Preparation Of Transition Metal Boride Powders

Boride ceramics are the main candidate materials applied in extreme environments such as ultra-high temperature, super-hard and electrometallurgy due to their high melting point, high hardness and resisitance to chemical substances. However, the presence of the covalent B-B bond makes it difficult for the boride ceramics to be fully densified with a low self-diffusion coefficienct. Smaller grain size with larger specific surface area induces smaller grain-boundary energy and larger driving force for sintering. As a result, it has important significants on both theory and application to investigate the synthesis of ultra-fine boride powders for the purpose of improving densification and mechanical properties of the ceramics.Traditionally, boride powders were synthesized at high temperatures with large particle sizes and high impurities containing carbon or element boron. In this dissertation, several transition metal boride powders were prepared using amphous boron as a source of boron and metal element, oxide and hydride as a source of metal, respectively. In addition, molten salts were introduced during the synthesis process and its effects on the as-synthesised powders were investigated. The major works and results are summarized as follows:Firstly, NbB2 and NbB powders were prepared by both solid-state reaction and molten salt method. The results indicated that NbB2 powders could be prepared above 1200 oC under vacuum with a Nb2O5/B molar ratio of 1:9, but there still had some unreacted B remaimed in the product. When the Nb2O5/B molar ratio was 3:22, pure NbB2 powders could be prepared by the molten salt method at a lowest temperature of 800 oC under the protection of flowing Ar gas. The presence of molten salts remarkablely improved the process of the borothermal reduction reaction and inhibited the grain growth of the as-synthesized NbB2 powers. The finest NbB2 powders with a particle size of only 32 nm could be prepared at 1000 oC with the weight of the salts 10 times that of the reactants. When the Nb2O5/B molar ratio was 3:16, pure NbB powders could be prepared above 1100 oC by the molten salt method.Secondly, Ta B2 and TaB powders were prepared by both solid-state reaction and molten salt method. The results indicated that TaB2 powders could be prepared by both solid-state method and molten salt method above 900 oC under the protection of flowing Ar gas with a Ta2O5/B molar ratio of 3:22. The addition of the molten salt affected the morphology of the as-synthesized TaB2 powders. Flower-like TaB2 powders composed of nanorods were obtained as the the weight of the salts 10 times that of the reactants. When the Ta2O5/B molar ratio was 3:16, pure TaB powders could be prepared above 1100 oC by the molten salt method.Finally, TiB2 powders could not be prepared by the molten salt method below 1100 oC with TiO2 and TiN as a metal suource. Howerver, with metal Ti and TiH2 as raw materials, Ti B2 powders were sucessfuly prepared. The addition of molten salt obviously decreased the particle size of the as synthesized powders. When the weight of the salts 20 times that of the reactants, TiB2 powders prepared by the molten salt method at 1000 oC with TiH2 as as raw material had an average particle size of only 60 nm.