Potassium Silicate Glasses Research Articles

Optical and crystallization studies of titanium dioxide doped sodium and potassium silicate glasses

Combined optical and FTIR spectral analysis were used to investigate prepared invert two alkali silicate glasses (Na2O·SiO2K2O·SiO2) containing high titanium oxide content. Glass ceramic derivatives were prepared through thermal two step regime of the parent glasses and they were characterized by FTIR and X-ray diffraction measurements. Experimental optical spectra indicate the appearance of an additional UV band due to absorption of tetravalent titanium ions beside the UV bands due to trace ferric ions impurities. FT infrared absorption spectra reveal composite vibrational bands due to vibrational modes of both silicate groups and TiO4 or SiOTi units. Such tetravalent groupings of titanium ions confirm the stability of such invert glasses containing as such two main glass forming oxides. FTIR spectra of the glass – ceramic derivatives show nearly the same IR vibrational modes as their parent glasses. X-ray diffraction analysis show the separation of crystalline sodium or potassium titanate phases beside a minor form of silica and the results confirm that TiO2 acts mainly as network forming units leading to the formation of crystalline titanate phases.

Investigation of smalt in cross-sections of 17th century paintings using elemental mapping by laser ablation ICP-MS

Microanalysis by laser ablation-ICP-MS was performed on 17th century paint cross-sections of two Dutch masters (Hendrick Avercamp and Caesar Boetius van Everdingen) and a Chinese coromandel lacquer (unknown artist) to study the multi-elemental composition of the blue pigment smalt. Smalt is a potassium silicate glass colored by cobalt ions which are often associated with other elements such as As, Ni, Bi and Fe. Due to interactions between pigment and medium, smalt often deteriorates as a result of potassium leaching, thus losing its color and causing chemical and physical changes in the paint film.Elemental mapping by laser ablation-ICP-MS was carried out for a suite of 54 elements using line scanning (20–50 parallel laser tracks). Elemental maps with pixel dimensions of 5μm were generated for sample areas containing all the layers of the cross-sections. Elemental quantification of the smalt particles was carried out by a sum normalization calibration protocol based on summation of the elements as their oxides to 100wt.% and using relevant glass standards. Elemental concentrations in the maps of 5–30μm sized smalt particles were visualized in pseudo-colors.Next to highlighting the microanalytical potential of laser ablation-ICP-MS for multi-elemental mapping (on trace, minor and major element levels simultaneously) of smalt in paint cross-sections, this work will elaborate (statistical) data processing protocols for elucidation of cobalt ore provenance, possible deterioration mechanisms and the conservation state of the three examined paintings.

Using the two-point bend technique to determine failure stress of pristine glass fibers

Two-point bending (2pb) is the simplest technique for the measurement of failure strains of glass fibers under a variety of experimental conditions. There is little chance of damage to fibers even when testing in liquid nitrogen, leading to reproducible and precise measurements of failure strain with Weibull moduli of greater than 100 measured routinely. However, a limitation of 2pb is that it measures failure strain not failure stress, and thus the Young's modulus of the sample must be known at the failure strain in order to evaluate the failure stress.In this paper the failure strains, under both inert and ambient conditions, for a number of conventional glasses (commercial silica, soda-lime silicate, and E-glass), as well as a number of simple glasses, including a nepheline glass and a range of binary sodium and potassium silicate glasses are presented. These strain values are converted to failure stresses using known or estimated non-linear modulus parameters and compared with strength values found in the literature. For silica optical fibers, the failure stresses calculated from 2pb failure strains vary from 12.1±0.2 to 14.4±0.3GPa in inert (liquid nitrogen, 77K) conditions and 7.0 to 7.3±0.1GPa in ambient conditions (room temperature, 50% RH), compared to reports of 11–14GPa for liquid nitrogen and 4–5GPa ambient tensile strength measurements. For a commercial E-glass, the calculated failure stress from 2pb, is 5.1 to 5.2±0.1GPa in inert conditions and 3.7 to 3.8±0.1GPa in ambient conditions, compared to reported tensile strengths of 5.3GPa and 3.0–3.8GPa, respectively. The failure stresses for binary alkali silicate glasses calculated from 2pb failure strains are 2–3 times greater than those reported in the literature.

Comment on “Spectroscopic studies of oxygen speciation in potassium silicate glasses and melts”

Recent O1s XPS studies suggest that significant free oxide (O2– or K–O–K) is present in potassium silicate glasses, in contrast with what was concluded from 29Si and 17O solid-state NMR data. An alternative peak assignment of the Qn peaks in the 29Si NMR spectra was proposed to bring the 29Si NMR data in line with the O1s data, but this reassignment (i) is not supported by any evidence other than the resulting agreement between the NMR and XPS data, (ii) is at odds with the spectral properties of the bands (peak position and width), (iii) ignores the strong evidence for the original peak assignment, and (iv) results in highly implausible equilibrium constants for the Qn speciation reactions. More likely causes for the apparent discrepancy between the XPS and NMR data are the incorrect estimation of the precision of the bridging oxygen content from the O1s XPS data, the systematic overestimation of the bridging oxygen content from the O1s XPS data, and (or) the incorrect estimation of the accuracy of the compositional analysis, which was based on the precision rather than the accuracy of the electron microprobe analysis. Thus, the available evidence strongly suggests that the original assignment of the 29Si NMR spectra is correct. Neither the 29Si NMR nor the O1s XPS data support the presence of significant amounts of free oxide in potassium silicate glasses with K2O/SiO2 ≪ 2.

Reply to the comment by Malfait on “Spectroscopic studies of oxygen speciation in potassium silicate glasses and melts”

Malfait challenges our XPS results by arguing that the XPS spectroscopic technique cannot be used to determine small amounts of O2– in potassium silicate glasses. Instead, he claims that there is no free oxide (O2–) in potassium silicate glasses based primarily on his 29Si MAS NMR spectroscopic results. In this rebuttal, we demonstrate that O1s XPS and well-resolved 2D 29Si MAF NMR spectral results of potassium disilicate (K2Si2O5) glass are consistent with each other and that both techniques indicate the presence of a few mol% of O2– in the glass. Neither of these techniques, however, supports the interpretation of the 29Si MAS NMR results presented in the comments of Malfait. The major difficulty relates to the low resolution of the 29Si MAS NMR spectra, which does not reveal the Q4 signal beneath a strong Q3 peak in these spectra. The proof is provided by the 2D 29Si MAF NMR spectrum of potassium disilicate glass in which both Q3 and Q4 peaks are revealed; the 2D 29Si MAF NMR results for potassium disilicate glass are far more informative than 29Si MAS NMR spectra. It demonstrates that the potassium disilicate glass (K2Si2O5) contains greater Q4 intensity, is more polymerized than previously considered, and that O2– is present at ∼2 (±1) mol% in the potassic glass. This O2– value confirms our O1s XPS results. Specific points raised by Malfait are rebutted in Appendix A .

Dynamic strain propagation in nanoparticulate zirconia refractory

Residual and intrinsic strains in granular materials have been studied extensively. However, understanding the dynamic strains that cause these resultant residual strains is key to developing better strain-resistant materials. This investigation demonstrates a method for characterizing dynamic strain propagation in granular materials. The specimen is a zirconia-based refractory composed of sol–gel-derived zirconia nanoparticles in a potassium silicate glass binder.In situsynchrotron X-ray powder diffraction in flat-plate geometry is used to characterize the sample structure on timescales of the order of 1 ms. A 125 W CO2laser is used to strain the sample with a 25 ms pulse length. To compensate for the poor flux on this timescale, a pump–probe method is repeated 1000 times and the resulting data are subsequently re-binned to improve statistics. A Gaussian weighting function is also used to introduce better contrast between strained and unstrained frames.TOPAS Academicis used for fitting with a Le Bail model in `batch mode'. Lattice parameters and sample height are refined during fitting, along with a Lorentzian line width for extracting microstrain broadening. Microstrains, ∊, in the range of 1.01 < ∊ < 1.46% are reported on a 1 ms timescale.

Spectroscopic studies of oxygen speciation in potassium silicate glasses and melts

To resolve discrepant results between O1s XPS and 29Si and 17O NMR techniques, O1s XPS spectra were collected and fitted for 15 potassium silicate glasses containing 10−37 mol% K2O. The effects of melting on the compositions of the glasses are documented and quantified, as are uncertainties associated with fitting the O1s spectra. These spectra are well resolved into two symmetric peaks: a narrow peak due to NBO (Si−O−K) plus “free oxide” (O2– or K−O−K) and a broader peak due to BO (Si−O−Si). Values of mol% BO (uncertainties of ±1%) are obtained from the computed peak areas and indicate 2.2 (±0.8) mol% O2– for glasses containing 32 (±1) mol% K2O. Reassignment of Q species for previously published 29Si NMR spectra leads to BO and O2– values in good agreement with these O1s XPS spectra. A recent 17O NMR study on potassium silicate glasses concluded that there is <1 mol% O2– in these glasses based mainly on the assumption that the 17O chemical shift for O2– should be similar in the glasses and the oxide in crystalline K2O. This assumption is questionable. Free oxide in these glasses should be highly reactive toward gases such as CO2 (via CO2 + O2– → CO32–) and it seems likely that the reactive species in silicate glasses is O2– rather than NBO as often assumed. The small amounts of CO2 found in reacted glasses are qualitatively consistent with the abundance of O2– obtained from these O1s XPS spectra of silicate glass.

On the origin of the mixed alkali effect on indentation in silicate glasses

The compositional scaling of Vickers hardness (Hv) in mixed alkali oxide glasses manifests itself as a positive deviation from linearity as a function of the network modifier/modifier ratio, with a maximum deviation at the ratio of 1:1. In this work, we investigate the link between the indentation deformation processes (elastic deformation, plastic deformation, and densification) and Hv in two mixed sodium–potassium silicate glass series. We show that the mixed alkali effect in Hv originates from the nonlinear scaling of the resistance to plastic deformation. We thus confirm a direct relation between the resistance to plastic flow and Hv in mixed modifier glasses. Furthermore, we find that the mixed alkali effect also manifests itself as a positive deviation from linearity in the compositional scaling of density for glasses with high alumina content. This trend could be linked to a compaction of the network when two types of alkali ions co-exist.

Development of an oxidation resistant glass–ceramic composite coating on Ti–47Al–2Cr–2Nb alloy

Three glass–ceramic composite coatings were prepared on Ti–47Al–2Cr–2Nb alloy by air spraying technique and subsequent firing. The aim of this work is to study the reactions between glass matrix and inclusions and their effects on the oxidation resistance of the glass–ceramic composite coating. The powders of alumina, quartz, or both were added into the aqueous solution of potassium silicate (ASPS) to form slurries used as the starting materials for the composite coatings. The coating formed from an ASPS-alumina slurry was porous, because the reaction between alumina and potassium silicate glass resulted in the formation of leucite (KAlSi2O6), consuming substantive glass phase and hindering the densification of the composite coating. Cracks were observed in the coating prepared from an ASPS-quartz slurry due to the larger volume shrinkage of the coating than that of the alloy. In contrast, an intact and dense SiO2–Al2O3-glass coating was successfully prepared from an ASPS-alumina–silica slurry. The oxidation behavior of the SiO2–Al2O3-glass composite coating on Ti–47Al–2Cr–2Nb alloy was studied at 900°C. The SiO2–Al2O3-glass composite coating acted as an oxygen diffusion barrier, and prevented the inward diffusion of the oxygen from the air to the coating/alloy interface, therefore, decreasing the oxidation rate of the Ti–47Al–2Cr–2Nb alloy significantly.

Oxide ion speciation in potassium silicate glasses: New limits from 17O NMR

In silicate glasses and liquids, “free” oxide ions (O2− ions not bonded to any network-forming cations such as Si) are required by stoichiometry for ratios of O/Si>4, for example in “sub-orthosilicate” compositions (<33.3% SiO2 in MO-SiO2 or M2O-SiO2 binaries). Measurements of oxide ion activities and other thermodynamic considerations suggest however, that at higher silica contents the concentration of such ions should rapidly become very low, particularly in systems with highly basic modifier oxides such as Na2O or K2O. Recent 17O NMR studies of alkaline earth ortho- and sub-orthosilicate glasses have indeed directly shown the presence of a few percent of “free” oxide ions, but did not detect this species at a silica content of 44mol%. In contrast, recent O 1s XPS analyses of bridging oxygen concentrations in Na- and K-silicate glasses have suggested as much as about 6% “free” oxide even at about 67% SiO2 [1,2]. In K-silicates, theoretical calculations presented here, as well as previous work on Ca and Mg silicates, suggest that this species should be readily and directly measurable by 17O. However, we show here that “free” oxide ions are not detectable in glasses with 34 and 40mol% K2O, and therefore are not likely to be present above the 0.1 to 1% level. Measured ratios of bridging to non-bridging oxygens are within errors of those expected from analyzed compositions and an assumption of negligible “free” oxide, and 29Si MAS spectra can be accurately fitted with constraints based on this same assumption.

Investigation of medium range order in silicate glasses by infrared spectroscopy

A new decomposition method of infrared spectra which takes into account the configurational and dynamical origins of the disorder allows retrieving structural information on short and medium range orders in potassium silicate glasses. The distribution of tetrahedral units, the occupation of cation sites, the ratio of SiO bond ionicities involving bridging and non bridging oxygen, and a measure of the impact of low frequency floppy modes on the high frequency dynamics are byproducts of the modeling process. The composition dependence of two vibrational modes clearly identified as spectral components signing medium range order, shows that the disruption of the silicate network follows selective schemes. The 3D silica like network completely disappears in glasses with K2O amounts greater than 11mol% and above this threshold a progressive appearance of 2D silicate sheets is evidenced.

Investigation of ion-exchange ‘stuffed’ glass structures by molecular dynamics simulation

Molecular dynamics simulations were used to examine ion-exchange ‘stuffed’ glass structures by substitution of Na+ ions with K+ ions in xNa2O·(100−x)SiO2, x=9, 16, and 23mol%. After substitution, relaxation was performed under varied boundary conditions, including NVT and NPT ensembles. Structural features of stuffed glasses were compared with those of the host glass and of the as-formed, compositionally-equivalent potassium silicate glass. Stuffing K+ ions had oxygen coordination number near that of the as-formed potassium silicate. Ion-exchange stuffed glasses were topologically confined, showing little Qn distribution or ring size distribution change compared to the host glass. Intertetrahedral re-orientation occurred to bear some of the network strain. Resulting molar volume change of ion-exchange stuffed systems was 58–67% of the difference between the as-formed sodium silicate host and the as-formed end-member potassium silicate glass. This accounted for a portion of the reduced experimental surface compression observed during ion exchange relative to that expected from the linear elastic suppression of the difference of as-melted molar volumes.

Potassium-silicate glass exposed to low energy H+ beam

Pristine surface of binary potassium silicate glass 85SiO2·15K2O was prepared in vacuum and irradiated with a 5keV proton beam within the range of 0.6–103C/m2. The response of glass surface was monitored by XPS and the evolution of atomic concentrations divided it into two stages. During the first one, amounts of both potassium and non-bridging oxygen (NBO) increase in the surface layer and are governed by surface relaxation. The second stage is characterised by a continuous decrease of NBO and K. Comparison of K and NBO concentrations yielded a constant surplus of K proving the existence of potassium elemental state on the glass surface. Ratio of bridging oxygen (BO) and silicon is conserved during proton bombardment. The extrapolation of the glass response to the enhanced irradiation predicts a formation of substoichiometric SiOx with some elemental K on the topmost surface.

Reduced Young modulus and hardness of electron irradiated binary potassium-silicate glass

Binary potassium silicate glass of composition 15mol%K2O+85mol%SiO2 was irradiated with 50keV electrons. Dependence of mechanical properties of the irradiated glass on electron doses were measured by means of nanoindentation. The reduced Young modulus and hardness decrease for higher doses. The changes in mechanical properties are dominantly determined by the density variation and silica network depolymerisation during the electron bombardment.

High resolution X-ray Photoelectron Spectroscopy (XPS) study of K2O–SiO2 glasses: Evidence for three types of O and at least two types of Si

High resolution O 1s, K 2p and Si 2p XPS Spectra were collected for a series of potassium silicate glasses ranging in composition from 10mol% to 35mol% K2O. The mole fraction of bridging oxygen (BO) has been accurately evaluated from the O 1s spectra. BO mole fractions of K-silicate glasses were calculated from Q-species distributions previously reported by 29Si MAS NMR data. The mole fractions of BO are identical for the two techniques (within experimental error) in glasses containing 13mol% to 25mol% K2O but in the compositional range between 25mol% and 35mol% BO mole fractions obtained by XPS are slightly greater than values derived from NMR data. The slight discrepancies between the two techniques at higher K2O content have not been resolved. The experimental data between 13mol% and 25mol% K2O indicate the presence of a third type of oxygen, O2−, in these glasses. A thermodynamic analysis indicates O2− is present at a few mol% in the glasses of low K2O content, but increases monotonically with increased K2O content.The O 1s XPS line widths for the BO peaks are highly variable. The variation in line widths may result from two types of BO contributing to the BO peak. As in the Na2O–SiO2 glass system, one type probably bridges two Si atoms (SiOSi) and the second type is O bonded to two Si atoms and one K atom.The Si 2p XPS spectra are distinctly non-symmetric, with low binding energy shoulders commonly present on the major peak, suggesting two contributions to the Si 2p signal. There is a strong correlation of Si 2p XPS peak and shoulder intensities with the abundances of the Q4 and Q3 species in glasses of the same composition suggesting that, with additional resolution, XPS may be capable of resolving individual Q-species in this system.

Volume fluctuations in potassium silicate glass studied by molecular dynamics

Potassium-silicate glass, 0.05K 2O·0.95SiO 2, was prepared by molecular dynamics under three different regimes (cooling rates, simulating box sizes) to study their influence to temperature regimes of volume fluctuations. Partial volume fluctuations (VF) were introduced with the help of Voronoi polyhedra tessellation. Dynamic and static fluctuations were suggested as a measure for time- and space-related fluctuations. Separation of the dynamic and static VFs anticipates glass transition. The glass transition was related with the change of the temperature course of the static VFs. The dynamic VFs changed their temperature regimes well below the glass transition and the transition temperature was related with the change of the transport regime. Glass transition cages the atoms but Voronoi polyhedra are definitely shaped well below the glass transition temperature.

Ion Transport Mechanism in Glasses: Non-Arrhenius Conductivity and Nonuniversal Features

In this article, we report non-Arrhenius behavior in the temperature-dependent dc conductivity of alkali ion conducting silicate glasses well below their glass transition temperature. In contrast to the several fast ion-conducting and binary potassium silicate glasses, these glasses show a positive deviation in the Arrhenius plot. The observed non-Arrhenius behavior is completely reproducible in nature even after prolonged annealing close to the glass transition temperature of the respective glass sample. These results are the manifestation of local structural changes of the silicate network with temperature and give rise to different local environments into which the alkali ions hop, revealed by in situ high-temperature Raman spectroscopy. Furthermore, the present study provides new insights into the strong link between the dynamics of the alkali ions and different sites associated with it in the glasses.

The determination of sulfate and sulfide species in hydrous silicate glasses using Raman spectroscopy

Raman spectroscopy is used to identify the sulfur speciation (sulfur valence state) in “technical” iron-free soda-lime and potassium-silicate glasses and in “natural” glass compositions such as basalt, andesite, and rhyodacite. The presence of sulfate (S6+) is marked in Raman spectra of oxidized synthetic and natural glasses by bands at ~990 and ~1000 cm-1, respectively. The presence of sulfide (S2-) in the reduced technical glasses is marked in Raman spectra by a band at 2574 cm-1 indicating that S2- is present as HS- in these Fe-free glasses. Such a band is absent in the Raman spectra of reduced basaltic, andesitic, and rhyodacitic glasses. However, an additional band at ~400 cm-1 appears in the Raman spectra of the reduced S-bearing “natural” glasses when compared to that of S-free reduced “natural” reference glasses indicating that S2- is most likely complexed with Fe in these glasses. Thus, the dissolution mechanism of S2- appears to be different in Fe-free and Fe-bearing glasses and S2- is dissolved as either HS--species or Fe-S complexes, respectively. The data shown here demonstrate the potential of Raman spectroscopy in identifying the sulfur valence state in silicate glasses. In addition, S2- is dissolved as completely different complexes when comparing “technical” iron-free and “natural” iron-bearing, hydrous silicate glass compositions.

Changes of structure in potassium depleted binary silicate glass studied by MD

Ab-initio (DFT) molecular dynamics was used to simulate the structural relaxation of 15K 2O·85SiO 2 glass after the removal of potassium ions. The simulations mimic the experimental findings for potassium-silicate glass irradiated with medium-energy electrons (tens of keV) during which glass becomes fully depleted of alkali ions after some time of the continuous irradiation. Simulations include both volume compression and expansion, found experimentally at different doses. Non-bridging oxygen atoms were created by potassium removal from the prepared glass structure. Changes after alkali atoms removal and the following structural relaxation were characterized in terms of PP RDFs and PP CNFs. The preferential formation of O–O bonds after alkali atom removal was confirmed. The peroxide/ozonide bonds were pretty stable; they do not decay until 2000 K. The relaxation is driven in way Continuous Random Network modified by allowed O–O bonds is formed. Important role of structural defects, namely 5-coordinated Si and sharing edge polyhedra, was confirmed by the direct view of network evolution.

Binary potassium-silicate glass irradiated with electrons

Binary potassium-silicate glass of composition 15 mol% K 2O + 85 mol% SiO 2 was irradiated with 2.5 MeV. The thickness of glass was chosen so as electrons stopped near the back surface. After the high-energy irradiation both, back and top surfaces, were depleted from potassium; the back surface was depleted more than the top one. The decay curves were recorded on pristine glass, and on top and back surfaces of the glass pre-irradiated with high-energy electrons. The incubation times of the top surface increased while the incubation time of the back one decreased in comparison with the pristine glass. Observation of the volume changes induced by the following impaction of 50 keV revealed the top surface is closer to the pristine glass than the back one.