Single Alkali Glasses Research Articles

Impact of substituting K2O for Na2O on physico-chemical properties and in vitro bioactivity of bioactive glass S53P4

The impact of substituting K2O for Na2O (0–100%) on the physico-chemical and bioactive properties of bioactive glass S53P4 was studied. The thermal properties were evaluated using differential thermal analysis and high-temperature microscopy. Ion release kinetics was measured in static and dynamic TRIS-buffer and SBF using inductively coupled plasma optical emission spectroscopy. Substituting K2O for Na2O showed a mixed-alkali effect (MAE) for the thermal properties with a minimum value for the glass containing around 20% K2O. The hot-working range increased with the potassium content. The glass with 66% of Na2O replaced with K2O exhibited MAE in static TRIS by giving the biggest pH change and ion release at 24 h. The same glass showed the lowest pH and Si species release in dynamic SBF at the longest dissolution time, 72 h. Both single-alkali glasses showed higher in vitro bioactivity in dynamic SBF. The results provide tools for tailoring new bioactive glass compositions with desired properties.

Ion conduction in beryllium-alumino-silicate glasses doped with sodium or sodium and lithium ions

Electrical properties of beryllium-alumino-silicate glasses containing sodium ions or sodium and lithium ions were studied with impedance spectroscopy technique over a frequency range from 10 mHz to 1 MHz and at temperature range from 213 to 473 K. The frequency- and temperature-dependent conductivity spectra of individual single alkali glasses were superimposed by means of the Summerfield scaling. Mixed-alkali glasses do not overlap into a single master curve. Glasses doped with sodium ions exhibit significantly higher values of D.C. conductivity and lower activation energy (~0.63 eV) than glasses doped with both sodium and lithium ions (~0.95 eV). The observed mixed-alkali effect can be described by the dynamic structure model (DSM). The conductivity pre-exponential factors and activation energy follow the Meyer-Neldel rule in both glass series. It was observed that the replacement of SiO2 by BeO in single cation glasses resulted in decrease in activation energy and pre-exponential factor σ0. In mixed cations glasses similar effect found for D.C. conduction process parameters was assigned to influence of both oxides BeO and Al2O3.

Communication: Dimensionality of the ionic conduction pathways in glass and the mixed-alkali effect.

A revised empirical relationship between the power law exponent of ac conductivity dispersion and the dimensionality of the ionic conduction pathway is established on the basis of electrical impedance spectroscopic (EIS) measurements on crystalline ionic conductors. These results imply that the "universal" ac conductivity dispersion observed in glassy solids is associated with ionic transport along fractal pathways. EIS measurements on single-alkali glasses indicate that the dimensionality of this pathway D is ∼2.5, while in mixed-alkali glasses, D is lower and goes through a minimum value of ∼2.2 when the concentrations of the two alkalis become equal. D and σ display similar variation with alkali composition, thus suggesting a topological origin of the mixed-alkali effect.

Enthalpy of mixing of mixed alkali glasses

The enthalpy of mixing was determined for various alkali silicate glasses where alkali species were Li, Na, and K and the total alkali oxide contents were 25 mol% and 33 mol%, using an ion-exchange equilibrium method. It was found that the enthalpy of mixing is negative for all the systems investigated and the magnitude of the enthalpy of mixing was found to increase with alkali content for the Na 2O–K 2O–SiO 2 glass system and also with the size difference of two alkali species for a given alkali concentration. Furthermore, for all the systems investigated the magnitude of enthalpy of mixing was found to be proportional to the difference of molar volumes of the corresponding single alkali glasses. The results were interpreted as a probable cause of the large activation energy of DC electric conductivity, the large mechanical relaxation loss peak, and the large thermometer effect observed for mixed alkali glasses.

Conductivity in alkali doped CoO-B2O3 glasses

Two series of cobalt-borate glasses doped with Li2O and K2O in single and mixed proportions have been synthesized by melt quenching method and investigated for ac conductivity in the frequency range of 50Hz to 5MHz and temperature range of 310K to 610K. From the measured total conductivity, the pure ac component and its frequency exponent, s were determined. In the single alkali doped glasses, for all the frequencies, the conductivity increased with increase of Li2O up to 0.4 mole fractions and decreased for further increase of Li2O. The temperature dependence of conductivity has been analyzed using Mott's small polaron hopping model and activation energy for ac conduction has been determined. Based on conductivity and activation behaviors, in single alkali glasses, a change over of conduction mechanism predominantly from ionic to electronic has been predicted. In mixed alkali doped glasses, the conductivity passed through minimum and activation energy passed through maximum for second alkali (K2O) content of 0.2 mole fractions. This result revealed the mixed alkali effect to be occurring at 0.2 mole fractions of K2O. The frequency exponent, s, was compared with theoretical models such as Quantum Mechanical Tunneling and Correlated Barrier Hopping models and found them to be inadequate to explain the experimental observations. Time-temperature superposition principle has been verified in both the sets of glasses.

Ionic conductivity in borate glasses with three types of mixed alkali cations

We have investigated the temperature- and frequency-dependent ionic conductivity in (Li 0.67 − x Na 0.33 Rb x ) 2B 4O 7 (LNRBO) glasses with x = 0, 0.07, 0.2, 0.33, 0.47, and 0.6. The mixed alkali effect of the ternary mixed alkali system LNRBO is compared with that of the binary mixed alkali systems (Li 1 − x Na x ) 2B 4O 7 (LNBO), (Li 1 − x Rb x ) 2B 4O 7 (LRBO) and the single alkali glass Rb 2B 4O 7 (RBO). From the results of the dc conductivity and dc activation energy, we observe that the LNRBO system exhibits the combined characteristic of binary mixed alkali systems LNBO and LRBO. It is found that the power-law exponent n for binary alkali glass is the same as that for ternary alkali glass but it is lower than that for single alkali glass. This indicates that the dimensionality of conducting pathway in the mixed alkali glasses of LNBO, LRBO and LNRBO is lower than that in the single alkali RBO. We discuss the concentration dependence of the dc conductivity and dc activation energy in the framework of the bond valence technique to reverse Monte Carlo produced structural model [Phys. Rev. Lett. 90, 155507 (2003)].

Ac conductivity studies in single and mixed alkali vanadophosphate glasses

Abstract Two different series of vanadophosphate glasses doped with sodium and sodium–potassium were investigated for ac conductivity in the frequency range 500 Hz to 500 kHz and temperature range 300–660 K. Mott's small polaron hopping has been used to explain the temperature dependence of electrical conductivity. The change over of conduction mechanism, occurring at 0.16 mole fraction of Na2O from predominantly electronic to ionic has been observed in single alkali glasses. The mixed alkali effect, in terms of electrical conductivity and activation energies associated with it, has been observed to be occurring at 0.12 mole fraction of second alkali (K2O) content in mixed alkali glasses. The results have been discussed.

Thermal conductivity of mixed alkali silicate glasses at low temperature

Thermal conductivity of glass is one of the fundamental properties of it. However, that has not been studied enough. That of only less than 20 compositions has been measured below the room temperature. In this study, we measured the thermal conductivity of xNa2O·(100−x)SiO2 and (33−y)Na2O·yCs2O·67SiO2 glasses by a transient heating method in the temperature range from about 150K to room temperature. The conductivity of xNa2O·(100−x)SiO2 is found to decrease with the increase in alkali content. The dominant factor of this behavior is the decrease in phonon mean free path, which is due to the increase of non-bridge oxygen. Thermal conductivity of (33−y)Cs2O·yNa2O·67SiO2 is decreased with the increase in Cs2O/(Na2O+Cs2O) ratio. The dominant factor of this behavior is the decrease of sound velocity. However, composition dependence of the phonon mean free path also affects the thermal conductivity. Phonon mean free path of 33Cs2O·67SiO2 glass is longer than that of 33Na2O·67SiO2 glass, and should be related to the change in distribution of structural unit in glass. In addition, phonon mean free path of mixed alkali glasses are shorter than that of single alkali glasses.

Electrical conductivity studies in single and mixed alkali doped cobalt–borate glasses

The glasses of the type (Li2O)x–(CoO)0.2–(B2O3)0.8−x and (Li2O)0.2–(K2O)x–(CoO)0.2–(B2O3)0.6−x were prepared by melt quench technique and their non-crystallinity has been established by XRD studies. The glasses were investigated for room temperature density and dc electrical conductivity in the temperature range 300–550 K. Molar volumes were estimated from density data. Composition dependence of density and molar volume in both the sets of glasses has been discussed. Conductivity data has been analyzed in the light of Mott’s Small Polaron Hopping (SPH) Model and activation energies were determined. Variation of conductivity and activation energy with Li2O content in single alkali glasses indicated change over conduction mechanism from predominantly electronic to ionic, at 0.4 mole fraction of Li2O. In mixed alkali glasses, the conductivity has passed through minimum and activation energy has passed through maximum at x = 0.2. This has been attributed to the mixed alkali effect. It is for the first time that a change over of predominant conduction mechanism in lithium–cobalt–borate glasses and mixed alkali effect in lithium–potassium–cobalt–borate glasses has been observed. Various physical and polaron hopping parameters such as polaron hopping distance, polaron radius, polaron binding energy, polaron band width, polaron coupling constant, effective dielectric constant, density of states at Fermi level have been determined and discussed.

Relaxation in mixed alkali fluoride glasses

We have investigated the dynamics of alkali cations as well as fluorine anions in non-oxide fluoride glasses with a total alkali fluoride content varying from 16 to 35mol% in the frequency range from 10Hz to 2MHz and in the temperature range from room temperature to just below the glass transition temperature. We have shown that the dc and ac conductivity, crossover frequency and conductivity relaxation frequency exhibit a minimum in mixed alkali fluoride glasses, where anions also participate in the diffusion processes in addition to cations, unlike mixed alkali oxide glasses and crystals. We have observed lower dimensionality of the conduction pathways in mixed alkali fluoride glasses compared to that in the single alkali glasses. We have shown that the relaxation dynamics in mixed alkali fluoride glasses is independent of temperature.

Investigation of the Mixed Alkali Effect in a Range of Phosphate Glasses

This study investigated the mixed alkali effect in a series of phosphate based glasses. These glasses were of the composition 0.5P2O5-0.2CaO-0.3-xNa2O-xK2O where x=0 to 0.3 in steps of 0.05. This study considered density measurements using Archimedes’s principle, thermal characterisation using differential scanning calorimetry, phase analysis following crystallisation using X-ray powder diffraction (XRD), and degradation studies combined with ion release. The results showed that these mixed alkali glasses showed a linear decrease in density, with the ternary single alkali glass with 0.3mol K2O showing a 3% reduction in density as compared to that with 0.3mol Na2O which correlated well with the difference in ionic diameter and atomic weight of both cations. These glasses also showed intermediate glass transition temperature (Tg) values, compared to those of the ternary single alkali glasses having the same alkali oxide content, and the minimum Tg value was recorded for equimolar amounts of both alkali oxides. However, they did not show any significant change in the degradation rate compared to the glass with 0.3mol Na2O with the exception of the 0.25mol K2O glass. The single alkali glass with 0.3mol K2O showed a significant increase in the degradation rate by an approximate one order of magnitude. For the mixed alkali glasses with low molar concentration of K2O, only sodium phosphate-rich phases [NaCa(PO3)3 and Na4Ca(PO3)6] were detected from XRD; at high molar concentrations however, potassium phosphate-rich phases [KCa(PO3)3 and KPO3] were detected. At equimolar concentration of both alkali cations, KCa(PO3)3 and Na4Ca(PO3)6 were identified. K+, Ca2+, and P3O9 3- release followed the degradation behaviour where the highly degrading glasses with 0.25 and 0.3mol K2O released the highest amount of these ions; however, there was no definite trend in the remaining glass compositions.

Isothermal mechanical relaxations in single and mixed alkali silicate glasses

Very low frequency damping measurements, carried out in various mixed alkali silicate glasses (1 − x)Na2O · xLi2O · 3SiO2 with x = 0, 0.25, 0.5 and 0.75, indicate new experimental features. A new relaxation peak at very low temperature is evidenced in the single alkali glass (x = 0) in addition to the two classical relaxation peaks previously described and assigned, respectively, to the motion of alkali ions (ionic peak) and a local rearrangement of the glassy structure involving non-bridging oxygen (NBO peak). For the three mixed alkali glasses, the mixed alkali relaxation peak is shown to be superimposed to two satellite peaks corresponding to the ionic and NBO peaks. Moreover, an Arrhenius plot of the mixed alkali peak shows that the apparent activation energy is different below and above 340 K. These experimental results are discussed using recent models proposed for the alkali effect and a model developed previously for mechanical relaxation in ionic glasses below T g.

A revised view on the mixed-alkali effect in alkali borate glasses

The mixed-alkali effect (MAE) of the ionic conductivity is traditionally defined as a negative deviation of the dc conductivity from that of an ‘ideal glass’ obtained according a linear mixing rule. An alternative definition is proposed in this paper which relates the conductivity of a mixed-alkali glass to the conductivity-sum of two single-alkali glasses with total alkali concentrations equal to the corresponding partial alkali concentrations of the mixed-alkali glass. A new reference glass denoted as independent component glass (ICG) is introduced. In contrast to the ‘ideal glass’ the conductivity of a real mixed-alkali glass is larger than the conductivity of the respective ICG resulting in a positive MAE. This fact manifests an enhancement of the conductivity when two single-alkali glasses are combined to a mixed-alkali glass. The conductivities and the tracer diffusivities of the mixed-alkali- and the corresponding single-alkali glasses are compared from this point of view. A subnetwork diffusion concept (SDC) is proposed to describe the diffusion dynamics in Na–Rb-borate mixed-alkali glasses.

Systematic modeling study of channel waveguide fabrication by thermal silver ion exchange

A systematic study of thermal silver ion exchange used for the fabrication of optical channel waveguides is reported in a single-alkali glass. The diffusion equilibrium and diffusion dynamics are experimentally studied, and the concentration-dependent diffusion coefficients are determined. The relationship between the fabrication conditions, i.e., time, temperature, and melt concentration, and the induced waveguide refractive index profile is established. It is demonstrated that the diffusion equation can be solved, without use of any free parameters, to predict the refractive index profiles of both planar and channel waveguides. A 1.6 cm diameter integrated optic ring resonator, with a propagation loss of 0.1 dB/cm, is fabricated in a glass by thermal silver ion exchange. The induced refractive index profile is related to the optical characteristics of the functional device.

Ion dynamics and mixed mobile ion effect in fluoride glasses

We report the ionic relaxation and mixed mobile ion effect in 50ZrF4−10BaF2−10YF3−(30−x)LiF−xNaF fluoride glass series, where fluorine anions participate in the diffusion process in addition to alkali cations, unlike mixed alkali oxide glasses and crystals. By analyzing the ion dynamics in the framework of a power-law model as well as modulus formalism we have observed mixed mobile ion effect in the dc conductivity and its activation energy, the crossover frequency and its activation energy, the conductivity relaxation frequency and its activation energy, and also in the decoupling index. We have correlated these phenomena with the fractal dimension of the conduction pathways in the mixed alkali fluoride glasses compared to the single alkali glasses. We have shown that the relaxation dynamics in mixed alkali fluoride glasses is independent of temperature but dependent on glass composition.

Structural analysis of bioactive glasses

In this study four series of single and mixed alkali glass systems were made and investigated using MAS NMR. Additionally the densities of the glasses were measured experimentally, as well as calculated theoretically using Doweidar’s model. MAS NMR was used to obtain a quantitative structural understanding of glasses by calculating the concentrations of bridging and non-bridging oxygens per silicon oxygen tetrahedron as a function of the alkali oxide concentration expressed as Q n . 29Si MAS NMR spectra exhibited a single resonance corresponding closely with Si in a Q 2 state. The chemical shift of the 31P MAS NMR peak was attributed to phosphate in an orthophosphate environment. The 29Si NMR spectra are in agreement with the density data. Using Doweidar’s model the proportions of Q 2 and Q 3 were calculated, showing that all glasses studied are predominantly Q 2 in structure, i.e. [ SiO 3 - ] n silica chains which readily dissolve. The changes in the chemical shifts of the Q 2 and Q 3 species with composition have been interpreted as resulting from the preferential association of Na + with Q 3 and Ca 2+ with Q 2.

Ion dynamics in mixed alkali cadmium fluoride glasses

We have investigated ion dynamics in cadmium fluoride nonoxide glasses, in the frequency range from 10 Hz to 2 MHz and in the temperature range from room temperature to just below the glass transition temperature. We have established the existence of mixed alkali effect for these glasses from different aspects of relaxation dynamics. We have observed lower dimensionality of the conduction pathways in mixed alkali cadmium fluoride glasses compared to that in the single alkali glasses. Although, both n and beta represent the interaction between the charge carriers, we have not found any theoretical correlation between them. Using different scaling approaches we have shown that the relaxation dynamics in mixed alkali cadmium fluoride glasses is independent of temperatures.

Molecular dynamics simulations of the dynamics of ions in single and mixed alkali glasses

Molecular dynamics simulations were carried out at 700K in single alkali and mixed alkali metasilicate glasses to reproduce the dramatic mixed alkali effect occurring in the dilute foreign alkali region. Previous experimental work showed that one foreign alkali appears to immobilize a large number of host ions. This large number can now be rationalized with the help of the simulations as being due to blockage by the less mobile foreign ions of the co-operative ion dynamics that originate from ion–ion interaction and correlation. Molecular dynamics simulations of Li metasilicate glass at 500K showed that the ubiquitous nearly constant loss (NCL) comes from the motions of caged Li ions that experience ‘dynamic anharmonicity’ caused by the motions of the caging matrix atoms, particularly the oxygen atoms. The length scales of the Li ion motions are distributed according to a Lévy distribution that has a long tail, which is responsible for the peculiar frequency dependence of the NCL.

Structure conductivity correlation in reverse Monte Carlo models of single and mixed alkali glasses

The correlation between the local structure of alkali glasses and their ionic conductivity is investigated by a bond valence analysis of reverse Monte Carlo (RMC) fits to experimental diffraction data. Our previously detected general relation between the volume fraction of the transport pathways and their ionic conductivity also permits to reproduce the mixed alkali effect and provides further insight into its structural origin. The drastic drop of the conductivity is mainly caused by the blocking by immobile unlike cations, which is highly effective due to the low dimensionality of the pathways on local length scales. Factors that affect the pathway dimensionality are discussed. The evolution of the transport pathways in time is studied by applying the BV approach to entire molecular dynamics trajectories.

Frequency dependent conductivity of single alkali and mixed alkali phosphate glasses

The frequency dependent conductivity σ(f) of single alkali glasses LiPO3 and RbPO3 has been compared with the conductivity of the mixed alkali glass Li0.5Rb0.5PO3. The low frequency plateau value σ0 of the conductivity drops by several orders of magnitude in the mixed alkali glass. In this work the effect of mixed alkali on the shape of the conductivity σ(f) in the dispersive region has been analyzed. The slope of the conductivity curve, in the logσ vs. logf plot, increases more rapidly in the single alkali glasses than in the mixed composition. The onset of the dispersive region is more gradual in the mixed alkali glass. The results indicate that the mixed alkali glass behaves as a dilute single alkali glass.