Our thoughts are ours, their ends none of our own: Are there ways to synthesize materials beyond the limitations of today?

Abstract

Historically, the following three approaches to synthesize materials with new atomic structures and, thus, new properties may be distinguished. In the first period – beginning with the discovery of metals about 5000 years ago – the atomic structure of crystalline metallic materials was modified by introducing crystal defects, e.g. by hammering or rolling. However, even at the maximum defect density achievable by this approach (about 10 12 dislocations per cm 2) only about 1% of the atoms are located in the cores of the crystal defects where the atomic structure deviates significantly from the one in the perfect crystal lattice. In other words, this approach does not permit the generation of crystalline materials, the atomic arrangements of which deviate significantly (i.e. in a large volume fraction of the material) from the atomic arrangement in perfect crystals with the same chemical composition. Many crystalline materials used commercially today are based on this approach. In the second approach, the way to a class of materials with new atomic structures was opened about 35 years ago by having up to 50% of the atoms located in cores of grain boundaries and/or interphase boundaries. Materials of this kind were obtained by reducing the crystal size of polycrystals to a few nanometers. These materials were called nanocrystalline or nanostructured materials. As the atomic arrangements in the cores of grain and/or interphase boundaries differ from the ones in perfect crystals, this approach led to materials with new properties (in comparison to the chemically identical single crystals). In the most recent, third approach, a new class of materials with a glassy structure is synthesized. Their novel feature is that the atomic structure throughout the entire volume of the material as well as the density of the entire material can be tuned. Materials of this kind are called nanoglasses. They are generated by introducing interfaces into metallic glasses on a nanometer scale. These interfaces delocalize upon annealing, so that the free volume associated with the interfaces spreads throughout the volume of the glass. This delocalization changes the atomic structure and density of the glass throughout the volume. In fact, by controlling the spacing between the interfaces introduced into the glass as well as their degree of delocalization (by modifying the annealing time and/or annealing temperature), the atomic structures as well as the density (and hence all structure/density-dependent properties) of nanoglasses may be controlled. A reduction of the density by up to 15% seems to be possible. A comparable tuning of the atomic structure/density of crystalline materials is not conceivable because defects in crystals (grain boundaries, dislocations, etc.) do not delocalize upon annealing.