Non-equilibrium viscosity and activation energy

This article considers the break in viscosity curves that appears in the glass transition region. It demonstrates that the only possibility to interpret properly the break within the frameworks of the activation energy theory is to assume that at least several independent processes are taking place. Single process cannot explain the break. This assumption was already developed in details in the jump frequency model of Avramov and Milchev.     

Observation of nucleation effect on crystallization in lithium aluminosilicate glass by viscosity measurement

The crystal nucleation effect in lithium aluminosilicate glasses was investigated by the viscosity measurement with aid of the fiber elongation method. The abrupt increase of viscosity due to the crystallization of glass was observed in viscosity-temperature curve but the minimum viscosity temperature (T_η) related with crystallization showed a strong dependence on the nucleation state such as nucleation temperature, nucleation time and heating rate. The results by viscosity agreed well with those of DTA. The nucleation effect on the microstructure of glass-ceramics was also discussed. Finally, the nucleation effect on the crystallization kinetics was approached quantitatively by calculating the crystal volume from viscosity value.     

Evaluation of activation energy of viscous flow of sol–gel derived phenyl-modified silica glass

The temperature-induced softening behavior in sol–gel derived phenyl-modified low-melting glass (phenyl glass) was investigated in terms of the activation energy for the viscous flow. The temperature dependence of the relative viscous flow was measured from the falling rate of a needle loaded with a constant weight. The activation energy for the viscous flow of phenyl-modified silica glass was found to be irrespective of the time of drying the sample phenyl-modified silica glass, which directly affects the extent of polymerization. Furthermore, the obtained activation energy was in considerably good agreement with that for the viscous flow of potassium alkali glass, and approximately twice larger than that of linear amorphous polymer (polystyrene). This result suggests the common microstructural feature of glassy materials interspaced by additive substances like Na/K or covalently bonded chemical functions such as phenyl groups.     

Effect of surface treatments on the chemical durability and surface composition of soda-lime glass bottles

Chemical analyses show that water and 1% KHCO₃ solutions extract material that resembles the bulk glass composition for both untreated and treated soda-lime bottles although much less material is extracted from treated bottles. Also, fluorine appears to be deeply dispersed at perhaps 1% concentration in a fluorocarbon-treated glass surface. Electron spectra obtained before and after ion milling detected no fluoride and indicated calcium and some sodium depletion for fluorocarbon-treated surfaces, and a depletion of calcium as well as sodium for sulfur-treated surfaces. There also is evidence from electron spectra of some sodium and calcium depletion in untreated glass surfaces.     

Relaxations in polystyrene below the glass transition studied by viscosity measurement

The shear viscosity of organic glass polystyrene has been determined under pure shear deformation mode from room temperature up to the glass transition temperature. A mechanical model of series connection of anelasticity and viscosity was used to determine the viscosity of the material. Relaxation time for the viscous flow was determined as a function of temperature. The relaxation was composed of two thermal-activation type relaxation processes: the high temperature relaxation (HTR) and the low temperature relaxation (LTR). In both relaxations the relaxation time was represented as τ=τ_o exp(E/k_BT), and the values of τ₀ and E were different in specimens treated differently – aged, loaded, and annealed. The observed τ₀ and E were not independent of each other but a compensation effect, a linear decrease of logτ₀ with increased E, was seen. The results were explained using the idea of cooperative relaxations of relaxing elements. HTR and LTR were considered to correspond to the structural and the slow relaxations, respectively, and the relaxing elements could respectively be a single atom or molecule and a segment in molecular chains.     

Effect of external field on bond energy and activation barrier for surface diffusion

Surface diffusion is one of the most common phenomena during the thin-film growth. The microstructure of the thin film depends on the rate of surface diffusion of the adatoms. In this paper, we theoretically show the effect of external field on surface diffusion barrier using gallium nitride as an example. We show, via density functional calculation, that the desorption energy of surface adsorbed atom and activation barrier for surface diffusion process can be altered by application of external field. Finally, based on the arguments of the electronic effect, we analyze the origin of the effect of the external field and its orientation on desorption energy and activation barrier for surface diffusion.     

Correlations between apparent activation energy and thermostability and glass forming ability for Fe based metallic glasses

Apparent activation energies (E_g) of glass transition, glass transition temperature (T_g) and crystallization temperature (T_x) of amorphous alloys of composition Fe_91?xB_xZr₅Nb₄ (FBZN, 5 ? x ? 30 at.%) and Fe_61?xCo_xZr₅B₃₀Nb₄ (FCZBN, 0 ? x ? 15 at.%) were obtained by using differential scanning calorimeter (DSC) measurement and Kissinger equation, and correlations between E_g and T_g, T_x and glass-forming ability (GFA) were studied in the present work. It was found that the T_g and T_x are not independent each other for each glass composition in the two alloy systems, but related by a formula, T_x = αT_g+β, where α and β are constants, and were measured by nonisothermally scanning in the DSC together with the Lasocka’s equation. The E_g was found to be directly proportional to α and β, respectively, and had a correlation with T_x and T_g, $T_{\text{x}}=\frac{E_{\text{g}}-1.09}{3.527}{T}_{\text{g}}-\frac{E_{\text{g}}-4.86}{0.0041}$, indicating that E_g determines linear relationship between T_x and T_g. Supercooled liquid region ΔT_x is used as characterization of GFA of the Fe based metallic glasses and related to E_g and T_g by a formula: $\mathrm{\Delta }{T}_{\text{x}}=E_{\text{g}}(\frac{T_{\text{g}}}{3.527}-234.9)+1185.37-1.309T_{\text{g}}$, indicating that E_g and T_g can characterize GFA of the Fe based metallic glass well.     

Apparent activation energy of structural relaxation for Se70Te30 glass

Differential scanning calorimetry and dc conductivity measurements were used to study structural relaxation of Se₇₀Te₃₀ glass. A single set of Tool-Narayanaswamy-Moynihan (TNM) parameters was obtained from the curve-fitting procedure. The value of apparent activation energy Δh∗ was further confirmed by two non-fitting techniques. Results of the Δh∗ evaluation from the T_g dependence on cooling rate are discussed in terms of how the T_f determination might be influenced by the material’s structure type and by the interference of the crystallization process.     

Effect of stirring on striae in glass melts

Chemical striae have often negative effect on the glass properties, and hence, elimination of striae has been a key issue in glass science and technology. To produce highly homogeneous glasses, it is necessary to stir melts during the melting process. To explore the physical origin of the stria elimination during stirring, and to optimise the homogenisation process, both simulations of striation and homogenisation experiments are performed. The results show that stirring broadens the stria size distribution in the melt through conversion of larger striae into smaller ones. Only the striae with a size below half the diffusion length in the melt can be eliminated during the melting process. Stirring itself does not homogenise the melt, but enhances the stria elimination rate by generating small striae.     

The effect of ion exchange on glass surface morphology and composition

The surface morphology and K⁺, Na⁺ concentration distributions of plate glass were investigated by atomic force microscope (AFM) and electron probe microanalyzer (EPMA) before and after ion exchange. The AFM images indicate that the surface microcrack of chemical strengthened glass can be closed to a certain extent and the surface smoothness can be improved. The EPMA results indicate that the relationship between the weight gain and time, temperature follows Fick’s law, which are same as that of the thickness of ion exchanged layer, the total exchanged amount. Consequently, a simple and accurate weighing method was first proposed to estimate the ion exchange state.     

Multiphonon energy gap law in rare-earth doped chalcogenide glass

The parameters for multiphonon relaxation of rare earth (RE) ions in the sulfide glass Ge₂₅As_8.33Ga_1.67S₆₅ have been re-evaluated using the temperature dependence of the fluorescence lifetime to separate out true multiphonon decay from other non-radiative processes. It is found that for energy gaps to the next lowest level greater than 2500 cm⁻¹, other non-radiative processes become dominant over multiphonon decay. The most likely process for this additional non-radiative decay is energy transfer to vibrational impurities such as OH and SH. The newly derived parameters lead to an electron–phonon coupling parameter ε=0.058, which is more in line with other glass types than the previously accepted value of ε=0.36. These new multiphonon relaxation parameters and the existence of additional non-radiative decay mechanisms has implications for the modeling of chalcogenide-based active devices.     

Effect of devitrification on ion motion in lithium-disilicate glass

Li-disilicate glass-ceramics consist of microcrystallites imbedded in the glassy Li₂O · 2SiO₂ matrix where the number and size of the crystallites depend on the devitrification heat treatment. To assess ion motion in these model glass-ceramics, we have measured the temperature dependence of the dc conductivity, σ_dc, and the ⁷Li nuclear spin relaxation (NSR) rate, 1/T₁, in samples with various crystalline fraction, c, ranging from c = 0 (pure glass) to c = 1 (fully devitrified polycrystalline ceramic). The Cole–Cole presentation of the complex impedance shows two separate arcs caused by the remarkable difference of the ionic motion in the glassy and crystalline phase. These two arcs correspond to a bi-exponential decay of the ⁷Li nuclear spin magnetization where the resulting two NSR rates are induced by the ionic motion in the two phases. Thus the NSR and σ_dc data provide a comprehensive picture of the ionic motion in the glassy and crystalline phases. In particular, the ionic motion is the fastest in the glass; then at lower values of c we observe a metastable crystalline phase with ionic motion much greater than in the stable (LS₂) crystalline phase existing at large c-values.     

Effect of dehydration on the viscosity of sol-gel processed silica

For short times and low temperatures, the dehydration of silica gels is limited by the diffusion of water species. For long times and temperatures 600°C and higher, dehydration shows first order kinetics. That is dehydration rates decrease with increasing temperature where silica gels have less surface area and consequently fewer hydroxyls.

For comparison with samples treated isothermally, samples were heated to 600, 700, 800 and 850°C at 100°C intervals and held 4 h at each temperature starting with 500°C. Comparison of isothermal and multistep treatments shows the contributions of structural relaxation and dehydration to the viscosity. At a given temperature, two gels of different water content may show the same viscosity, because the difference in water is matched by the difference in degree of structural relaxation. More complete dehydration is accomplished by constant rate heating to a single isothermal treatment rather than heating in steps.     

Effect of glass composition on Judd–Ofelt parameters and radiative decay rates of Er in fluoride phosphate and phosphate glasses

For fluoride phosphate and phosphate glasses, Judd–Ofelt Ω_t parameters, spontaneous emission probabilities, radiative lifetimes and branching ratios of several Er³⁺ transitions were determined from the absorption spectra by the Judd–Ofelt treatment. The compositional changes of Ω₂ and Ω₆ are attributed to changes in the bonding between Er³⁺ and surrounding ligand groups. The phosphate groups play a special role. They show electron donation from the 2p orbitals of the oxygen ions, as well π-electron donation from the resonating P–O π-bond. Larger electron donation from the ligand anions leads to larger Ω₂, but smaller Ω₆. By contrast, larger π-electron donation from the phosphate groups increases Ω₆. The changes of Ω₂ with increasing phosphate content and with variation of modifier ions are ascribed to changes in the content and polarizability of the oxygen ions, leading to changes in the electron donation from the ligands at the rare earth (RE) sites. In the case of Pb²⁺, changes in the degree of asymmetry at the RE sites could also be important. The smaller Ω₆ parameter of phosphate glasses compared with fluoride phosphate glasses is caused by larger σ-electron donation due to larger oxygen content and by smaller π-electron donation due to smaller delocalization of the π-bonds. The changes of Ω₆ with phosphate content, in the case of fluoride phosphate glasses and with variation of modifier ions, are dominated by changes in the π-electron donation.     

Effect of fragility on relaxation of density fluctuations in glass

Density fluctuations play a crucial role in governing the optical, mechanical, thermodynamic, and kinetic properties of glass. The relaxation of density fluctuations displays an inherently nonmonotonic behavior, yielding a minimum in fluctuations following a quench and isothermal hold. Here we investigate the impact of liquid fragility on the relaxation of density fluctuations in the nonequilibrium glassy state. While fragility has a direct impact on the kinetics of the relaxation process, the minimum level of density fluctuations is unaffected by changes in fragility alone. The magnitude of density fluctuations can be minimized by tailoring the thermal history of the glass.