Elastico-Mechanoluminescence of Thermoluminescent Crystals

Elastico-mechanoluminescence (EML) is a type of luminescence induced by elastic deformation of solids. The present paper reports the elastic-ML of thermoluminescent crystals such as X-or γ-irradiated alkali halide crystals, ZnS:Mn, and ultraviolet irradiated persistent luminescent crystals. Generally, all the elastico-mechanoluminescent crystals are thermoluminescent, but all the thermoluminescent crystals are not the mechanoluminescent. The elastico-mechanoluminescence spectra of crystals are similar to their thermoluminescence spectra. Both the elastico-mechanoluminescence and thermoluminescence arise due to the de-trapping of charge carriers. As elastico-ML of persistent luminescent crystals depends on both the density of filled traps and piezoelectric field, the intense thermoluminescent crystals may not be the intense mechanoluminescent crystals. When a sample of X-or γ-irradiated alkali halide crystal, UV-irradiated persistent luminescent microcrystals mixed in epoxy resin, or a film of ZnS:Mn nanoparticles is deformed in the elastic region by the pressure rising at fixed pressing rate for a particular time, or by a pressure of triangular form, or by a pressure pulse, then after a threshold pressure, initially the EML intensity increases with time, attains a maximum value and later on it decreases with time. In the first case, the fast decay time of EML is related to the time-constant for stopping the moving crosshead of the testing machine; in the second case, generally the fast decay does not appear; and in the third case, the fast decay time is equal to the rise time of the pressure pulse. However, in all the cases, the slow decay time is related to the lifetime of re-trapped charge carriers in the shallow traps lying in the region where the piezoelectric field is negligible. When the sample is deformed by the pressure rising at fixed pressing rate for a particular time, or pressure of triangular form, then the ML appears after a threshold pressure and the transient EML intensity increases linearly with the applied pressure; however, the total EML intensity increases quadratically with the applied pressure. The EML intensity of persistent luminescent crystals decreases with increasing number of pressings. However, when these crystals are exposed to UV light, then the recovery of EML intensity takes place. The mechanical interaction between the bending segment of dislocations and filled electron traps is able to explain the elastico-ML of X-or γ-irradiated alkali halide crystals. However, the piezoelectrically-induced de-trapping model is suitable for explaining the ML of persistent luminescent crystals and ZnS:Mn. The investigation of elastico-ML may be helpful in understanding the thermoluminescence and the investigation of thermoluminescence may be helpful in understanding elastico-ML. Furthermore, similar to the thermoluminescence, the mechanoluminescence may also find application in radiation dosimetry. Expressions are derived for the elastico-ML of thermoluminescent crystals, in which a good agreement is found between the experimental and theoretical results. Finally, the application of the elasticoML of thermoluminescent crystals in light sources, displays, imaging devices, sensing devices, radiation dosimetry and in non-destructive testing of materials are discussed.Contents of Paper