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Register a new userThermodynamic properties of liquid mercury to 520 K and 7 GPa from acoustic velocity measurements
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Ultrafast acoustics measurements on liquid mercury have been performed at high pressure and temperature in diamond anvils cell using picosecond acoustic interferometry. We extract the density of mercury from adiabatic sound velocities using a numerical iterative procedure. The pressure and temperature dependence of the thermal expansion, the isothermal compressibilty, the isothermal bulk modulus and its pressure derivative are derived up to 7 GPa and 520 K. In the high pressure regime, the sound velocity values, at a given density, are shown to be only slightly dependent on the specific temperature and pressure conditions. The density dependence of sound velocity at low density is consistent with that observed with our data at high density in the metallic liquid state.
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p-V-T equation of state of superhard boron suboxide B6O has been measured up to 6 GPa and 2700 K using multianvil technique and synchrotron X-ray diffraction. To fit the experimental data, the theoretical p-V-T equation of state has been derived in approximation of the constant value of the Anderson-Gruneisen parameter {delta}T. The model includes bulk modulus B0 =181 GPa and its first pressure derivative B0 = 6 at 300 K; two parameters describing thermal expansion at 0.1 MPa, i.e. a = 1.4x10-5 K-1 and b = 5x10-9 K-2, as well as {delta}T = 6. The good agreement between fitted and experimental isobars has been achieved to the absolute volume changes up to 5% as compared to volume at standard conditions, V0. The fitted thermal expansion at 0.1 MPa is well consistent with the experimental data, as well as with ambient-pressure heat capacity cp, bulk modulus B0 and {delta}T describing its evolution with volume and temperature. The fitted value of Gruneisen parameter {gamma} = 0.85 is in agreement with previous empiric estimations for B6O and experimental values for other boron-rich solids.
The high-pressure melting curve of tantalum (Ta) has been the center of a long-standing controversy. Sound velocities along the Hugoniot curve are expected to help in understanding this issue. To that end, we employed a direct-reverse impact technique and velocity interferometry to determine sound velocities of Ta under shock compression in the 10-110 GPa pressure range. The measured longitudinal sound velocities show an obvious kink at ~60 GPa as a function of shock pressure, while the bulk sound velocities show no discontinuity. Such observation could result from a structural transformation associated with a negligible volume change or an electronic topological transition.
In the present paper we performed the analysis of available data on structural, thermodynamic and mechanical properties of B6O. Although the compound is known for half a century and has been extensively studied, many properties of this boron-rich solid remain unknown or doubtful. Semi-empirical analysis of our experimental and literature data allowed us to choose the best values of main thermodynamic and mechanical characteristics among previously reported data, to predict the thermoelastic equation of state of B6O, and dependence of its hardness on non-stoichiometry and temperature.
We have measured the lattice volume of ice VIII in different pressure-temperature pathways and found isothermal compression at low-temperature conditions makes the volume larger. Ice VIII has become its high-pressure phase with the molar volume of 6.45 cm^3 at 10 K where the pressure can be estimated as 60.4 GPa based on the third-order Birch-Murnaghan equation with parameters determined in this study (K_0= 32.4 GPa, K_0= 3.7, and V_0= 11.9 cm^3). The present results indicate that this high-pressure state is paraelectric with tetragonal symmetry.
Computing the grain boundary (GB) counterparts to bulk phase diagrams represents an emerging research direction with potentially broad impacts. Using a classical embrittlement model system Ga-doped Al, this study demonstrates the feasibility of computing temperature- and composition-dependent GB diagrams to represent not only equilibrium thermodynamic and structural characters, but also mechanical properties. Specifically, hybrid Monte Carlo and molecular dynamics (MC/MD) simulations are used to obtain the equilibrium GB structure as a function of temperature and composition. Simulated GB structures are validated by aberration-corrected scanning transmission electron microscopy. Subsequently, MD tensile tests are performed on the simulated equilibrium GB structures. GB diagrams are computed for not only GB adsorption and disorder, but also interfacial structural and chemical widths, MD ultimate strength, and tensile toughness. A model is established to forecast the ductile-to-brittle transition. This study establishes a new paradigm to compute a spectrum of GB diagrams to enable the investigation of the unique GB composition-structure-property relationship.
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