Factors for the Service Temperature of Ceramic Fiber Materials

The common raw materials for ceramic fiber materials include flint clay, coal gangue, alumina powder, quartz sand and zircon sand. The service temperature of ceramic fiber materials is mainly affected by factors such as chemical composition, impurities, moisture content, masonry, fuel and atmosphere. 

The effect of the chemical composition on the service temperature of ceramic fiber materials is obvious. The service temperatures of the common, standard, high purity, high alumina and zirconia ceramic fiber materials successively increase. The high purity, high alumina and zirconia ceramic fiber materials, due to the synthetic materials in their raw materials, have low content of impurities and high content of alumina and silica (zircon), which increases their service temperature. 

 

The moisture content is an important factor for the service temperature of ceramic fiber products that is generally not taken serious. Pores account for 90% of the ceramic fiber products. The dominant factor for the insulation performance is the air in pores. At 0℃, the thermal conductivity of water is 0.52w/m.k, which is early 20 times higher than that of air (0.0247 w/m.k) under the same conditions. The presence of water will increase the thermal conductivity of the ceramic fiber materials. If water within the fiber is frozen into ice, the thermal conductivity will be further significantly increased, since the thermal conductivity of ice is 2.32 w / m.k, 94 times higher than that of air under the same conditions. Therefore, for insulation works, the moisture content of the ceramic fiber material must be strictly controlled to ensure their thermal insulation properties.

 

In general, ceramic fiber materials are tested under atmospheric conditions. The gas in the pores is air, which actually plays a key role in the insulation of the ceramic fiber. However, in some cases, the materials are used in a vacuum, protective atmosphere or controlled atmosphere, such as hydrogen, carbon monoxide, carbon dioxide, hydrocarbons and inert gases. In this case the thermal conductivity of the materials is quite different. The thermal conductivity of these gases is higher than that of air, resulting in the increasing of the thermal conductivity of the materials. In those atmospheres, fibers react with gases and the high temperature resistant elements are reduced into gas oxides, which change the microstructure and reduce the pulverized time and the service life of the materials. 

 

The masonry methods also have impact on the service temperature of the ceramic fiber materials. In general, the thermal conductivity of the tiled ceramic fiber materials is lower than that of the stacked ones, so the service temperature of the tiled ceramic fiber materials is 50℃ higher than that of the stacked ones. 

 

The working system can be divided into continuous and batch. Generally, the service temperature of the materials in continuous kilns is 50 ℃ lower than that in batch kilns.

 

Fuels also have significant effect on the service temperature of ceramic fiber. Coal and heavy oil will produce a lot of metal oxides and carbon monoxide during the combustion process. Those metal oxides and carbon monoxide can react with the oxides in the fiber at high temperature, which will damage the structure, accelerate the pulverization of the materials and reduce its service life. Fe2O3 and FeO in the fiber can be reduced into iron at about 300℃. TiO2 can progressively be reduced TiO2 → Ti3O3 → Ti2O3 → TiO → Ti. Such oxidation - reduction reactions are reversible. The changes of the dew point and temperature of the atmosphere can cause the repeat of oxidation - reduction reaction, accompanied by the volume changes of materials and damages to the structure.