Synthesis Of Cubic Boron Nitride Microcrystals And Exploration Of Growing Its Bulk Crystals

Because of the excellent merits, such as supreme hardness, wide band gap, high thermal stability, excellent chemical stability and inertness against ferrous metals etc, cBN is of high value in fabricating machining tools, electronic and optical devices etc. For preparing cBN, many methods have been developed, for example, high temperature & high pressure (HTHP) method, chemical vapor deposition (CVD) method, solvothermal method and hydrothermal method. In this thesis, results from systematical investigations on some key factors influencing the production of boron nitride by hydrothermal method will be reported. Furthermore, optimized parameters for synthesizing pure cBN samples will be given. Based on the above mentioned work, a new route to grow cBN bulk crystals by using cBN seed-crystals will be presented. The major results are as follows:(1) It was shown that the sort of nitrogen sources, their quantity and the ways to introduce them into the reaction autoclave could effect the phases and yields of BN samples.At 300℃and 10 MPa, the content of cBN increased with the increase of R_N value (R_N is defined as the molar ratio of NaN₃ in primary nitrogen source to it in the secondary one), and pure cBN could be obtained at Rn=3:1. The yield of BN samples would be increased slightly, mainly of hBN, if the quantity of N(CH₃)₃ or NH₃H₂O increased.(2) The temperature of the reaction solution and the rate of introducing secondary nitrogen source were found to have important effects on the phases and yields of BN samples. Pure cBN sample was prepared at 300℃. However, if the temperature was further elevated, the phase purity would be deteriorated. Introducing the secondary nitrogen source into the autoclave by multi-steps would cause both the temperature and pressure undulating frequently, resulting in to favorite the formation of hBN and, thus, the coexistence of hBN and cBN.(3) The reactants and additives played an important role during the synthesis of BN by hydrothermal method. The yield of BN sample could be improved by using NH₄HB₄O₇·3H₂O and C₃N₆H₆ as the reactants. In this case, hBN became the dominant phase. By adding some HC1 into reaction systems for preparing BN, pure cBN could be synthesized. Its crystalinity and reproducibility of the experiment could be improved significantly.(4) Pure cBN could be prepared by using more than two reactants as the nitrogen source simultaneously, and its crystalline perfection and yield could be improved very much. We call this phenomenon as "mixed nitrogen sources effect".(5) To mention at last, lamellar growth process was observed on the surface of some larger cBN seed-crystals introduced into the reaction system for preparing cBN under optimized conditions. This indicates the possibility to grow large single crystals of cBN under hydrothermal conditions.