Key Engineering Materials Vols. 368-372

Abstract: An innovative processing route for fabricating porous zirconia ceramics has been developed based on traditional injection molding method. Azodicarbonamide (AC) was used as the foaming agents and mixed with the zirconia powder and conventional binders (polypropylene, ethylene/vinyl acetate, paraffin wax and stearic acid). There were three stages in the foaming course: (1) Small bubbles nucleated when AC decomposed into N2 and CO in the barrel. (2) Viscosity and pressure drop led to the growth of the bubbles when the melt feedstock was injected into the die cavity. (3) The porous structure was kept in the solidified body. The AC content and injection parameters were optimized to control the pore density and size. The porous green body was debinded at the heating-up rate of 0.5 °C /min to 450°C and sintered at 1550°C. Samples with porosity of 40%-50% and pore sizes from 200-250μm were prepared when the addition of AC was 0.3% by weight. The results showed that ceramic injection molding method was also suitable for fabricating the porous ceramics.

Abstract: ZrO2 (3Y)-CaO-SiO2-TiO2 powders with excellent chemical homogeneity were synthesized by heating the alcohol-aqueous salt solutions. The powders have good dispersion, with particle size of 11~15nm and no hard agglomeration. The nanocomposite powders were hot-pressed in vacuum. The obtained zirconia-based multi-phase nanocomposites were characterized by using XRD and SEM, as well as relative density testing. The results demonstrated that, even though the sintering temperature was as low as 1250°C, a high relative density of 97.5% was achieved. The superplastic compressive tests of the as-sintered specimens demonstrate that this material behaves good deformability at 1350~1500°C, at which the maximum extrusion pressure is lower than 35MPa.

Abstract: ZrO2-mullite compound powders were synthesized with zircon and aluminum sulfate in molten sodium sulfate. Na2ZrSiO5 was prepared as intermediate with zircon and sodium carbonate at 900°C for 3h. Then the mixture of resultant Na2ZrSiO5 and aluminum sulfate was heated at different temperatures in molten sodium sulfate salt. The ZrO2-Al2O3-SiO2 compound powders were obtained after washing away the solvable salts. The results showed that the compound powders synthesized at 800°C were tetragonal ZrO2, SiO2 and Al2O3; the powders synthesized at 1000°C were monoclinal ZrO2 and mullite. The mullite in compound powders became to decompose when the reaction temperature increased over 1100°C.

Abstract: As a foundational research for microwave sintering or joining of ceramics related to ternary ZrO2-Al2O3-SiO2 system, effects of substitution SiC for SiO2 on rates of raising temperature, phase evolutions and microstructures were investigated. Influences of microwave power and holding time on heating were examined in means of detecting formation of mullite. It was proved to be an effective means to substitute SiC for SiO2 in microwave heating of ternary ZrO2-Al2O3-SiO2 system.

Abstract: The large-scale Al2O3/YSZ ceramic plates were prepared by combustion synthesis under high gravity, the ceramics were mainly composed of random-oriented rod-shaped grains, and within the rod-shaped grain aligned nano-submicron YSZ fibers were embedded. Compared to the high-performance directionally solidified ceramics, the hardness, flexural strength and fracture toughness of the eutectic ceramics obtained in the experiment increased by 40.7~55.1%, 9.6~26.0% and 172.0~240.0%, respectively. The increase in hardness and strength of the ceramics could be attributed to nano-submicron YSZ fibers and inter-phase spacing and the refinement of the eutectic grains; meanwhile, high-energy, large-angle boundaries between rod-shaped grains could introduce strong toughening mechanisms involving crack-bridging and pull-out of rod-shaped eutectics.

Abstract: SiCw/3Y-ZrO2/A12O3 composites were prepared. The composite powders were prepared by a precursor method. Samples were conventionally sintered at 1500°C and 1580°C for 2h in air. The density, phases, microstructure, hardness of the SiCw/3Y-ZrO2/A12O3 composites were investigated. It was found that the relative density of the samples decreased as the increasing content of SiCw. The optimum mechanical properties may be obtained in the SiCw/3Y-ZrO2/A12O3 composites with 5 vol.% SiCw.

Abstract: AlN-ZrO2 composites were prepared by solid state reaction in N2 atmosphere at 1500°C for 4h with PSZ and Al powders as raw materials. Effects of Al content on performance of the composites and the process of nitriding reaction were investigated. The results showed that surface layer reaction of samples happened between 450°C and 765°C, and diffusion reaction happened between 765°C and 1500°C. With increasing of Al content, thermal expansion coefficient decreased, bending strength increased, thermal shock resistance and oxidization resistance were improved. Finally, bending strength of 73.61 MPa and thermal expansion coefficient of 6.06×10-6 /°C were realized for AlN-ZrO2 composites.

Abstract: Pr-CeO2 nano-size powders with the average crystallites size of 12 nm and particle size of 18 nm were prepared by a co-precipitation-hydrothermal method using Ce(NO3)3·6H2O, Pr6O11 as raw materials and ammonia as the precipitation agent. The influence of hydrothermal treatment time, hydrothermal treatment temperature on the average crystallites size, color of powders and the solubility of praseodymium were investigated. The synthesized powders were characterized by X-ray diffraction, transmission electron microscopy and color measurements. Results showed that after calcining at 800°C for 4 hours, the color of the as-prepared powder changes from light red to red brown, the solubility of praseodymium and the crystallites size of the Pr-CeO2 powder both increase.

Abstract: A W/O microemulsion system composed of OP-emolsifier / water / cyclohexane / 1-Pentanol was adopted to prepare ultrafine Ce1-xPrxO2 powder via the reaction between the precipitants of cerium and praseodymium salt solved in the nano reactors. The influence of the annealing conditions on the preparation of Ce1-xPrxO2 powder was investigated. It was shown that, with the increase of calcination temperature from 400 °C to 800 °C, the average crystallite size of the particles increases from 9.5 nm to 25.8 nm. FE-SEM images showed that shape of the particles is layered and sheet-like.

Abstract: The stable region of microemulsion of Triton X-100/hexanol/cyclohexane/ water water-in-oil (W/O) system was investigated by using distilled water as aqueous phase. SiO2 nanopowders were prepared by hydrolysis of tetraethoxysilane (TEOS) in the designed reverse microemulsion. The prepared samples were characterized by XRD and TEM. The results showed that the size of particles is within a range of 40~50nm and the aggregation is not evident after heat treatment. The size of particle is minimal and the dispersal is optimal when the ammonia content is 11% in volume.