Polycrystalline Wool (PCW)
Commercialized in the early 1970s
Unique characteristics and higher production costs than RCF
Produced in relatively small quantities for very specific applications
Typically used from 1100°F (600°C) to 3092°F (1700°C)
PCWs are made predominantly of aluminum (content typically 72-99% in the form of oxide) with the remainder consisting of silicon. The fibers are manufactured by sol-gel technology using gel extrusion, spinning and/or drawing in controlled air streams, using equipment/processes specifically designed to produce fibers of defined dimensions. These are then subjected to high-temperature treatment to produce the required polycrystalline structure.
A range of chemistries is used, with alumina content varying from 72-97% but always with silica as the only other major component. PCW fibers have final dimensions such that the median diameter is more than 3 microns, and less than 2% of the total fiber-length is less than 1 micron in diameter. Within this definition, PCW is manufactured to a number of different specifications with different average fiber diameters, all within a relatively narrow range. Thus, in addition to standard products with median diameters typically in the range 3-of 4 microns, manufacturers have also developed larger diameter variants with diameters more typically in the range of 5-7 microns.
Key features are:
Chemical and thermal stability
Low linear shrinkage
Low thermal mass and good insulating properties
High tensile strength
Low content of ‘shot’ (non-fibrous particles)
It is these defining characteristics that have resulted in PCWs being used in the most demanding of thermo/ mechanical applications, for example in the ceramics, metals, petrochemicals, aerospace and automotive industry sectors.
Typical PCW applications include use as support mats in catalytic converters and diesel particulate filters to reduce exhaust emissions, and as insulation in industrial high temperature furnaces for energy conservation, particularly in high temperature and/or chemically aggressive environments.