Comparison of triboluminescent emission yields for 27 luminescent materials W.A. Hollerman a,⇑ , R.S. Fontenot a,b , K.N. Bhat b , M.D. Aggarwal b , C.J. Guidry a , K.M. Nguyen a a University of Louisiana at Lafayette, Department of Physics, P.O. Box 44210, Lafayette, LA 70504, USA b Alabama A&M University, Department of Physics, Chemistry, and Mathematics, P.O. Box 1268, Normal, AL 35762, USA article info Article history: Received 2 August 2011 Received in revised form 13 February 2012 Accepted 7 March 2012 Available online 11 April 2012 Keywords: Triboluminescence Mechanoluminescence ML TL Comparison of triboluminescence abstract In 1888, Wiedemann and Schmidt defined triboluminescence (TL) as the emission of light produced by mechanical action. In 1999, Sage and Geddes patented a design for a sensor capable of discerning the locations of impacts. Their design involved embedding a sensor inside a material coated with a tribolu- minescent crystal. The resulting impacts would produce light that would be analyzed to determine its location. Using this idea, the authors have been investigating the triboluminescent properties of 27 mate- rials for its possible use as an impact sensor. This paper gives a detailed comparison of the tribolumines- cent emission yields resulting from low energy drops for 27 luminescent materials. Collection of this data is only the first step towards the development of a practical TL-based impact sensor. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Triboluminescence (TL) is light generated by mechanical action [1]. It has been estimated that 30% of organic crystals and 50% of inorganic crystals are triboluminescent [1]. A good overview of related research can be found in Walton [1]. Recently, work has been completed to characterize materials, mechanisms, and possi- ble applications of triboluminescence [2–14]. In 1999, Sage and Geddes patented a design for a sensor capable of discerning the locations of impacts [15]. This design involved embedding a sensor inside a material coated with a triboluminescent crystal. The resulting impacts would produce light that would be analyzed to determine its location. Starting in 2003, the authors investigated the triboluminescent emission properties for 7.5 lm ZnS:Mn, since it emits copious amounts of light at impact and a large quantity was readily available [2–14]. The next logical step was to deter- mine what luminescent materials (LMs) were good candidates for use as the active element in an impact sensor. Any material used for this purpose should have a large triboluminescent emission yield, be readily available, and be environmentally benign. For this reason, relative triboluminescent emission yields were measured for 27 candidate powders. This paper dis- cusses the results of these measurements using a low energy drop tower. 2. Materials The emission characteristics for a LM usually depend on compo- sition, inherent physical properties such as grain size, and dopant concentration. The primary goal of this research was to compare the triboluminescent emission yields for ZnS-based materials as a function of grain size and dopants. Since 2003, the authors have concentrated on investigating the triboluminescent emission from inorganic ZnS:Mn [2–14]. ZnS:Mn has a hexagonal crystal structure that is considered ‘‘loose’’, mean- ing that they emit light with very little stress applied [2–14]. It can be seen emitting TL light simply by scratching it with a nail or other sharp object. Materials such as ZnS:Mn are usually made up of a semiconductor host and an impurity. For ZnS:Mn, manga- nese is the impurity and is called the dopant. The dopant concen- tration is usually a small fraction of the composition, its inclusion changes the band structure of the crystal by narrowing the energy gap between conduction and valence bands. With narrower energy gaps, transitions that emit light are more probable and this in- creases the opportunity for light to be emitted during excitation or relaxation of electrons. Sometimes luminescent materials con- tain more than one dopant to give the desired set of emission characteristics. 2.1. Micro-grained ZnS compounds Sample powders of ZnS:Mn, ZnS:Cu, ZnS:Mn,Cu, ZnS:Cu,Pb,Mn, and ZnS:Cu,Pb were purchased from Phosphor Technology, Limited of Great Britain. These microscale grain size samples are 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.03.011 ⇑ Corresponding author. E-mail address: hollerman@louisiana.edu (W.A. Hollerman). Optical Materials 34 (2012) 1517–1521 Contents lists available at SciVerse ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat