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Explosives

Explosives

:This article is concerned solely with chemical explosives. There are many other varieties of more exotic explosive material, and theoretical methods of causing explosions such as nuclear explosives and antimatter, and other methods of producing explosions, such as abrupt heating with a high-intensity laser or electric arc. Any explosive material has the following characteristics:
- It is chemically or otherwise energetically unstable.
- The initiation produces a sudden expansion of the material accompanied by the production of heat and large changes in pressure (and typically also a flash or loud noise) which is called the explosion.

Chemical explosives

Explosives are classified as low or high explosives according to their rates of decomposition. Low explosives burn rapidly (or deflagrate). High explosives undergo detonation. There is no sharp line of demarcation between low and high explosives, due to the difficulties inherent in precisely observing and measuring rapid decomposition. The chemical decomposition of an explosive may take years, days, hours, or a fraction of a second. The slower forms of decomposition take place in storage and are of interest only from a stability standpoint. Of more interest are the two rapid forms of decomposition, burning and detonation. The term "detonation" is used to describe an explosive phenomenon whereby the decomposition is propagated by the explosive shockwave penetrating the explosive material. The shockwave front is capable of passing through the high explosive material at massive speeds. Explosive force is released at 90 degree angles from the surface of an explosive. If the surface is cut or shaped the explosive forces can be focused directionally, and will produce a greater effect. This is known as a shaped charge. In a low explosive, the decomposition is propagated by a flame front which travels much slower through the explosive material. The properties of the explosive indicate the class into which it falls. In some cases explosives may be made to fall into either class by the conditions under which they are initiated. Almost all low explosives can undergo true detonation like high explosives in sufficiently massive quantities. For convenience, low and high explosives may be differentiated by the shipping and storage classes.

Explosive compatibility groupings

differentiated Shipping tags will include a UN or US DOT hazardous material class with compatibility letter as follows.
- 1.1 Mass Explosion Hazard
- 1.2 Nonmass explosion, fragment-producing
- 1.3 Mass fire, minor blast or fragment hazard
- 1.4 Moderate fire, no blast or fragment: consumer fireworks are 1.4G or 1.4S
- 1.5 Explosive substance, very insensitive (with a mass explosion hazard)
- 1.6 Explosive article, extremely insensitive A Primary explosive substance (1.1A, 1.2A) B An article containing a primary explosive substance and not containing two or more effective protective features. Some articles, such as detonator assemblies for blasting and primers, cap-type, are included. (1.1B, 1.2B, 1.4B) C Propellant explosive substance or other deflagrating explosive substance or article containing such explosive substance (1.1C, 1.2C, 1.3C, 1.4C) D Secondary detonating explosive substance or black powder or article containing a secondary detonating explosive substance, in each case without means of initiation and without a propelling charge, or article containing a primary explosive substance and containing two or more effective protective features. (1.1D, 1.2D, 1.4D, 1.5D) E Article containing a secondary detonating explosive substance without means of initiation, with a propelling charge (other than one containing flammable liquid, gel or hypergolic liquid) (1.1E, 1.2E, 1.4E) F Article containing a secondary detonating explosive substance with its means of initiation, with a propelling charge (other than one containing flammable liquid, gel or hypergolic liquid) or without a propelling charge (1.1F, 1.2F, 1.3F, 1.4F) G Pyrotechnic substance or article containing a pyrotechnic substance, or article containing both an explosive substance and an illuminating, incendiary, tear-producing or smoke-producing substance (other than a water-activated article or one containing white phosphorus, phosphide or flammable liquid or gel or hypergolic liquid) (1.1G, 1.2G, 1.3G, 1.4G) H Article containing both an explosive substance and white phosphorus (1.2H, 1.3H) J Article containing both an explosive substance and flammable liquid or gel (1.1J, 1.2J, 1.3J) K Article containing both an explosive substance and a toxic chemical agent (1.2K, 1.3K) L Explosive substance or article containing an explosive substance and presenting a special risk (e.g., due to water-activation or presence of hypergolic liquids, phosphides or pyrophoric substances) needing isolation of each type (1.1L, 1.2L, 1.3L) N Articles containing only extremely insensitive detonating substances (1.6N) S Substance or article so packed or designed that any hazardous effects arising from accidental functioning are limited to the extent that they do not significantly hinder or prohibit fire fighting or other emergency response efforts in the immediate vicinity of the package (1.4S)

Low Explosives

Low explosives are normally employed as propellants. Most low explosives are mixtures; most high explosives are compounds, but to both there are notable exceptions. They undergo deflagration at rates that vary from a few centimeters per second to approximately 400 meters per second. Included in this group are smokeless powders, and pyrotechnics such as flares and illumination devices.

High Explosives

High explosives are normally employed in mining, demolitions and military warheads. They undergo detonation at rates of 1,000 to 8,500 meters per second. High explosives are conventionally subdivided into two classes and differentiated by sensitivity:
- Primary explosives are extremely sensitive to shock, friction, and heat. They will burn rapidly or detonate if ignited.
- Secondary or Base explosives are relatively insensitive to shock, friction, and heat. They may burn when ignited in small, unconfined quantities, but detonation can occur. These are sometimes added in small amount to blasting caps to boost their power. Dynamite, RDX, PETN, HMX, and others are secondary explosives. Some definitions add a third category:
- Tertiary, also called blasting agents. These are so insensitive to shock that they cannot be detonated by practical quantities of primary explosive, and instead require an intermediate explosive booster of secondary explosive. Some examples would be an Ammonium Nitrate/Fuel Oil mixture commonly known as ANFO and slurry or 'Wet Bag' explosives. These are primarily used in large scale mining and construction operations. Note that many if not most explosive chemical compounds may usefully deflagrate as well as detonate, and are used in high as well as low explosive compositions. This also means that under extreme conditions, propellant can detonate. For example, nitrocellulose deflagrates if ignited, but detonates if initiated by a detonator.

Detonation of an Explosive Charge

Also called an initiation sequence or a firing train, this is the sequence of events which cascade from relatively low levels of energy to cause a chain reaction to initiate the final explosive material or main charge. They can be either low or high explosive trains. Low explosive trains are something like a bullet - Primer and a propellant charge. High explosives trains can be more complex, either Two-Step (e.g. Detonator and Dynamite) or Three-Step (e.g. Detonator, Booster and ANFO). Detonators are often made from tetryl and Fulminates.

Composition of the material

Mixtures of an oxidizer and a fuel
- Black powder: potassium nitrate, charcoal and sulfur
- Flash powder: fine metal powder (usually aluminium or magnesium) and a strong oxidizer (e.g. potassium chlorate or perchlorate).
- Ammonal: ammonium nitrate and aluminium powder.
- Armstrong's mixture: potassium chlorate and red phosphorus. This is a very sensitive mixture. It is a primary high explosive in which sulfur is substitute for some or all phosphorus to slightly decrease sensitivity.
- Sprengel explosives: a very general class incorporating any strong oxidizer and highly reactive fuel, although in practice the name most commonly was applied to mixtures of chlorates and nitroaromatics
- ANFO: ammonium nitrate and fuel oil.
- Cheddites: chlorates or perchlorates and oil
- oxyliquits: mixtures of organic materials and liquid oxygen Chemically pure compounds
- Nitroglycerin: an unstable liquid known as dynamite when mixed into sawdust, powdered silica or most commonly diatomaceous earth, which act as stabilizers.
- Acetone peroxide: A very unstable white organic peroxide
- TNT: Yellow insensitive crystals that can be melted and molded without detonation.
- Nitrocellulose: A variantly nitrated polymer which can be a high or low explosive depending on nitration level and conditions.
- RDX, PETN: Very strong explosives which can be used pure or in plastic explosives.
- C4: An RDX plastic explosive plasticized to be adhesive and malleable.

Chemical explosive reaction

A chemical explosive is a compound or mixture which, upon the application of heat or shock, decomposes or rearranges with extreme rapidity, yielding much gas and heat. Many substances not ordinarily classed as explosives may do one, or even two, of these things. For example, a mixture of nitrogen and oxygen can be made to react with great rapidity and yield the gaseous product nitric oxide; yet the mixture is not an explosive since it does not evolve heat, but rather absorbs heat. :N₂ + O₂ → 2NO - 43,200 calories (or 180 kJ) per mole of N₂ For a chemical to be an explosive, it must exhibit all of the following:
- Exhibit Rapid Expansion (eg. rapid production of gasses or rapid heating of surroundings)
- Evolution of heat
- Rapidity of reaction
- Initiation of reaction

Formation of gases

Gases may be evolved from substances in a variety of ways. When wood or coal is burned in the atmosphere, the carbon and hydrogen in the fuel combine with the oxygen in the atmosphere to form carbon dioxide and steam, together with flame and smoke. When the wood or coal is pulverized, so that the total surface in contact with the oxygen is increased, and burned in a furnace or forge where more air can be supplied, the burning can be made more rapid and the combustion more complete. When the wood or coal is immersed in liquid oxygen or suspended in air in the form of dust, the burning takes place with explosive violence. In each case, the same action occurs: a burning combustible forms a gas.

Evolution of heat

The generation of heat in large quantities accompanies every explosive chemical reaction. It is this rapid liberation of heat that causes the gaseous products of reaction to expand and generate high pressures. This rapid generation of high pressures of the released gas constitutes the explosion. It should be noted that the liberation of heat with insufficient rapidity will not cause an explosion. For example, although a pound of coal yields five times as much heat as a pound of nitroglycerin, the coal cannot be used as an explosive because the rate at which it yields this heat is quite slow.

Rapidity of reaction

Rapidity of reaction distinguishes the explosive reaction from an ordinary combustion reaction by the great speed with which it takes place. Unless the reaction occurs rapidly, the thermally expanded gases will be dissipated in the medium, and there will be no explosion. Again, consider a wood or coal fire. As the fire burns, there is the evolution of heat and the formation of gases, but neither is liberated rapidly enough to cause an explosion. For those who know something about electronics, this can be likened to the energy discharge of a battery, which is slow; to a flash capacitor, like that in a camera flash and releases its energy all at once.

Initiation of reaction

A reaction must be capable of being initiated by the application of shock or heat to a small portion of the mass of the explosive material. A material in which the first three factors exist cannot be accepted as an explosive unless the reaction can be made to occur when desired.

Military explosives

To determine the suitability of an explosive substance for military use, its physical properties must first be investigated. The usefulness of a military explosive can only be appreciated when these properties and the factors affecting them are fully understood. Many explosives have been studied in past years to determine their suitability for military use and most have been found wanting. Several of those found acceptable have displayed certain characteristics that are considered undesirable and, therefore, limit their usefulness in military applications. The requirements of a military explosive are stringent, and very few explosives display all of the characteristics necessary to make them acceptable for military standardization. Some of the more important characteristics are discussed below:

Availability and cost

In view of the enormous quantity demands of modern warfare, explosives must be produced from cheap raw materials that are nonstrategic and available in great quantity. In addition, manufacturing operations must be reasonably simple, cheap, and safe.

Sensitivity

Regarding an explosive, this refers to the ease with which it can be ignited or detonated—i.e., the amount and intensity of shock, friction, or heat that is required. When the term sensitivity is used, care must be taken to clarify what kind of sensitivity is under discussion. The relative sensitivity of a given explosive to impact may vary greatly from its sensitivity to friction or heat. Some of the test methods used to determine sensitivity are as follows:
- Impact Sensitivity is expressed in terms of the distance through which a standard weight must be dropped to cause the material to explode.
- Friction Sensitivity is expressed in terms of what occurs when a weighted pendulum scrapes across the material (snaps, crackles, ignites, and/or explodes).
- Heat Sensitivity is expressed in terms of the temperature at which flashing or explosion of the material occurs. Sensitivity is an important consideration in selecting an explosive for a particular purpose. The explosive in an armor-piercing projectile must be relatively insensitive, or the shock of impact would cause it to detonate before it penetrated to the point desired.

Stability

Stability is the ability of an explosive to be stored without deterioration. The following factors affect the stability of an explosive:
- Chemical constitution. The very fact that some common chemical compounds can undergo explosion when heated indicates that there is something unstable in their structures. While no precise explanation has been developed for this, it is generally recognized that certain groups, nitro dioxide (NO₂), nitrate (NO₃), and azide (N₃), are intrinsically in a condition of internal strain. Increased strain through heating can cause a sudden disruption of the molecule and consequent explosion. In some cases, this condition of molecular instability is so great that decomposition takes place at ordinary temperatures.
- Temperature of storage. The rate of decomposition of explosives increases at higher temperatures. All of the standard military explosives may be considered to be of a high order of stability at temperatures of -10 to +35 °C, but each has a high temperature at which the rate of decomposition becomes rapidly accelerated and stability is reduced. As a rule of thumb, most explosives become dangerously unstable at temperatures exceeding 70 °C.
- Exposure to sun. If exposed to the ultraviolet rays of the sun, many explosive compounds that contain nitrogen groups will rapidly decompose, affecting their stability.
- Electrical discharge. Electrostatic or spark sensitivity to initiation is common to a number of explosives. Static or other electrical discharge may be sufficient to inspire detonation under some circumstances. As a result, the safe handling of explosives and pyrotechnics almost always requires electrical grounding of the operator.

Power

The term power (or more properly, performance) as it is applied to an explosive refers to its ability to do work. In practice it is defined as its ability to accomplish what is intended in the way of energy delivery (i.e., fragments, air blast, high-velocity jets, underwater bubble energy, etc.). Explosive power or performance is evaluated by a tailored series of tests to assess the material for its intended use. Of the tests listed below, cylinder expansion and air-blast tests are common to most testing programs, and the others support specific uses.
- Cylinder expansion test. A standard amount of explosive is loaded in a cylinder usually manufactured of copper. Data is collected concerning the rate of radial expansion of the cylinder and maximum cylinder wall velocity. This also establishes the Gurney constant or 2E.
- Cylinder fragmentation test. A standard steel cylinder is charged with explosive and fired in a sawdust pit. The fragments are collected and the size distribution analyzed.
- Detonation pressure (Chapman-Jouget). Detonation pressure data derived from measurements of shock waves transmitted into water by the detonation of cylindrical explosive charges of a standard size.
- Determination of critical diameter. This test establishes the minimum physical size a charge of a specific explosive must be to sustain its own detonation wave. The procedure involves the detonation of a series of charges of different diameters until difficulty in detonation wave propagation is observed.
- Infinity diameter detonation velocity. Detonation velocity is dependent on landing density (c), charge diameter, and grain size. The hydrodynamic theory of detonation used in predicting explosive phenomena does not include diameter of the charge, and therefore a detonation velocity, for an imaginary charge of infinite diameter. This procedure requires a series of charges of the same density and physical structure, but different diameters, to be fired and the resulting detonation velocities extrapolated to predict the detonation velocity of a charge of infinite diameter.
- Pressure versus scaled distance. A charge of specific size is detonated and its pressure effects measured at a standard distance. The values obtained are compared with that for TNT.
- Impulse versus scaled distance. A charge of specific size is detonated and its impulse (the area under the pressure-time curve) measured versus distance. The results are tabulated and expressed in TNT equivalent.
- Relative bubble energy (RBE). A 5 to 50 kg charge is detonated in water and piezoelectric gauges are used to measure peak pressure, time constant, impulse, and energy. ::The RBE may be defined as K_x 3 ::RBE = K_s ::where K = bubble expansion period for experimental (x) or standard (s) charge.

Brisance

In addition to strength, explosives display a second characteristic, which is their shattering effect or brisance (from the French meaning to "break"), which is distinguished from their total work capacity. This characteristic is of practical importance in determining the effectiveness of an explosion in fragmenting shells, bomb casings, grenades, and the like. The rapidity with which an explosive reaches its peak pressure is a measure of its brisance. Brisance values are primarily employed in France and Russia. The sand crush test is commonly employed to determine the relative brisance in comparison to TNT. No single test is capable of directly comparing the explosive properties of two or more compounds; it is important to examine the data from several such tests (sand crush, trauzl, and so forth) in order to gauge relative brisance. True values for comparison will require field experiments.

Density

Density of loading refers to the unit weight of an explosive per unit volume. Several methods of loading are available, and the one used is determined by the characteristics of the explosive. The methods available include pellet loading, cast loading, or press loading. Dependent upon the method employed, an average density of the loaded charge can be obtained that is within 80-95% of the theoretical maximum density of the explosive. High load density can reduce sensitivity by making the mass more resistant to internal friction. If density is increased to the extent that individual crystals are crushed, the explosive will become more sensitive. Increased load density also permits the use of more explosive, thereby increasing the strength of the warhead.

Volatility

Volatility, or the readiness with which a substance vaporizes, is an undesirable characteristic in military explosives. Explosives must be no more than slightly volatile at the temperature at which they are loaded or at their highest storage temperature. Excessive volatility often results in the development of pressure within rounds of ammunition and separation of mixtures into their constituents. Stability, as mentioned before, is the ability of an explosive to stand up under storage conditions without deteriorating. Volatility affects the chemical composition of the explosive such that a marked reduction in stability may occur, which results in an increase in the danger of handling. Maximum allowable volatility is 2 ml of gas evolved in 48 hours.

Hygroscopicity

The introduction of moisture into an explosive is highly undesirable since it reduces the sensitivity, strength, and velocity of detonation of the explosive. Hygroscopicity is used as a measure of a material's moisture-absorbing tendencies. Moisture affects explosives adversely by acting as an inert material that absorbs heat when vaporized, and by acting as a solvent medium that can cause undesired chemical reactions. Sensitivity, strength, and velocity of detonation are reduced by inert materials that reduce the continuity of the explosive mass. When the moisture content evaporates during detonation, cooling occurs, which reduces the temperature of reaction. Stability is also affected by the presence of moisture since moisture promotes decomposition of the explosive and, in addition, causes corrosion of the explosive's metal container. For all of these reasons, hygroscopicity must be negligible in military explosives.

Toxicity

Due to their chemical structure, most explosives are toxic to some extent. Since the effect of toxicity may vary from a mild headache to serious damage of internal organs, care must be taken to limit toxicity in military explosives to a minimum. Any explosive of high toxicity is unacceptable for military use.

Measurement of chemical explosive reaction

The development of new and improved types of ammunition requires a continuous program of research and development. Adoption of an explosive for a particular use is based upon both proving ground and service tests. Before these tests, however, preliminary estimates of the characteristics of the explosive are made. The principles of thermochemistry are applied for this process. Thermochemistry is concerned with the changes in internal energy, principally as heat, in chemical reactions. An explosion consists of a series of reactions, highly exothermic, involving decomposition of the ingredients and recombination to form the products of explosion. Energy changes in explosive reactions are calculated either from known chemical laws or by analysis of the products. For most common reactions, tables based on previous investigations permit rapid calculation of energy changes. Products of an explosive remaining in a closed calorimetric bomb (a constant-volume explosion) after cooling the bomb back to room temperature and pressure are rarely those present at the instant of maximum temperature and pressure. Since only the final products may be analyzed conveniently, indirect or theoretical methods are often used to determine the maximum temperature and pressure values. Some of the important characteristics of an explosive that can be determined by such theoretical computations are:
- Oxygen balance
- Heat of explosion or reaction
- Volume of products of explosion
- Potential of the explosive

Oxygen balance (OB%)

Oxygen balance is an expression that is used to indicate the degree to which an explosive can be oxidized. If an explosive molecule contains just enough oxygen to convert all of its carbon to carbon dioxide, all of its hydrogen to water, and all of its metal to metal oxide with no excess, the molecule is said to have a zero oxygen balance. The molecule is said to have a positive oxygen balance if it contains more oxygen than is needed and a negative oxygen balance if it contains less oxygen than is needed. The sensitivity, strength, and brisance of an explosive are all somewhat dependent upon oxygen balance and tend to approach their maximums as oxygen balance approaches zero. The oxygen balance (OB) is calculated from the empirical formula of a compound in percentage of oxygen required for complete conversion of carbon to carbon dioxide, hydrogen to water, and metal to metal oxide. The procedure for calculating oxygen balance in terms of 100 grams of the explosive material is to determine the number of moles of oxygen that are excess or deficient for 100 grams of a compound. :

OB% = \frac \times (2X + (Y/2) + M - Z)

where X = number of atoms of carbon, Y = number of atoms of hydrogen, Z = number of atoms of oxygen, and M = number of atoms of metal (metallic oxide produced). In the case of TNT (C₆H₂(NO₂)₃CH₃), Molecular weight = 227.1 X = 7 (number of carbon atoms) Y = 5 (number of hydrogen atoms) Z = 6 (number of oxygen atoms) Therefore :

OB% = \frac \times (14 + 2.5 - 6)

:OB% = -74% for TNT Because sensitivity, brisance, and strength are properties resulting from a complex explosive chemical reaction, a simple relationship such as oxygen balance cannot be depended upon to yield universally consistent results. When using oxygen balance to predict properties of one explosive relative to another, it is to be expected that one with an oxygen balance closer to zero will be the more brisant, powerful, and sensitive; however, many exceptions to this rule do exist. More complicated predictive calculations, such as those discussed in the next section, result in more accurate predictions. One area in which oxygen balance can be applied is in the processing of mixtures of explosives. The family of explosives called amatols are mixtures of ammonium nitrate and TNT. Ammonium nitrate has an oxygen balance of +20% and TNT has an oxygen balance of −74%, so it would appear that the mixture yielding an oxygen balance of zero would also result in the best explosive properties. In actual practice a mixture of 80% ammonium nitrate and 20% TNT by weight yields an oxygen balance of +1%, the best properties of all mixtures, and an increase in strength of 30% over TNT.

Heat of explosion

When a chemical compound is formed from its constituents, the reaction may either absorb or give off heat. The quantity of heat absorbed or given off during transformation is called the heat of formation. The heats of formations for solids and gases found in explosive reactions have been determined for a temperature of 15 °C and atmospheric pressure, and are normally tabulated in units of kilocalories per gram molecule. (See table 12-1). Where a negative value is given, it indicates that heat is absorbed during the formation of the compound from its elements. Such a reaction is called an endothermic reaction. The convention usually employed in simple thermochemical calculations is arbitrarily to take heat contents of all elements as zero in their standard states at all temperatures (standard state being defined as the state at which the elements are found under natural or ambient conditions). Since the heat of formation of a compound is the net difference between the heat content of the compound and that of its elements, and since the latter are taken as zero by convention, it follows that the heat content of a compound is equal to its heat of formation in such nonrigorous calculations. This leads us to the principle of initial and final state, which may be expressed as follows: "The net quantity of heat liberated or absorbed in any chemical modification of a system depends solely upon the initial and final states of the system, provided the transformation takes place at constant volume or at constant pressure. It is completely independent of the intermediate transformations and of the time required for the reactions." From this it follows that the heat liberated in any transformation accomplished through successive reactions is the algebraic sum of the heats liberated or absorbed in the different reactions. Consider the formation of the original explosive from its elements as an intermediate reaction in the formation of the products of explosion. The net amount of heat liberated during an explosion is the sum of the heats of formation of the products of explosion, minus the heat of formation of the original explosive. The net heat difference between heats of formations of the reactants and products in a chemical reaction is termed the heat of reaction. For oxidation this heat of reaction may be termed heat of combustion. In explosive technology only materials that are exothermic — that is, have a heat of reaction that causes net liberation of heat — are of interest. Hence, in this text, heats of reaction are virtually all positive. Reaction heat is measured under conditions either of constant pressure or constant volume. It is this heat of reaction that may be properly expressed as "heat of the explosion."

Balancing chemical explosion equations

In order to assist in balancing chemical equations, an order of priorities is presented in table 12-2. Explosives containing C, H, O, and N and/or a metal will form the products of reaction in the priority sequence shown. Some observation you might want to make as you balance an equation:
- The progression is from top to bottom; you may skip steps that are not applicable, but you never back up.
- At each separate step there are never more than two compositions and two products.
- At the conclusion of the balancing, elemental forms, nitrogen, oxygen, and hydrogen, are always found in diatomic form. Example, TNT: :C₆H₂(NO₂)₃CH₃; constituents: 7C + 5H + 3N + 6O Using the order of priorities in table 12-1, priority 4 gives the first reaction products: :7C + 6O → 6CO with one mol of carbon remaining Next, since all the oxygen has been combined with the carbon to form CO, priority 7 results in: :3N → 1.5N₂ Finally, priority 9 results in: 5H → 2.5H₂ The balanced equation, showing the products of reaction resulting from the detonation of TNT is: :C₆H₂(NO₂)₃CH₃ → 6CO + 2.5H₂ + 1.5N₂ + C Notice that partial moles are permitted in these calculations. The number of moles of gas formed is 10. The product, carbon, is a solid.

Volume of products of explosion

The law of Avogadro states that equal volumes of all gases under the same conditions of temperature and pressure contain the same number of molecules. From this law, it follows that the molar volume of one gas is equal to the molar volume of any other gas. The molar volume of any gas at 0 °C and under normal atmospheric pressure is very nearly 22.4 liters or 22.4 cubic decimeters. Thus, considering the nitroglycerin reaction. :C₃H₅(NO₃)₃ → 3CO₂ + 2.5H₂O + 1.5N₂ + 0.25O₂ the explosion of one mole of nitroglycerin produces in the gaseous state: 3 moles of CO₂; 2.5 moles of O₂. Since a molar volume is the volume of one mole of gas, one mole of nitroglycerin produces 3 + 2.5 + 1.5 + 0.25 = 7.25 molar volumes of gas; and these molar volumes at 0 °C and atmospheric pressure form an actual volume of 7.25 × 22.4 = 162.4 liters of gas. (Note that the products H₂O and CO₂ are in their gaseous form.) Based upon this simple beginning, it can be seen that the volume of the products of explosion can be predicted for any quantity of the explosive. Further, by employing Charles' Law for perfect gases, the volume of the products of explosion may also be calculated for any given temperature. This law states that at a constant pressure a perfect gas expands 1/273.15 of its volume at 0 °C, for each degree Celsius of rise in temperature. Therefore, at 15 °C the molar volume of an ideal gas is, :V₁₅ = 22.414 (288.15/273.15) = 23.64 liters per mole Thus, at 15 °C the volume of gas produced by the explosive decomposition of one mole of nitroglycerin becomes :V = (23.64 l/mol)(7.25 mol) = 171.4 l

Explosive strength

The potential of an explosive is the total work that can be performed by the gas resulting from its explosion, when expanded adiabatically from its original volume, until its pressure is reduced to atmospheric pressure and its temperature to 15 °C. The potential is therefore the total quantity of heat given off at constant volume when expressed in equivalent work units and is a measure of the strength of the explosive. An explosion may occur under two general conditions: the first, unconfined, as in the open air where the pressure (atmospheric) is constant; the second, confined, as in a closed chamber where the volume is constant. The same amount of heat energy is liberated in each case, but in the unconfined explosion, a certain amount is used as work energy in pushing back the surrounding air, and therefore is lost as heat. In a confined explosion, where the explosive volume is small (such as occurs in the powder chamber of a firearm), practically all the heat of explosion is conserved as useful energy. If the quantity of heat liberated at constant volume under adiabatic conditions is calculated and converted from heat units to equivalent work units, the potential or capacity for work results. Therefore, if Q_mp represents the total quantity of heat given off by a mole of explosive of 15 °C and constant pressure (atmospheric); Q_mv represents the total heat given off by a mole of explosive at 15 °C and constant volume; and W represents the work energy expended in pushing back the surrounding air in an unconfined explosion and thus is not available as net theoretical heat; Then, because of the conversion of energy to work in the constant pressure case, :Q_mv = Q_mp + W from which the value of Q_mv may be determined. Subsequently, the potential of a mole of an explosive may be calculated. Using this value, the potential for any other weight of explosive may be determined by simple proportion. Using the principle of the initial and final state, and heat of formation table (resulting from experimental data), the heat released at constant pressure may be readily calculated. m n Q_mp = v_iQ_fi - v_kQ_fk 1 1 where: Q_fi = heat of formation of product i at constant pressure Q_fk = heat of formation of reactant k at constant pressure v = number of moles of each product/reactants (m is the number of products and n the number of reactants) The work energy expended by the gaseous products of detonation is expressed by: :W = P dv With pressure constant and negligible initial volume, this expression reduces to: :W = P·V₂ Since heats of formation are calculated for standard atmospheric pressure (101 325 Pa, where 1 Pa = 1 N/m²) and 15 °C, V₂ is the volume occupied by the product gases under these conditions. At this point W/mol = (101 325 N/m²)(23.63 L/mol)(1 m³/1000 L) = 2394 N·m/mol = 2394 J/mol and by applying the appropriate conversion factors, work can be converted to units of kilocalories. W/mol = 0.572 kcal/mol Once the chemical reaction has been balanced, one can calculate the volume of gas produced and the work of expansion. With this completed, the calculations necessary to determine potential may be accomplished. For TNT: :C₆H₂(NO₂)₃CH₃ → 6CO + 2.5H₂ + 1.5N₂ + C for 10 mol Then: :Q_mp = 6(26.43) - 16.5 = 142.08 kcal/mol Note: Elements in their natural state (H₂, O₂, N₂, C, etc.) are used as the basis for heat of formation tables and are assigned a value of zero. See table 12-2. :Q_mv = 142.08 + 0.572(10) = 147.8 kcal/mol As previously stated, Q_mv converted to equivalent work units is the potential of the explosive. (MW = Molecular Weight of Explosive) Potential = Q_mv kcal/mol × 4185 J/kcal × 10³ g/kg × 1 mol/(mol·g) Potential = Q_mv (4.185 × 10⁶) J/(mol·kg) For TNT, Potential = 147.8 (4.185 × 10⁶)/227.1 = 2.72 × 10⁶ J/kg Rather than tabulate such large numbers, in the field of explosives, TNT is taken as the standard explosive, and others are assigned strengths relative to that of TNT. The potential of TNT has been calculated above to be 2.72 × 10⁶ J/kg. Relative strength (RS) may be expressed as :R.S. = Potential of Explosive/(2.72 × 10⁶)

Example of thermochemical calculations

The PETN reaction will be examined as an example of thermo-chemical calculations. :PETN: C(CH₂ONO₂)₄ :Molecular weight = 316.15 g/mol :Heat of formation = 119.4 kcal/mol (1) Balance the chemical reaction equation. Using table 12-1, priority 4 gives the first reaction products: :5C + 12O → 5CO + 7O Next, the hydrogen combines with remaining oxygen: :8H + 7O → 4H₂O + 3O Then the remaining oxygen will combine with the CO to form CO and CO₂. :5CO + 3O → 2CO + 3CO₂ Finally the remaining nitrogen forms in its natural state (N₂). :4N → 2N₂ The balanced reaction equation is: :C(CH₂ONO₂)₄ → 2CO + 4H₂O + 3CO₂ + 2N₂ (2) Determine the number of molar volumes of gas per mole. Since the molar volume of one gas is equal to the molar volume of any other gas, and since all the products of the PETN reaction are gaseous, the resulting number of molar volumes of gas (N_m) is: :N_m = 2 + 4 + 3 + 2 = 11 V_molar/mol (3) Determine the potential (capacity for doing work). If the total heat liberated by an explosive under constant volume conditions (Q_m) is converted to the equivalent work units, the result is the potential of that explosive. The heat liberated at constant volume (Q_mv) is equivalent to the liberated at constant pressure (Q_mp) plus that heat converted to work in expanding the surrounding medium. Hence, Q_mv = Q_mp + work (converted). :a. Q_mp = Q_fi (products) - Q_fk (reactants) ::where: Q_f = heat of formation (see table 12-2) ::For the PETN reaction: :::Q_mp = 2(26.343) + 4(57.81) + 3(94.39) - (119.4) = 447.87 kcal/mol ::(If the compound produced a metallic oxide, that heat of formation would be included in Q_mp. :b. Work = 0.572N_m = 0.572(11) = 6.292 kcal/mol :As previously stated, Q_mv converted to equivalent work units is taken as the potential of the explosive. :c. Potential J = Q_mv (4.185 × 10⁶ kg)(MW) = 454.16 (4.185 × 10⁶) 316.15 = 6.01 × 10⁶ J kg :This product may then be used to find the relative strength (RS) of PETN, which is :d. RS = Pot (PETN) = 6.01 × 10⁶ = 2.21 Pot (TNT) 2.72 × 10⁶

External links

- [http://www.blasterexchange.com Blaster Exchange - Explosives Industry Portal]
- [http://www.fas.org/man/dod-101/navy/docs/fun/part12.htm Military Explosives]
- [http://globalsecurity.org/military/systems/munitions/explosives-class.htm UN hazard classification code]
- [http://environmentalchemistry.com/yogi/hazmat/placards/class1.html Class 1 Hazmat Placards]

References

- Army Research Office. Elements of Armament Engineering (Part One). Washington, D.C.: U.S. Army Material Command, 1964.
- Commander, Naval Ordnance Systems Command. Safety and Performance Tests for Qualification of Explosives. NAVORD OD 44811. Washington, D.C.: GPO, 1972.
- Commander, Naval Ordnance Systems Command. Weapons Systems Fundamentals. NAVORD OP 3000, vol. 2, 1st rev. Washington, D.C.: GPO, 1971.
- Departments of the Army and Air Force. Military Explosives. Washington, D.C.: 1967.
- USDOT Hazardous Materials Transportation Placards Category:Explosives ja:火薬

Chemical compound

A chemical compound is a chemical substance formed from two or more elements, with a fixed ratio determining the composition. For example, dihydrogen monoxide (water, ₂) is a compound composed of two hydrogen atoms for every oxygen atom. In general, this fixed ratio must be fixed due to some sort of physical property, rather than an arbitrary man-made selection. This is why materials such as brass, the superconductor YBCO, the semiconductor aluminium gallium arsenide, or chocolate are considered mixtures or alloys rather than compounds. A defining characteristic of a compound is that it has a chemical formula. Formulas describe the ratio of atoms in a substance, and the number of atoms in a single molecule of the substance (thus the formula for ethene is ₂₄ rather than ₂). The formula does not indicate that a compound is composed of molecules; for example, sodium chloride (table salt, ) is an ionic compound. Compounds may have a number of possible phases. Most compounds can exist as solids. Molecular compounds may also exist as liquids or gases. All compounds will decompose to smaller compounds or individual atoms if heated to a certain temperature (called the decomposition temperature). Every chemical compound that has been described in the literature carries a unique numerical identifier, its CAS number.

Types of compounds

- Acids
- Bases
- Ionic compounds
- Salts
- Oxides
- Organic compounds

Nuclear explosive

A nuclear explosive is an explosive device that derives its energy from nuclear reactions. Almost all nuclear explosive devices that have been designed and produced, and the two that have actually been used, are nuclear weapons intended for warfare; see that article for more detail. Other, non-warfare, applications for nuclear bombs have occasionally been proposed. For example, nuclear pulse propulsion is a form of spacecraft propulsion that would use nuclear bombs to provide impulse to a spacecraft. A similar application is the proposal to use nuclear bombs for asteroid deflection. From 1958 to 1965 The U. S government ran a project to design a nuclear bomb powered nuclear pulse rocket called Project Orion. Never built, this vessel would use repeated nuclear explosions to propel itself and was considered surprisingly practical. It is thought to be a feasible design for interstellar travel. On Earth, nuclear explosives were once considered for use in large-scale excavation. A nuclear explosion could be used to create a harbor, or a mountain pass, or possibly large underground cavities for use as storage space. It was thought that detonating a nuclear bomb in oil-rich rock could make it possible to extract more from the deposit. From 1958 to 1973 the U. S government exploded 28 nuclear test-shots in a project called the Operation Plowshare. The purpose of the Operation Plowshare was to use peaceful nuclear explosions for moving and lifting enormous amounts of earth and rock during construction projects such as building reservoirs. The Soviet Union conducted a much more vigorous program of 122 nuclear tests, some with multiple devices, between 1965 and 1989 under the auspices of Program No. 7-Nuclear Explosions for the National Economy. As controlled nuclear fusion has proven difficult to use as an energy source, an alternate proposal for producing fusion power has been to detonate fusion bombs inside very large underground chambers and then using the heat produced, which would be absorbed by a molten salt coolant which would also absorb neutrons. See the PACER project for more details. Failure to meet objectives, along with the realization of the dangers of nuclear fallout and other residual radioactivity, and with the enactment of various agreements such as the Partial Test Ban Treaty and the Outer Space Treaty, has lead to the temination of most of these programs.

External links

- [http://www.nuclearweaponarchive.org/Nwfaq/Nfaq0.html Nuclear Weapons Frequently Asked Questions] Category:Nuclear physics Category:Explosives

Laser

glass lasers (bottom) used for inertial confinement fusion.]] A LASER (Light Amplification by Stimulated Emission of Radiation) is an optical source that emits photons in a coherent beam. Laser light is typically near-monochromatic, i.e. consisting of a single wavelength or hue, and emitted in a narrow beam. This is in contrast to common light sources, such as the incandescent light bulb, which emit incoherent photons in almost all directions, usually over a wide spectrum of wavelengths. Laser action is understood by application of quantum mechanics and thermodynamics theory (see laser science). The verb "to lase" means "to produce coherent light" or possibly "to cut or otherwise treat with coherent light", and is a back-formation of the term laser.

Physics

back-formation A laser is composed of an active laser medium and a resonant optical cavity. The gain medium is a material of controlled purity, size, and shape, which uses a quantum mechanical effect called stimulated emission (discovered by Einstein while researching the photoelectric effect) to amplify the beam. For a laser to operate, the gain medium must be "pumped" by an external energy source, such as electricity or light (from a classical source such as a flash lamp, or another laser). The pump energy is absorbed by the laser medium to produce excited states in the medium. When the number of particles in one excited state exceeds the number of particles in some lower state, population inversion is achieved. In this condition, an optical beam passing through the medium produces more stimulated emission than stimulated absorption so the beam is amplified. An excited laser medium can also function as an optical amplifier. The light generated by stimulated emission is very similar to the input signal in terms of wavelength, phase, and polarization. This gives laser light its characteristic coherence, and allows it to maintain the uniform polarization and monochromaticity established by the optical cavity design. The resonant cavity (see also cavity resonator) contains a coherent beam of light between reflective surfaces so that each photon passes through the gain medium multiple times before being emitted from the output aperture or lost to diffraction or absorption. As light circulates through the cavity, passing through the gain medium, if the gain (amplification) in the medium is stronger than the resonator losses, the power of the circulating light can rise exponentially. However, each stimulated emission event returns a particle from its excited state to the ground state, reducing the capacity of the gain medium for further amplification. When this effect becomes strong, the gain is said to be saturated. The balance of pump power against gain saturation and cavity losses produces an equilibrium value of the intracavity laser power which determines the operating point of the laser. If the pump power is chosen too small (below the "laser threshold"), the gain is not sufficient to overcome the resonator losses, and the laser will emit only very small light powers. The beam in the cavity and the output beam of the laser, if they occur in free space rather than waveguides (as in an optical fiber laser), are often Gaussian beams. If the beam is not a pure Gaussian shape, the transverse modes of the beam may be analyzed as a superposition of Hermite-Gaussian or Laguerre-Gaussian beams. The beam often has a very small divergence (highly collimated), but a perfectly collimated beam cannot be created, due to the effect of diffraction. Nonetheless, a laser beam will spread much less than a beam of incoherent light. The distance over which the beam remains collimated increases with the square of the beam diameter, and the angle at which the beam eventually diverges varies inversely with the diameter. Thus, a beam generated by a small laboratory laser such as a helium-neon (HeNe) laser spreads to approximately 1 mile (1.6 kilometres) in diameter if shone from the Earth's surface to the Moon. By comparison, the output of a typical semiconductor laser, due to its small diameter, diverges almost immediately on exiting the aperture, at an angle that may be as high as 50°. However, such a divergent beam can be transformed into a collimated beam by means of a lens. In contrast, the light from non-laser light sources cannot be collimated by optics as well or much. lens at Univ. Paris 6. The glowing ray in the middle is an electric discharge producing light in much the same way as a neon light; though it is the gain medium through which the laser passes, it is not the laser beam itself which is visible there. The laser beam crosses the air and marks a red point on the screen to the right.]] The output of a laser may be a continuous, constant-amplitude output (known as CW or continuous wave), or pulsed, by using the techniques of Q-switching, modelocking, or gain-switching. In pulsed operation, much higher peak powers can be achieved. Some types of lasers, such as dye lasers and vibronic solid-state lasers can produce light over a broad range of wavelengths; this property makes them suitable for the generation of extremely short pulses of light, on the order of a femtosecond (10^-15 seconds). It should be understood that the word light in the acronym LASER is meant in the expansive sense, as photons of any energy; and is not limited to photons in the visible spectrum. Hence there are X-ray lasers, infrared lasers, ultraviolet lasers, etc. Because the microwave equivalent of the laser, the maser, was developed first, devices that emit microwave and radio frequencies are usually called masers. In early literature, particularly from researchers at Bell Telephone Laboratories, the laser was often called the optical maser. This usage has since become uncommon, and as of 1998 even Bell Labs uses the term laser [http://www.bell-labs.com/about/history/laser/].

History

In 1916, Albert Einstein laid the foundation for the invention of the laser and its predecessor, the maser, in a ground-breaking rederivation of Max Planck's law of radiation based on the concepts of spontaneous and induced emission. The theory was forgotten until after World War II. In 1953, Charles H. Townes and graduate students James P. Gordon and Herbert J. Zeiger produced the first maser, a device operating on similar principles to the laser, but producing microwave rather than optical radiation. Townes' maser was incapable of continuous output. Nikolay Basov and Aleksandr Prokhorov of the Soviet Union worked independently on the quantum oscillator and solved the problem of continuous output systems by using more than two energy levels. These systems could release stimulated emission without falling to the ground state, thus maintaining a population inversion. Townes, Basov and Prokhorov shared the Nobel Prize in Physics in 1964 "for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle." In 1957 Charles Townes and Arthur Leonard Schawlow, then at Bell Labs, began a serious study of the infrared maser. As ideas were developed, infrared frequencies were abandoned with focus on visible light instead. The concept was originally known as an "optical maser". Bell Labs filed a patent application for their proposed optical maser a year later. Schawlow and Townes sent a manuscript of their theoretical calculations to Physical Review, which published their paper that year (Volume 112, Issue 6). Simultanously, Gordon Gould, a graduate student at Columbia University, was working on a doctoral thesis on the energy levels of excited thallium. Gould and Townes met and had conversations on the general subject of radiation emission. After that meeting, Gould made notes about his ideas for a "laser" in November 1957. In 1958, Prokhorov proposed an open resonator which became an important ingredient of future lasers. The first introduction of the term "laser" to the public was in Gould's 1959 paper "The LASER, Light Amplification by Stimulated Emission of Radiation". Gould intended "aser" to be a suffix, to be used with an appropriate prefix for the spectra of light emitted by the device (e.g. X-ray laser = xaser, UltraViolet laser = uvaser). None of the other terms became popular, although "raser" is sometimes used for radio-frequency emitting devices. Gould's notes included possible applications for a laser, such as spectrometry, interferometry, radar, and nuclear fusion. He continued working on his idea and filed a patent application in April 1959. The U.S. Patent Office denied his application and awarded it to Bell Labs in 1960. This sparked a legal battle that spanned three decades, with scientific prestige and much money at stake. Gould won his first minor patent in 1977, but it was not until 1987 that he could claim his first significant patent victory when a federal judge ordered the government to issue a patent to him for each of the optically pumped and the gas discharge laser. The first working laser was made by Theodore H. Maiman in 1960 at Hughes Research Laboratories in Malibu, California, beating several research teams including those of Townes at Columbia University, and Arthur L. Schawlow at Bell Labs. Maiman used a solid-state flashlamp-pumped synthetic ruby crystal to produce red laser light at 694 nanometres wavelength. Maiman's laser, however, was only capable of pulsed operation due to its three energy level transitions. Later in the same year the Iranian physicist Ali Javan, together with William Bennet and Donald Herriot, made the first gas laser using helium and neon. Javan later received the Albert Einstein Award in 1993. The concept of the semiconductor laser was proposed by Basov and Javan; and the first laser diode was demonstrated by Robert N. Hall in 1962. Hall's device was constructed in the GaAs material system and produced emission at 850 nm, in the near-infrared region of the spectrum. The first semiconductor laser with visible emission was demonstrated later the same year by Nick Holonyak, Jr. As with the first gas lasers, these early semiconductor lasers could be used only in pulsed operation, and indeed only when cooled to liquid nitrogen temperatures (77 K). In 1970, Zhores Alferov in the Soviet Union and Hayashi and Panish of Bell Telephone Laboratories independently developed continuously operating laser diodes at room temperature, using the heterojunction structure. The first application of lasers visible in the daily lives of the general population was the supermarket barcode scanner, introduced in 1974. The laserdisc player, introduced in 1978, was the first successful consumer product to include a laser, but the compact disc player was the first laser-equipped device to become truly common in consumers' homes, beginning in 1982.

Recent innovations

1982 Since the early period of laser history, laser research has produced a variety of improved and specialized laser types, optimized for different performance goals, including
- new wavelength bands
- maximum average output power
- maximum peak output power
- minimum output pulse duration
- maximum power efficiency and this research continues to this day.
Lasing without maintaining the medium excited into a population inversion, was discovered in 1992 in sodium gas and again in 1995 in rubidium gas by various international teams. This was accomplished by using an external maser to induce "optical transparency" in the medium by introducing and destructively interfering the ground electron transitions between two paths, so that the likelihood for the ground electrons to absorb any energy has been cancelled. In 1985 at the University of Rochester's Laboratory for Laser Energetics a breakthrough in creating ultrashort-pulse, very high-intensity (terawatts) laser pulses became available using a technique called chirped pulse amplification, or CPA, discovered by Gérard Mourou. These high intensity pulses can produce filament propagation in the atmosphere.

Uses of lasers

At the time of their invention in 1960, lasers were called "a solution looking for a problem". Since then, they have become ubiquitous, finding utility in thousands of highly varied applications in every section of modern society, including consumer electronics, information technology, science, medicine, industry, law enforcement and the military. They have been widely regarded as one of the most influential technological achievements of the 20th century. The benefits of lasers in various applications stems from their properties such as coherency, high monochromaticity, capability for reaching extremely high powers. For instance, a highly coherent laser beam can be focused down to its diffraction limit, which at visible wavelengths corresponds to only a few hundred nanometers. This property allows a laser to record gigabytes of information in the microscopic pits of a DVD. It also allows a laser of modest power to be focused to very high intensities and used for cutting, burning or even vaporizing materials. For example, a frequency doubled neodymium yttrium aluminum garnet (Nd:YAG) laser emitting 532 nanometer (green) light at 10 watts output power is theoretically capable of achieving an intensity of megawatts per square centimeter. In reality however, perfect focusing of a beam to its diffraction limit is very difficult. square centimeter In consumer electronics, telecommunications, and data communications, lasers are used as the transmitters in optical communications over optical fiber and free space. They are used to store and retrieve data from compact discs and DVDs, as well as magneto-optical discs. Laser lighting displays (pictured) accompany many music concerts. In science, lasers are employed in a wide variety of interferometric techniques, and for Raman spectroscopy and laser induced breakdown spectroscopy . Other uses include atmospheric remote sensing, and investigation of nonlinear optics phenomena. Holographic techniques employing lasers also contribute to a number of measurement techniques. Lasers have also been used aboard scientific spacecraft. In medicine, the laser scalpel is used for laser vision correction and other surgical techniques. Lasers are also used for dermatological procedures including removal of tattoos, birthmarks, and hair; laser types used in dermatology include ruby (694 nm), alexandrite (755 nm), pulsed diode array (810 nm), Nd:YAG (1064 nm), Ho:YAG (2090 nm), and Er:YAG (2940 nm). In industry, laser cutting is used to cut steel and other metals. Laser line levels are used in surveying and construction. Lasers are also used for guidance for aircraft. Lasers are used in certain types of thermonuclear fusion reactors. In law enforcement the most widely known use of lasers is for lidar to detect the speed of vehicles. Military uses of lasers include use as target designators for other weapons; their use as directed-energy weapons is currently under research. Laser weapon systems under development include the airborne laser, the airborne tactical laser, the Tactical High Energy Laser, the High Energy Liquid Laser Area Defense System, and the MIRACL, or Mid-Infrared Advanced Chemical Laser.

Popular misconceptions

The representation of lasers in popular culture, especially in science fiction and action movies, is generally very misleading. For instance, contrary to their portrayal in movies such as Star Wars, a laser beam is never visible in the vacuum of space. In air the ray can hit dust and other particles in its path and scatter producing a glowing "ray", in much the same way that a sunbeam glows in dusty air. This effect can be intensified to make the beam more visible by increasing the amount of suspended particles in the air. Very high intensity beams can be visible in air due to Rayleigh scattering or Raman scattering. With even higher intensity beams, the air can heat up to the point where it becomes a plasma, which would be visible. This would however cause a loud explosion, and will cause a reflection of the ray back into the laser, probably damaging it (depending on the laser design). Furthermore, science fiction film special effects often depict laser beams propagating at only a few metres per second—i.e., slowly enough to see their progress, in a manner reminiscent of conventional tracer ammunition—whereas in reality a laser beam travels at the speed of light, and would be instantly visible along its entire length. Some action movies depict security systems using red lasers (and being foiled by the hero, typically using mirrors); the hero may see the path of the beam by sprinkling some white dust in the air. It is actually easier and cheaper to build infrared laser diodes rather than visible light laser diodes, therefore such systems would almost certainly not use visible light.

Laser safety

Even low-power lasers with only a few milliwatts of output power can be hazardous to a person's eyesight. At wavelengths which the cornea and the lens can focus well, the coherence and low divergence of laser light means that it can be focused by the eye into an extremely small spot on the retina, resulting in localised burning and permanent damage in seconds or even faster. Lasers are classified into safety classes numbered I, inherently safe, to IV, even scattered light can cause eye and/or skin damage. Laser products available for consumers, such as CD players and laser pointers are usually in class I, II, or III.

Common laser types

For a more complete list of laser types see list of laser types. list of laser types

Color	Wavelength interval	Frequency interval
red	~ 625 to 740 nm	~ 480 to 405 THz
orange	~ 590 to 625 nm	~ 510 to 480 THz
yellow	~ 565 to 590 nm	~ 530 to 510 THz
green	~ 520 to 565 nm	~ 580 to 530 THz
cyan	~ 500 to 520 nm	~ 600 to 580 THz
blue	~ 430 to 500 nm	~ 700 to 600 THz
violet	~ 380 to 430 nm	~ 790 to 700 THz

- Gas lasers
- HeNe (543 nm and 633 nm)
- Argon-Ion (458 nm, 488 nm or 514.5 nm)
- Carbon dioxide lasers (9.6 µm and 10.6 µm) used in industry for cutting and welding, up to 100 kW possible
- TEA Laser (UV Light, 337.1 nm)
- Nitrogen laser
- Carbon monoxide lasers, must be cooled, but extremely powerful, up to 500 kW possible
- Chemical lasers
- Chemical oxygen iodine laser (1315 nm)
- Hydrogen fluoride laser (2700-2900 nm)
- Deuterium fluoride laser (3800 nm)
- Excimer gas lasers, producing ultraviolet light, used in semiconductor manufacturing and in LASIK eye surgery; F₂ (157 nm), ArF (193 nm), KrCl (222 nm), KrF (248 nm), XeCl (308 nm), XeF (351 nm)
- Semiconductor lasers
- Laser diodes produce wavelengths from 405 nm to 1550 nm. Low power laser diodes are used in laser pointers, laser printers, and CD/DVD players. More powerful laser diodes are frequently used to optically pump other lasers with high efficiency. The highest power industrial laser diodes, with power up to 10 kW, are used in industry for cutting and welding.
- External-cavity semiconductor lasers have a semiconductor active medium in a larger cavity. These devices can generate high power outputs with good beam quality, wavelength-tunable narrow-linewidth radiation, or ultrashort laser pulses.
- VCSELs are semiconductor lasers whose emission direction is perpendicular to the surface of the wafer. VCSEL devices can achieve better beam quality and higher output power than conventional laser diodes, and potentially could be much cheaper to manufacture. The technology is not (as of 2005) as well developed, however.
- VECSELs are external-cavity VCSELs.
- Quantum cascade lasers are semiconductor lasers that have an active transition between energy sub-bands of an electron in a structure containing several quantum wells.
- Solid-state lasers
- Neodymium-doped YAG lasers (Nd:YAG), a high-power laser operating in the infrared spectrum at 1064nm, used for cutting, welding and marking of metals and other materials also used in spectroscopy and for pumping dye lasers. Can be frequency doubled from 1064nm to 532nm to produce a green laser.
- Ytterbium-doped lasers with crystals such as Yb:YAG, Yb:KGW, Yb:KYW, Yb:SYS, Yb:BOYS, Yb:CaF2, or Yb-doped glasses (e.g. fibers); typically operating around 1020-1050 nm; potentially very high efficiency and high powers due to a small quantum defect; extremely high powers in ultrashort pulses can be achieved with Yb:YAG
- Erbium-doped YAG, 1645 nm, 2940 nm
- Thulium-doped YAG, 2015 nm
- Holmium-doped YAG, 2097 nm; an efficient laser operating in the infrared spectrum, it is strongly absorbed by water-bearing tissues in sections less than a millimeter thick. It is usually operated in a pulsed mode, and passed through optical fiber surgical devices to resurface joints, remove rot from teeth, vaporize cancers, and pulverize kidney and gall stones.
- Titanium-doped sapphire (Ti:sapphire) lasers, a highly tunable infrared laser, used for spectroscopy
- Erbium-doped fiber lasers, a type of laser formed from a specially made optical fiber, which is used as an amplifier for optical communications.
- Dye lasers
- Hollow cathode sputtering metal ion lasers, generating deep ultraviolet wavelengths, of which there are two examples; Helium-Silver (HeAg) 224 nm and Neon-Copper (NeCu) 248 nm. These lasers have particularly narrow oscillation linewidths of less than 0.01 cm^-1 making them good candidates for use in fluorescence suppressed Raman spectroscopy.

External links

- [http://www.repairfaq.org/sam/lasersam.htm Sam's Laser FAQ] by Samuel M. Goldwasser
- [http://www.rp-photonics.com/encyclopedia.html Encyclopedia of laser physics and technology] by [http://www.rp-photonics.com/paschotta.html Dr. Rüdiger Paschotta]
- [http://www.aip.org/pnu/2002/596.html Liquid Light] by Phil Schewe, James Riordon, and Ben Stein
- [http://www.newscientist.com/article.ns?id=dn2497 Light turns into glowing liquid] by Eugenie Samuel
- [http://www.aip.org/pt/vol-54/iss-8/p17.html Experiments Detail How Powerful Ultrashort Laser Pulses Propagate through Air]
- [http://www.nrl.navy.mil/content.php?P=03REVIEW59 Filamentation and Propagation of Ultra-Short, Intense Laser Pulses in Air]
- [http://www.aip.org/pnu/1992/physnews.100.htm Lasing Activity without Population Inversion] by Phillip F. Schewe and Ben Stein
- [http://www.aip.org/pnu/1995/physnews.240.htm Lasing without Inversion] by Phillip F. Schewe and Ben Stein Category:Optical devices Lasers Category:Lighting Category:Acronyms ko:레이저 ms:laser ja:レーザー

Chemistry

Chemistry (derived from the Arabic word kimia, alchemy, where al is Arabic for the) is the science of matter that deals with the composition, structure, and properties of substances and with the transformations that they undergo. In the study of matter, chemistry also investigates its interactions with energy and itself (see physics, biology). Because of the diversity of matter, which is mostly composed of different combinations of atoms, chemists often study how atoms of different chemical elements interact to form molecules and how molecules interact with each other. molecules

Introduction

Chemistry is a large field encompassing many subdisciplines that often overlap with significant portions of other sciences. The fundamental component of chemistry is that it involves matter in some way (this explains its broad reach). It may involve the interaction of matter with non-material phenomena such as energy. More central to chemistry is the interaction of matter with other matter such as in the classic chemical reaction where chemical bonds are broken and made, forming new molecules. Matter, such as the chair you are sitting on or the air you breathe, is known today to consist of molecules. Each molecule consists of small bits of matter known as atoms that are connected together through chemical bonds. Each atom consists of smaller bits of matter known as subatomic particles. The structure of the world we commonly experience and the properties of the matter we commonly interact with are determined by the nature of this matter on the chemical level. Steel is hard because of how the atoms are bound together. Wood will burn because it can react with oxygen in a chemical reaction. Water is a liquid at room temperature because of how each molecule of water interacts with its neighbors. In fact, you are a thinking, sentient being because of an on-going series of chemical reactions and other chemical interactions. You can see this text because of how light interacts with molecules called proteins in the back of your eye. Chemistry is often called the central science because it is what connects most of the other sciences together. Chemistry is in some ways physics on a larger scale and in some ways is biology or geology on a smaller scale. Chemistry is used to understand and make better materials for engineering. It is used to understand the chemical mechanisms of disease as well as to create pharmaceuticals to treat disease. Chemistry is somehow involved in almost every science, every technology and every "thing". With such a large area of study, it is impossible to know everything about chemistry and very difficult to summarize the field concisely. Even the most knowledgable, experienced chemist only knows a very narrow area of chemistry better than others. Of course, most chemists have a broad general knowledge of many areas of chemistry as well. Chemistry is divided into many areas of study called subdisciplines in which chemists specialize. The chemistry taught at the high school or early college level is often called "general chemistry" and is intended to be an introduction to a wide variety of fundamental concepts and to give the student the tools to continue on to more advanced subjects. Many concepts presented at this level are often incomplete and technically inaccurate yet of extraordinary utility. Chemists regularly use these simple, elegant tools and explanations in their work when they suffice because the best solution possible is often so overwhelmingly difficult and the true solution is usually unobtainable. The science of chemistry is historically a recent development but has its roots in alchemy which has been practiced for millennia throughout the world. The word chemistry is directly derived from the word alchemy, however the etymology of alchemy is unclear (see alchemy).

Subdisciplines of chemistry

Chemistry typically is divided into several major sub-disciplines. There are also several main cross-disciplinary and more specialized fields of chemistry. ; Analytical chemistry : Analytical chemistry is the analysis of material samples to gain an understanding of their chemical composition and structure. Analytical chemistry incorporates standardized experimental methods in chemistry. These methods may be used in all subdiciplines of chemistry, exluding purely theoretical chemistry. ; Biochemistry : Biochemistry is the study of the chemicals, chemical reactions and chemical interactions that take place in living organisms. Biochemistry and organic chemistry are closely related f.e. in medicinal chemistry. ; Inorganic chemistry : Inorganic chemistry is the study of the properties and reactions of inorganic compounds. The distinction between organic and inorganic disciplines is not absolute and there is much overlap, most importantly in the sub-discipline of organometallic chemistry. ; Organic chemistry : Organic chemistry is the study of the structure, properties, composition, mechanisms, and reactions of organic compounds. ; Physical chemistry : Physical chemistry or physicochemistry is the study of the physical basis of chemical systems and processes. In particular, the energetics and dynamics of such systems and processes are of interest to physical chemists. Important areas of study include chemical thermodynamics, chemical kinetics, electrochemistry, statistical mechanics, and spectroscopy. Physical chemistry has large overlap with molecular physics. ; Theoretical chemistry : Theoretical chemistry is the study of chemistry via theoretical reasoning (usually within mathematics or physics). In particular the application of quantum mechanics to chemistry is called quantum chemistry. Since the end of the second world war, the development of computers has allowed a systematic development of computational chemistry, which is the art of developing and applying computer programs for solving chemical problems. Theoretical chemistry has large overlap with molecular physics. ; Other fields : Astrochemistry, Atmospheric chemistry, Chemical Engineering, Electrochemistry, Environmental chemistry, Geochemistry, History of chemistry, Materials science, Medicinal chemistry, Molecular Biology, Molecular genetics, Nuclear chemistry, Organometallic chemistry, Petrochemistry, Pharmacology, Photochemistry, Phytochemistry, Polymer chemistry, Supramolecular chemistry, Surface chemistry, and Thermochemistry.

Fundamental concepts

Nomenclature

Nomenclature refers to the system for naming chemical compounds. There are well-defined systems in place for naming chemical species. Organic compounds are named according to the organic nomenclature system. Inorganic compounds are named according to the inorganic nomenclature system. See also: IUPAC nomenclature

Atoms

Main article: Atom. An atom is a collection of matter consisting of a positively charged core (the nucleus) which contains protons and neutrons, and which maintains a number of electrons to balance the positive charge in the nucleus.

Elements

Main article: Chemical element. An element is a class of atoms which have the same number of protons in the nucleus. This number is known as the atomic number of the element. For example, all atoms with 6 protons in their nuclei are atoms of the chemical element carbon, and all atoms with 92 protons in their nuclei are atoms of the element uranium. The most convenient presentation of the elements is in the periodic table, which groups elements with similar chemical properties together. Lists of the elements by name, by symbol, and by atomic number are also available. See also: isotope

Compounds

Main article: Chemical compound A compound is a substance with a fixed ratio of chemical elements which determines the composition, and a particular organisation which determines chemical properties. For example, water is a compound containing hydrogen and oxygen in the ratio of two to one, with the Oxygen between the hydrogens, and an angle of 104.5° between them. Compounds are formed and interconverted by chemical reactions.

Molecules

Main article: Molecule. A molecule is the smallest indivisible portion of a pure compound that retains a set of unique chemical properties. A molecule consists of two or more atoms covalently bonded together.

Ions

Main article: Ion. An ion is a charged species, or an atom or a molecule that has lost or gained an electron. Positively charged cations (e.g. sodium cation Na⁺) and negatively charged anions (e.g. chloride Cl^-) can form neutral salts (e.g. sodium chloride NaCl). Examples of polyatomic ions that do not split up during acid-base reactions are hydroxide (OH^-), or phosphate (PO₄^3-).

Bonding

Main article: Chemical bond. A chemical bond is an interaction which holds together atoms in molecules or crystals. In many simple compounds, valence bond theory and the concept of oxidation number can be used to predict molecular structure and composition. Similarly, theories from classical physics can be used to predict many ionic structures. With more complicated compounds, such as metal complexes, valence bond theory fails and alternative approaches which are based on quantum chemistry, such as molecular orbital theory, are necessary.

States of matter

Main article: Phase (matter). A phase is a set of states of a chemical system that have similar bulk structural properties, over a range of conditions, such as pressure or temperature. Physical properties, such as density and refractive index tend to fall within values characteristic of the phase. The phase of matter is defined by the phase transition, which is when energy put into or taken out of the system goes into rearranging the structure of the system, instead of changing the bulk conditions. Sometimes the distinction between phases can be continuous instead of having a discrete boundary, in this case the matter is considered to be in a supercritical state. When three states meet based on the conditions, it is known as a triple point and since this is invariant, it is a convenient way to define a set of conditions. The most familiar examples of phases are solids, liquids, and gases. Less familiar phases include plasmas, Bose-Einstein condensates and fermionic condensates and the paramagnetic and ferromagnetic phases of magnetic materials. Even the familiar ice has many different phases, depending on the pressure and temperature of the system. While most familiar phases deal with three-dimensional systems, it is also possible to define analogs in two-dimensional systems, which is getting a lot of attention because of its relevance to biology.

Chemical reactions

Main article: Chemical reaction. Chemical reactions are transformations in the fine structure of molecules. Such reactions can result in molecules attaching to each other to form larger molecules, molecules breaking apart to form two or more smaller molecules, or rearrangement of atoms within or across molecules. Chemical reactions usually involve the making or breaking of chemical bonds.

Quantum chemistry

Main article: Quantum chemistry. Quantum chemistry describes the behavior of matter at the molecular scale. It is, in principle, possible to describe all chemical systems using this theory. In practice, only the simplest chemical systems may realistically be investigated in purely quantum mechanical terms, and approximations must be made for most practical purposes (e.g., Hartree-Fock, post Hartree-Fock or Density functional theory, see computational chemistry for more details). Hence a detailed understanding of quantum mechanics is not necessary for most chemistry, as the important implications of the theory (principally the orbital approximation) can be understood and applied in simpler terms.

Laws

The most fundamental concept in chemistry is the law of conservation of mass, which states that there is no detectable change in the quantity of matter during an ordinary chemical reaction. Modern physics shows that it is actually energy that is conserved, and that energy and mass are related; a concept which becomes important in nuclear chemistry. Conservation of energy leads to the important concepts of equilibrium, thermodynamics, and kinetics. Further laws of chemistry elaborate on the law of conservation of mass. Joseph Proust's law of definite composition says that pure chemicals are composed of elements in a definite formulation; we now know that the structural arrangement of these elements is also important. Dalton's law of multiple proportions says that these chemicals will present themselves in proportions that are small whole numbers (i.e. 1:2 O:H in water); although in many systems (notably biomacromolecules and minerals) the ratios tend to require large numbers, and are frequently represented as a fraction. Such compounds are known as Non-Stoichiometric Compounds More modern laws of chemistry define the relationship between energy and transformations.
- In equilibrium, molecules exist in mixture defined by the transformations possible on the timescale of the equilibrium, and are in a ratio defined by the intrinsic energy of the molecules—the lower the intrinsic energy, the more abundant the molecule.
- Transforming one structure to another requires the input of energy to cross an energy barrier; this can come from the intrinsic energy of the molecules themselves, or from an external source which will generally accelerate transformations. The higher the energy barrier, the slower the transformation occurs.
- There is a hypothetical intermediate, or transition structure, that corresponds to the structure at the top of the energy barrier. The Hammond-Leffler Postulate states that this structure looks most similar to the product or starting material which has intrinsic energy closest to that of the energy barrier. Stabilizing this hypothetical intermediate through chemical interaction is one way to achieve catalysis.
- All chemical processes are reversible (law of microscopic reversibility) although some processes have such an energy bias, they are essentially irreversible.

History of chemistry

- Alchemy
- Discovery of the chemical elements
- History of chemistry
- Nobel Prize in chemistry
- Timeline of chemical element discovery

Etymology

Old French: alkemie; Arab al-kimia: the art of transformation. See also: alchemy

External links

- [http://www.allchemicals.info/ Chemical Glossary]
- [http://chem.sis.nlm.nih.gov/chemidplus/ Chemistry Information Database includes basic information and some toxicity]
- [http://www.chem.qmw.ac.uk/iupac/ IUPAC Nomenclature Home Page], see especially the "Gold Book" containing definitions of standard chemical terms
- [http://www.cci.ethz.ch/index.html Experiments] videos and photos of the techniques and results
- [http://physchem.ox.ac.uk/MSDS/ Material safety data sheets for a variety of chemicals]
- [http://www.flinnsci.com/search_MSDS.asp Material Safety Data Sheets]

Heat

---- Heat (also improperly called heat change) is the transfer of thermal energy due to a temperature gradient. The SI unit for heat is the joule. Heat is a process quantity, and is to thermal energy as work is to mechanical energy. Heat flows between regions that are not in thermal equilibrium with each other; it spontaneously flows from areas of high temperature to areas of low temperature. All objects (matter) have a certain amount of internal energy --a

Explosives

Chemical explosives

Explosive compatibility groupings

Low Explosives

High Explosives

Detonation of an Explosive Charge

Composition of the material

Chemical explosive reaction

Formation of gases

Evolution of heat

Rapidity of reaction

Initiation of reaction

Military explosives

Availability and cost

Sensitivity

Stability

Power

Brisance

Density

Volatility

Hygroscopicity

Toxicity

Measurement of chemical explosive reaction

Oxygen balance (OB%)

Heat of explosion

Balancing chemical explosion equations

Volume of products of explosion

Explosive strength

Example of thermochemical calculations

See also

External links

References

Chemical compound

Types of compounds

See also

Nuclear explosive

External links

Laser

Physics

History

Recent innovations

Uses of lasers

Popular misconceptions

Laser safety

Common laser types

See also

External links

Chemistry

Introduction

Subdisciplines of chemistry

Fundamental concepts

Nomenclature

Atoms

Elements

Compounds

Molecules

Ions

Bonding

States of matter

Chemical reactions

Quantum chemistry

Laws

History of chemistry

Etymology

See also

External links

Further reading

Heat