An explosive is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of light, heat, sound, and pressure. An explosive charge is a measured quantity of explosive material. The material may either be composed solely of one ingredient or be a mixture containing at least two substances. An explosive can be used together with mechanical or electronic components to manufacture a bomb.
The potential energy can be stored in an explosive material in various ways. Most commonly it is stored as chemical energy, such as black powder, nitroglycerin or even grain dust. It can also be stored as pressurized gas, like a gas cylinder, aerosol can, or boiling liquid expanding vapor explosion, or less commonly as nuclear energy, such as in the fissile isotopes uranium-235 and plutonium-239.
Explosive materials may be categorized by the speed at which they expand. Materials that detonate, in which the front of the chemical reaction moves through the material faster than the speed of sound, are called “high explosives.” In contrast, materials that deflagrate, where the front of the reaction moves slower than the speed of sound, are known as “low explosives.” Generally, outcome of the reaction is a rapid burn rather than a proper explosion as seen with high explosives. Explosives may also be categorized by their sensitivity. Sensitive materials that can be initiated by a relatively small amount of heat or pressure are primary explosives, and materials that are relatively insensitive are secondary or tertiary explosives. Such materials require a detonator to initiate the explosion, without which they will deflagrate instead.
Only some of the manufactured chemicals are suitable to be used as explosives. The remainders are too dangerous, sensitive, toxic, expensive, unstable, or prone to decomposition or degradation over short time spans. In contrast, some materials are merely combustible or flammable if they burn without exploding. The distinction, however, is not always clear. Certain materials—dusts, powders, gases, or volatile organic liquids—may simply be combustible or flammable under ordinary conditions but become explosive in specific situations or forms, such as dispersed airborne clouds, confinement, or sudden release.
History
Early thermal weapons, such as Greek fire, have existed since ancient times. The history of chemical explosives is closely intertwined with the history of gunpowder. While searching for an elixir of eternal life, Taoist alchemists in China created the earliest form of gunpowder from coal, saltpeter, and sulfur. Gunpowder was the first form of chemical explosives, first seeing use in warfare in 1161. Early forms of explosives in warfare included bamboo firecrackers, which were explosives fired from bamboo or bronze tubes.
The first explosive stronger than black powder to see widespread use was nitroglycerin, developed in 1847. Since nitroglycerin is a liquid and highly unstable, it was replaced by nitrocellulose and trinitrotoluene (TNT) in 1863, smokeless powder and dynamite in 1867, and gelignite. World War I saw the adoption of TNT in artillery shells, while World War II saw extensive use of new explosives .
In modern weapons, these have largely been replaced by more powerful explosives such as C-4 and pentaerythritol tetranitrate (PETN), which are waterproof and malleable, though they may catch fire due to reactions with metals.Ankony, Robert C., Lurps: A Ranger's Diary of Tet, Khe Sanh, A Shau, and Quang Tri, revised ed., Rowman & Littlefield Publishing Group, Lanham, MD (2009), p.73.
Applications
Classification
By energy source
Chemical
An explosion is a type of spontaneous chemical reaction that, once initiated, is driven by both a large exothermic change and a large positive entropy change in going from reactants to products, thereby constituting a thermodynamically favorable process that propagates very rapidly. Thus, explosives are substances that contain a large amount of energy stored in chemical bonds. The energetic stability of the gaseous products, and hence their generation, comes from the formation of strongly bonded species like carbon monoxide, carbon dioxide, and nitrogen gas, which contain strong double and triple bonds having bond strengths of nearly 1 MJ/mole. Consequently, most commercial explosives are organic compounds containing –NO2, –ONO2, and –NHNO2 groups that, when detonated, release gases like the aforementioned (e.g., nitroglycerin, TNT, octogen (HMX), PETN, nitrocellulose).
Traditional explosives mechanics are based on the shock-sensitive rapid oxidation of carbon and hydrogen to carbon dioxide, carbon monoxide, and water in the form of steam. Nitrates typically provide the required oxygen to burn the carbon and hydrogen fuel. A sensitizer such as powdered aluminum may be added to an explosive to increase the energy of the detonation. Once detonated, the nitrogen portion of the explosive formulation emerges as nitrogen gas and toxic nitric oxides.
The chemical decomposition of an explosive may take years, days, hours, or a fraction of a second. The slower processes of decomposition take place in storage and are of interest only from a stability standpoint. Of more interest are the other two rapid forms besides decomposition: deflagration and detonation.
Nuclear
Exotic In addition to chemical and nuclear explosives, there are more exotic explosive materials.
By velocity An explosive is classified as a low or high explosive according to its rate of combustion: low explosives deflagrate (burn rapidly), while high explosives detonate. While these definitions are distinct, the problem of precisely measuring rapid decomposition makes practical classification of explosives difficult. For a reaction to be classified as a detonation as opposed to just a deflagration, the propagation of the reaction shockwave through the material being tested must be faster than the speed of sound through that material. The speed of sound through a liquid or solid material is usually orders of magnitude faster than the speed of sound through air or other gases.
Low
Low explosives (or low-order explosives) are explosive materials that deflagrate, meaning that the explosion is propagated by a flame front that proceeds through the material at a subsonic speed.
In deflagration, the decomposition of explosive material is propagated by a flame front at subsonic speeds within the substance (which is usually still higher than 340 m/s or in most liquid or solid materials), in contrast to a shock wave in detonation, which travels at supersonic speed. Under normal conditions, low explosives undergo deflagration at rates that vary from a few centimetres per second to approximately . It is possible for them to deflagrate more quickly, producing an effect similar to a detonation. This can happen under higher pressure (such as when gunpowder deflagrates inside the confined space of a bullet casing, accelerating the bullet to well beyond the speed of sound) or temperature.
A low explosive is usually a mixture of a combustible substance and an oxidant that decomposes rapidly; however, they burn more slowly than a high explosive, which has an extremely fast burn rate.
Low explosives are normally employed as propellants. Included in this group are petroleum products such as propane and gasoline, gunpowder (including smokeless powder), and light pyrotechnics such as flares and fireworks, but they can replace high explosives in certain applications, including gas pressure blasting.
High
High explosives (HE, or high-order explosives) are explosive materials that detonate, meaning that the explosion is propagated by an explosive shock front that passes through the material at supersonic speed. High explosives detonate with an explosive velocity of about . For instance, TNT has a detonation (burn) rate of approximately , detonating cord of , and C-4 about .
High explosives tend to have the fuel and oxidizer contained in the same molecule (e.g. TNT) while low explosives like gunpowder are usually mixtures of fuel (e.g. the sulfur and charcoal in gunpowder) and oxidizer (the saltpeter in gunpowder), causing the reaction front to propagate subsonically (deflagration).
High explosives are normally employed in mining, demolition, and military applications.
High explosives can be divided into two explosive classes differentiated by sensitivity: primary explosives and secondary explosives. Although tertiary explosives, such as ANFO at , can technically meet the explosive velocity definition, they are not considered high explosives in regulatory contexts.
Countless high-explosive compounds are chemically possible, but commercially and militarily important ones have included NG, TNT, TNP, IMX-101, RDX, HMX, PETN, EGDN, TATB, and HNS.
By sensitivity
Primary A primary explosive is an explosive that is extremely sensitive to stimuli such as impact, friction, heat, static electricity, or electromagnetic radiation. Some primary explosives are also known as contact explosives. A relatively small amount of energy is required for initiation. As a very general rule, primary explosives are considered to be those compounds that are more sensitive than PETN. As a practical measure, primary explosives are sufficiently sensitive that they can be reliably initiated with a blow from a hammer; however, PETN can also usually be initiated in this manner, so this is only a very broad guideline. Additionally, several compounds, such as nitrogen triiodide, are so sensitive that they cannot even be handled without detonating. Nitrogen triiodide is so sensitive that it can be reliably detonated by exposure to alpha radiation.
Primary explosives are often used in detonators or to trigger larger charges of less sensitive secondary explosives. Primary explosives are commonly used in blasting caps and percussion caps to translate a physical shock signal. In other situations, different signals, such as electrical or physical shock, or, in the case of laser detonation systems, light, are used to initiate an action, i.e., an explosion. A small quantity, typically milligrams, is sufficient to initiate a larger charge of explosive that is usually safer to handle.
Examples of primary high explosives are:
Acetone peroxide Alkali metal ozonides Ammonium permanganate Ammonium chlorate Azidotetrazolates Azoclathrates Benzoyl peroxide Benzvalene 3,5-Bis(trinitromethyl)tetrazole Chlorine oxides Copper(I) acetylide Copper(II) azide Cumene hydroperoxide Cycloprop(-2-)enyl nitrate (CXP or CPN) Cyanogen azide Cyanuric triazide Diacetyl peroxide 1-Diazidocarbamoyl-5-azidotetrazole Diazodinitrophenol Diazomethane Diethyl ether peroxide 4-Dimethylaminophenylpentazole Disulfur dinitride Ethyl azide Explosive antimony Fluorine perchlorate Fulminic acid Halogen azides: Fluorine azide Chlorine azide Bromine azide Iodine azide Hexamethylene triperoxide diamine Hydrazoic acid Hypofluorous acid Lead azide Lead styphnate Lead picrate Manganese heptoxide Mercury(II) fulminate Mercury nitride Methyl ethyl ketone peroxide Nickel hydrazine nitrate Nickel hydrazine perchlorate Nitrogen trihalides: Nitrogen trichloride Nitrogen tribromide Nitrogen triiodide Nitroglycerin Nitronium perchlorate Nitrosyl perchlorate Nitrotetrazolate-N-oxides Pentazenium hexafluoroarsenate Peroxy acids Peroxymonosulfuric acid Selenium tetraazide Silicon tetraazide Silver azide Silver acetylide Silver fulminate Silver nitride Tellurium tetraazide tert-Butyl hydroperoxide Tetraamine copper complexes Tetraazidomethane Tetrazene explosive Tetrazoles Titanium tetraazide Triazidomethane Oxides of xenon: Xenon dioxide Xenon oxytetrafluoride Xenon tetroxide Xenon trioxide
Secondary