Preparation Of Ultra - Fine Zirconium Boride Powder By Sol - Gel Carbothermal Reduction

With the rapid development of science and technology, aeroxpace faced with new challenges constantly and the requirements for materials are always increasingly demanding. Therefore, the research for the Ultra-high temperature ceramics materials is one of the most active international topic which has excellent physical and chemical properties. Due to a strong chemical bond, Zirconium diboride (ZrB2) has dual properties of metal and ceramic and comprehensive exhibits fine features, such as a high melting-point(3245℃), high hardness, high thermal conductivity, excellent electrical conductivity and low coefficient of thermal expansion in high temperature etc. As a result, ZrB2 will be the most promising candidate for the UHTCs.Ultra-fine ZrB2 powders were synthesized with sol-gel method collaborative carbothermic reduction which was depended on the basic principle of carbothermal reduction in ZrO2-B2O3-C system.The feasibility of the reaction pathway was demonstrated by the thermodynamics theories and experimental results.By comparing the two experimental program, the influences of different program, zirconium ratio of boron, zirconium ratio of carbon,reaction temperatures and reaction time on the phase compostion of the synthesized powders were investigated.According to the orthogonal experiment, factors of the process for preparing ZrB2 precursor by sol-gel were investigated, such as pH,metal Zr4+ and citric acid ratio,sol-gel temperature and dispersant content, defined the optimal synthesis conditions.Subsequent purification process for the synthesized products were researched preliminary.Conclusions are as following:Combining the thermodynamics theories and the TG-DSC with experimental results proved that ZrB2 powders were able to be synthesized in 1600℃.The optimum conditions for the preparation of ZrB2 powders used ZrOCl2·8H2O as the raw materials of zirconium are:n(B)/n(Zr)=4; n(C)/n(Zr)=10; pyrolysis 2h at 1600℃. As the pyrolysis temperature increased the ZrB2 content always increased gradually and the grain size groweing up significantly at the same time.There are not only small spherical grains but also some hexagonal crystals in the products when the pyrolysis temperature increased to 1600℃.The SEM results indicate that there are some agglomeration between particles in different pyrolysis temperature.The TEM results showed that a single grain size is about 15nm. ZrB2 powders have a large specific surface area of 88.1396m2/g.The optimum conditions for the preparation of ZrB2 powders used Zr(NO3)4·5H2O as the raw materials of zirconium are:n(B)/n(Zr)=3.5; n(C)/n(Zr)=12.5; pyrolysis 2h at 1600℃. The higher pyrolysis temperature, the higher purity of the ZrB2 powders.The longer pyrolysis time,the better crystalline powders and the bigger grain size. The SEM results indicate that the particle is spherical or spherical grains and the size is smaller and better dispersion at a lower temperature. In contrast, merging and growing up made the average particle size increased significantly. ZrB2 powders have a large specific surface area of 116.2643m2/g.By contrasting, the results showed that Zr(NO3)4-5H2O as the raw materials of zirconium has a easier synthesis process, the narrower particles size distribution of the powders, a better dispersion and sphericity and also a larger specific surface area.We made a crafs optimization for the synthesis conditions of the zirconium nitrate sol by designing a orthogonal experiment.The results showed that stable complex will be formed when the pH is 4. The concentration of complex salt molecules reach the strongest when metal Zr4+ and citric acid ratio is 2. Taking the efficiency into account,the most suitable sol-gel temperature is 50℃. Due to the dispersibility of polyethylene glycol,we can obtain better dispersed ZrB2 powders when the dispersant content is 2%.In this paper, a preliminary study about the subsequent purification process was done. At first the boron impurities was removed by washing with deionized water. Secondly, removed the carbonaceous impurities by heat treatment in the open air. The result indicated that ZrB2 could be oxidized and adsorpted oxygen from the air itself during heat-treatment in air which will lead to increase the oxygen content and it will be harmful to the properties of the ZrB2.In order to reduce the oxygen content in the products we should heat-treatment in an inert atmosphere protection and higher temperature.