Development of an optical thermometry system for phosphor materials

[摘要] English: This study is focussed on the development of a system that was used to investigate the emissionof thermographic phosphors at various temperatures. The photoluminescence (PL) system atthe Department of Physics at the University of the Free State was studied in detail and modifiedfor temperature measurements. Modifications include a purpose-built heating unit, used tomeasure and control the phosphor material´s temperature, a beam splitter together with a powermeter to be used as a reference detector for the excitation source and a sample holder togetherwith an XYZ stage that ensures position-stability and position-control for the samplesthroughout the measurements. A software program was developed to allow user-friendlycontrol and automation of the modified system. The wavelength, excitation energy andtemperature were calibrated.The modified system was used to measure the emission of commercially available lanthanumoxysulphide doped with europium(III) (La2O2S:Eu(III)) phosphor material at differenttemperatures. For the thermal quenching process, the average activation energies for theemission from the 5D2, 5D1 and 5D0 excited states were determined as 0.49 eV, 0.55 eV and0.77 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 thefluorescence intensity ratio of the emission from the 5D1 and 5D0 excited states. This workedwell 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 thatthe sulphur(II) to europium(III) (Eu(III)) charge transfer band absorbs ultraviolet radiation andtransfers the excited electrons to the excited states of the Eu(III) ions from where emission cantake place. Lifetime of luminescence results show that the higher excited states have a doubleexponential lifetime that results from the emission from both the conventional Eu(III) ions andEu(III) ions that are in the vicinity of a defect or impurity group. It was determined that in thecase of the La2O2S:Eu(III) phosphor material, the presence of defect or impurity groups is dueto the hydroxide groups that forms when the material is exposed to water vapour in theatmosphere at room temperature. The average emission decay constants of the 5D2, 5D1 and 5D0excited 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 ofthermographic phosphors at various temperatures. It was observed that the overallluminescence intensity of the La2O2S:Eu(III) increased with annealing time at a constanttemperature of 400 °C. The x-ray diffraction results indicate a decrease of the strain of thelattice as a function of period of annealing which is due to the removal of defects or impuritiesin the crystal lattice. The reduction of hydroxide impurities as a function of annealing time wasobserved using both x-ray photoelectron spectroscopy and measurement of the lifetime ofluminescence. The increase in luminescence intensity as a function of annealing time cantherefore be attributed to the reduction of the hydroxide impurities, however it was shown thatthese instability effects did not have an influence on the relative luminescence intensity fromthe different excited levels of the phosphor material and therefore La2O2S:Eu(III) can be usedas a stable optical temperature sensor.

[发布机构] University of the Free State