Special features of the heat capacity of detonation nanocrystalline diamond

The heat capacity of detonation nanocrystalline diamond has been studied by adiabatic calorimetry in the 60–300 K temperature range. It has been found that the heat capacity of commercial samples of nanodiamond exceeds that of bulk diamond by more than 30%. It has been shown that this excess is only partially caused by the impurities and for an impurity-free nanodiamond it is more than 15%. It has been proposed to explain this feature of the heat capacity by a contribution of surface carbon atoms into a low energy density of diamond phonon states. Based on the experimentally obtained temperature dependences of the heat capacity, the standard values of the enthalpy, entropy, and reduced Gibbs free energy of commercial and impurity-free nanodiamond have been calculated.