Synthesis And Characterization Of Boron Nitride And Carbon Nitride Nanomaterials

Cubic boron nitride (cBN) has many merits, such as high stability, high thermal conductivity, high hardness and wide band gap etc. Although its hardness is inferior to that of diamond, its stability is higher than the latter. Besides, unlike diamond, cBN dose not react with ferrous metals, which makes it the ideal material to fabricate cutting tools, protective coatings and grinding materials etc. In addition, cBN can also be used in the fabrication of high-power semiconductor devices, short wavelength optoelectronic devices, ultraviolet detectors, high power microwave devices, heat sinks and transparent windows etc.On the other hand,β-C₃N₄, which has the similar structure with that ofβ--Si₃N₄, has been expected has bulk modulus and hardness comparable or superior to that of diamond. It's believed thatβ-C₃N₄ will open a challenging field in material science, due to its novel mechanical, optical and tribological properties.However, almost all the methods used in the synthesis of cBN andβ-C₃N₄ have their intrinsic disadvantages. Especially the newly developed solvothermal and hydrothermal synthesis method, is urgently improved and optimized, in order to prepare cBN with high yield and crystalline perfection. Based on such a consideration, we have carried out a series of works:I. Optimization of preparation parameters of synthesizing cBN by hydrothermal synthesis method1 The influence of heating / cooling rate of the autoclave has been investigated. It was found that the cBN content and crystalline perfection of the sample was obviously improved when the heating / cooling rate is 0.5℃/min. Besides the heating / cooling rate, the influence of stirring rate was also investigated. The experimental results indicated that both the cBN content and crystalline perfection could be increased under high stirring speed, for example, 400r/min.During the synthesis of cBN by phase-selective hydrothermal method, the nitrogen source was divided into two parts: one of them was mixed with boron source at the beginning of the reaction, and the other part (it was called the "secondary nitrogen source") was introduced into the reaction solution at high temperature and pressure. We have found that if the secondary nitrogen source was introduced into the reaction solution at different temperature, cBN content of the prepared BN sample varied. When this temperature was 300℃, the cBN content reached its maxium value.2 Control of the cBN content and crystalline perfection of BN samples prepared by benzenethermal method.It was found that, varing the reaction temperature in the range of 180 to 240℃has almost no effect on the crystallinity and cBN content of the BN samples. However, the cBN content of the sample was increased by using mixed nitrogen source (for example, LisN+NaNH2), and at the same time, the crystalline perfection also improved. In order to explain this phenomenon, a preliminary model has been proposed in this thesis.3 Investigation on the phase stability of BN nanoparticies.It has been well known that the activity of a BN particle might increase to a large extent when its size minished to several nanometers. As a result, the temperature and pressure required for the phase transformation will decrease accordingly. However, the conditions of solid state phase transformation of BN nanoparticies are still rather harsh. In order to overcome this difficulty, we added benzene into the BN nanoparticies and carried out the phase transformation experiments in liquid medium. From the results we know that hBN nanocrystals could be transformed into cBN and wBN at 280℃and 80～120MPa. The higher the hot-press temperature and pressure, the higher the cBN content and crystalline perfection were. Besides, prolonging the hot-press time has also faciliated the above transformation.II. Preliminary attempt to synthesize C₃N₄ by benzenethermal methodC₃N₄ nanocrystals were prepared by the reaction of NaN₃ and C₃N₃Cl₃ (1,3,5-trichlorotriazine) in solvothermal conditions. The results showed that the major product is g-C₃N₄. In addition, it was found that the morphology of the sample can be controlled by adding metallic zinc powder as the catalyst. If no zinc was added, the sample is mainly composed of flakes in irregular shape. On the other hand, large amount of nanowires have been observed when zinc was added.