Alumina-silica (Al2O3-SiO2) fibers, frequently referred to as ceramic fibers, are formed by subjecting a molten stream to a fiberizing force. Such force may be developed by high-velocity gas jets or rotors or intricate combinations of these. The molten stream is produced by melting high-purity Al2O3 and SiO2, plus suitable fluxing agents, and then pouring this melt through an orifice. The jet or rotor atomizes the molten stream and attenuates the small particles into fine fibers as supercooling occurs.
The resulting fibrous material is a versatile high-temperature insulation for continuous service in the 538 to 1260°C range. It thus bridges the gap between conventional inorganic fiber insulating materials (e.g., asbestos, mineral wool, and glass) and insulating refractories.
Al2O3-SiO2 fibers have a maximum continuous use temperature of 1093 to 1260°C, and a melting point of over 1760°C. If the fiber is exposed to temperature in excess of 1093°C for extended periods of time, a phenomenon called devitrification occurs. This is a change in the orientation of the molecular structure of the material from the amorphous state (random orientation) to the crystalline state (definitely arranged pattern). Insulating properties are not affected by this phase change but the material becomes more brittle.
Most ceramic fibers have an Al2O3 content from 40 to 60%, and an SiO2 content from 40 to 60%. Also contained in the fibers are from 1.5 to 7% oxides of sodium, boron, magnesium, calcium, titanium, zirconium, and iron.
Fibers as formed resemble a cottonlike mass with individual fiber length varying from short to 254 mm, and diameters from less than 1 to 10 |im. Larger-diameter fibers are produced for specific applications. In all processes, some unfiberized particles are formed that have diameters up to 40 | m.
Low density, excellent thermal shock resistance, and very low thermal conductivity are the properties of Al2O3-SiO2 fibers that make them an excellent high-temperature insulating material. Available in a variety of forms, ceramic fiber is in ever-increasing demand due to higher and higher temperatures now found in industrial and research processes.
Applications
Ceramic fibers were originally developed for application in insulating jet engines. Now, this is only one of numerous uses for this material. It can be found in aircraft and missile applications where a high-temperature insulating medium is necessary to withstand the searing heat developed by rockets and supersonic aircraft. Employed as a thermal-balance and pressure-distribution material, ceramic fiber in the form of paper has made possible the efficient brazing of metallic honeycomb-sandwich structures.
Successful trials have been conducted in aluminum processing where this versatile product in paper or molded form has been used to transport molten metal with very little heat loss. Such fibrous bodies are particularly useful in these applications because they are not readily wet by molten aluminum.
Industrial furnace manufacturers utilize lightweight ceramic fiber insulation between firebrick and the furnace shell. It is also used for "hot topping," heating element cushions, and as expansion joint packing to reduce heat loss and maintain uniform furnace temperatures.
Use of this new fiber as combustion chamber liners in oil-fired home heating units has materially improved heat-transfer efficiencies. The low heat capacity and light weight, compared to previously used firebrick, improve furnace performance and offer both customer and manufacturer many benefits.
Sic Fibers
These fibers, capable of withstanding temperatures to about 1200°C, are manufactured from a polymer precursor. The polymer is spun into a fine thread, then pyrolized to form a 15-| m ceramic fiber consisting of fine SiC crystallites and an amorphous phase. An advantage of the process is that it uses technology developed for commercial fiber products such as nylon and polyester. Two commercial SiC fiber products are the Ube Industries Tyranno fiber and the Nippon Carbon Nicalon fiber.
