Characterization of Fine Nickel-Coated Powder as Potential Fuel Additive

M ETAL fuel additives to propellants, explosives, and pyrotechnic compositions help increase the energy density of respective formulations [1–3]. Among various metal additives, aluminum is used most widely due to its high combustion enthalpy and low cost [1,4]. Aluminum particles are coated with a thin protective oxide layer, so that the heterogeneous oxidation reaction leading to aluminum ignition is rate controlled by a relatively slow diffusion of aluminum and oxygen ions through this layer [5–8]. Respectively, extended ignition delays often present a bottleneck for the overall combustion rate of aluminum powders. In addition, the agglomeration of aluminum particles before ignition, in particular occurring in solid propellant formulations, results in further ignition delays and the incomplete combustion of metal particles [9,10]. One approach proposed to reduce ignition delays and increase the burn rate of aluminum powders is based on using nickel surface coatings [11–21]. Such coatings are produced using one of several chemical methods, including an electroless nickel plate solution for treating aluminum powder [22], using aluminum to reduce metals from solutions of its salts [23], and using a modified polyol process [24]. Limited information is currently available about the ignition and combustion processes for such nickel-coated aluminum powders. For example, it was reported that nickel-coated aluminum powders could be ignited on the surface of an electrically heated nickel– chromium filament whereas similar powders without such coating did not ignite [21]. Coated powders also exhibited a reduced ignition temperature compared to uncoated aluminum [21]. Burn rates for individual particles were reported to be unaffected by the Ni coating [21]; however, the bulk burn rates of the coated powders were reported to be greater than that of uncoated aluminum [18]. It was reported that Al–Ni intermetallic reaction is capable of supporting a flame in a cloud of coated Al–Ni powders aerosolized in inert gas [13]. Experiments with Ni-coated particles with sizes ranging from 32 m to 1 mm levitated in various oxidizing environments and ignited with a laser beam were reported in [14–16]. Those experiments established that, for Ni-coated particles with thick coatings ( 50 wt% of Ni), ignition is defined by reactions in the Ni–Al system and does not depend on the presence of an external oxidizer. A similar conclusion was made for large, 2.5 mm particles with Ni coatings comprising 29 and 58 wt% ignited under normal and microgravity conditions [19]. Forfiner particles and thinner coatings, the Ni–Al reaction was considered a critically important exothermic process assisting ignition; however, the role of selective oxidation of nickel or the effect of nickel on the oxidation of aluminum were not considered. Micron-sized particles with small bulk concentrations of Ni are expected to be used in practical energetic formulations, in which it is desirable to maximize the overall energy density of the fuel additives. Thus, heat release due to the intermetallic Al–Ni reaction may become comparable to or even smaller than the heat released due to the heterogeneous oxidation of aluminum. The rate of oxidation itself can be affected by the presence of the Ni coating instead of, or in addition to, the natural amorphous aluminum oxide coating present on surface of pure Al powders. The objective of the present study is to characterize heterogeneous reactions leading to ignition in fine Ni-coated aluminum powders. It is further desired to assess whether the presence of relatively small amounts of Ni affect ensuing particle combustion.