Mixed Alkali Effect Research Articles

ESR, physical and structural studies on Mn2+ doped in mixed alkali phosphate glasses

Glass formation and properties of manganese phosphate glasses containing both Na2O and K2O oxides with the general formula 49.95[xNa2O-(1-x)K2O]-0.1MnO2-49.95P2O5 with x = 0–1 mol%, have been investigated. The vitreous samples were prepared by standard melt quenching. To get an insight into their physical properties, the density and the glass transition temperature were procedure determined. The structure of the glass was performed by Infrared, Raman, and electron spin resonance (EPR) spectroscopies. The results obtained by these techniques have allowed us to explain their structure and properties upon the variation of the chemical composition. The variation of the glass transition temperature as a function of the composition presents a minimum for the ratio Na/Na + K = 0.5. Infrared and Raman spectroscopies has identified the presence of different structural grouping units in the glassy-network. It is found that the stretching vibration νs(PO2)− and νs(P–O–P) are more sensitive to the substitution of alkali elements. EPR experiments have shown the presence of Mn2+ centers in the glasses. The variation of the g-factor as a function of the composition is non-linear. The non-linearity behavior of the composition dependence of Tg, vibration bending mode, and the g-factor are a fingerprint of the mixed alkali effect in the glasses under study.

Modeling Ionic Transport and Manifestation of Mixed Alkali Effect in Glass and Glass-Forming Melts of Lithium and Sodium Niobium Phosphates

The diffusion coefficients of Li+ and Na+ ions in glass and glass-forming melts of lithium and sodium niobium phosphates, which are in satisfactory agreement with the experimental data available in the literature, are obtained by the molecular dynamics method. The nonlinear change in the migration parameters (manifestation of the mixed alkali effect) is confirmed at the equivalent replacement of Li+ ions by Na+ ions in this system. The analysis of the radial pair correlation functions demonstrates an increased formation of lithium-sodium ionic pairs in bimetallic compositions, responsible for manifestation of the mixed alkali effect.

Ab Initio Study of Hydrolysis Effects in Single and Ion-Exchanged Alkali Aluminosilicate Glasses

Hydrolysis in alkali-doped aluminosilicate glasses is one of the most complicated mechanisms in glass science. There remain many fundamental and unresolved issues with implications on their potential applications. Herein, we address this challenge by carrying out detailed calculations on the structure and properties of both anhydrate (dry) and hydrated alkali aluminosilicate glasses on carefully constructed models. Specifically, the Na-, (Na + K)- and K-doped aluminosilicate glasses with compositions (SiO2)0.6 (Al2O3)0.2 (Na2O)0.2-x (K2O)x (x = 0, 0.10 and 0.20) are simulated using ab initio molecular dynamics (AIMD). The local short- and intermediate-range order in these glasses are analyzed in terms of atomic pair distribution, coordination number, bond length and bond angle distributions to delineate the subtle variations due to different alkali size and hydrolysis. The electronic structure, interatomic bonding, mechanical and optical properties for these models are calculated and validated with available experimental data. We use the novel concept of total bond order density (TBOD), the quantum mechanically derived metric, to characterize the internal cohesion and strength in the simulated glasses. Detailed analysis of the hydrolysis mechanism enables us to provide information on the complex interplay of various participating elements and their interactions at the atomic level. Such detailed information provides new platform of knowledge which is crucial for understanding the issues related to glass corrosion and durability and ways and means for their special applications in commercial glass products. Both un-dissociated molecular water and dissociated water in the form of hydroxyl group exist in the hydrated models in the presence of alkali ions. For the first time, we observed the opposite mixed alkali effect in the Poisson's ratio for anhydrate and hydrated glasses.

Mixed-alkali effect on hardness and indentation-loading behavior of a borate glass system

In this work, we investigate the mixed alkali effect on both hardness and indendation-loading behavior of the 75B2O3•xLi2O•(25-x)Na2O (mol%) glass series. It is found that the mixed alkali effect manifests itself as a minimum in the extent of the indentation size effect (ISE). This minimum could be connected to that of the plastic deformation induced by pressure. We also study the structural responses to indentation using the Raman spectroscopy. The results show that indentation leads to both incorporation of three-coordinated boron (BIII) into medium-range order units and transformation of BIII into four-coordinated boron (BIV). This implies that the mixed-alkali effect on both hardness and indendation-loading behavior could be associated with the contributions of both short-range and medium-range order units to the structural changes.

Studies on spin Hamiltonian parameters and local structures for v4+ in BaCl2-BaO-B2O3 glasses at various BaCl2 compositions

ABSTRACT The spin Hamiltonian parameters (SHPs), local structure and their composition dependences for impurity V4+ in xBaCl2-(30–x)BaO-68B2O3-2V2O5 (x = 0, 2, 5, 10 mol%) glasses are theoretically analyzed by using the perturbation formulae of SHPs for octahedral 3d1 clusters with tetragonal compression. The power law composition functions for the relevant quantities (covalency factor N, relative tetragonal compression ratio ρ, core polarisation constant κ and reduction factor H) suitably recur the experimental composition dependences of the SHPs for V4+. N shows the firstly fast and then slow decreasing trend, whereas ρ, κ and H exhibit the firstly fast and then slow increasing tendencies. The above composition dependences can be attributed to the modifications of the local structure and electron cloud distribution near the impurity V4+ due to the doped chlorine in participating in the formation and modification of the glasses network and mixed alkali effect.

Influence of glass network ionicity on the mixed‐alkali effect

AbstractMost studies of the mixed‐alkali effect (MAE) have focused on relating the differences between the cations to the strength of the MAE; here we examine the effect of the glass former by comparing the MAE in aluminofluorophosphate (FP) and aluminosulfofluorophosphate (FPS) glasses. The sulfate anion in the FPS series does not bond directly to the aluminophosphate network, decreasing connectivity and increasing the ionicity of the FPS glasses. The increased degrees of freedom imparted by the sulfate are evident in the single‐alkali FPS glasses, which have lower Ea, hardness, shear moduli, and Young's moduli, and higher Tg than the corresponding single‐alkali FP glasses (without sulfate). Conversely, for the mixed‐alkali FPS compositions, the sulfate's mobility is reduced by the mixed cations, resulting in a magnified MAE for the FPS series. For dynamic properties, this phenomenon is explained by reduced plasticity and slippage along ion channels, while understanding Tg and elastic properties requires examination of the shape of the sulfate's potential energy well and the resulting regions of compressive and tensile stress. We posit that glass compositions with higher plasticity, that is, less covalent bonding, will exhibit a larger MAE, but, simultaneously, sufficient conventional glass network must be present to constrain the mobile ions.

Mixed Alkali Effect in Viscosity of CaO-SiO2-Al2O3-R2O melts

The mixed alkali effects of Li2O, Na2O, and K2O on viscosity of CaO-SiO2-Al2O3 melts are investigated by rotating cylinder method. It is found that the single addition of Li2O, Na2O, or K2O to CaO-SiO2-Al2O3 melt will reduce the viscosity of melt, regardless of the Al2O3 content, while the degree of reduction follows the sequence of Li2O > Na2O > K2O. For melts containing two alkali oxides, viscosity monotonically decreases when one alkali oxide is replaced by another at all temperature investigated in this paper, but no extrema of viscosity is found like the behavior of electrical conductivity. In addition, the extrema are not found in the slag containing three alkali oxides if changing their relative contents but keeping their total contents constant. Raman spectroscopy is used to illustrate the relationship between viscosity and structure of melt and it is found that degree of polymerization is the major factor affecting the viscosity of melt.

Effects of mixed alkali-thermal pretreatment on anaerobic digestion performance of waste activated sludge

In the present study, the effect of low-temperature thermal and alkali pretreatments on biodegradability of waste activated sludge was evaluated. Combined with low-temperature thermal pretreatment (80 °C), varied proportions of a mixed alkali (i.e., a mixture of NaOH and Ca [OH]2) were added. The pretreatment procedures were conducted in different orders; thermal-alkali pretreatment (TAP), simultaneous thermal and alkali pretreatment (STAP), and alkali-thermal pretreatment (ATP). Following pretreatment, the sludge with the alkali-thermal pretreatment was most considerably dissolved. The SCOD concentration was up to 18,000 mg/L in the WAS, and the dissolution rate reached 112.8%. Therefore, the anaerobic digestion (AD) performance of waste activated sludge (WAS) by mixed alkali-thermal pretreatment was investigated. With an increase in sodium hydroxide content in the mixed alkali, the cumulative methane production (CMP) of the sludge increased accordingly. The maximum CMP (i.e., 430 mL/g VS [volatile solids]), increased by 308.7% compared with untreated sludge and was obtained by the pretreatment of the mixed alkali (NaOH/Ca(OH)2 = 4:1) at 80 °C. The results show that low-temperature thermal treatment combined with the mixed alkali treatment can reduce the inhibitory effect of mixed alkali on AD, thereby shortening the inhibition period (6d) and increasing methane production (308.7%). The preliminary economic feasibility analysis also showed that the ATP was a cost-effective pretreatment method.

Anisotropic structure of alkali metaphosphate glasses

AbstractOrientation anisotropy, which is well known in organic polymers with appropriate network structures, is less common in oxide glasses. We present the intermediate‐range order in anisotropic alkali metaphosphate glass which consists of oriented PO4 tetrahedral chains and intervening alkali cations along the elongation direction. The X‐ray total structure factor S(Q) indicates that the inter‐chain spacing depends on the size of alkali cations and varies from 5.03 to 6.28 Å. The mixed alkali effect is primarily related to an increase of the separation. The total correlation function T(r) provides the first definite evidence that the anisotropic structure is composed of phosphorus‐bridging oxygen bonds (P–OB) lying along the elongation direction and phosphorus‐non‐bridging oxygen bonds (P–OT) oriented perpendicular to the elongation direction. The present result unveils fundamental aspects of the anisotropic structure of an oxide glass and provides essential information for the development of oxide glasses to control structural anisotropy.

Structural origins of the Mixed Alkali Effect in Alkali Aluminosilicate Glasses: Molecular Dynamics Study and its Assessment

The comprehension of the nonlinear effects provided by mixed alkali effect (MAE) in oxide glasses is useful to optimize glass compositions to achieve specific properties that depend on the mobility of ions, such as the chemical durability, glass transition temperature, viscosity and ionic conductivity. Although molecular dynamics (MD) simulations have already been applied to investigate the MAE on silicates, less effort has been devoted to study such phenomenon in mixed alkali aluminosilicate glasses where alkali cations can act both as modifiers, forming non-bridging oxygens and percolation channels, and as charge compensator of the AlO4− units present in the network. Moreover, the ionic conductivity has not been computed yet; thus, the accuracy of the atomistic simulations in reproducing the MAE on the property is still open to question. In this work, we have validated five major interatomic potentials for the classical MD simulations by modelling the structure, density, glass transition temperature and ionic conductivity for three aluminosilicate glasses, (25 − x)Na2O − x(K2O) − 10(Al2O3) − 65(SiO2) (x = 0, 12.5, 25). It was observed that only the core-shell (CS) polarizable force field well reproduces the experimentally measured MAE on Tg and the ionic conductivity as well as the higher conductivity of single sodium aluminosilicate glass at low temperature and the higher conductivity of single potassium aluminosilicate glass at high temperature. The MAE is related to the suppression of jump events of the alkaline ions between dissimilar sites in the percolation channels consisting of both sodium and potassium ions as in the case of alkaline silicates. The superior reproducibility of the CS potential is originated from the larger and the flexible ring structures due to the smaller Si-O-Si inter-tetrahedra angle, creating appropriate percolation channels for ion conductivity. We also report detailed assessments for using the potential models including the CS potential for investigating MAE on aluminosilicates.

Progress in modeling of glass properties using topological constraint theory

AbstractPrediction of glass properties as a function of composition has been a long‐standing problem. Topological constraint theory has built the quantitative relationships between composition and macroscopic properties, which can be used to predict compositional dependence of glass properties. In this review, we will summarize the recent important progress for the development of the theory. The topological model for glass transition temperature has been proposed from temperature‐dependent constraint theory. We will also discuss the effect of water on topological constraint, how to account for mixed alkali effect and mixed alkaline earth effect, and the application of the model to new glass family besides inorganic glasses. Topological models for hardness and thermal expansion have also been developed. Topological models for predicting elasticity have been recently proposed. We also show some example on the application of the theory in designing new glasses and we suggest some future directions of the theory.

Mixed alkali effect on borosilicate glass structure with vanadyl ion as spin probe

Mixed alkali borosilicate glasses doped with 2% vanadium oxide were synthesized by conventional melt-quench method. The Micro structure of the materials was analysed using XRD, SEM and EDX spectrum. Optical nature of glasses was studied by computing optical bandgaps with absorption spectral data. Covalent nature of glasses was estimated correlating optical bandgap energy with EPR spectral data. The metallization criterion of glasses was evaluated from the physical parameters. FTIR and Raman spectral techniques were used to identify the structural groups of the borates and silicates that are formed with addition of modifiers. Emission spectra were recorded at excitation wavelength of 350 nm and their thermal nature was also studied using TGA/DTA techniques.

Study on the preparation and influence of alkali metal oxides R2O on properties of microfiber glass with high water resistance

The microfiber glass with high water resistant was prepared by the centrifugation with synchronous water-repellent atomization spray. The influence of the alkali metal oxide Na2O/(Na2O+K2O) ratio on the fiber forming ability, chemical stability, thermal and acoustical insulation properties of the Si-Ca-Mg-Al-Fe-B multi-glass fiber system was studied by combing with the “mixed alkali effect”. It was found that when the ratio Na2O/(Na2O+K2O)=0.97, the fiber forming ability and chemical stability of the prepared microfiber glass were optimized. The transmission loss and the sound absorption coefficient are distributed at 2000Hz at a maximum of 9.1dB and 0.88. Therefore, under this paper preparation conditions the optimal Na2O and K2O distribution ratio of Si-Ca-Mg-Al-Fe-B multi-glass fiber system is 16wt% and 0.5wt%, respectively. Each prepared fiber is tightly connected together by the surface treatment of the water repellent to form a stable fiber network structure, demonstrating the feasibility of producing high water resistant microfiber glass by the centrifugation with synchronous water-repellent atomization spray in practical production applications.

Detailed Investigation of Phenomenon “MIXED ALKALI EFFECT” by Doping Alkali Ions with different Cationic Sizes with Equal Molar Concentration in Borate Glasses. An Ultrasonic Study at 10 MHz and Explanation by “Random Network Model”

Detailed Investigation of Phenomenon “MIXED ALKALI EFFECT” by Doping Alkali Ions with different Cationic Sizes with Equal Molar Concentration in Borate Glasses. An Ultrasonic Study at 10 MHz and Explanation by “Random Network Model”

Mechanical Properties Enhancement due to Network Modification by different Alkali Ions at Equal Concentration Doped in Borate Glasses-An Ultrasonic Investigation at 10MHz on the Phenomenon “Mixed Alkali Effect”

Mechanical Properties Enhancement due to Network Modification by different Alkali Ions at Equal Concentration Doped in Borate Glasses-An Ultrasonic Investigation at 10MHz on the Phenomenon “Mixed Alkali Effect”

Effects of mixed-alkali oxides on AC conductivity and dielectric properties of xNa2O-(20-x)K2O-30V2O5-50TeO2 glasses

The melt-quenching technique was used to prepare tellurovanadate glasses containing sodium and potassium oxides with compositions of xNa2O-(20-x)K2O-30V2O5-50TeO2 (x = 0–20 mol%) to examine the impact of mixed-alkali oxide on alternating-current (AC) conductivity and dielectric properties. Structural analysis revealed the presence of bridging oxygen (BO) and non-bridging oxygen (NBO). Thermal analysis results demonstrated a minimum value of glass transition temperature (Tg) at x = 10 mol% with an increase in Na2O content. The variation of AC conductivity (σac) revealed a non-linear behaviour with Na2O at temperature < 383 K, where σac increased to maximum value at x = 15 mol% and shifted to x = 10 mol% at temperature ≥ 383 K. This result may be attributed to mixed-alkali effect (MAE) because of the mixing of the two alkali oxides. The increase in σac with Na2O might be due to increase in the carrier pathway caused by the opening up of the glass network as NBO increased. Meanwhile, dielectric constant (ε') demonstrated an optimal increase at x = 10 mol% prior to the reduction greater than x = 10 mol%. The maximum ε' value at x = 10 mol% was in accordance with the σac anomaly, which may also related to the MAE that induced formation of heavy dipoles. Consequently, this facilitated the dipole orientation in the glass network. Conduction mechanism at the dispersion region for all glass samples was found to be the correlated barrier hopping (CBH) model. The electrical modulus of the investigated samples revealed the non-Debye type relaxation, which indicates presence of interaction between ions.

Origin of the mixed alkali effect in silicate glass

Silicate glasses have evolved from basic structural materials to enabling materials for advanced applications. In this article, we unravel the origin of the mixed alkali effect for alkali silicate 22.7R2O–77.3SiO2 glasses (R = Na and/or K) by identifying the variation in the alkali ion location around the non-bridging oxygen atoms. To do so, we constructed a state-of-the art structural model, which reproduces both diffraction and NMR data with a particular focus on the behavior of the alkali ions. A novel topological analysis using persistent homology found that sodium-potassium silicate glass shows a significant reduction in large cavities as a result of the mixed alkali effect. Furthermore, a highly correlated pair arrangement between sodium and potassium ions around non-bridging oxygen atoms was identified. The potassium ions can be trapped in K–O polyhedra due to the increased bridging oxygen coordination; therefore, the correlated pair arrangement is likely the intrinsic origin of the mixed alkali effect.

A new method for controlling structural, electrical and optical properties of phosphate glasses, containing transition metal ions (TMIs)

In this study, glassy samples with a composition of 40P2O5–10V2O5–(50−x) Li2O–xNa2O, (x = 5–45 mol%), were prepared and their structural, electrical, and optical properties were investigated for the mixed alkali effect (MAE). The density of the samples displayed nonlinear variation with Na2O content (x mol%), which confirmed the existence of the MAE. It was found that the conductivity in the present glassy system is more ionic rather than electronic, and the minimum dc conductivity at x = 25 mol% is due to the MAE on ionic conductivity. Some optical parameters such as indirect allowed optical band gap, Urbach energy, refractive index, dielectric constant, reflection loss, molar refraction, and electronic polarizability were evaluated from the absorption and reflection spectra. The values of the optical band gap showed nonlinear behavior upon substitution of Li2O by Na2O with a maximum at x = 25 mol%, which supported the existence of the MAE in the optical properties.

Relating Magnox and international waste glasses

Magnox and international radioactive waste glasses observe significantly different aqueous dissolution behaviours. Previously, the effects of Li on the kinetics of dissolution and B network of analogues of a complex Magnox waste glass were investigated (Li-Mg-EM). Here, an “International Simple Glass” with Li substituted for Na at two ratios (Li-ISG) was fabricated to compare the aqueous durability of Magnox and international glasses. 11B MAS-NMR, 23Na MAS-NMR, 6Li1H CP-NMR and SEM studies are presented. The IIIB/IVB ratio and the role of Na in the pristine glasses varied insignificantly with the Li:Na ratio. Further, the B network of Li-ISG was shown to leach congruently. Li, Na and Mg-containing secondary phases were present for Li-Mg-EM, whilst no precipitates were observed for Li-ISG. Li in ISG was shown to be detrimental but further substitution of Li improved long-term aqueous durability, which was attributed to a LiNa mixed alkali effect for Li-ISG but not Li-Mg-EM.

Effectively promoting the crystallization of lithium disilicate glass-ceramics by free oxygen in the glass

Lithium disilicate glass-ceramics were prepared with SiO2–Al2O3–Li2O–CeO2–P2O5-R2O as the base system through the traditional melting method and modified with K2O, Rb2O, Cs2O, and Tb4O7. The microstructure of the lithium disilicate glass-ceramics was characterized through differential thermal analysis, X-ray diffraction and scanning electron microscopy. The whiteness and fracture strength of the lithium disilicate glass-ceramics were investigated to validate their potential applications as dental materials. Results indicated that the mixed alkali effect exerted by the introduction of K2O, Rb2O, and Cs2O greatly improved the fracture strength of the modified lithium disilicate glass-ceramics. Furthermore, X-ray photoelectron spectroscopy and whiteness analysis proved that free oxygen increased through the introduction of larger alkali metal ions and terbium element in the glass-ceramics. The increase in free oxygen content through the introduction of Rb2O or Cs2O into the lithium disilicate glass-ceramics could effectively promote the formation of lithium disilicate crystals and the crystallization of lithium disilicate glass-ceramics. Research results indicated that lithium disilicate glass-ceramics with high fracture strength under pressureless sintering had been successfully fabricated.