On the mechanism of electrical relaxation in Na-conductive phosphate glasses

The crossover from a frequency independent to a frequency dependent ac response has been studied in glasses with the composition 37.2Na₂O-12.8CaO₅ · 50P₂O₅ and 30Na₂O-5CaO-7.5Al₂O₃-57.5P₂O₅ (mol%) containing 5 × 10⁻³ ≤ mol% Ag₂O ≤ 5 × 10⁻¹. Recently, we have established that in these glasses the diffusion coefficient of guest silver ions varies in space. In this case, as a first approximation, the diffusion coefficient may be considered as a constant within regions whose size is no less than 10 nm across. We assume that the diffusion of sodium ions can be given by the D(r) coefficient with the same spatial dispersion as that of silver ions. It is demonstrated that the frequency dependence of ac response is in fair agreement with the assumption.     

On the mechanism of dielectric relaxation in vanadium phosphate glasses

The mechanism of dielectric relaxation in xV₂O₅?(100?x)P₂O₅ glasses (x=50?70) has been investigated using the stimulated dielectric relaxation currents (SDRC) technique. The dielectric relaxation in metal-glass-metal systems is found to be non-linear with regard to voltage bias. Schottky barrier formation at the metal-glass interface, rather than a conventional Debye-type relaxation, is suggested to account for the observed behavior. The density of states N(E_F) as determined from the SDRC is in excellent agreement with the density of paramagnetic states found by the EPR technique.     

Rayleigh wave study of surface diffusion and stress relaxation in glass

Surface wave propagation on ion-exchanged glass surfaces has been systematically studied. The glass system was an alkali-alumino silicate glass, and the ion-exchange involved the replacement of lithium ions by sodium ions. Accurate velocity measurements were carried out on systems ion-exchanged for various times and at various temperatures. The effects of density and modulus changes on the velocity have been separated. Stress build-up and relaxation have been observed which correlate well with shear viscosity measurements. In addition, the diffusion coefficient for the Na⁺ ? Li⁺ exchange has been determined at a variety of temperatures.     

Vacancy type mechanism of the electrical relaxation processes in glass

This paper presents a model to account for the electrical relaxation behaviour of glasses in which a vacancy mechanism prevails. The vacancies existing in glass a priori are constant in number and usually are in excess over those generated by thermal motion in the sense of Frenkel's defect theory. The pathways of vacancies form a net system with junctions and impasses. The displacement of vacancies in the impasses is deemed especially responsible for the relaxation process observed when a glass is subjected to an external electric field. According to the model (i) the dispersion of permittivity is inversely proportional to the temperature, and (ii) as the temperature is varied the curves of the ratios of the permittivity and dielectric loss, versus the dispersion, as a function of the logarithm of angular frequency, remain constant in shape. It follows from the model that a certain proportion of alkali ions in glass cannot be replaced either in an ion exchange process or by electrolysis if the temperature is not very high.     

The influence of the distribution of water in silicate glasses on mechanical relaxation

When the internal friction of an alkali silicate glass is measured at a constant frequency and as a function of temperature, three major absorption regions are found. Little doubt exists about the mechanisms responsible for the low temperature peak or for the high mechanical loss observed at higher temperatures. On the other hand, the mechanism causing the peak observed at intermediate temperatures is still open to debate. Possible mechanisms responsible for this peak are reviewed and discussed.     

The role of stress development and relaxation on crystal growth in glass

The role of internal stresses energy is usually neglected when crystallization kinetics is considered. The common argument is that stress is relaxing too fast to affect the process. In this article we develop a generalized formalism to describe steady-state growth kinetics in viscoelastic media. The residual stress energy results from interplay between the rate of stress development (due to the propagation of a crystal throughout the matrix) and rate of stress dissipation (due to relaxation of the viscous matrix). The degree to which the stress energy can relax depends on the ratio of τ_c/τ_r the characteristic time for crystal growth, τ_c, and the relaxation time, τ_r. The present results challenge the widespread fallacy that internal stresses relax too fast to affect crystal growth. Our model explains the often observed lack of agreement between the theoretical predictions (without taking into account the stresses energy development) and experimental data.     

Frequency upconversion and its new mechanism in Tm-doped fluoroaluminate glasses

Upconversion fluorescences have been observed in the UV (363 nm) and the blue region (451 and 476 nm) in Tm³⁺-doped fluoroaluminate glass by pumping with a DCM dye laser. The two-photon absorption process of upconversion fluorescences was confirmed by the quadratic dependence of the emission intensity on incident pumping power. The excitation spectra for upconversion fluorescences show that the mechanism of upconversion is a simple ESA (excited state absorption) process. In addition to ESA, it was found that the energy transfer by cross-relaxation (³H₄, ³H₆ → ³F₄, ³F₄) is also involved in the mechanism of the 476 nm blue upconversion fluorescence which differs from that of the UV and the 451 nm blue upconversion fluorescence. The difference was confirmed by measuring both the radiative and the non-radiative decay rate of the ³H₄ level.     

Amorphous nanostructuring in potassium niobium silicate glasses by SANS and SHG: a new mechanism for second-order optical non-linearity of glasses

Potassium niobium silicate (KNS) glasses xK₂OxNb₂O₅(1−2x)SiO₂ with x=0.167; 0.182; 0.200; 0.220 and 0.250 have been subjected to prolonged heat treatments in a wide temperature range above T_g. As a result, glasses exhibiting liquid-type phase separation phenomena have been isolated. Moreover for each glass composition, the temperature zones have been determined to produce transparent, opalescent or opaque materials which have been studied by X-ray diffraction (XRD), small-angle neutron scattering (SANS) and second harmonic generation (SHG) techniques. SANS data unambiguously point at nanostructuring of KNS glasses in the scale of 5-20 nm under appropriate heat treatments near T_g. In contrast to initial KNS glasses, nanostructured glasses exhibit SHG activity. For earliest stages of phase separation SHG-active glasses are characterized by fully amorphous XRD patterns. Further development of phase separation in glasses with increasing of their opalescence leads to diminishing SHG, and subsequently partial crystallization takes place giving opaque materials. Since relative maximum of SHG efficiency corresponds to non-crystalline nanostructured glasses, such new transparent second-order non-linear media may be of both scientific and practical interest. With regard to non-crystalline structure of nano-inhomogeneities, SHG mechanism in the glasses is supposed to be due to a combination of third-order non-linearity with a spatial modulation of linear polarizability.     

A quantitative explanation of the difference between nuclear spin relaxation and ionic conductivity relaxation in superionic glasses

Electrical conductivity relaxation (ECR) experimental data represented by either the complex conductivity or the complex electric modulus are macroscopic in nature. In contrast to ECR, nuclear spin relaxation is a more direct probe of ionic movement and from its result we can infer the microscopic dynamics of the ions. Combined studies of ionic motion using ECR and nuclear spin-lattice relaxation (SLR) in several glassy ionic conductors have shown a large difference between the ECR and SLR times. Any theory that attempts to explain quantitatively the difference faces the dilemma of how to compare the SLR time with the ECR time. In this work we use a recent result [Phys. Rev. B 60 (1999) 9396] to find the ion hopping correlation time from the experimental ECR time. Next, we use the coupling model to calculate the SLR time from the ion hopping correlation time. Good agreements are obtained in three glassy ionic conductors and a crystalline ionic conductor.     

A combined measurement of thermal and mechanical relaxation

In order to describe relaxation the thermodynamic coefficient can be generalized into a complex frequency-dependent cross response function. We explore theoretically the possibility of measuring for a supercooled liquid near the glass transition. This is done by placing a thermistor in the middle of the liquid which itself is contained in a spherical piezoelectric shell. The piezoelectric voltage response to a thermal power generated in the thermistor is found to be proportional to but factors pertaining to heat diffusion and adiabatic compressibility κ_S(ω) do also intervene. We estimate a measurable piezoelectric voltage of 1 mV to be generated at 1 Hz for a heating power of 0.3 mW. Together with κ_S(ω) and the longitudinal specific heat c_l(ω) which may also be found in the same setup a complete triple of thermoviscoelastic response functions may be determined when supplemented with shear modulus data.     

Effects of Zn on the glass forming ability and mechanical properties of MgLi-based bulk metallic glasses

Through the addition of Zn element, Mg-Li-Cu-Zn-(Y, Gd) bulk metallic glasses (BMGs) with diameter of 2 mm have been successfully fabricated by conventional copper mold injection casting method. The X-ray diffractometer (XRD) patterns and differential scanning calorimeter (DSC) traces demonstrated that the as-cast Mg₆₀Li₅Cu₂₀Zn₅(Y, Gd)₁₀ BMGs were fully amorphous phase. By the minor addition of 5 at.% Zn, the supercooled liquid regions (SLR) of Mg₆₀Li₅Cu₂₀Zn₅(Y, Gd)₁₀ BMGs increased 6 K and 3 K respectively compared with that of Zn-free samples, which implied that their glass forming ability (GFA) and thermal stability were improved. It was also proved that the addition of Zn effectively enhanced the alloys’ fracture strength (453 MPa and 456 MPa) and elastic strain (0.75% and 0.92%), which were much higher than that of Mg₆₀Li₅Cu₂₅Y₁₀ BMG (403 MPa, 0.62%) and Mg₆₀Li₅Cu₂₅Gd₁₀ BMG (412 MPa, 0.82%).     

Aging of alkali-resistant glass and basalt fibers in alkaline solutions: Evaluation of the failure stress by Weibull distribution function

In alkaline solutions, the reaction of hydroxyl ions with Si–O–Si-groups of the glass network leads to the formation of hydrated surfaces and dissolved silicate. The rate of this corrosion depends on the chemical constitution of the fiber and the alkaline solution as well as on time and temperature. The investigation of the aging of glass and basalt fibers with different chemical constitutions in NaOH and cement solutions shows that the corrosion mechanism changes due to the inhibiting effect of calcium ions. The strength distributions have been evaluated using a Weibull distribution function. The mechanical behavior strongly depends on the chemistry of the solution and determines the parameters of the Weibull distribution function in terms of either single or mixed distributions. The corrosion in NaOH solution leads to a strong dissolution of the outer layer of the glass and basalt fibers, whereas during aging in cement solution at the same pH-value a limited, local attack was revealed.     

Structural relaxation in the glass: Evidence for a path dependence of the relaxation time

Using a recently developed novel temperature perturbation experiment, one of the primary assumptions underlying the phenomenological models of structural recovery, i.e., that the relaxation time depends on the instantaneous state of the material and not on its temperature history, is tested. The results of our experiments show that as the material gets close to equilibrium, this assumption is valid, but far from equilibrium, it is not.     

Surface structure and properties of NiZr model metallic glasses: A molecular dynamics simulation

A systematic study of the surface structure and properties of NiZr model metallic glasses is reported using atomistic simulations. It is found that at low temperatures below the glass transition temperatures, the surface retains the amorphous structure and the surface energy γ is significantly lower (∼50%) than that of the corresponding crystalline alloy constituents. The variation of alloy concentration has little effect on γ; but increase in cooling rate and annealing temperature can lead to large decrease in γ. At elevated temperatures, γ increases with temperature and surface melting occurs at a temperature about 30% below T_g. At all temperatures up to T_g, the surface remains atomically smooth.     

Surface modification and grafting of carbon fibers: A route to better interface

This review is an audit of various Carbon fibers (CF) surface modification techniques that have been attempted and which produced results with an enhancement in the interfacial characteristics of CFRP systems. An introduction to the CF surface morphology, various techniques of modifications, their results and challenges are discussed here. CFs are emerging as the most promising materials for designing many technologically significant materials for current and future generations. In order to extract all the physic-mechanical properties of CF, it is essential to modulate a suitable environment through which good interfacial relation is achieved between the CF and the matrix. The interface has the utmost significance in composites and hybrid materials since tension at the interface can result in a deterioration of the fundamental properties. This review is aimed to provide a detailed understanding of the CF structure, its possible ways of modification, and the influence of interfacial compatibility in physic-mechanical and tribological properties. Both physical and chemical modifications are illustrated with specific examples, in addition to the characterization methods. Overall, this article provides key information about the CF based composite fabrication and their many applications in aerospace and electronics- where light weight and excellent mechanical strength are required.     

A kinetic treatment of glass formation V: Surface and bulk heterogeneous nucleation

An analytical method is described for calculating the detailed distribution of crystallite sizes in a supercooled liquid, and the changes in this distribution as a function of temperature (time) while the liquid is cooled from above the melting point.This method, termed the analysis of crystallization statistics, is applied to the calculation of continuous cooling curves for anorthite and o-terphenyl as representative of inorganic and organic systems. In addition to homogeneous nucleation, bulk as well as surface heterogeneous nucleation are considered. The effects of distributions of heterogeneities with contact angles between 40 and 100° as well as overall concentrations of heterogeneities between 10³ and 10⁹ cm^?3 are considered. Heterogeneities with contact angles higher than about 100° are shown not to have an effect on the critical cooling rate for typical concentrations of heterogeneities.For liquids containing distributions of heterogeneities, the nucleation behavior is dominated by small concentrations of heterogeneities having small contact angles. Theoretical log (I_vη) versus (T_r³ΔT_r²)^?1 curves have been constructed for homogeneous nucleation + heterogeneous nucleation with a single type of heterogeneity and for homogeneous nucleation + heterogeneous nucleation with the heterogeneities distributed with regard to contact angle. In the former case, the curve is composed of two linear portions; and in the latter case, the curve shows pronounced curvature. The curvature reflects a continous change in the frequency of heterogeneous nucleation.Surface heterogeneous nucleation was assumed to originate at discrete surface heterogeneities and was shown to give rise to continuous cooling curves similar to those calculated for bulk heterogeneous nucleation.     

Surface and bulk residual stresses in Li2O · 2SiO2 glass–ceramics

In the present work surface and bulk residual stresses generated in partially crystallized Li₂O · 2SiO₂ glass–ceramics are analyzed after different heat treatments. The phase specific residual stresses in the crystalline Li₂Si₂O₅-phase are evaluated for the first time in the near-surface zone and the bulk of the samples using both medium and high energy synchrotron radiation. The results reveal that in the crystals within the bulk of the samples micro residual stresses generated by the thermal anisotropy of the isolated individual crystallites depend on the crystallographic direction. In contrast, the residual stress state in the near-surface zone is isotropic due to the superposition of thermal residual stresses in and around the crystals of the near-surface area. Residual stress calculations using a modified Selsing’s model yield a good estimate of the anisotropic residual stresses in the bulk crystallites, whereas the isotropic residual stress state in the crystallized surface layer can be described by an elastic stress model for thin films.     

Photostructural relaxation in As–Se–S glasses: Effect of network fragility

The effect of photoinduced structural relaxation in As–S–Se glass is investigated during sub-bandgap irradiation. It is shown that the glass undergoes rapid optically induced structural relaxation upon photoexcitation of bonding electrons. Fragile systems exhibit larger relaxation as expected from their enthalpy profile. This suggests that the process is thermodynamically driven and that the kinetic impediment to relaxation at low temperature is lifted through photoinduced softening of the glass matrix. Activation energy for enthalpy relaxation measurement and an annealing study near T_g show that the photorelaxation effect is not a thermally activated process. The 〈r〉 dependence of photostructural changes is addressed and explained using the energy landscape formalism.