br Reactive metals and metal

Reactive metals and metal carbonyls Yen and Wang reviewed several lipid metabolism pathway of reactive metals that have been considered for energetic applications [2]. These include elemental metals, thermites/intermolecular composites (MIC), encapsulated metals, metastable alloys and “surface activated” metals. Properties, processing techniques, ignition and combustion characteristics of these materials as well as their field performance of the reactive metals in explosive formulations were also reported (if available). Finally, some reactive metals were identified in their review as potential metals. Metals having high combustion enthalpies attract attention as high energy density materials. One of those metal additives is aluminum. Since the beginning of the 20th century aluminized explosives have been used in various formulations (e.g. Ammonal, Tritonal, Hexal, aluminized plastic bonded explosives, etc.). However, the potential benefits expected from aluminum additives have not been fully exploited. This is mainly due to the character of aluminum (the high melting point having oxide layer covers the surface, thus causing long ignition delays and slow combustion rate). Hence, researchers have attempted to overcome these drawbacks by improving material processing and searching for new materials. One of these material processing techniques is the mechanical activation (MA) which is a size reduction process by milling techniques. Note that fine particles are usually more reactive than relatively coarse ones. Reactive metals find application in air-blast and underwater explosives. Due to the high heat released from reactions of metals with the decomposition products of explosives in ambient air or water, a considerably huge increase in energy release can be achieved. The active metal particles react over a much longer timescale than the detonation of the explosive itself. Thus, they contribute a great deal to the work done by the expanding combustion products. It is known that in underwater applications, the reactions of metals with water also contribute to the bubble energy [2]. In the past, not many other elemental metal powders besides aluminum are taken into consideration for the formulation of explosives. Quite recently, boron has been considered for the same purpose. The literature indicates that boron has the highest gravimetric and volumetric heat of combustion compared to aluminum and many other metal fuels. When boron was incorporated in HMX-based explosive compositions (B/HMX), it was observed that slightly higher explosion heats (per unit mass) occur compared to aluminum-containing ones (Al/HMX) in a bomb calorimetric test [22]. Lee et al. [23] studied the use of mixtures of boron and aluminum in an explosive formulation (RDX/Al/B/HTPB, 45/10/20/25). The test was conducted in a confined chamber and quasi-static pressure was measured. Note that a quasi-static process is a thermodynamic process that happens slowly enough for the system to remain in internal equilibrium. The authors found that the formulation containing mixtures of boron and aluminum performed 1.3 times better as compared to the formulation containing pure aluminum (RDX/Al//HTPB, 45/38/17). This is the result despite the lower metal content. Therefore, it appears that boron is a potential candidate for use as fuel additive in energetic compositions. Nonetheless, there is also some experimental effort indicating that the high ignition temperature of boron is actually a drawback to its application [24]. Since the boron flame temperature is 2067 °C, while its boiling point is 3865 °C, boron burns at the particle surface, which consequently turns into a covered surface coated with the viscous oxide (B2O3) at such a high temperature. Because of that, this occurrence reduces the ability of the fuel to mix well with oxidizer and leads to inefficient burning. Schaefer and Nicolich studied the blast performance of boron-containing cast-cured, HMX-based explosive in a semi-confined structure [25].