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.     

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.     

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.     

An X-ray diffraction study of sodium borate glasses

X-ray diffraction measurements have been performed on two pairs of sodium borate glasses of differing compositions. Glasses of given composition were obtained with two different thermal histories. The radial distribution functions of samples of the same composition appear similar. The short range order is not affected by the range of quenching rates explored.     

Sodium tracer diffusion in sodium boroaluminosilicate glasses

Sodium tracer diffusion coefficients, D*_Na, have been measured using the radioactive isotope Na-22 in sodium boroaluminosilicate (NBAS) glasses containing either a small amount of As₂O₃ or Fe₂O₃. The chemical compositions of the first type of glasses are given by the formula [(Na₂O)_0.71(Fe₂O₃)_0.05(B₂O₃)_0.24]_0.2[(SiO₂)_x(Al₂O₃)_1 ? x]_0.8 and those of the second type of glasses correspond to the formula [(Na₂O)_0.73(B₂O₃)_0.24(As₂O₃)_0.03]_0.18[(SiO₂)_x(Al₂O₃)_1 ? x]_0.82. Tracer diffusion measurements were performed at different temperatures between 198 and 350 °C. Pre-annealing of the glass samples at their glass transition temperatures in common air was found to lead to changes in the values of sodium tracer diffusion coefficients. For the NBAS glasses containing Fe₂O₃, after pre-annealing for 5 h, the activation enthalpy derived for the sodium tracer diffusion increases almost linearly from 57.5 to 71.3 kJ/mol with a decrease in the alumina content while the pre-exponential factor of the sodium tracer diffusion coefficient increases from 2.1 · 10^? 4 to 5.3 · 10^? 4 cm²/s. For the iron-free NBAS glasses pre-annealed for 5 h, the activation enthalpy varies between 63.9 and 71.4 kJ/mol while the pre-exponential factor varies between 1.5 · 10^? 4 and 1.2 · 10^? 3 cm²/s. In addition, it was observed that the considered glasses take up water when annealed at 300 °C in wet air with P_H2O = 474 mbar.     

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.     

Transition from a single-ion to a collective diffusion mechanism in alkali borate glasses

Two distinct regions of the dc conductivity and its temperature dependence as a function of the mol fraction of alkali oxides, X, are observed in Na- and Li-borate glasses. At low alkali contents the dc conductivity σ_dc increases only moderately with X. However, at higher alkali contents, log σ_dc increases linearly and the activation enthalpy ΔH of σ_dc × T decreases linearly with log X, i.e. the dc conductivity reveals an effective power-law behavior. The transition between low alkali and high alkali behavior takes place at X ≈ 0.08 for Na-borate and at X ≈ 0.09 for Li-borate glasses. This behavior suggests that the diffusion mechanism changes at these alkali contents. The results are discussed in terms of ion separations and the transition from a single-ion jump to a collective diffusion mechanism. The vanishing of the mixed-alkali peak in Na–Rb borate and alumino-germanate glasses at sodium contents similar to that observed for the change in slope of σ_dc(X) in this work suggests that both phenomena share the same origin.     

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.     

Effects of pressure on the electrical conductivity of chalcogenide glasses

The effects of hydrostatic pressure (to ～2.4 GPa) on the electrical conductivity of AsTe, AsTeI and AsTeGe bulk semiconducting glasses have been determined. The electrical conductivity σ increases nearly exponentially with increasing pressure P. The Δ 1n σ/ΔP values are dependent upon composition and pressure, and vary from about 2 to 6 GPa^?1. This is a narrow range of values considering that the initial conductivies vary over five orders of magnitude for the compositions studied. Many of the glasses exhibited time-dependent conductivity changes both at high pressure and after cycling to high pressure. At high pressure the conductivity drifted to higher values over a period of several hours, initially following a logarithmic time dependence. Generally, the drifts were observed for P ? 0.8 GPa and for $\text{σ ? 10}{}^{\mathrm{t-1}}\text{(Ω-}\text{m}\text{)}{}^{\mathrm{?1}}$. Following the high-pressure experiment, the conductivity (and also the density) of some glasses were above that for the as-prepared material. These same samples had a slightly different conductivity temperature dependence. The conductivity slowly relaxed (over many months) toward the original conductivity state, again initially following a logarithmic time dependence. Much of our data can be interpreted consistently if we assumed that the conductivity changes depend primarily on “expected” volume changes. The kinds of behavior reported here are similar to those observed for a wide variety of glass systems. Any models developed for describing electrical transport under pressure must account for time-dependent as well as pressure-dependent effects.     

Electrical conductivity and 11B NMR studies of sodium borosilicate glasses

A large number of compositions within the SiO₂–NaO₂–B₂O₃ (SNB) ternary glass system were investigated by various complementary techniques. Impedance spectroscopy was used to determine the parameters related to sodium ion diffusion in the glass structure; high-field boron NMR measurements on a series of samples identified and quantified the boron coordination as a function of the composition; exhaustive DTA measurements gave the glass transition temperature for all the compositions studied. Based on these results we demonstrate that the activation energy of sodium ion diffusion is closely related to the boron coordination number and involves two types of structural motifs: one corresponding to the sodium associated to non-bridging oxygen atoms, and the other to sodium compensating B^IV motif. We also show that simple DTA measurements of the glass transition temperature can be used to define structural domains within this ternary composition range.     

Effect of thermal history on conductivity and electrical relaxation in alkali silicate glasses

Electrical conductivity σ₀ and electric field relaxation measurements have been carried out as a function of thermal history for two alkali silicate glasses, Na₂O3SiO₂ and K₂O3SiO₂. Specimens of each glass with three different thermal histories, two of the anneal-and-quench type and one of the rate-cool type, were studied. The average structural or fictive temperature T_f of each of the specimens was characterized by measuring their indices of refraction. Effects of thermal history on σ₀ and its activation enthalpy $\text{H}{}_{\mathrm{\sigma }}{}^1$ were in accord with results of previous investigators. That is, for a given type of thermal history σ₀ was lower and $\text{H}{}_{\mathrm{\sigma }}{}^1$ higher the lower T_f. In addition it was found that for two specimens with the same T_f or index of refraction but different thermal histories the rate-cooled specimen exhibited a lower conductivity than the annealed-and-quenched specimen, in accord with the results of Ritland. The distribution of relaxation times τ_σ for decay of the electric field due to ionic migration was found to be due primarily to a distribution in the pre-exponential term ln $\text{τ}{}_{\mathrm{\sigma }}{}^1$ in the equation $\text{ln}\text{τ}{}_{\mathrm{\sigma }}\text{=}\text{ln}\text{τ}{}_{\mathrm{\sigma }}{}^1\text{+ H}{}^1\text{/RT}$; the distribution in $\text{H}{}^1$ was extremely narrow. Differences in thermal history caused small differences in the distribution of τ_σ, but no difference in the average activation enthalpy $\text{〈H}{}^1\text{〉}$ for τ_σ. From this result it appeared that the dependence of the conductivity activation enthalpy $\text{H}{}_{\mathrm{\sigma }}{}^1$ on thermal history was due to the effect of thermal history on the temperature dependence of the distribution in τ_σ.     

Internal friction in phosphoaluminate and borate glasses

Internal friction in sodium phosphoaluminate and borate glasses was investigated by torsion pendulum, beam vibration and pulse-echo methods. The Arrhenian relationship was verified in a frequency interval covering eight decades. A detailed analysis of the first peak was made to obtain the distribution of relaxation times and the distribution of activation entropy ΔS and activation enthalpy ΔH. The log-normal distributions were further compared to the Doremus theory and the observed discrepancy discussed.     

Mixed-alkali effect in alkali borate glasses

The spectral absorption of Cu²⁺ and Ni²⁺, the density, molar volume, refractive index, molar refraction, thermal expansion coefficient, transformation and dilatometric softening temperatures of mixed alkali borate glasses are reviewed. These results indicate that in borate glasses, mixed alkalis do not produce gross alterations of the glass network, and that the weak deviations observed may be related to the boric oxide anomaly.     

Scattering losses in binary borate glasses

The purpose of this paper is to examine the potential of three binary borate glasses; namely PbOB₂O₃, K₂OB₂O₃ and Li₂OB₂O₃ as candidates for fabrication of low optical loss and low cost fiber-glass wave-guides.The importance of ultrasonic measurements as the first step in a systematic search for a glass with low optical loss, is discussed. Results of ultrasonic measurements of PbOB₂O₃ system are then presented. Using these results and the published results for the K₂OB₂O₃ and Li₂OB₂O₃ systems, estimates of the magnitude of density fluctuations as a function of composition have been made for each system. Comparison with the previously published results on the K₂OSiO₂ system suggests that out of the three systems chosen, only 50 mole % Li₂O50 mole % B₂O₃ glass is a likely candidate for the production of low optical loss glass fibers.     

Molar volume,glass-transition temperature,and ionic conductivity of Na- and Rb-borate glasses in comparison with mixed Na–Rb borate glasses

Mass densities, molar volumes, glass-transition temperatures, and ionic conductivities are measured in series of YNa₂O · (1 − Y)B₂O₃ glasses, with Y = 0.00, 0.04, 0.08, 0.12, 0.16, 0.20, 0.25, 0.30 and YRb₂O · (1 − Y)B₂O₃ glasses, with Y = 0.00, 0.12, 0.16, 0.20, 0.25, 0.30. Measurements of the molar volumes indicate that the incorporation of rubidium ions leads to a considerable expansion of the network, which is not observed for sodium ions. The glass-transition temperature increases with increasing alkali content and reaches a maximum near Y = 0.25 for both glass systems. These trends are attributed to changes in the glass network. For each glass composition an Arrhenius-activated increase of the product of dc conductivity and temperature is observed. The activation enthalpy decreases with increasing number density of ions. A comparison between the binary sodium- and rubidium-borate glasses from this work, with the ternary sodium–rubidium borate glasses studied earlier in our laboratory, provides interesting insights in the influence of the glass structure on ionic transport processes and the mixed-alkali effect.     

Dielectric relaxation in iron-containing borate glasses

The ac and dc conductivities, dielectric constant and dielectric loss of a series of calcium borate glasses containing Fe₂O₃ up to 10 mol% were measured as a function of temperature (300–700 K) and frequency (10²–10⁵ c/s). Glasses melted in platinum crucibles were found to show slightly higher dielectric constants and conductivities than glasses melted in zirconia crucibles under the same melting conditions. Relaxation effects with a distribution of relaxation times were observed for all the glasses of this system. The data show that the pre-exponential factor τ₀ is about 10^?11 s. Peaks in tan δ were found to shift to higher temperatures when the frequency of measurements was increased. Activation energy for dc conduction was found to be equal to that of ac conduction. No polarization effects were observed in these glasses, indicating that conduction is predominantly electronic.     

The role of spin-spin interaction in the electrical conductivity of vanadium phosphate glasses

Representative glass series of different vanadium content and valence state have been studied with special emphasis on V⁴⁺−V⁴⁺ spin-spin interaction. From temperature dependent EPR studies paramagnetic Curie temperatures and J values have been determined. We demonstrate that this method is informative in the whole glass formation range. It is shown that there is an antiferromagnetic → ferromagnetic transition at c = V⁴⁺/(V⁴⁺ + V⁵⁺) 0.17 (2.3×10²¹ spin cm⁻³). At this composition the glass is essentially homogeneous and displays maximal conductivity. In glasses of higher V⁴⁺ content, in which the dominating interaction is ferromagnetic, the conductivity gradually decreases. Therefore, ferromagnetically coupled V⁴⁺ − V⁴⁺ pairs seem to contribute to conductivity less effectively.