4:30 PM - 4:45 PM
[PCG22-11] Study on transport properties of liquid iron and iron alloy under high temperature and high pressure using the laser shock technique
Keywords:iron alloy
Information on the thermal and electrical transport properties of iron alloys under high-pressure and high-temperature conditions corresponding to the earth’s outer core is necessary to clarify the convection phenomena of the earth and the cooling history. In particular, the transport properties in the outer core–mantle boundary condition strongly influence the mantle convection, the outer core convection generating the earth's magnetic field, the growth and inner flow of the inner core, and so on. However, there are very few experimental examples concerning high-pressure and high-temperature liquid iron/iron-alloy which correspond to the outer core conditions, and only a few cases have been reported for pure iron [1]. It is required to investigate transport coefficients of liquid iron alloys containing light elements to simulate actual outer core, experimentally. Laser shock compression technique has several advantages to exploring iron-alloys under high-pressure and high-temperature condition. As there is no mechanical restriction on pressure generation, producing the pressure of even TPa is easy, and high heating rate exceeding 1015K / s is possible, and it is a closed system that is not affected by surroundings. Therefore the technique is effective for generation and observation of high-pressure and high-temperature liquid iron-alloys. In this research, we report optical reflectivity, pressure and temperature measurements of laser shock compressed iron and iron-silicon (Fe-Si) alloy at and above the earth's outer core conditions.
Target assemblies containing polypropylene (CH), aluminum, quartz, iron or iron-silicon alloy and magnesium oxide (MgO) window were shock compressed by laser ablation at the GEKKO XII laser facility, the Institute of Laser Engineering, Osaka University. Iron and iron-silicon alloy were formed by electron beam evaporation on the MgO window. Laser beams with a wavelength of 527 nm were focused onto the target. A focal spot was 1 mm in diameter with a flat-top spatial intensity distribution resulting in a planar shock front. Temporal shape of the laser pulse was approximately a flat-top with a full-width at half-maximum of 2.5 ns. The velocity interferometer system for any reflector (VISAR) measure a particle velocity and a reflectivity at the interface between iron or iron-silicon alloy and MgO. The Ppressure was evaluated using measured particle velocity and a known equation of state of MgO[2]. The optical pyrometer measures the temperature at the interface through the MgO window.
The pressures determined from the measurement results were from 200 to 400 GPa, and the temperatures were up to 15000 K. In the pressure range exceeding 200 GPa, in the case of iron, a decrease in reflectivity was observed as the pressure increased, whereas in the case of the iron-silicon alloy, a continuous increase following a discontinuous decrease in the reflectivity was observed. The decrease in reflectivity of iron suggests a decrease in electronic conductivity due to temperature rise with increasing pressure. On the other hand, the continuous rise seems to indicate an increase in the density of free electrons derived from silicon.
[1] K. Ohta, Y. Kuwayama, K. Hirose, K. Shimizu, Y. Ohisi: Nature, 534, 95-98 (02 June 2016).
[2] S. Root, L. Shulenburger, R. W. Lemke et al.: Phys. Rev. Lett, 115, 198501 (2015).
Target assemblies containing polypropylene (CH), aluminum, quartz, iron or iron-silicon alloy and magnesium oxide (MgO) window were shock compressed by laser ablation at the GEKKO XII laser facility, the Institute of Laser Engineering, Osaka University. Iron and iron-silicon alloy were formed by electron beam evaporation on the MgO window. Laser beams with a wavelength of 527 nm were focused onto the target. A focal spot was 1 mm in diameter with a flat-top spatial intensity distribution resulting in a planar shock front. Temporal shape of the laser pulse was approximately a flat-top with a full-width at half-maximum of 2.5 ns. The velocity interferometer system for any reflector (VISAR) measure a particle velocity and a reflectivity at the interface between iron or iron-silicon alloy and MgO. The Ppressure was evaluated using measured particle velocity and a known equation of state of MgO[2]. The optical pyrometer measures the temperature at the interface through the MgO window.
The pressures determined from the measurement results were from 200 to 400 GPa, and the temperatures were up to 15000 K. In the pressure range exceeding 200 GPa, in the case of iron, a decrease in reflectivity was observed as the pressure increased, whereas in the case of the iron-silicon alloy, a continuous increase following a discontinuous decrease in the reflectivity was observed. The decrease in reflectivity of iron suggests a decrease in electronic conductivity due to temperature rise with increasing pressure. On the other hand, the continuous rise seems to indicate an increase in the density of free electrons derived from silicon.
[1] K. Ohta, Y. Kuwayama, K. Hirose, K. Shimizu, Y. Ohisi: Nature, 534, 95-98 (02 June 2016).
[2] S. Root, L. Shulenburger, R. W. Lemke et al.: Phys. Rev. Lett, 115, 198501 (2015).