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Title: Phosphors for biomedical and environmental sensing technology
Author: Yap, Sook Voon
ISNI:       0000 0004 2683 6391
Awarding Body: Nottingham Trent University
Current Institution: Nottingham Trent University
Date of Award: 2009
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Temperature measurement in an electromagnetically hostile environment, such as a magnetic resonance imaging (MRI) scanner, is challenging. The monitoring of a patient's temperature within an MRI is necessary to ensure the patient is in a comfortable condition due to the following problem. Unfortunately, the radio frequency (RF) radiation in the imaging cavity of an MRI will induce heat into both the body tissue of a patient as well as into metallic elements of any sensors. This thesis presents a feasibility study on the production of an optical phosphor-based sensor for temperature monitoring within such a hostile environment. The concept of a desired optical sensor is based on the thermal dependence of phosphor luminescence. A pulsed ultraviolet light source is used to excite phosphors coated on a thin membrane wliich is in contact with the patient's skin. The skin temperature (over 27-37 °C) can then be determined by calculating the exponential decay of luminescence intensity with time after the excitation pulse has ceased. In this research, two thermographic phosphors have been investigated, namely europium doped lanthanum oxysulphide (La₂O₂S: Eu) and terbium doped lanthanum oxysulphide (La₂O₂S: Tb). A wide range of dopant concentrations, 0.005-15 mol% for La₂O₂S: Eu and 0.005-50 for mol% La₂O₂S: Tb, have been characterised in terms of photoluminescent emission, decay time and crystallinity in determining their temperature dependent characteristics. Both phosphors have shown a dependency to dopant concentration through variance of peak intensity and decay time measurements over a low temperature range of 5 to 60 °C. Although maximum brightness of the temperature dependent lines is achieved at dopant concentrations of 1 and 10 mol% for La₂O₂S: Eu and La₂O₂S: Tb respectively, results have shown that optimum temperature dependency is at a lower mol % of 0.1 for La₂O₂S: Eu with a high quenching rate of 24.03 m °C⁻¹. Therefore, 0.1 mol% La₂O₂S: Eu appears to be an ideal candidate for use within an optical sensor. A key aspect of this research is that in comparison to conventional phosphor temperature dependent characteristic, it has been shown for the first time that La₂O₂S: Tb has an increase in decay time with respect to temperature for concentrations above 2 mol%. A hypothesis of this discovery based on the differences in energy transfer pathways is described. X-ray diffraction (XRD) analysis has demonstrated a linear relationship between the c-axis lattice parameter of a unit cell of the phosphors with dopant concentration. As dopancy increases, the (100) and (002) reflections merge and there is a reduction in the c-axis parameter as well as the crystallite size. In addition, a positive thermal expansion behaviour is observed for a and c parameters as well as the volume of the unit cell over the temperature range of interest.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available