Temperature dependent onset of shear thinning in supercooled glass-forming network liquids

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 γ̇_c that is several orders of magnitude lower than the shear relaxation rate τ₀ ^-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 (=τ₀γ̇_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.

Rheological behavior of molecular vs network chalcogenide supercooled liquids

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.

Behavior of a supercooled chalcogenide liquid in the non-Newtonian regime under steady vs. oscillatory shear

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.

Communication: Non-Newtonian rheology of inorganic glass-forming liquids: Universal patterns and outstanding questions

All families of inorganic glass-forming liquids display non-Newtonian rheological behavior in the form of shear thinning at high shear rates. Experimental evidence is presented to demonstrate the existence of remarkable universality in this behavior, irrespective of chemical composition, structure, topology, and viscosity. However, contrary to intuition, in all cases the characteristic shear rates that mark the onset of shear thinning in these liquids are orders of magnitude slower than the global shear relaxation rates. Attempt is made to reconcile such differences within the framework of the cooperative structural relaxation model of glass-forming liquids.

Hysteretically reversible phase transition in a molecular glass

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 As(4)S(3) 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.

Structure, topology and chemical order in Ge-As-Te glasses: a high-energy x-ray diffraction study

High-energy x-ray diffraction is employed to study the atomic structure of bulk Ge(x)As(2x)Te(100-3x) glasses with compositions in the range 25 ≤ 3x ≤ 70. The coordination environments of Te atoms suggest significant violation of chemical order in these glasses. Analyses of the nearest-neighbor coordination environments and the parameters for the first sharp diffraction peak indicate that these telluride glasses are structurally and chemically more disordered as compared with their sulfide or selenide analogs. The compositional evolution of the structural parameters is shown to be consistent with the corresponding variation in molar volume and glass transition temperature.

Hierarchical dynamics of As2P2S8 quasi-molecular units in a supercooled liquid in the As-P-S system: a 31P NMR spectroscopic study

The dynamics of As2P2S8 quasi-molecular units caged in an As-S network in the supercooled chalcogenide liquid of composition (As2S3)90(P2S5)10 have been studied near the glass transition region (Tg=468<or=T<or=628 K) using 31P NMR line shape analysis and spin-lattice relaxation techniques. 31P NMR line shape analysis indicates the presence of isotropic rotational reorientation of As2P2S8 quasi-molecular units at frequencies on the order of tens of kilohertz at T<540 K. At higher temperatures, the time scale of intramolecular bond-breaking and rearrangement is coupled to that of shear/structural relaxation of the surrounding network. On the other hand, over the entire temperature range, the 31P NMR spin-lattice relaxation results from fast cage-rattling dynamics of the same molecules at frequencies in the megahertz to gigahertz range. When taken together, these results imply the presence of serial hierarchical dynamics in which the fast rattling of As2P2S8 quasi-molecular units trapped in their cages coexists with slower isotropic rotational reorientation. The shear or alpha-relaxation involves cooperative rearrangement of the surrounding As-S network and, consequently, relaxation of the cages that provides feedback to the fast rattling dynamics over the entire temperature range.

A Neutron Scattering and Nuclear Magnetic Resonance Study of the Structure of GeO2−P2O5Glasses

Germanophosphate (GeO2-P2O5) glasses were studied with neutron diffraction, phosphorus, and oxygen nuclear magnetic resonance, calorimetry, viscosity measurements, and first-principles calculations. These data sets were combined to propose a structural model of GeO2-P2O5 glasses, which includes tetrahedrally coordinated phosphorus, formation of octahedrally coordinated germanium as P2O5 content increases, an absence of trigonally coordinated oxygen, and hence an absence of rutile-like GeO2 domains. The structural model was then used to propose explanations for both the observed composition dependence of the glass transition temperature and the fragility of the GeO2-P2O5 liquids.

Observation of a pressure-induced first-order polyamorphic transition in a chalcogenide glass at ambient temperature

An apparently first-order polyamorphic transition has been observed with increasing pressure at ambient temperature in a molecular glass of composition Ge(2.5)As(51.25)S(46.25) Raman spectroscopic measurements on pressure-quenched samples and in situ x-ray diffraction measurements indicate that this transition corresponds to a collapse of the ambient-pressure molecular phase to a high-pressure network phase. The high-pressure phase first appears at a pressure of approximately 8-9 GPa and the transformation becomes complete at approximately 14-15 GPa. Calorimetric measurements indicate that the low- and high-pressure phases are thermodynamically distinct and that they coexist in the transition range.

Self-calibrating quantum efficiency measurement technique and application to Pr(3+)-doped sulfide glass

A measurement technique is described in which the radiative quantum efficiency of certain transitions in rare-earth-doped glasses can be determined based only on relative fluorescence measurements. We calibrate the emission from the level of interest by measuring emission into that level from a higher excited level. Application of the technique to Pr(3+)-doped sulfide glasses yields quantum efficiencies for the (1)G(4) ? (3)H(5) transition as high as 60%, in good agreement with measurements using the integrating sphere technique. Calculated efficiency values based on the Judd-Ofelt technique are shown to be subject to inherent uncertainties.