Synthesis Of ZrB2 Powders Research Articles

Microwave sintering of ZrB2-based ceramics: A review

Recently, microwave sintering has absorbed remarkable attention on the basis of enhanced microstructural/mechanical characteristics in comparison with conventional sintering techniques based on powder technology. This method not only can be employed for the processing of metals, alloys, and metal matrix composites but also for the manufacturing of advanced ceramics and ceramic matrix composites. Zirconium diboride (ZrB2) as an interesting member of ultrahigh temperature ceramics is one of the most undertaking candidates in modern structural ceramics applications. This paper reviews the processing-densification-mechanical properties correlations in microwave-sintered ZrB2-based ceramics and composites. The text concentrates on the microwave-assisted production of ZrB2 divided into two categories: synthesis of ZrB2 powders by microwave sintering and sintering of ZrB2-based ceramics and composites by microwave sintering. The effects of some additives and reinforcements, such as B4C, SiC, TiC, and MgO, on zirconium diboride's densification and mechanical properties are summarized.

Synthesis of ZrB2 powders by molten-salt participating silicothermic reduction

Abstract A novel molten-salt silicothermic reduction method was developed to prepare ZrB2 powders from raw materials of ZrO2, Na2B4O7 and silicon powders with the assistance of sodium orthosilicate in the reaction system. The influences of reaction temperature and quantity of silicon on the phase composition and morphology of final products were investigated. The results showed that the silicothermic reduction reaction for synthesizing ZrB2 was initiated at the temperature as low as 800 °C, and the reaction completed at 1200 °C for 3 h. Only ZrB2 was detected by X-ray diffractormeter in the final products prepared at only 1000 °C for 3 h with addition of excessive 20 wt% Si powders in the reaction system. Moreover, the sheet-like ZrB2 particles were observed by SEM and TEM, which were homogeneously distributed at the samples.

Synthesis of fine ZrB2 powders by new borothermal reduction of coarse ZrO2 powders

Effects of ZrO2 particle size (~50nm and ~2µm) on synthesis of ZrB2 powders via conventional or new borothermal reduction were investigated. The conventional process was a one-step heat treatment at 1600°C in vacuum. The newly developed process consisted of two-step heat treatment (first-step at 1000°C and second-step at 1600°C) plus intermediate step of water washing. Micron-sized ZrB2 powders were prepared by conventional process, independent of initial ZrO2 particle size. Based on new process, submicrometric ZrB2 powders were synthesized using nano-size ZrO2. However, submicrometric ZrB2 could also be synthesized using micron-size ZrO2 by prolonging the holding time during first-step heat treatment. Therefore, results of present study suggested that fine ZrB2 powders could be synthesized by controlling the time of first-step heat treatment of the new borothermal reduction process by converting ZrO2 as completely as possible independent of initial particle size.

Synthesis of ZrB2 powders by carbothermal and borothermal reduction

Zirconium diboride (ZrB2) powders were synthesized using ZrO2+B2O3+C (carbothermal reduction), ZrO2+B4C (boron carbide reduction), and ZrO2+B4C+C (combined reduction) with various compositions at 1250°C for 1–3h under flowing argon. ZrB2 powders synthesized using ZrO2+B2O3+C displayed rod shape growth. There was much residual impurity carbon in ZrB2 powders synthesized using ZrO2+B4C+C. When synthesized using ZrO2+B4C, there were the residual impurity B2O3 and little rod shape growth. Residual B2O3 impurities were easily removed by washing with methanol. We concluded that the ZrB2 powder synthesis method using boron carbide reduction is the most desirable way to produce ZrB2 powders among the three synthesis routes. ZrB2 powders synthesized using ZrO2+B4C have a particle size of 1.1μm and a hexagonal shape, and low oxygen content (0.725wt.%).

Synthesis of Ultra-Fine and High-Pure Zirconium Diboride Powders Using Solution-Based Processing

Ultra-fine and high-pure zirconium diboride powders were prepared by carbothermal reduction boronization of zirconia/boron carbide/carbon mixtures. Fine-scale mixing of the reactants was achieved by solution-based processing in which zirconium oxychloride (ZrOCl2⋅8H2O) as the zirconia-bearing precursor was precipitated in the suspension of boron carbide (B4C) and carbon powders in water. The carbothermal reduction boronization reaction was substantially completed at relatively low temperatures (<1600°C) and the resulting products (ZrB2 powders) had small average grain sizes (1~2 μm) and high purity (>99.6 wt%). The experiments indicated that excessive B4C and C were necessary during the carbothermal reduction boronization because of volatilization of boron and carbon elements. The effects of temperature and holding time on the synthesis of ZrB2 powders were also discussed.