Effect of NaBO2 on the Structural Organization of Phase-Separated Bioactive Glasses and the Clustering of Si, P, and F Atoms
FERNANDES H. 1, GADDAM A. 2, TRICOT G. 3, GOLEBIEWSKI P. 4,5, BUCZYNSKI R. 4,5, FERREIRA J. 1, ECKERT H. 2,6
1 CICECO - Aveiro Institute of Materials, University of Aveiro, Aveiro, Portugal; 2 Instituto de Física de São Carlos, Universidade de São Paulo, São Carlos, Brazil; 3 LASIRE, Université de Lille, Villeneuve d’Ascq, France; 4 ?ukasiewicz Research Network - Institute of Microelectronics and Photonics, Warsaw, Poland; 5 Faculty of Physics, University of Warsaw, Warsaw, Poland; 6 Institut fu?r Physikalische Chemie, Westfa?lische Wilhelms-Universita?t Mu?nster, Mu?nster, Germany
This work presents the effect of NaBO2 addition to a phase-separated alkali-free bioactive glass with a composition of 38.49 SiO2 ? 36.07 CaO ? 19.24 MgO ? 5.61 P2O5 ? 0.59 CaF2 (mol %) prepared by melt-quenching method. The characteristic temperatures (Tg, Ts, Tc, Tp and Tm) characterizing thermal behavior and the coefficient of thermal expansion (CTE) were obtained by dilatometry and differential scanning calorimetry (DSC), while the liquid-liquid phase separation (LLPS) was observed by scanning electron microscopy (SEM). Details of the short- and medium-range order were studied by 29Si, 31P, 11B, 19F, 25Mg, and 23Na nuclear magnetic resonance (NMR).
SEM revealed binodal phase separation involving two Si microphases with a droplet size of ~200 μm, driven by the thermodynamic LLPS mechanism. The local environments and spatial distribution of silicate, phosphate, and fluoride ions in this phase-separated system studied by NMR revealed that silicate units are dominantly of the metasilicate (Si2) type, phosphate units exist mostly as orthophosphate (P0) and borate units are present in the form of pyroborate (B1).Dipolar re-coupling experiments indicate that the minority components F, P, and Na all occur within a common phase, suggesting that atomic distribution scenarios involving the separation of these components into separate phases can be excluded.
Based on the analysis of 31P spin echo decay (SED) method along with Monte Carlo simulations, the phosphate component forms clusters of sizes 1-4 nm, which are embedded in an environment more dilute in phosphate. The 31P/19F dipolar recoupling experiments suggest a local environment resembling that of fluoroapatite, while 19F SED results indicate that the fluoride ions do not form clusters and are close to randomly distributed.