Blocking Grain Boundaries in Yttria-Doped and Undoped Ceria Ceramics of High Purity

CeO₂ samples doped with 10, 1.0, and 0.1 mol% Y₂O₃ and undoped CeO₂ samples of high purity were studied by impedance spectroscopy at temperatures <800°C and under various oxygen partial pressures. According to microstructural investigations by SEM and analytical STEM (equipped with EDXS), the grain boundaries were free of any second phase, providing direct grain-to-grain contacts. An amorphous siliceous phase was detected at only a few triple junctions, if at all; as a result, its contribution to the grain-boundary resistance was negligible. Nevertheless, the specific grain-boundary conductivities were still 2–7 orders of magnitude lower than the bulk conductivities, depending on dopant concentration, temperature, and oxygen partial pressure. The charge carrier transport across the grain boundaries occurred only through the grain-to-grain contacts, whose properties were then determined by the space-charge layer. The space-charge potential in acceptor-doped CeO₂ was positive, causing the simultaneous depletion of oxygen vacancies and accumulation of electrons in the space-charge layer. The very low grain-boundary conductivities can be accounted for by the oxygen-vacancy depletion; the accumulation of electrons became evident in weakly doped and undoped CeO₂ at high temperatures and under low oxygen partial pressures.     

Supported Cu Catalysts with Yttria-Doped Ceria for Steam Reforming of Methanol

The steam reforming of methanol was studied over Cu/Al₂O₃ catalysts with the addition of yttria-doped ceria (YDC). The YDC-modified catalysts were prepared by impregnating a -Al₂O₃ support with Y and Ce then with Cu. The addition of YDC drastically enhanced the activity of Cu/Al₂O₃ in the methanol reforming reaction. The enhanced activity was attributed to the increase of Cu⁺ species by YDC in the methanol reforming environment. However, the addition of YDC decreased the copper dispersion. The Cu dispersion could be enhanced by adding chromium oxide. The addition of YDC and Cr where Al₂O₃ was first impregnated with Cr then with YDC showed the most pronounced enhancement of the catalyst activity. At reaction temperatures of 200250 °C, the CO concentration in the products was smaller than 0.1%.     

氮化硅及其微粉的制备

氮化硅陶瓷以其优异的性能被用来制作发动机元件、刀具、轴承等器件。本文主要介绍了氮化硅的性能、应用范围以及硅粉直接氮化法、SiO2还原氮化法、液相法、气相法等氮化硅微粉的制备方法。提出要批量生产Si3N4粉体，应从产品质量高、成本低和生产规模大等几个基本原则加以综合考虑。     

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.     

Sintering Behavior of Fine Aluminum Nitride Powder Synthesized from Aluminum Polynuclear Complexes

The sintering behavior of fine AIN powder synthesized from an aluminum polynuclear complex was investigated. The focus of this work was to investigate the densification behavior of the AIN powder with different particle sizes (specific surface area: 3.2–22.8 m²/g). The AIN powder was synthesized from basic aluminum chloride and glucose mixed in a water solution. This powder was divided into two groups: one with 2 wt% Y₂O₃ added as the sintering aid and the other without such an additive. The AIN powder investigated possessed favorable densification potential. The density of the AIN powder with a surface area of 16.6 m²/g and without additives attained theoretical density at 1700°C. Adding Y₂O₃ further decreased the sintering temperature required for full densification to 1600°C. It is speculated that low-temperature sintering of our fine AIN powder with Y₂O₃ proceeds in two steps: in the initial stage, sintering proceeds predominantly through interdiffusion between yttrium aluminates formed on the AIN powder surface; in the second stage, the densification may occur by the interdiffusion between solid phases formed by a reaction between the yttrium aluminates and AIN. To investigate the effect of oxygen on sintering, the content of oxygen in AIN powder was varied while the particle size was kept constant. In this study, the difference in surface oxygen content scarcely affected the sintering behavior of fine AIN powder.     

氧化铈粉体的颗粒度控制与分级

综述了氧化铈颗粒度、颗粒度分布以及颗粒分散性的各种影响因素。爆炸法合成的氧化铈经过离心分级,达到纳米级颗粒度。     

废镁铬砖细粉的加入对镁质浇注料烧结性的影响

将水泥回转窑烧成带废镁铬砖经处理后制成细粉料,用它代替镁质浇注料中的镁砂原料,研究了废镁铬砖细粉的加入对SiO2微粉结合镁质浇注料烧结性的影响.结果表明:加入适量的废镁铬砖细粉对于提高镁质浇注料的烧结程度和强度均有作用 ,但加入量不能太多.本实验条件下加入量为8%时,其烧结性能良好.对高温烧后试样的SEM及XRD分析可知,由于废镁铬砖细粉的加入而生成的尖晶石固溶体是促进镁质浇注料烧结、增加其结构致密性的原因.     

Mechanism of Alumina-Enhanced Sintering of Fine Zirconia Powder: Influence of Alumina Concentration on the Initial Stage Sintering

The isothermal shrinkage behavior of 2.9 mol% Y₂O₃-doped ZrO₂ powders with 0–1 mass% Al₂O₃ was investigated to clarify the effect of Al₂O₃ concentration on the initial sintering stage. The shrinkage of the powder compact was measured at constant temperatures in the range of 950°–1050°C. The Al₂O₃ addition increased the densification rate with increasing temperature. The values of apparent activation energy ( nQ ) and apparent frequency-factor term (β₀ ⁿ ), where n is the order depending on the diffusion mechanism, were estimated at the initial sintering stage by applying a sintering-rate equation to the isothermal shrinkage data. The diffusion mechanism changed from grain-boundary diffusion (GBD) to volume diffusion (VD) by Al₂O₃ addition and both nQ and β₀ ⁿ increased with increasing Al₂O₃ concentration. The kinetic analysis of the sintering mechanism suggested that the increase of densification rate by Al₂O₃ addition largely depends on the increase of β₀ ⁿ , that is, the increases of n with GBD→VD change and β₀ with an increase in Al₂O₃ content, although the nQ also increases with Al₂O₃ addition. This enhanced sintering mechanism is reasonably interpreted by the segregated dissolution of Al₂O₃ at ZrO₂ grain boundaries.     

Low-Temperature Sintering of Mullite/Yttria-Doped Zirconia Composites in the Mullite-Rich Region

Mullite/Y-TZP composites in the mullite-rich region, using powders of mullite prepared via the hydrazine method and a commercially available zirconia compound (ZrO₂(2Y)), have been fabricated by sintering for 3 h at 1450°C in air. The grain sizes of mullite and ZrO₂(2Y) are changed with increased ZrO₂(2Y) content; the former decreases from 0.25 µm to 0.13 µm, and the latter increases from 0.21 µm to 0.35 µm. High strength (780 MPa) and fracture toughness (5.4 MPam^1/2) are obtained in the 50/50 (vol%) mullite/ZrO₂(2Y) composite with 99.6% of theoretical density.     

Fabrication of Titanium Carbide Ceramics by High-Pressure Self-Combustion Sintering of Titanium Powder and Carbon Fiber

Titanium carbide ceramics have been fabricated from the reactants titanium powder and carbon fiber by the highpressure self-combustion sintering method (HPCS) under two pressure conditions (65 MPa and 3 GPa). Porous TiC with a density of about 50% of theoretical was obtained under a pressure of 65 MPa. A possible model accounting for the formation mechanism of TiC is proposed on the basis of observation of the microstructure of the products formed from the incomplete reaction. On the other hand, dense TiC (>95% of theoretical) was fabricated under 3 GPa. The mean grain size varied, depending on the mixing molar ratio of the reactants. The influence of differences in the carbon (powder and fiber forms) on the structure of TiC compacts is discussed.     

Fabrication of Transparent Lead Lanthanum Zirconate Titanate Ceramics from Fine Powders by Two-Stage Sintering

Transparent lead lanthanum zirconate titanate (PLZT) ceramics were fabricated from fine powders using an inexpensive two-stage sintering technique. The powders were prepared by hydrolysis from low-cost inorganic precursors. In the two-stage sintering method, uniaxially pressed green pellets were densified to nearly theoretical values in an oxygen gas atmosphere during the first-stage sintering, at 1000°C for 1 h, and then residual, free lead oxide in the pellets was removed by second-stage sintering at 1100°C for 12 h. Transparent ceramic with an average grain size of 1.6 μm and a porosity of 1.3% was obtained. The transparency and dielectric characteristics of the present samples were compared with those of hot-pressed samples: The study of the polarization–field hysteresis loops of the present samples yielded a remanent polarization of 6.8 μC/m² and a coercive field of 1.6 kV/cm. The low coercive field of PLZT ceramics could potentially reduce the driving voltage of electrooptic devices in many applications.     

Sintering Analysis of Undoped and Cobalt Oxide Doped Ceria Solid Solutions

Pure and CoO_{1– x} -doped Ce_0.8Gd_0.2O_1.9 (CGO) have been analyzed with constant heating rate dilatometry. Doping changes the grain boundary structure of CGO resulting in enhanced mass transport and in increased densification rates. Significant changes in activation energies have been detected. While pure CGO exhibits a significant amount of surface diffusion, cobalt oxide doping enhances grain boundary diffusion. The obtained activation energies are 4.70±0.32 eV for CGO and 6.24±0.32 eV for CoO_{1− x} -doped CGO. The higher activation energy of CoO_{1− x} -doped CGO is a result of the formation of intergranular films. It is suggested that sintering of CoO_{1− x} -doped CGO occurs by rearrangement and grain boundary diffusion whereas grain boundary and volume diffusion govern the densification of pure CGO.     

混合烧结法在玻璃陶瓷制备中的应用

概述了混合烧结法的分类、致密化机理、影响因素和在玻璃陶瓷制备中的应用.和传统玻璃陶瓷制备工艺相比,混合烧结法的特点在于玻璃陶瓷中的晶体是直接加入或是通过外加晶体和玻璃反应析出而不是从母相玻璃中直接析出,因而对母相玻璃组分要求不十分严格,在直接利用废玻璃制备玻璃陶瓷和复相玻璃陶瓷方面有着独特的优势.本文重点介绍了近年来该工艺在这方面的应用和进展.     

Sintering Kinetics at Isothermal Shrinkage: Effect of Specific Surface Area on the Initial Sintering Stage of Fine Zirconia Powder

The isothermal shrinkage behaviors of fine zirconia powders (containing 2.8–2.9 mol% Y₂O₃) with specific surface areas of about 6 and 16 m²/g were investigated to clarify the effect of specific surface area on the initial sintering stage. The shrinkage of powder compact was measured under constant temperatures in the range of 1000°–1100°C. The increase in specific surface area enhanced the densification rate with increasing temperature. The values of activation energy ( Q ) and frequency-factor term (β₀) of diffusion at initial sintering were estimated by applying the sintering-rate equation to the isothermal shrinkage data. The Q of diffusion changes little but the β₀ increases with the increase in specific surface area. It is therefore concluded that the increase in the specific surface area of fine zirconia powder enhances the shrinkage rate because of an increase in the β₀ at the initial stage of sintering.     

Fabrication of Highly Porous Samaria-Doped Ceria by Acid Leaching Magnesia from Magnesia–Samaria-Doped Ceria Ceramics

Highly porous Ce_0.8Sm_0.2O_1.9 (CSO) was prepared by acid leaching MgO from CSO–MgO (CSM) ceramics with acetic acid solution. The effects of leaching conditions and MgO content were studied. Using 60 vol% acetic acid at 85°C for 8.5 h, the MgO in CSM ceramics was effectively leached out but no CSO was dissolved; thus highly porous CSO with an open porosity of 68% was successfully prepared. The open porosity of sufficiently leached pellet increased with the MgO content, but when the vol% of MgO exceeded 60%, the flexural strength of the leached pellets decreased from 25 to 1 MPa. The leaching approach allows the fabrication of porous CSO with the vol% of MgO up to 60%.     

Ceria doped alumina by Spark Plasma Sintering for optical applications

Al₂O₃–CeO₂ ceramics were found to be transparent in the IR range and high-traslucent in the visible range. The surface of nanometric α-alumina particles was modified by deposition–precipitation of small fractions of ceria nanoparticles. The powders were sintered using Spark Plasma Sintering. Values of Real In-line Transmittance up to 70% in the IR-Vis range have been measured. Transparency enhancement has been attributed to Cerium oxide nanoparticles located at the grain boundaries and triple points. These particles are hindering alumina grain growth during SPS at a temperature as high as 1430 °C. This effect is found to be effective under SPS low vacuum conditions and short dwell times. The optimum ceria content was found to be 0.7 wt.%. Diffusion in Al₂O₃–CeO₂ as a function of the atmosphere has been studied in diffusion couples. The results obtained by the proposed route are discussed considering the data reported in the literature for SPS_ed transparent alumina.     

Sintering of Ultrafine Silicon Powder

The sintering behavior of compacts of ultrafine silicon powder (0.02 to 0.1 μm particle size) was investigated. Two sintering modes occur: normal sintering associated with densification and subnormal sintering without densification. The micro-structure developed in normal sintering has a fine grain size (0.05 to 0.3 μm) and fine porosity; the grains contain stacking faults and twins. The microstructure developed in subnormal sintering exhibits larger grains (∼1 μm in size) and coarse pores. Green densities >42% of theoretical and temperatures >1100°C are required for densification. Densification follows an exponential time and temperature dependence with an activation energy of 470 kJ/mol, indicating bulk diffusion as the transport mechanism. Grain-boundary diffusion is thought to be inhibited by grain-boundary oxide films. The carbon phase-separates into discrete amorphous regions and is thought to have little effect on sintering behavior.     

Effect of Gallia Addition on the Sintering Behavior of Samaria-Doped Ceria

Samaria-doped ceria (SDC) was prepared by using the solid-state reaction method. Sintering of SDC was significantly promoted by adding a small amount of gallium. SDC that had 1% of gallium added, sintered at 1450°C, showed almost the same properties as SDC sintered at 1600°C. Measurements showed that the addition of gallia could reduce the sintering temperature by 150°C without deteriorating the properties of SDC as an electrolyte for solid oxide fuel cells.     

Formation and Sintering of Yttria-Doped Tetragonal Zirconia with 50 mol% Alumina Prepared by the Hydrazine Method

Al₂O₃/Y₂O₃-doped ZrO₂ composite powders with 50 mol% Al₂O₃ are prepared by the hydrazine method. As-prepared powders are mixtures of AlO(OH) gel and amorphous ZrO₂ solid solutions containing Y₂O₃ and Al₂O₃. The formation process leading to α-Al₂O₃- t -ZrO₂ composite powders is examined. Hot isostatic pressing is performed for 2 h at 1400°C under 196 MPa using θ-Al₂O₃- t -ZrO₂ composite powders. The resulting dense, sintered α-Al₂O₃- t -ZrO₂ composites show excellent mechanical strength.     

压力烧结炉的设计及制造

介绍了压力烧结炉结构机理、性能及应用，着重从炉体结构、简易的设计途径等做了说明，不断总结经验，提高烧结炉的设计质量。