electromagnetic spectrum (380-780 nanometers) , with emission ex-5.
B. J. Thomson and A. M. Wild, "Factors Affecting the Rate tending into the infrared and ultraviolet regions.
Researchers at
of Burning of a Titantium - Strontium Nitrate Based Compo-the Crane facility have performed extensive research on the theory sition," Proceeding of Pyrochem International 1975, Pyro-and performance of illuminating flares, especially the sodium nitrate/
technics Branch, Royal Armament Research and Development magnesium system. (NWSC, Crane, Indiana)
Establishment, United Kingdom, July, 1975.
5IGNITION AND PROPAGATION
I GNITION PRINCIPLES
Successful performance of a high-energy mixture depends upon; 1. The ability to ignite the material using an external stimulus, as well as the stability of the composition in the absence of the stimulus.
2. The ability of the mixture, once ignited, to sustain burning through the remainder of the composition.
Therefore, a composition is required that will readily ignite and burn, producing the desired effect upon demand, while remaining quite stable during manufacture and storage. This is not an easy requirement to meet, and is one of the main reasons why a relatively small number of materials are used in high-energy mixtures.
For ignition to occur, a portion of the mixture must be heated to its ignition temperature, which is defined as the minimum temperature required for the initiation of a self-propagating reaction.
Upon ignition, the reaction then proceeds on its own, in the absence of any additional energy input.
Application of the ignition stimulus (such as a spark or flame) initiates a complex sequence of events in the composition. The solid components may undergo crystalline phase transitions, melting, boiling, and decomposition. Liquid and vapor phases may be formed, and a chemical reaction will eventually occur at the surface 97
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Chemistry o f Pyrotechnics
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99
where the energy input is applied, if the necessary activation energy has been provided.
The heat released by the occurrence of the high-energy reaction raises the temperature of the next layer of composition.
If the heat evolution and thermal conductivity are sufficient to supply the required activation energy to this next layer, further reaction will occur, liberating additional heat and propagation of the reaction down the length of the column of mixture takes place. The rates, and quantity, of heat transfer to, heat production in, and heat loss from the high-energy composition are all critical factors in achieving propagation of burning and a self-sustaining chemical reaction.
The combustion process itself is quite complex, involving high temperatures and a variety of short-lived, high-energy FIG. 5.1 Burning pyrotechnic composition. Several major regions chemical species. The solid, liquid, and vapor states may all are present in a reacting pyrotechnic composition. The actual be present in the actual flame, as well as in the region imme-self-propagating exothermic process is occurring in the reaction diately adjacent to it. Products will be formed as the reaction zone. High temperature, flame and smoke production, and the proceeds, and they will either escape as gaseous species or ac-likely presence of gaseous and liquid materials characterize this cumulate as solids in the reaction zone (Figure 5.1).
region. Behind the advancing reaction zone are solid products A moving, high-temperature reaction zone, progressing formed during the reaction (unless all products were gaseous).
through the composition, is characteristic of a combustion (or Immediately ahead of the reaction zone is the next layer of com-
"burning") reaction. This zone separates unreacted starting position that will undergo reaction. This layer is being heated by material from the reaction products. In "normal" chemical re-the approaching reaction, and melting, solid-solid phase transi-actions, such as those carried out in a flask or beaker, the entions, and low-velocity pre-ignition reactions may be occurring.
tire system is at the same temperature and molecules react ran-The thermal conductivity of the composition is quite important in domly throughout the container. Combustion is distinguished transferring heat from the reaction zone to the adjacent, unre-from detonation by the absence of a pressure differential be-acted material. Hot gases as well as hot solid and liquid par-tween the region undergoing reaction and the remainder of the ticles aid in the propagation of burning.
unreacted composition [1].
A variety of factors affect the ignition temperature and the burning rate of a high-energy mixture, and the chemist has the ability to alter most of these factors to achieve a desired change in performance.
The oxidizers used in high-energy mixtures are generally ionic One requirement for ignition appears to be the need for solids, and the "looseness" of the ionic lattice is quite important either the oxidizer or fuel to be in the liquid (or vapor) state, in determining their reactivity [3]. A crystalline lattice has some and reactivity becomes even more certain when both are liq-vibrational motion at normal room temperature, and the amplitude uids. The presence of a low-melting fuel can substantially lower of this vibration increases as the temperature of the solid is raised.
the ignition temperature of many compositions [2]. Sulfur and At the melting point, the forces holding the crystalline solid to-organic compounds have been employed as "tinders" in high-gether collapse, producing the randomly-oriented liquid state.
energy mixtures to facilitate ignition. Sulfur melts at 119°C, For reaction to occur in a high-energy system, the fuel and oxy-while most sugars, gums, starches, and other organic polymers gen-rich oxidizer anion must become intimately mixed, on the ionic have melting points or decomposition temperatures of 300°C or or molecular level. Liquid fuel can diffuse into the solid oxidizer less (Table 5.1).
lattice if the vibrational amplitude in the crystal is sufficient.
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Chemistry o f Pyrotechnics
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101
Organic Fuels on Ignition
I
TABLE 5. 1 Effect of Sulfur and
and used the ratio of the actual temperature of a solid divided by Temperature
the melting point of the solid (on the Kelvin or "absolute" scale) to quantify this concept.
Ignition tempera-
a = T(solid) /T (melting point) (in K)
(5.1)
Composition
(% by weight)
ture, °C
Tammann proposed that diffusion of a mobile species into a IA.
KC1O,,
66.7
446a
crystalline lattice should be "significant" at an a-value of 0.5
Al
33.3
(or halfway to the melting point, on the Kelvin scale). At this temperature, later termed the Tammann temperature, a solid has IB.
KC1O,,
64
360
approximately 70% of the vibrational freedom present at the melt-Al
22.5
ing point, and diffusion into the lattice becomes probable [3].
S
10
If this is the approximate temperature where diffusion becomes SbZS 3
3.5
probable, it is therefore also the temperature where a chemical reaction between a good oxidizer and a mobile, reactive fuel be-IIA.
BaCrO
comes possible. This is a very important point from a safety 4
90
615a (3.1 ml per
B
10
gram of evolved
standpoint - the potential for a reaction may exist at surprisingly gas)
low temperatures, especially with sulfur or organic fuels present.
Table 5.2 lists the Tammann temperatures of some of the common IIB.