Phosphors for biomedical and environmental sensing technology

Yap, S.V., 2009. Phosphors for biomedical and environmental sensing technology. PhD, Nottingham Trent University.

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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 which 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 mo1% for La2O2S: Eu and 0.005-50 for mo1% La2OO2S: Tb, have been characterized in terms of photo luminescent 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 mo1% for La202S: Eu and La2O2S: Tb respectively, results have shown that optimum temperature dependency is at a lower mo1% of 0.1 for La2O2S: Eu with a high quenching rate of 24.03 m°C-1 Therefore, 0.1 mo1% La2O2S S: 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 mo1%. 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 behavior is observed for a and c parameters as well as the volume of the unit cell over the temperature range of interest.

Item Type: Thesis
Creators: Yap, S.V.
Date: 2009
ISBN: 9781369314366
Rights: This work is the intellectual property of the author. You may copy up to 5% of this work for private study, or personal, non-commercial research. Any re-use of the information contained within this document should be fully referenced, quoting the author, title, university, degree level and pagination. Queries or requests for any other use, or if a more substantial copy is required, should be directed in the owner(s) of the Intellectual Property Rights.
Divisions: Schools > School of Science and Technology
Record created by: Linda Sullivan
Date Added: 18 Sep 2020 11:07
Last Modified: 26 Jul 2023 12:56

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