Growing and application of single crystals
Systematic and comprehensive clarification of the physical and physicochemical properties of crystalline matter requires the availability of large crystals of high quality. Their growth and characterization is one of the most important tasks of crystallography in the development of new materials closely related to technological applications, e.g. in fields such as semiconductor electronics, integrated optics, optoelectronics, ultrasonic technology, high-frequency technology, solid-state lasers, radiation detectors, optical memories, piezo-electric displacement sensors, elements for energy conversion as well as synthetic gems and hard materials. Improving the technologically relevant properties of crystalline materials almost always requires a thorough knowledge of the corresponding properties of the monocrystalline bodies, including their defects (real structure).
Most materials such as metals, ceramics and semi-crystalline polymers have a polycrystalline structure. The same applies to almost all substances of the earth’s crust investigated in geosciences. A quantitative understanding of the properties of such substances must be based on the crystal structure and the properties of the individual crystallites. In addition, numerous other structural parameters characterize the polycrystalline aggregate. These are statistical distribution functions of the size, shape and arrangement (structural parameters) as well as the crystallographic orientation (texture parameters) of the crystallites in the aggregate. These parameters determine typical aggregate properties such as macroanistropy, microheterogeneity, grain boundary heterogeneity and porosity. The multi-crystal parameters are mainly investigated using microscopic imaging methods and diffraction methods using X-rays, electrons or neutrons (crystallography of the multi-crystal).
Polycrystalline aggregates are formed or modified by solid state processes of all kinds such as primary crystallization, plastic deformation, recrystallization, phase transformations and rigid rotation of crystallites. The study of the orientation parameters of the polycrystal in particular is therefore a very meaningful method for studying these processes themselves. In geology, these parameters provide information about processes that occurred millions of years ago. Solid state processes in polycrystal are the basis for the production of very different polycrystalline materials with desired macroscopic properties. This applies in particular to many new high-tech materials such as intermetallic phases, structural and functional ceramics, high-temperature superconductors, hard coating materials, and liquid crystal polymers, etc. These materials often consist of crystals with complicated crystal structures and strong anistropies. They are developed on a crystallographic basis. The crystallography of polycrystalline aggregates is therefore particularly closely linked to modern materials science.
A special working direction (powder diffraction) deals with the analysis of multi-crystal diffraction diagrams with the aim of crystal structure analysis, phase analysis, stress measurement, texture analysis and the determination of lattice defects. A central problem of this working direction is the mathematical “unfolding” of diffraction diagrams with any orientation distribution of the crystallites of the polycrystalline system.