Synthesis of Early Transition Metal Diboride Nanocrystals

Group IV and V metal diboride (MB2) have very high melting point (>3000 °C), high hardness, good chemical stability, thermal and electrical conductivity. CMCs based on these phases are expected to be potential candidate materials for applications at extreme temperature and harsh environments in the aerospace industry [1]. Availability of submicrometric/nanometric boride particles has indeed the potential to improve several stages of ceramic processing, [2] or for instance to facilitate the sintering of those ceramics, [3] due to enhanced particle reactivity. The synthesis of MB2 nanocrystals (M=Zr, Ti, Hf, Nb, Ta) using NaBH4 and the respective metal oxides at atmospheric pressure was studied at temperatures between 400 and 1000 °C. [7] Reaction products were analyzed by x-ray diffraction, the crystallite size was determined after Rietveld refinement of diffraction patterns, whilst the morphology was analyzed by scanning and transmission electron microscopy. The reaction occurs first via decomposition of NaBH4, followed by the formation crystalline ternary species Na2MO3 (M=Ti, Zr, Hf), NaMO3 (M= Nb, Ta) and several sodium borates at the early stage of the synthesis at 500-600 °C. Metal diboride (MB2) and sodium meta-borate (NaBO2) are the final products at 700 °C.