Synthesis Of Fine Hafnium Carbide Ceramic Powders

Hafnium carbide has a typical face-centered cubic NaCl crystal structure.The hafnium carbide structure exhibits both metallic and covalent bonding,which leads to favorable physical and chemical properties,such as high hardness,high strength,and high melting point.Hence,hafnium carbide has applications in the field of cutting tools and mold field due to its high hardness,and in the field of rocket nozzles due to its high modulus of elasticity,high thermal and electrical conductivities,and high melting point.Thus,hafnium carbide has good value in use and wide application prospect.In this paper,Hafnium dichloride oxide,salicylic acid and citric acid were used as the main raw materials.Ultra-fine hafnium carbide(HfC)powders were synthesized using a novel method combining liquid precursor conversion and spark plasma sintering(SPS).Solution-based processing was used to achieve a fine-scale mixing(atomic or molecular level)of the reactants,then placed in a tube furnace to obtain the precursor of hafnium carbide with high reactivity,and further treatment by SPS allowed fast formation of hafnium carbide powders.We investigated the effect of the type of acid used during the liquid precursor conversion on the synthesized powders,where mixtures were prepared using salicylic acid,citric acid,or a combination of these.The synthesis condition of hafnium carbide precursor and rapid synthesis process of SPS were studied.The suitable conditions of the tube furnace were as followed: The heat treatment temperature: 800?,holding time: 0.5h,atmosphere: argon.The study of SPS process was mainly focused on the factors as belowed: raw material ratio,synthesis temperature,heating rate,atmosphere pressure,holding time,milling time.We have come to the best synthetic conditions: the total ratio of hafnium and carbon was 5,synthesis temperature: 1550?,heating rate:300K/min,holding time:6min,atmosphere: argon,milling time of hafnium carbide precursor: 6h.The results show that pure hafnium carbide powders(with an average particle sizes of 350nm)were obtained at a relatively low temperature(1550?)using the hafnium dichloride oxide-salicylic acid-citric acid system.The oxygen content of the synthesized powders was only 0.97wt%.The different of the synthesis system had a significant effect on the synthesis product.When using the hafnium dichloride oxide-citric acid system,the temperature required to produce pure hafnium carbide increased to 1700?.In the case of the hafnium dichloride oxide-salicylic acid system,pure HfC was not obtained,even at a synthesis temperature of 1700?.Spark plasma sintering(SPS)has a rapid heating and cooling rate compare to the traditional sintering furnace,which has a potential advantages on the powder synthesis.It can reduce the time required for the carbothermal reduction,so it is beneficial to obtain the hafnium carbide powders with the smaller average particle size.In addition,the fast heating rate makes the reactants keep high active,which may reduce the temperature required for the carbothermal reaction.Therefore,this technique is very suitable for the synthesis of powders.