8 - Energetic materials

Summary

Introduction

This chapter will detail the response of a class of materials dubbed energetic to signify that they can break bonds and react under load. These substances contain a large amount of stored chemical energy that can be released if appropriate thermal thresholds are exceeded. Such materials combine a fuel and an oxidiser; fuels are typically carbon or hydrogen, oxidisers are oxygen or a halogen like chlorine, for example. Combining hydrogen and oxygen to form water liberates 13 260 J kg^–1 and burning petrol with oxygen (air) 30000 J kg^–1. Yet the high explosive TNT liberates only 4080 J kg^–1, less than 15% of the amount liberated by petrol. The difference is that fuel alone burns only where oxygen is present; a spillage will burn for minutes with oxygen from air, for example. Yet a TNT molecule contains oxygen within it and can liberate energy in the microseconds the reaction front takes to transit the molecule and break bonds. Therefore the difference between these fuels lies in the power that the molecule supplies in the form in which the material exists on ignition. Energetic materials may be solids, liquids or gases, but condensed-phase materials will be followed here as earlier in the book. Further, they need not necessarily be organic. There is increasing need for higher performance, lighter weight and safer composites which use reacting metals as well as more conventional materials and using new material morphologies which have increased surface areas, such as mixtures of nano-materials or designed nano-composites. However, the principal energetics used at the present time include a range of elements that react with oxygen and these will be discussed in what follows.