Metal foams (or porous metals) represent a new class of materials with unique combinations of properties of light weight high surface area, high electrical conductivity and low thermal conductivity that could potentially enable various new applications in electronics, thermal insulation, sensing, catalysis and energy storage. Conventional methods for producing metal foams include powder metallurgical process, combustion methods, de-alloying, or plating of metal films on existing porous templates. Although, these methods either demand critical manufacturing conditions (e.g. high temperature, high pressure, and/or strict oxygen exclusion), are expensive, or not scalable for practical device applications.
Recent advances in metal nanowire (NW) synthesis enable new methodologies of metal foam production under mild conditions can be scaled up and with flexible materials choices. Starting with an aqueous suspension of metal NWs, ultralight porous metal aerogels can be formed by freeze-drying or critical point drying.
Besides copper nanowire (CuNWs), there is also an attempt to use silver nanowire (AgNWs) to produce conductive aerogels because Ag is more conductive than Cu and more chemically stable against oxidation.
LLNL researchers have developed a new method of using silver nanowires for fabrication of ultralight conductive silver aerogel monoliths with predicable densities and excellent properties. Silver nanowire building blocks were prepared by polyol synthesis and purified by selective precipitation. Silver aerogels were produced by freeze-casting nanowire aqueous suspensions followed by thermal sintering to weld the nanowire junctions. As-prepared silver aerogels have unique anisotropic microporous structures with density precisely controlled by the nanowire concentration down to 4.8 mg/cm3 and electrical conductivity up to 51,000 S/m. Mechanical studies show AgNW aerogels exhibit "elastic stiffening" behavior with Young's modulus up to 16,800 Pa.
The new LLNL-developed method allows for fabricating ultralight AgNW aerogels with predicable densities, pore structures, electrical conductivity and mechanical properties. Using this method, LLNL produced high-performance Ag foams with an ultra-low density down to 4.8 mg/cm3, a high electrical conductivity up to 51,000 S/m and a high Young's modulus of 16,800 Pa.
This technology may be useful for electronics, energy storage, catalytic supports, fuel cells, sensing and medical materials.