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Polycrystalline materials consist of many single crystals or grains of various sizes and shapes. Grains are the unitary regions of a polycrystalline material, where the crystal lattice is continuous and the arrangement of atoms remains unchanged. An aggregate of grains constituting a real material always contains structural defects at the grain boundaries within and between grains, nearby and further away from the internal surfaces of the crystal lattices. Such material heterogeneity affects its macroscopic behavior, including its electrical, magnetic and optical properties.
Polycrystalline materials are used extensively in the semiconductor industry, as well as in various advanced manufacturing processes and technologies. The materials may be produced by different methods, such as crystallization from a liquid solution, precipitation, heat treatment combusting, sintering, and depositing. All of these methods cause variations in composition, shape and size of the grains.
The structure of polycrystalline material consists of several layers or domains of grains arranged in a lattice array adjacent to or on each other. The interfaces between these domains are called as grain boundaries. The grain boundaries are structurally distinct areas containing lower symmetry and higher concentration of defect sites than the internal regions of a single crystal lattice. They provide an important link between grains in the diffusion processes. As mentioned earlier, size and shape of the grains play an important role in the electrical, thermal and mechanical property of the polycrystalline material.
When a polycrystalline material is subjected to mechanical or thermal loading, there is a stress concentration at grain boundaries, regardless of the crystal orientation. The stress variation across the boundary- grain boundary regions is called as stress mismatch, it can be substantial, especially for layers that differ greatly in thickness or crystal orientation, resulting in dislocation of the grain boundary. This is an important phenomenon in relation to the failure or mechanical/thermal loading of polycrystalline materials.
When polycrystalline materials are subjected to thermal loading, the grain boundary regions become sites for rapid diffusion and precipitation. At even higher temperatures, grain boundaries can act as channels for fast diffusion of dissolution of individual or cumulated atoms or molecules. Due to this phenomenon, the properties of polycrystalline material, such as electronic conductivity, thermal conductivity, refractive index and other can be dramatically different from those of single crystal material.
Polycrystalline material is essential in a variety of advanced technologies and processes. Its physical structure, constituent structure and mechanical property are important in modern material science and engineering. It is becoming increasingly important to understand the fundamentals of polycrystalline materials in order to gain insight in their physical and mechanical response to thermal and applied loads. This has led to the development of several theories and models based on the grain structure of polycrystalline material that try to explain their behavior.
