The Preparation And Characterization Of Light Element In The B-C-N System Compound

New materials are the matter basis of scientific and technological progress and the precursor of the contemporary socio-economic. Any other subjects will rely on the progress and development of materials science. Along with the rapid development of science and technology, People have had more understanding to the function of a variety of materials and approach to the processing of materials. As hardness is one of the most important and fundamental properties of materials, the synthesis, characterization and properties of materials with super hardness become one of the highlights in condensed matter physics and materials science and technology at present. The recent progresses in materials design, as well as the discoveries of new phases, in the B-C-N system, has aroused great interest among scientists to seek materials with hardness comparable to or even higher than diamond, both experimentally and theoretically.Besides the potential super hardness, the compounds in the B-C-N system are characteristic of their short, covalent bonds, which also endows them with desired physical, chemical, electronic and photoelectronic properties, etc.. For example, diamon is a wide bandgap semiconductor with the highest saturated eletron drift velocity, the lowest dielectric constant, excellent thermal conductivity and transparence. Whereas cBN is a wide bandgap semiconductor with negative electron affinity, chemical inertness,etc..In this thesis, exploring works in the following three aspects such as the synthesis and characterization of hexagonal and cubic boron carbonitride compound under high temperature and high pressure, the synthesis and characterization of graphite C₃N₄, the in situ synchrotron radiation X-ray diffraction research under high pressures of g-C₃N₄ and the synthesis and the in situ synchrotron radiation X-ray diffraction research of cubic C₃N₄ under high temperature and high pressure.(1) Diamond and c-BN are two representative superhard materials with a wide range of potential applications. Although diamond possesses the highest hardness, it is neither stable in the presence of oxygen even at moderate temperatures, nor is it a suitable abrasive for machining ferrous alloys. The hardness of c-BN is only half that of diamond in spite of its higher thermal stability. Thus great efforts have been devoted to the preparation of a new material which combines the advantages of the hardness of diamond and the thermal stability of c-BN. Conceivably such a material may be found in the BCN family. Liu et al theoretically proposed three structural models of BCN with the first-principle calculations in 1989. According to their investigations, cubic BCN was claimed to be a new superhard materials possessing those properties. Furthermore, it was suggested that cubic BCN could be produced from hexagonal BCN using high pressure and high temperature techniques in similar ways as diamond and c-BN. Hexagonal BCN compounds were synthesized with C3H6N6 melamine and B2O3 as raw materials,combine with heat treatment and high pressure high temperature. An amorphous BCN precursor was prepared at 800 oC under vacuum for 30 min. At 1200 oC under 5.0 GPa, the amorphous BCN precursor crystallizes into hexagonal BCN compound by using the refinement module combined in the Materials Studio program analysis. The shape and SAD of sample also approved that the sample was h-BCN crystal by TEM analysis. The grain size about was 200 nm. XPS analysis was applied for the sample. The results confirmed bending energy of C-C,C-N,C-B,N-B.Well-crystallized BCN compounds have been prepared from melamine(C3H6N6) and boron oxidation(B2O3) at pressures above 25 GPa and temperatures higher than 2200 oC. The synthetic BCN samples were characterized transmission electron microscope(TEM) and electron energy-loss spectra(EELS). The atomic B/C/N ratio was determined by electron energy-loss spectra. Electron energy-loss spectroscopy studies indicated that the bonding of the B-C-N compound is in a sp3 configuration.(2) The vast surge of activity in carbon nitride material has arisen from the initial work by M. L. Cohen and his coworkers who proposed thatβ-C₃N₄ might be harder than diamond. The prediction of theβ-C₃N₄ phase has attracted more than thousands of laboratories in the world to synthesize this new superhard material. All kinds of techniques, including Sputtering, Chemical Vapor Deposition, Laser Ablation, Ion-beam Deposition, Plasma etc., have been used to synthesize this superhard material. However, we can see that lots of experimental researches concentrate on the deposition of CN film. In spite of all techniques used, it appeared clear that stoichiometric C₃N₄ single crystal is still not readily achieved by the deposition techniques. An important limitation in the production of carbon nitrides has shown to be the amount of nitrogen that can be incorporated in the structure. In fact, reports of nitrogen concentrations exceeding 40 at % in pure CN materials are very rare.Because of the difficulties in synthesizing stoichiometric C₃N₄ single crystal by the film-deposition methods. We select pyrolysis method to synthesize bulk C₃N₄ single crystal from the chemical reaction of organic compounds at the proper conditions.Flexible graphitic carbon nitride (g-C₃N₄) nanofibers have been synthesized via a two step pyrolysis of melamine (C3H6N6) at 800 oC for 2 h under vacuum. X-ray diffraction (XRD) patterns strongly indicate that the synthesized sample is g-C₃N₄. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) morphologies indicate that the product is mainly composed of flexible graphitic carbon nitride nanofibers. The stoichiometric ratio of C/N is determined to be 0.72 by elemental analysis (EA). The chemical bonding of the sample has been investigated by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR).(3)In situ synchrotron radiation angle-dispersive powder X-ray diffraction studies under high pressures and room temperature on g-C₃N₄ samples. Structural phase transition of g-C₃N₄ has been observed within 16.57 GPa pressure range, from graphitic to triclinic structure at 6.6 GPa. P-V data of the triclinic phases has been fitted to Birch-Murnaghan equation of state by using the least square method.The c-C₃N₄ was synthesized with g-C₃N₄ as raw materials and laser heating diamond anvil cell(LHDAC) technology at 25 GPa and 1700 oC. In situ synchrotron radiation angle-dispersive powder X-ray diffraction studies under high pressures and room temperature on c-C₃N₄ samples.