Key Engineering Materials Vols. 317-318

Abstract: Fine AlN powder doped with Y2O3 and CaO as sintering additives was ground by a ball mill, a planetary ball mill and a super-fine grinding mill in order to obtain fine homogenous powder for low-temperature sintering. The size reduction and the sinterability of ground powders at 1500oC for 6 h were investigated. The size and shape of the agglomeration showed no significant change after the ball mill and planetary mill processes, resulting in poor densification. On the contrary, AlN particles with size of 50~100 nm was pulverized and dispersed by a super-fine grinding mill with very small ZrO2 beads as a mill media. The microstructures of the specimen exhibited equiaxed and homogenous grains with size of 0.3~0.4 μm. Pores in the specimens were eliminated. The thermal conductivity was 70W/mK, which is better than that of Al2O3 ceramics (~20W/mK).

Abstract: Wet jet milling is a new technology for the pulverization of powders, in which streams of slurry, higher than 100m/s, collide in the channel made of diamond. The effect of processing parameters was investigated in the system of BN/SiO2 mixed powders. The particle size after milling was evaluated by the measurement of BET surface area. The BET surface area of BN increased with increase in the number of milling, in the mixture ratio of SiO2/BN, in the particle size of SiO2 and in the processing pressure, whereas no significant change in the size of SiO2 particle was found. The surface area of BN increased by about 2.1 times by wet jet milling for only about 20 minutes (5 times).

Abstract: Barium titanate powders were synthesized via a hydrothermal process at 120-180
using titanium oxide powder with different particle sizes as starting materials in this study. When micron-sized titanium oxide was used, pure perovskite phase was not formed because of the insufficient chemical reactivity of titanium oxide. However, as nano-sized titanium oxide was utilized, pure barium titanate was obtained after the hydrothermal process was performed at 180 oC. The effects of sintering temperature on the dielectric properties of barium titanate were also investigated. Densified ceramics with high dielectric constants were obtained after sintering at 1200oC.

Abstract: To overcome the difficulty of preparing the stoichiometric single phase calcium phosphate materials, ultrasonic spray pyrolysis was used to fabricate various calcium phosphates. Effect of the initial Ca/P ratios and pyrolysis temperature were investigated in preparation of single phase materials. Also thermal stability of the obtained powders was estimated with the heat treatment in the air. Crystallinity of the powders was analyzed using XRD and FT-IR. The HAp and β-TCP powders with single phase could be obtained from their stoichiometric Ca/P ratios of 1.50 and 1.67, respectively. These single phase powders with good crystallinity and stoichiometry were stable with respect to heat treatment up to 1000oC. The particle size of these powders would be controlled by the concentration of starting solution.

Abstract: The hypereutectic prealloyed Al-Si powders were prepared by gas atomization process. The microstructure and compacting ability of the atomized Al-Si powders were investigated. With increasing the gas pressure of the atomization, the average powder size and oxygen content were decreased. The size of primary eutectic Si particles in the as-atomized powders was about 8~10. As-atomized powders were hot-pressed into a cylindrical shape. Relative density of the hot-pressed samples was achieved about 96~99 % of theoretical density. Some properties of the samples were evaluated in this paper.

Abstract: Hollow calcium carbonate (CaCO3) particles were synthesized by bubbling CO2 in the solution of calcium chloride (CaCl2) with the presence of ammonia at room temperature. In this method, CO2 bubble, besides as reactant, has an important role as a template for forming hollow particles. The precipitation was carried out by controlling the precipitation condition, such as pH, flow rate and additives. Hollow calcium carbonate particles were precipitated with the major crystal phase of vaterite. An interesting finding is that magnesium ion (Mg2+) can suppress the transformation of vaterite to calcite by inhibiting the growth of the calcite.

Abstract: In the present work, effects of different salts (NaCl, 0.5NaCl-0.5KCl and KCl) on molten-salt-synthesized Sr0.5Ba0.5Nb2O6 (SBN50) powder were explored. A variety of particle morphology, compositions and lattice parameters of SBN50 were investigated. The results showed that SBN50 particles synthesized in NaCl salt have the finest diameter (0.1-0.2μm) and the highest aspect ratio (around 10), while the particles from 0.5NaCl-0.5KCl salt show the coarsest diameter (0.2-0.3μm) and the lowest aspect ratio (around 5). If only using KCl salt, trace Sr2Nb2O7 normally co-exists with SBN50 phase. In addition, Na+ and K+ cations from salts are easily incorporated with SBN structure and reach a very high content.

Abstract: Aerogels are sol-gel derived mesoporous materials with low apparent density and large specific surface areas. The mechanical property evaluation of pulsed electric current sintered (PECS) alumina and its composite aerogels with nanosized metallic dispersions is an area of interest. In the present work alumina, alumina-copper, alumina-nickel, alumina-cobalt and alumina-tungsten aerogels have been synthesized by super critical drying of respective the sols. The samples were pre-calcined under controlled sintering conditions and pulsed electric current sintered at two different temperatures: (1) at a lower sintering temperature with short residence time (1250°C for 5min) and (2) at a high temperature with prolonged soaking time (1450°C for 30 min) and their mechanical properties were evaluated.

Abstract: To improve the mechanical properties of Al2O3/ZrO2 composites, the homogeneous dispersion of ultra low size ZrO2 particles in Al2O3 ceramics have been controlled by partial dispersion of ZrO2 by chemical processes such as coprecipitation or polymeric precursor method(Pechini process). So nanosized Zr/Y hydroxide were coprecipitated or polymerized directly to the surfaces of commercial sub-micron size α-alumina powder(Sumitomo: AES-11
(0.4 μm)) using ZrOCl2 /Y(NO3)3 solution. By the partial coprecipitation method, dispersion of relatively small sized ZrO2 in Al2O3/ZrO2 composites could be achieved at 1500~1600° C of sintering temperature. In case of the polyesterization of Zr/Y(NO3)3-citric acid solution in ethylene glycol directly to the commercial sub-micron size α-alumina powder, more homogeneous dispersion of relatively low sized ZrO2 in Al2O3/ZrO2 composites could be obtained at 1450~1600°C of sintering temperature range and their mechanical strength was more enhanced.

Abstract: A novel CO2 separation technique that employs the chemical reaction of lithium-containing oxides with CO2 has been developed. Since this method is effective in the temperature range of 450oC to 700oC, it has the advantages of enabling CO2 separation in power plants without lowering the temperature and of absorbing CO2 from the steam-methane reforming process at the same time. Because the absorption is exothermic and the steam reforming is endothermic, the energy loss is expected to be significantly reduced by combining the reactions. Hydrogen yields are expected to be higher because the equilibrium may be shifted by the removal of the CO2 byproduct. We have therefore proposed a pre-combustion CO2 capture system using lithium silicate and steam reforming. Bench-scale experiments were performed to measure the methane conversion and CO2 removal efficiency in order to evaluate the feasibility of the pre-combustion CO2 capture system. At temperatures of less than 650oC, the methane conversion in the case of mixture of catalyst and absorbent was higher than that in the case of catalyst alone. In addition, the CO2 removal efficiency is almost 90%. These results appear to indicate that pre-combustion CO2 capture combined with steam reforming is feasible.