中国科学院过程工程研究所机构知识库

金属钛因优异的抗腐蚀性、生物相容性而具有广阔的应用前景，但受其高成本的制约，至今未广泛应用于抗腐蚀、生物医疗领域。化学气相沉积（CVD）制备金属钛膜因成本低，是最有望扩大金属钛应用的方法。然而在目前CVD金属钛膜主要的反应体系中，亚氯化钛前驱体的CVD反应原理仍不明晰，CVD过程也未优化，由此引发的沉积温度过高以致常用的金属基底无法耐受，沉积膜为氧化钛膜，钛膜基本的抗腐蚀性能缺乏优化等问题尤为突出，这些均阻碍了CVD钛膜的实际应用。针对上述问题，提出了合成亚氯化钛，直接通过亚氯化钛来研究CVD反应原理进而优化CVD过程的新思路。基于此思路，系统研究了优选前驱体的合成、热稳定性与气化行为，还实验研究了优选前驱体金属钛CVD的反应原理、沉积行为和相关CVD钛涂层的抗腐蚀性能。取得的主要创新性成果如下：（1）发现TiCl2较TiCl3是更优的金属钛CVD前驱体，基于流态化建立了低温、定向、快速合成TiCl2的方法。热力学上，TiCl3沉积钛效率极低（约2.4%）且沉积温度为900~1300 ‍°C，而TiCl2沉积钛效率高（约32%~35.5%）且沉积温度可降至900 ‍°C以下，由此优选TiCl2为金属钛CVD前驱体。采用TiCl4气体和钛粉流态化合成TiCl2时，TiCl4分压不高于12.2 ‍kPa可避免其过量引起的TiCl3生成，进而实现TiCl2的定向合成。反应温度不超过625 ‍°C可避免反应时钛粉表面生成TiCl2的熔化引起的失流，由此625 ‍°C下TiCl2合成速率高达以往1000 ‍°C下的2倍。最终得到化学反应控制下Ti与TiCl4合成TiCl2的动力学方程，其中活化能为102.92 ‍kJ/mol，TiCl4分压的反应级数为0.55。（2）探明了TiCl2的热稳定性，获得了TiCl2在更宽温度范围的蒸气压数据。固相TiCl2升温时，除经历晶型转变外不会发生明显的化学反应，升至500 ‍°C左右开始升华，升至640 ‍°C时发生熔化。升华、640 ‍°C以上汽化的焓值分别为257 ‍kJ ‍mol-1、‍112 ‍kJ ‍mol-1。通过TG法测定了TiCl2的蒸气压，将TiCl2蒸气压数据由以往文献中不高于627 ‍°C拓宽至865 ‍°C。（3）揭示了TiCl2 CVD金属钛的反应原理，优化获得了低氧钛膜的低温沉积。实验证实了TiCl2是金属钛CVD的前驱体，其CVD反应原理主要是3TiCl2(g)→Ti(s)+2TiCl3(g)，800~1400‍ ‍°C时还将出现TiCl2(g)→Ti(s)+TiCl4(g)。由于TiCl2低温气化后即可歧化沉积钛的特性，通过CVD沉积可观金属钛的温度由以往1000 ‍°C以上降至620 ‍°C。TiCl2的定向、高效合成还确保了沉积时其分压足够高，由此钛沉积速率远高于高纯氩气中微量氧溶解于钛的速率，故可沉积得到低氧含量的钛膜。（4）通过CVD钛涂层优化了铜、镍、316L不锈钢的抗点蚀性能，揭示了CVD钛涂层的腐蚀防护机制。CVD钛涂层的低孔隙率及其热扩散后仍保持的高钛含量是其优异抗点蚀性能的根本原因，3.5 wt.% NaCl溶液中涂层的高钛含量可形成性质稳定的主要成分为TiO2的钝化膜，而涂层的低孔隙率还可产生大的离子扩散阻力与电荷传递阻力，由此CVD钛涂层对基底起到充分的点蚀防护作用。例如，铜上钛涂层因孔隙率低且表层为CuTi，其钝态下腐蚀速率降至纯铜的1%左右；镍上钛涂层因孔隙率低且表面为NiTi层或薄NiTi2层，可使点蚀电位由纯镍的-‍0.07 ‍V分别升至0.08 ‍‍‍‍V、0.26 ‍V左右；316L不锈钢上钛涂层因表面为结构致密的NiTi2层，其点蚀电位高达1.2 ‍V，超过以往物理气相沉积的钛涂层。这些研究显示CVD钛膜可显著提升金属的抗盐水腐蚀能力，可望在抗腐蚀、生物医疗领域获得应用，本研究为CVD钛膜的实际应用奠定了基础。;Metallic titanium has numerous potential applications owing to its outstanding corrosion resistance and biocompatibility. Unfortunately, due to its high cost, it has not been widely used in anticorrosion and biomedical fields so far. Chemical vapor deposition (CVD) of titanium films is inexpensive and thus the most promising method for expanding the application of metallic titanium. However, in the main reaction system for CVD of titanium films, the CVD reaction principle of titanium subchlorides (TiCl2 and TiCl3) as titanium precursors is still not clear, and the CVD process is not optimized. Resultantly, several acute problems arise. First, the deposition temperatures are so high that the commonly used metal substrates cannot withstand. Second, the nature of the deposited films is titanium oxide rather than metallic titanium. Third, there is a lack of optimization for the basic anticorrosion performance of titanium film. These have hindered the practical application of the CVD titanium film.To address the aforementioned issues, the novel idea was proposed to synthesize titanium subchlorides, subsequently investigate the CVD reaction principle and optimize the CVD process directly using titanium subchlorides. Accordingly, the synthesis, thermal stability and vaporization behavior of the selected precursor between titanium subchlorides were systematically investigated. Moreover, the CVD reaction principle and deposition behaviors of the preferred precursor, and the anticorrosion performance of the deposited titanium coatings were experimentally studied. The main innovative findings achieved are as follows:(1) TiCl2 was found to be a better CVD precursor of metallic titanium than TiCl3, and a selective and rapid synthesis method of TiCl2 at low temperatures was established based on fluidization. Thermodynamically, titanium can be deposited using TiCl3 only at 900-1300 ‍°C and an extremely low deposition efficiency (approximately 2.4%), while titanium deposition using TiCl2 can reach a much higher deposition efficiency (approximately 32%-35.5%) at reduced temperatures (<900 ‍°C). TiCl2 was therefore selected as the preferable precursor. During the fluidized-bed synthesis of TiCl2 using TiCl4 vapor and titanium powder, the partial pressure of TiCl4 ≤‍‍ 12.2 ‍kPa prevented its excess and the resulting production of TiCl3, hence the selective synthesis of TiCl2. Reaction temperature ≤ 625 ‍°C inhibited the defluidization due to the melting of TiCl2 on the surface of titanium powder, yielding the formation rate of TiCl2 at 625 ‍°C twice that at 1000 ‍°C reported. Finally, the kinetic equation for the synthesis of TiCl2 from titanium and TiCl4 under chemical reaction control was obtained, in which the activation energy was 102.92 ‍kJ/mol, and the reaction order for partial pressure of TiCl4 was 0.55.(2) The thermal stability of TiCl2 was ascertained, and the vapor pressure data of TiCl2 in a wider temperature range was obtained. When solid TiCl2 is heated up, no obvious chemical reaction except the crystal transformation occurs, and TiCl2 begins to sublime at about 500 ‍°C and melt at about 640 ‍°C. The enthalpy values of sublimation and vaporization above 640 ‍°C were 257 ‍kJ ‍mol-1 and 112 ‍kJ ‍mol-1. The vapor pressure of TiCl2 was determined by the thermogravimetric method, expanding the temperature range for the vapor pressure data of TiCl2 from no higher than 627 ‍°C reported to 865 ‍°C.(3) The CVD reaction principle from TiCl2 to titanium was revealed, and the low-temperature deposition of low-oxygen titanium film was achieved through the optimization. TiCl2 was experimentally confirmed to be the CVD precursor of metallic titanium. Its CVD reaction principle was mainly 3TiCl2(g)→Ti(s)+2TiCl3(g), and TiCl2(g)→Ti(s)+TiCl4(g) occurred merely at 800-1400‍ ‍°C. Owing to the unique feature of TiCl2 that it disproportionates and produces titanium deposits immediately after its low-temperature vaporization, the temperature for CVD of an appreciable titanium film is substantially reduced from above 1000 ‍°C reported to 620 ‍°C. Furthermore, the selective and efficient synthesis of TiCl2 ensures its sufficient partial pressure during the deposition, allowing the deposition rate of titanium much higher than the dissolution rate of the trace oxygen in high purity argon into titanium. Consequently, a low-oxygen titanium film is deposited.(4) The pitting corrosion resistance of copper, nickel and 316L stainless steel was optimized by using CVD titanium coatings, and the corrosion protection mechanisms of CVD titanium coatings were revealed. The low porosity of the coating and its remaining high titanium content after the thermal diffusion are the fundamental causes of its superb pitting corrosion resistance. The high titanium content of the coatings forms stable passive surface films mainly composed of TiO2 in 3.5 wt.% NaCl solution. Additionally, the low porosity of the coatings generates large ion diffusion resistance and charge transfer resistance. Thus the CVD titanium coatings provide adequate protection of the substrates against pitting corrosion. For example, the titanium coating on copper has low porosity and the CuTi surface layer, reducing the corrosion rate in the passive state to about 1% that of pure copper. The titanium coating on nickel has low porosity and the NiTi or thin NiTi2 surface layer, increasing the pitting potential from -‍0.07 ‍V for pure nickel to about 0.08 ‍‍‍‍V or 0.26 ‍V, respectively. The titanium coating on 316L stainless steel has the dense NiTi2 surface layer, resulting in a higher pitting potential (1.2 ‍V) than those of the titanium coatings prepared by physical vapor deposition. These studies show that CVD titanium film can significantly improve the corrosion resistance of metals in salt water, and thus promises to be utilized in anticorrosion and biomedical fields. This study has laid a foundation for the practical application of CVD titanium film.