Synthesis, structure and properties of pure TeO2 glass, binary and ternary tellurite glasses.

Tagiara, Nagia S.

dc.contributor.author	Tagiara, Nagia S.
dc.date.accessioned	2022-01-19T07:13:21Z
dc.identifier.uri	https://dspace.lib.ntua.gr/xmlui/handle/123456789/54362
dc.identifier.uri	http://dx.doi.org/10.26240/heal.ntua.22060
dc.rights	Αναφορά Δημιουργού-Μη Εμπορική Χρήση-Όχι Παράγωγα Έργα 3.0 Ελλάδα	*
dc.rights.uri	http://creativecommons.org/licenses/by-nc-nd/3.0/gr/	*
dc.subject	Vibrational spectroscopy	en
dc.subject	Tellurite glasses	en
dc.subject	Density Functional Theory	en
dc.subject	Glass structure	en
dc.subject	pure TeO2 glass	en
dc.title	Synthesis, structure and properties of pure TeO2 glass, binary and ternary tellurite glasses.	en
heal.type	doctoralThesis
heal.classification	Physics	en
heal.classification	Chemistry	en
heal.classification	Glass science	en
heal.dateAvailable	2023-01-18T22:00:00Z
heal.language	en
heal.access	embargo
heal.recordProvider	ntua	el
heal.publicationDate	2021-11-19
heal.abstract	The present doctoral dissertation deals with the development and systematic investigation of pure TeO2 glass and TeO2–based binary and ternary tellurite glass systems. The main goal is to understand structural and physical properties and their correlation in such glasses, and to explore the possibility of triggering second harmonic generation by electrothermal poling. A new method for producing sizable quantities of pure TeO2 glass was developed, named the Intermittent Quenching technique (IQ-technique). This synthesis method involves melting TeO2 in Pt crucible and quenching several times the bottom of the crucible containing the viscous melt into room temperature water. The procedure gives monolithic pieces of pure TeO2 glass with approximate dimensions 2.5 cm x 1.5 cm x 2 mm for the size of the available platinum crucible (ca. 20 cm3). For the pure TeO2 glass synthesized using the IQ-technique, its glass transition temperature, Tg, density, ρ, and elastic moduli were measured and its structure was studied by Raman and infrared spectroscopy. The properties and structure of this pure TeO2 glass were found to be drastically different from those reported for TeO2 glasses melted in alumina crucible. In order to further understand the structure of pure TeO2 glass, tellurium oxide clusters (TeO2)6 were investigated through density functional theory (DFT). Among a large number of studied stable conformers, a cyclic and nonsymmetric structure was optimized without terminal Te=O double bonds. The dimer of this structure, (TeO2)12, gives calculated Raman and infrared spectra in very good agreement with the experimental ones, and its total pair distribution function was found also in agreement with results of neutron and high-energy X-ray diffraction studies. The (TeO2)12 cluster consists mainly of TeO4 trigonal bipyramid (tbp) units connected by asymmetric and nearly symmetric Te−O−Te bridges as in γ-TeO2, and involves also edge-sharing of TeO4 units through double-oxygen Te−O2−Te bridges as in the β-TeO2 polymorph. The optimized cluster structure is slightly unstable compared to the calculated global minimum structure, suggesting a kinetically stable product similar to its corresponding experimental TeO2 glass. The pure TeO2 glass (a network glass) was studied for the first time up to the record pressure of 70 GPa using Raman spectroscopy. The Boson peak frequency (ωb) exhibits a decrease of the ∂ωb/∂P slope at 5-6 GPa, indicating a modification towards a more compact tellurite network. Above 30 GPa, ωb reaches a “saturation” with a practically constant value up to 70 GPa. In the short-range order, both our experimental and theoretical results indicate that pressure up to 15 GPa induces the transformation of single Te-O-Te bridges to double Te-O2-Te bridges, leading to a more compact tellurite network. At higher pressures, a new Raman activity develops around 580 cm-1 and is associated with the increase of Te coordination to six-fold. Natural bond orbital analysis showed that the formation of double Te-O2-Te bridges favors the s→d transition, which promotes the increase of Te coordination number at higher pressures through a d2sp3 hybridization. This results to the formation of a practically canonical TeO6 octahedron, in strict difference with crystalline TeO2 at the same pressure range, and the development of a 3D network that freezes the medium range order. Our study highlights the interplay between pressure-induced electronic transitions and the increase in Te coordination number in the flexible tellurite glass network. Glasses in the binary system xZnO−(1−x)TeO2 (0≤x≤0.50) were also prepared by melting in Pt crucibles. Raman spectroscopy showed also that doping TeO2 with ZnO causes the progressive conversion of TeO4 trigonal bipyramids (tbp) to TeO3+1 polyhedra with two terminal oxygens, and then to TeO3 trigonal pyramids (tp) with three terminal oxygens. This structural transformation is reflected in the composition dependence of the volume per mole TeO2 evaluated from density data. The ZnO-dependence of this parameter is described by two linear parts with an inflection point at x=0.25, which indicates an increasing rate of forming terminal Te-O bonds at higher ZnO contents. The Tg was found to increase with ZnO content and this was attributed to the cross-linking ability of ZnO4 units, while density was found to decrease due mainly to replacement of the heavier TeO2 by the lighter ZnO. The results of this study are discussed in the dissertation with reference to previous works on zinc-tellurite glasses melted in alumina crucibles. Glasses in the binary system yAl2/3O−(1-y)TeO2 (0≤y≤0.43), and the ternary systems zR2/3O−(0.30-z)ZnO−0.70TeO2 (R = Al, B) were also prepared by melting in Pt crucibles and studied for correlations between structure and thermal as well as mechanical properties, whereby the glass composition is varied to tailor the short-range speciation of tellurite, aluminate, and borate groups. The glass structure was studied by Raman and infrared spectroscopy, and the measured properties include glass transition temperature (Tg), density (ρ), and ultrasonic longitudinal (VL) and transverse (VT) velocities. The longitudinal (VL) and transverse (VT) sound velocities were also measured for the binary zinc-tellurite glass system mentioned earlier. In addition, atomic packing density (Cg), elastic moduli, and Poisson’s ratio (σ) were evaluated from the measured properties. It was found that Al2/3O leads to cross-linked alumino-tellurite networks by strong Te−O−Al bonds, which cause a profound enhancement in Tg. The influence of ZnO and B2/3O on Tg is relatively smaller due to the weaker cross-linking effects of ZnO4 tetrahedra and of Te···O−B bonds. Short-range bonding characteristics, interatomic bonding energy differences, and atomic packing density were found to have a strong effect on VT and mostly on the VL sound velocity. The combined effects of structure and bonding are nicely expressed in the composition dependence of Poisson’s ratio; it exhibits decreasing trends with Al2/3O content in the binary and ternary glasses studied in this dissertation, but increasing trends with ZnO and B2/3O additions in glasses ZnO−TeO2 and B2/3O−ZnO−TeO2, respectively. The results for Poisson’s ratio and atomic packing density for the studied glasses were found to fit nicely in the global σ versus Cg correlation established previously for a range of glasses not including tellurites so far. Finally, the sound velocities and Poisson’s ratio of pure TeO2 glass were determined for the first time and found to differ markedly from those in the literature for TeO2 glass melted in alumina crucible; this is because the latter glass is highly doped with Al2O3 leached from the alumina crucible. In this dissertation, the viscosity of binary tellurite glasses is measured as a function of temperature and type of the oxide added to TeO2 (glass former or glass modifier) with the aim of understanding the effect of glass structure on the viscosity behavior and fragility parameter in the following systems: xZnO-(1-x)TeO2 (x=0.10, 0.20, 0.30, 04), yB2/3O-(1-y)TeO2 (y=0.10, 0.20, 0.30), zAl2/3O-(1-z)TeO2 (z=0.20, 0.30), and 0.20Na2O-0.80TeO2. It was found that for the same amount of the added second oxide ([ZnO]=[B2/3O]=[Al2/3O]=[Na2O]) the viscosity (η) measured at the same temperature varies in the order η(Al2/3O) > η(B2/3O) > η(ZnO) >> η(Na2O). The results were discussed in terms of the cross-linking ability and the strength of the bond with oxygen of the Zn2+, Al3+, B3+ and Na+ cations. Also, the fragility index m=dlog(η)/d(Tg/T) evaluated at T=Tg was determined according to C.A. Angell who introduced the log(η) vs. reduce temperature Tg/T plots. It was found that the fragility index varies as follows: 10ZT (m=97.3), 10BT (m=81.9), 20ZT (m=74.6), 20BT (m=73.5), 20AT (m=91.7), 20NT (m=68.7), 30ZT (m=70.2), 30BT (m=75.5), 30AT (m=82.8). Lastly, electro-thermal poling experiments and SHG measurements on various tellurite glass systems showed that the production and detection of second harmonic generation signal is not a trivial process. SHG was found to be associated with structural changes in a sub-anodic layer after electrothermal poling, which seems to be associated with creation of more Te-O-Te bridging bonds in the optically active layer. This subject is still an open topic under study that requires further future experimental efforts in order to find the most appropriate electro-thermal poling conditions that would ensure greater homogeneity, repeatability and stability over time of the poling-induced SHG signal.	en
heal.advisorName	Kyritsis, Apostolos
heal.committeeMemberName	Kamitsos, Efstratios I.
heal.committeeMemberName	Simandiras, Emmanouel D.
heal.committeeMemberName	Feller, Steve
heal.committeeMemberName	Chrysikos, George
heal.committeeMemberName	Stavrou, Elissaios
heal.committeeMemberName	Kontos, Athanasios
heal.academicPublisher	Σχολή Εφαρμοσμένων Μαθηματικών και Φυσικών Επιστημών	el
heal.academicPublisherID	ntua
heal.numberOfPages	271
heal.fullTextAvailability	false