Potassium Silicate Glasses Research Articles

Catalytic removal of soot particles over potassium glasses – the effect of doping with aliovalent redox metals

The catalytic performance of potassium silicate glass catalysts for soot combustion depends on potassium stability and can be controlled by metal doping. Dopants are located in the bulk and modify the structure of glasses.

Surface properties of alkali silicate glasses: Influence of the modifiers.

Using large-scale molecular dynamics simulations, we investigate the surface properties of lithium, sodium, and potassium silicate glasses containing 25 mol % of alkali oxide. The comparison of two types of surfaces, a melt-formed surface (MS) and a fracture surface (FS), demonstrates that the influence of the alkali modifier on the surface properties depends strongly on the nature of the surface. The FS exhibits a monotonic increase of modifier concentration with increasing alkali size while the MS shows a saturation of alkali concentration when going from Na to K glasses, indicating the presence of competing mechanisms that influence the properties of a MS. For the FS, we find that larger alkali ions reduce the concentration of under-coordinated Si atoms and increase the fraction of two-membered rings, implying an enhanced chemical reactivity of the surface. For both types of surfaces, the roughness is found to increase with alkali size, with the effect being more pronounced for the FS than for the MS. The height-height correlation functions of the surfaces show a scaling behavior that is independent of the alkali species considered: The ones for the MS are compatible with the prediction of the frozen capillary wave theory while the ones for the FS show a logarithmic growth, i.e., on the nanoscale these surfaces are not self-affine fractals. The influence of the modifier on the surface properties are rationalized in terms of the interplay between multiple factors involving the size of the ions, bond strength, and charge balance on the surface.

Analysis of the Formed Protective Layer Inhibiting Alkali Corrosion in Aluminosilicate Refractory Castables

This article analyzes the mechanism of the protective layer formation under the action of alkali in a refractory castable when ground quartz sand (GQS) is used as an admixture to produce refractory conventional castables (CC) and medium cement castables (MCC). It was found that, independently of the castable composition, the addition of GQS (2.5%) reduces the degree of K2CO3 dissolution at high temperature, and the released potassium reacts with the silica and forms a viscous potassium silicate glass, which reduces the mobility of alkali. The liquid phase formed filled some of the open pores and hindered the penetration of potassium into the deeper layers of the refractory castable. The thickness of the formed protective layer, after three cycles of the alkaline corrosion test, varies from 700 µm up to 1300 µm, depending on the castable composition.

Theoretical estimation of dielectric loss of oxide glasses using nonequilibrium molecular dynamics simulations

AbstractTo theoretically explore amorphous materials with a sufficiently low dielectric loss, which are essential for next‐generation communication devices, the applicability of a nonequilibrium molecular dynamics simulation employing an external alternating electric field was examined using alkaline silicate glass models. In this method, the dielectric loss is directly evaluated as the phase shift of the dipole moment from the applied electric field. This method enabled us to evaluate the dielectric loss in a wide frequency range from 1 GHz to 10 THz. It was observed that the dielectric loss reaches its maximum at a few THz. The simulation method was found to qualitatively reproduce the effects of alkaline content and alkaline type on the dielectric loss. Furthermore, it reasonably reproduced the effect of mixed alkalines on the dielectric loss, which was observed in our experiments on sodium and/or potassium silicate glasses. Alkaline mixing was thus found to reduce the dielectric loss.

Microstructure Study of Phase Transformation of Quartz in Potassium Silicate Glass at 900 °C and 1000 °C

Interfacial reaction between quartz and potassium silicate glass was studied at both 900 °C and 1000 °C. The results showed that no phase transformation was observed for the pure quartz at 900 °C or 1000 °C. Instead, for quartz particles in K2O-SiO2 glass, the transformation from quartz to cristobalite occurred at the quartz/glass interface at first, and then the cristobalite crystals transformed into tridymite. The tridymite formed at the interface between particles and glass became the site of heterogeneous nucleation, which induces plenty of tridymite precipitation in potassium silicate glass. The influential mechanism of firing temperature and size of quartz particles on transformation rate was discussed.

Potassium-silicate glass foil irradiated with electrons – Asymmetry in migration and space distribution given by the elastic scattering of electrons on potassium atoms

Binary potassium silicate glass 15K2O·85SiO2 was prepared in the form of a thin foil and was irradiated with electrons in the transmission mode. A defocused beam of 10, 20, 30, and 50 keV energy was used to prevent a significant temperature increase. Both, the front and back surfaces were depleted of potassium by the irradiation, but not to the same extent; the front surface was quickly fully depleted of potassium, but the backside only partially. The potassium depth distribution was not consistent with the assumed shape of the electric field established in the solid, but was significantly shifted from the middle of the foil. The found asymmetry is due to the momentum transfer from the elastic scattering of the incident electron to the potassium ion, which acts as an additional effective force.

Effect of SiO2-Al2O3 Glass Composite Coating on the Oxidation Behavior of Ti60 Alloy.

A SiO2–Al2O3 glass composite coating was prepared on Ti60 alloy via air spraying slurry and then a suitable baking process. It was composed of potassium silicate glass, alumina and quartz powders. The high temperature oxidation performance of the alloy with and without coating was evaluated in static air at both 800 °C and 900 °C. The results show that catastrophic oxidation occurs for Ti60 bare alloy. It had a mass gain of about 2 mg/cm2 after oxidation at 800 °C and 17 mg/cm2 at 900 °C for 100 h. On the contrary, the oxidation resistance of alloy coated with composite coating was much improved with the mass gain about 0.36 mg/cm2 and 0.95 mg/cm2 at 800 °C and at 900 °C, respectively. The microstructure evolution of the composite coating and the alloy was analyzed by scanning electron microscope and electron probe microanalyzer. The effect of the composite coating on the oxidation performance of the alloy is discussed especially in terms of oxygen diffusion and interfacial reaction.

Structure-Elasticity Relationship of Potassium Silicate Glasses from Brillouin Light Scattering Spectroscopy and Molecular Dynamics Simulations.

Brillouin light scattering (BLS) spectroscopy and molecular dynamic (MD) simulations allowed the identification of a relationship between the elastic properties and the structure of K-containing glasses of formula (K2O)x-(SiO2)1-x, having different K2O concentrations. Excellent agreement was observed between experimental data and simulations. The peculiar elastic properties observed for these potassium silicate glasses have been extensively discussed in terms of structural and energetic features of the materials. Elastic properties were shown to be strongly dependent on the asymmetry of potential energy in the K-BO interactions and the K-NBO interactions. A low K2O content (below 10-15% K2O) appeared to be in favor of K+-BO interactions and high asymmetry of potential energy, whereas a high K2O content (from 10 to 15% K2O) was in favor of K+-NBO interactions with lower asymmetry. Our results suggest a possible explanation to the observed anomalous dependence of elastic properties of potassium silicate glasses with K2O amount.

Raman spectroscopy study of glass corrosion

Binary sodium silicate and potassium silicate glasses with different alkali content were exposed to the corrosion environment of 35 % hydrochloric acid (HCl). Glass samples were measured by Raman spectroscopy and the spectra were compared with Raman spectra of original pristine glasses in order to observe structural changes induced by corrosion within the surface layer. The usage of confocal Raman spectroscope enabled the measurement of Raman spectra from the depths of few micrometres to document the range of structural changes. The results suggest the depolymerisation of silicate structure due to the growth of the corrosion layer on the surface. The decrease of D2 peak points at the decomposition of three-membered rings. In addition, it is assumed that the Si-O-Si angle changes within the surface layer.

Anionic speciation in sodium and potassium silicate glasses near the metasilicate ([Na,K]2SiO3) composition: 29Si, 17O, and 23Na MAS NMR

Abstract Alkali silicate glasses near to the metasilicate composition provide sensitive tests of anionic speciation. However, reports on 29Si NMR data vary considerably in their derived proportions of silicate species. In some cases, an apparent deficit in the content of non-bridging oxygen has been attributed to up to 5 to 10% “free” oxide ions. Here we provide new NMR data on a series of sodium silicate glasses ranging from 56 to 45 mol% silica with accurately analyzed compositions. High signal-to-noise data for a 29Si-enriched glass provide improved constraints on fitting models for spectra, and suggests a slight asymmetry in the main Q2 component. Data are consistent with conventional models in which “free” oxide contents are below detection limits of about 0.5 to 1%. Spectra for an 17O-enriched K-silicate glass with about 53% silica confirm that there is no obvious direct signature for “free” oxide species in this compositional range.

LA-ICP-MS analysis of corroded glass beads from Southern China: tackling highly inhomogeneous archaeological glass

ABSTRACT Excavated glass artifacts are usually environmentally impacted, causing spatial inhomogeneity which poses great challenges for accurate and non-destructive chemical characterization. Here, we present our study on accurate characterization of archaeological glass with Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), a high-precision and minimally destructive analytic method. Using three corroded glass beads excavated from Guangnan, Yunnan, Southern China as examples, we discuss the issues that may be involved when observing signal curves of highly inhomogeneous archaeological glass, as well as provide chemical characterization for the intact pristine body. Results indicate that these Indo-Pacific monochrome beads are of the potassium silicate glass type and used mineral potash source, and that they contain copper as the colorant. By discussing the breadth and depth types of inhomogeneity and focusing on perturbation endured by major elements, we identify the extent of environmental alteration and describe how different elements and matrices can respond differently to the archaeological environment, leading to various corrosion behaviors. For such samples LA-ICP-MS is shown to be an advantageous tool to provide archaeologically relevant information, or to probe artifact conditions for conservation purposes. A good understanding of sample conditions, and close attention to the experimental and calibration process are required to overcome inhomogeneity when tackling archaeological glass.

Predicting Composition-Structure Relations in Alkali Borosilicate Glasses Using Statistical Mechanics

Predicting the atomic-scale structure of multicomponent glasses from their composition and thermal history would greatly accelerate the discovery of new engineering and functional glasses. A statistical mechanics-based approach has recently been applied to predict the composition-structure evolution in binary oxide glasses by determining the relative entropic and enthalpic contributions to the bonding preferences. In this work, we first establish the network modifier-former interaction parameters in sodium silicate and sodium borate glasses to predict the structural evolution in sodium borosilicate glasses. Due to the significant fluctuations in the experimentally determined structural speciation in borosilicate glasses, we perform classical molecular dynamics (MD) simulations to establish and validate our structural model. We also show that the statistical mechanical model naturally accounts for the difference in structural speciation from MD simulations and NMR experiments, which in turn arises from the difference in cooling rate and thus thermal history of the glasses. Finally, we demonstrate the predictive capability of the model by accurately accounting for the structural evolution in potassium borosilicate glasses without using any adjustable model parameters. This is possible because all the interaction parameters are already established in the potassium silicate, potassium borate, and sodium borosilicate glasses, respectively.

Structural studies of potassium silicate glasses with fixed iron content and their relation to similar alkali silicates

Silicate glasses containing potassium oxide of molar percentages 12%, 16% and 20% each with 8% iron oxide have been prepared and examined by Raman, Extended X-ray absorption fine structure (EXAFS) and Mössbauer spectroscopy. For the 12% and 16% of iron potassium silicate glasses, analysis of the Raman data indicated a maximum of Q2 linkages of the silica tetrahedra while for the 20% iron potassium silicate sample, the Q3 and Q4 linkages were dominant. EXAFS data found the Fe ion to be in tetrahedral coordination with oxygen and a FeO distance of ~1.85 Å. The isomeric shifts in the Mössbauer spectra found the Fe3+ to Fe2+ ratios to vary with potassium content from 2:1 for the 12% to 8:1 for the 20% potassium glass. Spectral areas showed iron ions to be tetrahedrally coordinated to oxygen with quadrupole splitting showing distortion of these units. From the distribution of Qn linkages it is found that at 16 mol% of the alkali, the lithium iron silicates have more open structures than the sodium and potassium iron silicate glasses. The more open structure of sodium silicate glass when iron is added could have important implications for its use as an electrolytic material.

The fragility of alkali silicate glass melts: Part of a universal topological pattern

The compositional variation of the fragility of alkali silicate glasses is shown to mimic that of most all other network forming oxide glasses when graphed as a function of a coarse-grained network connectivity which incorporates rigid structural units associated with intermediate range order. Following procedures established previously, we demonstrate how this coarse graining for both sodium and potassium silicate glasses is achieved using a variety of intermediate range order structures reported in the literature.

Heat-Reflecting Ceramic Materials Based on Potassium Polytitanate and Silicon Oxide

The possibility of obtaining a porous ceramic composite with a low heat conductivity and high heat reflecting ability based on a mixture of raw materials including potassium polytitanate and different silicon oxide modifications is investigated. It is shown that the introduction of silicon oxide favors the formation of the potassium silicate glass phase acting as a binding agent and promoting the formation of a ceramic composite structure consisting of interwoven high-strength fibrous K2Ti6O13 crystals bound by the evenly distributed glass phase.

Pentacoordinated silicon in ambient pressure potassium and lithium silicate glasses: Temperature and compositional effects and analogies to alkali borate and germanate systems

Abstract Pentacoordinated silicon (SiO5) has long been considered as a possible reactive intermediate in bond-swapping reactions in even ambient pressure viscous flow, diffusion, nucleation and crystallization. In this paper, new results are presented for potassium silicate glasses and for two lithium silicates. In the former, SiO5 is readily detectable by 29Si MAS NMR on 29Si-enriched glasses at concentrations as low as about 0.02 mol%; in the latter this species is below detection limits. SiO5 concentrations are higher at higher fictive temperatures, and first increase, then decrease, as K2O is added to SiO2. This pattern in compositional variation resembles those long-known in alkali borate and germanates, although high coordinate species (BO4 and GeO5/GeO6) are orders of magnitude more abundant than in ambient pressure silicates. A simple thermodynamic model, considering only the non-ideal mixing of bridging and non-bridging oxygens, at least qualitatively predicts the shape of the compositional variation of SiO5 in the potassium silicates, and is sensible with respect to known tendencies for clustering and unmixing; such a model also predicts curves of network cation coordination vs. compositon that resemble those known for borates and germanates, suggesting an underlying similarity in the energetics of mixing of anionic species.

Pentacoordinated and hexacoordinated silicon cations in a potassium silicate glass: Effects of pressure and temperature

Silicon cations with five or six oxygen first neighbors (SiO5 and SiO6, pentacoordinated and hexacoordinated) are known primarily from alkali and alkaline earth silicate glasses melted at pressures up to about 12 GPa, where their formation is thought to be an important part of the densification of the liquid. These species, especially SiO5 groups, have also been proposed as high-energy intermediate structures in the bond-swapping that controls network cation and anion diffusion, and hence viscous transport, in high temperature, silica-rich melts, as well as in low-temperature liquid-state silicate reactions, and have thus been the subject of numerous recent theoretical studies. In spite of this interest, the amount of experimental data on controls of pressure, temperature, and composition on concentrations of penta- and hexacoordinated silicon is very limited. Here we report new results from 29Si MAS NMR on high pressure glasses with the model composition K2Si4O9, which show that because of transient pressure drop during quench, previous studies probably significantly underestimated SiO5 and SiO6 concentrations. In addition, we refine the earlier results on SiO5 even in 1 bar glasses, and demonstrate a strong increase in its concentration at higher fictive temperature. A positive enthalpy for the reaction forming this species is consistent with its role as an energetically important transition complex in melt dynamics.

Dielectric properties of ferroelectric nanocomposites: effects of thermal stresses and filler shape anisotropy

Ferroelectric nanocomposites are intriguing nonhomogeneous materials, which may have unusual phase states and specific physical properties useful for practical applications. Here we theoretically describe dielectric properties of nanocomposites comprising single-domain ferroelectric nanocrystals embedded into a linear dielectric medium. First, small-signal intrinsic permittivities of spheroidal PbTiO3 and BaTiO3 crystallites surrounded by an isotropic matrix with linear elastic properties are calculated with the aid of a nonlinear thermodynamic theory. It is shown that thermal stresses caused by differences in thermal expansion between the inclusions and the matrix may strongly influence the intrinsic permittivities of ferroelectric nanocrystals. Second, macroscopic dielectric responses of ferroelectric–dielectric composites are evaluated in the Maxwell Garnett approximation. For effective permittivities of such composites, generalized relations are derived, which allow for both the shape and dielectric anisotropies of ferroelectric nanocrystals. Numerical calculations of the effective permittivities are performed for composites comprising PbTiO3 and BaTiO3 nanocrystals embedded into representative dielectric matrices generating tensile (silica glass) or compressive (potassium silicate glass) thermal stresses inside ferroelectric inclusions. For nanocomposites involving randomly oriented and similarly aligned spheroidal inclusions, temperature dependences of the effective permittivities are determined with the account of phase transitions occurring in strained ferroelectric nanocrystals and suppression of their dielectric responses due to the depolarizing-field effect created by low-permittivity matrix. Our theoretical calculations show that effective permittivities of composites comprising needle-like or disc-shaped ferroelectric inclusions could strongly exceed the permittivity of the matrix. Most importantly, we predict that BaTiO3–K2O-SiO2 composites with such inclusions should exhibit a broad dielectric peak around the room temperature, which is associated with shifted structural transitions in strained BaTiO3 nanocrystals. The presented theory provides guidelines for the development of ferroelectric nanocomposites with enhanced dielectric responses, which could be suitable for applications in supercapacitors and other advanced electronic devices.

Phase diagrams of ferroelectric nanocrystals strained by an elastic matrix

Ferroelectric crystallites embedded into a dielectric matrix experience temperature-dependent elastic strains caused by differences in the thermal expansion of the crystallites and the matrix. Owing to the electrostriction, these lattice strains may affect polarization states of ferroelectric inclusions significantly, making them different from those of a stress-free bulk crystal. Here, using a nonlinear thermodynamic theory, we study the mechanical effect of elastic matrix on the phase states of embedded single-domain ferroelectric nanocrystals. Their equilibrium polarization states are determined by minimizing a special thermodynamic potential that describes the energetics of an ellipsoidal ferroelectric inclusion surrounded by a linear elastic medium. To demonstrate the stability ranges of such states for a given material combination, we construct a phase diagram, where the inclusion’s shape anisotropy and temperature are used as two parameters. The ‘shape-temperature’ phase diagrams are calculated numerically for PbTiO3 and BaTiO3 nanocrystals embedded into representative dielectric matrices generating tensile (silica glass) or compressive (potassium silicate glass) thermal stresses inside ferroelectric inclusions. The developed phase maps demonstrate that the joint effect of thermal stresses and matrix-induced elastic clamping of ferroelectric inclusions gives rise to several important features in the polarization behavior of PbTiO3 and BaTiO3 nanocrystals. In particular, the Curie temperature displays a nonmonotonic variation with the ellipsoid’s aspect ratio, being minimal for spherical inclusions. Furthermore, the diagrams show that the polarization orientation with respect to the ellipsoid’s symmetry axis is controlled by the shape anisotropy and the sign of thermal stresses. Under certain conditions, the mechanical inclusion-matrix interaction qualitatively alters the evolution of ferroelectric states on cooling, inducing a structural transition in PbTiO3 nanocrystals and suppressing in BaTiO3 inclusions some transformations occurring in their bulk counterpart. The constructed phase maps open the possibility to calculate dielectric properties of strained PbTiO3 and BaTiO3 nanocrystals and ferroelectric nanocomposites comprising such crystallites.

Mechanism of AZS Refractory Corrosion in Barium Soda Lime Potassium Silicate Glass Melt

Al2O3 - ZrO2 - SiO2 refractory corrosion in barium soda lime potassium silicate glass melt have been studied. The composition of glass phase of unused AZS and the concentration profiles of the principal oxides in the glass phases of corroded AZS materials were determined. The refractory samples have been taken from the melting parts of a furnace after production termination as well as from the material where the laboratory static corrosion tests have been conducted. SEM and EDX analysis have been provided on the samples located in the part above and below the metal line corrosion.The corrosion mechanism of refractories in glass melt consists of diffusion of alkalis (Na, K, Ca, Ba) from the glass melt into the refractory, dissolution of the Al2O3 and ZrO2 in the glass phase, diffusion of Zr and Al from the refractory into the glass melt, creation and erosion of the boundary layer area.In the part, where the refractory is not in contact with the glass melt, the mechanism is characterized by a dissolution of alumina in refractory glass phase, diffusion of alkalis in glass phase from the refractory part below the glass level into the upper part, condensation and diffusion of evaporated volatile components of glass melt into the refractory material.