HUANG Xiaoyang, LI Yage, QU Chao, PENG Jinqi, ZHANG Shaowei, ZHANG Haijun
(The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, Hubei, China)

Extended Abstract: [Significance] Boron nitride nanotubes (BNNTs) have a tubular nanostructure similar to carbon nanotubes (CNTs). Owing to their excellent thermal stability, good biocompatibility, excellent electrical insulation, high thermal conductivity and superhydrophobicity, BNNTs have broad application prospects in various fields, such as chemical, metallurgical and biomedical industries. The performance of BNNTs is determined by their purity, size and arrangement, which are dependent on the synthesis methods. The preparation methods for BNNTs include chemical vapor deposition (CVD), arc discharge, laser ablation, template, plasma-enhanced and ball milling–annealing methods. Compared with other preparation techniques, chemical vapor deposition (CVD) offers various advantages, such as simple equipment, low reaction temperature and high purity of the final product. The growth of BNNTs typically follows the vapor-liquid-solid (VLS) mechanism. The precursor powder reacts to produce B₂O₂ vapor, which then reacts with N₂ from NH₃ to form h-BN. Over time, h-BN continues to condense until it becomes supersaturated and precipitates as BNNTs. In this paper, the effects of preparation methods, precursors, experimental setup, gas flow rate and substrate position on the properties of BNNTs are summarized, while future research directions in the field are discussed.[Progress] First of all, the preparation of BNNTs via CVD can be divided into boron oxide chemical vapor deposition (BOCVD), argon-assisted thermal chemical vapor deposition (Ar-TCVD) and other CVD methods. The advantages and disadvantages of BNNTs prepared via different CVD methods are discussed in detail. Among them, BNNTs synthesized by BOCVD method usually use B as boron source and metal or its compounds as catalysts. Solid boron source has low toxicity, thus preventing environmental pollution, injury to experimental personnel and corrosion of equipment. The Ar-TCVD method has been widely used in the preparation of BNNTs. Compared with the BOCVD method, in the Ar-TCVD method, the issue of the too high flow rate of NH₃ could be readily addressed, so that longer BNNTs could be produced. At the same time, by introducing Ar gas instead of vacuum pumping, the test device is simplified and the test cost is reduced. The raw materials used in other CVD methods are dangerous, prone to spontaneous combustion and highly toxic. Furthermore, the yield of BNNTs is low. In addition, the effects of precursor type and its ratio, substrate material, temperature, atmosphere, pressure and gas flow rate on the quality, quantity, aspect ratio and structure of BNNTs were also discussed. It is found that using B as the boron source and MgO/FeO as catalysts is one of the most effective CVD methods to prepare BNNTs. Alkali metal borate, alkaline earth metal borate and transition metal borate can catalyze the growth of BNNTs. Low pressure is beneficial to the formation of uniform BNNTs on Si substrate. BNNTs were generated in the low-temperature zone using a dual-zone tube furnace, which improved the utilization rate of NH₃ and the formation rate of BNNTs. If the reaction time is too long, fibrous BN will be formed. If the reaction time is insufficient, flake BN will be produced. Only when the reaction time is moderate, BNNTs will be obtained. At present, the diameter of BNNTs prepared with CVD method is generally between 10 nm and 100 nm, while the length is greater than 10 μm.[Conclusions and Prospects] Chemical vapor deposition (CVD) is considered to be an effective method for the large-scale synthesis of high-quality BNNTs. This technology can not only improve the yield and quality of the products, but also facilitate the vertical arrangement and growth of BNNTs. At present, there are still problems in the preparation of BNNTs by using CVD method, such as low yield, presence of impurities that are not easy to remove by using catalysts, high reaction temperature without catalyst system and amorphous BNNTs synthesized at low reaction temperature. In the future, more effort should be directed towards searching new and efficient catalysts to achieve low temperature, high efficiency, high purity and controllable growth of BNNTs.
Key words: boron nitride; nanotubes; chemical vapor deposition method