High-pressure And High-temperature Properties Of Fe-S And Fe-Si System Under Shock Loading

In order to constrain the amount of S and Si in the outer core,the most effective method is to simultaneously measure densities and sound velocities of candidate core materials with high pressure and temperature technique,and compare the measured results to the geochemical and geophysical model under the outer core conditions.However,in-situ measuring the density and sound velocity of liquid iron alloys under high pressure and temperature conditions is still a huge challenge with static high pressure techniques.On the basis of the previous work of Professor Huang's group,we systematically investigated the melting behavior,equation of state and sound velocity of liquid Fe-11.8 wt.%S and Fe-8.6 wt.%Si using dynamic compression techniques.Applying the thermodynamics model,the density and sound velocity of liquid Fe-S and Fe-Si binary system were calculated under high pressure and temperature.Simultaneously,considering the geochemical and geophysical constraints,we estimated the content of S and Si in the outer core,which will provide useful references for the next experimental research on the Fe-S-Si ternary system.The main research work and innovations of the thesis are as follows:1.Using reverse-impact technique,it is very difficult to judge the elastic-plastic transition point from the history of the particle velocity,resulting in large errors in the data processing on the bulk sound velocity.A differential method was proposed in this thesis to determine the lastic-plastic transition point.Through the time derivative values of particle velocity,we can accurately determine the elastic unloading process,including the elastic-plastic unloading transition point,and the plastic unloading time.The accuracy of longitudinal wave,bulk wave and shear wave velocity was obviously improved.2.Based on the shock velocity versus particle velocity,and the transition of longitudinal sound velocity to the bulk sound velocity,the dynamic compressed Fe-11.8 wt.%S is completely melt at 111.3?2.3?GPa.The Initial density is?₀=6.50?0.02?g/cm³,Hugoniot parameters are C₀=3.68?0.11?km/s,?=1.55?0.04?.3.For shock temperature,it is usually calculated based on the equation of state along Hugoniot and 300K isotherm,which obviously increases the workloads of the experiment.On the basis of previous studies,we propose a new method to calculate the shock temperature of liquid based entirely on the Hugoniot data.Based on the model,the density and sound velocity of liquid Fe-11.8 wt.%S were calculated using this method,and the results are consistent with the static experimental data measured at low pressure.Hence,our thermodynamics model is feasible to calculate the shocking temperature.4.Using dynamic pressure technique,the equation of state and sound velocity data of Fe-8.6 wt.%Si were obtained.The measurements of the sound velocity implies that the sample completely melts at 260.3?4.6?GPa.Upon melting,there is no obvious changes in the linear correlation between shock velocity and particle velocity.The initial density of Fe-8.6 wt.%Si is?₀=7.386?0.021?g/cm³,Hugoniot parameters are C₀=34.603?0.055?km/s,?=1.505?0.017?.5.Effects of temperature on the longitudinal sound?V_P?,bulk sound?V_B?and shear sound?V_S?at high pressure were analyzed.We found that V_P increases linearly with density at high temperature,which is consistent with the experimental results measured at 300 K.It is shown that V_P of Fe-8.6 wt.%Si still satisfies Birch's law at high temperature.We also found V_B increases with temperature,but the amplitude is small,and V_S varies greatly with the temperature.On the basis of previous studies,a new model is proposed to describe the density and temperature dependence of V_s.6.Based on the thermodynamics model,the density and sound velocity of liquid Fe-11.8 wt.%S,Fe-8.6 wt.%Si and Fe-5.4 wt.%S-4 wt.%Si were calculated.The density and sound velocity of liquid Fe-5.4 wt.%S-4 wt.%Si are both consistent with PREM model,as well as satisfy the geochemical and geophysical constraints.Under the inner core condition,V_P and V_S of Fe-Si decreases with the increasing content of Si.However,V_P and V_S of Fe-4 wt.%Si are still higher than PREM model,although its density matches the value of inner core.Our results indicates that Si is less than 4wt.%in the inner core,and other light elements are needed to reduce the sound velocity of iron.