Thermal insulators are essential in industrial practices, domestic and equipment appliances. That type of materials is basically those that do not have free electrons in their structure and heat conduction in only because of lattice vibration in form of elastic waves. Those elastic vibrations are affected via impurity atoms and defects like grain boundaries, dislocations and vacancies. The interference among elastic waves and defects causes also anisotropy of heat conductivity. This is easy to know that in crystalline materials the interference will reduce along with decreasing temperature along with consequent increase in thermal conductivity as in figure of Schematic Variation of Thermal Conductivity. Such interferences do not exist in amorphous materials like plastics and glasses and their thermal conductivity therefore, increases with increasing temperature. Following figure of Thermal Conductivity as Functions of Temperature shows variation of thermal conductivity of some solids utilized at high temperatures. The normal insulating materials are mixtures of cellular, granular of fibrous bodies along with voids filled through air. Airs being a bad thermal conductor, such porous materials are good insulator.
Figure: Schematic Variation of Thermal Conductivity
The insulating materials are separated in four groups. Low temperature insulators are for employ up to 10oC. Porous polymeric materials might carry gases that freeze in pores to create vacuum. Vacuum is greatly better insulator because it terminates elastic wave. Such insulators are utilized for cryogenic applications. Building insulators are utilized in the range of 10oC to 150oC. Such are generally wood and plastics. Industrial insulators form the group that is utilized between 150oC to 315oC. Bricks, concrete are primarily utilized in this range but are often more insulated by minerals, slag wools, glasses and various inorganic powders filled between sheets. High temperature insulators are such group which is utilized above 315oC. Porous ceramic bricks for refractory and ceramic powders are utilized as insulators at such temperatures. Insulating bricks can't sustain flames directly but firebricks can withstand flame.
Figure: Thermal Conductivity as Functions of Temperature