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Electromagnetic properties of semiconducting metal oxides under external stimulation

Partridge, Samuel 2019. Electromagnetic properties of semiconducting metal oxides under external stimulation. PhD Thesis, Cardiff University.
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The main aim of this thesis is to investigate the realtime and long term electrical properties of semiconducting pigments when exposed to light an other environmental stimuli. A novel rectangular microwave resonant cavity is introduced for measuring the realtime dielectric changes of thin foil samples containing lacqueur and pigment while being exposed to a mostly uniform ultraviolet light source. This in an improvement to the invasive resistive measurements usually used to characterise resistive thin films. It also gives an insight into the localized conductivity of the material compared to the bulk conductivity of a traditional measurement. A novel temperature correction technique is described which splits degenerate cylindrical cavity modes into a measurement and reference mode. This was tested using a water sample, and was found to be very effective. A simplified analytical method of interpreting the resonant frequencies for temperature correction was also described, and experiments performed to check it’s accuracy. The properties of two sets of photosensitive materials were investigated. Titanium Dioxide (TiO2), which is known for being a brilliant white pigment and effective photocatalyst, was investigated as a powder under ultraviolet, green and red light. It was also investigated during thermal and humidity exposure. The results showed that Anatase TiO2 was more photoactive than Rutile under UV light. A photocatalytic grade of TiO2 with a smaller particle size and Carbon doping didn’t show a large change in dielectric loss, however its initial loss was much larger than the other samples suggesting it was already excited from ambient light. A pigment grade of TiO2 with a surface treatment of aluminium, silicon and polysiloxane compunds was also analysed which almost no response to UV light. None of the samples showed significant changes under green and red light. A range of pigments called Iriotec ® 7000, which are based around Antimony doped Tin Oxide coated onto mica/quartz/talcum substrates, were investigated using the rectangular resonant cavity due to their unusual darkening when exposed to ultraviolet light. The microwave measurements also showed a semi-permanent increase in dielectric loss suggesting an increase in trapped charge states. The larger particle sizes were in general more conductive than the smaller ones, and the addition of quartz/talcum also increased the conductivity slightly. Finally the in house built software used to run these experiments was described, including its low level architecture and available features such as a retracking algorithm to keep the frequency sweep close to the resonant frequency of the mode being measured.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: Semi conducting metal oxides; Microwave measurements; Light exposure; Degenerate mode; Titanium dioxide; Antimony tin oxide.
Date of First Compliant Deposit: 5 December 2019
Last Modified: 17 Oct 2020 01:27

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