Preparation and properties evaluation of zirconia-based/Al2O3 composites as electrolytes for solid oxide fuel cell systems

Yttria-doped zirconia powders containing 3 to 8 mol% Y₂O₃ and 0 to 20 wt% Al₂O₃ were prepared by both mixing commercial oxides and a coprecipitation method, and the mechanical and electrical properties have been examined as a function of the Al₂O₃ content. The bending strength of the composite at room temperature increased with increasing Al₂O₃ content. In the temperature range 500–1000 °C the bending strength increased with Al₂O₃ content up to 10 wt% and then decreased, the measured value at 1000 °C (200 MPa) being higher than those at lower temperatures for cubic zirconia materials. Fracture toughness (K_IC) decreased with increasing Y₂O₃ content in the Al₂O₃-free zirconia materials. Al₂O₃ additions enhanced the fracture toughness and this was maximum (7 MPa m^1/2) for the composite ZrO₂-3 mol% Y₂O₃/10 wt% Al₂O₃. The electrical conductivity of cubic ZrO₂/Al₂O₃ composites decreased monotonically with Al₂O₃ content, but in tetragonal ZrO₂/Al₂O₃ composites hardly varied or apparently increased up to 10 wt% Al₂O₃. At 1000 °C the highest electrical conductivity was 0.30 S cm^–1 for ZrO₂-8 mol% Y₂O₃, and this decreased up to 0.10 S cm^–1 for the composite ZrO₂-8mol% Y₂O₃/20 wt% Al₂O₃.     

Chemical stability of MgO/CaO/Cr2O3–Al2O3–B2O3–phosphate glasses in solid oxide fuel cell environment

The thermal properties, glass forming tendency and glass stability in solid oxide fuel cell (SOFC) relevant atmospheres have been determined for four different refractory MgO/CaO/Cr2O3–Al2O3–B2O3 phosphate based glasses. Also, the bonding ability and interaction as a function of temperature between the glasses and La0.8Ca0.22CrO3 interconnect material have been studied. The microstructural characterization and elemental analysis of reaction couples reveal that the bonding ability and interface interaction is closely related to the CaO content and the relative glass forming tendency. The reactions at 1000°C between the glass and the solid interconnect material for the high CaO or MgO containing compositions occur by a liquid phase or a vapour phase mechanism, respectively. The reaction at 1200°C in both cases occurs by a liquid phase mechanism. © 1998 Chapman & Hall     

Processing and properties of Y-TZP/Al2O3 composites

The processing and property measurement of Y-TZP/Al₂O₃ ceramic-ceramic composites was investigated. The wet chemical synthesis route was adopted for the preparation of 3Y-TZP matrix dispersed with Al₂O₃ in three different volume fractions. Characterization of the resultant powders was carried out and their densification behaviour was studied by sintering in air in the temperature range 1200–1600 °C. The role of alumina as grain-growth inhibitor for Y-TZP, and the mechanical response of these ultrafine-grain ceramic composites in terms of K_lc characteristics, have been discussed.     

Ti/Al2O3复合材料制备及其力学性能

利用放电等离子烧结(SPS)制备了性能优异的40%(体积分数)Ti/Al2O3复合材料,其弯曲强度、断裂韧性、显微硬度和相对密度分别为897.29MPa、17.38MPa·m1/2、17.13GPa和99.24%.SEM和HREM对复合材料的微观结构分析发现,晶粒细化、位错环强化等是材料强度提高的主要原因;裂纹的偏转和桥联是材料韧性提高的关键所在.     

Mechanical properties of Al2O3/Ni laminated composites

Al₂O₃/Ni laminated composites were prepared by aqueous tape casting and hot pressing with intent to study mechanical properties including the fracture strength and toughness. The residual stress was evaluated and proved. The relations of mechanical properties with the thermal residual stress, the ductility of metal layers and the layer thickness ratio were studied, respectively. It was found that the toughness and work of fracture of Al₂O₃/Ni laminar reached to 12.56 MPa m^1/2 and 12 450 J m^− 2, which are 3.6 and 478.8 times that of pure Al₂O₃.     

Preparation and mechanical properties of high-purity Al2O3 fibre/Al2O3 matrix composite

Al₂O₃ matrix composites with unidirectionally oriented high-purity Al₂O₃ fibre with and without carbon coating, were fabricated by the filament-winding method, followed by hot-pressing at 1573–1773 K. The composite with non-coated Al₂O₃ fibre exhibited a bending strength (594 MPa) comparable to that of monolithic Al₂O₃ (589 MPa). While the composite with a carbon-coated fibre had lower strength (477 MPa), it showed improved fracture toughness (6.5 MPa m^1/2) compared to the composite with an uncoated fibre (4.5 MPa m^1/2) and monolithic Al₂O₃ (5.5 MPa m^1/2). This toughness enhancement was explained based on the increased crack extension resistance caused by the fibre pull-out observed by SEM at the notch tip. This revised version was published online in November 2006 with corrections to the Cover Date.     

Gadolinia doped ceria/yttria stabilised zirconia electrolytes for solid oxide fuel cell applications

Solid oxide fuel cells are currently constructed using a yttria stabilised zirconia electrolyte membrane. However, zirconia has a number of disadvantages associated with its use, such as the high operational temperatures required for it to exhibit acceptable levels of ionic conductivity. Alternative ceramics such as doped cerium oxide show promise as electrolytes capable of operating at reduced temperatures, but introduce additional problems such as electronic conduction and inferior mechanical properties. This paper describes the manufacture and characterisation of a number of prototype electrolytes consisting of a mixture of yttria stabilised zirconia and gadolinium doped ceria. Traditional ceramic processing techniques were used to produce the samples, which were then examined using dilatometry, impedance spectroscopy and X-ray diffraction. Results show a lowering of the ionic conductivity of zirconia with the addition of doped ceria. X-ray diffraction patterns obtained from the samples suggested that this effect could be attributed to the formation of a solid solution of ceria in zirconia.     

Slip casting of Al2O3 and Al2O3/ZrO2 composites

Al₂O₃ and Al₂O₃/ZrO₂ composites have been fabricated by slip casting from aqueous suspensions. The physical and structural characteristics of the starting powders, composition of the suspensions, casting behaviour, microstructure of the green and fired bodies and the mechanical properties of the products were investigated. The addition of ZrO₂ to Al₂O₃ leads to a significant increase in fracture toughness when ZrO₂ particles are retained in the tetragonal form (transformation-toughening mechanism) but when microcracking (due to the spontaneous transformation of ZrO₂ from the tetragonal phase to the monoclinic one) is dominant, an excellent toughness value is accompanied by a drastic drop in strength and hardness.     

The mechanical properties of laminated microscale composites of Al/Al2O3

The tensile properties, at both room and elevated temperatures, of laminated thin films containing alternate layers of aluminium and aluminium oxide were investigated. At room temperature the strength of the films followed a Hall-Petch type relationship dependent on the interlamellar spacing, and the strength could be extrapolated from data for conventional grain size aluminium. At the finest interlayer spacing of 50 nm, the strength was equivalent to/70, where is the shear strength of aluminium and the samples exhibited very extensive ductility. At elevated temperatures, cavitation became an important deformation mechanism but it occurred preferentially at Al/Al rather than Al/Al₂O₃ boundaries. The microstructure of the films was probed using transmission electron microscopy and fractography was used to investigate deformation and fracture mechanisms.     

Fabrication of Al/Al2O3 composites and FGM

Al/Al2O3 composites of different ratios were hot-press sintered at 575 similar to 640℃ under a pressure of 30 MPa for 2 h in a vacuum furnace. It was found that the relative density of the Al/Al2O3 composites could be increased evidently with the rise of sinter temperature. No reaction occurred between Al and Al2O3 at the sinter temperatures. Under 640℃-30 MPa-2 h experimental condition, Al/Al2O3 system FGM was successfully fabricated, and its density range changed quasi-continuously from 2.887x10(3) kg/m(3) to 3.1909x10(3) kg/m3 within the middle 1.0 mm thickness range.     

Interfacial properties and structure stability of Ni/Y2 O3-ZrO2-TiO2 cermet anodes for solid oxide fuel cells

Ceramics of the ternary system Y₂O₃-ZrO₂-TiO₂ (YZT) and Ni/YZT cermets are evaluated in terms of application as anode electrodes in a Solid Oxide Fuel Cell. Wetting experiments in liquid Ni/YZT systems show that the increase of TiO₂ content in the ceramic phase improves the bond strength at the metal ceramic interface, due to the reduction of the interfacial energy. Ni(40 vol%)/YZT cermets are exposed at 1000°C for up to 1000 h in reducing atmosphere and exhibit an improved long term stability regarding to the electrical conductivity and the microstructure compared to the “state of the art” Ni/8YSZ (yttria(8 mol%)-stabilized zirconia) cermet. This is explained by the enhanced adherence at the Ni/ceramic interface, which suppresses the agglomeration rate of the Ni particles. The improvement of the interfacial properties diminishes the TEC values of the Ni/YZT cermets constraining the thermal expansion mismatch between the cermet anode and the 8YSZ electrolyte in the SOFCs.     

Microstructure and mechanical properties of Al2O3-Cr2O3-ZrO2 composites

Al₂O₃ is a popular ceramic and has been used widely in many applications and studied in many aspects. On the other hand, zirconia-toughened alumina (ZTA) is a desirable material for engineering ceramics because of its high hardness, high wear resistance and high toughness. In the present research, Al₂O₃-Cr₂O₃-ZrO₂ composites were produced by hot-pressing in order to harden the Al₂O₃ matrix in ZTA. Its microstructure and mechanical properties were studied by SEM, ESCA, XRD, Vickers hardness and bending strength test. It was found that addition of ZrO₂ inhibited the grain growth of Al₂O₃-Cr₂O₃ and the grain growth of ZrO₂ proceeded with increasing amounts of ZrO₂ in the Al₂O₃-Cr₂O₃-Zr₂ composite. The formation of solid solution Al₂O₃-Cr₂O₃ was also confirmed by XRD, and monoclinic ZrO₂ increased on addition of Cr₂O₃. Maximum hardness was at Al₂O₃-10wt% Cr₂O₃ with 10 vol% ZrO₂ and a stress-induced transformation was confirmed on the fracture surface of the specimen after the bending test.     

功能化Al2O3@SiO2/酚醛环氧-双马树脂复合材料的微观结构及性能

采用溶胶-凝胶法(Sol-gel)分别制备Al₂O₃和SiO₂,同时以KH560为架桥剂制得SiO₂包覆Al₂O₃(KH560-Al₂O₃@SiO₂)的增强体。以双马来酰亚胺树脂和酚醛环氧树脂(MBMI-EPN)为基体、4’4-二氨基二苯甲烷(DDM)为固化剂,采用原位聚合法制备了KH560-Al₂O₃@SiO₂/MBMI-EPN复合材料;表征KH560-Al₂O₃@SiO₂的微观结构及该增强体对复合材料性能的影响。结果表明:Al₂O₃@SiO₂粒子微观结构清晰,核壳结构完整,内核为短纤维状Al₂O₃,外壳为无定形SiO₂,二者通过化学键方式相连;Al₂O₃@SiO₂表面成功接枝上KH560基团,粒子堆积现象减弱。KH560-Al₂O₃@SiO₂/MBMI-EPN复合材料的微观形貌显示:KH560-Al₂O₃@SiO₂在MBMI-EPN基体中形成多相结构、分散性较好、界面作用稳定且断面形貌呈鱼鳞状,并未发现Al₂O₃@SiO₂粒子团聚体,整体结构完整。当KH560-Al₂O₃@SiO₂含量为1.5wt%时,复合材料的弯曲强度与冲击强度分别为126 MPa和14.7 kJ/m;,比树脂基体分别提高了21.2%和27.8%;材料的热分解温度为392.3℃,比树脂基体提高了14.5℃,力学性能和耐热性得到明显改善。     

Mechanical properties of pressure-sintered Al2O3-ZrC composites

Alumina-zirconium carbide composites containing up to 0.90 molecular fraction of carbide were prepared by pressure-sintering at 1800 K under pressures of 35 and 50 MPa. Both toughness (evaluated from the indentation cracks length) and flexural strength, first increase with carbide molecular fraction, then decrease. The influence of the sintering pressure is more marked for the toughness than for the flexural strength. The initial increase of the mechanical properties seems to be related to the alumina grain size diminution, and the subsequent decrease to the presence of poorly bonded zirconium carbide agglomerates.     

ZrO2/Al2O3多孔陶瓷的制备与力学性能

采用凝胶-发泡法制备了ZrO2/Al2O3多孔陶瓷,研究了陶瓷浆料的流变性,固相含量对多孔陶瓷坯体显微结构与力学性能的影响,以及烧结助剂MgO含量与多孔陶瓷抗压强度及气孔率之间的关系.结果表明,在分散剂含量为0.4％(质量分数),球磨4 h,pH值为4的条件下,陶瓷浆料的黏度较低,有利于凝胶注模.固相含量增加,坯体气孔率下降.过高的固相含量使浆料流动困难,注模时引入空气导致坯体内形成较大的气孔甚至裂纹,使坯体抗压强度下降.由ZrO2引起的相变增韧及微裂纹增韧可有效改善多孔陶瓷的力学性能.随烧结助剂含量增加,多孔陶瓷气孔支撑体致密化程度增大,气孔率降低,抗压强度明显升高.多孔陶瓷的抗压强度最高达30 MPa.引入适量的ZrO2及烧结助剂,可制备气孔率适中、抗压强度高的多孔陶瓷.     

Bone-like apatite coating on Mg-PSZ/Al2O3 composites using bioactive systems

A biomimetic method was used to promote bioactivity on zirconia/alumina composites. The composites were composed of 80 vol% Mg-PSZ and 20 vol% Al₂O₃. Samples of these bioinert materials were immersed in simulated body fluid (SBF) for 7 days on either a bed of wollastonite ceramics or bioactive glass. After those 7 days, the samples were immersed in a more concentrated solution (1.4 SBF) for 14 days. Experiments were also performed without using a bioactive system during the first stage of immersion. A bone-like apatite layer was formed on the surface of all the materials tested, using wollastonite the bioactive layer was thicker and its morphology was close to that observed on the existing bioactive systems. A thinner apatite layer consisting of small agglomerates was obtained using bioactive glass. The thickness of the ceramic layers was within the range of 15 to 30 μm.     

Mechanochemically synthesized CsH2PO4–H3PW12O40 composites as proton-conducting electrolytes for fuel cell systems in a dry atmosphere

Abstract

Cesium dihydrogen phosphate (CsH₂PO₄, CDP) and dodecaphosphotungstic acid (H₃PW₁₂O₄₀·nH₂O, WPA·nH₂O) were mechanochemically milled to synthesize CDP–WPA composites. The ionic conductivities of these composites were measured by an ac impedance method under anhydrous conditions. Despite the synthesis temperatures being much lower than the dehydration and phase-transition temperatures of CDP under anhydrous conditions, the ionic conductivities of the studied composites increased significantly. The highest ionic conductivity of 6.58×10^?4 Scm^?1 was achieved for the 95CDP·5WPA composite electrolyte at 170 °C under anhydrous conditions. The ionic conduction was probably induced in the percolated interfacial phase between CDP and WPA. The phenomenon of high ionic conduction differs for the CDP–WPA composite and pure CDP or pure WPA under anhydrous conditions. The newly developed hydrogen interaction between CDP and WPA supports anhydrous proton conduction in the composites.