Mixed Alkali Effect Research Articles

Correlative Activation Free Energy (CAFE) Model: Application to Viscous Processes in Inorganic Oxide Glass-Formers

We explore the concept of correlative activation free energy (CAFE) for the analysis and interpretation of viscosity data of a collection of 847 inorganic oxide glass formers that exhibit various degrees of non-Arrhenius behavior. The CAFE model formalism is strictly based on transition state theory and accounts for the variation in the activation barrier for the viscous dissipation due to the structural evolution the system undergoes upon traversing the glass transition regime. Thus, fitting parameters are meaningful in a statistical thermodynamic context. Compared to the VFT and MYEGA equations, fits using the CAFE model are more robust when extrapolating to infinite temperature because the latter encodes non-enthalpic contributions to the rate coefficient more realistically. The CAFE model-based analysis reveals a strong connection between melt fragility and the degree of change in the potential energy landscape with temperature. Accordingly, while the average ground-state potential energy of the glass forming liquid gradually increases with temperature, the energy of the activated state remains relatively invariant. Our analysis also allows one to estimate the number of atoms involved in the viscous relaxation process, which ranges from approximately 10 to 50 atoms for the oxide glass formers studied. We observe that thermally activated dynamic phenomena exhibited by glassy networks, such as the mixed alkali effect, persist into the supercooled liquid state.

Structural investigation and physical properties of RO-ZnO-Li2B4O7-K2B4O7 (RO= SrO and BaO) glasses

Glass samples 10RO-30ZnO-xLi2B4O7-(60-x) K2B4O7 (RO=SrO and BaO) with alkali tetra borates varying from 0 to 60 mol% were produced by traditional quenching procedure. Peak free broad X-ray diffraction patterns established the amorphous feature of glass samples. FTIR and Raman spectroscopic analysis had shown existence of BO3 and BO4 structural groups along with other borate units. The BO3 ⇔ BO4 conversion rate was not much affected with variation in one of the alkali-tetraborates. EPR spectra of copper doped glasses confirmed the ground state of Cu2+ ions as 2B1g. Physical and optical properties namely density, molar volume, refractive index, molar refractivity, optical band gap and Urbach energy values were found to be composition dependent. The inflections observed in density, and other optical properties around equal mol% of alkali oxides in the glass system were attributed to structural modifications and mixed alkali effect. These results exposed the structural variations caused due to competitiveness between the two different alkali and alkaline oxides in occupying the geometrical positions of the borate glass network.

Mixed alkali effect on the mechanical, thermal and biological properties of 58S bioactive glass

Bioactive glasses (BG) containing silicon, phosphorous, and boron have emerged as promising materials for bone tissue engineering. Modifying ions play a multifaceted role in BGs, influencing their structure, properties, and biological activity that depends on their size, charge, and coordination chemistry. This study investigates the addition of sodium borate (Na2B4O7) to the 58S BG and the mixed alkali effect (MAE) resulting from incorporating Na and B ions into the CaO–SiO2–P2O5 system. In specific concentrations, Na+ and Ca2+ ions function as network modifiers, introducing structural disruptions and generating defects that impact the glass transition temperature (Tg), crystallization temperature (Tc), hardness, and sintering characteristics. The physicochemical characterization of BGs confirms the incorporation of Na and B structures. Furthermore, the Na and B doped glass sample with a [Na/Si] ratio of 0.045 exhibits favorable properties for promoting cell attachment, including forming a hydroxyapatite (HAp) layer over the surface, thereby accelerating human placental mesenchymal stem cells (hPMSCs) proliferation. The findings demonstrate the potential of Na2B4O7 incorporation in BG for bone tissue engineering applications.

Mixed-Alkali Effect in Glasses of Different Classes in the Framework of a Microinhomogeneous-Glass-Structure Model

Mixed-Alkali Effect in Glasses of Different Classes in the Framework of a Microinhomogeneous-Glass-Structure Model

Topological understanding of the influence of mixed alkali components on the structure and properties of aluminosilicate glass

Herein, aluminosilicate (AS) glasses exhibit varying trends in Vickers hardness (HV), coefficient of thermal expansion, softening temperature (Ts) and glass transition temperature owing to the mixed alkali effect. To elucidate the mechanism of the mixed alkali effect, the glass structure was characterised using multiple methods. Employing temperature-dependent topological constraint theory, the variations in HV and Ts were investigated, and the constraint strengths of sodium (Na) and lithium (Li) in AS glasses were calculated. The findings indicated that an increase in alkali metal–ion content elevates the proportion of non-bridging oxygen, leading to a progressive decrease in constraint strength and the depolymerisation of the glass structure. For AS glasses with mixed alkali components, as the content of Li2O replacing Na2O increases, the constraint strength of the glass structure below the critical temperature (Tc) gradually rises, while above Tc, it undergoes unconventional changes. These alterations lead to non-linear variations in glass properties.

Effects of Na/K-Cl Salts on Hydrolysis of Aluminosilicate Glass Using Ab Initio Molecular Dynamics.

The structural and chemical modifications on the surface of pure and alkali-doped aluminosilicate (AS) glasses due to hydrolysis are investigated using ab initio molecular dynamics. The effects of water on the glass network are fully elucidated by analyzing the short- and intermediate-range structural orders embedded in the pair distribution function, bond length and angle distribution, coordination number, and interatomic bonding. A novel concept of total bond order is used to quantify and compare the strength of bonds in hydrated and unhydrated glasses. We show that AS glass is hydrolyzed by water diffusion near the surface and by proton (H+) transfers into the bulk, which increases with time. Hence, a dissolved glass-water interface becomes rich in Si-OH and Al-OH. The alkali ions associated with the nonbridging oxygen accelerate the hydrolysis by facilitating water and H+ diffusion. Al is more impacted by hydrolysis than Si, resulting in greater variation in the Al-O bond order than Si-O. Doping of NaCl and KCl enhances the ionization of water and the hydrolysis of ASs with increased salt concentration. The KCl doping ionizes more water molecules and causes more degradation of the glass network than NaCl. Co-doping of Na and K results in a mixed alkali effect due to complex interatomic bonding from different-sized ions. These exceptionally detailed findings in highly complex glasses with varying salt compositions provide new and unprecedented atomistic insights that can help to understand the hydrolysis and dissolution mechanisms of ASs and other silicate glasses.

Deciphering the Mixed-Alkali effect in supercooled oxide liquids: Results from dynamic and thermodynamic measurements

The fundamental nature of the mixed-alkali effect (MAE) on viscosity, fragility index and glass transition of supercooled borate and phosphate liquids is studied using a combination of dynamic and thermodynamic measurements. The negative deviation of Tg from linearity in mixed-alkali borates is found to be always the largest when two distinct alkalis are combined in equal proportions, consistent with the prediction of the matrix-mediated coupling model. However, this deviation is shown to vary nonmonotonically with the alkali size difference, which is attributed here to a balance between the strength and the reorientation probability of the mechanical dipoles associated with the hopping of alkalis to unlike sites. On the other hand, isobaric heat capacity measurements of mixed-alkali phosphate glasses and supercooled liquids indicate that the negative deviation of the liquid fragility index m from linearity may not be related to the configurational entropy of mixing of the alkalis. It is hypothesized that the bond scission-renewal dynamics of the network responsible for the viscous flow is catalyzed by the reorientation process of the alkali mechanical dipoles in mixed-alkali liquids, which results in a lowering of the activation energy of the former. This lowering of activation energy is manifested in a significant lowering of m of a mixed-alkali liquid, compared to its single-alkali end members that lack this catalytic feedback. The relative importance of this catalytic contribution diminishes at higher temperatures as the thermal contribution to the activation barrier crossing begins to dominate and thus the MAE on viscosity eventually disappears above a critical temperature.

Alkali and mixed alkali effect: Spectral investigations on Sm3+ and Dy3+ ions doped zinc tungstate borophosphate glasses

In this study, we provide the spectra data of borophosphate zinc tungstate glasses doped with Sm3+or Dy3+ ions based on alkali and mixed alkali oxides. In order to describe the spectroscopic properties of the manufactured Sm3+ and Dy3+ glasses, we used XRD, EDAX, FT-IR, FT-Raman, Absorption, and PL Spectra. The amorphous nature of glass has been studied by looking at its X-ray diffraction data. Glasses that have been doped with Sm3+ and Dy3+ ions have been shown to exhibit significant absorption bands at 401 nm in the Vis-NIR range, as well as three weak bands at 360, 373, and 473 nm, and bands centered at roughly 1079, 1216, 1363, and 1455 nm in the NIR area. Multiple absorption bands at various wavelengths, from 796 nm to 1667 nm, are present in both series C and D glasses. After being excited at 402 nm, the emission spectra of Sm3+ doped glasses show three emission transitions at 562, 598, and 645 nm. The transition at 598 nm has been proven to produce brilliant orange-red light. Excitation at 350 nm in the emission spectra of Dy3+ doped glasses causes three emission transitions to occur at 482, 574, and 664 nm. The 574 nm emission band offers the clearest evidence of yellow emission. Energy level diagrams have been used to describe the emission process that occurs in these glasses.

Defect Engineering of Nanocrystal‐In‐Glass Composites for Ultrashort Optical Pulse Monitoring

AbstractThe rational control of intrinsic defects in materials can significantly enhance their scientific and technological potentials, but it remains a long‐standing challenge in nanocrystal‐in‐glass composites (NGCs). Herein, a defect engineering strategy mediated by the mixed alkali effect is proposed and experimentally demonstrated for NGCs. Interestingly, the hybridization of large alkali ions can effectively increase the barrier of Li+ migration and reduce the Li‐related defects in LiNbO3 NGC. As a result, a novel LiNbO3 NGC with greatly reduced Li‐related defects, high crystallinity of over 60%, and excellent optical transmission are successfully fabricated. This unique NGC configuration facilitates efficient transverse second harmonic generation (TSHG) in a broad wavelength region. Based on the above effects, a standard TSHG device is fabricated and implemented to monitor ultrashort optical pulses with duration in the order of ≈10−13 s over a broad wavelength region, down to 780 nm. The proposed strategy not only provides a new idea for defect engineering in materials science but also has great significance for boosting the practical applications of NGCs in ultrashort optical pulse monitoring.

The unexplored role of alkali and alkaline earth elements (ALAEs) on the structure, processing, and biological effects of bioactive glasses.

Bioactive glass has been employed in several medical applications since its inception in 1969. The compositions of these materials have been investigated extensively with emphasis on glass network formers, therapeutic transition metals, and glass network modifiers. Through these experiments, several commercial and experimental compositions have been developed with varying chemical durability, induced physiological responses, and hydroxyapatite forming abilities. In many of these studies, the concentrations of each alkali and alkaline earth element have been altered to monitor changes in structure and biological response. This review aims to discuss the impact of each alkali and alkaline earth element on the structure, processing, and biological effects of bioactive glass. We explore critical questions regarding these elements from both a glass science and biological perspective. Should elements with little biological impact be included? Are alkali free bioactive glasses more promising for greater biological responses? Does this mixed alkali effect show increased degradation rates and should it be employed for optimized dissolution? Each of these questions along with others are evaluated comprehensively and discussed in the final section where guidance for compositional design is provided.

Alkali and mixed alkali effect: spectroscopic properties of Eu3+ and Tb3+ ions doped zinc tungstate boro phosphate glasses

Doped with trivalent rare-earth ions (Eu3+ and Tb3+), zinc tungstate borophosphate glasses are reported in this study, along with their structural, elemental, optical properties. Amorphous glass was identified via X-ray diffraction analysis. The elemental evaluation was finished using EDAX. The FTIR spectrum is used to identify ligands. The produced glasses’ optical features were studied using FT-Raman spectral analysis, photoluminescence excitation (PLE), and photoluminescence (PL) analysis. Emission spectra of glasses doped with Eu3+ ions reveal the bright red emission at 613 nm (5D0 → 7F2) with an excitation wavelength of 394 nm. (7F0 → 5L6) Green light is emitted at 545 nm (5D4 → 7F5) and 379 nm (7F6 → 5G6) is used for excitation in the emission spectra of glasses doped with Tb3+ ions. By analyzing decay curves, we may estimate how long Eu3+ or Tb3+ ions in BPWZL glasses will remain stable. The energy level diagram explains the emission procedure that takes place in (Eu3+ and Tb3+) doped zinc tungstate borophosphate glasses.

Mixed-Alkali Effect on Thermal Property and Elastic Behavior in Borosilicate Glasses

Mixed-Alkali Effect on Thermal Property and Elastic Behavior in Borosilicate Glasses

Impact of substituting K2O for Na2O on physico-chemical properties and in vitro bioactivity of bioactive glass S53P4

The impact of substituting K2O for Na2O (0–100%) on the physico-chemical and bioactive properties of bioactive glass S53P4 was studied. The thermal properties were evaluated using differential thermal analysis and high-temperature microscopy. Ion release kinetics was measured in static and dynamic TRIS-buffer and SBF using inductively coupled plasma optical emission spectroscopy. Substituting K2O for Na2O showed a mixed-alkali effect (MAE) for the thermal properties with a minimum value for the glass containing around 20% K2O. The hot-working range increased with the potassium content. The glass with 66% of Na2O replaced with K2O exhibited MAE in static TRIS by giving the biggest pH change and ion release at 24 h. The same glass showed the lowest pH and Si species release in dynamic SBF at the longest dissolution time, 72 h. Both single-alkali glasses showed higher in vitro bioactivity in dynamic SBF. The results provide tools for tailoring new bioactive glass compositions with desired properties.

Degradation-controlling mechanisms in natural organic acid solutions of CaO–Al2O3–SiO2–B2O3 glass-ceramics by partially substituting Ca2+ with Ba2+ and Sr2+

The degradation of environmentally friendly CaO–Al2O3–SiO2–B2O3 (CASB) glass-ceramics, which consist of anorthite and glass phase, was investigated in three natural organic acid solutions. The results indicated that citric acid had the most significant effect on the degradation of CASB glass-ceramics. While the chemical stability of anorthite is relatively poor, the glass phase also contributed significantly to the effective degradation of CASB glass-ceramics. Subsequently, Ba2+ or Sr2+ was used for full or partial substitution of Ca2+ in CASB glass-ceramics, and the degradation-controlling mechanism of the substituted CASB glass-ceramics was further researched. The full substitution of Ca2+ in CASB glass-ceramics by the two cations resulted in the occurrence of borate [BO4] units in the glass phases, and the interlinkage of [BO4] with broken silicate [SiO4] network structures caused a complementary network effect. Consequently, the degradation of CASB glass-ceramics by organic acids was reduced due to the improvements in the chemical stability of the modified glass-ceramics. Additionally, degradation control can also be achieved based on a mixed-alkali effect, originating from the partial substitution of Ca2+ in CASB glass-ceramics by Ba2+ or Sr2+. The degradable glass-ceramics have the potential to be applied in low-temperature co-fired ceramic technology because of their good physical properties, which include a dielectric constant of 3–5, a dielectric loss as low as 10−3, a coefficient of thermal expansion of 3–9 × 10−6/°C, and an average bending strength of about 47 MPa. Noticeably, the development of the degradable glass-ceramics is helpful to the low-cost and pollution-free recycling of valuable metal electrodes, which is significant for the sustainable development of electronic packaging technologies.

Physical, Optical, and Spectroscopic Characteristics of Mixed Alkali Tetraborate Glasses RO–ZnO–Li2B4O7-K2B4O7: Impact of Alkaline Earth Oxides (R = Mg, Ca)

Alkali tetraborate glasses 10RO-30ZnO-xLi2B4O7-(60-x) K2B4O7 (R = Mg, Ca) with x varying from 0 mole% to 60 mole% were synthesized by melt quenching approach at around 1150 °C. Broad and peak less X-ray diffraction spectra confirmed the amorphous nature of synthesized glass samples. Physical and optical properties namely density, molar volume, refractive index, optical band gaps, molar refractivity and Urbach energy values have been reported. The density of the present glass samples improved with increasing x mole% except at the presence of equal amounts of alkali oxides in the glass system. The molar volume decreased continuously with x mole%. Oxygen packing density (OPD) increased with increasing x mole%. The optical energy band gap Eopt values with increasing lithium tetraborate in the glass composition have shown a downward trend, where are refractive index values shown upward. FTIR and Raman studies revealed the variation of BO3 and BO4 units in the glasses with composition is marginal. EPR spectra of copper ions confirmed the existence of Cu2+ ions in the ground state dx 2–y 2 orbital (2B1g). The observed non-linear variations of various properties are attributed to the structural changes caused by mixed alkali effect.

Mechanism of mixed alkali effect in silicate glass/liquid: Pathway and network analysis

The mixed alkali effect in silicate glass/liquid causes two anomalies: a decrease in the self-diffusivity of alkali ions and a decrease in shear viscosity. Previous studies using molecular dynamics (MD) simulations proposed that the decrease in diffusivity of alkali ions is the result of pathway blocking. However, the actual statistics of the pathway have never been presented. In this study, we revisited the mixed alkali effect in sodium–potassium silicate glass/liquid using MD simulations. The distinct part of van Hove's function indicates that site exchange between different alkali species is significantly slower than between the same species. By introducing simplex sphere analysis and its cluster statistics with varying compositions, we clarified the pathway disconnection caused by alkali mixing. Moreover, we analyzed the geometry and topology of the corner-shared network of the SiO4 tetrahedra precisely. Both the geometry and topology change linearly with advancing alkali substitution, whereas the calculated shear viscosities have a minimum in the mixed alkali composition. Consequently, a method to quantitatively analyze the changes in relaxation dynamics is required to clarify the viscosity anomaly in mixed alkali liquids.

Mixed-Alkali Effect and Correlation to Glass Structure in Ionically Conductive P2O5-Al2O3-Na2O-K2O Glass System

In this study, the nature of the electrical transport and structural changes resulting from the systematic substitution of Al2O3 with K2O in 40P2O5-(25−x)Al2O3-35Na2O-xK2O, where x = 5.0, 7.5, 10.0, 12.5, and 15.0 mol% (PANxK), is investigated. The impact of the changes in glass structure and its correlation to electrical properties is presented. The mixed alkali effect (MAE) is observed due to the presence of two different alkali oxides, resulting in a non-monotonic trend in the studied glass properties. The infrared spectra show the shift and diminishing of the bands related to the P–O–P/P–O–Al bridges with increasing K2O content and changes in bands related to depolymerization of the glass network, which is confirmed by the trend of the Tg values. The minimum value of DC conductivity is obtained for glass with x = 12.5 mol%. With the overall increase in alkali content, the number of non-bridging oxygens increases, also affecting the conductivity values. Frequency-dependent conductivity spectra analyzed by Summerfield, Baranovskii-Cordes and Sidebottom scaling procedures revealed interesting features and signature of the MAE in the short-range dynamics of the potassium and sodium ions, both for individual glass composition and glass series as a whole. This study showed the impact of MAE and local glass structure on the electrical features and the prevailing of one effect over the other as a function of the glass composition. MAE dominates in a wider range, but with the significant increase in alkali content, MAE is consequently overpowered.

Thio and selenosilicates, sulfide and selenide counterparts of silicates: similarities and differences

Thiosilicate and selenosilicate glasses are the sulfide and selenide counterparts of the well-known silicate glasses. This paper reviews the main investigations carried out to shed light on the structural, thermal and electrical characteristics of these glasses, showing their particularities compared to their oxide counterparts, such as a more complex structure with the presence of both, corner- and edge-sharing tetrahedra and much higher ion conductivity due to the larger polarizability of the chalcogen and the more covalent nature of the bonds, but also, their similarities with the presence of both, bridging and non-bridging S or Se or the existence of a mixed alkali effect when two mobile Li + and Na + cations coexist in the glassy network. Structural consequences of competition between SiS 2 and another former, GeS 2 , PS 5/2 or BS 3/2 are also discussed.

Influence of Mixed Na2O/K2O on Chemical Durability and Spectral Properties of P2O5-Al2O3-BaO-K2O-Na2O-Nd2O3 Phosphate Glasses.

A series of 56P2O5-7.5Al2O3-5.9BaO-(28.56-x)K2O-xNa2O-1.51Nd2O3 phosphate glasses with different Na/(Na+K) ratios, which were specially designed for high-power laser application, were prepared by a high-temperature melting method. Except for the density, refractive index, glass transition temperature, and DC conductivity, the chemical durability and spectral properties, as emphasized by high-power and high-energy laser material, were further measured and analyzed. Regarding the chemical durability, the dissolution rates of these glasses do not show an evident mixed alkali effect with increasing the Na/(Na+K) ratio, although the effect is obvious for the glass transition temperature and DC conductivity. To better understand the nature of the dissolution mechanism, the ionic release concentrations of every element are determined. Both Na and K undergo ion exchange, but the ion exchange rate of K is much larger than that of Na. In terms of the spectral properties, the J-O parameters, emission cross-section, radiation lifetime, fluorescence lifetime, effective bandwidth, fluorescence branching ratio, and quantum efficiency are determined from absorption and emission spectra. The trend of Ω2 deviating from linearity indicates that the coordination environment symmetry of Nd3+ ions and the covalence of Nd-O also present an evident mixed alkali effect. The most important finding is that the emission cross-section and fluorescence lifetime of Nd3+ ions at 1053 nm were not affected by the change in the Na/K ratio. According to the above experimental results, the optimized value of the Na/K ratio was determined, based on which the 56P2O5-7.5Al2O3-5.9BaO-(28.56-x)K2O-xNa2O-1.51Nd2O3 glass maintains a high emission cross-section with good chemical durability.

Resolving the mixed-alkali effect on the viscoelastic behavior of supercooled liquids

Relaxation processes in mixed-alkali metaphosphate glasses and supercooled liquids are studied using a combination of calorimetric, electrical impedance spectroscopic and rheological measurements to investigate the atomistic nature of the mixed-alkali effect (MAE) on the glass transition temperature Tg and viscous flow. While Tg and fragility index of single-alkali liquids are controlled by the segmental motion of phosphate chains, the mixed-alkali liquids exhibit the presence of a second relaxation process, which is slower than chain motion and is characterized by a lower activation energy. This process is assigned to the scission and renewal of P-O bonds that relieve the accumulated stress in the network resulting from its matrix-mediated coupling with pairwise hopping of dissimilar alkalis. The viscous flow and glass transition in mixed-alkali phosphate liquids appear to be controlled by this P-O bond scission-renewal process, which is shown to be consistent with a lowering of their fragility index and a concomitant decrease in the stretching exponent of structural relaxation, compared to their single-alkali counterparts.