Preparation and properties of W-SiC/Cu composites by tape casting and hot-pressing sintering

In this study, W-SiC/Cu composites were prepared by tape casting and vacuum hot-pressing sintering. The microstructures and properties of the composites were studied by means of X-ray diffraction, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, Vickers hardness test, bending strength test and coefficient of thermal expansion (CTE) test. The results showed that W₂C, WC and WSi₂ formed in the composites. The effects of SiC particle size on the relative densities, Vickers hardness, bending strength and CTE of composites were investigated. Vickers hardness, bending strength and CTE of the composite with SiC particle size of 6?µm reached the optimal values, which were 445.2?HV, 726.1?MPa, 9.24?ppm?K^?1.     

Non-Newtonian fluid flow model for ceramic tape casting

The current device of miniaturisation and higher device counts in integrated circuit (IC) packages has significantly increased the use of both multilayer ceramic packages (MLCP) and multilayer capacitors (MLC). Currently, one of the main methods used for the manufacture of flat ceramic packages with precise thickness control and consistency is the tape casting technique. Since these tapes can be cast with thickness of about 100 μm, it is crucial that the control of green tape thickness is precise, and that these thickness values are reproducible consistently. The flow of the slurry onto the casting surface can be modelled as a two dimensional fluid flow through a parallel channel. By choosing a suitable constitutive model, the predictions of the proposed model and existing models were compared with experimental results. The proposed model accurately described the fluid flow characteristics of the process, and had good agreement with experimental results.     

Microwave synthesis of yttria stabilized zirconia

Yttria stabilised zirconia (YSZ) nanocrystals, with a mean size between 5 and 10 nm, were prepared by microwave flash synthesis. Flash synthesis was performed in alcoholic solutions of yttrium, zirconium chloride and sodium ethoxide (EtONa) using a microwave autoclave (RAMO system) specially designed by authors. Energy dispersive X-ray analysis (EDX), X-ray powder diffraction (XRD), BET adsorption technique, photon correlation spectroscopy (PCS) transmission and scanning electron microscopy (TEM and SEM) are used to characterized these nanoparticles. Compared with conventional synthesis, nanopowders can be produced in a short period (e.g. 10 s), both high purity and stoechiometric control are obtained. Nevertheless, this mean of production is more cheaper and much faster than the ones commonly used to produce yttria stabilized zirconia (YSZ) by conventional sol-gel techniques.     

Microwave annealing of yttria stabilized zirconia ceramics

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

流延法制备高热导率定向石墨/高分子复合片层材料(英文)

以天然鳞片石墨为原料,PVB为黏结剂,PEG和DBP混合物为增塑剂,通过流延工艺在室温下制备了定向排列的石墨/聚合物片层复合材料。系统分析了不同黏结剂用量和流延刀口高度下复合片层材料的定向排列状况,并探讨了定向排列程度对其热导率的影响。XRD和SEM的结果表明,石墨/聚合物复合片层材料显示了不同程度的定向排列。热导率测试结果表明,片层复合材料的热导率随着定向排列程度的提高而增大。通过优化黏结剂的用量和流延刀口高度制备了具有较高热导率的片层复合材料,其热导率最高可达490 W/(m.K)。     

Optical properties of transparent nanocrystalline yttria stabilized zirconia

The optical properties of transparent nanocrystalline zirconia produced using a current activated method were characterized over the entire visible spectrum. The resolutions of the samples were characterized using standard resolution targets. All of the samples produced were found to have as high a resolution as detectable from the test, i.e., they are transparent not translucent. Transmission, reflectance, and absorption coefficients are reported for various wavelengths. The absorption coefficients were found to be highly dependent on processing time. Annealing experiments helped determine that oxygen vacancies (with free electrons) are the primary absorption centers in the visible wavelengths. In addition it was found that grain boundary cores or their associated defects do not contribute significantly to light absorption in the visible range. The lack of an influence of the grain boundary regions is discussed in terms of low oxygen vacancy concentration in the grain boundary space charge layer.     

Fracture resistance of 8 mol% yttria stabilized zirconia

Anin situ technique for the assessment of fracture resistance employing double cantilever beam (DCB) specimens was developed in the present study. The side-grooved DCB specimens were loaded with pure bending moments in a specially designed and fabricated test fixture which went inside the specimen chamber of a scanning electron microscope. The study as conducted on a 8 mol% fully stabilized cubic phase yttria (Y₂O₃) stabilized zirconia (YSZ) ceramic. The powder processed sheets were sintered at 1600°C for 2 h in a zirconia tube furnace. The mode I applied energy release rate, G_I was determined for both pure YSZ and treated YSZ. Two sets of experiments were conducted for the complete characterization of the ceramics. Three fracture toughness values were determined for the pure and treated ceramics, viz. (i) at the onset of the crack initiation,G _ic, (ii) at the arrest of a subcritical crack, G_ia and (iii) at the onset of the fast fracture,G _if. Two analyses of the experimental data were carried out, viz. method of extrapolation and statistical analysis. In case of the pure YSZ, a transgranular mode of the stable crack growth was identified to be predominant. The porous coating treatment appeared to have positive effects as the crack initiation resistance increased due to electrode layers. The stable crack growth behaviours of the ceramics were investigated by monitoring the crack growth velocity as a function of appliedG values. The results obtained were of direct significance in designing and fabrication of SOFC stacks.     

Slip casting of partially stabilized zirconia

Casting behaviour and rheological properties are studied in order to define the appropriate conditions under which to prepare slips for the production of high-temperature ceramics. Various commercial powders have been used, which were characterized with respect to morphology, particle size distribution and specific surface area. Zirconia slips with 75 wt% solid content were prepared with distilled water and ethanol as dispersing agent, with and without deflocculant. Hydrochloric acid and tetramethylammonium hydroxide were used to control the pH. Investigations into rheology, i.e. the dependence of viscosity and shear stress on shear rate, were performed. The slip, green and sedimentation bulk densities were measured.     

(Y2O3,CaO)复合掺杂ZrO2材料的交流复阻抗研究

借助交流复抗技术对（1－0.08y-0.12x-ZrO2-0.08yY2O3-0.12xCaO(x y=1,x:0-1）系统在523－973K内的导电行为进行了研究,发现随着CaO含量增加,该系统的晶粒电导活化能的增加,而晶界电导活化能则呈现先降后升的趋势。本文对上述现象进行了初步分析。     

Plasma spraying of nanostructured partially yttria stabilized zirconia powders

Nanosized partially yttria stabilized zirconia particles, prepared using a co-precipitation method, were reprocessed into agglomerate powders using two methods for plasma spraying. The first method was to make micrometer-sized agglomerates directly following the grinding of the calcined yttria–zirconia agglomerates. The second method was to reconstitute the nanosized particles into micrometer agglomerates using spray drying. The deposition efficiency, porosity, microhardness and average grain size of the deposits made from these two reprocessed powders were studied. Distinct results related to the process parameters were obtained for the two types of powders. The second type of powder was more suitable for plasma spraying than the first one. Using the second type of powder, some unique results distinguished from those of the conventional partially yttria stabilized zirconia powders were observed and an optimized coating with a porosity of 3.8%, Hv_0.3 of 953 and mainly consisting of 1–3 μm columnar grains in the columnar direction and smaller than 100 nm in their cross-sections was achieved.     

Electrical properties of La2Mo1.98Nb0.02O8.99 oxide ionic conductors prepared by tape casting

La₂Mo_2-xNb_xO_9-δ thick films have been successfully prepared by using a tape casting technique. Partial stabilization of the high temperature cubic phase is revealed in Nb doped La₂Mo₂O₉ (LMO) films. The sintering temperature is decreased to 925?°C as compared with that of 1150?°C in bulk ceramics. The grain exhibits an oxide ionic conductivity of 0.014?S?cm^?1 at 603?°C for La₂Mo_1.98Nb_0.02O_8.99, which is 39% higher than pure LMO. Additionally, the Nb doped LMO films present low grain boundary resistance showing the potential application as solid electrolytes.     

Microstructure evolution and electrical properties of yttria and magnesia stabilized zirconia

The paper contains the results of microstructure and chemical composition analyses of micrograins, which were formed on initial grain boundaries in ZrO₂-Y₂O₃ and ZrO₂-Y₂O₃-MgO systems. It has been found that the micrograins appear in the process of diffusion induced by grain boundary migration (DIGM). The observed processes can be described as both liquid film migration (LFM) and chemically induced grain boundary migration (CIGM). New micrograins had an increased content of Y₂O₃ and a cubic symmetry. Zirconia-yttria solid solutions with magnesia particulate addition showed an increased amount of migration nuclei and bigger size of new grains. However, no change in the chemical composition of the grains has been detected. The ionic conductivity measurements have shown that the activation energy (E_a) of conductivity at lower temperatures is connected to a DIGM-like process and to the distance of grain boundary migration. In the case of materials with dominating LFM process an increased grain boundary migration distance leads to a lowering of the activation energy of conductivity. Contrary to that, in the materials with dominating CIGM process an increase of migration zone causes increase of E_a values. The data obtained with respect to the type of DIGM process (LFM or CIGM) indicate that the grain boundary conductivity contribution increases with the CIGM distance.     

Fabrication of yttria stabilized zirconia coatings by a novel slurry method

By adopting a ‘bricks and mortar’ approach, yttria stabilized zirconia (YSZ) coatings were produced on Fecralloy substrates by a novel slurry method. The slurry contained large preformed particles of 10–60 μm, as the ‘bricks’, and concentrated nano-particle slurry, as the ‘mortar’. Green coatings were prepared by spreading the slurry on substrates. Then sintering at 1200 °C was carried out to produce coatings with grain size up to 200 nm. The presence of larger preformed particles hindered the shrinkage of the coatings during drying and sintering. It was observed that the ‘mortar’ could form inter-preformed particle ‘bridging’, an underlying mechanism for the formation of ‘cemented’ preformed particle network. By using different preformed particles, different micro-architectures were obtained and reduced thermal diffusivities were achieved. The microstructure and density of the coatings can also be modified by infiltration of the nano-slurry.     

氧化钇稳定二氧化锆(YSZ)微波烧结陶瓷的复阻抗谱分析

研究了8mol%Y2O3掺杂ZrO2(8YSZ)材料微波烧结陶瓷在300～850℃温度范围内的交流复阻抗谱,获得了该材料的温度-离子电导率曲线,并与常规烧结的陶瓷体进行了比较.结果发现8YSZ的微波烧结陶瓷的晶界势垒在550℃被击穿,常规烧结陶瓷的晶界势垒在500℃被击穿.击穿后晶界电阻消失,离子电导率的变化主要由晶粒电导率的变化决定.在击穿温度点以下,陶瓷体的离子电导率随温度的升高呈波浪式上升,即曲线呈上升～下降～上升趋势.     

Structure of yttria stabilized zirconia beads produced by gel supported precipitation

Yttria stabilized zirconia (YSZ) is one of the inert matrix candidates selected for investigation as host matrix for minor actinide (MA) transmutation. The structural properties of (Zr_0.84, Y_0.16)O_1.92 beads prepared by a sol–gel method for MA infiltration, are characterized as calcined (850 °C) and sintered (1,600 °C) beads. The calcined YSZ beads are fine-grained and homogenous over the entire sphere and are surrounded by a uniform outer layer of approximately 30 μm thickness. After sintering at 1,600 °C, the beads are compacted to 51% of their initial volume and exhibit a granular structure. The thermal expansion is nearly linear for the calcined material, but shows a parabolic behavior for the sintered (1,400 °C) beads. In addition, the thermal expansion of calcined material is 20–25% less than after sintering. During heating up to 1,400 °C, two processes can be distinguished. The first occurs between 900 and 1,000 °C and is related to an increase in unit cell order. The second process involves grain-growth of the less crystalline calcined material between 1,100 and 1,300 °C. These results have implications for preparation of YSZ and its use as an inert MA transmutation matix.     

Composite ceramics thermal barrier coatings of yttria stabilized zirconia for aero-engines

Composite ceramics thermal barrier coatings(TBCs) are widely used in the aero-engines field due to their excellent thermal insulation, which improves the service life and durability of the inherent hot components. The most typical, successful and widely used TBCs material is yttria stabilized zirconia(YSZ). In this paper, fabrication methods, coating structures, materials, failure mechanism and major challenges of YSZ TBCs are introduced and reviewed. The research tendency is put forward as well. This review provides a good understanding of the YSZ TBCs and inspires researchers to discover versatile ideas to improve the TBCs systems.     

Transformation toughening of NiAl composites reinforced with yttria partially stabilized zirconia particles

The paper presents the results of a combined experimental and analytical study of transformation toughening in NiAl composites reinforced with 20 vol. % of 2 mole % yttria stabilized zirconia particles. The extent of stress-induced phase transformation is characterized using laser Raman spectroscopy techniques. The overall toughening increment due to stress-induced transformation is also predicted using micromechanics models that account for the role of dilatation only, or the combined effects of dilatation and shear.     

Transformation of yttria partially stabilized zirconia by low temperature annealing in air

The tetragonal-to-monoclinic phase transformation of yttria partially stabilized zirconia by low temperature annealing in air was investigated in the temperature range 100 to 650° C using a sintered body of zirconia containing 2 to 4 mol% Y₂O₃. The amount of monoclinic phase formed was maximum at about 200° C. Both the decrease in grain size and increase in the yttria concentration were effective in decreasing the critical temperature below which the monoclinic phase was formed. The relationship between the critical temperature (T _c) and the grain size was experimentally determined.     

YSZ纳米粉料流延成型电解质薄膜性能研究

探索了以YSZ纳米粉体为原料,采用流延成型的方法制备YSZ电解质薄膜的工艺过程,具体探讨了不同粒度粉体流延后坯体的性能,结果表明,纳米范围内颗粒粒度粗可以获得致密度较高的坯体。但在烧结过程中,细粒度粉料表现出更好的性能,实验制备的YSZ电解质薄膜的面积比电阻在1123K,比德国D样品大幅度下降。573～1023K的阻抗谱显示出实验制备的YSZ电解质试样的晶粒、晶界和电极处的阻抗都明显降低,综合性能明显优于国外同类产品。流延成型工艺可以获得尺寸为100mm×100mm×0.125mm的8YSZ电解质薄片,这为平板式SOFC在中国的快速发展做好了材料上的准备。