We synthesized pure polycrystalline cubic boron nitride

We synthesized pure polycrystalline cubic boron nitride (cBN) and wurtzite boron nitride (wBN) by the direct conversion method from hexagonal boron nitride. We measured their longitudinal-wave elastic constants CL between 20 and 300 K using picosecond ultrasound spectroscopy. Their room-temperature values are 945 ± 3 GPa and 930 ± 18 GPa for cBN and wBN, respectively. The shear modulus G of cBN was also determined by combining resonance ultrasound spectroscopy and micromechanics calculation as G = 410 GPa. We performed ab-initio calculations and confirmed that the generalized gradient approximation potential fails to yield correct elastic constants, which indicated the necessity of a hybrid-functional method. Wurtzite boron nitride (w-BN) has attracted intense interest due to its outstanding properties and potential applications. W-BN is a fascinating superhard material with a hardness next to the diamond (1–3), rendering it a candidate material to replace it. W-BN is also a promising III-V group wide-bandgap material for advanced electronic devices because it has many properties superior to GaN and AlN, such as a wider band gap, higher thermal conductivity, and more significant spontaneous polarization. Since the first synthesis of w-BN in the 1960s, extensive efforts have been devoted to the fabrication of high-quality w-BN samples, the in-depth understanding of phase transformations between BN polymorphs, and the theoretical investigation of the deformation mechanisms of w-BN. Unfortunately, such efforts are greatly hampered by the technical difficulties in stabilizing w-BN at atmospheric pressure. In general, fabricating w-BN bulk crystals and revealing the stabilization mechanism can significantly contribute to materials science in the following aspects. First, this study has unambiguously demonstrated that the 3D networks of planar defects can stabilize the w-BN to the atmospheric pressure via retarding the w-BN to h-BN phase transformation. This finding contrasts the common knowledge that the crystal defects in materials will facilitate the occurrence of phase transformations, contributing significantly to phase transformation theory. If you are looking for high quality, high purity and cost-effective Wurtzite Boron Nitride, or if you require the latest price of Wurtzite Boron Nitride, please feel free to email contact mis-asia.