Preparation Of High-quality Titanium Nitride Powders By Chemical Vapor Deposition In Fluidized-bed

Titanium nitride(TiN)is a typical non-stoichiometry metal nitride,which has excellent physical and chemical properties and shows great advantage in high temperature,super-hard and other special service environments.High quality TiN powders with near stoichiometry are key factor for fabricating high-performance TiN devices.There is an insurmountable diffusion process in the traditional gas-solid reactions for fabricating TiN powders in fluidized bed,leading to impure phase and low nitrogen contents.Chemical vapor deposition(CVD)is an important method for the preparation of high quality powders,which can control the reaction from the atomic or molecular scale.However,it is difficult to prepare homogeneous nucleation TiN powders by the CVD process based on the TiCl4-N2-H2 system.The products are always TiN coatings.Herein,to address the issue,a new fluidized bed chemical vapor deposition(FBCVD)process was developed to fabricate high quality TiN powders,that is,TiN is deposited on the surface of TiN seeds in the TiCl4-N2-H2 system.Moreover,TiC14(g)was reduced to TiC13(g)by H2(g)to break through the barrier of homogeneous nucleation,and high quality TiN powder was obtained directly in the gas phase.Based on this new idea,two new processes for preparing high quality TiN powders by FBCVD were developed.The following results have been obtained:(1)Thermodynamic analyses of the TiCl4-N2-H2 system were conducted.The effects of reaction composition and reaction temperature on the preparation of TiN powders were systematically investigated.The optimum parameters such as deposition temperature 1000 ? and n(TiCl4):n(N2):n(H2)=1:1:3 were obtained to realize high efficiency and low energy consumption.(2)The effect of particle size of TiN seeds on the fluidization quality was studied.It was found that when the average particle size of TiN seeds was larger than 52.95 ?m,the TiN seeds can realize longterm stable fluidization at 1000? even for 2 h.The new TiN particles with sub-micron nodular shapes were deposited on the surface of TiN seeds,and the TiN0.96 powders were obtained.The content of oxygen impurity was reduced by about 40%compared with the original seeds,and the deposition rate is above 2 times that of the conventional fixed bed CVD process.(3)Compared with the traditional TiCl4-N2-H2 system,the TiCl3-N2-H2 system has lower reaction Gibbs free energy and higher conversion rate,and the reaction equilibrium constant of TiCl3 synthesis is greater than 102.1 below 1000? which can break through the barrier of homogeneous nucleation and meet the requirement of homogeneous nucleation in the gas phase.(4)A modified FBCVD process with pre-reduction-nitridation was proposed,in which TiC14(g)was first reduced to form TiCl3(g)by H2(g),followed by the deposition of TiN with in-situ formed TiCl3(g)in the FBCVD reactor.The nearly spherical TiN0.98 fine powders with average size of 137.3 nm were obtained,and the Cl impurity was less than 0.001 wt.%.