Thermal and Structural Effects of Calcium in Borophosphate Bioactive Glasses

A series of borophosphate glasses with the composition (65-x) B ₂ O ₃  + 15 P ₂ O ₅  + 20 Na ₂ O + x CaO (x = 0, 2.5, 5, 7.5, 10, and 15 mol%) synthesized by melt-quenching technique. XRD analysis confirmed the amorphous nature of the glass. FTIR spectroscopy used to investigate structural changes in the borophosphate glass with varying CaO content. In BPNC0 sample, bands were observed between 500–650 cm⁻¹ (B-O-B), 1050 cm⁻¹ (P-O-P), 1465–1471 cm⁻¹ (H-O-H), and 3215–3446 cm ^− 1 (O-H). With CaO incorporation, notable shifts and new bands emerged. Broader bands from 400–1600 cm ^− 1 in higher CaO contents indicated the presence of metallic bonds, network depolymerization and structural rearrangements confirming the role of CaO as a network modifier. Physical parameters systematically evaluated, revealing a densification trend with increasing CaO content. Density fluctuated between 2.163 to 2.532 g/cm ³ , while molar volume fluctuated between 46.34 to 39.05 cm ³ /mol, indicating tighter atomic packing. OPD fluctuated from 69.73 to 62.58 mol/L, suggesting enhanced structural compactness. Optical absorption spectra analyzed via the UV-Vis-NIR spectrometer, the Urbach rule showed a redshift in the absorption edge, with the optical band gap fluctuating from 2.470 to 3.162 eV as CaO content increased, implying the generation of non-bridging oxygen atoms and expanded conduction bands. The metallization criterion calculated from refractive index and band gap values fluctuated from 0.351 to 0.397 by increasing CaO, indicating higher metallic character; these findings point to a tunable electronic structure. The thermal stability of the glass has been determined by means of Differential Scanning Calorimetry/Thermogravimetric Analysis (DSC/TGA). From the DSC/TGA curve, the glass transition temperature ( T _g ), crystallization temperature ( T _c ) and melting temperature ( T _m ) have been identified and thermal stability is also calculated. The tailored compositional design significantly affects bioactivity, positioning these borophosphate glasses as promising candidates for bone regeneration.