|
OXIDIZING AGENTS
Conkling, John A.,Chemistry of pyrotechnics,COPYRIGHT ©1985 by MARCEL DEKKER, INC. -51-54
Requirements
Oxidizing agents are usually oxygen-rich ionic solids that decompose
at moderate-to-high temperatures, liberating oxygen gas.
These materials must be readily available in pure form, in the
proper particle size, at reasonable cost. They should give a
neutral reaction when wet, be stable over a wide temperature
range (at least up to 100°C), and yet readily decompose to release
oxygen at higher temperatures. For the pyrotechnic chemist's
use, acceptable species include a variety of negative ions
(anions), usually containing high-energy Cl-O or N-O bonds:
NO3 - nitrate ion C1O3 - chlorate ion
ClO4 - perchlorate ion CrO=4 - chromate ion
O= - oxide ion
Cr2O7= - dichromate ion
The positive ions used to combine with these anions must form
compounds meeting several restrictions
- 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.
Sodium compounds in general are quite hygroscopic (e.g.,
sodium nitrate - NaNO3 ) and thus they are rarely employed.
Potassium salts tend to be much better, and are
commonly used in pyrotechnics. Hygroscopicity tends to
parallel water solubility, and solubility data can be used
to anticipate possible moisture-attracting problems. The
water solubility of the common oxidizers can be found in
Table 3.2. However, it should be mentioned that large
quantities of sodium nitrate are used by the military in
combination with magnesium metal for white light production.
Here, strict humidity control is required throughout
the manufacturing process to avoid moisture uptake,
and the finished items must be sealed to prevent water
from being picked up during storage.
- The oxidizer's positive ion (cation) must not adversely affect
the desired flame color. Sodium, for example, is an
intense emitter of yellow light, and its presence can ruin
attempts to generate red, green, and blue flames.
- The alkali metals (Li, Na, K) and alkaline earth metals
(Ca, Sr, and Ba) are preferred for the positive ion.
These species are poor electron acceptors (and conversely,
the metals are good electron donors), and they
will not react with active metal fuels such as Mg and Al.
If easily reducible metal ions such as lead (Pb+2 ) and
copper (Cu+2) are present in oxidizers, there is a strong
possibility that a reaction such as
Cu(NO3)2 + Mg -> Cu + Mg(NO3)2
will occur, especially under moist conditions. The pyrotechnic
performance will be greatly diminished, and spontaneous
ignition might occur.
- The compound must have an acceptable heat of decomposition.
A value that is too exothermic will produce explosive
or highly sensitive mixtures, while a value that is
too endothermic will cause ignition difficulties as well as
poor propagation of burning.
- The compound should have as high an active oxygen content
as possible. Light cations (Na+,
K+, NH+ ) are desirable
while heavy cations (Pb+2, Ba+2 ) should be avoided
if possible. Oxygen-rich anions, of course, are preferred.
- Finally, all materials used in high-energy compositions
should be low in toxicity, and yield low-toxicity reaction
products.
In addition to ionic solids, covalent molecules containing halogen
atoms (primarily F and Cl) can function as "oxidizers" in
pyrotechnic compositions, especially with active metal fuels. Examples
of this are the use of hexachloroethane (C2C16) with zinc
metal in white smoke compositions,
3 Zn + C2C16 -> 3 ZnC12 + 2 C
and the use of Teflon with magnesium metal in heat-producing
mixtures,
(C2F,,)n + 2n Mg -} 2n C + 2n MgF2 + heat
In both of these examples, the metal has been "oxidized" - has
lost electrons and increased in oxidation number -- while the carbon
atoms have gained electrons and been "reduced."
Table 3.2 lists some of the common oxidizers together with a
variety of their properties.
ДонНТУ|Портал магистров ДонНТУ|||назад в библиотеку |
  |