Nanoporous metal foams (NMFs) have been a long sought-after class of materials in the quest for high-surface-area conductive and catalytic materials. Herein we present an overview of newly developed synthetic strategies for producing NMFs along with an in-depth discussion of combustion synthesis as a versatile and scalable approach for the preparation of nanoporous, nanostructured metal foams. Current applications of NMFs prepared using combustion synthesis are also presented including hydrogen storage and catalysis.

The synthesis of low-density, nanoporous materials has been an active area of study in chemistry and materials science dating back to the initial synthesis of aerogels. These materials, however, are most often limited to metal oxides, e.g., silica and alumina, and organic aerogels, e.g., resorcinol/formaldehyde, or carbon aerogels, produced from the pyrolysis of organic aerogels. The ability to form monolithic metallic nanocellular porous materials is difficult and sometimes elusive using conventional methodology. Here we report a relatively simple method to access unprecedented ultralow-density, nanostructured, monolithic, transition-metal foams, utilizing self-propagating combustion synthesis of novel transition-metal complexes containing high nitrogen energetic ligands. During the investigation of the decomposition behavior of the high-nitrogen transition metal complexes, it was discovered that nanostructured metal monolithic foams were formed in a post flame-front dynamic assembly having remarkably low densities down to 0.011 g cm(-3) and extremely high surface areas as high as 270 m(2) g(-1). We have produced monolithic nanoporous metal foams via this method of iron, cobalt, copper, and silver metals. We expect to be able to apply this to many other metals and to be able to tailor the resulting structure significantly.

number of materials for which surface forces could be measPhosphocholine (PC) zwitterionic surfactants, with differured, it has been difficult to perform rheological and force ent hydrocarbon chain lengths (C 6 C 6 PC to C 9 C 9 PC), were measurements on the same system. The recent development of absorbed on the surface of silicon nitride near the isoelectric force measuring techniques for small particles 5 now enables us point (pH 6). Adsorption of the surfactants changed the to perform both measurements on the same material. lateral and normal surface forces, the rheology, and the A variety of empirical approaches, based on the expected consolidation behavior of the particles. The normal force form of the interparticle forces, have been devised for manipubetween two silicon nitride surfaces as a function of separalating the rheology of slurries so as to consolidate and process tion and the lateral (friction) forces were measured using ceramics without the use of binders. 6,7 In such cases, the nature an atomic force microscope (AFM). These measurements of the forces between particles has generally been assumed to indicated that surfactant adsorption reduced the magnitude be that described by the DLVO theory, 8,9 namely the van der of the long-range attractive van der Waals force and proWaals force and the electrostatic, "double-layer" force. The duced a repulsive short-range force. Although the adsorbed competition between these two provides a basis for underlayers provided a barrier to particle contact, they could be standing such phenomena as flocculation and dispersion of ejected with a critical force that increased with the hydromicrometer-sized particles. However, these forces alone are carbon chain length. The effect of an adsorbed layer on the inadequate to explain some desirable characteristics such as the viscosity and consolidation of slurries was also measured.plasticity of bodies consolidated from slurries. A short-range The viscosity of all slurries decreased with increasing shear repulsive force is required to obtain plastic bodies as well as to rate, indicative of attractive particle networks. The highest produce a yield stress, or to reduce the coefficient of friction. 10,11 viscosity was observed for slurries formulated at the isoelecThus, the plasticity of clays is generally attributed to the effect tric point without added surfactant. Much lower viscosities of water of hydration attached to the clay surface. 12 In support were observed when the surfactant concentration was of this hypothesis, when hydrated ions are added to one of the greater than the critical micelle concentration (cmc). A relaclay minerals, muscovite mica, the surfaces exhibit both a shorttive density of 0.46 was obtained via pressure filtration at range repulsive force 13 and a lower coefficient of friction. 14 4 MPa without a surfactant, and between 0.46 to 0.59 Similar plastic behavior has been achieved by the addition of (C 6 C 6 PC to C 9 C 9 PC, respectively) for surfactant concentrahigh concentrations of salt to alum...

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The properties of aqueous, dispersed, silicon nitride slurries, with an isoelectric point of pH 5.5, can be changed with additions of NH,CI. At pH 10 the effect of adding NH,Cl is similar to that suggested by DLVO theory; namely, for concentrations ?0.5M, the viscosity vs shear rate behavior, the elastic modulus, and the relative packing density are identical to those for slurries prepared at the isoelectric point. On the other hand, the effect of salt on dispersed slurries prepared at pH 2 differs from the behavior implied by classic DLVO theory; i.e., measurement of the same properties showed that the attractive interparticle potential was much weaker relative to that produced at the isoelectric point. As previously reported for alumina slurries, the results suggest that a short-range, repulsive interparticle potential is developed in salt-added slurries prepared at pH 2 which prevents attractive particles in the slurry from touching and aids particle packing. The same short-range potential apparently is not developed with salt additions at pH 10. The difference between silicon nitride and alumina slurries is apparent when the slurries are consolidated. Bodies consolidated from any silicon nitride slurry are elastic (i.e., they fracture before they flow) unlike salt-added alumina slurries, which are plastic.

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Nanoporöse Metallschäume (NMFs) werden seit langem als eine Klasse von Stoffen für leitfähige und katalytische Materialien mit großen Oberflächen untersucht. Dieser Aufsatz gibt einen Überblick über neue Strategien für die Herstellung von NMFs mit einem Schwerpunkt auf der Verbrennungssynthese und diskutiert aktuelle Anwendung von NMFs, unter anderem zur Speicherung von Wasserstoff und in der Katalyse.