Mixed Alkali Effect Research Articles

Homogeneity of modifier ion distributions and the mixed alkaline earth effect in MgO–CaO–SiO2 silicate glasses using molecular dynamics

Oxide glasses containing a mixture of modifier ions have a wide range of applications, and it is often assumed that these modifier ions mix homogenously in the melt. Silicate glasses are known for their significant industrial importance and are often doped with magnesium and calcium modifier ions to utilise their properties for applications. This study investigated the structural and dynamical impact imposed on a glass by the presence of two different types of alkaline earth cations. This was achieved through increasingly substituting magnesium for calcium in xMgO–(50–x)CaO–50SiO2 glasses which were modelled computationally using classical molecular dynamics simulation (MD). A mixed alkaline earth effect (MAEE) was found to cause the suppression of the modifier ion diffusivity. An absence of any significant deviation from the expected trends in the structural investigations confirmed that the MAEE is not a structural effect. This MAEE was therefore analogous to the mixed alkali effect (MAE), and was most prominent for roughly equimolar mixtures of calcium and magnesium ions. Interestingly, there was also evidence of the diffusivity being enhanced when only a small amount of a second alkaline earth ion was added.

Structural Investigation, Physical and Optical Properties of Mixed Alkali Bismuth Borate Glasses

Lead free, eco-friendly bismuth borate glasses doped with alkali oxides of composition xLi2O+(30-x)Na2O+55B2O3+15Bi2O3 (x = 5,10,15,20,25) were prepared by melt quenching technique. Amorphous nature of glasses was confirmed by X-ray diffraction. Density measurements were carried out at room temperature by standard principle of Archimedes with Xylene as an immersion liquid. The non-linear behavior of density, molar volume, glass transition temperature, direct and indirect optical band gaps confirm the mixed alkali effect. The FTIR analysis revealed that the network structure consists of BO3, BO4 units and BiO6 octahedral units. UV-vis spectroscopy shows no sharp absorption edges giving a clear indication of the amorphous nature of the glasses.

Molecular structure, configurational entropy and viscosity of silicate melts: Link through the Adam and Gibbs theory of viscous flow

The Adam and Gibbs theory depicts the viscous flow of silicate melts as governed by the cooperative re-arrangement of molecular sub-systems. Considering that such subsystems involve the silicate Qn units (n=number of bridging oxygens), this study presents a model that links the Qn unit fractions to the melt configurational entropy at the glass transition temperature Tg, Sconf(Tg), and finally, to its viscosity η. With 13 adjustable parameters, the model reproduces η and Tg of melts in the Na2O-K2O-SiO2 system (60≤[SiO2]≤100mol%) with 1σ standard deviations of 0.18 log unit and 10.6°, respectively.The model helps understanding the links between the melt chemical composition, structure, Sconf and η. For instance, small compositional changes in highly polymerized melts generate important changes in their Sconf(Tg) because of an excess of entropy generated by mixing Si between Q4 and Q3 units. Changing the melt silica concentration affects the Qn unit distribution, this resulting in non-linear changes in the topological contribution to Sconf(Tg). The model also indicates that, at [SiO2]≥60mol%, the mixed alkali effect has negligible impact on the silicate glass Qn unit distribution, as corroborated by Raman spectroscopy data on mixed Na-K tri- and tetrasilicate glasses. Such model may be critical to link the melt structure to its physical and thermodynamic properties, but its refinement requires further high-quality quantitative structural data on silicate and aluminosilicate melts.

Do properties of bioactive glasses exhibit mixed alkali behavior?

The effect of substituting K2O for Na2O on the physical and chemical properties of 15 glasses in the system Na2O–K2O–CaO–P2O5–SiO2 was studied for three series: low (52 mol% SiO2), medium (60 mol% SiO2) and high (66 mol% SiO2) silica. The SiO2 content expressed as weight-% varied from 46 to 64 wt%, thus suggesting that the compositions were either bioactive or biocompatible. The crystallization tendency and sintering behavior were studied using differential thermal analysis and hot stage microscopy. Formation of silica- and hydroxy-apatite-rich layers were studied for glass plates immersed in static simulated body fluid. The release of inorganic ions into Tris buffer solution was analyzed using inductively coupled plasma optical emission spectrometer in dynamic and static conditions. Substitution of K2O for Na2O suggested mixed alkali effect (MAE) for the thermal properties with a minimum value around 25% substitution. With increased share of K2O in total alkali oxides, the hot working window markedly expanded in each series. Silica and hydroxyapatite layers were seen only on the low silica glasses, while a thin silica-rich layer formed on the other glasses. In each series, greater dissolution of alkali and alkali earth ions was seen from K-rich glasses. Clear MAE and preferential ion dissolution were recorded for medium and high silica series, while for low silica glasses, the initial MAE dissolution trends become rapidly covered by other simultaneous surface reactions. MAE enables designing especially low silica bioactive glasses for improved hot working properties and medium and high silica glasses for controlled dissolution.

Influences of Na2O and K2O Additions on Electrical Conductivity of CaO-MgO-Al2O3-SiO2 Melts

The present study investigated the influences of Na2O and K2O additions on electrical conductivity of blast furnace type CaO-MgO-Al2O3-SiO2 melts by the four-electrode method. Both the single addition of Na2O or K2O and the double additions of Na2O and K2O were studied. It was found that electrical conductivity monotonously increased as the amount of Na2O addition was gradually increased, whereas, when K2O was added, there was a continuous decrease of electrical conductivity. With melts containing both Na2O and K2O, electrical conductivity first decreased but then increased when Na2O was gradually substituted for K2O while keeping the molar fractions of other components constant. In other words, the mixed-alkali effect took place in CaO-Mg-Al2O3-SiO2-ΣR2O melts.

The mixed modifier effect in ionic conductivity and mechanical properties for xMgO-(50-x)CaO-50SiO2 glasses

The mixed-modifier effect (MME), i.e., the tendency of the physical properties of a glass to deviate from the law of additivity when multiple network-modifying components are included, can cause large improvements in the mechanical properties of glasses. However, previous studies of the MME have been largely limited to systems containing alkali metal modifiers, and the origins of the MME for mechanical properties remain unclear. Here, a series of glasses with purely mixed alkaline earth modifiers, xMgO-(50−x)CaO-50SiO2, and prepared and used to verify the existence of the mixed-alkaline-earth effect (MAEE) in ionic conductivity and mechanical properties. While the deviations from linearity for the MAEE were not as large as is typical for the mixed-alkali effect (MAE), they were still significant (up to 30%). Unexpectedly, the ionic conduction in these glasses was found to deviate significantly from Arrhenius behaviour, with the activation energy decreasing at elevated temperature. Examination of the MME in mechanical properties showed that the activation energy for ion conduction was correlated with shear stiffness, but not compressibility, indicating that the shear modulus may serve as a reasonable predictor of the strain energy required for ion hopping during conduction. The connectivity of the glass network, as determined by Raman spectroscopic analysis, was used to relate the MME in static properties and dynamic properties.

Mixed Polyanion Glass Cathodes: Mixed Alkali Effect

In lithium-ion batteries, mixed polyanion glass cathodes have demonstrated high capacities (200–500 mAh/g) by undergoing conversion and intercalation reactions. Mixed polyanion glasses typically have the same fundamental issues as other conversion cathodes, i.e.: large hysteresis, capacity fade, and 1st-cycle irreversible loss. A key advantage of glass cathodes is the ability to tailor their composition to optimize the desired physical properties and electrochemical performance. The strong dependence of glass physical properties (e.g., ionic diffusivity, electrical conductivity, and chemical durability) on the composition of alkali mixtures in a glass is well known and has been named the mixed alkali effect. The mixed alkali effect on battery electrochemical properties is reported here for the first time. Depending on glass composition, the mixed alkali effect is shown to improve capacity retention during cycling (from 39% to 50% after 50 cycle test), to reduce the 1st-cycle irreversible loss (from 41% to 22%), and improve the high power (500 mA/g) capacity (from 50% to 67% of slow discharge capacity).

Influences of Na<sub>2</sub>O and K<sub>2</sub>O Additions on Electrical Conductivity of CaO-SiO<sub>2</sub>-(Al<sub>2</sub>O<sub>3</sub>) Melts

The present study investigated the influences of Na2O and K2O additions on electrical conductivity of blast furnace type CaO-MgO-Al2O3-SiO2 melts by the four-electrode method. Both the single addition of Na2O or K2O and the double additions of Na2O and K2O were studied. It was found that electrical conductivity monotonously increased as the amount of Na2O addition was gradually increased, whereas, when K2O was added, there was a continuous decrease of electrical conductivity. With melts containing both Na2O and K2O, electrical conductivity first decreased but then increased when Na2O was gradually substituted for K2O while keeping the molar fractions of other components constant. In other words, the mixed-alkali effect took place in CaO-Mg-Al2O3-SiO2-ΣR2O melts.

The mixed alkali effect in the MgO–Li2O–Na2O–K2O–B2O3 glass system

The mixed alkali effect in the MgO–Li2O–Na2O–K2O–B2O3 glass system

Optical Properties of K2O-Li2O-WO3-B2O3 Glasses: Evidence of Mixed Alkali Effect

Glass with compositions xK2O-(30 − x)Li2O-10WO3-60B2O3 for 0 ≤ x ≤ 30 mol.% have been prepared using the normal melt quenching technique. The optical reflection and absorption spectra were recorded at room temperature in the wavelength range 300–800 nm. From the absorption edge studies, the values of the optical band gap (E opt) and Urbach energy (ΔE) have been evaluated. The values of E opt and ΔE vary non-linearly with composition parameter, showing the mixed alkali effect. The dispersion of the refractive index is discussed in terms of the single oscillator Wemple Di-Domenico model.

Modifier Interaction and Mixed-Alkali Effect in Bond Constraint Theory Applied to Ternary Alkali Metaphosphate Glasses

Introducing an interaction parameter γ, we implement modifier interaction and the mixed-alkali effect into bond constraint theory, and apply this extension for simplistic property prediction on ternary phosphate glasses. The severity of the mixed alkali effect results from the interplay of two simultaneous contributions: Bond constraints on the modifier species soften or stiffen with decreasing or increasing γ, respectively. When the modifier size is not too dissimilar the decrease in γ reflects that the alkali ions can easily migrate between different sites, forcing the network to continuously re-accommodate for any subsequent distortions. With increasing size difference, migration becomes increasingly difficult without considerable network deformation. This holds even for smaller ions, where the sluggish dynamics of the larger constituent result in blocking of the fast ion movement, leading to the subsequent increase in γ. Beyond a certain size difference in the modifier pair, a value of γ exceeding unity may indicate the presence of steric hindrance due to the large surrounding modifiers impeding the phosphate network to re-accommodate deformation.

Structural and ion transport properties of [(AgI)x(AgBr)0.4−x](LiPO3)0.6 and (AgBr)x(LiPO3)(1−x) solid electrolytes

AbstractThe (AgBr)x(LiPO3)(1−x) (x=0.4 and 0.5) and [(AgI)x(AgBr)0.4−x](LiPO3)0.6 (x=0.1, 0.2, and 0.3) superionic electrolytes have been prepared by conventional melt quenching using a twin roller. These electrolytes are characterized by X‐ray diffraction, SEM, and energy dispersive X‐ray analysis (EDAX) for structural investigation. Electrical characterizations have been carried out by the AC impedance analysis. The conductivity of LiPO3 glassy system at room temperature is improved by doping with the silver bromide (AgBr)x(LiPO3)(1−x) and the mixture of silver iodide, silver bromide (AgI‐AgBr‐LiPO3 system) up to 10−5 and 10−3 Ω−1 cm−1, respectively (improvements by four or five orders of magnitude). The frequency response of ionic conductivity has been analyzed by universal dynamic response model (Jonscher's law) and AC conductivity data are fitted using the Jonscher's power law. The conductivity values obtained by the power law and impedance plots are comparable. The frequency exponent (n) has a value between 0 and 1. The AgI‐AgBr‐LiPO3 system shows the mixed alkali effect. Summerfield scaling master curve is temperature dependent, which may be due to the contribution of the both lithium and silver ions to ionic conduction.

Nanoindentation Study of the Surface of Ion-Exchanged Lithium Silicate Glass

Variations in mechanical properties (stiffness and hardness) were measured as a function of case depth in ion-exchange glasses using nanoindentation. A simple silicate composition, 30Li2O–70Si2O, was exchanged (Li+ ↔ K+) at several different temperatures, below and above the glass transition, to evaluate the effect of exchange temperature on mechanical properties throughout the ion-exchange layer. Significant enhancements in Young’s modulus and hardness were found near the edge in ion-exchanged glasses and attributed to compressive stress and the mixed-alkali effect. Indent size effect-like behaviors were observed in the untreated and ion-exchange samples alike near the sample edge; this was explained by surface-condition sensitivity and compressive stress found within this region. The elastic recovery and resistance to plastic deformation were calculated to assess the effect of ion exchange on elastic and plastic mechanical responses. Microhardness measurements of the exchanged samples were also made for...

EPR and optical studies on binary mixed alkali-alkaline earth oxide borate glasses doped with Cu2+ ions

Mixed alkali alkaline earth oxide borate glasses of the composition (25 – x)Li2O–xK2O–12.5BaO–12.5MgO–49B2O3–1CuO (x = 0, 5, 10, 15 and 20 mol %) were prepared by the melt quenching technique. The X-ray diffractograms of all the glass samples were recorded at room temperature. Peak free X-ray spectra revealed the amorphous nature of all the prepared glasses. Modulated differential scanning calorimetry (MDSC) was used to determine the glass-transition temperature (Tg). The probable mixed alkali effect was investigated using experimental techniques like density, molar volume, MDSC, electron paramagnetic resonance (EPR), and optical absorption studies. From the EPR spectra the spin-Hamiltonian parameters were evaluated. The spin-Hamiltonian parameter values indicated that the ground state of \(C{u^{2 + }}is{\kern 1pt} {d_{{x^2} - {y^2}}}\) orbital (2B1g) and the site symmetry around Cu2 is tetragonally distorted octahedral. The variation of g|| and A|| as a function of Li2O content was found to be nonlinear. A broad optical absorption band was observed in all the glasses containing Cu2 ions corresponding to 2B1g → 2B2g transition. From the optical absorption studies the values of the optical band gap (Eopt) for indirect, direct transitions and Urbarch energy (ΔE) have been evaluated. By co-relating the EPR and optical absorption data, bonding parameters α2, β2 and β12 were evaluated.

Diffusion and Ion Conduction in Cation-Conducting Oxide Glasses

In this Chapter we review knowledge about diffusion and cation conduction in oxide glasses. We first remind the reader in Section 1 of major aspects of the glassy state and recall in Section 2 the more common glass families. The diffusive motion in ion-conducting oxide glasses can be studied by several techniques – measurements of radiotracer diffusion, studies of the ionic conductivity by impedance spectroscopy, viscosity studies and pressure dependent studies of tracer diffusion and ion conduction. These methods are briefly reviewed in Section 3. Radiotracer diffusion is element-specific, whereas ionic conduction is not. A comparison of both types of experiments can throw considerable light on the question which type of ions are carriers of ionic conduction. For ionic conductors Haven ratios can be obtained from the tracer diffusivity and the ionic conductivity for those ions which dominate the conductivity.In the following sections we review the diffusive motion of cations in soda-lime silicate glass and in several alkali-oxide glasses based mainly on results from our laboratory published in detail elsewhere, but we also take into account literature data.Section 4 is devoted to two soda-lime silicate glasses, materials which are commonly used for window glass and glass containers. A comparison between ionic conductivity and tracer diffusion of Na and Ca isotopes, using the Nernst-Einstein relation to deduce charge diffusivities, reveals that sodium ions are the carriers of ionic conduction in soda-lime glasses. A comparison with viscosity data on the basis of the Stokes-Einstein relation shows that the SiO2 network is many orders of magnitude less mobile than the relatively fast diffusing modifier cations Na. The Ca ions are less mobile than the Na ions but nevertheless Ca is considerably more mobile than the network.Section 5 summarizes results of ion conduction and tracer diffusion for single Na and single Rb borate glasses. Tracer diffusion and ionic conduction have been studied in single alkali-borate glasses as functions of temperature and pressure. The smaller ion is the faster diffusing species in its own glass. This is a common feature of all alkali oxide glasses. The Haven ratio of Na in Na borate glass is temperature independent whereas the Haven ratio of Rb diffusion in Rb borate glass decreases with decreasing temperature.Section 6 reviews major facts of alkali-oxide glasses with two different alkali ions. Such glasses reveal the so-called mixed-alkali effect. Its major feature is a deep minimum of the conductivity near some middle composition for the ratio of the two alkali ions. Tracer diffusion shows a crossover of the two tracer diffusivities as functions of the relative alkali content near the conductivity minimum. The values of the tracer diffusivities also reveal in which composition range which ions dominate ionic conduction. Tracer diffusion is faster for those alkali ions which dominate the composition of the mixed glass.Section 7 considers the pressure dependence of tracer diffusion and ionic conduction. Activation volumes of tracer diffusion and of charge diffusion are reviewed. By comparison of tracer and charge diffusion the so-called Haven ratios are obtained as functions of temperature, pressure and composition. The Haven ratio of Rb in Rb borate glass decreases with temperature and pressure whereas that of Na in Na borate glass is almost constant.Section 8 summarizes additional common features of alkali-oxide glasses. Activation enthalpies of charge diffusion decrease with decreasing average ion-ion distance. The Haven ratio is unity for large ion-ion distances and decreases with increasing alkali content and hence with decreasing ion-ion distance.Conclusions about the mechanism of diffusion are discussed in Section 9. The Haven ratio near unity at low alkali concentrations can be attributed to interstitial-like diffusion similar to interstitial diffusion in crystals. At higher alkali contents collective, chain-like motions of several ions prevail and lead to a decrease of the Haven ratio. The tracer diffusivities have a pressure dependence which is stronger than that of ionic conductivity. This entails a pressure-dependent Haven ratio, which can be attributed to an increasing degree of collectivity of the ionic jump process with increasing pressure. Monte Carlo simulations showed that the number of ions which participate in collective jump events increases with increasing ion content – i.e. with decreasing average ion-ion distance. For the highest alkali contents up to four ions can be involved in collective motion. Common aspects of the motion process of ions in glasses and of atoms in glassy metals are pointed out. Diffusion in glassy metals also occurs by collective motion of several atoms.Section 10 summarizes the major features of ionic conduction and tracer diffusion and its temperature and pressure dependence of oxide glasses.

Studies of the Local Distortions and the EPR Parameters for Cu2+inxLi2O-(30–x)Na2O-69·5B2O Glasses

AbstractThe local distortions and electron paramagnetic resonance (EPR) parameters for Cu2+in lithium sodium borate (LNB) glassesxLi2O·(30–x)·Na2O·69.5B2O3(5≤x≤25 mol%) are theoretically studied at various concentrationsxin a consistent way. Owing to the Jahn–Teller effect, the [CuO6]10−clusters are found to experience the significant tetragonal elongations of 16% along C4axis. Despite the nearly unchanging observedgfactors, measured d–d transition band (or cubic field parameterDq) shows remarkable linear increases with concentrationx, whose influences ong‖andg⊥are actually cancelled by the linearly increasing covalency factorNand relative elongation ratioηwithx. The almost unvarying hyperfine structure constants are attributed to the fact that the influences of the linearly increasingNand the linearly decreasing core polarisation constantκlargely cancel one another. The microscopic mechanisms of the above concentration dependences for these quantities are illustrated from mixed alkali effect (modification of B2O3network by transforming some BO3units into BO4ones with variations in modifier Li2O concentration).

Sodium-free mixed alkali bioactive glasses

AbstractTwo sodium-free mixed alkali series of bioactive glasses based on compositions Bioglass 45S5 and ICIE1, containing lithium and/or potassium as alkali ions, were prepared by a melt-quench route. Thermal properties showed the well-known mixed alkali effect, with glass transition and crystallisation temperatures and the coefficient of thermal expansion going either through a minimum or a maximum for the mixed alkali composition, resulting in a wider processing window. Ion release, by contrast, was controlled by the modifier ionic radius, with ion release rates in dynamic and static dissolution studies increasing for potassium-substituted glasses compared to the composition containing lithium as the only alkali ion. This was caused by pronounced changes in oxygen packing density and molar volume of the glasses owing to the differences in ionic radii (76 pm for Li+ and 138 pm for K+). Partially substituting one alkali for another therefore helps to improve high temperature processing of bioactive glasses and can also be used to control or tailor ion release.

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.

Local structure of alkalis in mixed-alkali borate glass to elucidate the origin of mixed-alkali effect

We report the structural analysis of Na+ and Cs+ in sodium cesium borate crystals and glasses using 23Na and 133Cs magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The composition dependence of NMR spectra of the borate was similar to that of the silicate: (1) the peak position of cesium borate crystals shifted to upfield for structures with larger Cs+ coordination numbers, (2) the MAS NMR spectra of xNa2O-yCs2O-3B2O3 (x = 0, 0.25, 0.5, 0.75, 1.0, x + y = 1) glass showed that the average coordination number (CN) of both the alkali cations decreases with increasing Cs+/(Na+ + Cs+) ratio. However, the degree of decrement in borates is much smaller than that in silicates. We have considered that the small difference in CN is due to 4-coordinated B, because it is electrically compensated by the alkali metal ions resulting in the restriction of having various coordinations of O to alkali metal.

Investigation of mixed alkali effect on mechanical, structural and thermal properties of three-alkali borate glass system

In the present communication, the results of investigation on mixed alkali effect (MAE) in mechanical, structural and thermal investigation of alkali zinc borate glasses with nominal composition 5Li2O–(25−x)K2O–xNa2O–60B2O3–10ZnO (x = 0, 5, 10, 15, 20 and 25 mol%) are reported. The samples were prepared using standard melt quenching technique. Fourier transform infrared (FTIR) spectroscopy, differential scanning calarometry (DSC) and Vickers indentation studies were performed to investigate the mixed alkali effect in the samples. From the DSC studies, it was observed that the thermal parameters viz., glass transition temperature (Tg), glass melting temperature (Tm), glass crystallization temperature (Tc), glass stability (S) and fragility (F) exhibit a non linear variation with respect to increase in compositional parameter (RNa). This behavior clearly indicated the presence of strong MAE in the samples. FTIR studies confirmed the presence of both [BO3] and [BO4] units, indicating the present glass networks to be made up of these two units placed in different structural groups. The non linear variation of peak positions of B–O–B bending and stretching of [BO3] and [BO4] units of each glass sample explain the role of modifier alkali elements and validates the existence of strong MAE. The microhardness and fracture toughness of the samples were measured using Vickers micro indentation technique and non linear variation of both the properties have been observed confirming the presence of MAE in these glass samples.