Chapter 2

Energetics of Energetic Materials

Abstract

This chapter mainly discusses the thermochemistry of high-energy materials (HEMs). It first discusses the basic difference between fuels and HEMs, showing that the term "high-energy material" is actually a misnomer. It gives a lucid illustration of the importance of heat of formation and oxygen balance of HEMs and the difference between calorific and calorimetric values and adiabatic and isochoric flame temperatures. The role played by these thermochemical parameters in the ultimate performance of HEMs, such as propellants and explosives, is summed up in the HEM network chart given at the end of the chapter.

Keywords

Flame temperature; Heat content (enthalpy); Heat of combustion (calorific value); Heat of explosion (calorimetric value); Heat of formation; Hess's law; High-energy materials; Impetus (force constant); Oxygen balance

2.1. Are Explosives and Propellants High-Energy Materials?

Explosives are storehouses of energy. The potential energy lying dormant in the molecules of these materials is released when they are properly triggered or initiated and the release of this energy originates at the breaking of the energetic chemical bonds in the molecule of the explosive compound. The explosives (and propellants, which are categorized as low explosives) are sometimes referred to as "high-energy materials" (HEMs) or "energetic materials." Is one justified in using these terms for explosives? Let us compare the most powerful high explosive in use today-HMX (an abbreviation for High Melting Explosive; chemical name being cyclic tetramethylene tetranitramine)-with a well-known fuel-coal-in terms of their energetics. From Table 2.1, it is seen that for every gram, coal produces more than 5 times the heat produced by HMX. The heat evolved by 1 g of coal and HMX is illustrated as a chart in Figure 2.1. A similar comparison will show us that all fuels of day-to-day use release far more heat than any known explosive. Therefore, in a thermochemical sense, explosives and propellants are not really HEMs. However, we also observe that HMX detonates in much less time than a piece of coal takes to burn. The former undergoes the process of detonation accompanied by shock waves whereas the latter takes its own time for combustion with the help of oxygen available in air. If we take the rate at which the heat is released, then the power of HMX is approximately 5.6 × 109 W in comparison to 488 W of coal in the above example. This power generation by HMX is far more than the capacity of all of the power generators in the country put together. The better term for explosives may not be HEMs but "power-packed materials." In the above example, an arbitrary figure of 60 s has been given for the burning of coal. Under static wind conditions, the time taken for 1 g of coal to burn depends on its surface area exposed to air. As we go on breaking it into pieces, the burning time of coal comes down drastically. At its extreme, when the same 1 g of coal is finely powdered and dispersed as coal dust in air, facilitating the exposure of the maximum surface area to air, each such dust particle is in intimate contact with the oxygen molecules of air. When initiated, the combustion reaction takes place so fast that it is virtually converted into a violent detonation. Disastrous coal-dust explosions in coal mines are a result of this phenomenon. Such dust explosions are not uncommon in many other industries.

Table 2.1

Heat generated by coal and HMX.

1. Heat evolved 7000 cal (heat of combustion) 1355 cal (heat of explosion) 2. Time (burning/detonation) 60 s 10-6 s 3. Power 488 W 5.6 × 109 W
Figure 2.1 Comparison of Heat Evolved by 1 g of Coal and HMX.

2.2. Explosive: The Wonderful Lamp

An explosive is similar to the genie that we come across in the ever-fascinating tale of Aladdin and the Wonderful Lamp. It has great potential, but then it has to be kept under check or "bottled-up." Only when its services are needed do we open the bottle, and, in the case of explosives, we give the necessary trigger energy. An explosive is a substance in a metastable equilibrium, in a "ready-to-go-off" stage with huge potential energy. The relation between the energy needed to make an explosive and the energy released by it on explosion can be qualitatively understood by comparing it with a huge boulder brought to the apex of a cliff. Figure 2.2(a) and (b), respectively give an analogy between a boulder kept on the brink of a cliff and an explosive synthesized and "kept" in a metastable state. One has to make great efforts (or spend much energy) to place the boulder on the cliff (A) in Figure 2.2(a). The boulder continues to remain there until someone decides to push it (giving an energy equal to B) so that it falls off from a great height, converting the potential energy into kinetic energy, which is dissipated as heat and sound when it strikes the ground. Release of energy is equal to C. Likewise, the synthesis of an explosive molecule is done by packing in it a great amount of potential energy such as high bond energy, structural strain, etc., and it is kept in the metastable state as shown in Figure 2.2(b). (D-E) is the effective energy spent in such a synthesis. If the reactants are assumed as elements such as carbon, hydrogen, and oxygen, (D-E) is referred to as the heat of formation of the explosive. The explosive now needs only a trigger energy (generally called activation energy) equal to F, so that a net energy equal to G is liberated during the explosive process and the formation of stable products.
Figure 2.2 A Boulder on a Cliff and an Explosive Molecule. The chemist who wants to synthesize an explosive ensures that (1) as far as possible the product has a high positive heat of formation (i.e., the energy level of the explosive molecule is higher than that of the elements from which it is made), (2) it has its own supply of oxygen in the molecule to be independent of external or atmospheric oxygen to affect the process of explosion, and (3) the explosive reaction results in a large amount of gases. Factors 1 and 2 will ensure that the explosion process releases a large amount of heat (heat of explosion), thereby enormously increasing the temperature of the products, normally more than 2000 °C. Factor 3 will ensure that, with so many gases at a high temperature, there will be development of very high pressures. The gases expand rapidly from very high pressures to the atmospheric pressure, thereby performing a large amount of work in a short time; that is, the produced gases will work as a powerful working fluid to perform certain assigned tasks such as the blast effect produced by high explosives in microseconds, the work of throwing a projectile through a gun barrel in a few milliseconds, or the self-propulsion by a rocket in a time period varying from a few seconds to even a few minutes. Is an oxidation reaction always necessary in a chemical explosion? Although most of the chemical explosions involve fast oxidation of fuel elements, it need not be so in some cases. For example, lead azide (Pb(N3)2), a well-known primary explosive, does not contain any oxygen atoms in its molecule. However, it has a positive heat of formation. The azide (-N-NN) groups attached to the lead atom have weak linkages and are themselves at a higher energy level. Only a small trigger energy is necessary to rupture these linkages to produce more stable products with the evolution of energy.

(-N(-)-N(+)=N)2Pb+3N2+110.8kcal

2.3. Thermochemistry and Explosive Energy

Chemical reactions are accompanied by energy changes, mainly in the form of heat. The branch of science that deals with the heat changes during chemical reactions is called "thermochemistry." It is essential to remember certain basic concepts in thermochemistry to obtain better insight into the heat transactions during the formation and explosion of explosives. The concepts about the three important parameters-internal energy (E), heat content or enthalpy (H), and work (W)-should also be clear. The internal energy of a substance is the total quantity of energy it possesses by virtue of its kinetic portion of energy (due to translational, vibrational, and rotational motions associated with the molecules) and the potential portion of energy (due to various interatomic, intermolecular, and submolecular forces of attraction and repulsion). In a chemical reaction in which certain bonds of the reactant molecules are broken and certain bonds of the product molecules are formed, it is mostly the kinetic portion of the internal energy that undergoes a change and may be positive or...