Bulletin of the American Physical Society
18th Biennial Intl. Conference of the APS Topical Group on Shock Compression of Condensed Matter held in conjunction with the 24th Biennial Intl. Conference of the Intl. Association for the Advancement of High Pressure Science and Technology (AIRAPT)
Volume 58, Number 7
Sunday–Friday, July 7–12, 2013; Seattle, Washington
Session H2: CM.2 Phase Transitions: Tantalum and Molybdenum |
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Chair: Carl Greeff, Los Alamos National Laboratory Room: Grand Ballroom II |
Tuesday, July 9, 2013 9:15AM - 9:30AM |
H2.00001: Structures, properties, and phase Transformations of Ta at high pressures and temperatures Dennis Klug, Yansun Yao High pressure structures and phase transitions of dense Ta were studied with several theoretical methods to address recent controversies regarding the properties of this element. The objective is to characterize the structure of Ta at high temperatures and pressures where possible phase transitions could occur and have been reported. The techniques employed include structure search methods and the metadynamics method based on density functional theory, together with a detailed analysis of the mechanical and dynamical properties of candidate structures the may be stable near the melting temperature of Ta and to pressures up to several TPa which are currently obtainable in shock- compression experiments. This includes a characterization of anharmonic effects on the dynamical and mechanical stability of Ta over the temperature range from 0 K to the melting temperature. [Preview Abstract] |
Tuesday, July 9, 2013 9:30AM - 9:45AM |
H2.00002: Melting curve of Ta from the modified Z method in molecular dynamics simulation Wang Shuaichuang, Liu Haifeng, Zhang Gongmu, Song Haifeng, Tang Li Our recently proposed modified Z method to calculate the melting curve of metals has an obvious feature that a system can run naturally into its steady solid-liquid coexistence state from a perfect solid configuration in one running process. The method has been proved to be successful for face-centered cubic metals. Now we examine its validity for the melting curve of body-centered cubic metals, Ta as an example. A steady solid-liquid coexistence state can still be achieved for a system with only about 1000 atoms. The melting temperature and pressure results of Ta, extracted from the coexistence state, are in good agreement with those of the two-phase method in the literature. [Preview Abstract] |
Tuesday, July 9, 2013 9:45AM - 10:15AM |
H2.00003: Rarefaction wave propagation and longitudinal sound velocities in shock compressed tantalum Invited Speaker: Robert Scharff The purpose of this work is to investigate the bcc to hexagonal structural phase transition recently reported for shock compressed tantalum. Longitudinal sound velocities were obtained using a velocimetry diagnostic to record the shock and rarefaction wave arrival times at the sample/anvil interface in the reverse-ballistic plate impact geometry. This approach allows for the determination of the sound speed as a function of pressure and is sensitive to volume changes associated with phase transition behavior. The authors demonstrate that if elastic -- plastic wave interactions are correctly determined, then the high pressure structural phase transition that has been previously reported is notably absent. [Preview Abstract] |
Tuesday, July 9, 2013 10:15AM - 10:30AM |
H2.00004: The compressibility and sound velocity measurements of molybdenum up to $\sim $0.7 TPa Chengda Dai, Xiang Wang, Xiulu Zhang, Qingsong Wang, Ke Jin, Ye Tan, Hongxing Song, Feng Xi, Jianbo Hu, Hua Tan The compressibility (Hugoniot) and sound velocity data of matter are of particular importance for constructing high-pressure equation of state and/or detecting phase transitions. In this presentation, we report the Hugoniot measurements of Mo up to $\sim $0.7 TPa performed on a gas gun. A hypervelocity flyer launcher was fixed on a two-stage gun muzzle for a graded-density impactor to drive Ta secondary flyer up to $\sim $10 km/s. The simultaneous measurements of Ta flyer velocity and shock wave velocity of Mo in each shot yielded a Hugoniot data pair. The obtained results are in a good agreement with available data. The sound velocities of Mo were also measured under shock pressure from $\sim $60 GPa to $\sim $160 GPa using a backward or forward impact geometry based on rarefaction overtake method. The extracted data smooth in tendency the knee around 210 GPa, not supporting the interpretation as a polymorphic transition. Furthermore, the obtained Mo Hugoniot and sound velocity data are compared with the results calculated using QEOS model. [Preview Abstract] |
Tuesday, July 9, 2013 10:30AM - 10:45AM |
H2.00005: No solid-solid phase transition in Mo before melting: experiment and theory Xiulu Zhang, Zhongli Liu, Ke Jin, Feng Xi, Yuying Yu, Ye Tan, Chengda Dai, Lingcang Cai Whether there is a solid-solid phase transition plays an important role in shaping the phase diagram of Mo. Previous sound velocity measurements suggested one at 210 GPa[Phys. Rev. Lett. 62, 637(1989)], but the latest results [J. Nguyen, et al., In Shock Compression of Condensed Matter-2011] did not support it. In our work, adopting the ``reverse impact'' method[J. Geophys. Res. 100(B1), 529 (1995)] and the ``overtake'' method[Rev. Sci. Instrum. 53, 245(1982)], we obtained new data in the pressure range from 38 GPa to 160 GPa. Together with the latest results, it can be concluded that a solid-solid phase transition does not occur in Mo under shock loading pressures from 38 GPa to 240 GPa. With the crystal structure prediction techniques based on the genetic algorithms and density functional theory calculations, two metastable structures C2m and I4mmm are found, but we did not reproduce fcc phase of Mo in our searches. As their energy is much higher than that of bcc Mo, it is unlikely that they are more stable than bcc Mo at high temperatures. This is consistent with C. Cazorla's conclusion that fcc is less stable than bcc[Phys. Rev. B 85, 064113(2012)], and theoretically support the conclusion of our sound velocity measurements. [Preview Abstract] |
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