Explosive


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.
The potential energy stored in an explosive material may, for example, be:
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.” 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.
A wide variety of chemicals can explode; only some are manufactured specifically for the purpose of being 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, or confinement or sudden release.

History

, 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 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, which are waterproof and malleable, though they may catch fire due to reactions with metals.

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.
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 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:
A secondary explosive is less sensitive than a primary explosive and requires substantially more energy to be initiated. Because they are less sensitive, they are usable in a wider variety of applications and are safer to handle and store. Secondary explosives are used in larger quantities in an explosive train and are usually initiated by a smaller quantity of a primary explosive.
Examples of secondary explosives include TNT and hexogen.

Tertiary

Tertiary explosives, also called blasting agents, are so insensitive to shock that they cannot be reliably detonated by practical quantities of primary explosives, and instead require an intermediate explosive booster of secondary explosives. These are often used for safety and the typically lower costs of material and handling. The largest consumers are large-scale mining and construction operations.
Most tertiaries include a fuel and an oxidizer. ANFO can be a tertiary explosive if its reaction rate is slow.

By velocity

An explosive is classified as a low or high explosive according to its rate of combustion: low explosives deflagrate, 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.