Investigation of the effects of various boron oxide additives on vermiculite based cordierite glass-ceramic properties

Cordierite (2MgO.2Al2O3.5SiO2) is one of the important phases of the MgO-SiO2-Al2O3 system. Cordierite ceramics have low dielectric constant (ε = 5– 6), high resistivity (ρ>1012Ωcm) and very low coefficient of thermal expansion (α = 1-2x10-6 °C-1). These properties allow cordierite to be used as a semiconductor material in various high temperature applications and in the microelectronics industry. The characteristic properties of cordierite ceramics are affected by many factors such as the composition and structure of the initial raw materials used in their synthesis and the phases in their microstructures after synthesis, impurities and production technique. Secondary crystal phases such as mullite, corundum, spinel, forsterite, ensteatite, cristobalite can often be found together with the cordierite phase in the glassy cordierite matrix structure, and this multiphase microstructure determines the final properties of cordierite ceramics. In particular, in cordierite ceramics produced by solid state sintering of oxide powders, cristobalite and spinel phases that increase the thermal expansion coefficient inevitably accompany the cordierite phase. For the production of high-density cordierite ceramics, excessive sintering causes the crystalline cordierite to decompose into forsterite and mullite phases; this significantly increases thermal expansion. On the other hand, insufficient sintering cannot develop sufficient crystalline cordierite phase for low thermal expansion and high shock resistance (Akpinar et al. 2015). Cordierite, which has a melting point of 1460 °C, is difficult to sinter because it has a very narrow sintering temperature range (1300-1400 °C) (Kumar et al. 2015). For cordierite ceramics composed of various raw material mixtures, sintering aids are often needed to achieve full densification. However, the thermal and electrical properties of cordierite, which is vital in various fields such as circuit boards, catalytic converters and thermal insulators, are adversely affected by the formation of secondary crystalline phases in the case of the use of sintering additives (Akpinar 2019). Therefore, the need for sintering additives makes it very difficult to obtain single-phase cordierite ceramics due to their impurity sensitive nature (Oliveira and Fernandes 2002). Studies on cordierite show that it is very difficult to heat at sintering temperature without sintering aids (Thorp and Hutton 1981). n this thesis, for the production of single-phase cordierite ceramics, it is aimed to melt the Vermiculite raw material, which has a composition similar to cordierite cytochiometry, by doping with various boron oxide additives (borax, boric acid, colemanite) with good melting properties at a rate that does not significantly affect the purity and technical properties of the final product, and then melting the glass. It is aimed to produce cordierite glass-ceramic by transforming it into a structure and then crystallization of the glass structure at the appropriate temperature. In this context, the effects of boron oxide additives on the phase, microstructure, physical, mechanical and thermal properties of the final glass-ceramic structure will be investigated.