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|Keyword||流化床化学气相沉积, 氮化钛粉体, 化学计量比, 均相形核|
氮化钛（TiN）是一种典型的非化学计量比的金属氮化物，具有优异的物化特性，在高温、超硬等特殊服役环境中具有重要的应用价值。近化学计量比的高质量TiN粉体是制备高性能TiN器件的关键。基于气–固反应的传统流化床技术制备TiN粉体存在难以逾越的气固扩散过程，导致物相不纯，且氮含量较低。化学气相沉积工艺（Chemical Vapor Deposition, CVD）能够从原子或分子尺度调控反应，是制备高质量粉体的重要手段。但是，由于传统TiCl4-N2-H2体系无法在气相中均相形核，只能获得TiN涂层。基于此，本文提出了一种流化床化学气相沉积（Fluidized Bed Chemical Vapor Deposition, FBCVD）制备高质量TiN粉体的新思路，即利用TiCl4-N2-H2体系易成膜特性在TiN种子粉体表面沉积高质量TiN颗粒，并进一步通过将TiCl4还原为TiCl3以突破难以均相形核的壁垒，从气相中直接获得高质量TiN细粉。基于该新思路，发展了两种FBCVD制备高质量TiN粉体的新工艺，取得以下研究成果：（1）对TiCl4-N2-H2体系制备TiN进行热力学分析，系统考察了反应组份配比及反应温度对制备TiN粉体的影响规律，获得了沉积温度1000 °C、n(TiCl4):n(N2):n(H2)=1:1:3的优化工艺参数，实现了低杂质含量TiN粉体的高效率、低能耗制备。（2）研究了种子粉体粒径对沉积过程流化质量的影响，发现当TiN种子粉体粒径大于52.95 μm时，即使在1000 °C温度下沉积2 h也不会失流。利用FBCVD工艺在TiN种子粉体表面沉积出了亚微米级结节状的新生TiN颗粒，获得了TiN0.96粉体，其氧杂质含量比原始种子粉体降低了约40%，且沉积速率约为传统固定床CVD工艺的2倍以上。（3）通过热力学计算，发现与TiCl4-N2-H2体系相比，TiCl3-N2-H2体系制备TiN反应具有更小的吉布斯自由能和更高的转化率，低于1000 °C时TiCl3合成TiN的反应平衡常数大于102.1，满足气相均相形核的所需条件，可突破传统TiCl4-N2-H2体系难以均相形核的壁垒，为均相形核制备高质量TiN细粉探明了新方向。（4）提出一种TiCl4-N2-H2预还原-氮化的改进FBCVD工艺，即利用H2(g)将部分TiCl4(g)还原为活性的TiCl3(g)，同时与FBCVD制备TiN粉体工艺相耦合，实现TiCl3(g)的“即时生产和即时消耗”，获得了氯杂质含量低于0.001 wt.%的近球形TiN0.98细粉，其平均粒径为137.3 nm。;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, TiCl4(g) was reduced to TiCl3(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 °C 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 μm, the TiN seeds can realize longterm stable fluidization at 1000 °C 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 °C, 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 TiCl4(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.%.
|桑元. 流化床化学气相沉积法制备高质量氮化钛粉体研究[D]. 中国科学院大学,2020.|
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