Structural effects of Zn+2/Mg+2 ratios on crystallization characteristics and microstructure of fluorophlogopite mica-containing glass-ceramics

• Zn2+/Mg2+ substitution in SiO2–MgO–Al2O3–K2O–B2O3–MgF2 glass supports to develop Si–O–Zn network.
• These are crystallized at 1050 °C to form [KMg3(AlSi3O10)F2] which controls thermal, mechanical properties.
• When Mg2+ is substituted by Zn2+, the [Zn2SiO4] crystals also formed and become predominant with more ZnO content.
• Microhardness of glass/glass-ceramic first increased on addition of 5 mol% ZnO and then decreased.

We report the effect of Mg+2 substitution (by Zn+2) on crystallization kinetics, microstructure, thermal and mechanical properties of boroaluminosilicate glass. Zn2+ was selected for Mg2+ on the basis of similar ionic radius in six coordination system (Mg2+∼0.72 Å, Zn2+∼0.75 Å). The melt-quenched glasses with SiO2–(1 − x) MgO–Al2O3–K2O–B2O3–MgF2 (BPAS)/x ZnO system, have been investigated to establish the effect of Zn+2/Mg+2 ratios. It is found that the density of BPAS glass without zinc content is 2.52 g/cm3 and increased linearly on substitution of Mg2+ by 5–32 mol% ZnO. Tg and Td of BPAS glass initially increased on adding 5 mol% ZnO and then decreased on further addition. From DSC study, it is found that the crystallization exotherm changes significantly in the temperature range 750–1000 °C, where different crystalline phases are formed, and the activation energy of crystallization (EC) varies in the range of 254–388 kJ/mol. The crystalline phases formed in opaque BPAS glass-ceramic, derived by controlled heat treatment at 800 and 1050 °C (4 h), are identified as fluorophlogopite [KMg3(AlSi3O10)F2] mica and willemite (Zn2SiO4) by XRD technique, and confirmed by FTIR spectroscopy. The change of crystallization phenomena varying Zn+2/Mg+2 ratios correspond to significant microstructural change. A wide range of thermal expansion (CTE) values are obtained for the BPAS glasses and corresponding glass-ceramics. CTE (50–500 °C) of BPAS glass without zinc content is 7.76 × 10−6/K, and decreased sequentially on increasing Zn+2/Mg+2 ratio. The density of glass-ceramics after heating at 800 and 1050 °C increased linearly with increasing Zn+2 substitution for Mg+2. Microhardness of the BPAS glasses is in the range of 4.26–6.15 GPa and found to be increased to 4.58–6.78 GPa after crystallized at 1050 °C.

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