No description available.

Physics at extreme conditions is not a young field; there have been decades of developments that have allowed us to generate high-pressure and high-temperature conditions in a vast array of materials. Conventionally, these extreme conditions were generated using static compression techniques; compressing a material in a diamond anvil cell which could then be heated or cooled, with structural information deduced using synchrotron radiation. These techniques are still invaluable for extreme conditions research although the pressures and temperatures that are accessible to them are limited by the strength of the diamond anvil cells and their ability to withstand extreme temperatures. The necessity for access to pressure-temperature states that are beyond the scope of the conventional diamond anvil cell is driven by the need to characterise extreme environments such as planetary interiors. It was long believed that materials in high pressure-temperature states would exhibit relatively simple, high-symmetry crystal structures, but recent research has proven that, conversely, there is an abundance of complex structural behaviour at these extreme conditions. One means of attaining pressure-temperature states beyond those accessible using static compression techniques is to impart a large amount of energy into a material in a comparatively short period of time (milliseconds to nanoseconds); this is known as dynamic compression. Dynamic compression can be generated using impact techniques or, alternatively, via laser ablation. Access to the most extreme conditions is commonly achieved by generating a shockwave which compresses the sample with the fastest achievable compression wave. Not only does this type of compression facilitate access to the most extreme states, it also allows us to explore the physics of impact phenomena and other such situations involving rapid energy transfer. Dynamic compression occurs on short timescales and, as such, there is a considerable challenge in implementing diagnostics to study the behaviour of compressed materials. Furthermore, because complexity is commonplace in extreme conditions, it is vital that any diagnostics should be able to provide data of high enough quality that this complexity may be resolved. The advent of 4th generation light sources (x-ray free electron lasers) has afforded us the opportunity to obtain extraordinarily high quality data on dynamic compression timescales. In the interest of refining analytical techniques when utilising this novel technology, materials exhibiting complex crystal structures should be investigated. Antimony is an element which is known, under static compression, to transform from a Peierls-distorted rhombohedral phase (R-3m) to an incommensurately modulated host-guest structure (I'4=mcm(00γ)000s), a structure with an incredibly high level of complexity. The complexity of this host-guest phase, and the relatively low pressure at which it forms, makes antimony an ideal candidate for testing the resolution achievable using these 4th generation light sources. Furthermore, it is interesting to observe whether such a complex phase can form on the short timescales of dynamic compression. In this work antimony is both statically and dynamically compressed and the results of both experiments are compared. A static phase diagram is constructed for antimony up to 31 GPa and 835 K, confirming the location of a previously theorised triple point and suggesting the location of an additional triple point. Three solid phases are characterised and data are found to agree with the pre-existing static compression studies. The nature of the host-guest phase is investigated and the guest 'chains' are found to remain intact even at the highest temperatures and pressures, a result which has not previously been observed in high pressure-temperature host-guest structures. Dynamic data from shock-compression experiments at pressures up to 59.3 GPa are plotted alongside the static data and contrasting phase behaviour is discussed. Four solid phases are identified along with one liquid phase. Observation of the host-guest phase in shock-compressed antimony confirms that highly complex crystal structures are able to form on the nanosecond timescale.

<p>This thesis presents transverse magnetoresistance (MR) and Hall resistivity measurements of nickel thin films at temperatures between 280 and 455 K and pressures up to 6 GPa. An experimental system was developed for conducting precise magnetotransport measurements using the current reversal and van der Pauw techniques in combination with a 10 T superconducting magnet. Polycrystalline Ni_0.985O_0.015 thin film samples were manufactured with preexisting point contacts allowing highly reproducible magnetotransport measurements at pressure in the diamond anvil cell (DAC).</p><p>The magnetic resistivity above the technical saturation of the magnetization was found to decrease linearly to the highest applied fields, 10 T, while the field derivative, 0.010-0.018 µΩ cm T^-1 between 280 and 316 K, increased with temperature and decreased with pressure. The decrease in the magnetoresistance is attributed to spin wave damping of electron-magnon scattering processes at high fields. The magnon mass, 535(14) meV Ų at 0 K and 0 GPa, determined from longitudinal magnetic resistivity theory is a slightly increasing function of pressure. Correlation between the zero field resistivity and the extraordinary Hall coefficient (EHC) confirmed side jump scattering as the dominant diffusion mechanism at 0 GPa, however, skew scattering was found to become increasingly important with pressure.</p><p>The effect of oxygen and pressure on the density of states (DOS) at the Fermi level was investigated through total energy band structure calculations using a periodic supercell of 64 atoms to simulate the sample chemistry. The DOS of Ni_0.985O_0.015 at the Fermi level was found to increase by 27% at 10 GPa relative to 0 GPa. However, when compared to the results for pure Ni, decreases of 60% and 23% occurred for the corresponding calculations at 0 and 10 GPa. The relative differences in the magnetic resistivity are attributed to competing effects between the DOS, average magnetic moment and magnon mass.</p><p>The technique developed for conducting magnetotransport measurements at pressure is applicable to the study of electronic diffusion in ferromagnets as well as geophysical problems such as the geodynamo.</p>

This thesis presents transverse magnetoresistance (MR) and Hall resistivity measurements of nickel thin films at temperatures between 280 and 455 K and pressures up to 6 GPa. An experimental system was developed for conducting precise magnetotransport measurements using the current reversal and van der Pauw techniques in combination with a 10 T superconducting magnet. Polycrystalline Ni0.985O0.015 thin film samples were manufactured with preexisting point contacts allowing highly reproducible magnetotransport measurements at pressure in the diamond anvil cell (DAC). The magnetic resistivity above the technical saturation of the magnetization was found to decrease linearly to the highest applied fields, 10 T, while the field derivative, 0.010-0.018 µΩ cm T-1 between 280 and 316 K, increased with temperature and decreased with pressure. The decrease in the magnetoresistance is attributed to spin wave damping of electron-magnon scattering processes at high fields. The magnon mass, 535(14) meV Å2 at 0 K and 0 GPa, determined from longitudinal magnetic resistivity theory is a slightly increasing function of pressure. Correlation between the zero field resistivity and the extraordinary Hall coefficient (EHC) confirmed side jump scattering as the dominant diffusion mechanism at 0 GPa, however, skew scattering was found to become increasingly important with pressure. The effect of oxygen and pressure on the density of states (DOS) at the Fermi level was investigated through total energy band structure calculations using a periodic supercell of 64 atoms to simulate the sample chemistry. The DOS of Ni0.985O0.015 at the Fermi level was found to increase by 27% at 10 GPa relative to 0 GPa. However, when compared to the results for pure Ni, decreases of 60% and 23% occurred for the corresponding calculations at 0 and 10 GPa. The relative differences in the magnetic resistivity are attributed to competing effects between the DOS, average magnetic moment and magnon mass. The technique developed for conducting magnetotransport measurements at pressure is applicable to the study of electronic diffusion in ferromagnets as well as geophysical problems such as the geodynamo.

Elastic properties of the major constituent minerals in the Earth’s upper mantle at relevant high pressure-temperature (P-T) conditions are crucial for understanding the composition and seismic velocity structures of the region. In this study, we have measured the single-crystal elasticity of natural Fe-bearing pyrope, Mg2.04Fe0.74Ca0.16Mn0.05Al2Si3O12, using in situ Brillouin spectroscopy and X-ray diffraction at simultaneous high P-T conditions up to 20 GPa and 750 K in an externally-heated diamond anvil cell. The derived aggregate adiabatic bulk and shear modulus (KS0, G0) at ambient conditions are 168.2 (±1.8) GPa and 92.1 (±1.1) GPa, respectively, consistent with literature results. Using the third-order Eulerian finite-strain equation to fit the high P-T data, the derived pressure derivative of the bulk and shear moduli at constant temperature are (∂KS/∂P)T=4.4 (±0.1) and (∂G/∂P)T=1.2 (±0.1), respectively. Applying these pressure derivatives, the temperature derivative of these moduli at constant pressure are also calculated, yielding (∂KS/∂T)P=-18.5(±1.3) MPa/K and (∂G/∂T)P=-5.2(±1.1) MPa/K, respectively. Compared to literature values, our results show that addition of 25% Fe in pyrope increases the pressure derivative of the bulk modulus by 7%, but has a negligible effect on other elastic parameters. Extrapolation of our results shows that Fe-bearing pyrope remains almost elastically isotropic at relevant P-T conditions of the upper mantle, indicating that it may not have a significant contribution to seismic Vp and Vs anisotropy in the upper mantle. Together with the elasticity of olivine and pyroxene minerals in the upper mantle, we have constructed new velocity profiles for two representative compositional models, pyrolite and piclogite, along Earth’s upper mantle geotherm. These velocity models show Vs profiles consistent with seismic observations, although Vp profiles are slightly lower than in seismic models. / text

The presented thesis focuses on study of transport and thermoelastic properties of materials under conditions of planetary interiors by means of high-pressure experimental tools and finite element modeling, and their role in the dynamics and states of cores of terrestrial planets. Experiments in laser-heated diamond anvil cell (LHDAC) in combination with numerical simulations of heat transfer in DAC are shown to yield information on thermal conductivity of a pressurized sample. The novel technique consists of one-sided laser heating and double-sided temperature measurements and utilizes a precise determination of several parameters in course of the experiment, including the sample geometry, laser beam power distribution, and optical properties of employed materials. The pressure-temperature conditions at the probed portion of the sample are, however, not uniform. To address this problem, thermal pressure in the laser-heated diamond anvil cell and anisotropic thermal conductivity originating from the texture development upon uniaxial compression have been studied by means of numerical simulations. The method for determination of thermal conductivity is applied to iron at pressures up to 70 GPa and temperatures of 2000 K, meeting the Earth’s lower mantle conditions and covering Mercury’s entire core. The obtained results are extrapolated to the conditions of the Earth’s core-mantle boundary using a theoretical model of the density dependence of thermal conductivity of metals and published values on Grüneisen parameter and bulk modulus. After considering the effect of minor core elements, the obtained value at these conditions supports case for the downward revision of the thermal conductivity in the core. From the point of view of core dynamics and energy budget, the lower thermal conductivity implies more favorable conditions to drive the dynamo. Similar scenario applies for Mercury where, for high values of thermal conductivity, heat flux conducted along the iron-core adiabat exceeds the actual heat flux through the core-mantle boundary. This leads to a negative rate of entropy production in the core that makes it impossible to sustain the dynamo process presumably responsible for the observed magnetic field of Mercury.

Transition metals (Ti, Zr, Hf, Mo, W, V, Nb, Ta, Pd, Pt, Cu, Ag, and Au) are essential building units of many materials and have important industrial applications. Therefore, it is important to understand their thermal and physical behavior when they are subjected to extreme conditions of pressure and temperature. This dissertation presents: An improved experimental technique to use lasers for the measurement of thermal conductivity of materials under conditions of very high pressure (P, up to 50 GPa) and temperature (T up to 2500 K). An experimental study of the phase relationship and physical properties of selected transition metals, which revealed new and unexpected physical effects of thermal conductivity in Zr, and Hf under high P-T. New phase diagrams created for Hf, Ti and Zr from experimental data. P-T dependence of the lattice parameters in α-hafnium. Contrary to prior reports, the α-ω phase transition in hafnium has a negative dT/dP slope. New data on thermodynamic and physical properties of several transition metals and their respective high P-T phase diagrams. First complete thermodynamic database for solid phases of 13 common transition metals was created. This database has: All the thermochemical data on these elements in their standard state (mostly available and compiled). All the equations of state (EoS) formulated from pressure-volume-temperature data (measured as a part of this study and from literature). Complete thermodynamic data for selected elements from standard to extreme conditions. The thermodynamic database provided by this study can be used with available thermodynamic software to calculate all thermophysical properties and phase diagrams at high P-T conditions. For readers who do not have access to this software, tabulated values of all thermodynamic and volume data for the 13 metals at high P-T are included in the APPENDIX. In the APPENDIX, a description of several other high-pressure studies of selected oxide systems is also included. Thermophysical properties (Cp, H, S, G) of the high P-T ω-phase of Ti, Zr and Hf were determined during the optimization of the EoS parameters and are presented in this study for the first time. These results should have important implications in understanding hexagonal-close-packed to simple-hexagonal phase transitions in transition metals and other materials.

Complex concentrated alloys (CCAs) offer the unique ability to tune composition and microstructure to achieve a wide range of mechanical performance. Recently, the development of metastable CCAs has led to the creation of transformation-induced plasticity (TRIP) CCAs. Similar to TRIP steels, TRIP CCAs are more effective at absorbing high strain rate loads when TRIP is activated during the loading process. The objective of our study is to investigate the effect of copper on the critical pressure for activating TRIP and the high pressure stability of a Fe(40-X)Mn20Cr15Co20Si5CuX TRIP CCA, where x varies from 0 to 3 at.% Cu. To achieve this goal, diamond anvil cell testing during in-situ synchrotron radiation X-ray diffraction was performed using both a monochromatic wide angle X-ray scattering (WAXS) beam and, for the first time ever, a polychromatic Laue diffraction beam on a CCA. Laue diffraction allows for real-time phase evolution tracking of the γ-fcc → ε-hcp transformation in a high pressure environment. Based on the results, a new method for processing and preparation of high pressure samples without changing the microstructure of sample was developed. This new method can be used to prepare any CCA samples for high pressure testing.

No description available.

Phase equilibria and volumetric properties in the system water-sodium chloride-carbon dioxide were determined experimentally for pressures between about 1 to 6 kbar, temperatures of 300° to 800°C, and fluid compositions up to 40 wt% NaCl and 20 mol% carbon dioxide, both relative to water. This was accomplished by using the synthetic fluid inclusion technique in conjunction with conventional microthermometry, a hydrothermal diamond-anvil cell and Raman spectroscopy. At constant salinity, the high-pressure portion of the solvus migrates to higher pressures and temperatures with increasing carbon dioxide concentration. Immiscibility is possible in this ternary system over almost the entire range of crustal P-T conditions at salinities equal to or in excess of 20 wt% NaCl and carbon dioxide concentrations between about 30 and 70 mol% carbon dioxide. The dP/dT slopes of lines of equal homogenization temperature decrease nonlinearly with increasing homogenization temperature; at constant homogenization temperature, these slopes become steeper (higher) along pseudobinaries with addition of carbon dioxide and particularly with addition of sodium chloride. Up to concentrations of 20 wt% NaCl and 20 mol% carbon dioxide, a sharp rise in the critical temperature was observed with increasing salinity at a fixed water/carbon dioxide ratio. The critical point shifts rapidly towards higher pressures with increasing carbon dioxide concentration. Addition of carbon dioxide to an aqueous 40 wt% NaCl solution results in a slight elevation of the halite dissolution temperature under vapor-saturated conditions. A significant error can be associated with the calculation of molar volumes from measured densities of the carbonic phase of water-sodium chloride-carbon dioxide inclusions. To avoid such errors, phase diagrams were constructed based on the obtained lines of equal homogenization temperature for salinities between 6 and 40 wt% NaCl and carbon dioxide concentrations between 5 and 20 mol% relative to water. These diagrams are of direct applicability to the interpretation of natural fluid inclusions from a wide variety of geologic environments. / Ph. D.