Thermally Stimulated Currents in Sodium Silicate Glasses

Sodium-ion motion in three sodium silicate glasses and a sodium aluminosilicate glass was investigated by the thermally stimulated polarization (TSPC) and depolarization (TSDC) current techniques. The two TSDC peaks found in the sodium silicate glasses were attributed to localized sodium-ion movement around a nonbridging oxygen ion, a type of dipolar orientational polarization, and to a longer-range sodium motion leading to interfacial polarization at the immiscible phase boundaries. The high-temperature background (HTB) current corresponded to the sodium motion for dc conductivity and diffusion. The commonly observed dc absorption current was found to be related to the two TSDC peaks.     

Thermally Stimulated Polarization and Depolarization in Halogen-Containing Lead Silicate Glasses

Thermally stimulated polarization and depolarization current (TSPC/TSDC) measurements were made on lead halide silicate glasses having compositions (65 − x )PbO· x PbX₂·35SiO₂ where x = 0, 0.1, 2, 10 and X = F, CI, Br, and I. The addition of halogen ions to lead silicate glasses gives rise to a new high-temperature TSDC peak in the vicinity of the peak previously observed in binary lead silicate glasses. The integrated area of the new peak is dependent on the amount and type of halogen ion present in the glass and does not saturate in the temperature range of our measurements. This new peak is attributed to space charge polarization of halogen ions.     

Sodium Motion in Phase-Separated Sodium Silicate Glasses

The thermally stimulated polarization and depolarization current (TSPC/TSDC) technique was used to study Na^plus; ion motion in phase-separated silicate glasses containing between 10.3 and 20 mol% Na₂O. Generally, two TSDC peaks and a high-temperature background (HTB) current were observed. The two TSDC peaks were attributed to different types of Na^plus; motion, i.e. a short-range reorientation process and a longer-range translational motion. Changes in the microstructure affected only the size of the larger TSDC peak located at higher temperatures.     

Substructures in Silicate Glasses

Glass compositions are correlated with published data for refractive index, specific volume, and fluidity, which indicate the presence of different structural types in silicate glasses. These structural types are shown to be uniquely related to the composition areas of primary crystallization phase fields. It is suggested that these different structural types are to be understood as silicate framework structures or "substructures" in which other appropriate cations may be accommodated. The described data relations definitely establish the presence of different recognizable substructures in silicate glasses but do not permit judgments on their detailed characteristics.     

Thermally Stimulated Depolarization Current

Abstract

Thermally stimulated discharge or thermally stimulated polarization and depolarization current is becoming a valuable technique in discerning transitions having a small relaxation strength. Polymer liquid crystals (PLCs) are a group of such materials. This article provides a review of the different experimental techniques associated with the technique and their application to PLCs. Experimental results for thermotropic longitudinal PLCs are presented.     

Photoelastic Effects in Silicate Glasses

The photoelasticity of R_nO_mSiO₂(R_nO_m=Li₂O, Na₂O, K₂O, and ZnO), Li₂O-Al₂O₃-SiO₂, Na₂O-Al₂O₃-SiO₂, and Na₂O-ZnO-SiO₂ glasses was analyzed on the basis of the following equation relating the stress-optical coefficient and the strain-optical coefficient:
where C is the stress-optical coefficient, n is the refractive index, G is the shear modulus, pI2and pI1are the Pockels strain-optical coefticients, and p and q are the Neumann strain-optical coefficients. No simple relation was found between the stress-optical coefficient and the shear modulus, and the factor (p−q) was found to be an essential factor deter- mining the stress-optical coefficient of silicate glasses. It was found that (p−q) decreases markedly with increasing content of modifier oxide, and that the change of (p − q) with composition is governed by the change of the atomic effect, while the lattice effect does not change as much. It is argued that the bridging oxygen with covalent bonds increases the atomic effect and the nonbridging oxygen with ionic bonds reduces the atomic effect.     

Na-K Exchange in Silicate Glasses

Electrolytic Na-K displacement in commercial sheet glass was studied. Concentration profiles and compressive surface stresses were determined.     

Microstructure in Hydrated Silicate Glasses

A microstructure ranging from 5 to 20 nm in size was detected by small-angle X-ray scattering in Na₂O·3SiO₂ and 4EaO·28Na₂O·68SiO₂ glasses hydrated at ∼100° and 80°C, respectively. This observation suggests the presence of silica gel-solvent (water) phase separation.     

Microwave Dielectric Properties and Thermally Stimulated Depolarization Currents of MgF2‐Doped Diopside Ceramics

A new low‐fired dielectric material derived from CaMg_0.9Zn_0.1Si₂O₆ (CMZS) ceramics with high quality factor was synthesized by solid‐state reaction method. The effects of MgF₂ addition on the sinterability, phase composition, crystal defects, and microwave dielectric properties of CMZS were investigated. MgF₂ was proved not only to lower the sintering temperature to ~1000°C but also to remarkably modify the microwave dielectric properties of CMZS. In addition to the main diopside phase, forsterite was identified as the secondary phase in all MgF₂‐doped samples. Dielectric temperature spectra showed that MgF₂ induced significant dielectric relaxations associated with oxygen vacancy defects to CMZS. Thermally stimulated depolarization current was, therefore, considered to obtain the defects associated with extrinsic microwave dielectric loss mechanisms. Compared with undoped CMZS, although the concentration of oxygen vacancies showed a notable increase in the 5 wt% MgF₂‐doped CMZS, the Q×f values were still improved. Here, with proper MgF₂‐doping, it demonstrated that the microwave dielectric loss was basically influenced by phase composition. The excellent characteristics of ε_r = 7.78, Q×f = 151 800 GHz, and τ_f = ?26.40 ppm/°C were achieved from the 5 wt% MgF₂‐doped specimens sintered at 1000°C.     

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.     

Structure Transition in Alkali Silicate Glasses

The logarithm of measured high-temperature fluidities is related to the square of the calculated probability of finding a nonbridging oxygen in a randomly chosen SO4 group. This is interpreted in terms of a flow mechanism (gliding) dependent on juxtaposition of two such oxygens. A theory of the glass transformation is developed. On cooling, the coordination number of nonbridging oxygens increases until, ideally, at T , each touches three oxygens of an external Si04 group (stabilization). Below this temperature, thermal expansion/contraction contains no coordination change component. These changes constitute a second-order transition. Observed volume data are in agreement with hypothesis. On this view, stabilization creates vacant tetrahedral environments adjacent to existing ones. Migration of glass-forming ions to these empty sites can unlock the structure and permit the operation of a new set of flow processes. The coordination requirements for stabilization severely restrict the types of local arrangement possible at high silica contents, and therefore reduce the configurational entropy and induce unmixing. The overall transformation is composite in character. Rather extensive bond rearrangement and unmixing (when present) are features of first-order kind.     

Ionic Conductivity in Calcium Silicate Glasses

Glasses in the system CaO–SiO₂ were prepared with the composition varying from 40 to 55 mole % CaO. The Na₂O impurity content ranged from 0.01 to 1.30 mole %. The electrical resistivity was insensitive to the Na₂O content but decreased with increasing CaO concentration. The activation energy for conduction ranged from 33.54 to 31.23 kcal/mole as the CaO concentration increased. The conduction mechanism was considered in terms of three arbitrary parameters: (1) equivalent oxygen packing density, (2) ionic radius, and (3) ion-oxygen attraction.     

Undulating Structure in Complex Silicate Glasses

Complex silicate glasses (e.g. commercial plate, sheet, and ophthalmic compositions), mica, and fused SiO₂ were carefully examined in the electron microscope using Pt-C replicating techniques. The complex glasses show an undulating structure at 300 to 500 Å, whereas the mica and fused SiO₂ do not. The low contrast observed between peak and valley is consistent with a small difference in chemical composition. The observed structure is explained in terms of an extension of the anionic model for molten silicates whereby the peaks and valleys result from clustering of silicate anions accompanied by partial expulsion of cations to the periphery.     

Viscoelastic Indentation of Silicate Glasses

The time-dependent viscoelastic deformation and flow of various types of silicate glasses are examined by the use of a pyramidal Berkovich indenter. It is demonstrated that a pyramidal indenter is an efficient microprobe for viscoelastic studies of glass-forming materials at temperatures near the glass transition point. Some important rheological functions of silicate glasses are determined as functions of time on the basis of a linear viscoelastic constitutive equation for pyramidal indentation. The test results are theoretically related to the dimension of flow units of these glasses, suggesting that a model of thermally fragmented silicate clusters is appropriate for consistently understanding the present rheological test results. However, the microstructural details (crystallite-like or network-like microstructures) of fragmented clusters are not inferred from the rheological information.     

Substructure Classification of Silicate Glasses

It is proposed that silicate glasses be classified in terms of substructures uniquely identified with primary crystallization phases. All glasses having compositions within the ranges or areas covered by a given primary crystallization phase field should constitute a separate glass type or class. This classification is definitive for properties determined principally by the silica frameworks or substructures. Such composition-property relations can be reproduced by independent investigators if the properties are measured under temperature equilibrium conditions.     

Ionic Conduction in Alkali and Thallium Silicate Glasses

Electrical conductivities of binary alkali and thallous silicate glasses have been measured as a function of composition, temperature, and frequency. The best approximation to the activation energy for dc conduction can probably be obtained by extrapolating its frequency dependence to zero frequency, although values obtained at frequencies below 2500 hz do not differ greatly from the dc values. The plot of activation energy for conduction against modifier content consists of two straight lines of different slope. For all the alkali silicate glasses the break in slope occurs near 25 mole % alkali oxide and at an activation energy near 14.7 kcal/mole. The slope of the line below 25 mole % alkali oxide is proportional to the alkali ion radius. The behavior of thallous silicate glass is similar but not identical to that of the alkali silicates. It is concluded that (1) the reported conduction behavior does not result primarily from phase separation, (2) the principal contribution to the activation energy is probably the work required for a mobile ion to pass through the glass network rather than to leave its initial position, and (3) a structural change independent of the nature of the modifier cation occurs near a modifier content of 25 mole %.     

Diffusion and Internal Friction in Na-Rb Silicate Glasses

The internal friction and self-diffusion coefficients of sodium and rubidium ions for (l-X)Na₂O-XRb₂O-3SiO₂ glasses were measured. Diffusion was measured by radioactive isotopes and a thin-sectioning technique at 350° to 500°C. Internal friction was measured at −140° to 500°C and 0.05 to 6000 Hz. The maximum height for the mixed-alkali internal friction peak occurs at the composition at which the Na and Rb diffusion coefficients are equal. It is concluded that the mechanism responsible for the mixed-alkali peak is a coupled, cooperative rearrangement of sodium and rubidium ions, with the slower moving ion controlling the rate of rearrangement. This mechanism should also apply to other mixed-alkali silicate glasses since the internal friction for these systems is similar.