Uranium Diboride: Preparation, Enthalpy of Formation at 298.15°K, Heat Capacity from 1° to 350°K, and Some Derived Thermodynamic Properties

Abstract

A sample of uranium diboride was prepared and characterized as UB1.979±0.006 with 0.06 ± 0.03 wt % of identified impurities. The standard enthalpy of combustion in fluorine was determined to be − 1021.2 ± 1.1 kcal mole−1. The heat capacity was measured from 0.84° to 350°K. At 298.15°K the heat capacity CP°, entropy S°, and enthalpy increment H° − H°0 are 13.23 ± 0.03 cal K−1·mole−1, 13.17 ± 0.03 cal °K−1·mole−, and 2108 ± 4 cal mole−1, respectively. The following values were obtained for the standard enthalpy, entropy, and Gibbs energy of formation of UB2 at 298.15°K: ΔHf° = − 39.3 ± 4.0 kcal mole−1, ΔSf° = − 1.54 ± 0.05 cal °K−1·mole−, and ΔGf° = − 38.8 ± 4.0 kcal mole−1. These agree within experimental error with values calculated from high-temperature effusion measurements. The heat-capacity results below 4.2°K follow the equation CP = (9.40 ± 0.01)T + (3.18 ± 0.14) × 10−2T3mJ °K−1·mole−1. The relatively high value for the coefficient of the linear term indicates that uranium diboride is a good electrical conductor. There seems to be an anomaly in the heat capacity between 40° and 100°K, which may be due to a two-level Schottky anomaly with the upper level having a degeneracy twice that of the lower level and a separation of the levels equal to 175° ± 5°K or 122 ± 4 cm−1.