Abstract
This thesis describes the development and characterisation of a new exfoliated
graphitic substrate for low temperature studies on thin helium films. The most
commonly used graphite substrates for helium film experiments have been the commercially available Papyex, Grafoil and ZYX. These can be characterised by their specific surface areas and surface coherence lengths, the typical step-free size of their basal-plane adsorbing facets. The surface coherence length for Grafoil is 10-20 nm, whilst for ZYX it is of the order of 200 nm. The ultimate goal of this work was to produce a substrate with larger coherence lengths than previous substrates, to enable the observation and study of novel new phases in helium films which exhibit long range order.
The new substrate uses natural Madagascan flake graphite as the starting material. This flake is then intercalated with Potassium and Ammonia and then exfoliated at relatively low temperatures via thermal decomposition. The resulting exfoliated material is then compressed into foils. A comparative study of this new material has been performed, characterising them against existing substrates using a wide range of techniques. The bulk properties have been studied using: X-ray diffraction to compare the mosaic spread and the in-plane crystallite sizes; Glow discharge mass spectrometry to measure the impurity levels; Surface characterisation has been by: Vapour pressure isotherms to measure the specific surface area and surface quality; Various microscopy techniques to image surfaces.
The final stage of this work was to perform a preliminary experiment on a prototype substrate to demonstrate its viability for low temperature SQUID NMR
measurements. This involved measuring the NMR properties of 3He as the coverage was swept through monolayer completion to observe the characteristic T∗2 minimum.
This work took place in the context of efforts to reduce the time between the NMR tipping pulse and measurement, which is especially important around the T ∗2 minimum.
graphitic substrate for low temperature studies on thin helium films. The most
commonly used graphite substrates for helium film experiments have been the commercially available Papyex, Grafoil and ZYX. These can be characterised by their specific surface areas and surface coherence lengths, the typical step-free size of their basal-plane adsorbing facets. The surface coherence length for Grafoil is 10-20 nm, whilst for ZYX it is of the order of 200 nm. The ultimate goal of this work was to produce a substrate with larger coherence lengths than previous substrates, to enable the observation and study of novel new phases in helium films which exhibit long range order.
The new substrate uses natural Madagascan flake graphite as the starting material. This flake is then intercalated with Potassium and Ammonia and then exfoliated at relatively low temperatures via thermal decomposition. The resulting exfoliated material is then compressed into foils. A comparative study of this new material has been performed, characterising them against existing substrates using a wide range of techniques. The bulk properties have been studied using: X-ray diffraction to compare the mosaic spread and the in-plane crystallite sizes; Glow discharge mass spectrometry to measure the impurity levels; Surface characterisation has been by: Vapour pressure isotherms to measure the specific surface area and surface quality; Various microscopy techniques to image surfaces.
The final stage of this work was to perform a preliminary experiment on a prototype substrate to demonstrate its viability for low temperature SQUID NMR
measurements. This involved measuring the NMR properties of 3He as the coverage was swept through monolayer completion to observe the characteristic T∗2 minimum.
This work took place in the context of efforts to reduce the time between the NMR tipping pulse and measurement, which is especially important around the T ∗2 minimum.
| Original language | English |
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| Qualification | Ph.D. |
| Awarding Institution |
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| Supervisors/Advisors |
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| Award date | 1 Mar 2015 |
| Publication status | Unpublished - 2014 |