Controllable Synthesis Of Cubic Boron Nitride At Mild Conditions

Based on the hydrothermal phase selective in-situ method and solid-state metathesis reaction route, we explored low-temperature controllable synthesis of BN from two aspects:in hydrothermal system, we researched the phase transformation mechanism of BN in synthesizing process, self-agglomeration phenomenon of cBN microcrystals and halide ions inducing synthesis of cBN. As a result, we successfully prepared pure phase cBN microcrystals; secondly, via low-temperature solid state reaction method, we obtained BN powders with high-yield, discussed the formation mechanism of micro-morphology and properties of BN. Furthermore, we also successfully prepared cBN via low temperature constant-pressure solid state method. By improvement for reaction equipment and method, nearly pure cBN was obtained finally. The detailed results are as follow:For controlling the phase composition of BN in phase-selective in situ hydrothermal synthesis process, we firstly researched the phase transformation mechanism of BN with temperature variety. As a result, it was found that as temperature increase, hBN gradually transformed to cBN, and a little amount of wBN accompanied to exist in the phase transformation process. So, BN possibly carried out phase transformation from hBN to cBN via a puckering-dislocation mechanism, namely hBN→wBN→cBN. Furthermore, we found that the synthesized cBN microcrystals easily self-agglomerated to form secondary particles up to micrometer degee. With the reaction temperature, concentration and uniformity of solution improving, and pressure decreasing, the particles size of agglomerated grain got larger and more uniform. The phenomenon induced the cBN microcrystals easily separated from a mixed phase samples. Meanwhile, because of the agglomeration of primary crystallites of cBN, only the microcrystals sited on the surface effectively grow. Naturally, the primary crystallites of cBN composing the secondary agglomerated particles presented multi-degree size phenomenon.In hydrothermal reaction system, halide species had the effects of suppressing hBN and inducing cBN because of halide species selectively etching hBN and assuaging (111) plane of cBN. By predigesting reaction system, finally, pure phase cBN was prepared by traditional hydrothermal method. Furthermore, we attempted the halide ions inducing method at ammonia as nitrogen source hydrothermal system and cBN was successfully obtained. In final, via changing the cooling method and pre-pressing the solution with high pure nitrogen gas, nearly pure phase of cBN was prepared.In the process of low temperature solid state reaction, by introducing additive and pre-pressing the starting materials into dense blocks, high-yield of BN nanopowders was prepared. Besides, it was found that the strategy could be widely used to form BN with other boron sources and the highest yield was up to～90%. In addition, with NH4BF4 as boron source at 250℃, high-ratio (85～90%) of hollow structures of BN was presented and could be repeated well. Hollow structure of BN with high specific surface area displayed excellent hydrogen storage capacity:～2.2 wt% at 77 K and 20 bar.By further improving the low-temperature solid state method and optimizing experimental parameters, nearly spherical BN nanoparticles with low agglomeration and monodisperse nature formed at 300℃and without sulfur-assistant, and then we also discussed the formation mechanism and property of them. In addition, we found that the modified solid state method could be also applied to other boron sources reaction system for BN nanoparticles formation, such as NaBF4 and NaBH4.On the other side, in modified solid state reaction system without sulfur assistant, a spot of cBN was found in as-prepared BN samples. By controlling the reaction temperature, increasing pressure and prolonging time, the relative content of cBN was obviously increased and cBN became the dominant phase in some samples. Furthermore, with the help of structural inducing effect, in final, nearly pure phase cBN microcrystals were obtained at low temperature and mid-pressure.