Electrical Transport Property Of Boron, Ferric Oxide Under High Pressure

With the development of science and technology, more and more measurements can be performed in diamond anvil cells (DACs), 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 increasingly mature techniques have led to a great improvement in the research of high pressure physics. Recently, the electrical measurement on DAC obtains large development, and fills up a blank on the technology of high-pressure impedance spectroscopy (IS) measurement. It breaks through the previous highest pressure and temperature range at which traditional high-pressure apparatus of big sample chamber performed IS measurement. But, high pressure magnetoresistance (MS) measurement on DAC is still in slow development and therefore present MS research can only be done in low pressure region. In this thesis, to solve above problems, we designed a new DAC system to perform in-situ magnetoresistance measurement under high pressure, by using a mature film deposition and photolithograph technique and a direct integration of measurement circuit on diamond anvil. And then, with this technology, the electrical transport behavior is investigated systematically at high pressure on the samples ofβ-boron and ferric oxide.The electrical transport properties ofβ-boron under high pressure are listed as follow: From the DC measurement results we find that no abrupt conductivity change occurs with increasing pressure in 30GPa. As the electrical conductivity gradually increases from 293 to 473K, an inflexion which is independent on pressure occurs at 370K. And then, a possible reason is proposed to explain the result based on the theory of valence band structure. In addition, our experimental result indicates thatβ-boron still shows the transport behavior of a semiconductor.From the AC-IS measurement we find thatβ-boron shows two impedance semicircle arcs in the Nyquist representation, which repsent two conduction processes relative to grain interior conduction and grain boundary conduction. Because sample resistivity changes at high pressure, the profile of two impedance arcs is different. By choosing appropriate representation and equal circuit, the pressure dependence of grain interior and bulk conductance are obtained, and the grain boundary effect is observed. At high pressure, grain boundary resistance is smaller than grain resistance. The relaxation frequency of grain increases with pressure. Bulk resistance and relaxation frequency vs pressure almost presents linear variation, so the activation energy at grain can be obtained by fitting experiment data. Pressure can enhance the conduction in the grain boundary through gradually decreasing the potential barrier height. Namely, the grain boundary can serve as highly conductive pathways under high pressure. This indicates that pressure can affect the electrical property of grain boundary obviously, and induce the change of grain boundary resistance and relaxation frequency.From the MS measurement under magnetic field and high pressure we find that the magnetoresistance ofβ-boron is positive and presents symmetrical relation and peak value in measured region.For bulkγ-Fe2O3, from the DC measurement results we observe the sharp increase of electrical conductivity at 29.88GPa which is agreeable with previous X-ray diffraction, in which the structural phase transition fromγ-Fe2O3 toα-Fe2O3 is detected. Above 33.59GPa, the electrical conductivity increase slowly, due to the complete of phase transition. In the experiment, the variation of crystal structure is reflected by the change of electrical conductivity.From the AC-IS measurement we find that, for bulkγ-Fe2O3 there are two impedance semicircle arcs in the Nyquist representation. It also indicates that there exist two conduction processes, grain interior conduction at high frequency region and grain boundary conduction at low frequency region.The resistance slowly decreases with increasing pressure within 29.88GPa; and then, the resistance steeply decreases with increasing pressure from 29.88 to 33.59GPa; above 33.59GPa, the resistance starts to decrease slowly again. This process is due to structural phase transition of sample. The impedance spectroscopy study of bulkγ-Fe2O3 under high pressure shows that the grain boundary resistance and grain boundary relaxation frequency strongly depend on the structural transformation.For nanoγ-Fe2O3, from the DC measurement we observe a sharp increase of electrical conductivity taking place at 21.3GPa, corresponding to a transition from the maghemite to the hematite. Above 26.42GPa, the conductivity rises gradually with increasing pressure. No distinct abnormal change is observed during decompression, indicating transformation is irreversible when the pressure comes back to ambient. Grain boundary resistance gradually decreases with increasing pressure, and its decreasing extent beccomes more and more sharp. Above 7.41GPa, the impedance plot only shows a simple semi-circular arc which corresponds to electrical response of grain. It indicates that the block effect of grain to charge carrier is predominant. Otherwise, the temperature dependence of the DC electrical conductivity at high pressure shows a semiconducting transport behavior from ambient pressure to 31.37GPa.We also investigated the electrical transport property of Fe₃O₄/β-CD under high pressure: An inflexion of electrical conductivity is observed at～17.06GPa, so we speculate that Fe₃O₄/β-CD sample undergoes a phase transition at～17.06GPa. During decompression, the electrical conductivity of sample gradually decreases with pressure, but it does not return to original state when the pressure comes back to ambient, so this is an irreversible transformation. From the IS measurement results we observe that sample only indicates the relaxation process of bulk resistance (grain). In summary, we carried out in-situ conductivity measurement onβ-boron, ferric oxide under high pressure using a microcircuit fabricated on a DAC. The pressure, temperature and magnetic field dependence of the abundant electrical properties are obtained in our experiment.