Theoretical Study Of Hardness For A₃N₄(A=Si,C) And Synthesis Of B-C-N Compounds By High Pressure And High Temperature

One of the key areas that have attracted a lot of attention in current light elementmaterials research is the theoretical hardness study and experimental synthesis of novelC-N, B-C-N materials. Using a new microscopic hardness model and a first-principlescalculation method of Phillips ionicity, we investigate the hardness problem of the fivecarbon nitride polymorphs, beginning with the question of whether the spinel Si₃N₄ is asuperhard material. We further investigate the possibility of finding new superhardmaterials that are harder than diamond in simple-structured covalent materials. Assisted by the CASTEP code of the Materials Studio software, we performfirst-principles calculations based on density functional theory of two silicon nitridecrystals (β-Si₃N₄, cs-Si₃N₄) and five carbon nitride polymorphs (β-C₃N₄,α-C₃N₄,p-C₃N₄,c-C₃N₄ and cs-C₃N₄) to obtain useful parameters of the crystals. We further study theirhardness using a new semi-experimental microscopic model of hardness. Our calculationsshow that the Vickers hardness of β-Si₃N₄ and that of cs-Si₃N₄ are only 33.3GPa. This isin agreement with their experimental values, which indicates that the cs-Si₃N₄ is not asuperhard material. We find that, although the calculated density, bulk modulus, andshear modulus of cs-Si₃N₄ are all higher than those of β-Si₃N₄, its hardness is not as highas expected due to the larger bond lengths of Sioct-N bond in cs-Si₃N₄. Our calculations onthe five C₃N₄ polymorphs indicate that, even though they are all predicted to be superhardmaterials, they are not any harder than diamond. Among the five polymorphs, c-C₃N₄ isthe hardest. However, even its hardness is about 5.4% lower than that of diamond. Byconducting further analysis, we conclude that the possibility of finding novel superhardmaterials that are harder than diamond in simple-structured covalent materials is slim.On the experimental side, we successfully synthesize the B-C-N ternary compoundsunder high pressure and high temperature conditions with the use of catalysts andsystematically investigate how pressure and temperature affect the synthesizedcompounds.Under high pressure in the range of 3-6GPa and high temperature in the range of1000-1600℃, we synthesize hexagonal B-C-N ternary compounds using a turbostraticB₂CN compound as precursor, which is prepared from melamine and boric acid through achemical combination and pyrolysis process, and Fe as catalyst. The obtained compoundare composed of many small round flakes about 3-10μm in diameter and 2μm in thickness.Their chemical composition is determined to be B0.52C0.09N0.39. For the hexagonal B-C-Ncrystal obtained under 6GPa and 1600℃, its lattice parameters are a=0.2510 nm andc=0.6675 nm. Further Fourier transformed infrared measurement indicates that B-N, B-Cand C-N bonds exist in B0.52C0.09N0.39 compound.Using amorphous B2CN prepared from pure graphite, h-BN and amorphous boronpowder by mechanical alloying as precursor, we obtained a hexagonal B2CN compoundby treating the precursor directly under high temperature and high pressure conditions.The obtained compound is composed of many small round flakes. The lattice parametersof the hexagonal B2CN crystal obtained under 5.5Gpa and 1500℃ are a=0.2510 nm andc=0.6675 nm. Fourier transformed infrared measurement indicates that B-N, B-C, C-Nand C-C bonds exist in this hexagonal B2CN compound. By conducting high temperatureX-ray diffraction analysis, we find that the hexagonal B2CN crystal is thermally stablefrom room temperature up to 1200℃ in vacuum. When catalyst Ca3B2N4 is used to treatthe amorphous B2CN under high temperature and high pressure conditions, we obtain acubic B(CxN1-x) (x=0.1-0.11) crystalline compound under 5.5-6GPa and 1400-1600℃.The lattice parameter of the cubic B(CxN1-x) (x=0.1-0.11) crystal is a=0.3621 nm. Fouriertransformed infrared measurement indicates that sp3 hybridized B-N, B-C and C-N bondsexist in the cubic B(CxN1-x) (x=0.1-0.11) compound. By conducting high temperatureX-ray diffraction analysis, we find that the cubic B(CxN1-x) (x=0.1-0.11) crystal isthermally stable from room temperature up to 900℃ in vacuum .