Nanocrystalline Materials: Synthesis and Properties
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The synthesis of nanocrystalline bulk and powder materials is one of the problems facing the modern materials scientist. In recent decades, the interest paid to this problem has grown remarkably because it was found that the properties of nanocrystalline substances change considerably when the size of crystallites decreases below a threshold value. Such changes arise when the average size of crystal grains does not exceed 100 nm and are most pronounced when grains are less than 10 nm in size. Ultrafine-grain substances should be studied considering not only their composition and structure, but also particle size distribution. Ultrafine-grain substances with grains 300 to 40 nm in size on the average are usually referred to as submicrocrystalline, while those with grains less than 40 nm in size on the average are called nanocrystalline.
Nanosubstances and nanomaterials may be classified by geometrical shape and the dimensionality of their structural elements. The main types of nanomaterials with respect to the dimensionality include cluster materials, fibrous materials, films and multilayered materials, and also polycrystalline materials whose grains have dimensions comparable in all the three directions.
The main objective of this paper is to give a general idea about diverse nanocrystalline substances and materials. The chemical composition, the microstructure, the grain size distribution, and, consequently, the properties of nanosubstances largely depend on their production method. It is for this reason that the paper first describes main methods for production of powders and bulk samples in the nanocrystalline state and then considers specific features of the microstructure of nanocrystalline substances. The influence of the nanocrystalline state on properties of various substances and determination of causes of this influence present the objective of the final part of this paper.