Glasses - Amorphous solids

AMORPHOUS SOLIDS

Amorphous solids possess properties of incompressibility and rigidity to a  certain extent but they do not have definite geometrical forms.

Glasses

When certain liquids are cooled rapidly there is no formation of crystals at a definite temperature, such as occurring on slow cooling. The viscosity of the liquid increases steadily and finally a glassy substance is formed.

Thechiefcharacteristicsofaglassarehardness,rigidityandabilitytowithstand shearing stresses which are all properties of the solid state. On the other hand glasses are optically isotropic and on heating without any sharp transition passes into a mobile liquid. At a high temperature glasses undergo phase transition when crystals separate first as they do form supercooled liquid. Therefore, glasses are regarded as amorphous solids or super cooled liquids as well. Thus, glassy or vitreous state is a condition in which certain substance can exist, lying between the solid and liquid states.

PROPERTIES OF CRYSTALLINE SOLIDS

Crystalline solids exhibit an important property called conductivity. Conductivity is the property of a material by which it allows the flow of electric current. Conducting materials are generally classified into three types namely

conductors, semi conductors and super conductors.

Typical metals are good conductors of electricity while elements like silicon  and germanium are non conductors at ordinary temperature. However, they exhibit appreciable conductivity upon addition of impurities as Arsenic and Boron. The resulting materials are called semiconductors. Semi conductors which exhibit conductivity due to the flow of excess negative electrons are called n-type semiconductors. The conductivity due to the positive holes are called p-type semiconductors. Semiconductors find application in modern devices as recitifiers, transitors and solar cells.