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 C_p=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.