Formula by weight
mixture
kcal/gram
behavior.
Upon ignition, a thermite mixture will form aluminum oxide and the metal corresponding to the starting metal oxide: Silicon dioxide
SiO 2
53
37
. 56
Fe
Chromium(III) oxide
Cr
2 0 3 + 2 Al -} A1 2 0 3 + 2 Fe
2 0 3
32
26
. 60
Thermite mixtures have found application as incendiary compo-Manganese dioxide
MnO 2
37
29
1.12
sitions and spot-welding mixtures. They are also used for the Iron oxide
Fe
intentional demolition of machinery and for the destruction of 2O 3
30
25
. 93
documents.
Thermites are usually produced without a binder Iron oxide
Fe 30 4
28
24
. 85
(or with a minimum of binder), because the gaseous products Cupric oxide
CuO
20
19
. 94
resulting from the combustion of the organic binder will carry away heat and cool the reaction.
Lead oxide (red)
Pb 3 O 4
9
10
. 47
Iron oxide (Fe 2O 3 or Fe 3O 4 ) with aluminum metal is the classic thermite mixture.
The particle size of the aluminum should be
a
somewhat coarse to prevent the reaction from being too rapid.
Reference 3.
Thermites tend to be quite safe to manufacture, and they are rather insensitive to most ignition stimuli. In fact, the major problem with most thermites is getting them to ignite, and a strong first fire is usually needed.
Calorific data for a variety of aluminum thermite mixtures are starting materials, used one source of charcoal, and did given in Table 6.9.
not vary the extent of mixing or the amount of moisture contained in their product.
2.
Black powder has a relatively low gas output. Only about PROPELLANTS
50% of the products are gaseous; the remainder are solids.
3.
The solid residue from black powder is highly alkaline The production of hot gas to lift and move objects, using a pyro-
(strongly basic), and it is quite corrosive to many materi-technic system, began with the development of black powder.
als.
Rockets were in use in Italy in the 14th century [51, and cannons were developed at about the same time. The development of aerial
"Pyrodex" is a patented pyrotechnic composition designed to ful-fireworks was a logical extension of cannon technology.
fill many of the functions of black powder. It contains the three Black powder remained the sole propellant available for mili-ingredients found in black powder plus binders and burning rate tary and civilian applications until well into the 19th century. A modifiers that make the material somewhat less sensitive and slower number of problems associated with the use of black powder stim-burning.
A greater degree of confinement is required to obtain ulated efforts to locate replacements
performance comparable to "normal" black powder [6].
The advantages of black powder and Pyrodex include good ig-1.
Substantial variation in burning behavior from batch to nitibility, moderate cost, ready availability of the ingredients, batch.
The better black powder factories produced good and a wide range of uses (fuse powder, delay mixture, propellant, powder if they paid close attention to the purity of their and explosive) depending on the degree of confinement.
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Chemistry of Pyrotechnics
Heat and Delay Compositions
139
As propellant technology developed, the ideal features for a GENERAL
better material became evident
1. A propellant that can safely be prepared from readily-available materials at moderate cost.
2. A material that readily ignites, but yet is stable during storage.
3. A mixture that forms the maximum quantity of low molecular weight gases upon burning, with minimum solid residue.
4. A mixture that reacts at the highest possible temperature, to provide maximum thrust.
The late 19th century saw the development of a new family of
"smokeless" powders, as modern organic chemistry blossomed and the nitration reaction became commercially feasible. Two "esters" - nitrocellulose and nitroglycerine - became the major components of these new propellants. An ester is a compound formed from the reaction between an acid and an alcohol. Figure 6.1 illustrates the formation of NC and NG from nitric acid and the pre-
( maximum of 3 -ON02 groups
cursor alcohols cellulose and glycerine.
per glucose unit)
"Single base" smokeless powder, developed mainly in the United States, uses only nitrocellulose. "Double base" smokeless powder, FIG. 6.1 The nitration reaction. Organic compounds containing developed in Europe, is a blend of nitrocellulose and nitroglycer-the -OH functional group are termed "alcohols." These com-ine. "Cordite," a British development, consists of 65% NC, 30%
pounds react with nitric acid to produce a class of compounds NG, and 5% mineral jelly. The mineral jelly (a hydrocarbon ma-known as "nitrate esters." Nitroglycerine and nitrocellulose are terial) functions as a coolant and produces substantial amounts among the numerous explosive materials produced using this re-of CO
action.
2 , CO, and H 2O gas to improve the propellant characteristics. "Triple base" smokeless powder, containing nitroguanidine as a third component with nitroglycerine and nitrocellulose is also manufactured.
An advantage of the smokeless powders is their ability to be extruded during the manufacturing process. Perforated grains can be produced that simultaneously burn inwardly and outwardly such that a constant burning surface area and constant gas production are achieved.
Nitrocellulose does not contain sufficient internal oxygen for complete combustion to C0 2 , H2O, and N2 , while nitroglycerine contains excess oxygen [7]. The double base smokeless propellants therefore achieve a slightly more complete combustion and benefit from the substantial exothermicity of NG (1486 calories/
gram) [7].
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Chemistry o f Pyrotechnics