No description available.

The application of pressure to chemical substances can change their physical properties (optical, magnetic, and electrical) and it can also be used to alter some chemical reactions. The need for compatible pressure generating instruments is constantly growing in various high pressure (HP) researches. The work described in this thesis is focused on development, construction, testing of several high pressure cells of novel design. These designs were developed to meet the requirements of different research collaborations. The main objective of this project is to develop high pressure cells for magnetic studies in the magnetic properties measurement system known as MPMS, which is the most popular commercial magnetometer nowadays. Three high pressure cells were designed and tested for different type of magnetic measurements. The first design presented in this thesis is a cylinder type pressure cell which is specially designed to measure the magnetic susceptibility of the pressure-sensitive material under pressure. The cell is driven by compressed helium gas which allows the internal pressure to be adjusted with small increments (1 MPa) through the regulator of the external gas cylinder. The cell was made of non-magnetic beryllium copper alloy and designed to work up to 100 MPa at 400 K temperature. The design was verified with finite element analysis (FEA) simulation and its sample volume was optimised to provide large sample capacity which allows high quality data to be collected in the MPMS. Modified from the earlier turnbuckle magnetic diamond anvil cell (TM-DAC) reported in Konstantin V. Kamenev (KVK) group, the second high pressure cell presented in this thesis is an opposed diamond anvil pressure cell. The working mechanism of this cell is based on the turnbuckle principle. The cell was specifically developed for iHelium3 system which is a add-on cryostat of the MPMS. The cell was coded TM-3He-DAC to distinguish with the original TMDAC. The cell is 6 mm in diameter and 7 mm in length, which are smaller than the dimensions of the predecessor (TM-DAC). Copper titanium alloy was used in building the cell to further reduce the magnetic background from the cell. The cell is capable of achieving close to 5 GPa sample pressure in the loading test and the magnetic background is significantly lower than the TM-DAC. The development of this cell enables high pressure magnetic measurements to be performed at extreme low temperature (0.5 K) in the iHelium3 system. The third high pressure cell developed for the MPMS is also a turnbuckle diamond anvil cell, however, all the material used in the cell is non-metallic to enable high-pressure ac magnetic measurement to be performed. An advanced high strength polymer was assessed using finite element analysis and experimental testing. The performance and failure modes for the key components of the cell working in tension and in compression were evaluated and the ways for optimising the designs were established. The cell is coded PTM-DAC in this thesis and the composite gasket was also developed and tested for the PTM-DAC. The cell is approximately 14 mm long, 8.5 mm in diameter and was demonstrated to reach pressures of 5.6 GPa. Ac susceptibility data collected on Dy2O3 and U6Fe demonstrated the performance of the cell in magnetic property measurement and confirmed that there was no screening of the sample by the environment which typically accompanies used of conventional metallic high pressure cells in oscillating magnetic fields. Based on the experience of from the development of above two turnbuckle diamond anvil cell, a turnbuckle sapphire anvil cell (T-SAC) was developed in this project for high-pressure neutron scattering. Commercial spherical sapphire were used as anvil in the cell as they are much more cost effective if compared to the diamond anvil. The developed T-SAC can generate and maintain sample pressures above 6 GPa with a sample volume 6 X 10-² mm³ which is 6 times that of conventional diamond anvil cell (DAC). Failure analysis was performed on the sapphire anvil to gain a better understanding of the failure mechanism of the spherical sapphire anvil. The cell had been used in measuring the crystal structure of single crystal niobium at 1.6 GPa through small angle neutron scattering (SANS) technique. The cell is less than 16 mm in length and 14 mm in diameter, it is the smallest sapphire anvil cell to date. The miniature feature allow it can be fit into most cryostat of modern scientific instrument without difficulties. Lastly, two piston-cylinder type high pressure cells were developed for high-pressure chemistry studies. These cells were designed to pressurise large amount of liquid sample (particular for water-based sample) up to 800 MPa in a controllable manner. Each design is presented separately with stress analysis in FEA and a description of the working mechanism. Hoop strain at the external surface of the cell was measured and then the internal pressure was calculated through the Lam´e equation. After that, the load and attainable internal pressure was calibrated for the users. These cells have been used in the high-pressure study of salicylaldoximes process, bio-diesels decomposition and crystallization, material polymerisation and pharmaceutical experiments.

Binary element-hydride systems have become a pertinent topic for high pressure research, following the measurement of record high temperature superconductivity in the dense hydrogen-sulfur system. The experimental study followed predictions of superconductivity with high transition temperature (Tc) in (H2S)2H2 at high pressures, leading to the current consensus that the high Tc phase is H3S, produced from the decomposition and recombination of H2S at high pressures. However, conjecture over the behaviour of hydrogen sulfide upon compression, and experimental limitations, cast significant ambiguity over interpretations of the structure and mechanism of the superconducting phase. Nonetheless, theory also predicts high Tc superconductivity in the dense hydrogen selenide and telenide systems; both experimentally uncharted at high pressures prior to this study. This thesis explores and maps the phase diagrams of hydrogen-chalcogen (S, Se, Te) systems using a combination of high pressure Raman spectroscopy and x-ray diffraction techniques. Gaining a comprehensive understanding of the behaviour of these systems under pressure is crucial to the eventual elucidation of the true nature of high Tc superconductivity. Hydrogen sulfide (H2S) and hydrogen selenide (H2Se) are appreciably toxic. A simple in situ synthesis technique is reported for producing hydrogen-chalcogenides directly from their constituent elements within diamond anvil cells, circumventing the need to condense toxic gases. This technique is also utilised to provide excess hydrogen, in order to produce the hydrogen-rich cocrystals thought to be vital to the formation of the high Tc phase. The hydrogen-sulfur system is most thoroughly investigated, and first presented. High quality Raman spectroscopic data provides an experimental review of pure H2S. Studies of (H2S)2H2 evaluate the current known ambient temperature phases and reveal three novel low temperature phases. Phase II0 is identified on cooling of phase I to 173 K (10 GPa), via splitting of both the single S-H stretching mode and low-frequency H2 vibron; sharp stretching modes indicate a significant reduction in orientational disorder. Successive splitting of the low-frequency H2 vibrons indicates two additional phase changes at 29 GPa (phase III0) and 53 GPa (phase- IV0) respectively, at 80 K. Phase IV0 is associated with an overall increase in symmetry. Evidence is also presented for a tentative fourth novel low temperature phase at ~160 GPa (20 K) and for the formation of an exceptionally stable hydrogen-sulfur compound with potentially novel stoichiometry. The behaviour of the H2S and (H2S)2H2 mixed molecular system is also reported; demonstrating that the coexistence of (H2S)2H2 and H2S can influence the hydrogen-bonding within both systems at high pressures. The first high pressure studies of the hydrogen-selenium system at ambient temperature are reported. The high pressure phase sequence of H2Se (I { I0 - IV) is identified by Raman spectroscopy, mirroring that of H2S. The isothermal boundaries for phases I0 and IV are found at 7 and 12 GPa respectively, at 300 K. Phase IV may have higher symmetry than phase IV H2S. X-ray diffraction and Raman spectroscopy demonstrate that the H2Se:H2 mixtures form cocrystals of (H2Se)2H2 from 4.2 GPa, with tetragonal space group I4=mcm, analogous to (H2S)2H2. Both H2Se and (H2Se)2H2 are shown to decompose into their constituent elements above 24 GPa. Attempts to synthesise the elusive H2Te directly from hydrogen and tellurium are reported. No reaction occurs upon heating Te in H2 at 0.2 GPa to 573 K. No visible reaction occurs between H2 and the high-pressure phases of Te, upon laser-heating. No photoreaction occurs upon exposure of tellurium in hydrogen to intense laser light (532 nm) at 0.2 GPa and 300 K, but formation may be stabilised at lower temperatures.

Thesis (Family Medicine)-- University of Limpopo, 2010. / Summary not available

Thesis (M.D.S.) -- University of Adelaide, Department of Oral Biology, 1973.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1969. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1969. / Vita. Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Bismuth is among the most studied of all elements, but its behaviour under pressure exhibits myriad unexpected puzzles even after many decades of research. Bismuth narrowly avoids being an insulator: a Peierls-type distortion almost completely gaps the electronic energy bands, producing a rhombohedral metal with a tiny overlap of conduction and valence bands. The resulting solitary free electron per 100,000 atoms can travel large distances in high-purity crystals, leading to a host of unusual properties. We show that the rhombohedral structure can be tuned with pressure, driving the carrier concentration to nearly zero. We compare our measurements to recent experimental advances implying the formation of novel electronic order driven by the pairing of low-density electrons and holes, and show evidence for a previously unseen phase at very low temperatures in the semiconducting state. We also present a method for calculating the carrier density and resistivity as a function of pressure, based on phenomenological band parameters and a simple charge-balance argument, and demonstrate that this approach can quite well describe most - but not all - of the observed behaviour of the resistivity. At higher pressures, bismuth undergoes a transition into a quasiperiodic host-guest structure. Here, two distinct crystal lattices coexist and interpenetrate, but the lattice parameters are incommensurate. This crystal thus lacks a single unit cell - an unexpected complexity for a simple element. The discovery of such unusual structures in elements is a new phenomenon and their physical properties are rather unexplored. We present experimental measurements of the resistivity and magnetic susceptibility in the incommensurate host-guest state. We argue that the experimental data (in particular, the shape of the normal-state electrical resistivity, and the high value of the low-temperature upper critical field) may be evidence for strong electron-phonon coupling. This strong coupling is consistent with theoretical predictions which suggest the presence of a low-energy phonon mode arising due to the vanishing energy cost of moving guest atoms through the host lattice.

No description available.

Hydrogen, being the simplest and most abundant element in the Universe, is of fundamental importance to condensed matter sciences. Through advances in high pressure experimental technique, hydrogen (and its isotope deuterium) has been contained and studied using in situ optical spectroscopy to 315 (275 GPa) at 300 K, pressure and temperature conditions previously thought to be inaccessible. At 200 GPa, hydrogen undergoes a phase transformation, attributed to phase III, previously observed only at low temperatures. This is succeeded at 220 GPa by a reversible transformation to a new phase, IV, characterized by the simultaneous appearance of the second vibrational fundamental mode, new low-frequency phonon excitations, and a dramatic softening and broadening of the first vibrational fundamental mode. To impose constraints on the P-T phase diagram, the temperature stability of phase IV is investigated through a series of low temperature experiments, where the phase IV-III transformation is observed. Analysis of the Raman spectra suggests that phase IV is a mixture of graphene-like layers, consisting of elongated H2 dimers experiencing large pairing fluctuations, and unbound H2 molecules. Isotopic comparisons reveal spectral differences between the phase IV-III transition of hydrogen and deuterium, which strongly indicates the presence of proton tunnelling in phase IV. Optical transmission spectra of phase IV reveals an overall increase of absorption and a closing band gap reaching 1.8 eV at 315 GPa. No differences between the isotopes were observed in absorption studies, resulting in identical values for the band gap. Extrapolation of the band gap yields 375 GPa as the minimum transition pressure to a metallic state of hydrogen (deuterium).