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It is for this reason that monocrystalline solids are used as semiconductor materials of the

highest quality. Due to an absence of defects that usually accompany grain
boundaries, monocrystals have unique mechanical and electrical properties. Therefore, they
are widely used for technological applications like optics and electronics. These properties
also make them precious in some gems.

Not surprisingly, the almost perfect crystalline structure yields the highest light-to-electricity
silicon conversion efficiency in solar panels. The primary use for single crystal superalloys is
to manufacture jet engine turbine blades. Galvanic corrosion is induced by the porous silicon
formation found in both monocrystalline and polycrystalline silicon, and results in a thickly

corroded surface layer.

Polycrystalline solid

Not all solids are single crystals (e.g. silicon semiconductors). Most crystalline solids
are composed of a collection of many small crystals or grains of varying size and orientation.
These have random crystallographic orientations. When a metal starts with crystallization,

the phase change begins with small crystals that grow until they fuse, forming
a polycrystalline structure. In the final block of solid material, each of the small crystals
(called “grains“) is a true crystal with a periodic arrangement of atoms, but the whole
polycrystal does not have a periodic arrangement of atoms, because the periodic pattern is

broken at the grain boundaries. Figure 2.16, shows grain and grain boundaries. Grains and
grain boundaries help determine the properties of a material.

Grains also known as crystallites, are small or even microscopic crystals which form,

for example, during the cooling of many materials (crystallization). A very important feature
of a metal is the average size of the grain. The size of the grain determines the properties of
the metal. For example, smaller grain size increases tensile strength and tends to increase
ductility. A larger grain size is preferred for improved high-temperature creep properties.
Creep is the permanent deformation that increases with time under constant load or stress.

Creep becomes progressively easier with increasing temperature.

Grain Boundaries refers to the outside area of a grain that separates it from the other
grains. The grain boundaries separate variously-oriented crystal regions (polycrystalline) in

which the crystal structures are identical. Grain boundaries are 2D defects in the crystal
structure and tend to decrease the electrical and thermal conductivity of the material. Most
grain boundaries are preferred sites for the onset of corrosion and for the precipitation of new

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