Development of an optical thermometry system for phosphor materials

Abstract

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.

Afrikaans: Dié studie fokus op die ontwikkeling van ´n stelsel wat gebruik kan word vir die bestudering van die emissie van termografiese fosfors by verskillende temperature. Die Fotoluminesensie (FL) sisteem in die Fisika Departement by die Universiteit van die Vrystaat is volledig bestudeer en aangepas vir temperatuur afhanklike metings. Die aanpassings sluit in, ´n doelgeboude verhittingsisteem, wat gebruik is om die fosformateriaal se temperatuur te meet en te beheer, ´n lig-verdeler tesame met ´n intensiteitsmeter wat gebruik word as ´n verwysingsdetektor om die opwekkingsbron te monitor en ´n monsterhouer tesame met ´n XYZ staander wat posisie-stabiliteit en posisie-beheer van die monsters verseker gedurende metings. ´n Sagtewareprogram is ontwikkel om gebruikersvriendelike beheer en outomatisering van die gewysigde stelsel moontlik te maak. Die golflengte, opwekkingsintensiteit en temperatuur is gekalibreer. Die aangepaste stelsel is gebruik om die emissie te meet by verskillende temperature van die kommersiële fosfor, lantaanoksisulfied wat gedoteer is met europium(III) (La2O2S:Eu(III)). Vir die termiese kwyning proses is die gemiddelde aktiveringsenergie van die 5D2, 5D1 en 5D0 opgewekte toestande bepaal as 0,49 eV, 0,55 eV en 0,77 eV onderskeidelik, terwyl die gemiddelde voor-eksponensiële-konstante bepaal is as 9.0×107 s-1. Dit is ook bepaal dat La2O2S:Eu(III) as ´n temperatuursensor gebruik kan word, deur gebruik te maak van die fosforiese-intensiteitsverhouding van die emissie vanaf die 5D1 en 5D0 opgewekte toestande. Die werk goed vir die temperatuur gebied vanaf 80 °C tot 180 °C. Die optiese bandgaping van La2O2S:Eu(III) is bepaal as 2,75 eV. Dit is vasgestel dat die swawel(II) na europium(III) (Eu(III)) ladingsoordragband ultravioletstraling absorbeer en dra die opgewekte elektrone oor na die opgewekte toestande van die Eu(III)-ione van waar emissie kan plaasvind. Die leeftyd van elke oorgang is ook bestudeer en dit is waargeneem dat die hoër opgewekte toestande dubbele eksponensiële leeftye het. Die rede hiervoor is emissie wat afkomstig is van die konvensionele Eu(III)-ione, asook die Eu(III)-ione wat in die omgewing van onsuiwerhede is. Dit is bevind dat in die geval van La2O2S:Eu(III) fosformateriaal, die teenwoordigheid van defekte of onsuiwerhede afkomstig is vanaf hidroksiedgroepe wat vorm wanneer die materiaal by kamertemperatuur blootgestel word aan waterdamp in die atmosfeer. Die gemiddelde emissieleeftydkonstantes van die 5D2, 5D1 en 5D0 opgewekte toestande is bepaal as 0,01 ms, 0,08 ms en 0,34 ms respektiewelik. Die aangepaste FL stelsel is ook ontwerp om die emissie-stabiliteit van termografiese fosfors by verskillende temperature te bestudeer. Daar is opgemerk dat die algehele fosforieseintensiteit van die La2O2S:Eu(III) toeneem met uitgloeityd by ´n temperatuur van 400 °C. Die x-straaldiffraksie resultate dui op die afname van die kristalroosterspanning as ´n funksie van uitgloeityd wat te wyte is aan die verwydering van defekte of onsuiwerhede in die kristalrooster. Die vermindering van hidroksieddefekte as ´n funksie van uitgloeityd is waargeneem met behulp van beide x-straalfoto-elektron-spektroskopie en ook die meting van leeftyd van luminessensie. Die toename in fosforiese-intensiteit as ´n funksie van uitgloeityd kan daarom toegeskryf word aan die vermindering van die hidroksieddefekte. Dit is egter vasgestel dat dié onstabiliteits-effekte nie ´n invloed op die fosforiese-intensiteitsverhouding van die fosformateriaal het nie en die materiaal kan dus as suksesvolle optiesetemperatuursensor benut word.