧结性能的影响

分别以化学法和电熔法制备的氧化锆及二者混合共磨粉为原料,通过造粒、成型、干燥、烧成制得样品,对烧成样品物理性能和微观结构进行分析,研究原料活性对烧结性能的影响.研究结果表明:在加入同样稳定剂的条件下,电熔氧化锆活性较差,烧结后气孔率较高,化学氧化锆烧结后收缩大,开裂严重,两者都无法满足作为制备氧化锆质定径水口基质原料要求,当二者的混合比例为1:1时,试样的烧结温度合适,气孔率、烧成收缩、耐压强度等各项理化指标较好,满足用作制备氧化锆质定径水口基质要求.     

Sintering Behavior of Fully Agglomerated Zirconia Compacts

Fully agglomerated superfine zirconia powders were prepared with the coprecipitation and spray-drying method. The compaction of such powders shows no fragmentation of the agglomerates. The sintering behavior of the compacts was studied and two sintering stages were identified: densification within agglomerates at temperatures not higher than 1250°C and the removal of interagglomerate pores at temperatures above 1600°C. The interagglomerate pores are difficult to remove, and sintering between agglomerates even at 1600°C is still insignificant. Heating of the compacts at temperatures above 1600°C leads only to grain growth and the entrapping of pores in large grains.     

Compaction and Pressureless Sintering of Zirconia Nanoparticles

Four nanometer-sized zirconia powders stabilized by 3 mol% Y₂O₃ were used for the preparation of dense bulk ceramics. Ceramic green bodies were prepared by cold isostatic pressing at pressures of 300–1000 MPa. The size of the pores in ceramic green bodies and their evolution during sintering were correlated with the characteristics of individual nanopowders and with the sintering behavior of powder compacts. Only homogeneous green bodies with pores of <10 nm could be sintered into dense bodies (>99% t.d.) at a sufficiently low temperature to keep the grain sizes in the range <100 nm. Powders with uniform particles 10 nm in size yielded green bodies of required microstructure. These nanoparticle compacts were sintered without pressure to give bodies (diameter 20 mm, thickness 4 mm) with a relative density higher than 99% and a grain size of about 85 nm (as determined by the linear intercept method).     

Two-Stage Sintering of Alumina with Submicrometer Grain Size

This work verifies the applicability of two-stage sintering as a means of suppressing the final stage grain growth of submicrometer alumina. The first heating step should be short at a relatively high-temperature (1400°–1450°C) in order to close porosity without significant grain growth. The second step at temperatures around 1150°C facilitates further densification with limited grain growth. Fine-grained alumina with a relative density of 98.8% and a grain size of 0.9 μm was prepared by two-stage sintering. A standard sintering process resulted in ceramics with identical relative density and a grain size of 1.6 μm.     

加压辅助闪烧工艺制备四方相氧化锆

研究了加压辅助闪烧烧结工艺参数(温度、电压、电流、压力)对钇稳氧化锆致密度、微观组织结构和成分组成的影响,该工艺在热压烧结基础上叠加闪烧效应,利用高电场强度使高温导电氧化锆瞬间发生密实化烧结.结果表明:钇稳氧化锆的闪烧临界温度为880℃,在相同的电场强度条件下,闪烧临界温度处烧结可获得最大的闪烧收缩量.氧化锆在临界烧结...     

反应烧结制备氧化锆增韧莫来石陶瓷

本文首先回顾了反应烧结的发展历史,在此基础上对描述反应烧结过程的相关参数、反应烧结的影响因素以及烧结体的显微结构特征进行了概述,并分析了反应烧结工艺的发展方向.     

Sintering of Zirconia Nanopowder by Microwave-Laser Hybrid Process

A new hybrid sintering process has been developed by replacing all but one laser by microwaves in the existing simultaneous multiple laser process (SIMPLE). Microwave energy has been used to preheat the material before laser radiation, and the synergism between microwave and laser energies could effectively heat the material to temperatures of 1700°C and beyond in just a few minutes. Using this process, rapid sintering of 3Y–ZrO₂ (3Y–TZP) pellets has been achieved in a few minutes. Microstructural investigations reveal that the microwave–laser hybrid sintered pellets of 3Y–ZrO₂ have nanograins averaging about 20 nm. The microwave–laser hybrid sintering process can clearly be a new approach for fabrication of nanoceramics and nanocomposites.     

两步烧结法制备纳米氧化钇稳定的四方氧化锆陶瓷

采用共沉淀法制备纳米氧化钇稳定的四方氧化锆（yttria stabilized tetragonal zirconia,3Y-TZP）粉体。利用X射线衍射、N2吸附–脱附等温线,透射电子显微镜对3Y-TZP粉体的物理性能和化学性能进行表征。研究了纳米3Y-TZP粉体的烧结曲线,分析了3Y-TZP素坯在烧结过程中的致密化行为和显微结构,探讨了两步烧结工艺对3Y-TZP纳米陶瓷微观结构的影响。结果表明：采用共沉淀法,在600℃煅烧2h后,可获得晶粒尺寸为13nm、晶型发育良好、团聚较少的纳米3Y-TZP粉体;采用两步烧结法,将素坯升温至1200℃保温1min后,再降温到1050℃保温35h,可获得相对密度大于98%,晶粒尺寸约为100nm的3Y-TZP陶瓷。两步烧结法通过控制煅烧温度和保温时间,利用晶界扩散及其迁移动力学之间的差异,使晶粒生长受到抑制,样品烧结致密化得以维持,实现在晶粒无显著生长前提下完成致密化。     

Zirconium Nitride Precipitation in Nominally Pure Yttria-Stabilized Zirconia

Nominally pure yttria-stabilized zirconia alloys are shown to contain unexpectedly large amounts of dissolved nitrogen. Its presence in the lattice was detected through the observation of large precipitates in alloys with three different concentrations of yttria deformed in compression in argon in the temperature range 1600°–1800°C. Electron diffraction, EDS and PEELS analyses, and Moiré imaging were used to identify the precipitates as ZrN. The possible origin of the nitrogen, its likely effects on properties, and the role of annealing atmosphere are briefly discussed.     

Estimate of the Activation Energies for Boundary Diffusion from Rate-Controlled Sintering of Pure Alumina, and Alumina Doped with Zirconia or Titania

Sintering experiments at constant heating rates were employed to estimate the activation energy for sintering in alumina and in alumina containing 5 vol% zirconia or 5 vol% titania. Grain growth, which can complicate the analysis of sintering kinetics data, was suppressed by using uniformly and densely packed grain compacts prepared by colloidal processing. Grain-boundary diffusion is believed to have been the dominant sintering mechanism. The activation energies were 440 ± 40 kJ/mol for pure alumina, 585 ± 40 kJ/mol for alumina (titania), and 730 ± 60 kJ/mol for alumina (zirconia). The alumina and alumina (titania) results are in agreement with the values reported in the literature. The possibility that the higher activation energies for doped alumina reflect a stronger bonding at alumina interfaces in the presence of zirconium and titanium is discussed.     

Effect of Phase Structure on Sintering Behavior of Zirconia Nanopowders

Zirconia nanopowder compacts with comparable particle sizes and pore size distributions but different phase structures were prepared. The sintering behavior of monoclinic, tetragonal, and cubic zirconia nanopowders was directly compared. The densification and microstructural changes during sintering were investigated. The tetragonal and cubic nanopowders showed similar sintering behavior whereas the monoclinic nanopowder exhibited a more difficult densification and coarser microstructure compared with tetragonal and cubic powders. The differences in the densification of zirconia nanopowders resulted from significant differences in the microstructure evolution during sintering. The microstructural changes in nanopowder compacts during sintering were described and a correlation between microstructural changes and interfacial energies associated with different crystal structures was discussed.     

纳米颗粒粒径对等离子喷涂法制备氧化锆纳米涂层的影响

利用扫描电镜(SEM)、场发射扫描电镜(FESEM)、透射电镜(TEM)、比表面积吸附法(BET)等分析测试技术,研究了3种不同纳米尺寸氧化锫粉粒的造粒性能、沉积效率以及对等离子喷涂涂层晶粒大小、涂层熔融性能、结合强度的影响.结果表明:纳米氧化锆粉体一次颗粒粒径大小显著影响纳米粉体的喷雾造粒性能、沉积效率、涂层表面粗糙度、涂层晶粒粒径和结合强度大小.本试验中,利用颗粒一次粒径范围为50～70 nm的纳米氧化锆粉体,等离子喷涂制备了晶粒粒径范围为80～120 nm,沉积效率为43%,涂层表面粗糙度为5.92 μm,结合强度为27 MPa的纳米结构氧化锆涂层.     

Agglomerate and Particle Size Effects on Sintering Yttria-Stabilized Zirconia

The initial-, intermediate-, and final-stage sintering of fine crystallite yttria-stabilized zirconia was studied. Experiments were conducted on powder lots of differing agglomerate size and one specially prepared agglomerate-free powder. Initial-stage sintering kinetics were compared with a sintering study on larger crystallite size calcia-stabilized zirconia to access the Herring scaling law. It was found that agglomerates limit attainable green density, interfere with the development of microstructure, impede initial-stage sintering kinetics, and limit the potential benefit of fine crystallites on final-stage sintering. An gglomerate free powder centrifuge-cast to 74% green density was sintered to 99.5% of theoretical density in a 1 h 1100°C cycle, which is ∼300°C lower than necessary for an agglomerated but equal crystallite size powder.     

Sintering Behavior of Nanocrystalline Zirconia Prepared by Chemical Vapor Synthesis

Powder synthesis and ceramic processing methods have to be improved to take full advantages of new, improved properties of nanocrystalline ceramics. Sintered nanocrystalline ceramics of pure, undoped zirconia are formed from nanocrystalline powder of optimized quality obtained by the chemical vapor synthesis (CVS) method. The as-synthesized CVS ZrO₂ powder is nonagglomerated with a crystallite size of about 5 nm, narrow size distribution, and high crystallinity. On uniaxial compaction a transparent green body of ultrafine, uniform microstructure and narrow pore size distribution corresponding to the grain size distribution is formed, which is sintered under vacuum at 950°C into a transparent, fully dense ZrO₂ ceramic with a grain size of 60 nm.     

Microwave Sintering of Zirconia Materials: Mechanical and Microstructural Properties

Commercially, 3 mol% Y₂O₃‐stabilized tetragonal zirconia (70–90 nm) compacts were fabricated using a conventional and a nonconventional sintering technique; microwave heating in a resonant mono‐mode cavity at 2.45 GHz, at temperatures in the 1100–1400°C range. A considerable difference in the densification behavior between conventional (CS) and microwave (MW) sintered materials was observed. The MW materials attain a full density of 99.9% of the theoretical density (t.d.) at 1400°C/10 min, whereas the CS reach only 98.0% t.d. at the same temperature and 1 h of dwelling time. Therefore, the MW materials exhibit superior Vickers hardness values (16.0 GPa) when compared with CS (13.4 GPa).     

Sintering Kinetics at Constant Rates of Heating: Mechanism of Silica-Enhanced Sintering of Fine Zirconia Powder

The sintering behavior of 3 mol% Y₂O₃-doped ZrO₂ powders with and without a small amount of SiO₂ was investigated to clarify the effect of SiO₂ addition on the initial sintering stage. The shrinkage behavior of a powder compact was measured under constant rates of heating (CRH). The sintering rate increased remarkably with the addition of a small amount of SiO₂. The apparent activation energy ( nQ ) and apparent frequency factor     , where n is the order depending on the diffusion mechanism, were estimated at the initial sintering stage by applying the sintering-rate equation to the CRH data. The diffusion mechanism changed from grain-boundary diffusion (GBD) to volume diffusions (VD) on SiO₂ addition, and both nQ and     increased with the GBD→VD change. It is, therefore, concluded that the sintering rate increases by SiO₂ addition because the increase in     rather than nQ is predominant.     

Microstructural Development during the Microwave Sintering of Yttria—Zirconia Ceramics

The microstructural development of 3 and 8 mol% yttria—zirconia ceramics during microwave sintering was studied and compared to conventionally heated samples. Microwave heating enhanced the densification processes occurring during constant-rate heating for both materials. No change was found in the grain size—density relationship for the 8 mol% yttria—zirconia. However, a small, but statistically significant shift favoring densification was found for the 3 mol% yttriazirconia. Differences in the responses of the two materials are not completely understood but may be due to the differences in the activation energy for grain growth and grain-boundary mobility.     

Activation Energy for the Sintering of Two-Phase Alumina/Zirconia Ceramics

In an earlier paper we reported measurements of the activation energy for sintering from constant-heating-rate experiments with alumina/5% zirconia. Here, results from a full range of compositions in this two-phase system are described. They show that the activation energy remains in the range 700 ± 100 kJ/mol when the composition changes from 5 to 95 vol% zirconia. In comparison, pure zirconia sinters with an activation energy of 615 ± 80 kJ/mol and pure alumina with the energy of 440 ± 45 kJ/mol. The addition of 2.8 mol% yttria to zirconia does not have a measurable effect on the activation energy. The grain size dependence of the sintering rate suggests boundary-diffusion-controlled sintering. These activation energies are phenomenologically correlated with the interfacial energies in alumina, zirconia, and two-phase alumina/zirconia, suggesting that the bonding at the interface influences diffusional transport.