Hybrid Approach of Phase Separation and Selective Leaching Processes for Multicomponent Alloy Nanostructures and its Application to Hydrogen Storage Alloys

In contrast to classical bulk alloying processes represented by melting and casting of metal mixtures, the fabrication of multicomponent alloy (MCA) nanostructures such as nanoparticles and nanofoams with more than three elements is often challenging. A few methodologies for directly synthesizing alloy nanostructures up to denary systems have been suggested recently. However, forming alloy nanoparticles inside another metal matrix, instead of inside aqueous media in wet solution-based chemical synthesis, is a fairly well understood strategy in terms of physical metallurgy. Extracting those alloy nanophases chemically from the matrix could provide an alternative way for fabricating novel MCA nanostructures. In this presentation, we introduce a hybrid approach of metallurgical bottom-up and chemical top-down processes for fabricating MCA nanostructures including nanoparticles and nanofoams. The former utilizes the liquid-state phase separation phenomenon that resembles “oil and water”, but occurs at nanoscale due to thermodynamic mixing relations among alloying elements and a rapid quenching process. Thermodynamic prediction of immiscible boundary in a temperature-composition space (miscibility gap) plays a key role in designing precursor alloys with MCA nanostructures. Selective leaching, the chemical top-down process for extracting the alloy nanostructures from the precursors, uses the chemical reactivity difference between the embedded nanostructures and the matrix phase against a certain chemical solution. We discuss here how the precise control of alloy composition and cooling rate based on thermodynamic assessments enables to prepare phase-separating precursor alloys for fabricating both nanoparticles and nanofoams with a broad size range. Depending on alloy systems, the atomic structure of alloy nanostructures could be controlled from fully amorphous to nanocrystalline and even into quasicrystalline structure. This unique approach for fabricating nanosized alloys provides an extended methodology to discover novel metallic nanomaterials with promising properties in diverse compositional spaces of MCA systems. We demonstrate how this approach can be applied to fabricate nanostructured alloys for reversible hydrogen storage (e.g. TiFe nanofoams). We also present how controlling the processing parameters influences the ligament size of the nanofoams and eventually manipulates their hydrogen adsorption and desorption behavior by nanoscale size effects.