WHITE LIGHT COMPOSITIONS
For white-light emission, a mixture is required that burns at high temperature, creating a substantial quantity of excited atoms or molecules in the vapor state together with incandescent solid or liquid particles. Incandescent particles emit a broad range of wavelengths in the visible region of the electromagnetic spectrum, and white light is perceived by the viewer. Intense emission from sodium atoms in the vapor state, excited to higher-energy electronic states by high flame temperature, is the principal light source in the sodium nitrate /magnesium /organic binder flare compositions widely used by the military.
Magnesium or aluminum fuels are found in most white-light compositions. These metals evolve substantial heat upon oxidation, and the high-melting magnesium oxide (MgO) and aluminum oxide (A1203 ) reaction products are good light emitters at the high reaction temperatures that can be achieved using these fuels. Ti- tanium and zirconium metals are also good fuels for white-light compositions.
In selecting an oxidizer and fuel for a white-light mixture, a main consideration is maximizing the heat output. A value of 1.5 kcal/gram has been given by Shidlovskiy as the minimum for a usable illuminating composition. A flame temperature of less than 2000°C will produce a minimum amount of white light by emission from incandescent particles or from excited gaseous sodium atoms.
Therefore, the initial choice for an oxidizer is one with an exothermic heat of decomposition such as potassium chlorate (KC1O3). However, mixtures of both chlorate and perchlorate salts with active metal fuels are too ignition-sensitive for commercial use, and the less-reactive - but safer - nitrate compounds are usually selected. Potassium perchlorate is used with aluminum and magnesium in some "photoflash" mixtures ; these are extremely reactive compositions, with velocities in the explosive range.
The nitrates are considerably endothermic in their decomposition and therefore deliver less heat than chlorates or perchlorates, but they can be used with less fear of accidental ignition. Barium nitrate is often selected for white-light mixtures. The barium oxide (BaO) product formed upon reaction is a good, broad-range molecular emitter in the vapor phase (the boiling point of BaO is ca. 2000°C), and condensed particles of BaO found in the cooler parts of the flame are also good emitters of incandescent light.
Sodium nitrate is another frequent choice. It is quite hygroscopic however, so precautions must be taken during production and storage to exclude moisture. Sodium nitrate produces good heat output per gram due to the low atomic weight (i.e. , 23) of sodium, and the intense flame emission from atomic sodium in the vapor state contributes substantially to the total light intensity. Potassium nitrate, on the other hand, is not a good source of atomic or molecular emission, and it is rarely - if ever - used as the sole oxidizer in white-light compositions.
Magnesium metal is the fuel found in most military illuminating compositions, as well as in many fireworks devices. Aluminum and titanium metals, the magnesium /aluminum alloy "magnasium," and antimony sulfide (Sb2S3 ) are used for white light effects in many fireworks mixtures. Several published formulas for white light compositions are given in Table 1.
TABLE 1. White Light Compositions
The ratio of ingredients, as expected, will affect the performance of the composition. Optimum performance is anticipated near the stoichiometric point, but an excess of metallic fuel usually increases the burning rate and light emission intensity. The additional metal increases the thermal conductivity of the mixture, thereby aiding burning, and the excess fuel - especially a volatile metal such as magnesium (boiling point 1107°C) - can vaporize and burn with oxygen in the surrounding air to produce extra heat and light. The sodium nitrate/magnesium system is extensively used for military illuminating compositions. Data for this system are given in Table 2.
TABLE 2. The Sodium Nitrate /Magnesium Systema
The anticipated reaction between sodium nitrate and magnesium is
Formula A in Table 2 therefore contains an excess of oxidizer. It is the slowest burning mixture and produces the least heat. Formula B is very close to the stoichiometric point. Formula C contains excess magnesium and is the most reactive of the three; the burning of the excess magnesium in air must contribute substantially to the performance of this composition.
A significant altitude effect will be shown by these illuminating compositions, especially those containing excess metal. The decreased atmospheric pressure - and therefore less oxygen - at higher altitudes will slow the burning rate as the excess fuel will not be consumed as efficiently.