Papers by Author: Yong Jie Yan

Abstract: Ultra-high temperature ceramic composites of ZrB2-SiC were densified by pressureless sintering. Ultra-fine ZrB2-SiC composite powders synthesized by sol-gel method were mixed with commercial ZrB2 and SiC powders. The sintered body of hybrid powders (combined commercial and synthesized composite powders) showed excellent properties not only in the relative density but also the flexure strength. Dry-pressed compacts using 4wt% Mo as a sintering aid were sintered to nearly full density at 2200°C/2h. The average strength was ~560MPa and the maximum was ~632MPa. SEM and TEM showed that SiC particles were distributed homogenously in the ZrB2 matrix and the average particle size was ~5μm. From HRTEM observations, the grain boundaries were apparently free of glassy phases and no intermediate phases existed.

Abstract: Stable ZrB2-based ultra-high temperature ceramic slurries were prepared and characterized with a solid content 40 vol% by aqueous gelcasting which was suitable to form high quality and complexshaped ceramic parts. In the present work, Ammonium Citrate Tribasic (ACT) was used as the dispersant. The properties of ZrB2 slurry, and the influencing factors were investigated by conventional techniques such as sedimentation tests, particle size distribution measurements, electrokinetic measurements and rheological analysis. High solids loading and low viscosity slurry was obtained by controlling the optimal conditions for the ZrB2-based ceramic powders.

Abstract: Ultra-fine titanium diboride (TiB2) powders have been prepared by carbothermal reduction reactions in TiO2-B2O3-C system using tetrabutyl titanate, boric acid and phenolic resin as the solution-derived precursors. The reactions were substantially completed at relatively lower temperature (<1400°C) and the resulting products had a smaller average crystallite size (< 200 nm). However, below 1100°C, titanium carbide was the predominant phase and the relative content decreased with the rise of temperature. The thermodynamic change in TiO2-B2O3-C system was mainly studied by TG-DTA and the mechanism of synthesis of TiB2 was discussed. The crystallite size and morphology of the synthesized powders were characterized by SEM and TEM.