Mixed Alkali Effect In Glasses Research Articles

Mixed-Alkali Effect in Glasses of Different Classes in the Framework of a Microinhomogeneous-Glass-Structure Model

Mixed-Alkali Effect in Glasses of Different Classes in the Framework of a Microinhomogeneous-Glass-Structure Model

Mixed Alkali Effect in Silicate Glass Structure: Viewpoint of 29Si Nuclear Magnetic Resonance and Statistical Mechanics

The mixed alkali effect in glasses is the deviation from linear property changes when alkali cations are mixed. The extent of this effect and its structural origin remain topics of interest. In this work, we use a statistical mechanics approach to predict the composition-structure relationship in mixed modifier Na2O-K2O-SiO2 glasses. This is achieved by accounting for the enthalpy between each pairwise alkali ion and silicate unit interaction. The initial enthalpy parameters are obtained based on experimental structural data for binary Na2O-SiO2 and K2O-SiO2 glasses, which can be transferred to predict the short-range order structure of mixed modifier glasses without additional free parameters. To this end, we have performed 29Si magic angle spinning NMR spectroscopy measurements on y(xNa2O-(1 - x)K2O)-(100 - y)SiO2 glasses with x = 0, 0.25, 0.5, 0.75, and 1 and y = 34, 42, and 50. Good agreement between experimental data and model predictions are observed. Finally, we use this information to discuss the relative entropic and enthalpic contributions to the mixed modifier effect in silicate glass structure.

Internal Friction and Vulnerability of Mixed Alkali Glasses

Based on a hopping model we show how the mixed alkali effect in glasses can be understood if only a small fraction c(V) of the available sites for the mobile ions is vacant. In particular, we reproduce the peculiar behavior of the internal friction and the steep fall ("vulnerability") of the mobility of the majority ion upon small replacements by the minority ion. The single and mixed alkali internal friction peaks are caused by ion-vacancy and ion-ion exchange processes. If c(V) is small, they can become comparable in height even at small mixing ratios. The large vulnerability is explained by a trapping of vacancies induced by the minority ions. Reasonable choices of model parameters yield typical behaviors found in experiments.

The dynamics of ions in glasses: importance of ion–ion interactions

We present frequency dispersion of two oxide glasses with widely different ion concentrations to show that ion–ion interaction and correlation plays an important role in determining the dc conductivity and the low frequency cooperative part of the dispersion. The origin of the near constant loss, which always appears at higher frequencies/lower temperatures, is traced to few and infrequent independent jumps of the ions out of their cages without the effect of ion–ion correlation. The coupling model gives quantitative support to this interpretation of the entire dispersion. The largest effect of the mixed alkali effect in glasses occurs in the dilute foreign-alkali region, where one foreign alkali immobilizes a large number of host ions. Results show that this feature of the mixed alkali effect can only be explained if ion–ion interaction/correlation is included in the description of the ion dynamics.

Mixed alkali effect in glasses.

We have applied the bond-valence technique to reverse Monte Carlo produced structural models of mixed alkali phosphate glasses in order to elucidate the mixed alkali effect (MAE) in glasses. For the first time, the MAE is reproduced and understood directly from structural models in quantitative agreement with available experimental results. The two types of alkali ions are randomly mixed and have distinctly different conduction pathways of low dimensionality. This implies that A ions tend to block the pathways for the B ions and vice versa, and this is the main reason for the MAE.

Random ion distribution model: A structural approach to the mixed-alkali effect in glasses

We have performed structural studies of a series of mixed-alkali phosphate glasses using diffraction experiments and reverse Monte Carlo simulations. We find that the alkali ions tend to preserve their local structural environment regardless of the glass composition. Furthermore, the distribution of the two types of cations in the structure is predominantly random. We propose that the long debated mixed-alkali effect is a natural consequence of the structural findings; a large energy mismatch for ionic jumps to dissimilar alkali sites resulting in blocking of low-dimensional migration pathways.

Mixed alkali effect in glass irradiated by 50 keV electron beam

A series of three sodium–potassium-silicate glasses with different alkali contents were prepared. The glasses were exposed to a 50 keV electron bombardment of defocused beams of various current densities. Decay curves (alkali X-ray intensity versus time) have been measured and analysed to obtain incubation periods and maximum transport rates. Obtained data have been compared with those known for binary alkali-silicate glass. The results confirm the influence of the mixed alkali effect on the transport behaviour of alkali ions under the electron bombardment.

Towards a theory for the mixed alkali effect in glasses

A general theoretical approach for the mixed alkali effect in glasses is discussed based on the idea that the covalent host network creates different structural energy landscapes for different types of mobile ions. By deriving a simplified model from this approach, it is shown how the experimentally observed changes in ion mobilities both upon mixing of two types of ions and upon changing the total concentration of mobile ions can be understood. Computer simulations of the model suggest that Coulomb forces have to be taken into account for explaining the differing behavior of activation energies in single modified and mixed ion glasses. It is further shown how the mixed alkali internal friction peak results from diffusional exchange processes of unlike ions in the glassy network.

Electrical and mechanical mixed alkali effect in a lithium/sodium borate glass at GHz frequencies

We present results of electrical and mechanical studies of the mixed alkali effect in glass, at frequencies up to 60 GHz. The measurements have been performed on glassy 0.3[ xNa 2O·(1− x)Li 2O]·0.7B 2O 3, where x has beem varied from 0.0 to 1.0.

Nuclear spin relaxation in glassy ionic conductors

The nuclear spin relaxation behaviour in disordered ionically conducting materials is ascribed to a mechanism based on diffusion-controlled relaxation. In this target-diffusion model, relaxation is assumed to occur at interstitialcy sites, triggered by the arrival of diffusing ions. The dependence of the spin-lattice relaxation rate on the temperature, Larmor frequency and ionic concentration can all be understood with this model. The results of a nuclear spin-lattice relaxation study of the mixed-alkali effect in glasses can, on this basis, be interpreted in terms of changes in ionic mobility.

A nuclear spin-lattice relaxation study of the mixed-alkali effect in glasses

The 7Li nuclear spin-lattice relaxation time has been measured as a function of temperature for a series of mixed-alkali Li-Na disilicate glasses. No evidence of a pronounced non-linear mixed-alkali effect was found for the low-temperature activation energy, ELT, of the spin-lattice relaxation (SLR) rate, nor of the ratio ELT/Eσ (where Eσ is the dc conductivity activation energy). These results can be interpreted, in terms of Elliott and Owens' diffusion-controlled relaxation model for SLR, as indicating that there is no significant change in the number of mobile ions in mixed-alkali glasses; instead, the mixed-alkali effect must be due to changes in the ionic mobility.

Study of the mixed alkali effect in glasses and its relation to glass structure and alkali earth ion content

The mixed alkali effect in borosilicate glasses has been studied by diffusion and interpreted in terms of the glass structure parameters. Sodium and potassium diffusion couples were prepared and the interdiffusion zone studied. The glass structure influences the alkali ion diffusion; a mixed alkali effect is present. The smaller alkali ion, sodium, has a greater influence on the diffusion than the larger, potassium ion. The influences of the alkali earth ions on the superimposed existing mixed alkali effect have been studied. The presence of magnesium, calcium and barium further decrease sodium and potassium diffusion coefficients by one or two orders of magnitude, in the interdiffusion zone. An explanation of these results lies in Dietzel's field strength theory of the mixed oxide effect in glasses.

East ion conduction: some theoretical issues

Recent progress in the theoretical understanding of fast ion conduction in solids is discussed, with emphasis placed on mechanistic behavior and on the characteristic features of particular sorts of solid electrolytes. We consider soft framework materials such as α-AgI, and hard framework materials such as β″-alumina. In each case, we discuss which theoretical methods have been used to investigate mechanisms of conductivity and diffusion, and some of the physical insights which have been gleaned on the mechanism of ionic conductivity. Comments are also made on glassy conductors such as glassy lithium aluminosilicate, and polymeric ionic conductors such as polyethylene oxide/lithium triflate. Since different characteristic timescales, and characteristic energies, are appropriate for these different classes of materials, varying theoretical methods have been used, and should be used, to understand the ionic motion. Particular concepts, such as dynamic percolation in polymer electrolytes, strong memory effects in soft framework materials, strongly correlated liquid-like diffusion in hard framework materials and disorder-induced weadening of correlations in glassy materials are pointed out. We speculate briefly on the role of very strong interionic correlations in causing possible domain-wall conduction, a process that goes well beyond any hopping description. We briefly discuss some special behavior observed in certain classes of solid electrolytes, such as fractal behavior, “universal dielectric response”, the mixed alkali effect in glasses, and the “Liang effect”, which is the enhancement of ionic conductivity by inclusion of an insulating second phase. Remarks are ventured both on theoretical methodology and on the usefulness of models for understanding, predicting and designing solid electrolyte behavior.

Mixed alkali effect in glasses: A new model using the thermodynamics of irreversible processes

The mixed alkali effect in glass has been the subject of several papers, but no theory has received universal acceptance yet. In the present work we introduce an analysis of electrical conductivity in binary silicate glasses and in mixed alkali glasses using the Onsager formalism in the frame of thermodynamics of irreversible processes for the ionic transport. On the basis of a new theoretical analysis of existing experimental data, an original interpretation of the mixed alkali effect will be attempted.

A theoretical model for mixed alkali effect in ionic conduction in β-aluminas and glasses

Ionic conductivity of a binary system consisting of two kinds of monovalent ions in the two dimensional honeycomb lattice has been calculated by the Path Probability Method of irreversible statistical mechanics in order to gain insight into the probable cause of the mixed alkali effect in glasses and β-aluminas. It is shown that, if the interaction between the two components is attractive, one can expect the existence of a sharp minimum in the conductivity at a certain composition. The minimum is mainly due to the percolation difficulty created by mutual arrangements of two kinds of ions. Relations between the percolation difficulty and the mixed alkali effect are discussed.

ChemInform Abstract: CONDUCTION AND DIELECTRIC LOSS MECHANISMS IN β‐ALUMINA AND GLASS: A DISCUSSION BASED ON THE PAIRED INTERSTITIALCY MODEL

A paired interstitialcy model is used as a basis for qualitative comparisons of conductivity and dielectric phenomena in β-alumina crystals and in glass. Thus, the increase in the conductivity of sodium silicate glasses with increasing Na2O activity can be explained if the concentration of (Na2*)2+ interstitial pairs increases with increased polarizability of O2- ions, expressed in terms of the optical basicity parameter, Δ. Similarly, the occurrence of the pronounced minima in conductivity isotherms (the mixed-alkali effect in glass) is attributed to disappearance of mobile interstitial pairs, e.g. (Li2*)2+ or (K2*)2+, and the stabilization (by polarization interactions) of apparently immobile mixed-alkali pairs, (LiK*)2+. The phenomenon of coionic conduction in certain β-alumina crystals is an interesting departure from this general pattern. The orientation dependence of the electrical modulus spectrum of monocrys-talline β-alumina highlights the presence of a bimodal distribution of relaxation times, in which the low-frequency component (v0=1011 Hz) may arise from the rearrangement of interstitial pairs and the high-frequency component (v0=2×1012 Hz) may arise from less hindered ionic motions. It is suggested that the motions of interstitial pairs and surrounding cations are mutually catalytic and that some form of combined motion is responsible for both the electrical and mechanical relaxations in β-alumina and glass.

Mixed alkali glasses — Their properties and uses

The mixed alkali effect in glass refers to the large, orders of magnitude, changes in many properties when a second alkali oxide is added. Properties most affected are those associated with alkali ion movement such as electrical conductivity and loss, alkali diffusion, internal friction, viscosity and chemical durability. The compositional dependence of these properties is briefly reviewed and examples are given of the relevance of the mixed alkali effect to the manufacture of commercial glasses. Also reviewed are various theories for this scientifically interesting effect.

Optical Spectra of Chromium in Mixed Alkali Silicate Glasses

As an approach to the mixed alkali effect in glass, the optical absorption spectra of Cr3+ were determined in mixed alkali silicate glasses. The values of the crystal field strength Dq and the Racah parameters B and C for the glasses were calculated from the observed band positions. The crystal field strength Dq is plotted in Fig. 1 against alkali concentration ratio, R1/(R1+R2) where (R1, R2)=(Li, Na), (Na, K) or (K, Li). The curves in Fig. 1 show that Dq deviates downwards from linearity predicted by combining the values for the end members, i.e., there is a minimum for Dq. The behavior in the glasses shown by curves in Fig. 1 may be interpreted based on the presence of mixed alkali effect.

Nuclear magnetic resonance studies of 7Li ionic motion in alkali silicate and borate glasses

Nuclear magnetic resonance (NMR) studies of 7Li ionic motion in four lithium and lithium-potassium silicate glasses of the R2O·4SiO2 and R2O·2SiO2 composition were made before and after heat treatment, and a newly developed equation was used for the analysis of the data. Other data from alkali borate glasses of the 3R2O·7B2O3 composition were reanalyzed using the equation, which eliminates the problem of a distribution of ionic relaxation times. It is shown how heterogeneities in the form of barriers to alkali ion motion in the glasses can be detected and studied by NMR. Although all glasses with higher alkali contents appear to be homogeneous, both the borates and silicates with lower alkali contents appear to be heterogeneous. These heterogeneities appear to be on a smaller scale in the borates than in the silicates. In addition to long-range 7Li ionic motion observed in homogeneous glasses (and intermediate-range motion in heterogeneous glasses), there is evidence of short-range motion within severely restricted regions in nearly all glasses studied. The data show that the long-range 7Li ionic motion in glasses proceeds by an interstitial process. The 7Li NMR data of several crystallized lithium silicate glasses in which polycrystalline lithium disilicate was formed reveal that the local atomic structure in the crystal differs from that of the glasses. A large difference found between the activation energies for 7Li long-range motion in glassy (0.648 eV) and crystalline (2.19 eV) lithium disilicate supports this conclusion. Changes of the 7Li activation energy in lithium borate glasses upon partial substitution of lithium-7 by other alkali species follow a trend in accord with an explanation of the mixed-alkali effect in glass previously proposed by the authors.

The mixed alkali effect in the Na 2OCs 2OSiO 2 glasses

As an approach to the mixed alkali effect in glass, the self-diffusion coefficients of sodium and cesium ions in Na 2OCs 2OSiO 2 glasses were measured at temperatures 350–550°C. Electrical conductivity of the glasses and the transport number for sodium ions were also measured. The substitution of the alkali ions in the glass by different alkali ions caused the mobility of each alkali ion to decrease pronouncedly and the activation energy for migration to increase rapidly. The increase of activation energy was attributed to an increase in alkali-oxygen bond strength resulting from the presence of two kinds of alkali ions. This is related to the expectation that the activity of the alkali ions decreases when two alkali ions are mixed.