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The development of novel cermet composites based on porous ceramics with high porosity, interconnected pore structure and good mechanical property has attracted considerable attention in engineering application. In this work, water-based freeze casting process was employed to fabricate ZrB2-SiC porous ceramic with aligned lamellar-channels structure using PAA-NH4 as the dispersant. The results revealed that the well-dispersed suspension with best rheological behavior was obtained using 1.0 wt% PAA-NH4 at pH 9. The crack-free porous ceramic exhibited small volume shrinkage ranging from 2.59 % to 1.87 %. By varying the solid loading, the fabricated samples displayed a tailored porosity ranging from 76.12% to 59.37% and an excellent compressive strength of 7 MPa to 78 MPa. After oxidation, the samples displayed a decreased porosity and an increased compressive strength. The ZrB2­SiC porous ceramic fabricated in this work will be a promising candidate for the framework of cermet composite. more...

In this work, a ZrB2-SiC-Ta4HfC5/Ta4HfC5 oxidation-resistant dual-layer coating was fabricated to improve the oxidation resistance of carbon–carbon (C/C) composites. The coating was prepared by pack cementation combined with the slurry paste method. Spark plasma sintering was used in the procedure. The as-prepared dual-layer coating was approximately 110 μm, and no distinct interface was observed between the coating and the matrix. The isothermal oxidation test results showed that the dual coating effectively prevented C/C composites from oxidizing. The weight loss of the coated samples was only 3.3 and 9.5% after oxidation at 1773 K for 20 h and ten thermal shock cycles between 1773 K and room temperature in air, respectively. The pores and microcracks were the main reason for the failure of the coating. more...

To improve the thermal cycling resistance of ZrB 2 –SiC/SiC dual layer coating, MgO was added into the pack powders used to prepare ZrB 2 –SiC outer coating by pack cementation. The composition, microstructure and thermal cycling resistance of these coatings were studied. The results show that as-received coatings were mainly composed of SiC, ZrB 2 and Si. ZrB 2 –SiC outer coating contained 5 wt%, 7 wt%, 9 wt% MgO additive showed better thermal cycling resistance with 97.6%, 98.8% and 96.5% weight retention after 15 thermal shock cycles between 1500 °C and room temperature. more...

Acidic silica sol was used as precursor to prepare SiO2 aerogels using the atmospheric pressure drying technology. The influence of pH, water bath temperature and concentrations of silica sol on the structure parameters and morphology were systematically studied. SiO2 aerogel prepared with the sol concentration of 20% possessed uniform structure and the best structure parameters. Water bath temperature exhibited an obvious effect on morphology, specific surface area and porosity. Moreover, the optimal SiO2 aerogel showed a good heat resistance up to 700 °C. A lower thermal conductivity was 0.019 W/m · K at room temperature (25 °C) and 0.044 W/m · K at 600 °C. more...

ZrB2 grains were prepared on graphite by chemical vapor deposition using a gas mixture of ZrCl4, BCl3, H2 and Ar at 1150 °C. The microstructure and phase composition of the coating were investigated by SEM, XRD, XPS and TEM. The results indicated that ZrB2 grains were deposited and grown directly by a vapor–solid growth mechanism on graphite without intermediate phase. Granulated ZrB2 grains were composed by many sub–grains with different crystal orientation. The rod–like or pagoda–like ZrB2 grains with lamellar structures were grew on the granulated ZrB2 particles and composed of two parts: the internal part with axial stacking faults and annular part with radial stacking faults. more...

In order to further improve the oxidation protective ability of (ZrB 2 –SiC)/SiC coated graphite, ferrocene was introduced into the outer ZrB 2 –SiC coatings during pack cementation process. The ferrocene addition increased the porosity of the outer ZrB 2 –SiC coatings, which was favorable to thermal shock resistance of (ZrB 2 –SiC)/SiC coated specimens. The ZrB 2 –SiC coating with 3 wt% ferrocene (ZS50-3f) has optimum oxidation protective ability. Its weight retention after 15 thermal shock cycles between 1500 °C and room temperature was 96. 2%. The weight loss of ZS50-3f was only 0.2% after 19 h oxidation at 1500 °C. more...