Mechanically Activated Magnesiothermic Combustion Synthesis of Zirconium and Hafnium Diborides
Magnetohydrodynamic (MHD) generation of electric power has the potential to increase the thermal efficiency of fossil-fuel burning power plants. Electrode materials in MHD generators must possess a high melting point, high electrical and thermal conductivities, chemical stability, and resistance to thermal shock, oxidation, and plasma sparks/arcs. Ultra-high temperature ceramics based on diborides of zirconium and hafnium (ZrB2 and HfB 2) are promising materials for this application. Self-propagating high-temperature synthesis (SHS) is an attractive method for their large-scale fabrication, but SHS of ZrB2 and HfB2 from elemental Zr, Hf, and B is not economically viable because of the high cost of the raw materials. Magnesiothermic SHS from less expensive oxides ZrO2 and B 2O3 is a more attractive route, but these mixtures are difficult to ignite because of their low exothermicity. One promising approach to enabling combustion of low-exothermic mixtures is mechanical activation-assisted self-propagating high-temperature synthesis (MASHS), which involves a short-duration, high-energy milling step before the combustion process. In the present work, magnesium, ZrO2, and B2O3 were milled with sodium chloride (NaCl) and magnesium oxide (MgO) additives in a planetary mill, compacted into pellets, and ignited in an argon environment. The reaction mechanisms in these mixtures were studied by using thermogravimetric analysis and differential scanning calorimetry. Combustion experiments with HfO2-based mixtures were also conducted. After the MASHS process, MgO and NaCl were removed from the products by acid leaching. The effects of excess Mg and the additives (i.e., MgO and NaCl) on the combustion characteristics and products were investigated. It has been shown that NaCl prevents loss of the mixture during milling, lowers the combustion temperature, and decreases the particle size of the products, while the excess magnesium improves the oxide-to-boride conversion.
Mechanical engineering|Materials science
Cordova, Sergio, "Mechanically Activated Magnesiothermic Combustion Synthesis of Zirconium and Hafnium Diborides" (2017). ETD Collection for University of Texas, El Paso. AAI10283704.