Kuopei Yu, Siqi Zhang, Yuhang Liu, Wen Ni
This study systematically explores the impacts of germanium (Ge) substitution for aluminium (Al) on the structure and properties of calcium fluoro-alumino-silicate glass (4.5SiO2-3Al2O3-1.5P2O5-3CaO-2CaF2) and its derived glass-ceramics, aiming to mitigate Al-induced biological toxicity in the materials. Glass samples with 0-60 mol% Ge substitution were prepared via melting-quenching, and comprehensively characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDS). Results show that the base glass contains three crystalline phases: fluorapatite (FAp, Ca5(PO4)3F), anorthite (CaAl2Si2O8), and aluminium phosphate (AlPO4). Ge4+ integrates into the glass network as [GeO4] tetrahedra without forming independent Ge-based phases; with rising Ge substitution ratio, FAp mass fraction increases from 36% to 65%, anorthite decreases from 46% to 18%, and glass transition temperature (Tg) drops from 678 °C to 617 °C. High Ge substitution (≥40 mol%) triggers network relaxation and spherical particle formation, and local Ca/P ratio reduction drives FAp morphological reconstruction, while low Ge content (≤20 mol%) promotes regular phase crystallisation. The optimal Ge substitution level is 20 mol%, which cuts Al content by 20% to alleviate toxicity, moderately reduces enthalpy for improved sinterability, and realises synergistic mechanical reinforcement via FAp, anorthite and AlPO4. This material integrates low toxicity, favourable sinterability and excellent bioactivity, providing a novel strategy for developing low-Al, high-bioactivity glass-ceramics for dental and bone repair applications.