Papers by Author: Ke Xin Chen

Abstract: In this article, the Al/Ni reactive multilayer foils that can be used for joining materials in an efficient and energy-saving way were prepared by a hot rolling method, during which the Al/Ni multilayer foils were kept in furnace at a certain temperature for a period before the multilayer foils were rolled for each pass. After the cold rolling and hot rolling procedure at the temperature of 200 oC and 300 oC, no reaction between Al and Ni could be detected. The effect of rolling temperature on the self-propagating high temperature synthesis (SHS) reaction of the Al/Ni multilayer foils was investigated. As the rolling temperature rose to 300 oC, the SHS reaction of the Al/Ni multilayer was hard to be ignited and was even quenched before the SHS reaction completed. The XRD patterns and SEM images results demonstrated that the higher temperature increased the deformation of the samples during the rolling passes so the Ni particles increased, and thereby led to the decrease of the mix homogeneity of Al and Ni foils.

Abstract: The big gap between the thermal conductivity of Si3N4 ceramics and the theoretical value of beta-Si3N4 single crystal implies that the low thermal conductive oxide grain-boundary phases in the ceramics possibly have a great influence on the thermal conductivity of the ceramics. In this work, yttrium chloride (YOCl) is introduced as sintering additive instead of the commonly-used Y2O3, in order to decrease the amount of the grain-boundary oxide; and the influence of YOCl on the sintering and the properties of Si3N4 ceramics was studied. Results show that YOCl additive can react with Si3N4 powders to form similar Y-Si-O-N grain-boundary phases just like Y2O3 while the Cl disappears during sintering. The thermal conductivity of the ceramics sintered with YOCl additive (85 W/m•K) increases about 15% as comparing with the ceramics sintered with Y2O3 additive, even though the relative density of the former reaches merely 97%. The microstructure of the sample sintered at different temperatures was investigated in the work, for elucidating the sintering process and the relationship between the properties and the microstructure of the YOCl added ceramics.

Abstract: In this paper, α-Si₃N₄ was mixed with Y₂O₃ and MgO by ball milling. Spray granulation was adopted to fabricate spherical powder. The influence of the sintering temperature and the holding time on the thermal conductivity of the powder was studied. The results show that α-Si₃N₄ raw powder was transformed into β-phase after sintering. Spherical powder, of which the diameter is about 20 μm, was fabricated. Some compounds such as YMgSi₂O₅N, Y₄Si₂N₂O₇ and Y₂Si₃N₄O₃ were generated in the samples. The thermal conductivity of the pressed powder first increased then decreased with the sintering temperature rising, showing a maximum at 1800°C. Also the thermal conductivity first increased then decreased as the holding time getting longer, showing a maximum at a sintering time of 2 hours.

Abstract: High thermal conductive Si3N4 is a promising candidate for producing high quality ceramic substrates used in vehicles. Although a lot of researches have been carried out on the tape casting of Si3N4, they were mainly restricted in fabricating thin films with a thickness less than 200μm which are not sufficient for producing substrate. With larger thickness, tapes tend to crack during drying. In this work, slurry of α-Si3N4 with Y2O3-MgO as additives was prepared using an azeotropic mixture of methylisobutylketone/methyehtylketone/cyclohexanone as media. To obtain thick green tapes, the amount of dispersant, binder and plasticizer in the slurry were optimized, and the effects of drying temperature on the cracking behavior of the tape were studied. By controlling the composition of the slurry and the drying conditions, thick green tapes with thicknesses up to 1.8mm were successfully fabricated, and Si3N4 substrates were produced after subsequent gas-pressing sintering, which were flat and had a relative density as high as 98.2%.

Abstract: β-Si3N4 and β-SiAlON powders were prepared by combustion synthesis with SrCO3 and NH4F used as additives. The resultant β-Si3N4 and β-SiAlON powders consisted of elongated prismatic microcrystals. By adding SrCO3, the anisotropic growth of β-Si3N4 and β-SiAlON crystals is improved and their aspect ratios increase. The addition of NH4F enhanced nitridation reactions and reduced the residual Si in combustion products. It was proposed that the elongated prismatic β-Si3N4 and β-SiAlON crystals grew from liquid phase and the composition and property of this liquid was affected by the addition of SrCO3.

Abstract: Effect of iteration times on mechanically-activated combustion synthesis of high α-content Si3N4 powders was investigated. Properties of the as-synthesized powders such as α-content (Cα) as well as specific surface area (As) were examined. Results showed that both of Cα and As became higher after iteration reactions. The mechanical properties of the sintered bulk ceramics from as-synthesized powders were also tested to reveal the sinterability of the powders. Results showed that relative density of all the sintered bulk ceramics were higher than 97%. Furthermore, fracture toughness had a trend of becoming higher, which reached a value of 10.2 MPam0.5. Correspondingly, bending strenth became a bit lower.

Abstract: New electric molding composites were fabricated with hybridizing epoxy and several fillers. The examined fillers were β-Si3N4, BN and fused silica or their combinations. The thermal conductivity of β-Si3N4 filler was compared with BN as filler for advanced epoxy molding compounds. The influence of pure resin matrix and mixture matrix on the thermal conductivity of EMC was discussed. The results were explained with Maxwell equation.

Abstract: Spinel LiMn2O4 was prepared by solution combustion synthesis. The effect of fuel content and calcination procedure on phase composition and microscopic structure of LiMn2O4 was studied. X-ray diffraction patterns showed that fuel content had no obvious influence on the grain size and phase purity of LiMn2O4. Higher calcination temperature led to higher phase purity, lager grain size, and better crystallization of resultant LiMn2O4. Below 600°C the effect of calcination time was inconspicuous, which became notable above 700°C. Scanning electron microscope images showed that nanocrystalline LiMn2O4 was obtained when the calcination temperature was lower than 600°C and the grain size increased at higher temperatures.

Abstract: Spinel LiMn2O4 powders were prepared by solution combustion synthesis using nitrate and acetate salts as raw materials and urea as fuel. The phase composition of as-synthesized powders was identified by XRD and the microscopic structure was examined by SEM. Single-phase spinel LiMn2O4 was prepared when acetate salts were used, and the incorporation of nitrate salts resulted in the formation of Mn2O3. The products consisted of slight agglomerations of fine particles with the size of 50-200nm. It was found that the addition of nitrate salts increased the reaction rate and the yield of LiMn2O4 was depressed when more nitrate salts were used as a reactant.

Abstract: Stir-froth-polymerization is a novel technique for producing highly porous ceramics, in which bubbles are formed by a mechanical frothing in aqueous slurry of ceramic powders. Many work had be done about the porosity, the permeability and the mechanical strength of the porous ceramics derived from this method, but there are still a lot of things remain unclear. This paper focuses on the mechanism of frothing, and the influence of parameters in the mechanical frothing. It was found that the stir time, the stir speed and the viscosity of the slurry have influence on the average cell-size and the distribution of them respectively. The average cell-size can be controlled in the range from 60 to 160 μm by adjusting the parameters.