Numerical Simulation And Experimental Study On Physical Properties Of Transition Metal Mo

There has been a longstanding controversy on the high pressure phase diagram of transition metals, for the great difference between the results obtained from dynamic and static high pressure experiments. As one of the often used pressure carlibration materials, the high pressure phase transition of Mo (a kind of4d metal) is still under to be clarified. Besides, it is also necessary to determine the structure configuration before investigating other physical properties. Based on the preheated shock wave experiments and numerical simulations, we would attempt to explain the transition paths of phases for Mo, and investigate its thermodynamic, mechanic and some other properties. The main contents of this dissertation are listed as follows:1. Based on the density functional theory (DFT), the relative stableness of bcc, fcc, hep, and dhcp structures for Mo is systematically investigated. A bcc-dhcp transition was found at～620GPa, which is lower than former reported result [Phys. Rev. Lett.101(2008)049602] by～40GPa. The quasiharmonic Debye (QHD) model was used to investigate the thermodynamic properties of bcc and dhcp phases. The results show that an obvious jump at～620GPa is observed along293K isotherm, and there is also obvious change of thermodynamics near the critical point of structural transition.2. By taking the thermo electron excitation into consideration, the Hugoniot shock temperatures with different initial temperatures (293,933,1698, and2600K) are calculated, respectively. We can get the conclusion that the TEC could reduce the shock temperature effectively. Based on the LLNL analytical model for calculating elastic constants at finite temperature, the Hugoniot sound velocities from different initial temperatures at the range of0～400GPa are calculated, respectively. The calculations agree with the newest experimental results [Phys. Rev. B89(2014)174109]. Besides, the method developed in this dissertation can be used to calculate the Hiigoniot sound velocity of other metals, and provide predictions for experiments.3. Based on the EAM many body potential, the solid-liquid coexistence technology has been employed to perform the molecular dynamics (MD) simulation of the melting curve for Mo within NPT ensemble. The MD results agree with the melting temperatures (MT) obtained from reported dynamic experiments and theoretical calculations, but are obviously higher than the MT derived from DAC experiments. By using the adaptive common neighbor analysis (CNA) technology, the phase transition of Mo at high P-T is investigated. It is observed that the bcc structure is stable before melting up to-200GPa; when higher than200GPa, Mo is under a stable multi-phases coexistence state at lower temperature, and transforms into bcc structure at higher temperature before melting.4. Based on the preheated SW loading technique, the shock MT (at324.6and373.3GPa) and release MT (at153.1and176.6GPa) of preheated Mo (T0=933K) are measured, respectively. The results agree with reported theoretical calculations well, and provide new data to clarify the great difference of melting curves obtained from dynamic and static high-P experiments.