Transition metal foams offer low density, high permeability and thermal conductivity which makes them attractive for acoustical insulation, fuel cell and catalytic applications. [1][2][3] They have limited availability, however, as their synthesis in prior work was by elaborate processes such as templating on dextran / polyurethane [4,5] or by the dissolution of one component of an alloy. [6] Recent investigations applied combustion techniques with Hunt et al. [7] burning nanoscale nickel and aluminum to form porous structures, while Tappan et al. [2] utilized a multi-step process to obtain a metal complex used as foam precursor. In the latter, the reactive compound was ignited under high pressure (20 atm) in an inert environment, yielding foam structures consisting of $50% metal with the balance being carbon/nitrogen compounds. The authors attributed the foam formation to the steady burning behavior of the particular metal complex, as compared to the more explosive combustion of similar energetic materials.In the present work, we demonstrate the synthesis of nickel, copper, cobalt, Ni-Cu/Ni-Co alloy, and cermet foams by combustion in an open container (i.e., in air at ambient pressure). This was achieved by tuning a single experimental parameter during solution combustion synthesis, a technique applied previously to oxide powder formation. [8,9] In this method, metal precursors, in the form of nitrates, are dissolved in water along with a fuel (in the present case, glycine) which also acts a complexing agent. The solution is then heated in an open container on a hotplate, evaporating the water and, upon reaching a critical temperature, causing the decomposition of the nitrate which induces fuel ignition. The advantages of this synthesis technique include low external heating requirements (as the fuel provides energy for product formation) and precursor mixing at the molecular level, ensured by the reagent dissolution in water. This permitted the synthesis of complex oxides, such as highly substituted perovskites, [10] in prior work. Thus, we hypothesized that, if the solution combustion method could be modified to enable foam formation, a whole range of new materials could be obtained as foam structures, including pure metals, alloys and cermets. As Tappan et al. [2] attributed the foam synthesis to the slow burning behavior of their specific metal complex, we sought to control the reaction wave propagation in the present method. As an example, let us consider the reaction for synthesis of NiO, using glycine as fuel:Here, the fuel to oxidizer ratio, denoted by w (where w ¼ 1 implies that all oxygen required for fuel combustion derives from the oxidizer), represents a tunable parameter. Varying w has previously been shown to alter the combustion behavior [9] as well as product oxidation state [11][12][13] and was therefore investigated as a method to obtain reaction conditions conducive to foam formation. Combustion of nickel nitrate/fuel mixture was investigated first, where the propagation of the flame front de...
