Carbothermal Reduction And Preparation Of Ti (c, N) Whiskers

Ti(C, N) whiskers were fabricated through carbothermal reduction of TiO₂ in protecting nitrogen at different temperatures. The raw materials were TiO₂, carbon black, NaCl and NiCl₂, among which NaCl was added to form the chlorides of Ti and NiCl₂ was used for whisker growth. X-ray diffraction (XRD) was carried out to analyze the phases of the synthesized samples and the corresponding phase-transformation behavior; the micrographs of the synthesized samples were characterized by scanning electron microscopy (SEM), the EDS was used to analyze the C, N atom contents of the Ti(C,N) whiskers, furthermore, the free carbon black and the oxygen contents of the samples were tested. The phases and micrographs of the synthesized samples influenced by the synthesis temperatures and times were discussed, and the free carbon black and the oxygen contents of the samples synthesized at different temperatures were studied; furthermore, the phases, micrographs and Ti(C,N) whiskers yields of the synthesized samples with different NaCl, carbon black and NiCl₂ contents added in the raw materials were compared; otherwise, the chemical reaction in the process of synthesis Ti(C,N) whiskers grown by VLS mechanism was analyzed, the Ti(C,N) whiskers grown model and dynamics equation were built up, and studied the relationship between the whisker diameter and catalyzer property. The mainly conclusions and findings can be summarized as follows: 1) TiO₂ directly reacts with carbon black and nitrogen to form TiN phase at 1200℃; while the temperature was in the range of 1250-1450℃, NaCl was decomposed and functions as a halogenation reagent of TiO₂ to forming TiCl₃ and NiCl₂ acts as catalysts for the Ti(C,N) whisker formation, there were some Ti oxides and carbon black in the synthesized samples when the synthesis temperature was in the range of 1250-1350℃,while the synthesis temperature was in the range of 1400-1500℃,the synthesized samples was composed by Ti(C,N) and Ni phases.2) The heat preservation time was an important parameter of Ti(C, N) whisker formation. There were a lot of Ti oxides and carbon black in the synthesized samples for short time heat preservation; with the heat preservation time increase the TiN and Ti oxides disappeared and the free carbon black contents were reduction. 3) There were two functions for NaCl added in the raw materials: one was forming the TiCl₃ gas for Ti(C,N) whisker growing, the other one was to restrain the deoxidize reaction of TiO₂, C and N₂; if the added carbon black content was low there were some Ti oxides in the samples, while increase the carbon black contents there were some remnant carbon black, and the atom rates of C, N in the Ti(C,N) whiskers would be changed with the carbon black contents added in the raw materials; the length of Ti(C,N) whiskers would be changed with the catelyzer contents ,while the catelyzer content was 3.77% the whiskers obtained were about 40μm in length and when the catelyzer content was 8.93%wt the length of Ti(C,N) whiskers were about 10-15μm. 4) The dynamics equation of growing Ti(C,N) whiskers based on VLS mechanism model was R=(hv/πr²) exp(-△G_f/kT)·δ, according to the dynamics equation the growing rate was determined by the degree of catelyzer super saturation and the whisker diameter when the synthesis temperature was affirmatory; the whisker diameters were determined by the surface tension between the whiskers and catelyzer, and the catelyzer volume also determined the whisker diameters.5)The optimization process of fabrication Ti(C,N) whisker by carbothermal reduction method was synthesis at 1400℃for 180min, and the optimization contents of NaCl and carbon black were 42.72%wt and 14.56%wt, respectively.