What are energetic materials?

Energetic materials are an essential and often unappreciated vital component of all ammunition, without energetic materials an army battalion would be just become a military parade. Energetic materials are used in every step of the munition functioning from the decision to fire until the destruction of the target.

Energetic materials are not remarkable in terms of their energy content, 1lb of butter will release more than twice as much energy when burnt than 1 lb. of Trinitrotoluene (TNT) ¹ but you can't demolish a bridge with a stick of butter. Energetic materials' utility lies in the rate they can release energy, TNT has a “rate of reaction” , detonation velocity, of 6,900 m/s ~ 15,434 mph (think London to New York in 13 minutes)

The reason that energetic compounds are able react so quickly is because they do not depend on the availability of atmospheric oxygen, it is either, bound up in their chemical composition or present in other compounds in the formulation. TNT, our all-time favorite energetic material, has most of the oxygen it needs to react right there in its molecular structure close to the carbon and hydrogen atoms which are so desperate to get their hands on some. It doesn’t quite have enough oxygen for complete reaction, see oxygen balance, but it has enough so that it doesn't have to wait around for the surrounding air to give up its oxygen to start reacting. However, it is not just the proximity of oxidizers that enables this energy to be released, some energetic materials have energy bound up in their structural bonds that is released on decomposition. Some high nitrogen containing materials don't technically have fuels or oxidizers but nonetheless release considerable energy forming nitrogen e.g. Lead Azide, PbN₆→ Pb + 3N₂ , figure 2.

The functional groups in organic chemistry that give organic compounds explosive properties such as the "nitro" in Trinitrotoluene and the "azide" in Lead Azide are known as Explosophores. Energetics are defined as either high or low explosives depending on whether their burning rates are less than or greater than the speed of sound, see deflagration and detonation A simple classification of energetics is shown in Figure 3.

Pure explosive compounds are listed in Explosive Compounds along with many of their important properties. TNT is one of the few energetic materials that can be used, as is, in artillery, mortar and demolition charges² without additional ingredients. However, most high explosives are mixtures of explosive compounds, fuels, oxidizers, binders and other additives, these formulations are listed in Explosive Formulations.


¹ ΔH_c Butter = 30.5 kJ/g compared to ΔH_c TNT = 15.0 kJ/g
² TNT melts at 80.4°C (178°F) which allows it to be melted in water jacketed vessels and then frozen into a solid for operational use.