Insulating materials play a crucial role in high-voltage power transmission, protecting equipment and ensuring the reliability of the power grid by effectively isolating current and voltage. Conventional ceramic insulators (such as porcelain or aluminium oxide insulators) are widely used in high-voltage power transmission, but they have inherent limitations, including a short service life, high manufacturing costs and many surface defects, which are likely to cause electrical breakdown in the material. Furthermore, defects such as internal porosity in ceramics can lead to deterioration of their mechanical and electrical properties. In contrast, glass insulators, owing to their exceptional mechanical strength and excellent stability, have become one of the materials of choice for long-span transmission lines. Among these, sodium-calcium silicate (NCS) glass offers significant advantages, including high transparency, excellent physicochemical stability, ease of manufacture, and low cost, and thus holds great potential for application in the field of high-voltage power transmission and transformation equipment. However, the mechanical strength and electrical properties of NCS matrix glass are insufficient to meet the demands of modern power equipment for high electrical strength, high mechanical strength and low dielectric loss. Therefore, the glass composition was optimized, and the NCS matrix glass system was modified by doping with B₂O₃ and K₂O. The evolution of the network structure, optical transparency, mechanical strength and electrical properties with varying doping ratios was investigated to develop a material suitable for high-performance, long-life, high-voltage insulators.

The matrix glass composition was an NCS system. Two sets of glass samples were prepared using the high-temperature melting-quenching method. In Group B, B₂O₃ was added at varying molar fractions (0-20%(mole fraction)) while keeping the other components constant. In Group N, using the mixed alkali effect, K₂O was introduced to partially replace Na₂O, while maintaining the total content of Na₂O and K₂O constant. Subsequently, high-purity raw materials (SiO₂, CaCO₃, Na₂CO₃, H₃BO₃, K₂CO₃) were thoroughly mixed, placed in an alumina crucible and melted for 2 h at 1400 ℃. The melt was cast into preheated copper molds and annealed at 500 ℃ for 2 h to relieve internal stresses. The glass was cut into samples measuring 10 mm × 10 mm × 1 mm and 20 mm × 20 mm × 1 mm, and the surfaces were ground flat using a metallographic grinding and polishing machine. The material structure was characterized using differential scanning calorimetry (DSC), X-ray diffraction (XRD), and Raman spectroscopy. The optical properties were determined by UV-vis spectrophotometry. Mechanical strength was measured using a three-point bending test. The electrical properties included volume resistivity (measured at room temperature using a resistivity meter), dielectric loss, and dielectric breakdown strength (under DC voltage).

The addition of B₂O₃ significantly alters the glass network structure. When a small amount of B₂O₃ is introduced, its [BO₃] triangular units tend to replace some of the [SiO₄] tetrahedra of SiO₂, forming an irregular boron-oxygen network structure. This structural transformation reduces the proportion of bridging oxygen atoms in the glass network, leading to a decrease in network connectivity. As the B₂O₃ content increases further, [BO₄] tetrahedra gradually form, the number of bridging oxygen atoms increases, the glass network structure becomes denser, and the structural stability of the glass is enhanced. This structural change enhances the rigidity of the network. When 5%(mole fraction) B₂O₃ was added, the breakdown strength of the sample reached a maximum of approximately 31.93 kV/mm, with a flexural strength of 108.37 MPa. However, when the B₂O₃ content was excessive, a large number of [BO₃]/[BO₄] units appeared in the glass, leading to the breaking of the Si-O-Si chains and a decrease in network polymerization, which severely affected glass performance.

Compared to Na⁺ ions, the larger radius of K⁺ ions means that introducing K₂O increases steric hindrance within the glass network, resulting in a denser glass structure and thereby improving flexural strength. When 4% K₂O was added, the sample exhibited a flexural strength of 110.55 MPa, and the dielectric loss reached its lowest value (0.0027). The results indicate that an appropriate amount of K₂O doping for partial replacement of Na₂O can improve the glass's flexural strength and stiffness; however, the introduction of excessive K₂O may lead to an increase in defects within the glass network, thereby reducing the material's mechanical properties.

In summary, the incorporation of an appropriate amount of B₂O₃ into NCS glass, combined with the mixed alkali effect resulting from the substitution of Na₂O by K₂O, is an effective strategy for enhancing the mechanical strength and electrical properties of glass insulating materials. The results indicate that as the B₂O₃ content increases, the material's flexural strength exhibits a marked nonlinear change; when 5% B₂O₃ is added, both the flexural strength and electrical breakdown strength of the material are enhanced. The introduction of an appropriate amount of K₂O (partially replacing Na₂O) alters the polarizability of the glass via the mixed alkali effect, thereby enhancing the material's electrical breakdown strength while maintaining low dielectric loss. When the B₂O₃ and K₂O contents were 5% and 4%, respectively, the prepared sample exhibited high mechanical strength (flexural strength > 110 MPa), excellent insulation properties (room-temperature volume resistivity > 10¹² Ω·m, dielectric loss < 0.003 at 50 Hz) and high electrical strength (breakdown strength > 32 kV/mm). These findings provide a theoretical basis and material-design guidance for glass insulating materials used in high-performance electrical equipment.