Effect of Germanium Substitution for Aluminium on the Properties of Calcium Fluoro-Alumino-Silicate Glass-Ceramics

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.