IPMD 14th Edition 2010-2011, 7 pages, 5193 words

Author: Dr Thomas Weissgärber, Fraunhofer IFAM, Winterbergstrasse 28, D-01277, Dresden, Germany.

Nanotechnology is one of the most innovative fields in science and industry worldwide. Powder metallurgy (PM) approaches can be used to produce nanostructures either by processing a nanosized powder into porous or dense materials or by refining the microstructure in high energy milling of coarser powders. Dr. Bernd Kieback and Dr. Weissgärber review new processes suitable for the fabrication of nanopowders as well as nanocrystalline materials and their mechanical and functional properties. They also discuss processing and potential applications for nanostructured light weight materials, dispersion strengthened materials and materials for hydrogen storage and thermoelectrics.

The methods to produce nanostructured materials can be divided into two groups (Fig.1). In the bottom-up approach nano-building blocks (powders) are compacted to form the dense material. The top-down approach uses bulk materials and by a following treatment (mechanical, chemical) it is transferred to a nano-structured one (e.g. high-energy milling, severe plastic deformation, co-reduction of oxide solid solutions). The two approaches are suitable to produce both single-phase, nanostructured materials and nano-composites.

While the influence of the free surface and interfacial energies in a material in the micrometer scale is limited to the well known in powder metallurgy phenomena of driving forces for sintering, wetting behaviour and grain growth, by moving to nanometer dimensions even characteristics that generally are considered to be constant will change. So the melting temperature is lowered by hundreds of degrees, the lattice parameter decreases, while the vapour pressure over the solid surface strongly increases if the particle size is as small as 10nm [2]. With the decreasing particle size the part of atoms located in surfaces or interfaces increases (Fig.2) and the surface and grain boundary diffusion with coefficients 103 - 104 times higher than those of volume diffusion [3] become dominant in thermally activated processes. Important factors for the properties of nanostructured materials are the interactions between lattice defects and interfaces or surfaces as well as dimensional electric and magnetic effects. More fundamentals on this matter are given in [4, 5].

A systematic classification of nanostructured materials was proposed by Siegel [6] as shown in Fig. 3 by choosing the dimension of the nanocrystalline phase as a criterion.

Production of nanopowders

Today the most wide-spread methods to produce nano-powders in a controlled way are gas-phase processes and sol-gel techniques [1]. The first method that was extensively studied for many metals and alloys was developed by Gleiter [7] using the evaporation of the material and its controlled condensation under partial pressure in an inert gas atmosphere. For the transfer of metal atoms into the gas phase, thermal evaporation, sputtering techniques, PVD-processes, laser ablation and other processes can be used. The metal atoms cooled down by collisions with the atoms of the noble gas, condense and form particles, the size of which is controlled mainly by the intensity of evaporation and by the partial pressure of the inert gas. The powder is collected inside the equipment and periodically removed. To prevent agglomeration it is possible to introduce the powder directly into a moving liquid in the VERL-process (vacuum evaporation on rotating liquid).......

Further sections of this article include:

• Production of nanopowders
  - Powder processing, compaction and sintering
• Properties and applications of nanocrystalline materials
  - Mechanical Properties
  - Soft-magnetic properties
  - Hard-magnetic properties
  - Cemented carbides
  - Nano-dispersion strengthened materials
  - Nanostructured thermoelectric materials
  - Hydrogen storage materials
• Outlook

Figures and Tables:

Fig. 1 Principle methods to produce nanostructured materials [2]

Fig. 2 Nanocrystalline material, demonstrating the high proportion of ‘grain boundary atoms’ [7]

Fig. 3 Principle classification of nanostructured materials according to Siegal [6]

Fig. 4 Microsilver production device [10]

Fig. 5 Ag powder prepared by the method of condensation in inert gas (“IGV”-method) – top: morphology of the Ag powder, bottom: highly porous structure of these powders are shown

Fig. 6 SEM of commercially produced WC-10wt%Co nanocomposites powder manufactured by Nanotech (Korea) using a patented liquid spray conversion process followed by hydrogen reduction and carburisation. Co and grain growth inhibitors can be incorporated into the liquid starting material. (31)

Fig. 7 Properties of soft-magnetic materials

Fig. 8 Increase of hardness of WC due to the reduction of grain size [74]

Fig. 9 A printed circuit board drill with a tip diameter of 0.25 mm [83]

Fig. 10 Typical curves of primary wear of the nano-WC and fine-grain PCB drills vs. the number of holes drilled. According to this evidence, nanodrill’s tool life is 2.5 times that of standard drills [85]

Fig. 11 Dispersion strengthened Cu, a) TEM micrograph of a Cu+3vol.-%TiC alloy, dispersoids are formed in-situ due to the reaction between Ti and C (dTiC=10-30nm, dCu=200nm), b) TEM micrograph of a Cu+3vol.-%TiC alloy, dispersoids are incorporated as nano-sized TiC powder, dTiC=30-40nm, dCu=400nm, c) semi-coherent interface between Cu and TiC (to a)), incoherent interface between Cu and TiC with amorphous interlayer (to b))

Fig. 12 Comparison of the mechanical properties and SEM pictures of cleaved samples of an as-grown crystal (lower right) and an SPS wafer (upper right)

Fig. 13 Hydrogenation (left, 20 bar) and dehydrogenation (right, vacuum) kinetics of melt-spun Mg80Ni10Y10 measured in a magnetic suspension balance.