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Title: High resolution multinuclear nuclear magnetic resonance studies of oxide glasses
Author: Lockyer, Martin William Geoffrey
ISNI:       0000 0001 3612 0336
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 1993
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Multinuclear Magic Angle Spinning nuclear resonance has been used as the main technique in the investigation of the structure of glasses from three different systems. A series of K2O. 4SiO2 based glasses with P2O5 addition have been investigated via natural abundance 29Si, 31P and 27A1 (present as an impurity) MAS NMR. The 29Si spectra reveal the occurrence of octahedrally coordinated silicon in addition to the normal tetrahedrally coordinated silicon for glasses with analysed P2O5 contents in excess of 28 mol%. Increasing the P2O5 concentration increases the relative proportion of the higher coordination species. IRAS also detected the presence of Sivi but only for concentrations >10% of the total Si content. 27A1 MAS NMR has shown the presence only of four coordinated aluminium for 5 mol% P2O5, whereas for 10 and 15 mol% small amounts of six and possibly five coordinated aluminium species occur. Higher concentrations of P2O5 also increase the amount of higher coordinated aluminium until 28 mol% when practically all the aluminium is octahedrally coordinated with phosphorus in the next nearest shell. The 31P MAS NMR spectra show the formation of metaphosphate chains for P2O5 concentrations greater than 10 mol%, which increase in length and become more disordered as the P2O5 content rises. The relative concentration of octahedral silicon has been found to be a function of fictive temperature, with the proportion of Sivi higher for higher Tf. An estimated energy for the conversion Siiv to Sivi was obtained, of 9.98 kJ/mol, indicating the reaction is exothermic. The relative proportions of Siiv and Sivi also affect the measured density of the glasses, density increasing as the proportion of Sivi increases. Potassium phosphosilicate and silicon phosphate xerogels were also prepared, but it did not prove possible to prepare amorphous samples free from water or organic residue. MAS NMR indicates that no Si - O - P bonding was formed below 500°C and XRD shows the presence of both SiP2O7 and Si5O(PO4)6 as crystal phases in the heated potassium phosphosilicate xerogel and Si3(PO4)4 in the silicon phosphate xerogel. Multinuclear MAS NMR investigations of Na2O. CaO. SiO2 . P2O5 glasses has indicated a preferential association of Na+ ions with the Q3 species and Ca2+ with Q2 units. The glasses are believed to consist of a mixture of two amorphous silicate environments similar to calcium metasilicate and sodium disilicate, which has been interpreted in terms of a possible model for the structural requirements for bioactivity. The glasses were subsequently converted into glass- ceramics via a controlled heat treatment process and the resulting structures investigated via MAS NMR and XRD. It has been established that all compositions prepared from within the bioactive region of the phase diagram contain Na2CaSi3O8 as the major crystalline phase. Certain crystal phases that went undetected by XRD were observed via MAS NMR. Glasses prepared from the PbO . P2O5. A12O3 system have shown some interesting structural species. Compositions with >10 mol% A12O3 contained Al in 4,5 and 6 coordination with a variety of next nearest neighbours. It is supposed that this multiple coordination stabilises longer metaphosphate chains than previously observed in glasses. In PbO . P2O5. glasses with >46.6 mol% PbO the network contains (PbO4) tetrahedra which act in an apparent intermediate-like role in a similar manner to that observed in PbO . SiO2 glasses. This study has emphasised that multinuclear MAS NMR is an ideal technique for the investigation of completely or partially amorphous inorganic materials, particularly when employed in conjunction with other techniques such as thermal analysis, IRAS and XRD.
Supervisor: Not available Sponsor: Science and Engineering Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics ; QD Chemistry