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University of Birmingham Sol-gel synthesis and formation mechanism of ultrahigh temperature ceramic

HfB 2 is an ultra-high temperature ceramic that holds great potential for use in thermal protection systems because of favourable temperature stability, good mechanical properties and oxidation resistance. In this work HfB 2 powder has been synthesised via a sol-gel based route using phenolic resin, hafnium chloride and boric acid as the source of carbon, hafnium and boron respectively, though a small number of comparative experiments involved amorphous boron as boron source. The effects of calcination dwell time and Hf:C and Hf:B molar ratio on the purity and morphology of the final powder have been studied and the mechanism of HfB 2 formation investigated using FTIR, TGA, DSC and XRD. The results showed that whilst temperatures as low as 1300 o C could be used to produce HfB 2 particles, the calcination needed to last for about 25 h and the long duration resulted in particle growth along the c-axis of the HfB 2 crystals yielding tube like structures of about 10 µm long. Equiaxed particles 1 – 2 µm in size were obtained when the precursor was calcined at 1600 o C for 2 h. The reaction mechanism involved carbo/borothermal reduction and the indications were that the formation of HfB 2 at 1300 o C is through the intermediate formation of an amorphous B or boron sub oxides, although at higher temperatures more than one reaction mechanism may be active.