Impulsive excitation of mechanoluminescence in SrAl2O4:Eu, Dy phosphors prepared by solid state reaction technique in reduction atmosphere

Abstract

The present paper reports the impulsive excitation of mechanoluminescence (ML) in SrAl2O4:Eu, Dy phosphor prepared by solid state reaction technique in a reduced atmosphere of 95% Ar+5% H2. The phosphors prepared exhibit such as intense ML that it can be seen in day light with naked eye. When a sample consisting of compact mass of the phosphor is deformed impulsively by the impact of a moving piston, then initially the mechanoluminescence intensity increases linearly with time, attains a peak value Im at a particular time tm, and later on it decreases with time, initially at a fast rate and then at a slow rate. The peak ML intensity Im and total ML intensity IT increase quardratically with the applied pressure and impact velocity. The ML spectra of SrAl2O4:Eu, Dy are found to be similar to the photoluminescence spectra. This result indicates that the ML is emitted from the Eu2+ ions as photoluminescence does. The ML intensity decreases with successive impact of the load onto the phosphors, whereby the diminished ML intensity can be recovered approximately by UV-irradiation. retain-->In the impulsive excitation, the complete recovery of ML intensity does not take place because the probability of radiative transition of luminescence centres formed near the newly created surfaces may decrease due the formation of surface traps. The values of tm, fast decay time and slow decay time do not change significantly with the impact velocity vo. The piezoelectrically-induced detrapping model is able to explain the characteristics of ML. According to this model, the local piezoelectric field near the activators causes the detrapping of electrons trapped in the negative oxygen vacancy and subsequently the detrapped electrons moving in the conduction band are captured by the Eu3+ ion, whereby the excited Eu2+ ions are produced and the subsequent de-excitation of excited Eu2+ ions gives rise to the light emission. It seems that, in addition to the change in crystal structure, in UV-irradiated SrAl2O4:Eu, Dy crystals, some localized electric dipoles present in the vicinity of Eu2+ ions may also enhance the piezoelectric constants of the crystals. The day light visible mechanoluminescence in SrAl2O4:Eu, Dy phosphors may be used for designing the impact stress and impact velocity sensors. Furthermore, the present investigation may be useful for determining the rise time of unknown pressure pulses and for determining the lifetime of charge carriers in shallow traps. The intense ML pulses may also be used in some specific photocatalytic reaction. The present study may also be helpful in preparing the ML-based light source by increasing the trap-depth of shallow traps. If the ML emission in certain materials will be in the ultraviolet range, then the ML pulses of short duration may be used to excite the phosphorescence and determining the decay time.