Crystallinity

From Plastics Wiki, free encyclopedia

Crystallinity is the degree of structural order in a solid, often represented by a fraction or percentage as a measure of how likely atoms or molecules are to be arranged in a regular pattern, namely into a crystal. This property can often be measured directly using diffraction techniques, such as x-ray diffraction: greater crystallinity yields a sharper diffracted beam.

Amorphous solids such as glass have crystallinity very near zero, while perfect gemstones and semiconductor wafers have a crystallinity approaching one. Most structural materials, including the vast majority of alloys and a slim majority of plastics, show their best performance at some intermediate level of crystallinity.

Anything that breaks the pattern of repeating atoms will reduce crystallinity, including the surface of a material; all other things being equal, a smaller object is less crystalline. However, this effect is only significant for extremely fine powders. Even if the object itself is relatively large, the crystallites that it consists of may be quite small; nanocrystalline materials often have crystallinity much less than one. Impurity atoms and the presence of different phases also contribute to reduced crystallinity.

In materials that crystallize slowly from an amorphous solid, such as glass-ceramics and crystalline polymers, crystallinity can be adjusted fairly precisely and over a very wide range by heat treatment.

In geology, there are four levels of crystallinity. Holocrystalline rocks are completely made of crystals and have no glass. Hypocrystalline rocks are mostly crystals, but have some glass. Hypohyaline rocks are mostly glass, with some crystals. Holohyaline rocks are completely made out of glass, such as obsidian.