• Monteiro, R. C. C.

Abstract

The crystallization kinetics of two glass compositions from the Li2O-K2O-ZrO2-Al2O3-SiO2-P2O5 system having different SiO2/Li2O ratios was investigated. The crystallization parameters (crystallization activation energy (Ec) and Avrami exponent (n)) were evaluated from the data obtained by differential scanning calorimetry (DSC) performed at different heating rates, which were combined with the results from X-ray diffraction (XRD) and scanning electron microscopy (SEM). DTA curves for both glass compositions, S1 (SiO2/Li2O = 2.39) and S2 (SiO2/Li2O = 3.39), exhibited two exothermic crystallization peaks whose maximum peak temperatures of crystallization (Tp1 and Tp2) were lower for S1 than for S2. XRD results revealed that lithium metassilicate (Li2SiO3) was the first crystalline phase to be formed and that the second phase to appear was lithium disilicate (Li2Si2O5). For both glass compositions, the activation energy for crystallization calculated on the basis of the obtained Tp1 values (Ec1), that is related to Li2SiO3 formation, was much lower (275±5 kJ/mol for S1 and 251±7 kJ/mol for S2) than the activation energy for crystallization determined from Tp2 data (Ec2), related to Li2Si2O5 formation (434±12 kJ/mol for S1 and 450±8 kJ/mol for S2). This was in agreement with the results determined from the study of the effect of crystallization fraction (c) on the crystallization activation energy (Ec(c) that, for 0.1 ≤ c ≤ 0.9, showed Ec1(c) values in the range 256-230 kJ/mol and 220-240 kJ/mol for S1 and S2, respectively, and Ec2(c) values in the range 462-424 kJ/mol and 446-510 kJ/mol for S1 and S2, respectively. The Avrami exponent (n) calculated for the crystallization of Li2SiO3 in both S1 and S2 compositions varied between 1.1 and 1.5, corresponding to a surface crystallization mechanism with one-dimensional crystal growth. This agrees with the needle-like habit of the crystals observed by SEM in the microstructures of S1 and S2 glass samples heat-treated at 800 °C. The value of n calculated for the formation of Li2Si2O5 in both S1 and S2 compositions was close to 3 (varied between 2.7 and 3.1) indicating a bulk crystallization mechanism with three-dimensional crystal growth. This was confirmed by SEM analysis, which revealed a fully crystalline microstructure with multi-shaped and spherical crystals that were present in the glass samples after heat-treatment at 900 °C

Topics

• impedance spectroscopy
• microstructure
• surface
• scanning electron microscopy
• x-ray diffraction
• crystalline phase
• glass
• glass
• differential scanning calorimetry
• Lithium
• activation
• crystallization
• one-dimensional
• differential thermal analysis