Cordierite bricks and kiln furniture: Cordierite bricks are a type of refractory material with cordierite (2MgO·2Al2O3·5SiO2) as its main component. Pure cordierite contains 13.7% magnesium oxide (MgO) and has the following characteristics: a low coefficient of thermal expansion of 3×10⁻⁶/℃. Due to its low coefficient of thermal expansion, cordierite materials generally exhibit excellent thermal shock resistance.
Pure cordierite is an expensive material, so lower purity semi-cordierite materials are often used as substitutes. Semi-cordierite materials also exhibit a low coefficient of thermal expansion.
Semi-cordierite materials are frequently used as kiln furniture, kiln car fixing blocks, and ceramic components in ceramic kilns. In some cases, alumina-silicate materials are also used for the same product applications. Table 7 lists the characteristics of some semi-cordierite products used for kiln furniture/kiln blocks. The maximum service temperature of these products is typically 1200℃ or higher.
Table 7 shows the relationship between magnesium oxide content and the coefficient of thermal expansion. When the magnesium oxide content is very low, the coefficient of thermal expansion is close to that of clay bricks or mullite [approximately 6 × 10⁻⁶/℃]. Figures 5 and 6 show the microstructure of such typical products.
 |
| Figure 5: Microstructure of hemicordierite extruded into pyrophyllite |
|
| Figure 6: Microstructure of clay-based hemicordie bricks |
The microstructure of cordierite exhibits "crack-like" pores, a typical characteristic of extruded clay products (Figure 5). The large size of the pyrophyllite aggregate particles, sometimes approaching 3 to 4 mm, is not clearly visible in this micrograph. In contrast, the microstructure of pressed cordierite shows that the refractory aggregate particles are surrounded by a sintered clay matrix, with predominantly circular pores (Figure 6). Many experts believe that the presence of fine circular pores improves thermal shock resistance. Therefore, pressed products with approximately 0.9% MgO content (Table 7) may have similar properties to extruded products with 3.0% MgO content.
A typical impurity in cordierite bricks is quartz, which is converted from "free" silica (SiO2) or quartz in the raw materials when the refractory is fired at a temperature of at least 1300°C. By referring to the MgO-Al2O3-SiO2 phase equilibrium diagram (not shown), it can be seen that free cristobalite is an equilibrium phase unless the product composition is exactly the same as pure cordierite. Excessive cristobalite content can lead to "caking" and reduce the lifespan of kiln furniture/kiln blocks. Crizobalite can be conveniently determined using X-ray diffraction (XRD) or thermal analysis (TA) techniques.
