1. Cordierite
Cordierite (Lolite, Dichroite) is an aluminum-magnesium silicate. Its formula is (Mg, Fe2+)2Al3 [AlSi5O18]·H2O, usually light blue or light purple, glass luster, transparent to translucent.
Cordierite occurs in schist, gneiss, and altered igneous rocks. Magnesium cordierite can be synthesized artificially for refractory materials.
Characteristics and Application
It has a fairly low density and a moderately low thermal conductivity among oxide-based engineering ceramics. So it is widely used in ceramics and glass industries to improve its ability to resist sudden cooling and rapid heating.
2. Mullite
Mullite (Cyanite; Disthene; Kaopolite; Kyanite) is a series of minerals composed of aluminum silicates. It can be presented in two stoichiometric forms: 3 (Al2O3) 2 (SiO2), or 2 (Al2O3) SiO2
It is rarely found in nature and is usually made synthetically and sold as a white powder.
Characteristics and Application
Based on two of its main features, it has two large groups of applications. On the one hand, its hardness, allows it to be used as a basic component for abrasion, for specific applications of high hardness.
On the other hand, its resistance to high temperatures makes it a perfect refractory material, both as a contribution to ceramics or clays to improve its properties and to work as a refractory in its synthetic form.
3. Cordierite-Mullite
Most often, ceramic kiln shelves are made from cordierite and mullite powder, which is mixed with other ceramic materials like clay, and then shaped, dry, and heated to form kiln shelves.
With (SiO2)n ring structure with a large gap, cordierite has the characteristics of a low thermal expansion coefficient, while mullite has a large thermal expansion coefficient, which leads to fine cracks forming at the interface between the two phases due to a mismatch between the expansion coefficients of the two materials.
The formation of fine cracks at the phase interface can effectively reduce the thermal stress caused by temperature changes, effectively avoid the spread of catastrophic cracks, and greatly improve the thermal stability of cordierite-mullite refractories.
At the same time, due to the high refractoriness of mullite crystals (melting point is 1810 degrees) and high mechanical strength (bending strength at room temperature is 49MPa), the combination of the two can effectively improve the high-temperature strength of kiln furniture.
