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Our Thoughts Are Ours, Their Ends none of Our Own - Are there Ways to synthesize Materials beyond the Limitations of Todays?
|Herbert D. Gleiter|
Forschungszentrum Karlsruhe, Institute of Nanotechnology, P.O.B. 3640, Karlsruhe D-76021, Germany
The methods available today to modify the structure and the properties of crystalline materials may be divided in the following two groups: Modifications resulting from the introduction of lattice defects and modifications resulting from alloying of two or more components.
In materials with grain sizes of 1 mm or more, the introduction of lattice defects modifies the microstructure. However, the modifications of the atomic structure are limited to less than 1 volume % of the material. The way to modify the atomic structure in up to 50 volume% of a material by introducing defects was opened by reducing the crystal size of polycrystalline materials to a few nanometers. Materials of this kind are called nanocrystalline or nanostructured materials. The step towards modifying the entire atomic structure of solid materials seems to be possible by means of nanoglasses. Nanoglasses are glasses that are generated by consolidating nanometer-sized glassy spheres at high pressures (several GPa). The existing structural investigations on metallic nanoglasses as well as studies by means of molecular dynamics suggest that nanoglasses consist - in the as prepared state - of the following two structural components. Glassy regions – resulting from the consolidated spheres – and interfaces between these glassy regions. In these glass/glass interfaces, the free volume is enhanced and the nearest neighbor co-ordinations deviate from the ones in the glassy regions. If these nanoglasses are annealed, the enhanced free volume in the glass/glass interfaces seems to delocalize and, thus, modifies the atomc structure of entire material. In fact, it is found that -after long annealing times- nanoglass specimens consist of a surface region with an enhanced densitiy (due to a high hydrostatic pressure) and a glassy core region with a significantly (up to about 10%) reduced density. In other words, nanoglasses may pave the way to tune the free volume (density) of glasses at constant chemical composition.
The modifications of solid materials by alloying may be divided in the following two groups. Components that can be alloyed by melting followed by solidification, and alloys of components that are immiscible in the solid state, e.g. alloys of metals and ionics such as Au-NaCl. The preparation of alloys of this type seems attractive because they are likely to exhibit new properties. So far, apparently two approaches have been considered for preparing such alloys. In the first approach (applicable to systems with mobile charge carries), electronic screening effects at interphase boundaries are utilized. If nanocomposits of immiscible components are prepared with a crystal size comparable to the electronic screening length, the electronic structure of the entire specimen is modified (due to the screening effects). As was shown, this modification may result in the formation of solid solutions of conventionally immiscible components, e.g. of Ag and Fe. In systems without mobile charge carries, vapor deposition of ions of one of the components on an electrically charged substrate may be used to generate solid solutions.
Presentation: Plenary talk at E-MRS Fall Meeting 2007, Plenary session, by Herbert D. Gleiter
See On-line Journal of E-MRS Fall Meeting 2007
Submitted: 2007-06-28 18:44 Revised: 2007-07-04 17:19