Research On High-Pressure Sound Velocity And Melting Of Porous Irom

The earth's core is dominantly composed of iron element. The pressure at the boundary between the liquid outer core and the solid inner core is determined to be 330GPa, and the corresponding temperature should be the melting temperature of iron under the pressure. Therefore, determination of iron's melting curve is an important topic in the field of either high-pressure physics or geophysics. In laboratory, both the shock wave technique (SW) and the diamond anvil cell technique (DAC) can be applied to compress iron up to very higher pressure, and the melting temperature can be experimentally or theoretically investigated at the various pressure ranges. When the melting curve at high pressure is extended to the pressure at IOB, the temperature at IOB could be known. However, there is a systematic discrepancy between the results from the two experimental methods. The results from SW are generally higher than that from DAC, and the difference will be enlarged when they are extended to 330GPa. Therefore, more attentions were recently paid on understanding the systematic deviation and determination of a unified melting curve of iron at high pressure. In this work, some new sound velocity data of porous iron samples are obtained, and improvements are made on investigation of melting curve, which are summarized as follows:(1) The measurement technique of the rarefaction sound velocity is applied to porous iron sample under shock compression, and two new shock-induced melting pressures are found at 134GPa and 87GPa, which are respectively related to the samples of two different porosities. This is the most important primary innovation of the present work. By combination with shock-induced melting pressure (260GPa) of dense iron from Brown and McQueen, and Nuygen and Holmes, it becomes possible that the melting curve of iron at high pressures could be determined just by the data from SW instead of DAC data.(2) Based on the correlation between the shock temperature and the initial porosity of sample, eight new sound velocity data are obtained in awider pressure and temperature range. For the samples with an average initial density of 6.9 g/cm3, four sound velocity data under shock pressure of 100-135GPa are measured. For the sample with an average initial density of 6.3 g/cm3, four sound velocity data under shock pressure of 75-90GPa are also obtained. All the sound data are of importance for constraining the equation of state of iron.(3) A new sound velocity measurement method is successfully used in present work, which is called the reverse impact technique. It may be a complement for the conventional multi-step optical analysis technique. But for porous sample, the characteristic of the recorded signal is easier to be distinguished. Therefore, the accuracy of time interval is relatively improved.(4) In this work, the minimum points on the sound velocity vs pressure curve (Cp-P curve), which generally located at the bulk velocity vs pressure curve, is supposed to be completely melt, it is defined the complete melting point, and its corresponding temperature is considered the equilibrium melting temperature, which is determined by Grover model applicable for the liquid metal. The equilibrium melt curve constrained by such iteration is found to be in good agreement with the recent DAC data. The result should be valuable for investigation the systematic deviation between SW and DAC measurement.(5) During calculations, the Hugoniot equation of state of dense iron is adopted, and the newly determined Gruneisen parameter of solid iron is cited in, and also the ab initio calculation results of thermal electrons and an-harmonic effects of lattice vibration are used. By this way, the problem of the cold pressure and cold energy formula, the problem of solid-solid transformation at lower pressures, even the difficulties on determining the thermodynamic parameters at expanding volumes will naturally avoided. So, the results in this work are more credible.(6) In the first appendix, two-phase molecular dynamic method with the canonical ensemble (NVT) is adopted to simulate the melting curve of Argon. It shows that the form of the atomic interaction potential will evidently affect the melting curve, and that the simulation result can explain the newly DAC experiment data. It is considered that the canonical ensemble (NVT) simulation will take advantage over others when the melting of the meta-stable phase is studied, which possibly exists during shock compression.(7) In the second appendix, a flyer impact method and an electrical pin technique are designed to measure the amplitude decay of perturbations on a corrugated shock front, and it is applied to study the viscous properties of aluminum at the high temperatures and pressures generated by two-stage light-gas gun. When compared with that of Sakharove et al, which is designed for detonation shock conditions, the new scheme has advantages of simplicity and easily application. The results are of importance for further researches on this topic.