Interrelation between transport properties in phosphate glasses through their atomic structure

The development of the processing and design of phosphate glasses in a wide range of new technological applications requires the complete understanding of their physical and chemical properties. The progress on science and glass technology increasingly depends on the ability to optimize the glass composition and on controlling the structure of the materials, through the characterization of the structure and the physico-chemical properties. The relationship between composition, structure and properties is an essential methodology in the modern glass science. The aim of this thesis has been to study the full process that runs from the generation of basic knowledge, to its application, in this case on their processing as bioactive phosphate glass fibres. For this porpoise, the relationships between chemical composition, structure and dynamics of the processes taking place within the phosphate glass network, which directly influence the processing and design of new glasses, has been investigated in depth. This assumption is based on the hypothesis that it is possible the modelling of the transport properties from structural data, and that a common model for the viscosity-T curves can be obtained through the study of the rheology of phosphate glasses when a certain composition is given. The particular milestone has been to establish a relationship between the main structural parameters, such as glass network polymerization degree, coordination number of the modifying cations or the nature of the metal-oxygen bonds; and the properties, such as electrical conductivity, chemical durability and fragility. To that end, a series of binary and ternary metaphosphate glasses with different modifying oxides of alkali, alkaline earth and zinc elements have been studied together with glasses with different glass former content (P2O5) and with more than one network former (P2O5 and B2O3). First of all, in order to get more insights into the compositional and structural influence on transport properties in pure phosphate glasses, different glass systems have been prepared to determine the role played by the different modifying oxides and by the proportion of P2O5 present in the glass. The ionic conductivity (σ) and viscosity have been studied and related with the short and medium range order structure by means of Nuclear Magnetic Resonance and Raman spectroscopies. The kinetic fragility (m) of each glass has been determined by fitting the viscosity experimental data to Vogel-Fulcher-Tamman (VFT), Avramov-Milchev (AM) and Mauro-Yue-Ellison-Gupta-Allan (MYEGA) equations. Finally, the activation energy of viscous flow, and its correlation with composition and structure, has also been studied. It has been found that the presence of cations with lower cationic potential (Z/a) in the glass network, leads to less covalent MO bonds. The decrease on the covalent character of the glass network and, therefore, the weakening and subsequent decrease of the overall network’s rigidity, are reflected on the short and medium range order structure and, as a consequence, the main glass properties will also be influenced up to some extent by those. On the other hand, variations on the glass network connectivity, especially the appearance of Q units in the glass network, causes a more pronounced increase on the NBO per tetrahedral unit, which seems to provoke a preferential arrangement of the Q groups units in chains instead of rings. A higher proportion of chains seems to lead to a denser glass network that influences on the flow of the PO4 units through it and thus on the properties. The prediction of properties with enough accuracy has been for many years one of the most important goals that the glass science community wanted to achieve in order to offer an alternative to the time consuming experimental studies. On the basis of the relationships found between the structure and properties, the first steps towards the modelling of glass compositions with desired fragility trough the prediction of the viscosity-temperature curves in the whole temperature range has been achieved. In particular, the similarity found between the calculated and experimental viscosity points in the melt has validated the use of the relationships found and allowed establishing an alternative to the experimental determination of viscosity data at high temperatures. Once the influence of the addition of different modifying oxides and variation on the connectivity degree of the glass network has been investigated and established, studies on glasses with more than one network former oxide were carried out. In recent years, borophosphate glasses have received increasing attention because of their interesting optical and electrical properties. It is well known that these glasses, where two glass formers are mixed, show a pronounced non-linear change in their properties, which is commonly known as the so-called mixed network former effect (MGFE). In the light of the wide existing literature found about structural studies in borophosphate glasses, it is noteworthy the effort made by the glass community to better understand the effect of composition on structure and on physical properties, and to bring some light to the MGFE in order to predict the compositional dependence of the properties. Nevertheless, to the best of the author ́s knowledge just few works address a systematic study on the rheological properties of alkali borophosphate glasses through the determination of their fragility. In this thesis, the