Structural,thermal, dielectric and ac conductivity properties of lithium fluoro-borate optical glasses

Transparent and stable glasses in the chemical composition of Li₂O–LiF–B₂O₃–MO (M = Zn and Cd) have been prepared by a conventional melt quenching method. For these glasses, absorption spectra, structural (XRD, FT-IR, and Raman spectra), thermal (TG–DTA and DSC), dielectric (?′, ?″, tan δ), ac conductivity (σ_ac), and electric modulus (M′ and M″) have been investigated. Amorphous nature of these glasses has been confirmed from their XRD profiles. The LFB glasses with the presence of ZnO or CdO an extended UV-transmission ability has been achieved. The measured FT-IR and Raman spectra have exhibited the vibrational bands of B–O from [BO₃] and [BO₄] units and Li–O. The dielectric properties (tan δ, dielectric constant (?′), dielectric loss (?″)), electrical modulus and electrical conductivity (σ_ac) of these glasses have also been studied from 100 Hz to 1 MHz at the room temperature. Based on the trends noticed in the ac conductivities, the present glasses could be found useful as battery cathode materials.     

Structural and proton conductivity study of P2O5-TiO2-SiO2 glasses

Proton conducting membranes for hydrogen fuel cell applications are being developed using sol–gel-derived P₂O₅-TiO₂-SiO₂ glasses. The present work is devoted to the structural analysis of P₂O₅-TiO₂-SiO₂ glasses containing controlled pore sizes and the surface area of the glasses reaching a maximum 410 m²/g, while the average pore diameter is about 2.4 nm. They were characterized by nitrogen adsorption–desorption measurements, thermal measurement as well as FTIR spectroscopy. The structural studies were based on the evolution of the intensities and profiles of certain characteristics bands in the FTIR spectra. In order to increase proton conductivity at low relative humidity, the result suggests that the mobility of proton in the P₂O₅-TiO₂-SiO₂ glasses increases with decreasing OH bonding strength. An H₂/O₂ fuel cell was constructed using the P₂O₅-TiO₂-SiO₂ glasses as electrolyte and Pt/C-loaded carbon paper sheets as electrodes. Results show that the performance of the cell is an output power of 88 μW/cm² at 30 °C with humidity.     

Structural interpretation of concentration dependences of molar volume of glasses in R2O-SiO2 and RO-SiO2 systems

A method is considered for structural interpretation of concentration dependences of properties of binary silicate glasses (with the example of glass molar volume) based on establishing a correlation between the values of a certain property and the degree of depolymerization of the silicon-oxygen skeleton (structural lattice) of glassR* which reflects the number of non-bridge oxygen ions per silicon-oxygen tetrahedron. Partial values of molar volumes are determined for a series of oxides of the first and second groups of the periodic system, which preserve their constant values over the entire glass-formation area. The estimate error in calculating the molar volume of glasses according to the proposed method is around 1%.     

Dielectric polarizability of SiO2 in niobiosilicate glasses

Understanding the mechanisms contributing to dielectric properties of glasses is critical for designing new compositions for microwave frequency applications. In this work, dielectric permittivity was measured using a cavity perturbation technique at 10 GHz for a series of niobiosilicate glasses with the compositions (100-2x)SiO₂- xNb₂O₅- xLi₂O where x = 32.5, 30, 25, and 15 mol%. Permittivity measurements and glass compositions were used to calculate the polarizability of each cation-anion unit in the glass network using the Clausius-Mossotti equation. The SiO₂ polarizability in niobiosilicates was calculated to be 6.16 ų, which is much higher than the SiO₂ polarizability in fused silica glass (5.25 ų), alkali modified silicates (5.37 ų), and aluminosilicates (5.89 ų). The increasing trend in SiO₂ polarizability is attributed to the disruption in the connectivity of the SiO₄ tetrahedral network as it accommodates different network formers. The high SiO₂ polarizability of 6.16 ų accurately predicts measured dielectric permittivity when Nb₂O₅ = 25, 30, and 32.5 mol%, but overpredicts measured permittivity when Nb₂O₅ ≤ 15 mol%, which is attributed to a decrease in SiO₂ polarizability as the percentage of corner sharing SiO₄ tetrahedra with NbO₆ octahedra goes down. This work demonstrates that SiO₂ polarizability depends on chemistry and connectivity of the glass, which has important implications in designing glass compositions for microwave frequency applications.     

Dielectric relaxation and ac conductivity in magnetoelectric YCrO3 ceramics: A temperature dependent impedance spectroscopy analysis

Here, we report on the temperature and frequency dependent electrical conduction and dielectric behaviour of YCrO₃ ceramics. Dielectric studies reveal a peak in the dielectric constant ～230?K, suggesting presence of spin-charge coupling. Also, an additional broad peak found at ～450?K is reminiscent of a relaxor like behaviour for YCrO₃, attributed to a diffused phase transition. The nature of dc conductivity is of Arrhenius type and shows an abrupt change in the activation energy at ～230?K and ～450?K. The activation energy suggests that the polaronic hopping mechanism stabilizes at low temperature while, at higher temperatures, the process is associated with the diffusion of double ionized oxygen vacancies. However, ac conductivity suggests that the overlapping large polaron tunnelling conduction mechanism drives the ac conduction below 300?K and above 300?K, the conduction behaviour is consistent with the correlated barrier hopping conduction mechanism.     

Structure and electrical conductivity of Li2O-B2O3 glasses

The temperature-concentration dependence of the electrical conductivity of Li₂O-B₂O₃ glasses in the temperature range of ～180–310°C has been studied. For pure boron anhydride, the dependence logσ = f([1/T]) is linear, whereas for glasses with ～2 mol % ≤ [Li₂O] < ～10 mol %, similar curves are kinked. At higher Li₂O concentration the kinks disappear. Occurrence of kinks is attributed to variation of essence of current carriers from proton pattern for B₂O₃ to mixed proton-ion pattern for low-alkali glasses. Conductivity of glasses at [Li₂O] ≥ 20 mol % is stipulated by the formation of a continuous sublattice of polar structuralchemical entities (entities) [BO_4/2]^?Li⁺ and the migration of lithium ions.     

Dielectric characteristics of polyimide CP2

High-performance aerospace-grade polyimides such as CP2 fulfill many important roles in a wide range of applications. A thorough understanding of the polymer matrix’s physiochemical properties is an important consideration when developing polymer nanocomposite materials. In this work, we report the dielectric properties of polyimide CP2 including the primary and two secondary dipole relaxations, and their thermal characteristics by way of temperature variable impedance spectroscopy (10⁻² to 10⁶ Hz, −40 to 225 °C). Special emphasis has been placed on detailing the characteristic phenomena near CP2′s glass transition (199 °C). The consequences of residual DMAc solvent on CP2′s overall loss and relaxation characteristics are also discussed.     

Electrical conductivity of mixed-alkali fluorozirconate glasses

The features of the mixed-alkali effect in fluorozirconate glasses have been studied. It is demonstrated that this effect in oxygen-free halide glasses can be interpreted within the theory of inhomogeneous glass structure. The revealed structural features of mixed-alkali fluorozirconate glasses make it possible to elucidate the role of all ions in the glass formation process. Deceased.     

Dielectric spectroscopy and dissolution studies of bioactive glasses

The first and rate-limiting step in the degradation of bioactive glasses is thought to be the ion exchange of hydrated protons in the external fluid with alkali metal cations in the glass. The activation energy (E_a) for alkali ion hopping can be followed by dielectric spectroscopy. The replacement of CaO by Na₂O resulted in a reduction in the E_a for ion hopping. In contrast, increasing the glass network connectivity or reducing the nonbridging oxygen content of the glass resulted in an increase in E_a. Substitution of K₂O for Na₂O had little influence on E_a. Mixing alkali metals increased the E_a as expected on the basis of the mixed alkali effect. There was no correlation between the E_a for ion hopping and the dissolution behavior of the glass. Furthermore, the activation energy for Si, Ca Na, and K ion release was found to be approximately a factor of three lower than that for ion hopping suggesting that another rate-controlling mechanism is important in the degradation of bioactive glasses. The presence of a second relaxation process suggested that bioactive glasses undergo amorphous phase separation into silica-rich and orthophosphate-rich phases and the two relaxation processes are due to ion hopping in the two phases.     

Electrical conductivity of mixed-alkali fluorozirconate glasses

The features of the mixed-alkali effect in fluorozirconate glasses have been studied. It is demonstrated that this effect in oxygen-free halide glasses can be interpreted within the theory of inhomogeneous glass structure. The revealed structural features of mixed-alkali fluorozirconate glasses make it possible to elucidate the role of all ions in the glass formation process. Deceased.     

A quantum-chemical study of the mechanism of ionic conductivity in alkali borate glasses

The incorporation of alkali metal oxide molecules into a continuous random network of vitreous boron oxide is considered within the cluster approximation at a semiempirical MNDO level. It is found that the oxygen atom of the Li₂O molecule easily forms an additional bond with the threefold-coordinated boron atom with the formation of the BO₄ tetrahedron. Different types of metal cation motion in alkali borate glasses are treated. It is revealed that the M⁺ cation in neutral systems is predominantly located near the edges of a BO ₄ ^- tetrahedron on the outside of the B-O-B corner fragments and can readily change its position. The M⁺ cation in the neighborhood of the pentaborate grouping makes a complete turn without considerable expenditure of energy. The vibrations with a change in the rotation angle from 180° to 270° are possible in tetraborates, and, only in the neighborhood of triborate, one position is substantially more stable than the other positions. The cation can migrate from center to center along chains of a continuous glass network on the outside of the B-O-B corner fragments. In the pentaborate and tetraborate centers, the cation can migrate into another chain and change its direction of motion. From the materials of the paper reported at the International Conference “Glasses and Solid Electrolytes” (St. Petersburg, 1999, May 17–19).     

A quantum-chemical study of the mechanism of ionic conductivity in alkali borate glasses

The incorporation of alkali metal oxide molecules into a continuous random network of vitreous boron oxide is considered within the cluster approximation at a semiempirical MNDO level. It is found that the oxygen atom of the Li₂O molecule easily forms an additional bond with the threefold-coordinated boron atom with the formation of the BO₄ tetrahedron. Different types of metal cation motion in alkali borate glasses are treated. It is revealed that the M⁺ cation in neutral systems is predominantly located near the edges of a BO ₄ ^- tetrahedron on the outside of the B-O-B corner fragments and can readily change its position. The M⁺ cation in the neighborhood of the pentaborate grouping makes a complete turn without considerable expenditure of energy. The vibrations with a change in the rotation angle from 180° to 270° are possible in tetraborates, and, only in the neighborhood of triborate, one position is substantially more stable than the other positions. The cation can migrate from center to center along chains of a continuous glass network on the outside of the B-O-B corner fragments. In the pentaborate and tetraborate centers, the cation can migrate into another chain and change its direction of motion. From the materials of the paper reported at the International Conference “Glasses and Solid Electrolytes” (St. Petersburg, 1999, May 17–19).     

Dielectric and conductivity studies on cobalt phthalocyanine tetramers

Electrically conductive organic and metalloorganic polymers are of great interest and they have applications in electronic, optical, photonic, photoelectric, electrochemical, and dielectric devices. Tetrameric cobalt phthalocyanine was prepared by conventional chemical method. The dielectric permittivity of the tetrameric cobalt phthalocyanine sample was evaluated from the observed capacitance values in the frequency range 100 KHz to 5 MHz and in the temperature range of 300 to 383°K. It is found that the system obeys the Maxwell Wagner relaxation of space charge phenomenon. Further, from the permittivity studies AC conductivity was evaluated. The values of AC conductivity and DC conductivity were compared. Activation energy was calculated. To understand the conduction mechanism Mott's variable range hopping model was applied to the system. The T^?1/4 behavior of the DC conductivity along with the values of Mott's Temperature (T₀), density of states at the Fermi energy N (E_F), and range of hopping R and hopping energy W indicate that the transport of charge carriers are by three‐dimensional variable range hopping. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2529–2535, 2004     

Effect of heat treatment on AgI-rich chalcogenide glasses with enhanced ionic conductivity

The glass-ceramics of AgI-based electroconductive chalcogenide system was realized using an appropriate heat treatment at a fairly high temperature (T_g + 40°C) and different times ranging from 4 to 20 hours. The crystallization behavior and electroconductive properties of the heat-treated samples were studied in detail. Transmission study was performed, and the results show that the cut-off edge of the short wavelength is red-shifted at prolonged annealing time but remains an excellent transmittance in the mid-infrared (IR) region. XRD and scanning electron microscopy results indicated that the precipitated crystalline phases are mainly β-/γ-AgI. Moreover, a small amount of α-AgI, which rarely existed at room temperature, is precipitated in the AgI-rich chalcogenide glass-ceramics. The ionic conductivity of all glass-ceramics was enhanced by heat treatment in contrast to that of base glass. Raman analysis exhibited the structure variation in the glass sample after heat treatments. This study provided an observation of crystallization in chalcogenide glass containing large amounts of AgI and be of good guidance to fabricate novel AgI-based chalcogenide glass-ceramics that can be candidates in infrared optics and solid electrolyte applications.     

溶胶—凝胶法制备Li2O—SiO2凝胶玻璃

用溶胶－凝胶法制备了Ｌｉ２Ｏ－ＳｉＯ２系统凝胶及其玻璃。研究了溶液水硅比,醇,硅比对溶液胶凝时间的影响,利用ｘ－射线衍射与扫描电镜技术研究了１８Ｌｉ２Ｏ－８２ＳＩＯ２组分的析晶与分相特征。     

Critical evaluation and thermodynamic optimization of the Li-O,and Li2O-SiO2 systems

A critical evaluation and thermodynamic optimization of all available experimental data of the Li-O and Li₂O-SiO₂ systems were performed to obtain one set of consistent Gibbs energy functions for all phases in the systems. The obtained Gibbs energy functions can reproduce all available and reliable experimental data from 298 K to above liquidus temperatures at one atm total pressure. It is the first time, to the best of our knowledge that the Gibbs energy of stoichiometric phases like Li₂O, Li₄SiO₄, Li₂SiO₃, and Li₂Si₂O₅ was comprehensively evaluated and optimized. The liquid oxide solution was modeled using the Modified Quasichemical Model to describe its thermodynamic behavior accurately considering the short range ordering. Discrepancies observed in the metastable liquid immiscibility and the liquidus in the SiO₂-rich region of the Li₂O-SiO₂ system was resolved. The phase diagram and thermodynamic data of all the solid and liquid phases were well reproduced within experimental error limits.