Mixed Alkali Effect Research Articles

Thermal and electrical properties of mixed alkali in Li 2O–Na 2O–WO 3–P 2O 5 glasses

A series of tungsten-phosphate glasses, 0.5[ xNa 2O–(1 − x)Li 2O] − 0.5[0.25(WO 3) 2 − 0.75P 2O 5] with x varying between 0 and 1, were prepared using the melt quenching technique. They were studied by using powder X-ray diffraction (XRD), differential thermal analysis (DTA) and impedance spectroscopy. The total alkali oxide content is chosen in such a way that the electrical conductivity of the glasses under study is entirely ionic. It was evidenced that the conductivity minimum corresponds to the maximum of the activation energy. The glass transition temperature presents a minimum for the ratio Na/(Na + Li) = 0.5. On the other hand, the density varies linearly versus Na/(Na + Li). The mixed-alkali effect (MAE) is resulting here in a conductivity minimum observed for the composition corresponding to Na/(Na + Li) = 0.5, ( x = 0.5). It could be attributed to the fact that the two types of alkali ions are randomly mixed and have distinct conduction pathways.

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.

Diffusion and ionic conduction in oxide glasses

The ion transport properties of soda-lime silicate and alkali borate glasses have been studied with complimentary tracer diffusion and impedance spectroscopy techniques in order to investigate the ion dynamics and mixed-alkali effect (MAE). In soda-lime silicate glasses the tracer diffusivity of 22Na alkali ions is more than six orders of magnitude faster than the diffusivity of earth alkali 45Ca ions. This observation is attributed to a stronger binding of bivalent earth alkali ions to the glass network as compared to that of alkali ions. The conductivity of the investigated standard soda-lime silicate glasses is mostly due to the high mobility of sodium ions and a temperature independent Haven ratio of about 0.45 is obtained. For single alkali sodium-borate glasses, the Haven ratio is also temperature independent, however, it is decreases with decreasing temperature for rubidium-borate glass. The MAE was investigated for Na-Rb borate glasses and it was observed that the tracer diffusivities of 22Na and 86Rb ions cross, when plotted as function of the relative alkali content. This crossover occurs near the Na/(Na+Rb) ratio of the conductivity minimum due to MAE. The authors suggest that this crossover and the trend of diffusion coefficients is the key to an understanding of the MAE.

Mixed alkali effect in galliogermanate glasses

The relaxation of alkali ions in 20(Li 2O-Cs 2O)-15Ga 2O 3-65GeO 2 glasses is investigated in both the ac conductivity and the electric modulus formalisms. The power law exponents that are obtained from ac conductivity are close to one at relatively low temperatures and decrease with increasing of temperatures. The coupling parameters obtained from the electric modulus decrease with increasing temperatures. However, for samples with pronounced mixed alkali effect, the coupling parameters are less sensitive with temperature changes. In those samples, the ion–ion coupling and the coupling between ion and structural-relaxation are both important for ionic relaxation. The near constant loss found at low temperatures is originated from the vibrational relaxation of ions within their cages. This may also be the origin of the ion and structural-relaxation coupling. The dc conductivities indicate that the mixed alkali effect is most pronounced in the sample containing 8 mol% of Li 2O.

Conductivity studies of lithium zinc silicate glasses with varying lithium contents

The electrical conductivity of lithium zinc silicate (LZS) glasses with composition, (SiO2)0.527 (Na2O)0.054(B2O3)0.05(P2O5)0.029(ZnO)0.34−x (Li2O) x (x = 0.05, 0.08, 0.11, 0.18, 0.21, 0.24 and 0.27), was studied as a function of frequency in the range 100 Hz–15 MHz, over a temperature range from 546–637 K. The a.c. conductivity is found to obey Jonscher’s relation. The d.c. conductivity (σ d.c.) and the hopping frequency (ω h), inferred from the a.c. conductivity data, exhibit Arrhenius-type behaviour with temperature. The electrical modulus spectra show a single peak, indicating a single electrical relaxation time, τ, which also exhibits Arrhenius-type behaviour. Values of activation energy derived from σ d.c., ω h and τ are almost equal within the experimental error. It is seen that σ d.c. and ω h increase systematically with Li2O content up to 21 mol% and then decrease for higher Li2O content, indicating a mixed alkali effect caused by mobile Li+ and Na+ ions. The scaling behaviour of the modulus suggests that the relaxation process is independent of temperature but depends upon Li+ concentration.

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.

Ionic Conductivity in Mixed Alkali-Borate Glasses

The temperatureand frequency-dependent ionic conductivity in (Li1−xNax)2B4O7 (LNBO), (Li1−xKx)2B4O7 (LKBO) and (Li1−xRbx)2B4O7 (LRBO) glasses have been investigated using Xray diffraction (XRD) and an impedance/gain-phase analyzer. We have specifically focused on the dielectric properties in frequency and temperature domains, and those have been analyzed by using a power law. We have found that the dc conductivity value decreases with heavy elements on mixing the alkali ions in the glass structure. For the LNBO, LKBO and LRBO glasses, the mixed alkali effect is observed to become stronger with decreasing temperature and increasing mismatch of the element size. The concentration dependence of the activation energy of the dc conductivity is characterized in the framework of reverse-Monte-Carlo-produced structural models in combination with the bond-valence technique.

Properties of mixed Li 2O and Na 2O molybdenum phosphate glasses

The present study concerns the system 0.5[ xNa 2O–(1 − x)Li 2O]–0.5[0.4(MoO 3) 2–0.6P 2O 5], with x varying between 0 and 1. The elaborated glass samples have been characterised using powder X-ray diffraction (XRD), differential thermal analysis (DTA) and impedance spectroscopy. The total alkali oxide content is chosen in order to ensure that the electrical conductivity of the glasses under study is entirely ionic. It was evidenced that the conductivity minimum corresponds to the maximum of the activation energy. The glass transition temperature seems to present a minimum for the ratio Na/(Na+Li) = 0.5. On the other hand, the density appears very linearly versus Na/(Na+Li). The mixed alkali effect (MAE) is resulting here in a conductivity minimum observed for the composition corresponding to Na/(Na+Li) = 0.5, ( x = 0.5). It could be attributed to the fact that the two types of alkali ions are randomly mixed and have distinct conduction pathways. This implies that Li ions tend to block the pathways for the Na ions and vice versa.

Effect of lithium-potassium mixed alkali on spectroscopic properties of Er3+-doped aluminophosphate glasses

Er3+-doped lithium-potassium mixed alkali aluminophosphate glasses belonging to the oxide system xK2O-(15-x)Li2O-4B2O3-11Al2O3-5BaO-65P2O5 are obtained in a semi-continuous melting quenching process. Spectroscopic properties of Er3+-doped glass matrix have been analysed by fitting the experimental data with the standard Judd–Ofelt theory. It is observed that Judd–Ofelt intensity parameters Ωt (t = 2, 4 and 6) of Er3+ change when the second alkali is introduced into glass matrix. The variation of line strength Sed[4I13/2,4I15/2] follows the same trend as that of the Ω6 parameter. The effect of mixed alkali on the spectroscopic properties of the aluminophosphate glasses, such as absorption cross-section, stimulated emission cross-section, spontaneous emission probability, branching ratio and the radiative lifetime, has also been investigated in this paper.

EPR and optical studies of Mn 2+ ions in Li 2O–Na 2O–B 2O 3 glasses – An evidence of mixed alkali effect

EPR and optical absorption spectra of 0.5 mol% MnO 2 doped xLi 2O–(30 − x)Na 2O–69.5B 2O 3 (5 ⩽ x ⩽ 25) glasses have been studied. The EPR spectra exhibit resonance signals characteristic of Mn 2+ ions. The resonance signal at g ≅ 2.0 is due to Mn 2+ ions in an environment close to octahedral symmetry, whereas the resonances at g ≅ 4.3 and g ≅ 3.3 are attributed to the rhombic surroundings of the Mn 2+ ions. The ionic character ( A), the number of spins participating in resonance ( N), optical band gap energies ( E opt) and Urbach energies (Δ E) show the mixed alkali effect (MAE) with composition. The present study gives an indication that the size of alkalis we choose, is also an important contributing factor in showing the MAE. The variation of N with temperature obeys the Boltzmann law. The optical absorption spectra show a single broad band at ∼21 000 cm −1 corresponding to the transition 6A 1g(S) → 4T 1g(G) which exhibits a blue shift with x. The theoretical values of optical basicity ( Λ th) have also been evaluated.

Electrical conductivity and relaxation in mixed alkali tellurite glasses

The authors have reported the electrical conductivity and the conductivity relaxation in mixed alkali tellurite glasses of compositions of 70TeO2-xNa2O-(30-x)Li2O 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. They have analyzed the relaxation data in the framework of different models. They have observed the mixed alkali effect in the dc and ac conductivities, the crossover frequency, and the conductivity relaxation frequency as well as in their respective activation energies in these glasses. They have also observed the mixed alkali effect in the decoupling index. The scaling property of the modulus spectra of these mixed alkali glasses shows that the conductivity relaxation in the mixed alkali tellurite glasses is independent of temperature but depends on the glass compositions.

Structure of single and mixed alkali Li–Rb borate glasses by neutron diffraction

We have compared the local structure of Li–, Rb– and Li–Rb diborate glasses using neutron diffraction and reverse Monte Carlo simulations. The fraction of tetrahedral boron decreases as the Rb content increases, but is similar in the single and mixed alkali glasses at diborate compositions. Rb borate glasses exhibit an extensive cation ordering with Rb–Rb correlations at 4.7 and 7 Å. The environment of alkalis is slightly modified between the single and mixed glasses while their relative distribution is essentially random. This indicates that the alkali environment is affected by the presence of a different alkali and this cation–cation interaction could be a fundamental mechanism, together with the site mismatch, to explain the mixed alkali effect.

Mixed alkali effect in boroarsenate glasses

We report, for the first time the study of mixed alkali effect (MAE) in boroarsenate glasses. Density, DSC, DC electrical conductivity and IR studies have been carried out for xK 2O–(40− x)Na 2O–50B 2O 3–10As 2O 3 glasses. The DC electrical conductivity was measured in the temperature range 100 °C to below the glass transition temperature. The strength of the MAE in T g, DC electrical conductivity and activation energy has been determined. It is observed that the strength of MAE in DC electrical conductivity is less pronounced with increase in temperature. The results are explained by the structural model recently proposed by Swenson and coworkers, supporting molecular dynamic results. The IR studies show that the glass system contains BO 3 and BO 4 units in the disordered manner.

Optical characterization of the phosphate glasses containing pair transition ions

The paper deals with optical and electronic properties of the aluminophosphate glasses containing Fe–Mn and Fe–Cr ion pairs in different concentration. The influence of the mixed alkali ions over the electronic properties has been investigated. The optical behavior (optical transmission) of the glass samples has been studied by UV-VIS spectroscopy and the refractive index dependency on wavelength has been discussed. The transmission spectra show features specific for the doping transition ions (TM), revealing different oxidation states of iron (Fe2+/Fe3+), manganese (Mn2+/Mn3+) and chromium (Cr3+/Cr6+) in the vitreous network. Mossbauer spectroscopy offers information regarding the TM oxidation states, redox processes and the iron coordination symmetry in the vitreous network. In the case of Fe–Mn doped glasses, the percentage of Fe2+ is about 40% and a doubled iron content leads to an increasing of Fe2+ percentage up to 53%. The replacing of lithium ions by natrium ions (mixed alkali effect) provides an increasing of the Fe2+ percentage up to 56%. The occurrence of the tetrahedral or octahedral symmetry of Fe2+ ions bonded by O2− ions depends on the transition ion nature and Li+/Na+ ratio. Infrared absorption spectra of the pair transition ions-doped aluminophosphate glasses reveal optical phonons specific for the phosphate glass matrix.

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.

The mixed alkali effect in ionically conducting glasses revisited: A study by molecular dynamics simulation

When more than two kinds of mobile ions are mixed in ionic conducting glasses and crystals, there is a non-linear decrease of the transport coefficients of either type of ion. This phenomenon is known as the mixed mobile ion effect or Mixed Alkali Effect (MAE), and remains an unsolved problem. We use molecular dynamics simulation to study the complex ion dynamics in ionically conducting glasses including the MAE. In the mixed alkali lithium-potassium silicate glasses and related systems, a distinct part of the van Hove functions reveals that jumps from one kind of site to another are suppressed. Although, consensus for the existence of preferential jump paths for each kind of mobile ions seems to have been reached amongst researchers, the role of network formers and the number of unoccupied ion sites remain controversial in explaining the MAE. In principle, these factors when incorporated into a theory can generate the MAE, but in reality they are not essential for a viable explanation of the ion dynamics and the MAE. Instead, dynamical heterogeneity and "cooperativity blockage" originating from ion-ion interaction and correlation are fundamental for the observed ion dynamics and the MAE. Suppression of long range motion with increased back-correlated motions is shown to be a cause of the large decrease of the diffusivity especially in dilute foreign alkali regions. Support for our conclusion also comes from the fact that these features of ion dynamics are common to other ionic conductors, which have no glassy networks, and yet they all exhibit the MAE.

Ion diffusion and mechanical losses in mixed alkali glasses

Based on general theoretical considerations we show that the fraction cV of vacant sites for the mobile ions in glasses should be small in order to understand the peculiar behavior of the internal friction in the mixed alkali effect. Moreover, for small cV, the steep fall of the conductivity in the dilute foreign alkali region can be explained by a trapping of vacancies induced by the minority ions. To demonstrate our general analysis we study a simple hopping model and show that it allows one to recover both the typical behavior of the internal friction (shifts of single and mixed alkali peak positions with the mixing ratio and associated changes in activation energies, changes of peak heights) and the typical behavior of tracer diffusion coefficients (drastic fall of mobility of one type of ion by replacement with a second type of mobile ion and associated changes in activation energies). Reasonable values of model parameters can be estimated from measured data and predictions are made for experiments.

Degradation behaviour of borosilicate glass: some studies

PurposeTo study the degradation behaviour of borosilicate glass, which is suitable for hermetic sealing with Molybdenum and Kovar (Fe/Co/Ni) alloys, as a function of concentration and temperature in both acidic and alkaline media for long durations, up to 160 h.Design/methodology/approachThe degradation (weight loss in mg/cm2 of the glass sample) was determined by immersing the glass sample in HCl and NaOH solutions at different temperatures for different periods extending up to 300 h. The damage to the glass surface was seen under an optical microscope and the chemical species on the corroded surface were identified by electron spectroscopy for chemical analysis.FindingsThe borosilicate glass, having the nominal composition 0.70 SiO2, 0.039 Na2O, 0.028 K2O, 0.21 B2O3, 0.01 Al2O3 was synthesized by melt and quench techniques. Degradation (corrosion) behaviour of this glass was investigated by immersing glass samples in 5 and 10 per cent HCl and 5 per cent NaOH solutions at different temperatures up to 90°C, for different periods and measuring dissolution rate (weight loss in mg/cm2 of the sample). Dissolution rates were found to be 5.47 mg/cm2 and 46.77 mg/cm2 in 5 per cent NaOH at 60 and 90°C, respectively, whereas they were comparatively low (2.59 and 5.80 mg/cm2 at 60 and 90°C, respectively, in 5 per cent HCl medium) after 160 h of total immersion period. The plot of dissolution rates against the temperatures showed the nonlinear behaviour at higher temperatures, probably due to the change in mechanism of corrosion. XPS studies exhibited the chemical species on the corroded surfaces. The optical microscopy of the corroded surface revealed that the corrosion mechanisms were different in acid and alkali media.Research limitations/implicationsThe degradation behaviour of borosilicate glass having a specific composition has been investigated as a function of concentration and temperature in both acid and alkaline media. The mixed alkali effect on the degradation behaviour may be studied by varying relative amount of Na2O and K2O in the glass composition.Practical implicationsThe glass composition under the present study has been used for fabrication of matched type glass‐to‐metal (GM) seals with kovar alloy. In this respect the present study is significant in deciding the environmental conditions for its use.Originality/valueThe degradation behaviour of borosilicate glass having alkali and alkaline earth metal oxides has been investigated as a function of concentration and temperature in both acid and alkali media. The findings in this paper have the potential implications in deciding the environmental conditions for use of GM seals fabricated using this glass.

Mixed alkali effect in Li2O–Na2O–B2O3 glasses containing CuO – An EPR and optical study

Electron Paramagnetic Resonance (EPR) and optical absorption spectra of 0.5mol% CuO doped xLi2O–(30−x)Na2O–69.5B2O3 (5⩽x⩽25) mixed alkali glasses have been investigated. The EPR spectra of all the investigated samples exhibit resonance signals characteristic of Cu2+ ions in octahedral sites with tetragonal distortion. It is found that the spin-Hamiltonian parameters do not vary much with x. It is interesting to observe that the number of Cu2+ ions participating in resonance (N) and its paramagnetic susceptibility (χ) exhibits the mixed alkali effect with composition. It is observed that the temperature dependence of paramagnetic susceptibility (χ) obeys Curie–Weiss law. The paramagnetic Curie temperature (θp) is negative for the investigated sample, which suggests that the copper ions are antiferromagnetically coupled by negative super exchange interactions at very low temperatures. A broad band corresponding to the transition 2B1g→2B2g in the optical absorption spectrum shows a blue shift with x. By correlating the EPR and optical absorption data, the molecular orbital coefficients α2 and β12 have been evaluated. It is interesting to observe that the optical band gap (Eopt) and Urbach energies (ΔE) exhibit the mixed alkali effect. The theoretical values of optical basicity (Λth) have also been evaluated.

Disorder induced anomalous behavior: Cation conductivity of glasses and amorphous solids

While the classical version of the mixed alkali effect may be observed for glasses prepared from a melt, the weak mixed alkali effect occurs after a cation exchange below the glass transition temperature. In this paper we will give a theoretical approach of this phenomenon on the basic of the Landau–Vlassov approximation.