Preparation Of Ultrafine ZrB₂-SiC Composite Powders By A Combined Sol-gel And Microwave Boro/Carbothermal Reduction Method

ZrB2-SiC composite materials are expected to be the most promising candidatematerials for ultra–high temperature applications. In this work, a combined sol-gel andmicrowave boro/carbothermal reduction method was developed to synthesize ultrafineZrB2, ZrB2-SiC and ZrC powders. The effects of various processing parameters includingreaction temperature, ratios of raw materials, kinds and amounts of additions and heatingmethods on the synthesis of ZrB2-SiC composite powders were investigated. Moreover,ZrB2, ZrC and ZrB2hollow spheres powders were also prepared by using the sol-gel andcarbothermal reduction method, and oxidation resistance of prepared ZrB2, SiC andZrB2-SiC ultrafine powders prepared were also investigated.The results indicate that:(1) Ultrafine ZrB2powders were synthesized by a combined sol-gel andboro/carbothermal reduction method using zirconium oxychloride, boric acid andglucose as starting materials. The optimized molar ratio of n(B)/n(Zr) was2.5andn(C)/n(Zr) was5.0. Pure ZrB2phase can be prepared by using conventional heatingmethod with the boro/carbothermal reduction temperature of1773K. The grain sizes ofthe prepared ZrB2at1773K were about1~2μm and the crystalline sizes were calculatedto be about55nm by using the Scherrer equation. However, pure ZrB2phase can beprepared by using microwave heating method at a temperature as low as1573K,indicating that microwave heating method could reduce the synthesis temperature ofZrB2ultrafine powders by at least200K. The average crystalline sizes of the preparedZrB2were about44nm.(2) Ultrafine ZrB2-SiC composite powders were prepared by a combined sol-gel andboro/carbothermal reduction technique. The optimum molar ratios of n(B)/n(Zr) andn(C)/n(Zr+Si) for the preparation of pure ultrafine ZrB2-SiC composite powders were2.5and8.0, respectively. The required heating temperature was1773K by using theconventional boro/carbothermal reduction method. The grain sizes of the prepared ZrB2at1773K/2h were about500nm and the formed flocculent SiC were uniformlydistributed around the ZrB2grains. The crystalline sizes of ZrB2and SiC in thecomposite powders were about88nm and37nm, respectively. For comparison, pureZrB2-SiC ultrafine composite powders can be prepared by using microwave heating method when the boro/carbothermal reduction temperature as low as1573K. Theaverage crystalline sizes of ZrB2and SiC in the as-prepared samples were about58and27nm, respectively. A back propagation artificial neural networks (BP ANNs) was usedto establish the non-liner relationship between the relative content of ZrB2and SiC infinal products and the carbothermal reduction temperature, molar ratio of n(B)/n(Zr) andn(C)/n(Zr+Si) and the amount of additions. The test experiment verifies that theprediction ability of networks is quite satisfactory.(3) The oxidation kinetics of ZrB2and SiC powders, and ZrB2-SiC compositepowders was investigated by using non-isothermal thermogravimetry method. The resultsshowed that the oxidation activity energy of pure ZrB2and SiC was248.96kJ·mol-1and216.47kJ·mol-1, respectively. However, the oxidation activity energy of ZrB2in theZrB2-SiC composite powders was307.90kJ·mol-1, which was higher than that of pureZrB2, indicating that the addition of SiC into ZrB2can enhance the oxidation resistanceof ZrB2.(4) Hollow ZrB2spheres were successfully synthesized using a combinationhydrothermal and hard template-assisted technique with zirconium oxychloride, boricacid and glucose as starting materials. The results showed that the hollow ZrB2ultrfinespheres which were synthesized by template-assisted technique had a relativly smallersize and a more uniform hollow croe-shell structure than that prepared by one-stepprocess. The influence of the temperature greatly affects the formation of hollowultrafine ZrB2spheres. The optimized heating temperature and molar ratio ofn(C)/n(ZrO2+B2O3) were1773K and6.5, respectively.(5) ZrC ultrafine powders were synthesized by a combined sol-gel and microwavecarbothermal reduction method by using zirconium oxychloride and citric acid as startingmaterials. The optimum molar ratios of n(C)/n(Zr) for the preparation of phase pureultrafine ZrC composite powders was5.0, and the firing temperature required was1573K. This temperature was lower than that require for conventional carbothermal reductionmethod at least200K. The grain sizes of ZrC in the samples prepared at1573K for3hwere about200nm, and the average crystalline sizes of ZrC were about59nm.