Cordierite

Cordierite is a magnesium alumino silicate material with excellent thermal, electrical and chemical properties. It has a very low thermal expansion coefficient. Cordierite also has a very low dielectric constant and high melting point of 1460ºC. Due to these properties, researchers have found cordierite to be a potential material for use in gas turbines, industrial and lab-scale furnaces, and electronics packing materials. Other uses for cordierite include integrated circuits due to its low dielectric constant . As stated above, cordierite is a magnesium alumino silicate system, and Rice husk ash can therefore be used as a silica source for cordierite. It is used an alkali extraction method for extraction of silica from rice husk ash followed by solid-state mixing of the raw materials for development of cordierite. For the later step, i.e. development of cordierite, they mixed all the raw materials in an alcohol medium in an appropriate ratio of 2:2:5. The dried mass was palletized using uniaxial hydraulic pressing and sintered at temperatures of 1050–1350ºC. Similarly, Kurana used solid-state mixing of raw materials for development of the cordierite phase. They used RHA in place of kaolinite for development of cordierite. The milling was conducted in a planetary ball mill, however, and the medium was water. A sample was palletized at 1.96 MPa and sintered at 950-1350ºC. The ramp rate for temperatures up to 1000ºC was 5ºC/min and 2.5ºC/min for higher temperatures with a holding time of 1 h. They first prepared separate sols of silica and other ingredients and then mixed them. The mixtures were dried at 90ºC, followed by calcination at 400–1400ºC. In all three studies, formation of α–cordierite occurred at temperatures above 1200ºC. µ-cordierite was formed at lower temperatures, i.e. below 1200ºC, but the transition from µ-cordierite to α–cordierite occurred only at above 1200ºC. The formation of µ-cordierite at lower temperatures indicates that a diffusion reaction between cristobalite and spinel (MgAl2O4) starts at this temperature [83]. Moreover, At high temperatures > 1200ºC this µ-cordierite starts converting into α-cordierite, moreover, due to the formation of an Mg–O–Al–O–Si bond during phase transformation [57,82,83]. It can therefore be observed that active silica present in RHA promotes conversion of µ-cordierite to α–cordierite at low temperatures (1300ºC) compared to other conventional sources (1400ºC) due to its high surface area and fluxing properties. Besides this, application of RHA leads to a decrease in the activation energy required for crystallization of α–cordierite.

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Source - https://www.tandfonline.com/doi/full/10.1080/21870764.2018.1539210