URANIUM DIBORIDE - PREPARATION, ENTHALPY OF FORMATION AT 298.15 DEGREES K, HEAT CAPACITY FROM 1 DEGREE TO 350 DEGREES K, AND SOME DERIVED THERMODYNAMIC PROPERTIES

A sample of uranium diboride was prepared and characterized as UB 1.979±0006 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-1mole-1, 13.17±0.03 cal °K -1mole-1, 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-1, 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-1mole-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.