Study On The Synthesis Of TaC And NaTaO₃ Ceramic Powders Via A Molten Salts Mediated Method

Tantalum carbide?TaC?possesses excellent physical,chemical and high temperature properties,which can be widely used in many fields such as cemented carbide,cutting-tools and aerospace materials.Sodium tantalate?NaTaO₃?belongs to the perovskite type semiconductor material,which can be used in photocatalytic water splitting or degradation of pollutants due to its unique band structure.However,there are several problems in their synthetic process,such as extremely high reaction temperature,heterogeneous dispersion of particles,uncontrollable morphologies.This work aims to synthesis TaC and NaTaO₃ podwers at relatively low temperature and control their morphologies via a combination of molten salt assisted carbothermal reduction and solid reaction method.The effects of process parameters,raw materials,amount and type of molten salts on the preparation and microstructure were investigated in detail.The properties of powders were also evaluated.The results are shown as follows:TaC powders were synthesized by carbothermal reduction method using Ta₂O₅,phenolic resin or nano carbon black as raw materials.When using phenolic resin as carbon source,the average particle size of TaC powder is 376 nm after firing at 1300°C for 3 hours;When carbon black serves as carbon source,some agglomerate powders are obtained even at 1500°C,and in which some sintering characteristics were found.Based on the previous work about carbothermal reduction,TaC powders were synthesized by adjusting the temperature and the amount of molten salt using Ta₂O₅ and phenolic resin as raw materials in NaCl-NaF molten salt medium.The results show that the introduction of NaCl-NaF can further reduce the synthesis temperature,and excessive molten salt can promote the conversion of the intermediate to the target product.Besides,the reaction process and mechanism of TaC powder in NaCl-NaF molten salt were analyzed by TG-DTA.The results show that the Ta₂O₅ exhibits a high solubility in NaCl-NaF molten salt,and there are two endothermic peaks at 1252°C and 1326°C.Therefore,it can be inferred that Ta₂O₅ may react with NaCl or NaF.This process may be dominated by the “dissolution precipitation mechanism”,which can be inferred from the morphology of NaTaO₃.The endothermic peak of carbon thermal reduction occurs at 1444°C,and this process may be dominated by the “diffusion mechanism”,which can be deduced from the morphology of TaC.TG-DTA is used to test the oxidation behavior of the as-synthesized powder in air atmosphere.The results show that TaC powders is apt to oxidized in the range of 500-792°C severely,and completely oxidized to tantalum oxide beyond 792°C.NaTaO₃ powders are synthesized by molten salt assisted solid state reaction using Ta₂O₅ and Na₂CO₃ as raw materials.The synthesis temperature of NaTaO₃ powder tends to reduce by changing the type and amount of molten salts.When NaCl-KCl-NaF is used as the molten salts,the liquid phase environment can be formed at 600°C,which promotes the conversion of reactants into Na TaO₃ completely and is about 100°C lower than that of solid phase method,while the temperature rise to 700°C when NaCl-KCl is used as the molten salts.The SEM images show that powders synthesized by solid state reaction method are spherical particles,but the NaTaO₃ powders can grow into cubic shape in NaCl-KCl and NaCl-KCl-NaF composite molten salt at 900°C and 1000°C,respectively.The band gap of NaTaO₃ is 4.0 eV,which can be calculated by UV-vis diffuse reflectance spectroscopy.And the experimental results show that the as-synthesized powders exhibit photocatalytic activity.The powders with better crystallinity and smaller particle size have a higher photocatalytic activity.