Prediction of formation of cubic boron nitride by construction of temperature-pressure phase diagram at the nanoscale

The size-dependent phase diagram of BN was developed on the basis of the nanothermodynamic theory. Our studied results suggest that cubic BN (c-BN) is more stable than hexagonal BN (h-BN) in the deep nanometer scale and the triple point of c-BN, h-BN and liquid shifts toward the lower temperature and pressure with decreasing the crystal size. Moreover, surface stress, which is determined by the experimental conditions, is the main reason to influence the formation of c-BN nuclei. The developed phase diagram of BN could help us to exploit new techniques for the fabrication of c-BN nanomaterials.