Development of an optical thermometry system for phosphor materials

English: This study is focussed on the development of a system that was used to investigate the emission of thermographic phosphors at various temperatures. The photoluminescence (PL) system at the Department of Physics at the University of the Free State was studied in detail and modified for temperature measurements. Modifications include a purpose-built heating unit, used to measure and control the phosphor material´s temperature, a beam splitter together with a power meter to be used as a reference detector for the excitation source and a sample holder together with an XYZ stage that ensures position-stability and position-control for the samples throughout the measurements. A software program was developed to allow user-friendly control and automation of the modified system. The wavelength, excitation energy and temperature were calibrated. The modified system was used to measure the emission of commercially available lanthanum oxysulphide doped with europium(III) (La2O2S:Eu(III)) phosphor material at different temperatures. For the thermal quenching process, the average activation energies for the emission from the 5D2, 5D1 and 5D0 excited states were determined as 0.49 eV, 0.55 eV and 0.77 eV respectively and the average pre-exponential constant was determined as 9.5×107 s-1. It was also shown that La2O2S:Eu(III) can be utilised as a temperature sensor by using the fluorescence intensity ratio of the emission from the 5D1 and 5D0 excited states. This worked well for the temperature range from 80 °C to 180 °C. The optical band gap of La2O2S:Eu(III) was determined as 2.75 eV. It was also established that the sulphur(II) to europium(III) (Eu(III)) charge transfer band absorbs ultraviolet radiation and transfers the excited electrons to the excited states of the Eu(III) ions from where emission can take place. Lifetime of luminescence results show that the higher excited states have a double exponential lifetime that results from the emission from both the conventional Eu(III) ions and Eu(III) ions that are in the vicinity of a defect or impurity group. It was determined that in the case of the La2O2S:Eu(III) phosphor material, the presence of defect or impurity groups is due to the hydroxide groups that forms when the material is exposed to water vapour in the atmosphere at room temperature. The average emission decay constants of the 5D2, 5D1 and 5D0 excited states were determined as 0.01 ms, 0.08 ms and 0.34 ms respectively. The modified PL system was also designed to study the stability of the emission of thermographic phosphors at various temperatures. It was observed that the overall luminescence intensity of the La2O2S:Eu(III) increased with annealing time at a constant temperature of 400 °C. The x-ray diffraction results indicate a decrease of the strain of the lattice as a function of period of annealing which is due to the removal of defects or impurities in the crystal lattice. The reduction of hydroxide impurities as a function of annealing time was observed using both x-ray photoelectron spectroscopy and measurement of the lifetime of luminescence. The increase in luminescence intensity as a function of annealing time can therefore be attributed to the reduction of the hydroxide impurities, however it was shown that these instability effects did not have an influence on the relative luminescence intensity from the different excited levels of the phosphor material and therefore La2O2S:Eu(III) can be used as a stable optical temperature sensor.