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3.4.2 Short description

Nanostructured metals and alloys are a material class comprising a multitude of nanoscale particles and powders as well as nanocrystalline solids and nanostructures surfaces. The majority of these materials is currently in a more or less advanced research state with only some aluminium alloys being already in more widespread commercial use. However, individual areas are still in very early development states.

In the following some of the most relevant nano-metals and nano-alloys will be shortly described.

Nanoparticles
The most relevant metallic nanoparticles are titanium and noble metal nanoparticles. However, nanoparticles of e. g. cobalt are of interest due to their ferromagnetic properties. Amorphous Co-nanoparticles with diameters around 20 nm show an extremely strong ferromagnetism giving rise for utilization as ferrofluid or for magnetic datatstorage.

Titanium nanoparticles get an increasing importance as alloy compound to steel. The resulting material shows improved properties with respect to robustness, ductility corrosion and temperature resistance.

Noble metal nanoparticles summarize gold-, silver-, platinum- and palladium-nanoparticles. They are predominantly manufactured by the chemical reduction of acids in aqueous solution, where the selection of various process parameters and chemical stabilizeers define size, shape and surface properties of the particles. They mostly appear as spherical or rod-like particles in the size range of 1 nm-100 nm. Noble metal nanoparticles show surface-plasmon-resonances (see below), making them suitable for optical detection and sensing in analytical chemistry and most of all in molecular biol-ogy. Various chemical methods allow for chemical and biological functionalizations of the particles´ surface enabling chemical binding of organic molecules such as e. g. antibodies. Main utilizations are thus in medical imaging applications but also as catalysts for chemical reactions. Due to their optical properties noble metal nanoparticles are even investigated for solar photovoltaics.

Fig. 5: Electron micrograph of Au-nanoparticles (Source: Cornell Uni-versity).

Nanoparticle metal powders
Nanoparticulate metal powders typically consist of microscale particles. However, these micro-particles show nano crystalline down to amporphous internal structure. In R&D and in application powders of Al, Mg, Ni, Ti, Fe, Cr, Co and their alloys may be found. Nanoscale metal powders are mainly utilized due to their good electrical, magnetic and thermal properties. The most typical processing method is "spray atomisation". Nano metal powders are applied in powder metallurgy (e. g. in sintering processes) an in coat-ing technologies (e. g. flame spraying). Fabrication methods for metal nanoparticles are the chemical reduction of metal ions in presence of stabilizers, lithography, self organ-ized growth on surfaces, aerosol processes plasma deposition and high energy milling.

Fig. 6: Electron micro-graph of nanoporous platinum (Source: NanoMat).

Solutions of ferromagnetic particles in liquids enables the production of "ferrofluids", whose viscosity is controllable by the application of external magnetic fields. This property is giving rise for multiple application possibilities in vehicle suspension and breaking systems, in the bio-medical sector as well as in the ICT-area (e. g. in novel hard disc drives).

High energy milled metal hydrides are of particular interest for hydrogen storage. They offer a high storage density, which is due to their "nanoscale microstructure". Because of their extraordinary storage capacity of up to 7,6 % nanoparticles based on Mg-alloys are of special interest. Due to their chemical kinetic properties, nanostructured hydrides are additionally particular relevance for NiMH-batteries to achieve higher energy densities.

Nanocrystalline metals Nanocrystalline metals are classical metals and alloys with an ultra-fine internal crystallite structure of below 100 nm. Nanostructured metallic solids of this type may be manufactured e. g. by repeated swaging, consolidation of noble-gas condensed clusters or electrolytic deposition. They exhibit extraordinary mechanical and physical properties with considerable application potential. Very high robustness steels and lightweight components based on aluminium or magnesium are applied. Nanocrystalline magnesium furtheron is investigated for hydrogen storage.

Fig. 6: Schematic of the atomic structure of nanocrystalline metals. Black: atoms within the crys-tallite lattice - Whithe: atoms at the grain boundaries.

First concepts of nanocrystalline metals have already been presented in the nineteen eighties. Since then the interest has steadily increased. Different manufacturing methods and material properties have been investigated. The special properties of nanocrystalline metals are due to a basic principle: Due to the extremely small grains the metal body consists to a very large portion of grain boundaries. These grain boundaries exhibit a less arranged structure and different characteristics than conventional metals, which results in modified ductilities and firmnesses of the material. Some nanocrystalline met-als e. g. show an extremely high firmness combined with superplasticity, which results in stable materials allowing for novel flexible shaping possibilities. Examples of real-ized nanocrystalline metal materials are e. g. Al, Mg and Al-Mg alloys which offer a high strength and are light weight. Other ones are titanium based (Ti- and Ti-Al-alloys) and Fe-Cu-Nb-Si-B-alloys. The latter is particularly interesting because of its magnetic properties like the high magnetic induction and its large permeability.