Heat output, pyrotechnics

Pyrotechnic mixtures may also contain additional components that are added to modify the bum rate, enhance the pyrotechnic effect, or serve as a binder to maintain the homogeneity of the blended mixture and provide mechanical strength when the composition is pressed or consoHdated into a tube or other container. These additional components may also function as oxidizers or fuels in the composition, and it can be anticipated that the heat output, bum rate, and ignition sensitivity may all be affected by the addition of another component to a pyrotechnic composition. An example of an additional component is the use of a catalyst, such as iron oxide, to enhance the decomposition rate of ammonium perchlorate. Diatomaceous earth or coarse sawdust may be used to slow up the bum rate of a composition, or magnesium carbonate (an acid neutralizer) may be added to help stabilize mixtures that contain an acid-sensitive component such as potassium chlorate. Binders include such materials as dextrin (partially hydrolyzed starch), various gums, and assorted polymers such as poly(vinyl alcohol), epoxies, and polyesters. Polybutadiene mbber binders are widely used as fuels and binders in the soHd propellant industry. The production of colored flames is enhanced by the presence of chlorine atoms in the pyrotechnic flame, so chlorine donors such as poly(vinyl chloride) or chlorinated mbber are often added to color-producing compositions, where they also serve as fuels. 

Calorimetric tests are probably of very small value in any pyrotechnic device that burns in air. Such testing is discussed in Chapter 34 in connection with processes where the heat output as such is of interest. 

Compositions of highest heat output using boron, aluminum, and titanium or zirconium, with oxidizers such as potassium nitrate or perchlorate, are not normally needed in strictly pyrotechnic lire transfer, but they are important in the initiation of solid propellants, be it for small items such as gas cartridges or for larger grains. Formulas 182, 183, and 184 are some of the better-known examples. 

At low ambient temperatures, dry cells and other galvanic cells lose their faculty to give their nominal current output. Pyrotechnic heat sources can be employed to remedy this deficiency. Experimentally, one such system consists of cheap, easily molded, relatively cool burning pellets, inserted into metal tubes, which are immersed in the electrolyte. The pellets consist of fine iron powder and sulfur in the proportion 70/30 (theory 64/36 calc. 250 cal/g). They are easily ignited and while expanding in a semiliquid state, glow at bright red heat... 

It is not possible to produce nitrous oxide (NjO) in the pyrotechnic manner. The heat output of the reaction... 

The requirement for a specified quantity of calories to be delivered and therefore exactly measured is the exception rather than the rule in the applications of pyrotechnic heat production. How-evCTj with the mounting interest in the basic behavior of pyrotechnic heat sources and their relation to radiant output, dissemination, and other objectives, the question of how many calories a fuel or a mixture delivers is frequently in the mind of the modern practitioner. Heat output has the advantage that (as with physical constants) it often can be measured exactly. However, as has been emphasized pieviously, the number of calories as such is only one of the factors that determine usefulness or efficacy of a mixture. 

The figures for the heat of combustion as such play a subordinate part in pyrotechnics. They arc of importance in those formulations that burn in air and contain an excess of a fuel beyond the equivalent amount of oxidizer. The excess fuel is most often magnesium or an organic binder. Also, in many incendiaries, the major fuel such as magnesium or magnesium alloy bodies as well as most of the incendiary filler materials (kerosine, phosphorus) burn in ambient air. Table 20 furnishes a list of elements and Table 21 a list of compounds whose caloric output and other data may be of interest. Figures for the heat output per unit volume are theoretical, realized only if the substance bums as a compact non-porous solid or liquid. 

The newest oxidizer to appear in significant use in pyrotechnics, anunonium perchlorate (AP), has found considerable use in modem solid-fuel rocket propellants and in the firewoiks industry. The Space Shuttle alone uses approximately 2 million pounds of solid fuel per launch the mixture is 70% ammonium perchlorate, 16% almninum metal, and 14% organic polymer/epoxy, with a trace of iron oxide catalyst that can be varied to modify the bum rate. The aluminum powder is a surprising ingredient to find in a propellant, since it generates solid rather than gaseous reaction products, but its substantial heat output as a fuel and its excellent thermal conductivity both contribute to an enhancement of the bum rate of this propellant formulation. 

The selection of an oxidizer and a fuel (and binder) for a pyrotechnic or propellant composition, together with the weight ratio to be used, determines the heat output as well as the gas output for the mixture under consideration. It is possible to select components so that no significant gas is evolved, or to select components to produce all gaseous products. 

Illuminant compositions are typically mixtures of an oxidizer, metal fuel, and binder. They provide a classic example of how to modify the performance of a pyrotechnic mixture. For a faster (and brighter) reaction, several options are available use a higher percentage of the metal fuel, use a finer particle size metal, or reduce the binder content to raise the heat output. Criteria such as cost, performance, and specification limitations will determine which approach is best for a given situation. 

Several readily-oxidized nonmetallic elements have found widespread use in the field of pyrotechnics. The requirements again are stability to air and moisture, good heat-per-gram output, and reasonable cost. Materials in common use include sulfur, boron, silicon, and phosphorus. Their properties are summarized in Table 3.5. 

Squib. 1) (General). Any of various small size pyrotechnic or explosive devices 2) (Specific). A small explosive device similar in appearance to a detonator, but loaded with low explosive, so that its output is primarily heat (flash). Usually electrically initiated, and provided to initiate action of burning type munitions, pyrotechnic devices and rocket propellants. An electrical squib consists essentially of a tube containing a flammable material, and a small charge of powder compressed around a fine resistance wire connected to electrical leads or terminals (Ref 40a, p 135)... 

Flash F use. A small explosive device similar in appearance to a detonator, but loaded with LE (low explosive), so that its output is primarily heat (flash). Usually electrically initiated, and employed to initiate action of pyrotechnic devices and rocket propellants. 

The delay detonator (Figure lb) uses lead azide as the output charge and converts and augments the heat fiom a gasless delay column to a detonation. The small amount of azide used in this example would require the use of a relay or flash detonator as the next element if the sequence is to terminate in the detonation of a secondary explosive. Otherwise the output from the 20 mg of lead azide is adequate only to ignite a sensitive pyrotechnic or another lead azide charge. 

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