All gases are opaque materials

Although this classification system makes sense at first glance, it is not without its problems:It's not unique or consistent. could e.g. also be described as a composite material of minerals, quartz, mica and feldspar and "rubber" could be both a natural material and a non-metal.What is more important, however, is that the materials that are most important to us cannot be classified at all. Where should the semiconductors (Si, Ge, GaAs, ...) stand? Or that? The ?Of course, one could say that the questions put the in the foreground; the question is, however, whether properties is much better as a classification criterion. One can also try to classify a bit more clearly while partially maintaining the above systematics, e.g. more technical, or more chemical (arranged according to bonds). However, no classification can be found that is really satisfactory. Most of the classifications of the above type date from the time when the mechanical properties were ultimately in the foreground. The first association that arises with the listed material groups is almost always something mechanical or structural: hardness, brittleness, strength, ... With this knowledge and with the background that very clean and systematic definitions of material groups are not possible, life is increasingly made easy for oneself and the definition only distinguishes between: Structure materials: All materials for which the mechanical properties are in the foreground, and Functional materials: All materials that belong to a specific functional group, e.g. semiconductors, magnetic materials, sensor materials, ion conductors, superconductors, ... This is not overly helpful either, but you can clearly delimit work areas and orientations. The materials science of the technical faculty of the CAU Kiel, for example, is practically only concerned with functional materials, while the research partners in the research center GKSS in Geesthacht almost only work on structural materials.

© H. Föll (MaWi 1 script)