Thermodynamics of the Vaporization of Hf and HfO2: Dissociation Energy of HfO

The vaporization of Hf and HfO2 have been studied using Langmuir vaporization and Knudsen effusion methods in combination with radioactive tracer and mass spectrometric techniques. The vapor pressure of hafnium over the temperature range from 2050° to 2300°K is represented by the equation: logpmm=9.765–31250/T. Third-law analysis of the data yields the heat of vaporization, ΔH2980=145.5±1.0 kcal/mole. The estimated boiling point of hafnium is 4575±150°K. The only hafnium containing species observed when HfO2 was vaporized was HfO. For the reaction HfO2(s)→HfO(g)+O(g)ΔH2980 was found to be 349±3 kcal/mole. The dissociation energy of HfO was estimated to be D00 (HfO)=182.6±6 kcal/mole.