The Thermodynamics of Boron Nitride; Low-Temperature Heat Capacity and Entropy; Heats of Combustion and Formation

Abstract

The low-temperature heat capacity of microcrystalline boron nitride has been measured up to 300°K. It has been found that in the range from 20 to 65°K the heat capacity follows a T2 relationship rather than the usual Debye T3 law, and this has been explained as resulting from the layered, quasi-two-dimensional lattice of boron nitride. The Debye-type characteristic (two-dimensional) temperature is θ2=598°K. At 298.16°K Cp=4.783 cal/mole deg and S0=3.673 eu, of which 0.034 has been extrapolated below 20°K. Derived thermodynamic functions have been tabulated at 10° intervals between 20 and 300°K. The heat of combustion of boron nitride in a conventional bomb calorimeter to yield amorphous boric oxide and elemental nitrogen has been determined to be 90.2±0.5 kcal/mole. From this value and other established thermal data the heat of formation of boron nitride is exothermic 60.7±0.7 kcal/mole.