High-Temperature Evaporation and Thermodynamic Properties of Zirconium Diboride

Abstract

Vapor pressures of zirconium over zirconium diboride have been measured by the Knudsen technique over the temperature range 2150° to 2475°K. A new type of apparatus was constructed and used successfully in the study. Zirconium diboride was determined to evaporate congruently at a composition of ZrB_{1.906(+0.025 or —0.010)} by heating solid pressed plugs of both zirconium‐rich and boron‐rich material to constant composition at 2400°C to 2500°C. The over‐all reaction is $${\mathrm{ZrB}}_{\text{1.906}}(\mathrm{s}\mathrm{)=}\mathrm{Zr}(\mathrm{g}\text{)+1.906\hspace{0.17em}}\mathrm{B}(\mathrm{g})$$ Three series of measurements were made using tungsten crucibles and different orifice sizes. Second‐law and third‐law treatments of the data did not agree. Thermodynamic calculations were made which indicated that water vapor at low background pressures would produce volatile oxides of both zirconium and boron. This reaction was investigated by adding water vapor to the system and the increased transport of zirconium was clearly demonstrated. Accordingly, each pressure measurement was corrected by a factor β, constant for each series, related to the background pressure. From the corrected pressures, values of ΔH₀^o for the reaction were computed by the third‐law treatment. From the three series a vaporization coefficient of 0.025±0.010 was computed, leading to an equilibrium ΔH₀^o of 458.3±6.5 kcal/mole for reaction 1, or a partial pressure of Zr over ZrB_1.906 of 2.38×10^—10 atm±20% at 2000°K. The variance between this and an expected value of 477.4 kcal/mole is presumed to be related, at least in part, to discrepancies in the heat of vaporization of zirconium. The results verify the prediction that the Zr–B system exhibits a congruently evaporating phase and suggest that the congruently evaporating composition in many high‐temperature systems will occur at nonstoichiometric compositions.