chemical compounds or mixtures of substances that can undergo rapid chemical reaction, with the liberation of a large amount of heat and the formation of gases. This reaction, having started at some point as a result of heat, shock, friction, the detonation of another explosive, or some external action, spreads through the charge by the transmission of energy from layer to layer by heat and mass transfer processes (combustion) or a shock wave (detonation). The rates of combustion of different explosives vary from fractions of a millimeter per second to tens and hundreds of meters per second; the rate of detonation can exceed 9 km/sec.

Condensed (solid and liquid) substances, gases, and suspensions of solid or liquid particles in gases can be explosives. Condensed and water-filled explosives, primarily those with a considerable energy concentration per unit volume, are used in explosives engineering. In conjunction with a high speed of the process, high concentration makes it possible to produce intense explosive forces. Thus, with a charge of 1 kg of cyclonite, whose volume is 0.6 liter and whose heat of explosion is 5.4 megajoules (1,300 kilocalories), detonation can occur in 10 microseconds (1 x 10^-5 sec), which corresponds to a power of 500 million kilowatts (tens of times greater than the power of the largest electrical power plant). The reaction during detonation takes place so quickly that the gaseous products at a temperature of several thousand degrees are compressed into a volume almost equal to the initial volume of the charge, at pressures up to tens of giganewtons per sq m (hundreds of thousands of kilograms-force per sq cm). Expanding sharply, the compressed gas applies a shock of great force to the surrounding medium. An explosion oc-curs. Material near the charge is pulverized and undergoes extremely powerful plastic deformation (the local or brisant action of the explosion); the destruction far from the charge is less intense, but the zone in which it occurs is rather large (the total or mine effect of the explosion). The pressure p developed by the detonation, which determines the brisance of the explosive, depends on the charge density and rate of detonation. The work capacity of explosives is determined by the heat, as well as by the volume of the gaseous products of the explosion; it is usually expressed in relative units with trinitrotoluene (TNT), blasting gelatin, or No. 6 ammonite as the standard or in units of energy.

Apart from the capacity to perform one or another type of work, the sphere of application of explosives is determined by their chemical and physical stability (that is, the ability to retain their properties during loading, transporting, and storage) and by their sensitivity to external actions (which is characterized by the minimum quantity of energy needed to cause explosion). Another important characteristic of explosives is their detonation capacity, which is measured by the critical detonation diameter—that is, the minimum diameter of a cylindrical charge in which detonation will still propagate despite the scattering of material from the reaction zone. The smaller the critical detonation diameter of an explosive, the greater its detonation capacity.

The basic source of explosive energy is oxidation. The usual oxidant is oxygen, which is a component of explosives and which makes possible their combustion and explosion without the admission of air. The larger the amount of oxygen in an explosive, the higher its oxygen balance. If the oxygen suffices to convert all the carbon of an explosive to CO₂ and all the hydrogen to H₂O, the oxygen balance is zero. In explosives with insufficient oxygen, the balance is negative; if there is excess, it is positive. Some substances that do not contain oxygen—for example, azides, acetylene, acetylides, diazo compounds, hydrazine, nitrogen chloride and iodide, and mixtures of combustible substances with halogens, “frozen” free radicals, and inert gas compounds—can explode. Most of these, just as many oxygen-containing compounds (peroxides, ozonides, organic salts of perchloric and chloric acids, nitrites, and nitroso compounds), are substances that can explode but which are not used as explosives because of high sensitivity, low chemical stability, toxicity, or expense. Some mixtures of combustible substances with oxidizing agents (chromates, dichromates, peroxides, oxides, nitrates, or chlorates) are used as pyrotechnic compounds.

Of the many substances that can explode, only 20-30 are used as explosives or components of explosive mixtures. The most important of these are nitro compounds (trinitrotoluene, tetryl, cyclonite, octogen, nitroglycerin, pentaerythritol tetranitrate [PETN], cellulose nitrate, and nitro-methane) and salts of nitric acid, particularly ammonium nitrate. As a rule these substances are used not in pure form but in the form of mixtures—for example, mixtures of cyclonite, octogen, and PETN with TNT; nitroglycerin with nitroglycol, diethylene glycol dinitrate, and cellulose nitrate; TNT with ammonium nitrate (ammonite); and ammonium nitrate with liquid combustibles (solar oil) or powdered combustibles (sawdust or aluminum powder). To reduce sensitivity and danger in handling, powerful explosives are mixed with paraffin, ceresin, and other fusible additives (desensitization of explosives). Powdered aluminum or magnesium is mixed in to raise the heat of explosion. Mixed explosives made from nonexplosive (or weakly explosive) combustibles and oxidizing agents (for example, igdanit, granulit, black powder, and perchlorate explosives, which are mixtures based on salts of perchloric and chloric acids or on liquid oxygen [oxyliquits]) are of great importance.

Depending on their explosive properties (the conditions of transition from combustion to detonation) and the spheres of application governed by these properties, explosives are divided into initiators (primary), high explosives (secondary), and propellants (powders).

Initiator explosives are characterized by a very high rate of explosive transformation. Their sensitivity is high, and their combustion is unstable, leading to detonation even at atmospheric pressure. An explosion can be started by ignition, shock, or friction. Initiator explosives are used to start the explosive transformation of other substances. Outstanding examples of initiating explosives are lead azide, mercury fulminate, lead trinitroresorcinate, and tetrazine. Brisant explosives are more inert—they are much less sensitive to external influences than initiators. The combustion can become detonation only in the presence of a strong casing or a large amount of explosives; hence they are relatively safe to handle. The brisant explosives most often used are nitro compounds and the abovementioned explosive mixtures based on nitrates, chlorates, perchlorates, and liquid oxygen. The main mode of their explosive transformation is detonation, which is caused by a small charge of initiator explosive. Brisant explosives are used in blasting operations, as well as in projectiles and other ammunition. Propellant explosives burn in an even more stable manner than brisant explosives: they do not detonate during combustion even under the most severe conditions (large charges; pressures on the order of tens and hundreds of meganewtons per sq m—that is, hundreds and thousands of kilograms-force per sq cm). The main mode of the explosive transformation of propellant explosives is combustion. The difference between propellant and high explosives lies basically in their physical structure (charge density and strength) rather than in their chemical composition. (The characteristics of some explosives are given in Table 1.)

Explosives are used extensively in the national economy in blasting operations, blast welding, explosive hardening of metal, and explosive forming. The explosives used in the mining industry are distinguished as nonsafety explosives, which are used for open and underground mines (except

mines that are dangerous because of gas or dust; explosives for open mines usually have a higher detonation capacity than those for closed operations, and they yield fewer toxic gaseous products—nitrogen oxides and carbon monoxide), and permissible (safety) explosives (for mines that are dangerous because of gas or dust). The majority of industrial explosives are ammonites and granulits. Dynamite and TNT—primarily in the granulated form (granulotol) and sometimes with the addition of aluminum (aliumotol)—and water explosives are used in lesser quantities.

In military practice, explosives are used to load ammunition. Secondary explosives are used for the explosive charges of mines, shells, and aerial bombs; the warheads of rockets, torpedoes, and hand and rifle grenades; and so on. Propellant explosives are used as powder charges in artillery and mortar projectiles, in small-arms cartridges, and in solid-fuel rocket engines. Initiator explosives are used for devices that cause the detonation of explosive charges or the ignition of powder charges (in detonators, electric detonators, and detonating cords). Explosives are also used to make high-pressure gas generators (powder charges to supply components to the combustion chamber of liquid-fuel rocket engines, for flamethrowers, and so on) and to make engineering explosive barriers (minefields, land mines). They are an important part of atomic and thermonuclear warheads: the explosion of secondary explosive charges makes it possible to attain supercritical mass in a nuclear charge.

Explosives are also used extensively in scientific research as a simple and convenient means of obtaining high temperatures, high speeds, and ultrahigh pressures. Some of the trends in explosive development are the extensive use of friable granular explosives, which are suitable for mechanical charging; the exploding of charges without the use of initiators (for example, by means of a powerful electrical discharge); the development and introduction of new types of explosives [for example, compounds containing the oxygen-rich trinitromethyl group C(NO₂)₃]; and the use of nitroparaffins and explosive mixtures based on liquid oxidants (tetranitromethane, nitrogen tetroxide, and others).

The first explosive was black powder (gunpowder), which appeared in Europe in the 13th century. Secondary explosives began to be used only in the 19th century. The use of pyroxylin, picric acid, and TNT in military practice and of nitroglycerin and dynamites in the mining industry began. The use of PETN and cyclonite began just before World War II; octogen began to be used after the war. Smokeless powder—which became the main propellant explosive for firearms and, beginning in the 1930’s, for rocket shells as well (in addition to mixed powders)—was invented in the 1880’s. The beginning of the 19th century saw the beginning of the use of the first initiating explosive, mercury fulminate, to ignite black powder. It was later discovered that detonation of explosives can be obtained by increasing the charge of mercury fulminate. This made possible the large-scale use of explosives that are difficult to set off without a detonatorammonite, dinamony (a mixture of 85 percent ammonium nitrate and 15 percent flour), and water explosives. The use of explosives became increasingly economical and less dangerous. There was also substantial improvement in the means of utilization.

Modern blasting technology makes it possible to explode large charges (several thousand tons) of explosives with very great useful effect while ensuring the complete safety of people and surrounding structures. The annual world production of explosives is several million tons.