Glass - Back to the Future!

Presenting Author:
Delia S. Brauer

article posted 21 March 2016

Delia S. Brauer is a junior professor at the Otto Schott Institute, University of Jena, Germany. After finishing her studies in environmental chemistry she completed her PhD on phosphate glasses at the University of Jena. She worked as a postdoctoral researcher at the University of California, San Francisco, Nagoya Institute of Technology, Japan, and Imperial College and Queen Mary University of London, UK, before returning to Jena in 2012. Her research focuses on the structure–property relationship in glasses, with a focus on degradable and highly disrupted glass systems including phosphate glasses and phospho-silicate glasses. She is a member of Technical Committee 04 (Bioglasses) of the International Commission on Glass, member of the Basic Sciences and Technology Committee of the Society of Glass Technology, Associate Editor of a new journal Biomedical Glasses and winner of the Gottardi Prize of the ICG in 2015.

The role of fluoride in the chemical nanoheterogeneity of phospho-silicate glasses

Delia S. Brauer
Otto Schott Institute of Materials Research, Friedrich Schiller University Jena, Germany

Phospho-silicate glasses of high modifier (up to 50 mol%) and low phosphate content (1 to 6 mol%) consist of a disrupted silicate network (around 90% Q2 groups), with phosphate present as orthophosphate,(1) charge-balanced by modifier ions (Figure 1a), rather than linked to the silicate network by Si–O–P bonds. While (silicate-free) phosphate glasses with an orthophosphate structure show a very high tendency to crystallise, amorphous ortho-phosphate can be obtained easily within the silicate matrix of the glasses presented here. If fluoride is introduced into these glasses, it complexes modifier cations (Figure 1b) and is present in a more fluorite-like environment.(1)

Figure 1: (a) 31P and (b) 19F MAS NMR spectra of SiO2–P2O5–CaO–(Na2O)–CaF2 glasses showing (a) phosphorus present as orthophosphate charge-balanced by Na+ and Ca2+ and (b) fluoride complexing Na+ and Ca2+ ions.(1)

Molecular dynamics simulation experiments have shown phosphate(2) and fluoride(3) clusters in the glass, while electron microscopy images suggest the presence of droplet-shaped phase separation in some compositions. This structure allows for crystallisation of orthophosphate and fluoride containing phases including fluorapatite(4) and fluorite(1) upon heat treatment, with the crystals being embedded in an amorphous silicate matrix. In addition, both phosphate and fluoride ions are released readily from these glasses when in contact with aqueous solutions. This is particularly interesting for fluoride: despite the fluoride environment in the glass showing strong similarities to the chemical structure of fluorite (CaF2), a mineral of very low solubility, the fluoride-phase of the glass possesses a relatively high solubility, most likely owing to its amorphous structure.

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3. G. Lusvardi et al., Elucidation of the structural role of fluorine…, J Phys Chem B 112 (2008) 12730-12739.
4. D.S. Brauer et al., Fluoride-containig bioactive glass-ceramics, J Non-Cryst Solids, 358 (2012) 1438–1142.