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The onset of shear thinning and the transition from Newtonian to non-Newtonian behavior in the viscous flow of select chalcogenide and oxide network glass-forming liquids in the deeply supercooled regime and its temperature dependence are studied using parallel plate rheometry. In all cases, the onset occurs at a shear rate γ ̇ c that is several orders of magnitude lower than the shear relaxation rate τ 0 − 1 and the former increases with increasing temperature. These results are in good qualitative agreement with the predictions of the existing models of shear relaxation and shear thinning based on the nonlinear Langevin equation theory, random first order transition theory, and the free volume model. However, in contrast to the theoretical predictions, the reduced shear rate W0 (= τ 0 γ ̇ c ) at the onset is found to range between 10−3 and 10−5 and decrease with increasing temperature. This temperature dependence becomes stronger with increasing fragility of the liquid. These results likely indicate that the shear thinning mechanism in network liquids could be fundamentally different from those in molecular, metallic, or polymeric glass-formers. [ABSTRACT FROM AUTHOR]

The viscoelastic behavior of supercooled glass-forming liquids along the binary join As4S3–GeS2 with As4S3 contents varying from 81.25 to 9 mol. % and correspondingly with structures varying from predominantly molecular to a three-dimensional tetrahedral network is studied by small-amplitude oscillatory shear parallel plate rheometry. The storage shear modulus G′ shows a scaling behavior of G′(ω) ∼ ωn in the terminal (low-frequency) regime, where n varies between 1 and 2 and shows an increasingly anomalous departure from the expected value of 2 (Maxwell scaling) with increasing molecule content. A concomitant departure from the Maxwell scaling is also observed for the loss modulus G″ at frequencies above the G′–G″ crossover. On the other hand, the variation in the phase angle δ with the complex modulus indicates that the molecular liquid does not display a purely viscous response even at the lowest frequencies. These results, combined with an analysis of the relaxation spectra of these liquids, suggest that the anomalous behavior of molecular liquids may be linked to their rather broad relaxation spectrum and the presence of slow relaxation processes associated with molecular clusters. Additionally, these liquids are also characterized by a wide high-frequency plateau in the relaxation spectral density that can be linked to the rotational dynamics of the constituent molecules. Such fundamental differences between the rheological behavior of molecular and network liquids may explain the significantly higher fragility of the former. [ABSTRACT FROM AUTHOR]

The steady and oscillatory shear rate dependence of viscosity of a supercooled chalcogenide liquid of composition As10Se90 is measured at Newtonian viscosities ranging between 10³ and 107 Pa s using capillary and parallel plate rheometry. The liquid displays strong violation of the Cox-Merz rule in the non-Newtonian regime where the viscosity under steady shear is nearly an order of magnitude lower than that under oscillatory shear. This behavior is argued to be related to the emergence of unusually large (6-8 nm) cooperatively rearranging regions with long relaxation times in the liquid that result from significant structural rearrangements under steady shear. [ABSTRACT FROM AUTHOR]

Pressure induced densification in a molecular arsenic sulfide glass is studied at ambient temperature using x-ray scattering, absorption and Raman spectroscopic techniques in situ in a diamond anvil cell. The relatively abrupt changes in the position of the first sharp diffraction peak, FSDP, and the pressure-volume equation of state near ∼2 GPa suggest a phase transition between low- and high-density amorphous phases characterized by different densification mechanisms and rates. Raman spectroscopic results provide clear evidence that the phase transition corresponds to a topological transformation between a low-density molecular structure and a high-density network structure via opening of the constituent As4S3 cage molecules and bond switching. Pressure induced mode softening of the high density phase suggests a low dimensional nature of the network. The phase transformation is hysteretically reversible, and therefore, reminiscent of a first-order phase transition. [ABSTRACT FROM AUTHOR]

Presents information on a study wherein the electro-optic Kerr effect and its wavelength dispersion have been measured in glasses representing several compositional systems. Methods; Results; Discussion.

Structure of glasses in the pseudobinarysystem Ga2Se3–GeSe2with Ga2Se3content ranging from 6.3 to 30 mol % is investigatedusing a combinationof Raman and multinuclear (71Ga, 77Se) solidstate nuclear magnetic resonance (NMR) spectroscopy. The results indicatethat the structure of these glasses consists primarily of a cornersharing network of (Ge/Ga)Se4tetrahedra with some fractionof edge-sharing GeSe4tetrahedra and of ethane-like (Se3)Ge–Ge(Se3) units, in which the Ga, Ge,and Se atoms adopt coordination numbers of 4, 4, and 2, respectively.As expected, the concentration of metal–metal bonds increaseswith addition of Ga2Se3as the glass structurebecomes too deficient in Se to satisfy the tetrahedral coordinationof both Ga and Ge by Se atoms alone. These metal–metal bondsare mostly limited to Ge–Ge homopolar bonds, indicating a violationof chemical order. At relatively high degrees of Se-deficiency, however,spectroscopic evidence suggests the formation of triply coordinatedSe atoms as an alternate mechanism to accommodate the tetrahedralcoordination of Ga and Ge atoms. This observation indicates a violationof the 8-N coordination rule and is reminiscent of oxygen triclustersin isoelectronic Al2O3–SiO2glasses. Compositional variation of physical properties such asdensity, molar volume, optical band gap, glass transition temperature,and fragility are shown to be consistent with the proposed structuralmodel. [ABSTRACT FROM AUTHOR]