chromate
cm /second
Mixture
BaCrO 4
KC1O 4
Mo
cm /second
I.
44
53
3
0.69
I.
10
10
80
25.4
II.
39
47
14
0.44
II.
40
5
55
1.3
III.
37
43
20
0.29
III.
55
10
35
0.42
IV.
33
36
31
0.19
IV.
65
5
30
0.14
a Reference 2. Data from H. Ellern, Military and Civilian Pyro-aReference 2. Data from H. Ellern, Military and Civilian Pyrotechnics, Chemical Publishing Co., Inc., New York, 1968.
technics, Chemical Publishing Co. , Inc. , New York, 1968.
be more stable at the high reaction temperature of the burning de-down the column of pyrotechnic material, and the thermal conduc-lay mixture [2].
tivity of the mixture plays a significant role. As the density of A small percentage of fuel in excess of the stoichiometric amount the mixture increases due to increased loading pressure, the com-increases the burning rate for most delay mixtures, presumably ponents are pressed closer together and better heat transfer oc-through increased thermal conductivity for the composition. The curs.
Table 4.6 presented data for the barium chromate/boron propagation of burning is enhanced by the additional metal, es-system, showing the modest increase that occurs as the loading pecially in the absence of substantial quantities of hot gas to aid pressure is raised.
in the propagation of burning. Air oxidation of the excess metal fuel can also contribute additional heat to increase the reaction rate
I
if the burning composition is exposed to the atmosphere.
GNITION COMPOSITIONS AND FIRST FIRES
The rate of burning of ternary mixtures can similarly be affected by varying the percentages of the components. Table 6.6
Compositions with high ignition temperatures (i.e., above 600°C) presents data for a three-component delay composition. In this can be difficult to ignite using solely the "spit" from a black pow-study, a decrease in the burning rate (in cm/second) is observed der fuse or similar mild ignition stimulus. In such situations, an as the metal percentage is lowered (giving poorer thermal conduc-initial charge of a more-readily-ignitible material, called a "first tivity) and the percentage of higher-melting oxidizer (BaCrO 4 ) fire," is frequently used. The requirements for such a mixture is increased at the expense of the lower-melting, more reactive include [ 3] :
lead chromate, PbCrO 4.
Table 6. 7 illustrates this same concept for the molybdenum /
1.
Reliable ignitibility from a small thermal impulse such as a barium chromate /potassium perchlorate system. Here, KC1O 4 is fuse.
The ignition temperature of a "first fire" should be the better oxidizer.
500°C or less.
Contrary to the behavior expected for "gassy" mixtures, the 2.
The mixture should attain a high reaction temperature, rate of burning for gasless compositions is expected to increase well above the ignition temperature of the main composi-
(in units of grams reacting per second) as the consolidation pres-tion.
Metal fuels are usually used when high reaction tem-sure is increased. "Gasless" delays propagate via heat transfer peratures are needed.
134
Chemistry o f Pyrotechnics
Heat and Delay Compositions
135
3. A mixture that forms a hot, liquid slag is preferred. Such slag will provide considerable surface contact with the main composition, facilitating ignition. The production of hot gas will usually produce good ignition behavior on the ground, but reliability will deteriorate at higher altitudes.
Liquid and solid products provide better heat retention to aid ignition under these conditions.
4. A slower-burning mixture is preferred over a more rapid one. The slower release of energy allows for better heat transfer to the main composition. Also, most "first fires"
are pressed into place or added as moist pastes (that harden on drying), rather than used as faster-burning loose powders.
Potassium nitrate is frequently used in igniters and first fires.
Compositions made with this oxidizer tend to have low ignition temperatures (typically below 500 1C), and yet the mixtures are reasonably safe to prepare, use in production, and store. Potassium chlorate formulations also tend to have low ignition temperatures, but they are considerably more sensitive (and hazardous).
Potassium nitrate mixed with charcoal can be used for ignition, as can black powder worked into a paste with water and a little dextrine. Shidlovskiy reports that the composition KNO 3 , 75
Mg, 15
Iditol, 10 (iditol is a phenol/formaldehyde resin) works well as an igniter mixture [3] ; the solid magnesium oxide (MgO) residue aids in igniting the main composition. Boron mixed with potassium nitrate is a frequently-used, effective igniter mixture, as is the combination of iron oxide with zirconium metal and diatomaceous earth (commonly known as A-lA ignition mixture).
Table 6.8 lists a variety of formulations that have been published.
THERMITE MIXTURES
Thermites are mixtures that produce a high heat concentration, usually in the form of molten products. Thermite compositions contain a metal oxide as the oxidizer and a metal -- usually aluminum - as the fuel, although other active metals may be used.
136
Chemistry o f Pyrotechnics
Heat and Delay Compositions
137
A minimum amount of gas is produced, enabling the heat of reacTABLE 6. 9 Calorific Data for Thermite Mixturesa tion to concentrate in the solid and liquid products. High reaction temperatures can be achieved in the absence of volatile ma-
% Al by
terials; typically, values of 2000-2800°C are reached [3]. A
% Active
weight in
metal product such as iron, with a wide liquid range (melting oxygen
thermite
~Hreaction,
point 1535°C, boiling point 2800°C) produces the best thermite Oxidizers