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Isothermal crystallization kinetics of bulk gahnite glass-ceramics

Crystallization behavior and kinetics of bulk glass, with the composition: 55.6 mol% SiO2, 22.8 mol% Al2O3, 17.7 mol% ZnO and with 3.84 mol% of TiO2 as nucleating agent, has been studied by differential thermal analysis (DTA) and X-ray diffraction (XRD) in isothermal conditions. It was determined that the isothermal DTA scans are characterized with two overlapped exothermal crystallization peaks. According to XRD data, two crystalline phases: zinc-aluminosilicate s.s. with high-quartz structure, Znx/2AlxSi3-xO6, and gahnite, ZnAl2O4, are formed after isothermal heat treatment. The high degree of overlapping disabled the kinetic analysis using DTA scans, therefore X-ray diffraction method was employed for the crystallization path determination and kinetic parameters calculation. It was determined that crystallization of zinc-aluminosilicate s.s. and gahnite are competitive processes. The crystallization process of Zn-aluminosilicate is much slower compared to gahnite. The occurrence of Zn-aluminosilicate is possible due to a long induction period for gahnite crystallization. After commence of the gahnite crystallization process, sudden retardation and halt of Zn-aluminosilicate crystallization process occurs. The activation energies of zinc-aluminosilicate s.s. and gahnite crystallization have been determined by fitting the Johnson-Mehl-Avrami (JMA) equation to experimental XRD data and by application of Arrhenius equation. The differentiated JMA curves are than compared with DTA curves and very good match has been observed. The activation energies for zinc-aluminosilicate s.s. and gahnite crystallization have been determined to be 449± ; 59 and 334± ; 42 kJmol-1, respectively. The obtained values are in concordance with previously determined values for powder sample in non-isothermal conditions and literature data.

crystallization; DTA; gahnite; isothermal kinetics; zinc-aluminosilicate; XRD

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