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Structural characterization of Ge-As-S chalcogenide glasses and understanding structural relaxation in oxide glasses

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  1. Tez No: 400023
  2. Yazar: SEZEN SOYER
  3. Danışmanlar: PROF. SABYASACHI SEN
  4. Tez Türü: Doktora
  5. Konular: Kimya Mühendisliği, Chemical Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 2009
  8. Dil: İngilizce
  9. Üniversite: University of California DAVIS
  10. Enstitü: Yurtdışı Enstitü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 219

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This dissertation reports detailed structural characterization of short- andintermediate-range order in multi-component Ge-As-S chalcogenide glasses and someresults on atomic scale structural relaxation and density fluctuations in the glass transitionregion, in oxide network glasses.The structures of chalcogenide glasses in the Ge-As-S system with Ge:As = 1:1and with S concentration varying between 33.3 and 70.0 atom% are investigated usingneutron/x-ray diffraction. The nearest-neighbor coordination numbers of Ge and Asatoms are always found to be 4 and 3, respectively, irrespective of glass composition. Geand As atoms are primarily heteropolar bonded to S atoms in stoichiometric and S-excessglasses with x ? 18.2. Low and intermediate levels of S-deficiency (20? x ? 25) areaccommodated via the formation of homopolar As-As bonds while Ge atoms remainprimarily bonded to 4 S atoms, resulting in As-rich regions in the glass structure. Gestarts to participate in metal-metal bonding only in the highly S-deficient glasses with27.5? x ? 33.3. Additionally, large scale three-dimensional structural models obtainedfrom reverse Monte Carlo (RMC) simulations are used to investigate the nature andcompositional evolution of intermediate-range structural order in these ternary glasses.The intermediate-range order and its topological influence on atomic packing in thesethree compositional regions, in the order of increasing S-deficiency, are controlled by (a)a mixed GeS2 and As2S3 network, (b) the coexistence of GeS2 network and As clusters,and (c) large Ge-As metal-rich regions. This evolution of the intermediate-rangestructure with composition is consistent with the corresponding variation of the position,intensity and width of the first sharp diffraction peak in the structure factor.Combined neutron/x-ray diffraction, Ge and As K-edge EXAFS and Ramanspectroscopy are employed to study the compositional dependence of the short- andintermediate- range structures of As-rich GexAsyS100-x-y glasses with a constant Ge:Asatomic ratio of 1:17.3. The structures of glasses with compositions near stoichiometry (35? x+y ? 43) are dominated by the presence of a predominantly heteropolar-bonded As2S3network. However, increasing metal content (x+y = 55) results in a novel glassconsisting predominantly of As4S3 molecules, held together by van der Waals forces. Theformation of this molecule-rich glass implies an apparently anomalous situation of nearzeroconnectivity and dimensionality with increasing average coordination number.Formation of As-As homopolar bonded structural regions that coexist with As4S3molecules is observed with further increase in metal content (60 ? x+y ? 65). Suchunusual evolution of intermediate-range order is shown to be closely related to thecompositional variation of thermophysical properties and density fluctuation in theseglasses.Structural mechanisms of densification of a molecular chalcogenide glass ofcomposition Ge2.5As51.25S46.25 are studied in situ in a panoramic cell equipped withMoissanite (SiC) anvils at pressures ranging from 1 atm to 11 GPa at ambienttemperature as well as ex situ on a sample quenched from 12 GPa in a multi-anvil celland ambient temperature using high-energy x-ray diffraction. The height of the first sharpdiffraction peak in x-ray structure factors decrease in intensity and principal diffractionpeak increase in intensity with a concomitant shift to higher Q-values with increasingpressure. At low pressures of at least up to 5 GPa the densification of the structureprimarily involves an increase in the packing of the As4S3 molecules. At higher pressuresthe As4S3 molecules break up and reconnect to form a high-density network withincreased extended-range ordering at the highest pressure of 11 GPa indicating astructural transition.11B MAS NMR spectroscopy is employed to monitor relaxation kinetics of boroncoordination environments in a borosilicate glass in response to temperature jumps. Therelaxation timescale of the BO4:BO3 ratio is found to be closely comparable with those ofthe bulk properties, namely refractive index, density and viscosity, showing a closecorrelation with one another. Samples partially equilibrated at a low temperature, whensubjected to a positive temperature jump, display an initial rapid decrease followed by anincrease in the BO4:BO3 ratio. This reversal in the trend of the variation of the BO4:BO3ratio is interpreted to be the signature of a memory effect and the implications arediscussed within the framework of the potential energy landscape model of glassydynamics.The behavior of density and enthalpy fluctuations during glass relaxation isinvestigated theoretically and experimentally employing the Mauro-Loucks enthalpylandscape model and in situ small angle x-ray scattering, respectively. It is shown that themagnitude of these fluctuations relaxes non-monotonically as glass anneals at constanttemperature. These results provide direct evidence for dynamical heterogeneities inglassy dynamics and their close association with density fluctuations.

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