Chemical Vapor Synthesis of Ultrafine Niobium Powder Via Sodiothermic Reduction of Chloride
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RODUCTION
HIGH-PERFORMANCE capacitors used in miniaturized electronic devices are manufactured from fine and high-purity tantalum (Ta) powders. However, Ta is expensive because of limited resources in the earth’s crust. Niobium (Nb) has similar physical and chemical properties to those of Ta and is the most promising alternative to Ta.[1,2] Nb powder metallurgy is used to prepare the anode in a Nb capacitor. The capacitance is proportional to the anode surface area. That is, the synthesis of fine Nb powders is critical to design new capacitors with high volumetric efficiency.[3,4] A high capacitance exceeding 0.2 FV/g has been reported for a Ta capacitor with an anode-specific surface area above 5.0 m2/g (which is equivalent to an average anode particle size below 70 nm).[5] Therefore, it is necessary to develop a novel synthesis for fine Nb powders, particularly Nb nanopowders with diameters below 100 nm.
CHAO DU, YUSONG PAN, BIAO HU, RUN HUANG, and SHAODING SHENG are with the School of Materials Science and Engineering, Anhui University of Science and Technology, Huainan 232001, Anhui, China. Contact e-mail: [email protected] CHENGLING PAN and CHAO ZHANG are with the School of Materials Science and Engineering, Anhui University of Science and Technology and also with Institute of Environment-friendly Materials and Occupational Health, Anhui University of Science and Technology (Wuhu), Wuhu 241003, Anhui, China. Contact e-mail: [email protected] Manuscript submitted on March 7, 2020, accepted September 22, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Table I shows various synthesis methods for Nb powders that have been investigated in recent years: metallothermic reduction (aluminothermic,[6] carbothermic,[6] sodiothermic,[7] magnesiothermic[8–10] and calciothermic reduction methods[11]) and electrochemical reduction (EMR,[12] OS[13–15] and FFC methods[16]). Strong exothermal reactions in metallothermic reduction processes induce the growth of particles that agglomerate and sinter into large particles. Therefore, it is difficult to produce Nb nanopowders (below 100 nm in size), and metallothermic reduction produces Nb powders with a non-uniform particle size distribution. Electrochemical reduction (EMR, OS and FFC methods) is relatively low-energy reaction, and the heat is dissipated by molten salts. However, washing the reaction mixture with water to separate the product and the molten salt increases the oxygen content of the Nb powder. It is also difficult to synthesize Nb nanopowders with a uniform particle size distribution using electrochemical reduction. Chemical vapor synthesis (CVS, derived from chemical vapor deposition (CVD))[17–20] is a commonly used method to produce metal nanopowders, for which it is easy to control the process conditions, particle size, particle crystal structure, and purity. In CVS, the metal chloride raw material is boiled and introduced into the reaction zone as a gas. An excess of the H2 reductant is used to ensure high reaction rate and conversion rate. In an earlier study by Lampre
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