Fundamental studies on non radiative processes in ceramic selective emitter materials for TPV applications

In order to engineer a rare-earth based selective emitter, having both good optical and mechanical properties, a detailed theoretical study of both the excitation and the decay processes is required at the microscopic level. In this work we present a theoretical model by which the high temperature quantum efficiencies can be evaluated on the basis of the correlation between the local symmetry of the potential around the rare earth atoms and the probability of their non-radiative decay. The model, based on a point charge approach, provides the electric field and the crystal field parameters (CFPs) and consequently, the non radiative decay rates for a few representative configurations. The main result is that different local symmetries around, for instance, Er3+ atoms may provide dramatically different non radiative decay probabilities. An experimental confirmation of the theoretical model was obtained by the analysis of a set of ceramic samples containing the same Erbium fraction and treated at different sintering temperature (in 780K-1400K range) in order to obtain different crystal phases. The radiative transition involving the orbital I-4(11/2) -> I-4(15/2) which is expected to be quite sensitive to the different ions configurations, has been studied by means of temperature dependent photoluminescence experiments (in the range of 20K-300K) performed on samples sintered at different temperatures. The results show that the decay rate of I-4(11/2) -> I-4(15/2) transition (not observable at room temperature) increases with increasing sintering temperature manly. due to the development of a garnet configuration which strongly inhibits non radiative decay processes.