Refractory Materials

Refractory materials are those, which stubbornly resists softening, fusion, the action of fluxes and slag’s, and other effects, physical and chemical, of high temperatures. Usually, a material that can withstand a temperature of 1100° C without softening is termed as refractory material. The ability to withstand exposure to elevated temperatures without undergoing appreciable deformation is measured in terms of refractoriness. When heat is utilized and high temperatures are encountered and needs to be contained, refractory materials are used. Furnace linings and thermal barriers are typical applications. These heat treatment and containment products are used extensively for high technology applications in aerospace, automotive and industrial environments.

Refractory materials usually consist oxides of elements such as silicon, aluminum, magnesium, calcium and zirconium of single or mixed high melting point. There are also some non-oxide refractories like carbides, nitrides, borides and graphite. Atmosphere, temperature, and the materials that it will be in contact with are some of the operating factors that determine the composition of refractory materials.

Refractory materials are available in a variety of different densities and porosities. Usually, thermal conductivities are higher for refractories with low porosity. High porosity materials tend to be highly insulating as a result of high volume of air they trap, because air is a very poor thermal conductor. Such materials, however, do not work with higher temperatures and direct flame impingement, and are likely to shrink when subjected to such conditions. As a result, low porosity materials are generally used in hotter zones, while the more porous materials are usually used for thermal backup.

The key properties of refractory materials are:
1 The ability to withstand high temperatures and trap heat within a limited area, like a furnace.
2 The ability to maintain sufficient dimensional stability at high temperatures and after/during repeated thermal cycling.
3 To maintain sufficient mechanical properties (e.g. compressive strength) at high temperatures.

Some refractory materials commonly used are:
Magnesia: It is a white solid mineral formed by an ionic bond between one magnesium and one oxygen atom. It is used as a basic refractory material for lining crucibles.
Lime: It is a naturally occurring mineral and the materials derived from it are used in various industrial applications.
Fire Clay: This is a specific kind of clay used in manufacturing firebricks. It has a high percentage of silicon and aluminum oxides, which enables it to withstand temperatures upto 1600° C. Hence it is usually used for lining furnaces.
Silicon Carbide: It is a ceramic compound made of silicon and carbon. With a melting point of nearly 2700 ° C, and being highly chemically inert, it is widely used to make furnace parts for heating and casting metals.
Zirconia: This is a white crystalline oxide of zirconium, with a melting point of 2715 ° C. It is used as a refractory material in insulation, abrasives, and ceramic glazes.
Thus, as can be seen, refractories are usually used to line furnaces in order to keep the heat within the hearth of the furnace. As a result of using refractories as insulation, furnace efficiency is increased.