Calcium aluminogermanate glasses for optical fiber applications

This study investigates the structural, thermal, optical, and mechanical properties of calcium aluminate glasses upon incremental substitution with germania (GeO₂). A series of glasses with the molar formula xGeO₂·(1−x)(0.64CaO·0.36Al₂O₃), where x = 0–0.50 mole fraction, was synthesized and characterized. Germanium speciation was analyzed, showing a transition from Q² to Q⁴ and N₆ species with increasing GeO₂ content, which results in enhanced network connectivity. The incorporation of GeO₂ increases glass density and oxygen packing efficiency due to the germanate anomaly, with Ge shifting to higher coordination states. The liquid fragility index is observed to decrease with GeO₂ addition, indicating improved thermal stability and broader processing windows. Mechanical testing revealed a reduction in moduli and hardness with GeO₂ content, attributed to the replacement of stronger Al–O bonds with more flexible Ge–O bonds. Optical measurements showed improved ultraviolet and visible transparency, with a progressive red shift in the UV cutoff and a slight decrease in the optical band gap. Notably, infrared transmission was enhanced by GeO₂ incorporation, extending the transparency window and reducing OH⁻-related absorption. These results suggest that partially germanated calcium aluminate glasses offer a promising platform for cost-effective mid-IR applications, combining thermal robustness, improved processability, and expanded IR transmission with relatively low material cost.