Models for intrinsic and extrinsic elastico and plastico-mechanoluminescence of solids

Abstract

A large number of solids exhibit extrinsic elastico–mechanoluminescence (ML) and extrinsic plastico ML, only a few solids exhibit intrinsic elastico ML and intrinsic plastico ML. Whereas the elastico ML and plastico ML of a large number of solids are caused by the electron trapping and electron bombardment mechanisms, the elastico ML and plastico ML of only a few solids are caused by the chemiluminescence and other mechanisms. The elastico ML of rare earth doped strontium aluminate, rare earth doped strontium silicates and certain other crystals is so intense that it can be seen in day light with naked eye. For a long time, the detection of luminescence of metals was considered to be impossible; however, now-a-days, it has been found that the intrinsic ML in certain metals can be produced by their plastic deformation at fast rate. In an ultraviolet-irradiated CdS single crystal immersed in liquid nitrogen, the intrinsic elastico ML resembling edge emission can be produced simply by tapping it along its c-axis. Of the order of a 1000 flashes may be obtained from a well-stimulated crystal by tapping more or less uniformly over the whole surface. After the stimulation is exhausted, the crystal can be re-activated by a brief ultraviolet illumination, and the process of obtaining multiple flashes by tapping can be repeated. The spectra of the plastico ML of certain non-irradiated alkali halide crystals resemble the plastico ML spectra of the corresponding X- or γ-irradiated crystals. Such result can be understood on the basis of the formation of F-centres and hole centres during the intersection of the forest of immobile and mobile dislocations. Depending on the prevailing conditions, either the continuous plastico ML or pulsed plastico ML is observed in II–VI semiconductors and in certain non-irradiated alkali halide crystals. Whereas the spectra of elastico ML resemble the solid state luminescence spectra, depending on the solids the spectra of plastico ML consist of either only the solid state luminescence spectra or the combination of solid state luminescence spectra and discharge spectra of the adsorbed molecules of surrounding gases. Till now the elastico ML and plastico ML have been studied only in a limited number of solids, and therefore, they need further investigation. The study of the elastico ML and plastico ML of rubbers and polymers may be very interesting. The understanding of the mechanisms of intrinsic and extrinsic elastico ML and plastico ML may be useful in preparing suitable elastico and plastico mechanoluminescent materials and it may be helpful in designing the devices based on elastico ML and plastico ML. Theorists need the models or mechanisms of the phenomena to propose and develop their theories and experimentalists also need the models and mechanisms of the phenomena for improving their materials and devices. In this regard, the present study may also be useful and important.