The Electrical Properties And Phase Transition Of Water/ice Under High Pressure

With the development of experimental technology under high pressure, more and more in situ experiments can be performed in diamond anvil cell (DAC), such as synchrotron X-ray diffraction, neutron scattering, Mossbauer spectrum, laser Raman scattering, photoluminescence, optical absorption and so on. These increasingly matured techniques have led to a great improvement in the research of high pressure physics. In this paper, we integrated microcircuits on the diamond anvils and developed an effective method for In-situ electrical conductivity accurate measurement on liquid matters. In-situ electrical conductivity measurement on water was carried out which is crucial for physics, chemistry, biology, geophysics and planetary science. The electrical properties of water have been investigated extensively in the past decades, but they were limited mainly to shock–wave compression investigations. Although shock-wave compression investigations provide valuable information for understanding puzzling properties of this intriguing system, they inescapably bring about higher temperature (>600K) . While, under static pressure, researchers can achieve more moderate P-T conditions( < 500K). Static diamond anvil cell (DAC) experiments only investigated the physical properties in solid phase below the melting curve (Ice-VII above 2.2Gpa),however, the results of melting curves obtained by different physical phenomena show a large discrepancy. One of the main tasks of this paper is to analyze the conduction mechanism of water/ice and determine the melting curve of Ice-VI and VII accurately.We integrated the microcircuit on a diamond anvil by using sputtering film and micro fabrication technology. It exhibits some outstanding advantages as following: (1) the sputtered probes can be designed with various regular shapes by photolithography and exactly placed at a desirable place on the diamond culet; (2) Probes can remain unchanged under high pressure. These advantages make it possible for the accurate measurement of conductivity in DAC. van der Pauw method is adopted in our experiment. This arrangement can eliminate most of contact resistance between the sample and the probes. In our microcircuit we chose molybdenum (Mo) for the conductor and alumina (Al2O3) for the insulator and protective materials in the conductivity measurement. A flake of mica which was fabricated figure as the indention was chosen for further insulation and protection. The current was provided by KEITHLET 2400 Source Meter and the voltage was measured by a KEITHLEY 2700 Multimeter. The impedance spectroscopy was measured by SOLATRON 1260 associated with 1296In the course of compression from ambient pressure to 0.68GPa at room temperature, the electrical conductivity increases linearly several orders of magnitude. As well known water chemical ionization: 2H2O→H3O++OH-, water ionic dissociation increases steeply with rising pressure and temperature. The ionic mobility is very high and the conduction is induced by hopping of H +and OH ? ions from molecule to molecule overcoming hydrogen bonds energy～0.1eV. Consequently, we can safely get the conclusion that as increasing pressure the rapid increase in electrical conductivity is attributed to an enhancement of chemical ionization induced by high pressure. Our result is similar to that of Hamann and Linton under dynamical pressure. However, the increasing trend of conductivity in compression is more slower than Hamann's conclusion, which may be attributed to constant temperature under static pressure hinders chemical ionization of water molecules more than increasing higher temperature combined with dynamical pressure.In the range of 0.96-2.6GPa, the electrical conductivity of ice decreases with increasing pressure, but at 2.9GPa an inflexion appears and the pressure dependence of conductivity becomes relative evenness until 30GPa. We also detected the transition from Ice-VIII to Ice VII by electrical measurement at region of 77K - 300K and 1 - 3GPa. As a whole, the relationship of electrical conductivity of ice VS pressure is discontinuous, which is basically corresponding to the polymorph of ice. The distinctly electrical conductivity discrepancy between liquid and solid phase can reliably characterize the phase bound as other optical determinations. We determine the phase boundary of Ice-VI and VII by the sharp increase of electrical conductivity with increasing pressure when phase transition of Ice-VI to VII occurs. We determined that the triple point VI-VII-liquid is at 2.2GPa and 358K which is close to average of the values given in the literatures (2.17GPa and 354.8K).Below the triple point our melting line of VI-liquid phase is in good agreement with the measurement by Bridgman et al. However, the observed pressures of phase boundary of Ice VII are higher than Dubrovinskaia's determination who measured the phase boundary of Ice-VII and liquid by the appearance and disappearance of EDXD 110 peaks of Ice-VII. The melting temperature of ice VI and VII are monotonic increasing functions of pressure in the studied range. The data can also be expressed by a Simon equation. The Simon equation corresponding to the data of Ice-VI is: (P-2.17)/15.11=(T/354.8)0.48-1, and the Simon equation of Ice-VII melting line is: (P-2.17)/2.27=(T/354.8)2.04-1Water, being dipolar, can be partly aligned by a constant electrical field, but a mild alternating current stop the system from polarization. In the impedance spectroscopy we can clearly find that the frequency response of liquid water has two conduction processes. We can separate the charge transfer resistance from interface impedance and Warburg impedance by equivalent circuit fitting using Zview2 software. It is evident that pressure hinders the charges transfer and minimizes the conduction between electrode interface and sample near the electrode.