Papers by Author: Xue Jian Liu

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: Solid state sintered silicon carbide (S-SiC) ceramic is one of the top optical materials for high space reliability and other excellent properties. Two microstructures were produced by sintering under different conditions. The effects of microstructure on removal rates of SiC ceramics during polishing processes were studied. The material removal mechanisms during polishing were analysed and modeled. With the increase of the aspect ratio and grain diameter size during polishing, grain pull-out is more difficult in elongated grains than in exquiaxed grains. The SiC ceramic with high hardness has high removal resistance leading to get bad surface quality under the same mechanical procedure. The samples with elongated microstructure have low hardness and surface toughness.

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: The CNTs/Si3N4 ceramic matrix composites were prepared by the reaction bonded processing. The phase compositions, chemical compatibility, mechanical properties, and microwave attenuation properties of the composites were investigated. XRD analysis shows the composites consist mainly of the α- and β-Si3N4, with a trace of unreacted silicon. The SEM micrograph displays the fractured surface of the composites studs with intact CNTs, indicating that CNTs and Si3N4 are chemically compatible. The composites with 1.0wt.% CNTs have a strength of 280 MPa, hardness of 8.2 GPa and toughness of 2.3 MPa·m0.5. The average value of the transmission attenuation reaches 6 dB at X band, indicating the composites have a potential for application in electromagnetic adsorbing or shielding.

Abstract: Reticulated porous ceramics (RPCs) with uniform structure were fabricated by a two-step centrifuging process. The two-step centrifuging process includes two centrifuging steps. In the first stage, the polyurethane sponges are coated with thin slurry through high speed centrifuging process. In the second stage, the as-prepared dried green body was recoated with thixotropic slurry through centrifuging process with an appropriate centrifuging speed. According to the results, the RPCs with controlled structure can be fabricated by modulating the rotation speed in the second centrifuging process.