Sound Velocity And Equation Of State Of Ta, Bi And Mo Under Shock Loading

Phase diagram and equation of state (EOS) of materials are important topics in condensed matter physics. Theoretical modeling of a universal multi-phase EOS is usually constructed by such an approach:Proper physical models for different regimes are combined effectively to obtain a universal multi-phase EOS model; then, theoretical calculations of the model are carried out to compare with experimental data. These regimes are usually determined by the phase boundaries and triple points on the phase diagram, or some special physical conditions. The phase transformation is critical to the construction of the universal multi-phase EOS model. It is still a challenge to understand the shock melting mechanism of transition metals. Especially, there is large difference between the melting curves of the transition metals measured by shock wave (SW) experiments, static high-pressure experiments, and ab initio calculations. One of the involved issues might be whether the body centered cubic (BCC) structure phase of the transition metal is stable up to the shock melting pressure or not. On the other hand, bismuth (Bi) is a typical polymorphous material. In the liquid phase beyond the shock melting pressure, the EOS experimental data of Bi are scattering. Available experimental data are also scarce. The precision of the multi-phase EOS model of Bi at high pressure is limited severely by these problems.In order to detect the presumptive solid-phase transformation in Ta indicated by sound velocity and shock recovery experiments reported in the literature, high preci-sion sound velocity measurements of Ta are performed by using a symmetric impact method with the step-formed sample (SST) and a direct-reverse impact method (DRIT). Shock recovery experiments of Ta are also preformed, and the influence of the elastic-plastic property on the sound velocity measurement are investigated experimentally. Combined with the experimental results of Ta obtained by diamond anvil cell (DAC) with external heating and double laser heating, and the rotation-cell, respectively, and the experimental data and the theoretical results of Ta reported in the literature, our experimental results show that the BCC phase of Ta is stable up to the shock melting pressure and no evidence of the structural phase transformation is observed.In the liquid phase beyond the shock melting pressure of Bi, the experimental EOS data are scarce and scattering. In this work, highly accurate Hugoniot and sound veloc-ity of Bi are measured by using the asymmetric impact method with the step-formed sample (AST). These experimental data of Bi could be used to determine the shock melting pressure, to construct or verify the theoretical EOS model at high pressures, and to cross-check the experimental and theoretical results at low pressures. It also pro-vides an experimental evidence to carry out the multi-physical parameter measurement with high precision in one shot.Some challenges have to be overcome in the preheating experiment under shock loading, such as the large temperature gradient of the preheated sample and the low signal-noise-ratio (SNR). The Mo/LiF interface particle velocity profiles with a good SNR are obtained at a preheated temperature of-500 K by improving the experimental setup and the preheating technique, as well as a good sample-window interface with high reflecting obtained by using a special metal layer coated on the impact surface of the window. Taking into account the measurement uncertainties, the sound velocity of the preheated Mo confirms the conclusion that no evidence of the structural phase transformation was found up to the shock melting pressure. The experimental tech-niques to measure the Hugoniot and sound velocity of preheated metals under shock loading are developed, which provides a technology support to the dynamic compres-sion experiments with high precision and high preheated temperature.The reliability of the measurement uncertainty of the experimental data plays an important role in science researches. An appropriate and scientific evaluation method is crucial to improve the reliability of the measurement uncertainty. But detailed anal-ysis of the measurement uncertainty of the sound velocity is very few in the literature up to now. Because of the high reliability of the Monte Carlo (MC) method at the con-ditions where the GUM method doesn't perform well, the combination of the GUM and MC method is constructed and carried out to analyze our experimental data, which improves the reliability of the measurement uncertainty greatly. Based on this method, the data measured in different experiments could be compared directly, and the phys-ical parameter measurements under shock loading and the scientific evaluation of the measurement uncertainty could be performed at a high confidence level.