<p> The responses of structure and properties to composition and temperature have been investigated for glasses in TiO<sub>2</sub>-SiO<sub>2</sub> and Na<sub>2</sub>O-TiO<sub>2</sub>-SiO<sub>2</sub> systems. Additionally, the response of Na<sub>2</sub>O-TiO<sub>2</sub>-SiO<sub>2</sub> glasses to plastic deformation has been studied. (x)TiO<sub>2</sub>-(1-x)SiO<sub>2</sub> glasses were prepared through the sol-gel process with compositions 0 ≤ x ≤ 10 mol% and compared to commercial glasses prepared through flame hydrolysis deposition with x = 0, 5.4, and 8.3 mol%. (x) Na<sub>2</sub>O - (y) TiO<sub> 2</sub> - (1-x-y) SiO<sub>2</sub> glasses were prepared with x = 10, 15, 20, and 25 mol% and y = 4, 7, and 10 mol% through a melt-quench process. Density and index of refraction of glasses was measured through the Archimedes's method and using a prism coupler, respectively. The glass transition temperature of Na<sub>2</sub>O-TiO<sub>2</sub>-SiO<sub>2</sub> glasses was measured through differential thermal analysis. </p><p> The structure and elastic moduli have been studied through Raman spectroscopy and Brillouin light scattering, respectively, at room temperature and in-situ up to 1200 °C for TiO<sub>2</sub>-SiO<sub>2</sub> glasses and up to 800 °C for Na<sub>2</sub>O-TiO<sub>2</sub>-SiO<sub>2</sub> glasses. Young's modulus was observed to decrease from 72 GPa to 66 GPa with the addition of 8.3 mol% TiO<sub>2</sub> in TiO<sub>2</sub>-SiO<sub>2</sub> glasses and to increase from 65 GPa to 73 GPa with the addition of 10 mol% TiO<sub>2</sub> in 10 Na<sub>2</sub>O - (0-10) TiO<sub>2</sub>-SiO<sub>2</sub> glasses. The addition of TiO<sub>2</sub> was observed to shift the 460, 490, and 600 cm-1 Raman peaks to lower frequencies in TiO<sub>2</sub>-SiO<sub>2</sub> glasses, suggesting a more open and flexible network, and the 720, 800, and 840 cm<sup> -1</sup> Raman peaks to higher frequencies in Na<sub>2</sub>O-TiO<sub>2 </sub>-SiO<sub>2</sub> glasses, suggesting a lower free volume and stiffer network. The addition of TiO<sub>2</sub> has little effect on the temperature response of the elastic moduli in either system, but decreases the thermal expansion and increases the frequency shifts in the 950 and 1100 cm<sup> -1</sup> Raman peaks in the TiO<sub>2</sub>-SiO<sub>2</sub> system while the thermal expansion increases with initial additions of TiO<sub>2</sub> and then remains constant in the Na<sub>2</sub>O-TiO<sub>2</sub>-SiO<sub> 2</sub> system. </p><p> Changes in structure and property with composition have been discussed, and structural models were proposed. The reduction of thermal expansion and elastic moduli in TiO<sub>2</sub>-SiO<sub>2</sub> glasses occurs through the promotion of cooperative, inter-tetrahedral rotations facilitated by the longer and weaker Ti-O bonds. The increase in elastic moduli in the Na<sub>2</sub>O-TiO<sub> 2</sub>-SiO<sub>2</sub> glasses occurs through the formation of small clusters with local, relatively high Ti and Na concentrations, promoted by Ti adopting a five-fold coordination in a square-pyramidal geometry. These clusters work to shield the silica network from non-bridging oxygens from the presence of Na while simultaneously increasing the volume bond density of the glass. </p><p> For Na<sub>2</sub>O-TiO<sub>2</sub>-SiO<sub>2</sub> glasses, the response to mechanical damage and plastic deformation has been examined through Vickers indentation experiments at loads from 10 mN to 49 N. Fracture toughness was measured through the single-edge precracked beam method. The permanent deformation volumes around Vickers indents were investigated through atomic force microscopy. Critical loads for crack initiation and cracking patterns were systematically investigated and correlated with the elastic properties of glass. Vickers indents were observed to change from a mixture of radial/median and cone cracks to radial/median and lateral cracks as Poisson's ratio increases. As Poisson's ratio increases hardness decreases from 5.5 GPa to 4.5 GPa, the average radial/median crack length roughly doubles, and fracture toughness remains constant. A minimum in the critical crack initiation load was observed at ν = 0.21–0.22. The volume of glass deformed through shear flow during indentation increases gradually with increasing Poisson's ratio, becomes larger than the densified volume at ν = 0.237. The densified volume increases between ν = 0.18 and ν = 0.21 and decreases rapidly from 16.5 µm<sup>3</sup> to 8.7 µm<sup>3</sup> at ν = 0.235–0.237. A correlation between the minimum in crack initiation load and the change in deformation mechanisms over the same Poisson’s ratio range was observed.</p>
| Identifer | oai:union.ndltd.org:PROQUEST/oai:pqdtoai.proquest.com:10158591 |
| Date | 29 September 2016 |
| Creators | Scannell, Garth |
| Publisher | Rensselaer Polytechnic Institute |
| Source Sets | ProQuest.com |
| Language | English |
| Detected Language | English |
| Type | thesis |
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