Various materials can be selected for the insulation of high-temperature furnaces. In the specific case the maximum application temperature, furnace atmosphere and purity requirements have to be taken into account in the selection of the material. For example insulation materials made of oxidic compounds can usually only be used up to a temperature of 1650°C in oxidizing atmospheres. At higher temperatures, the lifetime is restricted by crystal growth and embrittlement or reduction reactions.

Non-oxidic materials usually with a composition from within the Si-C-B-N-O system reach the limit of their applicability at approx. 1800°C. They cannot be used in processes with high purity requirements, e.g. in the semiconductor industry.

In the form of graphite, on the other hand, carbon has an extremely high temperature stability of 2800°C in an inert atmosphere (nitrogen, argon or vacuum). Very high percentage purities can be achieved by means of high-temperature treatments in this field. Soft and hard felts made of carbon fibres are typical insulation materials. After shaping the soft felts are fixed into self-supporting structures with the help of chemical vapour infiltration with carbon. Hard felts are frequently laminated with graphite or carbon fibre films in order to reduce corrosion reactions and the emission of fibres into the area of the furnace and/ or the product. CFC constructions are suitable as supporting material since these can also be manufactured as high-purity materials and with a high temperature stability.

In addition to felts, carbon foams can also be used for the insulation. Just like the felts these are also characterized by a high temperature stability of up to approx. 2800°C and an excellent thermal fatigue resistance. On account of their high purity these foams are also extremely well suited for use in semiconductor technology. Since the foams are self-supporting and dimensionally stable, blocks can be cut into three-dimensional structures which can be used for insulation purposes in the furnace without a frame which is not possible in the case of soft felts. In order to improve the oxidation stability the foams can be coated and/ or impregnated with pyrocarbon and/ or -graphite.