Electrical Transport Property Of AIN Nanowires And Fe₃O₄ Powders Under High Pressure

With the development of science and technology, more and more measurements can be performed in diamond anvil cells(DAC), such as X-ray diffraction, neutron scattering, Raman scattering, optical absorption, m?ssbauer spectrum, photoluminescence, Brillouin scattering and so on. And all of these measurements can be in-situ conducted at high pressure and high temperature. These techniques have led to a great improvement in the research of high-pressure science. However, due to the restriction of the structure of DAC, it is difficult to measure the electrical properties of matter under high pressure, such as resistivity, magneto-resistance, and impedance spectra. In this thesis, for solving above technical problems, a designed DAC system has been made to perform in-situ electrical measurement under high pressure. By using the film deposition and photolithograph technique, a measurement microcircuit was integrated directly on a non-magnetic DAC. In-situ resistivity measurement, magneto-resistance measurement and impedance have been progressed under high pressure.The experimental samples were AlN nanowires and Fe3O4 powders. The in-situ X-ray diffraction experiments indicated that the AlN nano-wires have a pressure induced phase transition at 25.89GPa. The transition is from wurtzite phase to rocksalt phase. As pressure went to 47.44GPa, no new phase transition was observed. During the decompression process from 47.44GPa to ambient, the diffraction peaks of sample only shift to the left and no new peaks appear, that indicates the structural phase transition is an irreversible process. From AC in-situ impedance measurement of AlN nano-wires, two impedance semicircle arcs in the Nyquist representation were found, which indicates there are two conduction processes, i.e., the grain interior conduction at high frequency region and the grain boundary conduction at low frequency region. As pressure changed, the profiles of two impedance were different. The grain interior resistance decreases slowly with increasing pressure up to 31.83GPa, and the pressure have little effect on grain boundaries resistance.From the resistivity measurement under high pressure, it was found that the resistivity of Fe3O4 powder decreased gradually with pressure increasing. At 6GPa, an abnormal change occurred. Under pressure lower than 6GPa, the resistivity decreased faster when pressure increased. After 6GPa, it decreased slowly. We conclude that the micro-mechanism of Fe3O4 changed at 6GPa. The magneto-resistance measurement under high pressure showed that the resistivity increases with increasing magnetic field before 6GPa, showing a positive magneto-resistance effect. After 6GPa, the resistivity decreases with increasing magnetic field, showing a negative magneto-resistance effect. This further confirmed the micro-mechanism of Fe3O4 changed at 6GPa, and this resulted from the change of spin in Fe3O4 powders under high pressure. This phenomenon does not change when magnetic field direction changes.To sum up, we carried out in-situ high pressure structural and impedance measurement on AlN nanowires, and electrical resistivity and magnetoresistance measurements on Fe3O4 powders respectively on DAC. The electrical transportation properties of these materials under high pressure were studied.