Investigation On Physical Properties Of Methane, Nitrogen, Oxygen And Their Hydrate Under High Pressure And High Temperature Conditions

Gas hydrate extensively exist in the natural world, which form inclusion compounds, the clathrate hydrates under the condition of high pressure and low temperature with water and gas molecules. The structure of gas hydrate is that gas molecules (the'guest') occupy'host'cages formed of hydrogen-bonded water molecule. As the crisis of energy sources, people begin to widely exploit and investigate the gas hydrate as a potential source of energy. Varieties of gas hydrates have different crystal structures and many diversity interactions. They contain profound physical problems and become an ideal system to study transformation of the structure, the interaction between atoms (molecules). At present many scientific researchers have broadly interests in gas hydrates, so the study of high pressure structure phase transition and stability becomes into hot subject. In this paper, we systematically study the effect of high pressure and high temperature on crystal structure elastic property, etc. We observe many significant results by using the recently advanced experimental techniques of high temperature and high pressure diamond anvil cell, stigmatic focusing Raman and Brillouin scattering to investigate the methane, oxygen, nitrogen, methane hydrate, oxygen hydrate and nitrogen hydrate.The results of this paper have been listed as follow:By studying the Brillouin scatting of liquid methane under high temperature and high pressure at five isotherms(298K, 354K, 405K, 465K, 539K) , we obtain the acoustic velocity, refractive index, density, bulk modulus of liquid methane as a function of pressure at each isothermal line. The highest pressure is up to 5.2GPa. The acoustic velocity of liquid samples increase monotonously with increased pressure at constant temperature, but temperature has a small effect on the acoustic of liquid oxygen at same pressure. Combining the observed refractive index to Lorentz-Lorenz equation, we achieve the equation of state and bulk modulus. In all range of temperature, bulk modulus increase monotonously with pressure, the slope decrease with the increased temperature. Compared with the adiabatic bulk modulus of water, ammonia and argon, we observe that the adiabatic bulk modulus of methane is smallest and water's is biggest. This phenomenon suggest that methane is readily compressed. At the same time dB/dP of methane is also smallest. Through high pressure and low temperature method, we successful synthetize the methane hydrate which is burnable. This work provides important information for exploiting and utilizing new energy sources.By studying the Brillouin scatting of liquid nitrogen under high temperature and high pressure at three isotherms (295K, 339K, 437K), we obtain the acoustic velocity, refractive index, density, bulk modulus of liquid nitrogen. Nitrogen hydrate has been studied by Raman scattering, the highest pressure is up to 13GPa and 23GPa, respectively. We resolve the disputable problems about the structure of nitrogen hydrate, high pressure phase and decompose.We observe that the NH- I phase transforms into NH-II at 0.99GPa, when the pressure up to 1.65GPa nitrogen hydrate transforms from NH-II phase into NH-III phase. This has been reported in earlier work. With the pressure increasing, we first find that the nitrogen hydrate transforms into NH-IV at 5.2GPa, and happen to a solid-solid transition named NH-V at 6.6GPa. X-ray diffraction studies have shown that NH-III belongs to orthorhombic and NH-IV belongs to monoclinic. Studies also find that nitrogen hydrate is not decomposed at high pressure. Upon release pressure we do not observe any hysteresis in Raman spectra and X-ray diffraction, which suggest that the phase transition of nitrogen hydrate is reversible.By studying the Brillouin scatting of liquid oxygen under high temperature and high pressure at five isotherms(298K, 316K, 339K, 370K, 442K) , we obtain the acoustic velocity, refractive index, density, bulk modulus of liquid oxygen as a function of pressure. To acoustic velocity of longitudinal wave, we find that plots of acoustic velocity and density yield straight lines in isothermality, temperature has no effect on this linear relation. We first obtain the linear relation of adiabatic bulk modulus as a function of pressure at high temperature and high pressure, the slope decrease monotonously with increasing temperature. This suggests that the bulk modulus of liquid oxygen as a function of pressure changes larger at low temperature than high pressure.Using diamond anvil cell, we successful synthetize the oxygen hydrate, the highest pressure is up to 7GPa. When the pressure up to 0.34GPa, the Raman vibration spectrum of O-O stretching mode split into two peaks with same intensity, which suggest that a solid-solid(OH-I to OH-II) phase transition take place on oxygen hydrate, OH-II belongs to hexagonal crystal(SH). The oxygen hydrate do not composed and still stable. Oxygen hydrate decomposed when the pressure up to 1.18GPa.The studies of gas hydrate not only include the abundant content of physics but also supply important experimental data for technology development and practical applicants in many fields, for example, gasoline, natural gas, energy source, chemical industry, biotechnology engineering and environmental protection.