Carbon fibers in felted or woven form are the preferred materials for
high-temperature insulation forfurnaces operating above 1500C in vacuum
or inert atmosphere. However, these materials have a tendency to sag and
compact with time which eventually results in non-uniform insulation
characteristics. This problem is partially solved by the use of rigid insulation
which consists of a felt impregnated with a polymer, subsequently pyrolyzed,
and fired and outgassed at 2000°C under vacuum. The material is
available in the form of cylinders, boards, disks and special shapes to suit
furnace demands