Equilibrium Composition of Products in a Hafnium Dioxide–Calcium–Nitrogen–Carbon Mixture at Adiabatic Combustion Temperature

This paper presents the results of thermodynamic calculation of adiabatic temperature and the equilibrium composition of products of HfO₂ reduction with calcium depending on carbon and calcium content at different pressures. The formation of solid HfN–HfC solutions is identified with the formation of hafnium carbonitride. It is shown that adiabatic temperatures lie in a range of 2000–2900 K, and its elevation is limited by the melting of CaO at 2900 K. The introduction of carbon often reduces the adiabatic temperature, and a pressure rise leads to its increase. A connection is revealed between the composition of products and the type of temperature curves. The main reason why adiabatic temperature rises along with pressure is a displacement of equilibrium toward the formation of condensed phases and an increase in the fraction of HfN in the products.