Crystallization kinetics of gehlenite glass microspheres

Abstract

The glass of gehlenite composition was prepared by flame synthesis in the form of microspheres. The powder precursor was synthesised by standard solid-state reaction method using SiO 2 , Al 2 O 3 and CaCO 3 . The prepared glasses were characterized from the point of view of surface morphology, phase composition and thermal properties by optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC), respectively. The prepared samples contained only completely re-melted spherical particles. SEM did not reveal any features indicating the presence of crystalline phases. However, traces of crystalline gehlenite were detected by XRD. The high-temperature XRD measurements (HT XRD) were carried out to identify the phase evolution during glass crystallization. In the studied temperature range, gehlenite phase was identified as the main crystalline phase. Non-isothermal DSC analysis of prepared glass microspheres was carried out from room temperature up to 1200 °C at five different heating rates: 2, 4, 6, 8 and 10 °C/ min to determine the thermal properties of microspheres. In order to study the crystallization kinetics, the DSC curves were transformed into dependence of fractional extent of crystallization (α) on temperature. The Johnson-Mehl-Avrami-Kolmogorov model was found to be suitable for description of crystallization kinetics. Frequency factor A = 5.56 × 10 29 ± 1.73 × 10 29 min −1 , apparent activation energy E app = 722 ± 3 kJ mol −1 and the Avrami coefficient m = 2 were determined. In the studied system, the linear temperature dependence of nucleation rate, diffusion controlled crystal growth interface and a 2D crystal growth were confirmed.

Glass‐ceramic: Controlled crystallization of glasses obtained from biomass ash

“…As for the glass‐ceramic sample treated before the crystallization peak, that is, at temperatures of 970 and 990°C, the presence of low‐intensity diffraction peaks was observed, corresponding to the beginning of the tetragonal gehlenite phase crystallization (2CaO.Al 2 O 3 .2SiO 2 or Ca 2 Al 2 SiO 7 ) (JPCDF 2–823). Majerová et al 16 . used high‐temperature XRD and showed that the gehlenite crystallization starts at 910°C, although low‐intensity peaks were observed from 800°C in their study.…”

“…14 In general, gehlenite has a hardness between 5 and 6 Mohs, which gives the material characteristics needed for various applications, such as its use as a constituent of ceramic construction materials, bio-soluble glass fibers, and glass ceramics. 16 There are studies in the literature that report on gehlenite production; however, few describe the production of glass ceramics containing this phase and obtained from the controlled crystallization of glass produced with biomass ash, as in our study. Li et al 7 prepared glasses from blast furnace slag varying the Al 2 O 3 :SiO 2 ratio, and they obtained glassceramics from the sintering of pellets produced with glass.…”

“…However, the Al 3+ ion can also occupy a regular tetrahedral site, (Figure 1B), while the Ca 2+ ions occupy octahedral sites (Figure 1C) in the structure. 15,16 The interaction between silica tetrahedra is low, which is why double tetrahedra chemically interacts with a wide variety of ions and structural groups along the crystal lattice. 14 In general, gehlenite has a hardness between 5 and 6 Mohs, which gives the material characteristics needed for various applications, such as its use as a constituent of ceramic construction materials, bio-soluble glass fibers, and glass ceramics.…”

“…The Al 3+ and Si 4+ ions can be statistically distributed at many distorted tetrahedral sites (Figure 1A). However, the Al 3+ ion can also occupy a regular tetrahedral site, (Figure 1B), while the Ca 2+ ions occupy octahedral sites (Figure 1C) in the structure 15,16 …”

“…The interaction between silica tetrahedra is low, which is why double tetrahedra chemically interacts with a wide variety of ions and structural groups along the crystal lattice 14 . In general, gehlenite has a hardness between 5 and 6 Mohs, which gives the material characteristics needed for various applications, such as its use as a constituent of ceramic construction materials, bio‐soluble glass fibers, and glass ceramics 16 . There are studies in the literature that report on gehlenite production; however, few describe the production of glass ceramics containing this phase and obtained from the controlled crystallization of glass produced with biomass ash, as in our study.…”