Tracer diffusion and electrical conductivity in sodium-cesium silicate glasses

The tracer diffusion coefficients of ²²Na and ¹³⁷Cs, and the electrical conductivity have been measured in the (Na, Cs)₂O:3SiO₂ glasses as a function of temperature and Cs/Na ratio. Complex impedance analysis was used for the conductivity measurements. The Haven ratio at 396.5°C increases from 0.3–0.4 in single-alkali glasses to 0.8 for the mixed-alkali compositions. The results are explained in terms of a single-jump mechanism; interactions between alkali ions and non-bridging oxygen ions, and between different alkali ions, produce the observed correlation effects.     

Electrical relaxation from hydrated surface layers in alkali silicate glasses

Complex impedance plots for a 25 Na₂O?75 SiO₂ glass exposed to the ambient atmosphere indicate the presence of a relaxation at frequencies well below those for the electrical relaxation of the bulk material, but above the frequencies where electrode polarization takes place. This low frequency relaxation is shown to occur in a high resistivity surface layer having a thickness a few times 0.1 μm and formed by the action of atmospheric water vapor. The rate of growth of the surface layer could be monitored by means of the impedance measurements and was found to decrease with the duration of exposure of the glass to the laboratory atmosphere. Complex impedance plots for a chemically durable lithium aluminosilicate glass indicate that the hydrated surface layer thickness is much smaller than in the 25 Na₂O?75 SiO₂ glass.     

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 150 K 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.     

Influence of thermal history on the structure and properties of silicate glasses

We studied a set of float glass samples prepared with different fictive temperatures by previous annealing around the glass transition temperature. We compared the results to previous measurements on a series of amorphous silica samples, also prepared with different fictive temperatures. We showed that the modifications on the structure at a local scale are very small, the changes of physical properties are moderate but the changes on density fluctuations at a nanometer scale are rather large: 12% and 20% in float glass and silica, for relative changes of fictive temperature equal to 13% and 25% respectively. Local order and mechanical properties of silica vary in the opposite way compared to float glass (anomalous behavior) but the density fluctuations in both glasses increase with temperature and fictive temperature.     

Multichannel conduction in alkali silicate glasses

The qualitative change of the electrical conduction of alkali silicate glasses with alkali mixing is discussed in connection with dielectric and mechanical relaxation. The observed conduction is understood as the additive effect of two kinds of independent diffusion processes — I and II. In single alkali glass, diffusion process-I is always observed as the dominant conduction process. However, when one alkali is progressively substituted by another, the dominant conduction process is altered from diffusion process-I to II. In the mixed alkali glass containing equal amounts of two alkali ions, diffusion process-I disappears, so that only diffusion process-II is observed. Consequently, it becomes evident that the mixed alkali effect is not only the successive change of conductivity, but the alternation of the dominant conduction process.     

The structure of alkali silicate glasses

Pulsed neutron diffraction has been used to study the structure of three alkali silicate glasses of nominal composition SiO₂ · (M₂O)_0.5, with M = K, K_0.46Li_0.54 and Li. The data are quantitatively consistent with the modified random network model. Lithium is coordinated by two oxygen atoms with an interatomic distance of 1.97 Å. The potassium-oxygen distance is similar to the tetrahedral oxygen-oxygen distance, 2.66 Å, with potassium being coordinated by approximately four oxygen atoms.     

Effect of alumina on enthalpy of mixing of mixed alkali silicate glasses

The enthalpy of mixing of mixed alkali (Na₂O and K₂O) silicate glasses containing various concentrations of alumina was determined using an ion-exchange equilibrium method. For glasses with a constant alkali concentration, the enthalpy of mixing was found to become less negative with alumina addition. Consistent with our previous results on the enthalpy of mixing of alumina-free mixed alkali silicate glasses, the magnitude of enthalpy of mixing exhibited a good correlation with the molar volume mismatch of the corresponding two single alkali glasses as well as with the extent of conductivity mixed alkali effect, e.g. excess activation energy of conductivity, ΔE. The reduction of the magnitude of the enthalpy of mixing with alumina addition can be attributed to the reduction of non-bridging oxygen and ionic field strength. Combining the present results with results obtained earlier, the magnitude of the enthalpy of mixing for all mixed alkali (Na₂O and K₂O) silicate glasses with and without alumina was expressed by a simple function of a modified Tobolsky parameter, which takes into account the alkali concentration and the difference in cation-to-effective anion distances. The enthalpy of mixing data of the mixed alkali glasses was then compared with reported experimental data on the conductivity of mixed alkali aluminosilicate glasses. What appears to be conflicting experimental data can be understood in terms of the magnitude of the enthalpy of mixing and we can conclude that the mixed alkali effect is closely correlated with the negative enthalpy of mixing.     

The mixed alkali effect in sodium and potassium galliosilicate glasses II. DC electrical conductivity

The DC electrical conductivities of several series of mixed alkali galliosilicate glasses have been measured. The appearance of a minimum in the electrical conductivity of these glasses, independent of the gallium content, suggests that the mixed alkali effect is independent of the non-bridging oxygen content. These results are discussed in terms of current theories proposed to explain this anomalous behaviour.     

Densification of alkali silicate glasses at high pressure

The densification behavior of a number of alkali silicate glasses has been investigated using a Bridgman anvil high pressure device. The pressure range of the investigation was 10 to 60 kilobars; the temperature range was 100 to 400°C. In all cases, the densification was found to increase with increasing temperature and pressure. In the two systems where compositional variations were explored, Na₂OSiO₂ and K₂OSiO₂, a pronounced maximum in densification in the vicinity of 10 mole % alkali oxide was observed (for a given temperature and pressure). These maxima are taken to reflect a competition between two processes, one of which — the variation of molecular mobility with composition — should lead to increasing densification with increasing alkali concentration. Several possibilities are discussed for the decrease in densification with alkali oxide concentrations greater than about 10% (to values smaller than that of SiO₂ in some cases). The present results are related to those obtained in previous investigations of the densification of oxide and polymeric glasses.     

Optical and electrical properties of lead silicate glasses

The optical and electrical properties of PbO-SiO₂ glasses containing 24.6 to 65 mole % PbO have been determined. While the short-wavelength cutoff increases continuously in wavelength with increasing PbO concentration, an abrupt lightening in color is observed in the vicinity of 38–40% PbO. The darker color of glasses containing 30–38% PbO has been associated with a scattering process. The origin of this scattering is discussed and is tentatively identified with the formation of microcrystals of SiO₂ in the glasses. The alternative process of liquid-liquid phase separation seems less likely, but cannot be ruled out with certainty.The electrical conductivity of all glasses investigated exhibits an Arrhenian temperature dependence. The activation energy decreases from about 1.23 eV for the 30% PbO composition to about 1.11 eV for the 50% PbO composition, and remains about constant for glasses containing higher concentrations of PbO. Addition of NH₄Cl or Sb₂O₃ to the batch or use of Pb(NO₃)₂ or Pb₃O₄ instead of PbO as a raw material does not affect the electrical properties. The dielectric constant at 1kHz increases continuously with increasing PbO concentration, from a value in excess of 20 for the 55% PbO composition. The implications of these results for the conduction mechanism in PbO-SiO₂ glasses are discussed.     

DC conductivity in single and mixed alkali vanadophosphate glasses

Three different series of single and mixed alkali vanadium phosphate glasses have been prepared by a melt quench method. DSC studies were carried out on few of these samples and their glass transition temperatures were determined. The glass transition temperatures were found to decrease with alkali content in single alkali systems and increase with second alkali content in mixed alkali systems. The dc electrical conductivity has been measured as a function of temperature. The data has been analyzed in the light of Mott’s small polaron hopping model and activation energies were determined. In one set of single alkali glasses activation energies were found to increase with alkali content and in another set of single alkali systems a transition from predominantly electronic to ionic conduction has been observed above 0.16 mol fraction of alkali content. The mixed alkali glasses have shown higher activation energies and lower conductivities compared to single alkali doped glasses and this has been attributed to a mixed alkali influence on the electrical conduction in these systems.     

Activation volumes and site relaxation in mixed alkali glasses

Evidence is presented for site relaxations occurring in mixed alkali (cation) glasses based on activation volumes, V_A(σ) = −RT[d ln σ/dp]_T, which are determined for sodium aluminoborate glasses of varying Na₂O content, and for mixed alkali glasses where Na⁺ is partially replaced by Li⁺, K⁺ or Cs⁺ ions. In accordance with the ‘updated’ dynamic structure model, activation volumes are identified here with local expansions that accompany the opening up of C′ sites to admit incoming ions. ‘Anomalous’ increases in activation volume in mixed cation glasses correlate with the size of minority (guest) cations. This anomaly is interpreted in terms of a ‘leader follower’ mechanism that involves dynamic coupling between the faster (majority) and slower (minority) cations. Because of mismatch effects in mixed cation glasses this coupling requires the opening up of additional cation sites by the slower follower cations. The resulting disturbances in the glass network are responsible for many characteristic features of the mixed alkali effect, including the appearance of high temperature internal friction peaks and observed minima in glass transition temperatures and melt viscosities.     

Fast and slow dynamics in single and mixed alkali silicate glasses

A molecular dynamics (MD) simulation for single and mixed alkali metasilicates has been performed. The mixed alkali effect (MAE) in the diffusion coefficients is reproduced. The motion of lithium ions in Li₂SiO₃ can be divided into slow (type A) and fast (type B) categories clearly in the glassy state. It turns out that the slow dynamics are caused by localized jump motions with a long tail of the waiting time distribution. On the other hand, fast dynamics of the lithium ions are caused by successive cooperative jump motions and are identified as Lévy flights. The MAE in LiKSiO₃ occurs due to mutual interception of jump paths of both kinds of mobile ions. In such paths, the fast component is considerably suppressed. Namely, the cooperative forward-correlated jump process is sensitive to the blockage of the jump paths. Propagation of the immobilization in the MAE is discussed in relation with the mechanism of the cooperative jump motion. The ratio of the fast and slow dynamics plays an important role to determine the transport properties in the glassy state.     

Low temperature dielectric dispersion and electrical conductivity studies on Fe2O3 mixed lithium yttrium silicate glasses

Lithium yttrium silicate glasses mixed with different concentrations of Fe₂O₃ of the composition (40 ? x) Li₂O–10Y₂O₃–50SiO₂: x Fe₂O₃, with x = 0.3, 0.5, 0.8, 1.0, 1.2 and 1.5 (all in mol%) were synthesized. Electrical and dielectric properties including dielectric constant, ε′(ω), loss, tan δ, ac conductivity, σ_ac, impedance spectra as well as electric moduli, M(ω), over a wide continuous frequency range of 40 Hz to 10⁶ Hz and in the low temperature range 100 to 360 K were measured as a function of the concentration of Fe₂O₃. The dc conductivity is also evaluated in the temperature range 100 … 360 K. The temperature and frequency dispersions of dielectric constant as well as dielectric loss have been analyzed using space charge polarization model. The ac and dc conductivities have exhibited increasing trend with increasing Fe₂O₃ content beyond 0.5 mol%, whereas the activation energy for the conductivity demonstrated decreasing tendency in this dopant concentration range. Both quantum mechanical tunneling (QMT) and correlated barrier hopping models (CBH) were used for clarification of ac conductivity origin and the corresponding analysis has indicated that CBH model is more appropriate for this glass system. For the better understanding of relaxation dynamics of the electrical properties we have drawn the scaling plots for ac conductivity and also electric moduli. The plots indicated that the relaxation dynamics is independent on temperature but depends on concentration of Fe₂O₃. The dc conductivity is analyzed using small polaron hoping model. The increase of conductivity with the concentration of Fe₂O₃ beyond 0.5 mol% is explained in terms of variations in the redox ratio of iron ions in the glass network. The results were further analyzed quantitatively with the support of experimental data from IR, optical absorption and ESR spectral studies. The overall analysis has indicated that Li₂O–Y₂O₃–SiO₂ glasses containing more than 0.5 mol% of Fe₂O₃ are more suitable for achieving good electrical conductivity in these glasses.     

Dielectric study and ac conductivity of iron sodium silicate glasses

A series of glasses with formula (SiO₂)_0.7−x(Na₂O)_0.3(Fe₂O₃)_x with ( 0.0 ≤ x ≤ 0.20) were prepared and studied by means of AC measurements in the frequency range 20 kHz to 13 MHz at room temperature. The study of frequency dependence of both dielectric constant ε' and dielectric loss ε" showed a decrease of both quantities with increasing frequency. The results have been explained on the basis of frequency assistance of electron hopping besides electron polarization. From the Cole-Cole diagram the values of the static dielectric constant ε_s, infinity dielectric constant ε_∞, macroscopic time constant τ, and molecular time constant τ_m are calculated for the studied amorphous samples. The frequency dependence of the ac conductivity obeys a power relation, that is σ_ac (ω) = Α ω^s. The obtained values of the constant s lie in the range of 0.7 ≤ s ≤ 1 in agreement with the theoretical value which confirms the simple quantum mechanical tunneling (QMT) model. The increase in ac conductivity with iron concentration is likely to arise due to structural changes occurring in the glass network. The structure of a glass with similar composition was published and showed clustering of Fe²⁺ and Fe³⁺ ions which favor electron hopping and provide pathways for charge transport.     

DC electrical conductivity of semiconducting cobalt-phosphate glasses

The dc conductivity of semiconducting cobalt-phosphate glasses has been measured at temperatures ranging from 213 to 530 K. Four bulk samples of CoO-P₂O₅ glasses of different compositions were produced by melting dry mixtures of analytical reagent grades of CoO and P₂O₅ at temperatures between 1200-1250 °C for 2 h using a press-quenching method from glass melt. Samples were annealed at 400 °C for 1 h. The dc conductivity was found to be dependent on the CoO content in the glass. At temperatures from 213 to 444 K, however, both Mott's variable-range hopping (VRH) and the Greaves' intermediate range hopping models are found to be applicable. VRH at this range of temperatures is attributed to large values of the disorder energy of these glasses.     

Alkali diffusion and electrical conductivity in sodium borate glasses

The diffusion coefficient of sodium, ²²Na, and silver, ¹¹⁰Ag, and electrical conductivity for sodium borate glasses (4–24 mol% Na₂O) has been measured from 100°C to slightly below the glass transition temperature, T_g. Unlike silicate glasses where the self-diffusion coefficient is much larger than the impurity diffusion coefficient, D_Na and D_Ag had close to the same magnitude in the sodium borate glasses. In some glasses, D_Ag was slightly larger than D_Na. Na₂O-Ag₂O-B₂O₃ glasses show a relatively small mixed alkali effect, despite the significant mass difference between Ag(≈108) and Na(≈23). It is concluded that the mixed alkali effect is more dependent upon the difference in ionic radii rather than differences in mass.     

Mixed isotope electrical conductivity in lithium borate glasses

Electrical conductivity of (Li⁶/Li⁷)₂O·3B₂O₃ glasses with varying Li⁶/Li⁷ ratios has been measured as a function of temperature. The conductivity obtained after complex impedance analysis decreases continously and the activation enthalpy increases slightly as Li⁷ is substituted for Li⁶. This suggest that the difference in mass of the alkali ions is not the cause of the mixed alkali effect.