(465b) Fabrication Of Metal Matrix Nanocomposites By A Bottom Up Approach – The Design Of Mechanical And Electronic Properties

The preparation of metal/ceramic nanocomposites has been suggested to result in tremendously improved mechanical properties of metals. If the problem of material mixing and wetting at the nm-level could be circumvented, materials between classical metals and classical ceramic would open a new class of high-temperature stable hybrid materials.

The present contribution investigates the one-step preparation of a metal/ceramic nanocomposite. As an example, we demonstrate how the soft metal bismuth can be modified to the hardness of steel.

Bismuth/ceria metal matrix composites with various loadings (0 ? 35 vol% ceria) were synthesized by the compaction of reducing flame synthesis derived nanopowders. The problem of powder mixing was circumvented by simultaneously producing the metallic (bismuth) and ceramic (ceria) phase in the off-gas stream of the flame enabling the formation of highly homogenous metal matrix composites (Fig. 1).

The mechanical properties could be influenced by degree of ceramic loading achieving a hardness of coarse grained steel still maintaining the characteristic electronic properties of bismuth (Fig. 1). The experimental findings are accompanied by theoretical considerations including percolation limitations, mixing and indirect strengthening models. (Fig. 2)

The applied approach can be adapted for a wide range of materials offering novel nanocomposites with tunable mechanical and electronic properties.

References:

[1] R. N. Grass, T. F. Albrecht, F. Krumeich, W. J. Stark, J. Mater. Chem. 2007, 17, 1485.

[2] R. N. Grass, M. Dietiker, R. Spolenak, W. J. Stark, Nanotechnology 2007, 18, 035703.

[3] R. N. Grass, W. J. Stark, J. Nanopart. Res. 2006, 8, 729

Figure 1: Photograph of a nanocrystalline 15 vol% ceria ? bismuth composite sample (~5 g) in an electrical circuit. A bulk pill of CeO2/Bi shows good electrical conductivity (~ 100 S m^-1) enabling the illumination of a LED. The surface of the compact investigated in a scanning electron microscope (SEM, left) revealed the nano-scaled structure of the compact.

Figure 2: Evolution of the Vickers hardness for increasing ceria content in a metal-matrix nano-composite (squares). The red dot represents the Vickers hardness of the compact formed from commercial bismuth micro-powder. The full blue line shows the theoretical hardness calculated using the rule of mixture and measured densities. The broken red line shows a dispersion model fitted to the experimental data. Dotted lines show reference hardness values for bismuth and coarse- and fine- grained ferritic steels.