As phase-engineered composites integrating amorphous and crystalline domains, glass-ceramics synergistically retain a high transparency of vitreous materials, while achieving ceramic-like mechanical robustness. Transparent glass-ceramics include systems such as ZnO-MgO-Al₂O₃-SiO₂, Li₂O-Al₂O₃-SiO₂, etc., in which the precipitation of spinel crystalline phase can be controlled in ZnO-MgO-Al₂O₃-SiO₂ (ZMAS) system. The spinel crystal phase has a high Mohs hardness as well as a high modulus of elasticity, so the ZMAS system glass-ceramics with a spinel crystal phase has superior mechanical properties. The introduction of Li₂O into ZMAS glass-ceramics can refine the grain size and also promote the precipitation of crystal phases such as β-quartz solid solution with a low coefficient of thermal expansion, which can adjust the overall coefficient of thermal expansion of the glass-ceramics and improve the thermal shock resistance of the glass-ceramics. β-quartz solid solution crystals are close to the refractive index of the glass, which can significantly reduce the scattering of light and improve the transparency of the glass-ceramics. In this work, Li₂O-ZnO-MgO-Al₂O₃-SiO₂ glass system was selected as a research subject to investigate the effect of Li₂O content on the crystallization behavior and properties of the glass. The study revealed that the introduction of Li₂O exerted a modulating effect on the crystallization behavior, optical properties, and mechanical performance of the glass-ceramics.

The parent glasses were prepared by a melting method, and the components were designed as (54-x) SiO₂-17 Al₂O₃-x Li₂O-3 Na₂O-14 ZnO-7 MgO-3 ZrO₂-2 SnO₂ (%, x=0, 2, 4, 6). The glass was melted at 1600 ℃ for 3 h. The glass solution was poured into a copper abrasive tool that was preheated at 590 ℃ for moulding. The formed glass blocks were annealed in an annealing furnace at 590 ℃ for 5 h to remove the internal stresses in the glass, and the glass was cut into blocks with the thickness of 1.5 mm for subsequent testing. The chemical state of the elements in the base glass was characterized by a model ESCALAB250Xi X-ray photoelectron spectrometer (XPS, Thermo Fisher Co., USA). A glass powder was used for testing, corrected using the electron binding energy of C1s. The DSC curve of glass was determined by differential scanning calorimetry (DSC).. The crystalline phase of the prepared samples was examined by an X-ray diffractometer. The base glass and microcrystalline glass microstructures were analyzed by a model Ultra Plus field emission scanning electron microscope (FESEM, Zeiss Co., Germany) with an additional Ultim Max 65 energy spectrometer (JSM-IT800). The optical properties of the glass and microcrystalline glass samples after double-sided optical grade polishing were characterized by a model UV3600 UV-visible spectrophotometer (UV-vis, Shimadzu Co., Japan). The Vickers hardness of the polished glass samples was measured by a model HV-1000A microhardness tester (Huayin Co., China) with a load of 1.96 N and a dwell time of 10 s. Each glass specimen underwent five replicate measurements to minimize experimental errors. The coefficient of thermal expansion (CTE) was determined by a model DIL 402C thermal dilatometer (Netzsch Co., Germany). The glass was cut into some rectangular bars with the sizes of 4 mm × 4 mm × 25 mm for dilatometric analysis.

The free oxygen content in the glass network increases with the increase of Li₂O content from 0 to 6% (in mole), leading to a reduction in the degree of network polymerization. This structural change can reduce the glass transition temperature from 745 ℃ (i.e., at 0 mol% Li₂O) to 664 ℃ (i.e., at 6% Li₂O). The β-quartz solid solutions form as the Li₂O content increases to 2%. The temperature required for the formation of β-spodumene crystals decreases as the Li₂O content further increases, leading to an increase in the β-spodumene crystalline phase content and suppression of spinel crystal precipitation. The transmittance also undergoes a sharp decline due to the influence of large-sized β-spodumene crystals. The glass-ceramics with 2% Li₂O content, after crystallization at 850 ℃, exhibit a superior performance (i.e., a Vickers hardness of 8.62 GPa, a transmittance of 84%, and a flexural strength of 199 MPa).

Increasing Li₂O content from 0 to 6% could promote the depolymerization of the glass network, leading to a reduction in the glass transition temperature. At 2% Li₂O, this composition effectively suppressed grain agglomeration and promoted the formation of β-quartz solid solution crystalline phases, thereby enhancing the transparency of the glass-ceramics. The coexistence of spinel phases and β-quartz solid solution improved the comprehensive properties of the glass-ceramics (i.e., a Vickers hardness of~8.62 GPa and a flexural strength of~199 MPa).