Combustion in the ZrF4-Mg-Si and ZrF4-Al-Si systems for preparation of zirconium silicides

Abstract

The exothermic reactions in the ZrF4−Mg-Si and ZrF4-Al-Si systems are investigated by a fast temperature recording (thermocouple) technique, differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). A quenching method is used to arrest the combustion process and conduct a layer-by-layer analysis of the products by x-ray diffraction (XRD) and electron microscopy. Two seemingly similar reactive systems exhibited considerably different combustion characteristics, composition, and morphology. Based on these investigations, we propose and discuss phase formation mechanisms at the early stages for each system. Three different pathways involving the reaction of ZrF4 with other reagents and the Mg2Si intermediate are identified to occur in the ZrF4−Mg-Si system. Contrary to the complex mechanism in the ZrF4−Mg-Si system, the early stage of the combustion process for the ZrF4-Al-Si system involves the interaction of ZrF4 with Al-Si eutectic melt. The exothermic reaction between reduced solid Zr and Si melt is the primary heat-generating step for both systems in spite of substantial differences in the early stages of the reactions. The silicon content in the reactive mixtures governs the phase composition of products. The ZrSi2 phase, with a high growth rate, forms first on the Zr particle surfaces and then grows by a reactive diffusion mechanism. The ZrSi2+Zr reaction produces silicon-lean phases (e.g., ZrSi) when the silicon supply is limited. The combustion temperature also has a considerable influence on the phase compositions of the products. High combustion temperature in the ZrF4+2Mg+Si mixture enables the formation of multiphase products (α-ZrSi and β-ZrSi), whereas the relatively lower temperatures in the 3ZrF4+4Al+3Si mixture yields a single-phase α-ZrSi. Lower combustion temperatures also make the ZrF4-Al-Si system more advantageous for the preparation of zirconium silicides.