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Pyrotechnic Principles
Published in John A. Conkling, Christopher J. Mocella, Chemistry of Pyrotechnics, 2019
John A. Conkling, Christopher J. Mocella
Magnesium metal (Mg): This is an excellent fuel and produces brilliant illuminating mixtures. The metal is water-reactive, however, suggesting short shelf life and possible spontaneous ignition if magnesium-containing mixtures become wet (storage, safety). Whenever possible, replace magnesium with the more stable aluminum or “magnalium” alloy of aluminum with magnesium. If magnesium gives the best effect, and must be used, the metal can be found coated with an organic, water-repelling material.
Combustion Characteristics of Boron-HTPB-Based Solid Fuels for Hybrid Gas Generator in Ducted Rocket Applications
Published in Combustion Science and Technology, 2019
Syed Alay Hashim, Srinibas Karmakar, Arnab Roy
Higher specific impulse as well as higher regression rate can be achieved by using propellants comprising of high-energy minute particles (Davenas, 1993; Kubota et al., 1991; Liu et al., 2014). These particles are usually aluminum (Al), magnesium (Mg), alloys like magnalium (Mg/Al), boron (B), lithium (Li), and beryllium (Be) etc. Boron has been considered as the most attractive fuel for ducted rockets in spite of some limitations in undergoing effective burning. Though Be is more energetic than B, it produces noxious compounds on combustion. Another potential additive is Li; however, it is very reactive. Therefore, the application of Be and Li is limited (Yuasa, 2003). Theoretically, boron can produce high energy on both gravimetric and volumetric basis (59.3 kJ/g and 131.6 kJ/cm3, respectively) (Davenas, 1993; Liu et al., 2014) and high combustion temperature; however, practically it is not possible due to problems associated with its ignition and combustion, which is caused mainly due to the presence of native oxide layer (Chintersingh et al., 2016). Since boron particles are coated with oxide layer having lower melting and boiling point (MP: 450 °C and BP: 1860°C) than boron particles (MP: 2075°C and BP: 4000°C); therefore, during first stage combustion (ignition) boron particles are covered by molten oxide layer. This prevents the diffusion of oxygen and causes the boron to burn heterogeneously which makes overall combustion process inefficient (Liang et al., 2016). In fact, the two-stage combustion of boron was reported few decades ago by the pioneering study of Macek and Semple (1969). The first stage of boron combustion is considered as the removal of the native oxide layer from the particle surface. This oxide removal process is a slow, kinetic and/or diffusion controlled process, which constitutes a significant portion of the overall burning time of the particle. After removal of the oxide layer, the second stage begins with the actual combustion of bare boron. The full energy release of boron occurs when the final products condense to B2O3 (l), which is thermodynamically favored below 2400 K (Macek and Semple, 1969). They also found that ignition temperatures to be consistently in the range of 1850–2000 K and completed in a two-stage process. Schadow (1969) also suggested that a minimum temperature of 2300 K is required for very efficient combustion of the boron particle laden fuels. Indeed, during ignition and combustion, although boron particles are always with boron oxide layer, portion of oxygen continuously diffuses with the exposed core boron (due to cracks or on melting of boron oxide) and formed boron oxide adjacent to the boron particle. Actually, this process eats up local oxygen and requires additional oxygen for the further reaction. In this context, to enhance the boron combustion, high temperature accelerates conversion process of solid boron oxide to liquid boron oxide and more amount of oxygen maintains sufficient quantity of oxygen for reaction with boron.