Low-Temperature Synthesis of VB 2 Nanopowders by a Molten-Salt-Assisted Borothermal Reduction Process
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VANADIUM diboride (VB2) is one of the most promising ultrahigh-temperature ceramics (UHTC), owing to its excellent physical properties such as high melting point, outstanding wear and corrosion resistance, high thermal conductivity, good chemical stability, and so on.[1,2] These unique properties constitute VB2 and VB2-based ceramics as potential candidates with respect to high-temperature structural materials and surface protection coatings.[3,4] It is well known that the particle size and impurity (oxygen, carbon, or other metal) are the most important factors to assess the characteristics of VB2 powders. Ultrafine and high-purity powders can effectively increase the driving force for sintering and improve the densification behavior.[5,6] Matthew Thompson et al.[7] concluded that oxygen impurities in boride ceramics caused grain coarsening during the sintering process. The decrease of oxygen impurities promoted densification, reduced boride grain size, and led to increase room-temperature flexure strength.
YUE-DONG WU, GUO-HUA ZHANG, YU WANG, and RUI XU are with the State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China. Contact e-mail: [email protected] Manuscript submitted November 26, 2018. Article published online June 7, 2019. 1696—VOLUME 50B, AUGUST 2019
VB2 powders can be generally obtained via the following synthesis routes: direct reaction of elemental precursor powders,[8] mechanochemical synthesis,[9] self-propagating high-temperature reaction (SHS),[10,11] and solid-state reaction between vanadium chloride and MgB2 or NaBH4.[12,13] Wang et al.[8] prepared VB2 powders by a direct combination of elemental vanadium with boron in a high-energy shaker ball-miller. Hassanzadeh-Tabrizi et al.[9] reported that vanadium boride nanopowders were synthesized through mechanochemical method in a high-energy ball mill. Rao et al.[10] produced VB2 powders with a large size via self-propagating high-temperature technique. As mentioned above, most of these publications did not seek to prepare fine and high-purity starting powders, leading to the decrease of the driving force for sintering and the increase of the densification temperature. Therefore, it is necessary to develop a new technique to produce high-purity and fine VB2 powders. In an effort to develop a new synthesis of ultrafine VB2 powders with a high purity, this work tried to prepare it at low temperatures (1123 K to 1273 K (850 °C to 1000 °C)) via a molten-salt-assisted borothermal reduction reaction. V2O3 and boron powders were used as the raw materials, and NaCl salt was introduced to provide a liquid environment. The addition of NaCl salt could facilitate the diffusion of reactants, which assisted in reducing the reaction temperature and shortening the time required for complete reaction. A similar method had been also adopted by Ran et al.[14] by adopting NH4VO3 as vanadium source. The METALLURGICAL AND MATERIALS TRANSACTIONS B
utilization of V2O3 could decrease the consumption of boron due to the
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