School of Chemistry - Theses
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ItemThe environmental degradation of pyrotechnic magnesium powderde Yong, Leo ( 2002)Magnesium powder is one of the most common and important metal fuels used in military pyrotechnic systems. However, the major problem with magnesium based pyrotechnic systems is the susceptibility of the magnesium powder to react with moisture in the environment, particularly at elevated temperatures and relative humidities, typical of military storage conditions found in Northern Australia. This study of the bulk and surface chemical and physical changes associated with the environmental degradation of commercial pyrotechnic magnesium powder at elevated temperature and elevated relative humidity results in the formation of magnesium hydroxide (Mg(OH)2, magnesite or hydrated magnesium carbonate (MgCO3.xH2O), magnesium oxide (MgO) and hydromagnesite or basic magnesium carbonate (Mg5(C03)4(OH)2.4H2O). This was confirmed by auger and x-ray photoelectron and infrared spectroscopy and x-ray diffraction. All of these products form as a result of the reaction of the magnesium with moisture and air but they each form in different amounts and at different rates depending on the grade (particle size/surface area) and the type (particle shape) of the magnesium powder. For all the powders, the major degradation product is magnesium hydroxide, which was confirmed by all the analysis techniques. The temporal ageing of magnesium and the subsequent formation of bulk magnesium hydroxide follow a three stage sigmoidal relationship. The first stage represents surface nucleation; the second or main stage represents the progression of the reaction from the surface to the particle interior and exhibits extensive surface and sub-surface cracking; and the third stage represents the exhaustion of the available magnesium. The equation for the second (main) stage of the reaction for the formation of Mg(OH)2 is given by the pseudo first order rate equation where the rate of the reaction is independent of relative humidity and shows Arrhenius dependence. The formation of Mg(OH)2 proceeds from the outside of the particle inwards but is not diffusion controlled; the reaction is controlled by the rate at which the magnesium/reaction products interface moves into the particle. The surface morphology of the particles shows the formation of hexagonal and trigonal plates and trigonal pyramids and prisms. Crystallite formation is also observed in the interior of the particles. The formation of Mg(OH)2 is a complex heterogeneous process involving many separate steps which follows a Rideal type mechanism; the water diffuses to the particle surface, is chemisorbed and then reacts directly with the metal surface to form Mg(OH)2.