The electron
transfer reaction is expected to be rapid (even violent) in such species, upon application of the necessary activation energy to a small portion of the composition. A variety of compounds possessing this intramolecular reaction capability are shown in Table 3. 1.
The output from the exothermic decomposition of these compounds is typically heat, gas, and shock. Many of these materials detonate - a property quite uncommon with mixtures, where the degree of homogeneity is considerably less.The high-energy chemist can greatly expand his repertoire of materials by preparing mixtures, combining an oxidizing material with a fuel to produce the exact heat output and burning rate needed for a particular application. Bright light, colors, and smoke can also be produced using such mixtures, adding additional dimensions to the uses of high-energy materials.
For these effects to be achieved, it is critical that the mixture 49
50
Chemistry o f Pyrotechnics
Components of High-Energy Mixtures
51
TABLE 3.1
Compounds Containing Intramolecular
OXIDIZING AGENTS
Oxidation -Reduction Capability
Requirements
Compound
Formula
Oxidizing agents are usually oxygen-rich ionic solids that decompose at moderate-to-high temperatures, liberating oxygen gas.
Ammonium nitrate
NH,,NO 3
These materials must be readily available in pure form, in the Ammonium perchlorate
NH,,ClO,,
proper particle size, at reasonable cost. They should give a neutral reaction when wet, be stable over a wide temperature Lead azide
Pb(N,),
range (at least up to 100°C), and yet readily decompose to re-Trinitrotoluene (TNT)
C 7H SN 306
lease oxygen at higher temperatures. For the pyrotechnic chemist's use, acceptable species include a variety of negative ions Nitroglycerine (NG)
C 3H SN 3O 9
(anions), usually containing high-energy Cl-O or N-O bonds: Mercury fulminate
Hg(ONC) 2
NO3
nitrate ion
C103
chlorate ion
=
Note: These compounds readily undergo explosive C10
perchlorate ion
4
Cr0 4
chromate ion
decomposition when sufficient ignition stimulus is 0=
oxide ion
Cr
=
2 0 7
dichromate ion
applied. A shock stimulus is frequently needed to activate the nonionic organic molecules (e.g., TNT) ; The positive ions used to combine with these anions must form these compounds will frequently merely burn if a compounds meeting several restrictions [1]
flame is applied.
1. The oxidizer must be quite low in hygroscopicity, or the tendency to acquire moisture from the atmosphere. Water can cause a variety of problems in pyrotechnic mixtures, and materials that readily pick up water may not be used.
burn rather than explode. Burning behavior is dependent upon Sodium compounds in general are quite hygroscopic (e.g., a number of factors, and the pyrotechnist must carefully con-sodium nitrate - NaNO 3) and thus they are rarely em-trol these variables to obtain the desired performance.
ployed. Potassium salts tend to be much better, and are Pyrotechnic mixtures "burn," but it must be remembered that commonly used in pyrotechnics. Hygroscopicity tends to these materials supply their own oxygen for combustion, through parallel water solubility, and solubility data can be used the thermal decomposition of an oxygen-rich material such as po-to anticipate possible moisture-attracting problems. The tassium chlorate
water solubility of the common oxidizers can be found in Table 3.2. However, it should be mentioned that large heat
2 KC1O3
2 KCI + 3 0
(3.1)
quantities of sodium nitrate are used by the military in 2
combination with magnesium metal for white light produc-Thus, a pyrotechnic fire can not be suffocated - no air is needed tion. Here, strict humidity control is required through-for these mixtures to vigorously burn. In fact, confinement can out the manufacturing process to avoid moisture uptake, accelerate the burning of a pyrotechnic composition by producing and the finished items must be sealed to prevent water an increase in pressure, possibly leading to an explosion. Ade-from being picked up during storage.
quate venting is quite important in keeping a pyrotechnic fire 2. The oxidizer's positive ion (cation) must not adversely af-from developing into a serious explosion.
fect the desired flame color. Sodium, for example, is an A variety of ingredients, each serving one or more purposes, intense emitter of yellow light, and its presence can ruin can be used to create an effective composition.
attempts to generate red, green, and blue flames.
52
Chemistry of Pyrotechnics
Components of High-Energy Mixtures
53
TABLE 3.2
The Common Oxidizers and Their Properties
Water solu-
Heat of
Heat of
Grams of oxy- Weight of oxidizer
bility,
decompo- formation, gen released required to evolve one gram of
Formula
Melting point,
grams /100
sition,
kcal /
per gram of
ml @ 20°Ca kcal/mole
molea
oxidizer
oxygen
Compound
Formula
weight
oca
. 60 (total 0)
Ammonium nitrate
NH,,N0 3
80.0
170
118 (0°C)
-
-87.4
Approx. 0.28
Approx. 3.5
Ammonium perchlorate
NH,,C1O,,
117.5
Decomposes
37.2 c
-70.6
. 32
3.12
Barium chlorate
Ba(C10 3 ) 2 • H 20