Mechanoluminescence excited by hypervelocity impact of a projectile on a target coated with ZnS:Mn phosphor

Abstract

When a small aluminium projectile moving with a velocity of several km/s makes an impact on to an aluminium target whose opposite side is coated with ZnS:Mn phosohor using polyphenyl methyl siloxane (PPMS) binder, then initially the ML intensity increases with time, attains a peak value Im, at a particular time tm, and later on it decreases with time. The value of peak ML intensity increases with the increasing impact velocity of the projectile. For a high piezoelectric field on to the ZnS:Mn phosphor, the bending of bands takes place, and the trapped electrons tunnel to the conduction band, and subsequently, the energy released during the electron–hole recombination and the electron capture in vacant deep traps excites the Mn2+ ions and de-excitation gives rise to the light emission characteristic of the Mn2+ ions. Expressions are derived for different characteristics of ML, in which a good agreement is found between the experimental and mathematical results. Using impact-induced ML the lifetime of excited Mn2+ ions, lifetime of electrons in the shallow traps, and decay time of impact pressure can be determined. Using the calibrated curve between the ML intensity and impact velocity of the projectile, the unknown impact velocity of the projectile can be determined, and thus, the ML can be used as an impact sensor. With increasing impact velocity of the projectile the kinetic energy of the projectile increases. In this case, the expressions suggest that, both the peak ML intensity and total ML intensity should increase with increasing energy of the projectile. From the ML intensity versus time curve, the pressure versus time curve can be obtained as follows: (i) Normalizing the peak ML intensity as maximum impact pressure, and (ii) after tm, replacing the decay of ML intensity by the decay of pressure. It is to be noted that the pressure versus time curve plays important role in applied mechanics. Micrometeoroid and ultra high speed (hypervelocity) debris impacts pose a significant threat to spacecraft. As the mechanoluminescence has the potential to be used to detect impacts and other phenomena, it can be used as the active element for an impact detection system for spacecraft or bases on the Moon or other places. Thus, further studies on the ML at high impact velocities are required. The study of ML at hypervelocity of impact may also be helpful to understand the earthquake lights, pre-earthquake phenomena, post-earthquake phenomena, etc.