Preparation and Characterization of Ultra-Fine ZrB<sub>2</sub> Powders from Inorganic-Organic Hybrid Precursors via Carbothermal Reduction

Abstract

In the present study, we report on the synthesis and carbothermal reduction of ultra-fine zirconium diboride powders by using inorganic-organic hybrid precursors of Zirconium (IV) nitrate pentahydrate, boric acid and citric acid as sources of zirconia, boron oxide and carbon, resoectively. The effect of molar ratio of reactants and reaction temperatures on the as-synthesized precursors were investigated. The thermodynamic change in the ZrO2-B2O3-C system was mainly studied by thermogravimetric and differential scanning calorimetry. The phase compositions and crystalline state of the products after heat treatment was determined by X-ray diffraction and the crystallite size and morphology of the synthesized powders were characterized by scanning electron microscopy. It was found that the as-synthesized precursor with B/Zr molar ratio of 3.5 can transform into zirconium diboride and zirconium carbide by heating in an argon atmosphere with temperatures as low as 1400°C and the synthesized powders exhibited near-spherical morphology with a samll average crystallite size of about 200nm and dispersed relatively uniformly. Moreover, with the reaction temperature increased, the purity of the zirconium diboride powders are higher. The mixture was finally transformed into pure zirconium diboride at 1600°C. However, the grain sizes increased significantly and tended to be aggregated with the reaction temperature increased to 1600°C. The synthesized ZrB2 powders showed a porous structure and the grain sizes on the exterior is larger than the interior because of the higher heat treatment temperature. The finally single ultra-fine ZrB2 grain sizes were distributed from 190nm to 690nm in two-dimensions and have a larger specific surface area of 88.14m2/g.