Propagation of Burning

Reaction velocity is primarily determined by the selection of the oxidizer and fuel (but there are a large number of other factors, as we shall see). The rate-determining step in many high-energy reactions appears to be an endothermic process, with decomposition of the oxidizer frequently the key step. The higher the decomposition temperature of the oxidizer, and the more endothermic its decomposition, the slower the burning rate will be (with all other factors held constant). 

Shimizu reports the following reactivity sequence for the most common fireworks oxidizers  

Shimizu notes that potassium nitrate is not slow when used in black powder (in which the homogeneity is maximized by extended energetic mixing) and metal-containing compositions in which a hot fuel is present. Sodium nitrate is quite similar to potassium nitrate in reactivity (but is considerably more hygroscopic). 

Source A. A. Shidlovskiy, Principles of Pyrotechnics, 3rd ed., Moscow, 1964. (Translated by Foreign Technology Division, Wright-Patterson Air Force Base, Ohio, 1974.) 

Shidlovskiy has gathered data on burning rates for some of the common oxidizers. Table 5.5 contains data for oxidizers with a variety of fuels. Again, note the high reactivity of potassium chlorate. 

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The compound must have an acceptable heat of decomposition. A value that is too exothermic will produce explosive or highly sensitive mixtures, while a value that is too endothermic will cause ignition difficulties as well as poor propagation of burning. 

The amount of heat liberated is so high that it melts iron or steel with which it comes in contact The liberated heat is readily used in thermite welding. A lower ferrous oxide (FeO) content of the hammer scale produces a considerably better flame and better propagation of burning. However, the improvement is slight for values below 22% FeO. 

Propagation. This proceeds when the share of the intact material in the combustion heat plus the incidental heat input from the external source is sufficient for heating the adjacent materials to the combustion temperature. Propagation of burning is more efficient on the surface, due to its direct exposure to the external heat energy and its close contact with the atmospheric oxygen. 

A thick-walled metal tube is particularly effective at removing heat from a burning composition, and propagation of burning down a narrow column can be difficult for all but the hottest of mixtures if metal (particularly a thick metal tube) is used for the container material. On the other hand, the use of a metal wire for the center of the popular... 

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