Aluminum (Al)
The most widely used metallic fuel is probably aluminum, with magnesium running a close second.
Aluminum is reasonable in cost, lightweight, stable in storage, available in a variety of particle shapes and sizes, and can be used to achieve a variety of effects. Aluminum has a melting point of 660°C and a boiling point of approximately 2500°C. Its heat of combustion is 7.4 kcal/gram.
Aluminum is available in either "flake" or "atomized" form. The "atomized" variety consists of spheroidal particles. Spheres yield the minimum surface area (and hence minimum reactivity) for a given particle size, but this form will be the most reproducible in performance from batch to batch. Atomized aluminum, rather than the more reactive flake material, is used by the military for heat and light-producing compositions because the variation in performance from shipment to shipment is usually less.
Large flakes, called "flitter" aluminum, are widely used by the fireworks industry to produce bright white sparks. A special "pyro" grade of aluminum is also available from some suppliers. This is a dark gray powder consisting of small particle sizes and high surface area and it is extremely reactive. It is used to produce explosive mixtures for fireworks, and combinations of oxidizers with this "pyro" aluminum should only be prepared by skilled personnel, and only made in small batches. Their explosive power can be substantial, and they can be quite sensitive to ignition.
Aluminum surfaces are readily oxidized by the oxygen in air, and a tight surface coating of aluminum oxide (A1203) is formed that protects the inner metal from further oxidation. Hence, aluminum powder can be stored for extended periods with little loss of reactivity due to air oxidation. Metals that form a loose oxide coating on exposure to air - iron, for example - are not provided this surface protection, and extensive decomposition can occur during storage unless appropriate precautions are taken.
Compositions made with aluminum tend to be quite stable. However, moisture must be excluded if the mixture also contains a nitrate oxidizer. Otherwise, a reaction of the type
can occur, evolving heat and ammonia gas. This reaction is accelerated by the alkaline medium generated as the reaction proceeds, and autoignition is possible in a confined situation. A small quantity of a weak acid such as boric acid (H3B03) can effectively retard this decomposition by neutralizing the alkaline products and maintaining a weakly acidic environment. The hygroscopicity of the oxidizer is also important in this decomposition process. Sodium nitrate and aluminum can not be used together, due to the high moisture affinity of NaNO3 , unless the aluminum powder is coated with a protective layer of wax or simi- lar material. Alternatively, the product can be sealed in a moisture-proof packaging to exclude any water. Potassium nitrate/ aluminum compositions must be kept quite dry in storage to avoid
decomposition problems, but mixtures of aluminum and non-hygroscopic barium nitrate can be stored with a minimum of precautions, as long as the composition does not actually get wet. Mixtures of magnesium metal with nitrate salts do not have this alkaline-catalyzed decomposition problem. A magnesium hydroxide Mg(OH) 2 coating on the metal surface apparently protects it from further reaction. This protection is not provided to aluminum metal by the alkaline -soluble aluminum hydroxide, Al(OH) 3.