Electrical conductivities of mixed cation glasses

The electrical conductivities of (1−x) Li₂O · x BaO · 2 SiO₂, (1−x) Na₂O · x MgO ·2 SiO₂, (1−x) Na₂O · x CaO · SiO₂ and (1−x) Na₂O · x BaO · 2SiO₂ glasses were measured at temperature ranging from room temperature to 450°C. The transport numbers for Na⁺ ion in (1−x) Na₂O · x BaO · 2 SiO₂ glasses were measured. It was found that the alkali ion carried a significant part of the current in these glasses except one that had no alkali ions, and the conductivity decreased markedly as the alkali oxide was substituted by an alkaline earth oxide. The results of conductivity measurements combined with the data hitherto reported on mixed alkali glasses led to the proposal that the so-called mixed alkali effect could be explained on the basis of the independent path model in which it is assumed that cations can move only through vacant sites left by those of the same type.     

Mössbauer studies on some glasses and crystals in the Na2O---Fe2O3---SiO2 system

Mössbauer absorption measurements have been made at room temperature on ⁵⁷Fe in iron sodium silicate glasses containing 3–15 mol% Fe₂O₃ and various iron alkali silicate crystals in order to study the state of iron in these glasses. The spectra of all the glasses gave one doublet with a quadrupole splitting varying from 0.73–0.78 mm s⁻¹, while those of Na₂O · Fe₂O₃ · 4 SiO₂ and 5 Na₂O · Fe₂O₃ · 8 SiO₂ crystals showed much smaller quadrupole splitting, 0.28 mm s⁻¹ and 0.10 mm s⁻¹, respectively, and an asymmetrical doublet of much narrower linewidth. When sodium was replaced by other alkali metals of larger size, such as K and Cs, in MFeSi₂O₆ and MFeSi₃O₈ crystals, the quadrupole splitting became wider and approached to 0.73 mm s⁻¹. Such a variation was not observed for glasses. These results suggest that a larger number of non-identical sites exist in iron sodium silicate glasses than in the corresponding crystals.     

The effect of MgO on the thermodynamics of the Fe/Fe-redox equilibrium and the incorporation of iron in soda-magnesia-aluminosilicate melts

Melts with the basic compositions 10Na₂O · 10MgO · xAl₂O₃ · (80−x)SiO₂ (x=0, 5, 10, 15 and 20), 10Na₂O · xMgO · 10Al₂O₃ · (80−x)SiO₂ (x=5, 10, 15 and 20) and xNa₂O · 10MgO · 10Al₂O₃ · (80−x)SiO₂ (x=5, 10 and 15) all doped with 0.25 mol% Fe₂O₃ were studied using square-wave voltammetry. The temperatures applied were in the range of 1000–1600 °C. The square-wave voltammograms recorded show peaks caused by the reduction of Fe³⁺ to Fe²⁺. The attributed peak potentials measured decreased linearly with decreasing temperatures. Increasing the MgO-concentration led to more negative peak potentials. Introducing alumina in the melt first resulted in less negative peak potentials. If the molar Al₂O₃-concentration is equal to that of Na₂O (=10 mol%) the peak potentials are least negative. Further increase of the Al₂O₃-concentration led to more negative peak potentials. The variation of the Na₂O-concentration led to a maximum in the peak potentials at an Na₂O-concentration of 10 mol%. An empirical formula which allows the calculation of standard potentials from the chemical composition is proposed. Furthermore, a structural explanation for the effect of the chemical composition is given. Especially, the incorporation of Al₂O₃ as AlO₄⁻-tetrahedra at [Al₂O₃] < [Na₂O] and as network modifier at larger concentrations was structurally explained by the similarities of Fe²⁺ and Mg²⁺, with respect to cation radii and metal–oxygen bond lengths.     

The effect of mixed alkaline earths on the diffusivity and the incorporation of iron in 5Na2O · xMgO · (15−x)CaO · yAl2O3 · (80−y)SiO2 melts

Diffusion coefficients of iron were measured in glass melts with the basic compositions 5Na₂O · xMgO · (15−x)CaO · yAl₂O₃ · (80−y)SiO₂ with x=5, 10 and y=0, 5, 7.5, 10 and 15. The melts were doped with 0.25 mol% Fe₂O₃ and studied in the temperature range from 1000 to 1600 °C using square-wave voltammetry. The voltammograms exhibited distinct peaks attributed to the reduction of Fe³⁺ to Fe²⁺, from which peak currents mixed diffusion coefficients of iron were calculated. Diffusion coefficients in all melt compositions which did not show crystallization could be fitted to Arrhenius equation. The diffusivities measured in different melt compositions were related to the same viscosity, i.e. not the same temperature. Increasing the alumina concentration from 5 to 10 mol% resulted in an increase of the viscosity corrected diffusivities. At further increasing alumina concentrations, the diffusivities get smaller again. This can be explained by the stabilizing effect of Na⁺ and Ca²⁺ on FeO⁻₄ and AlO⁻₄-tetrahedra, which strengthens the incorporation of Fe³⁺ into the glass structure.     

Structural distribution and rigidity of the glass network determined by EPR of Cu

The linewidth-broadening of the EPR spectra of Cu²⁺ in silicate, borate and phosphate glasses was analyzed in terms of the distribution of g_| and A_| (δ_| and δA_|) and related to the distribution of the rigidity of the network structure. X- and K-band spectra were measured for the glasses doped with ⁶³Cu²⁺ (93% abundance). The linewidth of the HFS shoulders with parallel orientation to H increased linearly with increasing m or microwave frequency. δg_| and δA_| showed a marked dependence on glass composition. For example, in Na₂O---B₂O₃ glasses, on going from x (mol% of Na₂O) being small through intermediate to large, δg_| varied from small through large to negligibly small. In contrast to these glasses δg_| was extremely large for 75PbO · 25B₂O₃ glass. The large δg_| for the Na₂O---B₂O₃ glassesof intermediate x was attributed to the coexistence of various borate groups competitively coordinating to Cu²⁺. Negligibly small δg_| for 70Na₂O · 30B₂O₃ glass and extremely large δg_| for 75PbO ·25B₂O₃ glass, both with a narrower structural distribution, reflect regidity of the glass network. The Pb---O bonding is strong enough to distort the coordination of Cu²⁺-complex. The situation is the reverse in Na₂O---B₂O₃ glasses.     

Glass formation and optical properties of glasses in the systems (R2O or R''O)-Ta2O5---Ga2O3

Gallate glasses containing no conventional glass formers were obtained in the systems (Na₂O, K₂O or Cs₂O)-Ta₂O₅---Ga₂O₃ and (Sr or BaO)-Ta₂O₅---Ga₂O₃ by an ordinary crucible-melting technique. The glasses showed high optical transmission in the infrared as well as in the visible region. Infrared spectroscopic analysis suggested that the Ga³⁺ ions are tetrahedrally coordinated in the glasses. The glass-forming tendency of the melt and the infrared transmission of the glasses are discussed in terms of the glass structure.     

Influence of the mixed alkali effect on the chemical durability of Na2O---TiO2---SiO2 glasses

The influence of the mixed alkali effect on the chemical durability of Na₂O---TiO₂---SiO₂ glasses during substitution of K₂O for Na₂O in 21Na₂O---26TiO₂---53SiO₂ glasses was investigated. The best chemical durability was found at K₂O/Na₂O = 2.5 where the minimum was close to K⁺ ions of larger size. It was shown that the water corrosion process of the system was predominantly controlled by both the mobility and the exchange function of K⁺ ions resulting in the generation of a titanium-rich and silicon-rich layer at the surface. The mixed alkali effect can therefore be applied to lower the rate of water corrosion and increase chemical durability so that optical glasses with higher chemical durability can be obtained.     

B NMR investigations in xV2O5–B2O3 and xV2O5–B2O3–PbO glasses

11B (I=3/2) MAS NMR in the binary glass system xV₂O₅–B₂O₃ (x=0.053, 0.43) and the ternary glass system xV₂O₅–B₂O₃–PbO (0.1x1.5) has been investigated at room temperature. In the xV₂O₅–B₂O₃ glasses, one NMR line due to BO₃ unit was observed. Meanwhile in the xV₂O₅–B₂O₃–PbO, two NMR lines which arise from BO₃ and BO₄ units were detected, where the appearance of BO₄ units is produced by the presence of PbO. From the computer-simulation of the ¹¹B NMR central transition line (m=−1/2↔1/2), the quadrupole parameters (e²qQ/h and η) for BO₃ units in xV₂O₅–B₂O₃, and those for BO₃ and BO₄ units in xV₂O₅–B₂O₃–PbO were obtained as a function of x. As the V₂O₅ content increases in xV₂O₅–B₂O₃–PbO, the e²qQ/h and η values of the BO₃⁻ associated resonance are found to slightly decrease and increase, respectively. Meanwhile, the e²qQ/h and η values of BO₄⁻ associated resonance in xV₂O₅–B₂O₃–PbO are found to slightly increase and decrease, respectively. By comparing the intensities of the total transitions (m=−3/2↔−1/2,m=−1/2↔1/2, and 1/2↔3/2) for the ¹¹B NMR line of BO₃ and BO₄ units contained in xV₂O₅–B₂O₃–PbO with those of respective standard samples of 0.053V₂O₅–B₂O₃ and NaBH₄, the quantitative fractions of BO₃ and BO₄ in xV₂O₅–B₂O₃–PbO were obtained as a function of x.     

Internal friction in vanadium-phosphate glasses doped with Na2O

The internal friction of _xNa₂O·(0.5−x)V₂O₅·O.5P₂O₅(x = 0.025–0.3) glasses was studied using the low-frequency torsion pendulum technique. The temperature spectrum of internal friction reveals three maxima. Maximum 1, the so-called “electron” maximum, is the same as observed in binary vanadium-phosphate glasses. The origin of maximum 2 can be attributed to ion migration. Maximum 3 appears for glasses containing more than 10 mol.% Na₂O and is probably connected with sodium-proton interactions.     

An infrared study of crystallization in sodium-disilicate glasses containing iron oxides

The infrared absorption spectra of sodium-disilicate glasses containing various amounts of Fe₂O₃ ([Na₂O · 2 SiO₂]_1−x [Fe₂O₃]_x, where X = 0.05, 0.1 and 0.2) were investigated in the wavenumber range from 200–2000 cm⁻¹. The addition of Fe₂O₃ to the sodium-disilicate glass does not seem to introduce any new absorption band as compared with the spectrum of a pure sodium-disilicate glass; nevertheless, a general shift of the existing absorption bands toward lower wavenumbers is observed. The amount of shift is, in fact, proportional to the content of Fe₂O₃ in the glass. This observation is consistent with the recently proposed structural model for the bonding of Fe³⁺ ions in the iron-sodium-silicate glass system.

Annealing of 20 mol% iron oxide glasses at 550 and 580°C produced an extra sharp infrared absorption peak at about 610 cm⁻¹ wavenumber. This new peak is believed to be related to the crystallized particles of the glass as concluded from both a scanning electron micrograph and an electron diffraction pattern.     

Hydrolytic resistance of Na2O---B2O3---SiO2 gels prepared by sol-gel process

Alkoxide derived gels were prepared in the system Na₂O---B₂O₃---SiO₂. The gel compositions were situated in the liquid-liquid immiscibility area of the phase equilibrium diagram.

Hydrolytic resistance tests were performed on the gels heat-treated at temperatures ranging between 120 to 850 °C. The Na₂O, B₂O₃ and SiO₂ extracted from the attack gels were analyzed. The experimental results indicate that the amount of B₂O₃ has a significant influence on the chemical durability of the heat-treated gels. At temperatures of 850 °C the greater the B₂O₃ mol% the greater are the amounts of Na₂O and B₂O₃ extracted. Different behaviour was observed for gels heat-treated at 600 °C where the amounts of B₂O₃ and Na₂O extracted slightly increases as the B₂O₃ mol% increases. Small amounts of extracted SiO₂ were always observed.

These results are complemented with other measurements so that an explanation of the controlling mechanism is given.     

Cation site occupation by Ag/Na ion-exchange in R2O–Al2O3–SiO2 glasses

Ag⁺/Na⁺ ion-exchanged R₂O–Al₂O₃–SiO₂ glasses with uniform concentration profile of Ag⁺ and Na⁺ were prepared by heat treatment in molten silver salt followed by holding at the same temperature in an ambient atmosphere. Their glass transition temperature (T_g) and thermal expansion coefficient (TEC) were measured and structures were investigated using ²⁹Si-MAS NMR, ²⁷Al-MAS NMR, IR and Raman spectroscopies. Both T_g and TEC decreased with increase of the exchange ratio, but T_g was still above the ion-exchange temperature of 400°C even for the fully exchanged sample. The ²⁹Si- and ²⁷Al-MAS NMR spectra were mostly unchanged and no sign of the structural alteration of the glass network was observed. On the other hand, the vibrational spectra showed remarkable peak shifts depending on the exchange ratio. From these structural results, it was found that when the exchange ratio was low, the introduced Ag⁺ ions were stabilized at the non-bridging oxygen (NBO) site, and then Na⁺ ions in AlØ₄ site were exchanged by Ag⁺ ions after full replacement of NBO sites, where Ø represents the bridging oxygen.     

NMR studies of Na2O---B2O3---SiO2 glasses of mid-alkali content

¹¹B Fourier transform spectra have been used to study the structure of Na₂O---B₂O₃---SiO₂ glasses of mid-alkali content. Based on the measurements of the fraction N₄ of four-coordinated borons, it has been found that for K = mol.% SiO₂/mol.% B₂O₃ 8 and R = mol.% Na₂O/mol.% B₂O₃ = 1, N₄ is obviously smaller than 1 rather than equal to 1 as assumed in the relevant literature. Only when R reaches a value appropriately greater than 1, can the case where N₄ = 1 occur. A structural model suggested in this paper can satisfactorily explain the fact.     

Precipitation of In2O3 nano-crystallites from glasses in the system Na2O/B2O3/Al2O3/In2O3

Glasses in the system Na₂O/B₂O₃/Al₂O₃/In₂O₃ were melted and subsequently tempered in the range from 500 to 700 °C. Depending on the chemical composition, various crystalline phases were observed. From samples without Al₂O₃, In₂O₃ could not be crystallized from homogeneous glasses, because either spontaneous In₂O₃ crystallization occurred during cooling, or other phases such as NaInO₂ were formed during tempering. The addition of alumina, however, controlled the crystallization of In₂O₃. Depending on the crystallization temperature applied, the crystallite sizes were in the range from 13 to 53 nm. The glass matrix can be dissolved by soaking the powdered glass in water. This procedure can be used to prepare nano-crystalline In₂O₃-powders.     

On the mechanism of electrical relaxation in Na-conductive phosphate glasses

The crossover from a frequency independent to a frequency dependent ac response has been studied in glasses with the composition 37.2Na₂O-12.8CaO₅ · 50P₂O₅ and 30Na₂O-5CaO-7.5Al₂O₃-57.5P₂O₅ (mol%) containing 5 × 10⁻³ ≤ mol% Ag₂O ≤ 5 × 10⁻¹. Recently, we have established that in these glasses the diffusion coefficient of guest silver ions varies in space. In this case, as a first approximation, the diffusion coefficient may be considered as a constant within regions whose size is no less than 10 nm across. We assume that the diffusion of sodium ions can be given by the D(r) coefficient with the same spatial dispersion as that of silver ions. It is demonstrated that the frequency dependence of ac response is in fair agreement with the assumption.     

Study of PbO films on sodium borate glasses part II: Formation of interdiffusion layers at the glass surface

By selecting carefully controlled conditions for the thermal treatment of sodium borate glasses coated with PbO films, it is possible to prevent Pb²⁺ ions from penetrating deeply into the glass. For low alkali glasses, an interdiffusion layer can be formed, which sub-surface cation profiling (by ion beam induced radiation) shows is a solid solution of Na₂O and B₂O₃ in PbO which acts as solvent. Experiments with 18.0 mol-% Na₂O glass show that it is possible to transform such an interdiffusion layer into a second type in which Na₂O and B₂O₃ together act as solvent and the concentration of PbO solute varies through the layer. For both types of layer the Na₂O : B₂O₃ ratio is different from that of the glass substrate, and for the production of the second type of layer it is shown that an important factor connected with penetration of PbO into the glass surface is a “sweating” process in which thermal treatment of the glass, even in the absence of PbO, results in migration of Na⁺ ions so that the glass surface has a Na₂O content higher than that of the bulk glass. Changes in the UV spectra of the Pb²⁺ ion are correlated with the formation of the interdiffusion layers, and results show that types of layer have optical basicities significantly greater than that of the glass substrate, through either the high PbO or high Na₂O content.     

Vapor phase epitaxy of pure VO2 and V1−xCrxO2

The vapor phase epitaxy of thin epilayers of VO₂ and V_1−xCrxO₂ on TiO₂ transparent substrates is described. Chemical vapor deposition occurs by reacting a (VOCL₃/CrO₂Cl₂/H₂O/H₂) mixture at about 800°C using argon as a carrier gas. The preparation of pure VO₂ requires special care to make it homogeneously stoichiometric and to obtain steep concentration profiles at the TiO₂/VO₂ interface. Layers were obtained which had electrical and optical properties comparable to the best bulk crystals grown by other techniques. Homogeneous solid solutions of V_1−xCr_xO₂ epilayers were also grown for the first time in the range o < x < 0.17. Chromium concentration and homogeneity were determined by electron microprobe analysis. The separation coefficient k was also found to vary with x. It is close to unity below x = 0.001 and above this value Cr is incorporated more easily. High quality heteroepitaxial layers (1 cm² area, 1 to 30 μm thickness) of V_1−xCr_xO₂ have for the first time allowed the measurement of the optical absorption coefficient.     

XAS study of lanthanum coordination environments in glasses of the system K2O---SiO2---La2O3

The La L₁ and L₃ XANES and L₃ EXAFS have been investigated for the series of glasses 10K₂O---50SiO₂---x La₂O₃ (x = 1, 5, 10) and (10 − x)K₂O---40SiO₂−(x/3)La₂O₃ (x = 7.5, 5, 2.5) and model compounds La₂O₃, LaAlO₃, LaPO₄, La₂NiO₄, La₂CuO₄ and La(OH)₃. An edge resonance at 25 eV above the L₁ edge in the glass spectra is concentration-dependent, decreasing in intensity with increasing lanthanum concentration. The 2s → nd forbidden transition increases with La₂O₃ concentration, indicating a reduction in the ‘average’ site symmetry of the first coordination shell of La. Mapping _X(k) space, which is a new and promising technique, was employed to extract bond distance, coordination number and thermal parameters from the EXAFS. By this method, one calculates the complete _X(k) space a function of all physically reasonable values of the adjusted parameters in all possible combinations. The advantage in this method is the assurance of a global minimum. Bond lengths were comparable to those obtained by Fourier transforming the phase corrected EXAFS. The values are 2.42 Å (± 0.03 Å) for La---O. The coordination numbers (N ≤ 7 ± 1.5) were derived by mapping and comparison to the published structures for other La compounds. _X(k) mapping is compared with least-squares fitting the data, and the correlation between the Debye-Waller factor and coordination number is also discussed.     

Nuclear magnetic resonance studies of alkaline earth phosphosilicate and aluminoborosilicate glasses

The ¹¹B, ²⁷Al, ²⁹Si and ³¹P magic angle spinning (MAS) NMR spectra of MO–P₂O₅, MO–SiO₂–P₂O₅ and MO(M^′₂O)–SiO₂–Al₂O₃–B₂O₃ (M=Mg, Ca, Sr and Ba, M^′=Na) glasses were examined. In binary MO–P₂O₅ (M=Ca and Mg) glasses, the distributions of the phosphate sites, P(Q_n), can be expressed by a theoretical prediction that P₂O₅ reacts quantitatively with MO. In the ternary 0.30MO–0.05SiO₂–0.65P₂O₅ glasses, the 6-coordinated silicon sites were detected, whose population increases in the order of MgOxCaO–0.05SiO₂–(0.95−x)P₂O₅ glasses, its population increases with an increase in f (=([P₂O₅]−[MO]−[B₂O₃]−[Na₂O])/[SiO₂]) and has maximum at f=9. The signal due to the 5-coordinated silicon atoms is also observed when x is smaller than 0.45. When three network-forming oxides such as SiO₂, Al₂O₃ and B₂O₃ coexist, Al₂O₃ reacts preferably with MO. The populations of 4-coordinated boron atoms, N₄, are expressed well with r/(1−r), where r=([Na₂O]−[Al₂O₃])/([Na₂O]−[Al₂O₃]+[B₂O₃]). The correlation of the Raman signal at 1210 and 1350 cm⁻¹ with the NMR signal of Si(Q₆) at −215 ppm is also seen.     

New solvents for the growth of substituted BaFe12O19 single crystals from high-temperature solutions

The solubility of a series of hexaferrite derivatives of BaFe₁₂O₁₉ in solvents of the system Na₂O-B₂O₃ with oxide ratios of 7:3 and 3:2 has been investigated. The temperature dependences of the saturation concentration in these solvents are determined for ferrites with the nominal compositions Ba_0.8Pb_0.2Fe₁₂O₁₉, BaFe₁₀Ga₂O₁₉, BaFe₁₀Al₂O₁₉ and BaFe₈Mn₂ Ti₂O₁₉. Single crystals of BaFe₁₂O₁₉, in which part of the metal ions are replaced by various amounts of Pb²⁺, Ga³⁺, Al³⁺ and Mn²⁺ + Ti⁴⁺ ions, are g rown from the solutions by the slow cooling technique. The distribution coefficients of the substituting ions and the compositions of the crystals obtained are established by microprobe analysis (EPMA). Information on the position of substitution is obtained from the Mössbauer spectra.