PERFORMANCE FIBERS, INC.

FIBROUS INSULATION MATERIALS
FOR
MICROWAVE APPLICATIONS
by Mr. Ralph Grizzel and Dr. Gernot Mevec

Ceramic fiber insulation materials have been used for many years to reduce energy consumption in a variety of industrial furnace applications. More recently, these types of insulation materials are beginning to be used in microwave applications with increasing success as production scale shuttle and tunnel kilns are being built for ceramic drying and firing. This article will provide an overview of the types of insulation materials (ceramic fiber products) used in this new and evolving industry of microwave furnaces and kilns.

1. Introduction
The use of microwave energy is one of the latest areas for increasing production rates in ceramic applications through the rapid and homogeneous input of energy into the fired goods. The use of light weight insulation materials such as ceramic fibers plays an integral part in the apparent success of this new technology.  Thanks to the low apparent density, low heat capacity and resistance to thermal shock, ceramic fiber insulation allows rapid heating and cooling while minimizing the energy required to reach operating temperature. [1, 6].

2. Microwave Energy
Microwaves represent a small frequency band of electromagnetic waves.  Most common industrial applications operate between 1 and 30 GHz.  The basic idea of microwave energy is well known: Deposit the energy directly within the product and create a uniform temperature throughout the body. This technique has been used in household appliances for years. However, it took many years to translate this technique so it could be utilized in the ceramic industry for drying, heating and sintering products.  The advantages of this technology are very obvious.

• The thermal gradient within the stack of fired goods as well as within the product itself can be lowered, especially if used in conjunction with radiant energy (gas firing or radiant electric heating elements) from outside [1, 3, 5].
• Firing time can be reduced up to a quarter compared to regular radiant firing techniques. 
• Energy cost can be dropped by up to 50 percent. 
• Microstructure and crystalline phases can be influenced to modify specific properties, i.e. forming more uniform grain sizes throughout the ceramic body.
• Lower fluorine emissions in the brick industry, as well as lower hydrocarbon emissions are possible based on shorter soak times.
• Pores incorporated in the green (unfired) ceramic body will remain open longer during the firing process and allow the body to sinter to maximum density.

3. Formulation of the insulation problem
The use of microwave energy in industrial furnace applications creates new and often times demanding requirements for furnace and kiln insulation materials.  Depending on the service temperature and the energy through-put needed, these requirements limit the use of many common insulation materials. In addition to service temperature, the following are the major properties for insulating materials, which must be considered in microwave applications.

• Chemistry -- In order to prevent coupling (conversion of microwave energy into heat) of the insulation material, it is required to minimize the amount of dipole-molecules, such as zirconia, and the available free- charge carriers (free electrons), such as metals (Fe, Mn, Cr).
• Micro impurities -- For high temperature applications (> 1400°C (2550°F)) as well as equipment with high power requirement it is very important to control the level of micro-impurities (Na, K, Ca). Such micro-impurities can be found not only in the ceramic raw materials, but also in organic additives or even water.
• Dielectric properties -- Depending on temperature and microwave frequency, the dielectric properties of the insulation change and usually cause coupling at higher temperatures. For optimum performance in the electromagnetic field with minimum coupling of the insulation, materials with low dielectric constant must be selected (Al₂O₃, SiO₂).
• Electric conductivity -- For optimum dielectric properties it is also necessary to use insulation materials with low electric conductivity.

In addition to the requirement of the insulation material, the corresponding anchoring system, as well as the furnace (kiln) geometry is likely to be critical. Many common refractory anchors may not be used any more due to the fact they are metallic and therefore force the furnace manufacturers to design new anchoring systems. The furnace (kiln) geometry (see fig. 1) must be adjusted to the microwave heating system [7] as well as to the fired goods, their shape and stacking in order to provide uniform energy input into the fired goods.

4. Fibrous Materials
All the ALTRA^® insulating materials developed by RATH are based on high purity synthetic raw materials with silica and alumina as basic oxides. As mentioned above, one of the most important requirements is purity.

The two basic fibers in use are alumina (polycrystalline) ceramic fibers (>80% alumina) and regular synthetic alumino-silicate fibers (50 % silica and 50 % alumina).  It is not recommended to use any kind of so-called "HTZ" fibers, which contain between 10 and 20 % zirconia. The latest materials to enter this market are non-respirable alumina or silica fibers with a fiber diameter larger than 5 - 7 m in order to eliminate health concerns, which might appear in conjunction with common ceramic fibers with mean fiber diameters of 3 m.

In many large industrial gas fired or electrically heated kilns, blanket or module linings are the economical ways of insulating these units. Similar insulation systems are also used in microwave applications.  The major differences usually happen in the anchoring system.  No metal parts are allowed in the microwave field. Here, ceramic anchoring systems or high temperature adhesives are typically used to secure the insulation materials.

Fiber boards and shapes are manufactured from the fibers mentioned above. Besides the fibers, other raw materials are utilized to produce these materials such as alumina powders, colloidal binders and organics. All these ingredients must be pure enough, just like the fiber, to minimize any coupling effect. When using these materials in high energy microwave fields, it is even necessary to use deionized water for the production of such insulation materials to minimize trace metal impurities. These boards and shapes are basically used in the construction of smaller units, where no anchoring systems are necessary [4].

Ancillary products may be used to optimize the use of fibrous insulation materials.  Reflective coatings [2] may be applied on the hot faces of the lining for optimum insulation.  These coatings may also help to minimize any chemical attack onto the lining. Adhesives can be used to assemble board modules, glue shapes together or veneer insulation layers on top of each other.  The adhesive is one way to eliminate metal anchoring systems. Small insulation parts or prototype parts may be formed from ceramic fiber moldables, such as RATH's Fiberfoam. 

5. Applications
European as well as American companies are working on new ovens, kilns and furnaces utilizing microwave energy.  There is almost no limit to the type of industry or material, which could utilize microwave energy somewhere in it's process.  Current laboratory applications focus on rapid cycling furnaces, thermo-shock testing equipment, paper ashing furnaces and metal as well as ceramic sintering furnaces.

Industrial scale equipment is already used to manufacture parts from ceramic oxides, carbides and borides, heavy clay products, construction bricks, ceramics for electronic and electric industry, fuel cell ceramics, fiber reinforced structural ceramics, refractories, glass products, industrial ceramics, as well as parts based on metal powder technology.

6. Summary
The use of microwave energy is only one more step to faster firing cycles, lower energy input and finally more cost-effective productions. This trend , which began in household appliances, is now expanding into all kinds of industries, which have some type of heat treatment process. To make this development possible, new electric and electronic equipment, as well as insulation materials had to be developed. The RATH group has been part of the international effort to bring this technology into a commercial success. 

7. References
[1] Microwave Assisted Gas Firing of Ceramic Powders and Components (MAGF)
 EA Technology Ltd., Great Britain

[2] Optimizing of Fibre Insulation Materials up to 1800°C using Coatings
 G. Mevec, S. Bruckbacher
 cfi/Ber. DKG 70 (1993) No. 8

[3] Improving Energy Efficiency in Firing of Ceramics
 Materials World, August 1993

[4] Ceramic Fiber Insulation Boards with Improved Thermal Shock Resistance
 G. Mevec
 Am. Cer. Soc. Bulletin, Vol. 72, No. 10, October 1993

[5] Further Step for Microwave Assisted Firing
 Global Ceramic Review No. 3/95 Autumn 1995

[6] Industrial Microwave Drying Makes the Breakthrough
 MIPRO / Oeberbueren
 cfi/Ber. DKG 72 (1995) No. 5

 [7] Einsatz von Mikrowellen zum Sintern pulvermetallurgischer Produkte
 M. Willert Poranda
 Pulvermetallurgie in Wissenschaft und Praxis, Bd. 11, 1995

8. Figures
[1] 1700°C Laboratory Furnace, which can be adapted for microwave applications.

[2] ALTRA^® alumina fiber - used for fabricating insulation materials for microwave applications.

For any further questions, please give us a call at 1-800-458-RATH

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International Coordinator: info@rath-group.com

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