Chemical stability is a core characteristic of optical glass, enabling it to resist acid and alkali corrosion and maintain stable performance. This is especially crucial in complex environments, such as chemical reagent storage containers, optical instrument lenses, or industrial equipment viewing windows, where compositional optimization is necessary to enhance its corrosion resistance. Strengthening chemical stability requires a multi-dimensional, collaborative design approach, including adjusting the basic composition, introducing network modifiers, adding special oxides, and doping with rare earth elements, to construct a dense glass network structure and reduce the risk of environmental corrosion.
Silica (SiO₂) is a fundamental component for improving chemical stability. As a glass network former, high silica content can construct a three-dimensional framework structure dominated by silicon-oxygen tetrahedra, making the glass network denser and thus enhancing its resistance to acid and alkali media. For example, quartz glass, composed entirely of silicon-oxygen tetrahedra, has extremely high silicon-oxygen bond energies and is almost impervious to water or weak acids. However, it is important to note that if the glass also contains large amounts of alkali metal oxides (such as Na₂O and K₂O), these components can disrupt the silicon-oxygen network and reduce chemical stability. Therefore, a high silica content must be combined with a low alkali metal oxide content to effectively improve corrosion resistance.
The introduction of alumina (Al₂O₃) can significantly enhance the chemical stability of glass. Alumina exists in glass as aluminum-oxygen tetrahedra, filling the gaps in the silicon-oxygen network and forming a more stable mixed network structure through the alternating connection of [AlO₄] and [SiO₄]. This structure effectively hinders the penetration of water molecules or ions (such as H⁺, OH⁻), thereby slowing down the corrosion rate. Furthermore, alumina can improve the mechanical strength and thermal stability of glass, allowing it to maintain structural integrity under high temperature or mechanical stress environments, further reducing the risk of corrosion.
The addition of boron oxide (B₂O₃) requires precise control following the "boron anomaly." At low concentrations, boron exists as [BO₄] tetrahedra, which can partially replace [SiO₄] to strengthen the network structure and improve chemical stability. However, when the concentration exceeds a critical value (e.g., Na₂O/B₂O₃ < 1), boron exists as [BO₃] trigonometric forms, resulting in a loose network structure and reduced corrosion resistance. Therefore, it is necessary to adjust the ratio of boron to other oxides (e.g., the synergistic effect of B₂O₃ and SiO₂ in borosilicate glass) to achieve a balance between chemical stability and processing performance.
Zirconium oxide (ZrO₂) is a key component for improving the acid and alkali corrosion resistance of glass. Zirconia exists in glass as zirconium-oxygen tetrahedra, and its high bond energy (Zr-O bond energy is significantly higher than Si-O bond energy) can form locally highly stable regions, effectively blocking the intrusion of corrosive media. Furthermore, zirconium oxide can also reduce the risk of localized corrosion caused by compositional inhomogeneity by inhibiting glass phase separation. For example, adding zirconium oxide to phosphate optical glass significantly improves its hydrolysis resistance, making it suitable for high humidity or acidic environments.
Doping with rare earth oxides (such as La₂O₃ and CeO₂) can further optimize the chemical stability of the glass. Rare earth ions, with their high charge and large ionic radius, can form strong bonds within the glass network, enhancing its resistance to corrosion. For instance, cerium oxide (CeO₂) not only improves the chemical stability of the glass but also inhibits electrochemical corrosion reactions on the glass surface through its redox properties. Furthermore, rare earth oxides can improve the optical properties of the glass (such as high refractive index and low dispersion), making it more widely applicable in high-end optical instruments.
The introduction of phosphates (P₂O₅) can construct unique glass network structures. Phosphate glasses, with phosphorus-oxygen tetrahedra as the main chain, can be combined with other oxides (such as Al₂O₃ and ZrO₂) to form glass materials that possess both high chemical stability and excellent optical properties. For example, SZP phosphate glass powder (SrO-ZnO-P₂O₅-based system) constructs a stable framework through a high content of P₂O₅, while adding ZnO and SrO to adjust properties, ensuring its stability within a pH range of 2-12, making it suitable for complex chemical environments.
Enhancing the chemical stability of optical glass requires the synergistic effect of multiple components such as silica, alumina, boron oxide, zirconium oxide, rare earth oxides, and phosphates to construct a dense and stable glass network structure. This process must balance chemical stability, optical properties, and processing performance to meet the needs of different application scenarios.
