Ceramic fiber insulation is made in a variety of shapes, sizes, densities, and compositions. These materials possess a range of characteristics, properties, and utility. Successful applications of fibrous ceramic thermal insulation in rapid cycle furnace chambers operating at 1700°C (3092°F) is critically dependent on its proper use. Consideration must be given to the unique physical, thermal, and chemical properties of these materials and their fabricability.

Different shapes of ceramic fibers are formed, by drawing water base slurry of fiber and binders onto a screen. Fig. 1 shows the production of a standard vacuum-formed cylinder shape.

Typically, flat board and cylinder shapes are produced using this method, although an unlimited number of custom shapes can be formed. Once dried, and/or fired, these simple pieces can be machined into very complex geometries for a specific application. Fig. 2 shows the shapes that are produced.

During the forming process, colloidal binders adhere to the fiber surfaces. This bonds fiber to fiber and facilitates the build-up of large volumes. Fig. 3 shows the morphology of a typical vacuum-formed ceramic fiber material.

Vacuum-formed fibrous ceramic insulation has a laminar type internal structure, with the fiber generally oriented parallel to the vacuum-formed surface. This results in the fiber plane being perpendicular to the board or cylinder thickness, Fig. 4, thus creating anisotropy with respect to physical properties.

Vacuum-formed shapes can be made in combination with the zirconia, alumina or silica binders. Virtually, any combination is possible with certain variations being preferable for any given application.

Table I shows the most common ceramic fiber insulation materials currently used in high temperature laboratory furnace construction.

The high temperature stability of fibrous materials can be greatly influenced by the presence of impurities or by the atmosphere in the furnace chamber.

Fibrous zirconia materials are generally considered inert, and are unaffected by most furnacing conditions. Although, products with silica in the zircon bond can be adversely affected by materials which react with it, such as sodium chloride or sodium oxide or by furnace atmospheres which will cause its reduction. Zirconia-bonded zirconia fiber materials are less prone to be attacked by impurities and furnace atmosphere.

Silica-bonded alumina insulating materials exhibit the greatest high temperature stability. They can be adversely affected by impurities or atmospheric conditions, which attack silica. All alumina variations of these materials have been produced; they are used primarily in vacuum or reducing conditions where, silica in the insulation is attacked by the furnace atmosphere, or where the presence of silica cannot be tolerated with respect to load contamination.

Fibrous ceramic insulation is extremely low density relative to traditional furnace refractories. Porosities range from 70%-93%. The low density and associated low thermal mass imparts excellent thermal shock resistance (TSR). When used as furnace insulation, material TSR is affected by factors including chemistry, density, temperature cycle and geometrical configuration.

As insulation panel size is increased the overall thermal shock resistance decreases. Insulation panels of 80% A1₂O₃ – 20% SiO₂ that are larger than 12" x 12" (30.4 cm x 30.4 cm), tend to exhibit thermal shock cracking. Thus, for rapid cycle furnace application is important to limit the size of the hot-face insulation panel. Small panels are more resistant to thermal shock and mechanical failure due to their ability to expand and contract independently of other panels in the system.

Thermal shrinkage of fibrous insulating materials is due mainly to the sintering and consolidation of the porous network of fibers, which occurs at elevated temperatures.

Shrinkage, as determined for specimens under no load conditions in an isothermal soak, begins immediately upon exposure to elevated temperatures and continues for sometime (up to or in excess of 100 hours). Shrinkage under these conditions is largely due to the chemistry and associated phase changes.

The shrinkage of fibrous ceramic insulating materials is greatest in the direction perpendicular to the fiber plane. This is believed due mainly to the sintering of adjacent fibers and linear contraction perpendicular to the fibers. Certain materials, such as 80% A1₂O₃ – 20%SiO₂ insulation, exhibit expansion parallel to the fiber plane due to the formation of mullite in the body during exposure to elevated temperatures.

The mechanical stability of fibrous ceramic furnace insulation is highly influenced by the chemistry of the insulating material and furnace, as well as the use temperature, material shrinkage, and mechanical stress placed on the material. These low-density insulating materials are generally soft and non-self-supporting in large configurations at elevated temperatures. In their successful application in furnace chambers operating at 1700°C (3092°F), consideration must be given to all the factors mentioned above.

Furnace roofs, which experience bending stress, usually are the first part of the furnace lining to fail. Furnace walls under compressive loading, when relied on to support the roof, can buckle and slump.

Fibrous ceramic insulation becomes pyro-plastic at elevated temperatures. Materials exposed to isothermal soak conditions approaching their ultimate use temperature, will fail due to slumping under their own weight. The magnitude of slumping or sag increases dramatically as the span, load and temperature are increased and as the thickness of the material is decreased.

Fig. 5 shows the sag characteristics of simply supported beams of alumina insulation Type AL-30AA exposed to a 24-hour isothermal soak at 1600°C (2912°F). The thinner beam shows much more sag than the thicker one.

Fig. 6 shows the sag of alumina insulation Type 80% A1₂O₃ – 20% SiO₂ beams of equal size, subjected to isothermal soaks of one (1) hour @ 1600°C & 1650°C (2912°F & 3000°F).

Fibrous insulating materials subjected to isothermal conditions will fail at temperatures well below their recommended maximum use temperatures. Materials installed as insulation in furnace walls do not show the severe slumping of isothermally soaked beams because a temperature gradient exists through the thickness of the material.

Due to the "low hot strength" nature of these materials an appropriate thermal gradient must be maintained through the thickness of the furnace wall. This thermal gradient assures that a sufficient portion of any insulating layer exists at a cool enough temperature to provide mechanical support to the pyro-plastic "hot-face."

Fig. 7 shows the temperature gradient that exists in the walls and roof of a typical laboratory furnace. Note that the temperature drops sharply in both roof and sidewall, and that the cold face side of the hot-face insulation operates at a temperature well below that at which slumping will occur.

There are several common problems associated with improper application of insulation including: (1) Use of minimum thickness hot-face insulation causes "hot-face intermediate" interface temperatures to run too high. This often results in instability and failure of the hot-face lining. (2) The addition of excess back-up insulation to lower the furnace shell temperature and minimize hat losses often raises the hot-face insulation interface temperature to unacceptable levels. This also can results in instability and failure of the hot-face insulation. One of the best ways to achieve low shell temperatures is by the use of an air gap between the back-up insulation and furnace shell. Natural convective cooling or forced air can be employed. (3) Use of excessively thick hot-face insulation in an attempt to provide a "more stable" configuration often results in differential thermal expansion induced cracking and requires more "costly" hot-face insulation than is required. (4) Installing of hot-face roof insulation in a configuration where the edges extend from the hot-face all the way to the cold exterior. This often produces gradients in excess of 600°C/in. (1112°F/in.), often resulting in insulation cracking. The hot-face insulation should be fully encapsulated by the intermediate and back-up insulation layers where possible.

Porous ceramic fiber insulation possesses low thermal conductivity. Thermal conductivity is a function of product type (chemistry and density), operating temperature and fiber orientation. Thermal conductivities of ZIRCAR fibrous ceramic material range from a low of .07 W/m°K (.5Btu/hr ft²°F/in.)@400° C (1000°F) to .52 W/m°K (3.7Btu/ft²°F/in.)@1650° C (3000°F). Slightly higher thermal conductivities have been measured in the direction of the fiber plane. The direction of lowest conductivity is perpendicular to the plane of fiber or perpendicular to the thickness of the board of cylinder wall.

As thermal conductivity for a given material in any application is often difficult to arrive at, due to has permeability, losses at element terminals, and losses around door openings, temperature drop data is often accumulated for given configurations of insulation.

Wall losses for the furnace chambers to be discussed in Part II of this article have been determined to range from 3.1-3.5 w/sq in. of chamber hot-face surface area. This is an average value, which is independent of insulation orientation and wall configuration.

The insulation configuration and panel layout in the design and construction of high temperature, rapid cycle box furnaces is based on experience with chambers providing heating rates on the order of 60°C/min (140°/min) and extended soak at 1700°C (3092°F). Less rigorous design can be used for units operating at lower temperatures and with slower heating rates.

The basic chamber utilizes a multiplayer fibrous ceramic insulation system enclosed in a furnace shell, which provides an air space for convective or forced air-cooling of the cold-face. Molybdenum disilicide heating elements (Kanthal Super 33°) are installed through the roof of the chamber.

Typical multiplayer insulation is comprised of the materials shown in Fig. 8. 

The most refractory, highest cost material, is used as the hot-face layer. It is backed-up with less refractory, lower cost intermediate, and back-up insulation layers.

A proper thermal gradient is important to maintain so that an acceptable degree of rigidity exists in the furnace insulation layers. Table II shows the recommended hot-face, intermediate, and back-up insulation thicknesses in roof and wall panels required achieving the proper thermal gradient and stable insulation layers.

In small chambers, monolithic hot-face panels are adequate. As the chamber size increase, the magnitude of mechanical stress the panels are subjected to and the severity of thermal expansion induced stress increase. Due to these factors and the pyro-plastic nature of the materials there is a practical limit to the size and thickness of hot-face panels that can be utilized. Additional reinforcement in the form of internal support rods is required in larger chambers.

Table III relates recommended insulation thickness and internal support requirements to hot-face panel dimensions.