Controllable Synthesis And Phase Transformation Of Boron Nitride Nanoparticles In Liquid Mediums

On the basis of analyzing the previous results of synthesizing BN nanoparticles by solvothermal method, we have prepared BN nanoparticles with specific particle size and crystallinity by using different solvents. Besides, a new constant-pressure solvothermal method was applied to the synthesis of tBN, and the key factors in the synthesis procedure were investigated. Furthermore, the phase transformation of BN nanoparticles when being hot-pressed in solvents was systematically studied, and the key affecting factors have been explored. On the other hand, the phase transformation of hBN induced by hydrothermal hot-pressing process was firstly studied. The detailed results are as following:By optimizing the preparation parameters, two kinds of aBN with different particle size and crystallinity were synthesized at 250℃by solvothermal method, using BCl3 and Li3N as the starting materials. After that, a constant-pressure solvothermal method was firstly used in the reaction of BCl3 and Li3N. By using this route, tBN was synthesized in a controllable way, and the yield was as high as 63%. Based on the experimental results, a preliminary model for explaining the reaction mechanism was proposed.In order to synthesize pure cBN nanoparticles under mild conditions, the phase transformation of boron nitride nanoparticles in solvents was investigated, and the key factors were comprehensively explored. The results indicate that the required temperature and pressure for the formation of wBN and cBN were greatly decreased in solvents, and the phase transformation completed at rather moderate conditions. Besides, the temperature and pressure for BN transformation could be further decreased by optimizing solvent and decreasing the particle size and crystallinity of BN nanoparticles. When sample SM-II was used as the starting material, the content of wBN and cBN in the sample prepared by hot-pressing in pyridine, aniline and chlorobenzene 34.7%,37.1% and 45%, respectively. This result reveals that the induction effects on the transformation from sp2 to sp3 BN was as follows: chlorobenzene> aniline> pyridine. Additionally, the initial state of starting material also played a key role on the formation of wBN and cBN. Among the three starting materials SM-Ⅰ, SM-Ⅱand SM-Ⅲ, BN in SM-Ⅱtransformed into wBN and cBN under the modest conditions, and the content of wBN and cBN in the sample obtained by hot-pressing SM-II at 260℃and 150 MPa was as high as 91.3%. In comparison, the transformation from tBN to wBN and cBN in SM-III was rather difficult, and the maximum content of wBN and cBN was only 13.5%.On the purpose of synthesizing wBN and cBN at more moderate conditions, the reaction between hBN nanocrystals and H2O, as well as the phase transformation induced by hydrothermal hot-pressing process, were firstly studied by using commercial hBN nanocrystals as the starting material. The results indicate that hBN reacted with H2O and ammonium borate formed under very mild condition. Due to the decrease of the particle size, this reaction was greatly accelerated with the increase of temperature and pressure, as well as the prolonging of hot-pressing duration. Furthermore, a phase transformation of hBN→tBN→aBN occurred during the hydrothermal hot-pressing of hBN nanocrystals, which was transformed into tBN at the temperature above 270℃, and then transformed into aBN as the temperature further increased to 310℃. Meanwhile, a small amount of cBN nucleus was observed in the BN sample. In order to increase the content of cBN, LiCl/NaF aqueous solution was used as the liquid medium, which resulting in the formation of relative large amount of sp3-bonded BN.With the purpose of synthesizing pure cBN using commercial hBN nanocrystals under mild conditions, we developed a new constant-pressure solvothermal synthesis method and a hydrothermal hot-pressing method, the phase transformation of BN nanoparticles under moderate conditions was also explored. Although these methods are still far from being perfect, at least they provided us the possibility of synthesizing cBN in a moderate and controllable way.