Gas-loading apparatus for large-volume high-pressure cell
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The Paris-Edinburgh cell (PEC) is a widely used opposed-anvil device for neutron scattering. Since its development, it has been used to study a number of samples loaded as solids or liquids. However, studying gases at room temperature has not yet been possible. Up until now only a few gases could be loaded as liquids, in cryogenic conditions. Thus, it was impossible to study many gases and gas mixtures and also it was difficult to use gases as pressure-transmitting media (PTM). In order to overcome these limitations, a technique that would enable loading of gases into the PEC was required. The work described in this thesis was focused on the design and use of a gas-loading system for the PEC. The challenge of designing such a system comes from the fact that the gases need to be loaded into the gasket at sufficient density in order to achieve any significant pressure during further compression in the cell. This can be achieved by using a separate pressure vessel. Because the whole PEC is too large to be placed inside the vessel, a technique of loading gas into the anvils separated from the rest of the cell had to be devised. Designing the holder for the anvils, which would make this possible, presented a major challenge as it should allow the anvils to be transferred between the vessel and the PEC, with the gasket filled with high-pressure gas. Then it needs to allow further compression of the gasket inside the PEC. The developed system consists of a pressure vessel and a locking clamp for the anvils. The pressure vessel is a closed-end thick-walled cylinder with a top cover which has an opening for a piston. The vessel is placed on the table of a hydraulic press and the piston, sealed by a high-pressure reciprocating seal, is used to transmit the force from hydraulic ram onto the anvils which are held by the clamp and placed inside the vessel. One of the anvils is fixed to the clamp and the other one is supported by spring-loaded latches - the latches engage when the anvils are pushed towards each other. Thus, when the force is applied onto the anvils to compress the gasket, latches lock the anvils in their positions stopping them from retracting and maintaining the gasket compressed after the force is released. The clamp allows the gasket to be filled with the gas and then deformed to seal the compressed gas. The locking mechanism keeps the gasket compressed, which enables the clamp to be transferred from the vessel to the PEC. After the system was built and tested, it was installed at ISIS neutron source (Oxfordshire, UK), where it has been used in several experiments. The first experiment prepared with the gas-loading system was a neutron diffraction study of nitrogen at high pressure. Nitrogen was chosen as a sample material because its high-pressure structural phase diagram is well established. Nitrogen was loaded into the gasket using the gas loader and then it was compressed in increments to 6 GPa in the PEC. β and δ phases of solid nitrogen were clearly seen in the collected neutron diffraction data. The experiment proved the usability of the gas-loading system and verified its expected performance. The second experiment utilizing the gas-loading system was to study singlecrystal and powder samples of sodium chloride (NaCl) and squaric acid (H2C4O4). For these studies argon was used as a PTM, replacing the conventionally used methanol-ethanol mixture (ME). Up until this experiment the highest pressure reported in single-crystal neutron-scattering experiments was 12 GPa. This limit was set by the solidi cation pressure of ME. With argon as the PTM, the samples were compressed to 15 GPa without any damage to the crystals. Another advantage of replacing ME with argon is improved hydrostaticity. The highest pressure that ME remain hydrostatic to is 11 GPa. Compressing beyond this point causes sheer stress acting on the sample which affects the quality of the neutron scattering data manifested in the appearance of peak-broadening in the diffraction patterns. With use of argon, the powder samples have been compressed to 18 GPa while maintaining quasi-hydrostatic pressure conditions, resulting in clean and sharp diffraction patterns without any noticeable peak-broadening.