A nuclear magnetic resonance investigation of the structure of some alkali silicate glasses
The potential for the use of MAS and static NMR in the investigation of alkali silicate and alkali phosphosilicate glasses and glass ceramics is the main theme of this thesis. MAS NMR of binary lithium silicate glasses containing 24-29 mol% Li20 shows that their structure follows the constrained distribution model. However addition of Li20 in excess of 29 mol% causes deviation from the model and a concentration dependent disproportionation of Q3 to Q4+Q2 occurs. This is observable from MAS NMR in combination with static NMR results. The devitrification of these glasses produces two polymorphs of lithium disilicate. The structural changes during heat treatment occur aNve the glass transition temperature and are observed from the Si chemical shift and full width at half maximum. The static Li spectra for the crystallised samples exhibit a Pake doublet indicative of the presence of Li pairs. The effect of paramagnetic impurity from 0-0.8 mol% on the 29Si T1 relaxation times in Na20.2SiO2 base glasses is discussed. The heat treatment of the sodium disilicate glasses shows a -, ß-, Y- and 6- Na2Si2O5 as the devitrification products depending upon heat treatment. A way of estimating the unknown Si-O-Si mean bond angle of the polymorphs is presented. The effect of the addition of 0-70 moll P205 to alkali disilicate glasses is described. Small amounts of P205 (1-10 mol%) in the glasses results in the scavenging of alkali metal ions by phosphate groups. In sodium and potassium disilicates the phosphate groups resemble orthophosphate and pyrophosphate but only orthophosphate like units are formed in lithium disilicate glasses. As a consequence of the scavenging, the silicate network partially repolymerises. A structural model for the disilicates with (1-10) moll P2OS is presented. For larger concentrations of P205 (>10 mol%) in sodium disilicate glasses only metaphosphate species are observed and phosphorus occurs as next nearest neighbour of silicon. However this arrangement gradually changes to Si-O-Si bond with - 25 moll P 05 and the length of metaphosphate chain increases. On addition oft greater than about 30 mol% P205 some of the network silicon changes radically from its conventional four coordination to six coordination. Both the tetrahedral and octahedral environments of silicon at these concentrations are characteristic of mainly Si-O-P bonds. The six coordinated silicon occupies a SiPZO7 like environment. The proportion of "SiP2O7" depends upon the alkali content and the cooling rate. The cooling rates cause structural relaxation and are used to measure fictive temperature. This gives an estimate of the change of enthalpy for the conversion of one mole of six coordinated silicon from four coordinated silicon. The 29Si T1 relaxation times in alkali silicate glass, glass ceramics and alkali phosphosilicates are presented. The T1 in sodium disilicate glasses as a function of MnO content is a single exponential but a two component T1 is observed after heat treatment. The single component relaxation times for the disilicate composition range becomes two component as the Na20 content is reduced. The Tl values in all the lithium silicate glasses, glass ceramics and alkali phosphosilicate systems are two component. The conversion of single component exponential to two component because of heat treatment or Na20 content could be due to either nucleation or glass-in-glass phase separation. Thus the possibilities for obtaining new information about phase separation in glasses are also briefly discussed with the help of TEM micrographs.