Self-sustaining reaction

Flanumbility limits (or explosion limits) for a flammable gas define tlie concentration range of a gas-air ini. ture witliin wliich an ignition source can start a self-propagating reaction. Tlie minimmn and maximmn fuel concentrations in air tliat will produce a self-sustaining reaction mider given conditions are called tlie lower Jlammability limit (LFL) and tlie upper Jlammability limit (UFL). (The abbreviations LEL and UEL, for lower and upper explosivity limits, are sometimes used.) The flanunability limits are functions of... 

This test can be used to give early detection of the initial exothermicity. It is possible to estimate thermokinetic parameters (e.g., the activation energy and the adiabatic self-heat rate) and to estimate how the initial temperature for self-sustaining reactions will vary with the quantity of material present. 

It is a self-sustained reaction in which the energy is transmitted from the burning to the unburnt layers of the substance by means of surface transport properties consisting of burning, which is a relatively slow process. The linear deflgrn rate can be considered to be a function of ambient pressure only consequently, steady states are attainable at constant pressures. Specifically, for condensed expls the linear deflgrn rate is a monotone increasing function of pressure, a fact which plays an important role in the self-acceleration of explosive reactions... 

A Hydrox mixture can be initiated either by heating it at one point by a heat-producing fuse, by an electrically-heated wire or a blackpowder igniter. A few drops of acid produce a self-sustained reaction, after an induction period. The induction period depends on the nature of the acid solution, its concentration and on the amount of alkaline stabilizer in the Hydrox powder. 

Neither hydrochloric nor nitric acid initiates a self-sustained reaction in powder stabilized with sodium carbonate or magnesium oxide. 

Note in this reaction that one neutron starts the fission of the uranium nucleus and that the fission produces 3 neutrons. (It is also possible for a given fission event to produce either fewer than 3 neutrons or more than 3.) These product neutrons can cause the fissioning of 3 other uranium atoms, releasing 9 more neutrons. If each of these 9 neutrons succeeds in splitting a uranium atom, the next step in the reaction produces 27 neutrons, and so on. Such a sequence, illustrated in Figure 4.22, is called a chain reaction—a self-sustaining reaction in which the products of one reaction event stimulate further reaction events. 

Chain reaction A self-sustaining reaction in which the products of one fission event stimulate further events. 

A theoretical analysis of combustion synthesis of refractory nitrides was presented by Munir and Holt in 1987.37 They predicted the existence of an activation energy due to chemical reaction or mass-transport. Glassman et al. in 1987,38 in their thermodynamic analysis of TiN formation, examined the possibility of creating TiN by a self-sustained reaction of the metal particles and nitrogen gas in a rocket motor. They reported that for the stoichiometric ratio of 0.5 mole N2/mole titanium, the reaction has... 

In the early 1990s, Brenner and coworkers [163] developed interaction potentials for model explosives that include realistic chemical reaction steps (i.e., endothermic bond rupture and exothermic product formation) and many-body effects. This potential, called the Reactive Empirical Bond Order (REBO) potential, has been used in molecular dynamics simulations by numerous groups to explore atomic-level details of self-sustained reaction waves propagating through a crystal [163-171], The potential is based on ideas first proposed by Abell [172] and implemented for covalent solids by Tersoff [173]. It introduces many-body effects through modification of the pair-additive attractive term by an empirical bond-order function whose value is dependent on the local atomic environment. The form that has been used in the detonation simulations assumes that the total energy of a system of N atoms is ... 

Early explanations about the effect of mechanical energy on the reactivity of solids are the hot-spot-model [23] and the magma-plasma-model [8]. The generation of hot-spof may be used to explain the initiation of a self-sustained reaction such as explosion, deflagration, or decomposition. Temperatures of over 1000 K on surfaces of about 1 pm2 for KM to 10-3 s can be created. These temperatures can also be found near the tip of a propagating crack [24]. Typically nonequilibrium thermodynamics are used to describe these phenomena. The magma-plasma-model allows for local nonequilibrium states on the solid surface during impact however, due to the very short time scale of 1(H s of these states only statistical thermodynamics can describe the behavior. 

Self-sustained reaction rate oscillations have been shown to occur in many heterogeneous catalytic systems Cl—8]. By now, several comprehensive review papers have been published which deal with different aspects of the problem [3, 9, 10]. An impressive volume of theoretical work has also been accumulated [3, 9, ll], which tries to discover, understand, and model the underlying principles and causative factors behind the phenomenon of oscillations. Most of the people working in this area seem to believe that intrinsic surface processes and rates rather than the interaction between physical and chemical processes are responsible for this unexpected and interesting behavior. However, the majority of the available experimental literature (with a few exceptions [7, 13]) does not contain any surface data and information which could help us to critically test and further Improve the hypotheses and ideas set forth in the literature to explain this type of behavior. 

In both of these reactions, the metal nitrate and the organic component are prepared as an aqueous solution. The solution is boiled, dried, and the residue is heated, typically to 500°C. At this point the reaction begins and converts to a self-sustained reaction. Both of these examples are oxidation—reduction reactions, which derive their large exothermic character from the formation of molecular nitrogen as a product. 

Given the speed of these combustion reactions, establishing a detailed mechanism presents experimental challenges. Nonetheless, a few basic principles have emerged. In those reactions where a salt is a by-product, the heat released from the reaction melts the salt and provides a liquid medium in which the product and reactants can combine. This has been inferred from the results of syntheses that were deliberately loaded with additional product salt, which increased the heat capacity of the system. This additional heat capacity resulted in a lower peak reaction temperature. If enough salt was added, the peak reaction temperature was insufficient for reaching a self-sustaining reaction. 

The temperature that a self-sustaining reaction could reach can be estimated, on the high side, by calculating the adiabatic flame temperature (McQuarrie and Simon, 1997). The adiabatic flame temperature is routinely calculated in physical chemistry. An example with the combustion of methane gas will illustrate the salient points. The balanced chemical reaction for the complete combustion of methane is, again... 

Experimentally, the moment of ignition is recorded according to the appearance of light emission or a rapid rise of the temperature in the oxidation zone. The ignition is considered established as soon as the combustion process becomes steady. Shortterm glow is regarded as a flash. In the steady-state combustion the heat flux from the exothermal reaction zone to the fresh polymer surface makes up for all the heat necessary for the self-sustaining reaction. 

In 1922 Henderson [1] postulated that a-particles might be used to detonate explosives, and there has been a continuing interest in whether self-sustaining reactions can be initiated by nuclear or optical radiation of selected energies. The molecular or electronic aspects of the subject are described in Volume 1 this chapter, therefore, is devoted primarily to macroscopic effects. 

The can, in turn, attack the minerals causing more Fe(II) to be released that once oxidized and hydrolyzed could produce more protons, thus initiating a self-sustaining reaction, provided enough O2 is present. 

BROMOTRIFLUOROMETHANE (75-63-8) CBrFj Noncombustible gas. Can react violently with barium, sodium, and potassium chemically active metals powdered metals. The reaction with aluminumproduces a self-sustaining reaction... 

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