Influence of Strain Energy on Kinetics of Ion Exchange in Glass

The kinetics of the K⁺⇌ Na⁺ exchange in Na₂O·3SiO₂ glass at 350°C were studied. The distribution of K in the glass specimens after exchange in molten KNO₃ was determined with an electron microprobe. The calculated interdiffusion coefficients, which varied with the local composition in the glass, were compared with those obtained using the Nernst-Planck equation and Fleming and Day's self-diffusion data for xNa₂O. (1-x)K₂O·3SiO₂ glasses. Deviations of the observed interdiffusion coefficient from the Nernst-Planck formula were noted. These deviations are explained on the basis of changes in the average size of the alkali ion sites and in the strain energy that occur during ion exchange.     

Preparation and Properties of Quenched Li2O-BaO-Nb2O5 Glasses

Glasses were prepared in the wide variety of compositions in the system Li₂O-BaO-Nb₂O₅, which contains no "glass-former," by the twin-roller quenching technique. The crystallization temperature ranges from 500° to 700°C and increases as Li₂O is replaced by BaO. The dielectric constant increases steeply with temperature, reaching an anomalously large value (10⁴) in glasses which contain a large amount of Li₂O. Such a temperature dependence of dielectric constant is very similar to that of conductivity in each glass, indicating that the dielectric behavior is caused by polarization due to the ionic motion in glass. The conductivity at 500 K ranges from 10⁻¹³ to 10⁻⁵Ω⁻¹·cm⁻¹ and the activation energy for conduction from 120 to 40 kJ/mol. The concentration of Li⁺ ions mainly controls the conductivity and the addition of Ba²⁺ ions slightly lowers it.     

Copper ⇌ Alkali Ion Exchange of Alkali Aluminosillcate Glasses in Copper-Containing Molten Salt: I, Monovalent Copper Salt, CuCl

Cu⁺⇌ R⁺ (R = Li, Na, and K) ion exchange experiments were conducted for 20R₂O·10Al₂O₃·70SiO₂ glasses in molten CuCl at 550°C in air and nitrogen atmospheres. The depth profiles of the copper incorporated into glasses were determined with an electron microprobe X-ray analyzer. The total amount of diffusing copper, M _t, strongly depended on the type of alkali ion in the glass and the ion-exchange atmosphere; i.e., M _t increased with increasing cationic size in the order Li < Na < K and M _t was greater in air than in nitrogen. The Cu ⇌ R⁺ ion exchange kinetics are discussed in detail.     

AC Conductivity and Electrical Modulus Studies on Lithium Vanadophosphate Glasses

The lithium vanadophosphate (LVP) glasses were synthesized through a conventional melt quench method for various modifier ( m ) to formers ( f ) ratios ( m / f ) [ m %Li₂O/ f %{0.3V₂O₅+0.7P₂O₅}, where the m / f =0.43, 0.66, 1.0, 1.5, and 1.63]. Nature, structure, and glass transition T _g temperature for the prepared LVP samples were characterized by X-ray diffraction, FTIR, and DSC techniques, respectively. Impedance measurements were made for the various m / f ratios of LVP glasses at 423 K and above temperatures. The bulk conductivity (σ) and the activation energy ( E _a) for the LVP glass samples were calculated, respectively, from the analyzed impedance data of the various m / f ratios measured at different temperatures, using Boukamp equivalent circuit software. The best conducting [σ=4.83 × 10⁻⁸ S/cm at 423 K] m / f ratio of LVP was found to be 60%Li₂O–40% [0.3V₂O₅+0.7P₂O₅] and its activation energy ( E _a)=0.59 eV. AC conductivity was calculated from the impedance data and analyzed using Jonscher's power law for various m/f ratios of the LVP glasses at different temperatures. The power law exponent s , evaluated from AC conductivity of the LVP glasses, exhibited a non-linear behavior with temperatures. Kohlrausch–William–Watts stretched exponential function was used to fit the calculated modulus data and ion relaxation behavior was studied for the LVP glasses.     

Copper ⇌ Alkali Ion Exchange of Alkali Aluminosilicate Glasses in Copper-Containing Molten Salt: II, Divalent Copper Salts, CuCl2 and CuSO4

The ion-exchange mechanism between copper and alkali ions, when 20R₂O · 10Al₂O₃· 70SiO₂ (R = Li, Na, and K) glasses are immersed in divalent copper-containing molten salts in air and nitrogen at 550°C, has been investigated. In molten CuCl₂, the ion-exchange behavior in both air and nitrogen was very close to that in molten CuCl in air reported previously. This is explained by assuming that CuCl₂ decomposes into CuCl and Cl₂ at 550°C and the Cu⁺ ions thus formed mainly diffuse in glasses to replace alkali ions, where Cl₂ acts as an oxidizing agent just like oxygen. In the case of molten CuSO₄─₂SO₄, a small amount of Cu⁺ which is present in the molten state plays a primary role in the Cu ⇌ R⁺ ion exchange process, although the contribution of direct Cu²⁺⇌ 2R⁺ ion exchange cannot be ignored.     

Kinetics of Crystallization in Li2O-SiO2 Glasses

The kinetics of crystallization of Li₂Si₂O₅ from glasses in the Li₂O-SiO₂ system were studied using quantitative X-ray diffraction. Analysis of the data using the Johnson-Mehl-Avrami equation showed that crystallization occurred through the nucleation and growth of rods. The spherulitic nature of the crystals was substantiated by petrographic examinations of the partially crystalline glass. Analysis of the temperature dependence of the crystallization rate using a modification of the Johnson-Mehl-Avrami equation showed that the activation energy for nucleation was a function of composition and thermal treatment.     

Crystallization of Whitlockite from a Glass in the System CaOP2O5SiO2MgO

The kinetics of crystallization of a glass with composition 30SiO₂·24.14P₂ O₅·28.61CaO·17.25MgO (wt%) was studied with variable temperature, by differential thermal analysis. These results were interpreted according to revised theoretical models which describe the dependence of fraction crystallized on temperature and on the heating rate, and the dependence of the peak temperature on the rate of change in temperature. These models emphasize the differences in crystallization behavior between "as-prepared" glass samples, and the corresponding previously nucleated samples. These differences in behavior suggest that the kinetics of crystallization comprises a nucleation stage at temperatures of about 730°C, and particle growth for temperatures higher than 800°C. The temperature dependence of the fraction crystallized corresponds to a very high value of activation energy for growth (≈763 kJ/mol), but this is still lower than the activation energy evaluated from the temperature dependence of viscosity.     

Crystallization Kinetics of Lithia-Silica Glasses: Effect of Composition and Nucleating Agent

The kinetics of crystallization of a 25Li₂O · 75SiO₂ (wt%) glass doped with Pt, Cu, Au, TiO₂, and P₂O₅ were studied using nonisothermal techniques. The activation energy, E , and the frequency factor, v , for the overall crystallization process depended directly on the critical cooling rate for the glass formation, R_c , of these glasses. The crystallization kinetics of several other glasses in the lithia-silica system were also studied as a function of composition, critical cooling rate, and liquidus temperature, T_m. E and v for these glasses were more dependent on the liquidus temperature than on R_c .     

Bismuth Phosphate Glasses

Homogeneous glasses are formed in the Bi₂O₃-P₂O₅ system up to 35 mol% (63.8 wt%) Bi₂O₃. In property vs composition plots, the thermal expansion coefficient and tan δ exhibit minima, and hardness and activation energy for conduction show maxima at about 20 mol% Bi₂O₃. The deformation temperature of the glasses also increases abruptly at this composition. This anomalous behavior is interpreted in terms of a change in the function of Bi³⁺ ions from network formers to network modifiers.     

Two Peaks in the Internal Friction as a Function of Temperature in Some Soda Silicate Glasses

Measurements of the internal friction of the glasses 17% Na₂O-83% SiO₂, 25.6% Na₂O-74.4%, SiO₂, and 34% Na₂O-66% SiO₂ were made as a function of temperature, composition, and frequency in the range 1 to 3 cycles per second and from –90° to 500°F. The internal friction as a function of temperature revealed two distinct peaks for each glass tested. One peak occurred in the temperature range −55° to +50°F. and the other in the range 300° to 500°F. All the observed peaks were found to be best represented by an assumed Gaussian distribution of heats of activation with the high-temperature peaks exhibiting a much wider distribution than the low-temperature peaks. It was therefore concluded that none of the peaks was a manifestation of a single heat of activation. The temperatures at which all peaks occurred increased with increasing frequency. From this shift of "peak" temperatures with frequency, the most likely heats of activation associated with each peak were calculated. The temperature dependence of the dynamic shear modulus for each glass was used to calculate the "relaxation strength" of the relaxation process for each peak. An analysis of all these data strongly indicated that the high-temperature peaks as well as the low-temperature ones were related to cooperative action among sodium ions.     

Kinetics of Enthalpy Relaxation at the Glass Transition in Ternary Tellurite Glasses

The kinetics of enthalpy relaxation (recovery) at the glass transition in x K₂O·(20− x )MgO·80TeO₂ glasses has been examined from heat capacity measurements using differential scanning calorimetry to clarify the features of the structural relaxation in ternary TeO₂-based glasses. Ternary glasses such as 10K₂O·10MgO·80TeO₂ show high thermal resistance against crystallization compared with binary glasses. The degree of fragility m estimated from the activation energy for viscous flow E _η and the glass transition temperature T _g is m = 55–62, indicating a fragile character in TeO₂-based glasses. Large heat capacity changes of 43.1–48.2 J·mol⁻¹·K⁻¹ are also observed at the glass transition. The activation energy for enthalpy relaxation Δ H is evaluated from the cooling rate dependence of the limiting fictive temperature, and values of Δ H = 897–1268 kJ·mol⁻¹ are obtained. Negative deviation from additivity in Δ H is also observed. Values of the Kovacs–Aklonis–Huchinson–Ramos (KAHR) parameter θ estimated from Δ H and T _g are 0.33–0.42 K⁻¹. It has been proposed that ternary glasses have more homogeneous and constrained glass structure compared with binary glasses.     

Magnetic Properties of an Amorphous Antiferromagnet

The magnetic and physical properties of the amorphous antiferromagnetic MnO-P₂O₅ glass system were investigated. The glasses were found to be microstructurally homogeneous and amorphous within the detection limit of vacuum Guinier-De Wolff X-ray tenchiques. The magnetic properties of the glasses are interpreted in terms of antiferromagnetically coupled Mn²⁺ ions, which experience negative exchange interactions with 1,2, or 3 near neighbors, and structurally isolated Mn²⁺ ions, which experience weak exchange interactions. The ions which experience weak exchange interactions produce a downward curvature of the inverse-susceptibility-vs-temperature plot at low temperatures. The magnetization-vs-field data exhibit curvature toward the field axis, and the magnetization increases as the temperature is decreased to 1.73°K. Below this temperature, the magnetization becomes weakly temperature-dependent, and the M-vs-H curves almost superimpose. These results are interpreted in terms of a 2-spin Heisenberg model of antiferromagnetism.     

Leaching of Lead and Connectivity of Plumbate Networks in Lead Silicate Glasses

The formation of a plumbate network in binary lead silicate glasses was examined based on the leaching behavior of Pb²⁺ in lead silicate glasses over a wide composition region. The effective diffusion coefficient of Pb²⁺ at 40°C was on the order of 10⁻¹⁷ m²/s for PbO<35 mol% glasses, and increased three orders of magnitude for 35–50 mol% PbO contents. Such a steep composition dependence is considered to be because of changes in the medium or longer range structure. That is, it is proposed that the plumbate network forms a percolative 3D network in the composition region to form diffusion paths for the lead ions. In addition, the present results indicate that the lead ion exists as a network former over the entire glass forming composition range of the binary silicate glasses.     

Redox Equilibria of Multivalent Ions in Silicate Glasses

Experimental studies were made on the compositional dependence of the redox equilibrium of Eu in synthetic silicate liquids, together with an empirical model describing the observed compositional dependence. Electron paramagnetic resonance (EPR) was used to measure the concentration ratio of Eu²⁺ to Eu³⁺ in various glasses formed by rapidly quenching silicate liquids. The compositional field studied comprised mixtures of SiO₂, TiO₂, Al₂O₃, CaO, MgO, and Na₂O. The proposed model describes the Eu²⁺/Eu³⁺ ratio over the entire compositional field in terms of parameters easily related to each glass composition. The general applicability and utility of the model is further demonstrated by its application to the Fe²⁺-Fe³⁺, Ce³⁺-Ce⁴⁺, and Cr³⁺-Cr⁶⁺ redox reactions in binary alkali oxide silicate glasses of Li, Na, and K.     

Kinetics of Nonisothermal Crystallization of Bi2Sr2CaCu2Ox Glasses with Different Copper Valence States

The kinetics of the nonisothermal crystallization process in Bi₂Sr₂CaCu₂O_x glasses with different copper valance states, R(Cu⁺) = Cu⁺/(Cu⁺+ Cu²⁺) = 0.79–0.99, were examined by using differential scanning calorimetry. Four different kinetic equations were used for the data analyses. It was found that the values of activation energy for crystal growth, E _a, decreased with increasing Cu⁺ content. The value of E _a, which was estimated from the modified Ozawa equation, for the glass with R(Cu⁺) = 0.79 was 483 kJ·mol^-1 and for the glass with R(Cu⁺) = 0.99 was 353 kJ·mol^-1. In the former glass, the number of crystal nuclei was almost independent of the heating rate. But, in the latter glass, the number of crystal nuclei varied significantly with the heating rate. The crystallization mechanism in the glasses was closely related to the thermal stability and viscosity of glasses: the glass with R(Cu⁺) = 0.99 showing a high thermal stability and a low activation energy for viscous flow had a small value of E _a. The plot of In ( T _p²/α) against 1/ T _p, where T _p and α are crystallization peak temperature and heating rate, respectively, gave a reasonable value of E _a for the glass with R(Cu⁺) = 0.79 but was unsuitable for the evaluation of E _a for crystal growth of the glass with R(Cu⁺) = 0.99.     

Electrical Resistivity of Titanium-Containing Glasses

Titanium-containing glasses were prepared by fusion of a base glass (BaO·B₂O₃SiO₂) and TiO₂ and/or Ti₂O₃ in Ar. Their resistivities did not vary with melting time and temperature. Interaction of Ti⁴⁺ and Ti³⁺ in the glasses was deduced by spectroscopy, but the valence states in the batch compositions were preserved in the glasses, according to the chemical analysis. Glasses containing either Ti⁴⁺ or Ti³⁺ had very high resistivities, whereas the glass prepared by melting a mixture of a Ti⁴⁺-containing and a Ti³⁺-containing glass had much lower resistivity. All results confirmed the possibility of controlling the resistivity by batch composition for these glasses.     

Mössbauer Spectroscopy of Borate Glasses Containing Divalent Europium Ions

Borate glasses containing 0.99–30.0 mol% EuO have been prepared under a reducing atmosphere and ¹⁵¹Eu Mössbauer effect measurements have been carried out at room temperature in order to examine the chemical state of Eu²⁺ ions in these glasses. Mössbauer spectra indicate that most of the europium ions are present as the divalent oxidation state. In dilute Eu²⁺-containing borate glasses, spectra are split due to paramagnetic hyperfine interactions in the glasses with low and high sodium contents. In concentrated Eu²⁺-containing borate glasses, line broadening results from the contribution of quadrupole splitting due to asymmetrical oxygen coordination around Eu²⁺ and the contribution of inhomogeneous broadening due to site-to-site variation. The compositional dependence of isomer shift and quadrupole interaction parameter can be related to the structural changes in the borate glass. The variation of the isomer shift has a good correlation with the optical basicity of glass, and the trend can be explained in terms of the covalent admixture with 6 s character. The Eu-O bond in the borate glasses is more ionic than that in EuTiO₃ and EuZrO₃, where the oxygen coordination number for Eu²⁺ is 12. The average coordination number for Eu²⁺ is found to be 12 in all of the present glasses.     

Compositional Dependence of Absorption Spectra of Ti3+ in Silicate, Borate, and Phosphate Glasses

Absorption spectra of Ti³⁺ were measured for silicate, borate, and phosphate glasses doped with 0.5 mol% Ti₂O₃.The absorption coefficient at the peak wavelength of the ₂T₂→₂E transition of Ti³⁺ is used as a parameter showing the relative content of Ti³⁺ions in glass samples. The effect of glass composition on Ti³⁺/Ti⁴⁺ redox was studied. For multicomponent glasses, a basicity parameter calculated from glass composition is proposed in terms of coulomb force between the cation and the oxygen ion. The value of the absorption coefficient depends on basicity in silicate and borate glasses; however, it is independent of composition in phosphate glasses.     

Influence of Dopants on Nucleation and Growth of High-Quartz Solid Solution in Lithium Aluminosilicate Glass

The kinetic parameters of nucleation and crystal growth of high-quartz solid solution in multicomponent lithium aluminosilicate glasses doped with various transition-metalions were studied by nonisothermal DTA. The crystallization of glasses nucleated at different temperatures was carried out, and plots of the DTA peak versus the nucleation temperatures were used to determine the maximum nucleation rate temperature. Peak temperature data of nucleated samples at varying heating rates (5–20 K/min) were used to determine the activation energy for crystallization via the JMA equation. The temperature of maximum nucleation rate depends greatly on the doped transition- metal ions present. The activation energy for crystallization obtained for undoped glass or glasses doped with Fe₂O₃ is of the same order as that already published, and the Avrami exponent is consistent with predominantly three-dimensional crystal growth. The much higher activation energy values for glasses doped with CoO could be a consequence of two crystallization processes proceeding simultaneously.     

Stannous-Stannic Equilibrium in Molten Binary Alkali Silicate and Ternary Silicate Glasses

The stannous-stannic equilibrium in binary alkali silicate and ternary silicate glasses was studied by equilibrating glassmelts with air at 1400°C. The Sn²⁺-Sn⁴⁺ equilibrium shifts more toward the oxidized state with increasing ionic radii of the alkali ions or with increasing concentration of the alkali ions in the same series of glasses. The slope of the straight lines obtained on plotting log (Sn⁴⁺)/(Sn²⁺)( pO₂ )^n/2 vs mol% R₂O increased in the order Li→Na→K. In ternary silicate glasses having the base glass composition 20Na₂O·10RO·70SiO₂, the Sn²⁺-Sn⁴⁺ equilibrium shifts more toward the reduced state, with increasing bond strength between the divalent cations and the nonbridging oxygens. With increasing temperature, the equilibrium shifts more toward the reduced state.