Glass forming regions and dielectric properties of Na2O-Bi2O3-TiO2-P2O5 glasses

The glass forming regions have been determined in the quaternary systems Na₂O-Bi₂O₃-0.05TiO₂-P₂O₅ and Na₂O-0.05Bi₂O₃-TiO₂-P₂O₅. The largest vitreous do main has been found in the diagram with 5mol% of titanium oxide TiO₂- The variation of dielectric constant has been followed along two lines inside the glass regions. e'_r increases with increasing amount of sodium oxide Na₂O and diminishes with increasing percentage of either Bi₂O₃ or TiO₂.     

Structural,dielectric and impedance properties of NaCa2V5O15 ceramics

Polycrystalline sample of NaCa₂V₅O₁₅ (NCV) with tungsten bronze structure was prepared by a mixed oxide method at relatively low temperature (i.e. 630 °C). Preliminary structural analysis of the compound showed an orthorhombic crystal structure at room temperature. Microstructural study showed that the grains are uniformly and densely distributed over the surface of the sample. Detailed studies of dielectric properties showed that the compound has dielectric anomaly above the room temperature (i.e. 289 °C), and shows hysteresis in polarization study. The electrical parameters of the compound were studied using complex impedance spectroscopy technique in a wide temperature (23–500 °C) and frequency (10²–10⁶ Hz) ranges. The impedance plots showed only bulk (grain) contributions, and there is a non-Debye type of dielectric dispersion. Complex modulus spectrum confirms the grain contribution only in the compound as observed in the impedance spectrum. The activation energy, calculated from the ac conductivity of the compound, was found to be 0.20–0.30 eV. These values of activation energy suggest that the conduction process is of mixed type (i.e. ionic–polaronic).     

Threshold switching in V2O5-ZnO-SrO-FeO glasses

Density and current–field characteristics related to switching phenomenon in zinc-doped strontium iron vanadate glasses having the formula (70 ? x)%V₂O₅–x%ZnO–10%SrO–20%FeO, have been investigated over a wide composition range [0 ≤ x ≤ 15] at different thicknesses and temperatures. The samples are found to show threshold switching behavior. The switching fields are found to increase with the increase of zinc content while they decrease when the temperature is increased. The activation energy for switching was found to increase with the increase of ZnO content. Finally, the switching results are discussed in terms of the electrothermal model depending on the samples thicknesses and power calculus.     

Optical absorption in films of V2O5 and vanadium-phosphorus glasses

The results of an investigation of the spectral dependence of the absorption coefficient of amorphous and polycrystalline films of V₂O₅ and films of vanadium-phosphorus glasses in the 0.3–5 eV energy range are presented in this paper. It is shown that the fundamental absorption edge is determined by direct forbidden transitions; the nature of the absorption bands detected below the fundamental absorption edge is discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No.2, pp. 102–106, February, 1976.     

Structural and dielectric properties of Ba3V2O8 ceramics

Polycrystalline sample of Ba₃V₂O₈ was prepared by a high-temperature solid-state reaction technique. Preliminary X-ray diffraction (XRD) analysis confirms the formation of single-phase compound of hexagonal (rhombohedral) crystal structure at room temperature. Microstructural analysis by scanning electron microscope (SEM) shows that the compound has well defined grains, which are distributed uniformly throughout the surface of the sample. The dielectric properties of the compound studied in a wide frequency range (10²–10⁶ Hz) at different temperatures (25–400 °C), exhibits that they are temperature dependent. Detailed analysis of impedance spectra showed that the electric properties of the material are strongly dependent on frequency and temperature. The activation energy, calculated from the temperature dependence of ac conductivity (dielectric data), was found to be 0.23 eV at 50 kHz in the higher temperature region.     

Electrical properties of flash-evaporated V2O5 films

Electrical properties of vanadium-pentoxide flash-evaporated films are reported. Transport parameters are investigated as functions of growth conditions and post-deposition treatments. The conduction mechanism is analyzed using the small polaron hopping model. The main features of the electrical conductivity such as the polaron hopping energy, the disorder energy term and the density of states are determined. Paper presented at 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Preparation and properties of transparent SnO–P2O5 glasses

Colorless, transparent SnO–P₂O₅ (SP) glasses with 60–70 mol% SnO compositions were prepared by melting at 880–1000 °C in Ar atmosphere using commercial SnO and P₂O₅ powders as raw materials and vitreous carbon crucibles. SP glasses are characterized by glass transition temperature, onset of crystallization, thermal expansion coefficient and weight loss after immersion test. The viscosity of 67SnO–33P₂O₅ glass was measured by a penetration method in the range of 10^7.9–10^10.5 Pa s at 267–290 °C. The results of optical properties show that the transparent SP glasses have high refractive indexes over 1.75 and high transmission over 80% in the visible and IR region of 380 and 2700 nm.     

Electrical properties of V2O5-doped TlI

This paper reports the effect of heterogeneously doped vanadia on the ionic conductivity of thallium iodide (TlI). These compounds have been prepared and studied by X-ray diffraction, differential scanning calorimetry, scanning electron microscopy and electrical conductivity. The materials were found to be binary phase systems with the vanadia particles dispersed in the grains of TlI. The electrical conductivity shows an upward trend in the composition range x=0.1–0.5; however, with further increase in vanadia content, conductivity decreases. This behavior is attributed to disordering phenomena at interface boundaries and space-charge layers formed in the bulk grains of TlI. In addition to this, the increased content of vanadia in the system leads to the disappearance of the β?α phase transition of TlI, which is usually observed in the pure compound.     

Mössbauer spectra and electrical conductivity ofxFe2O3−(40-x)V2O5−60P2O5 glasses

Mössbauer spectra and electrical conductivities were measured for the purpose of studying on the conduction mechanism of xFe₂O₃?(40-x)V₂O₅?60P₂O₅ glasses. The ratios Fe²⁺/Fe²⁺+Fe³⁺ in xFe₂O₃?(40-x)V₂O₅?60P₂O₅ glasses were determined by Mössbauer spectroscopy. On the composition dependence of D. C. conductivities in these glasses, the minimum of log σ at each temperature was obtained at 25Fe₂O₃?15V₂O₅?60P₂O₅. The conductivities of ternary glassses in Fe rich region could be explained only by the changes of carrler (Fe²⁺) concentration and the hopping conduction between Fe²⁺ ions and Fe³⁺ ions in binary glasses. In V rich region, the saturation tendency of D. C. conductivities are observed. It was suggested to be explained by increasing of V⁴⁺ ions due to the influence of Fe ions.     

Transport and dielectric properties of lisicon

The ionic and electronic conductivities of Lisicon with compositions Li₁₄Zn(GeO₄)₄-(A) and Li₁₂Zn₂(GeO₄)₄-(B) have been determined in the temperature range 306–578 K. It has thus been shown that the ionic transport number is nearly unity throughout the temperature range. The activation energies for lithium motion were found to be 0.6 eV and 0.47 eV for compounds A and B. These values are very close to the values of the heat of transport q¹_Li⁺ 0.59 eV and 0.50 eV respectively, obtained from thermoelectric power (θ_t) measurements. The total ionic conductivity of the pellets of Lisicon was found to decrease with increasing average grain size in the range 20–78 μ. Electrode effects on the ionic conductivity are also reported for silver and TiS₂ electrodes. The electron/hole conductivity was studied using the Wagner cell technique. The activation energies for electron and hole transport for A were estimated as 1.02 eV and 1.39 eV and for B as 0.89 eV and 1.49 eV, respectively. The variation of the dielectric parameters ?', ?” and tan δ have been studied for sample A between 10 Hz-100 kHz. The high frequency dielectric constant (at 100 kHz) was 21.4. The ?' versus ?” (Cole-Cole plot) is also presented.     

Mixed conductivity of glasses in the P2O5-V2O5-Na2O system

The conductivity of glasses in the P₂O₅−[(1−x) V₂O₅−x Na₂O] system is studied as a function of temperature and composition. For all compositions, the conductivity variations as a function of temperature follow an Arrhenius type relationship: . The activation energies and pre-exponential factors corresponding to the V₂O₅ richest compositions are lower than that corresponding to the ionic ones. Isothermal variations of the conductivity as a function of composition show a deep minimum for a molar ratio x near 0.65. On either side of this minimum, the conductivity is mainly electronic (x<0.7) or ionic (x>0.8). The variations are interpreted assuming a prevailing diluting effect of the non predominantly present oxide without any interactions between the electronic and ionic charge carriers. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Nonlinear Optics properties of Er2O3- doped 75Nb2O5-20TeO2-5ZnO glasses

We have performed time-resolved degenerate four-wave mixing(DFWM)experiments in 75Nb2O5-20TeO2-5ZnO glasses doped by Er2O3 at different excitation intensities and lattice temperatures.DFWM signalexhibits three peaks at high excitation intensities,where a main peak appears at zero time delay and tworather weak side peaks locate symmetrically at the negative and positive time delay,respectively.Themain peak is attributed to local-field effect and two side peaks are attributed to Coulomb interaction(CI).     

AC conductivity and dielectric properties of sulphate glasses

AC conductivity and dielectric properties of sulphate glasses have been studied as a function of temperature, frequency and variation in interalkali concentration. AC conductivity at frequencies well beyond the dielectric loss peaks seems to arise from local motion of alkali ions within the neighbouring potential wells. Activation energies for AC conductivity were found to be very much lower than those for DC conductivity. Further, AC conductivity seems to be independent of interalkali variation, whereas ε′ and tan δ show a mild degree of mixed alkali effect. The observations made here have been explained on the basis of a structural model earlier proposed by us for these glasses. Communication No. 167 from Solid State and Structural Chemistry Unit.     

Fabrication and Li+-intercalation properties of V2O5-TiO2 composite nanorod arrays

A capillary-enforced template-based method has been applied to fabricate V₂O₅-TiO₂ composite nanorod arrays via filling mixture of VOSO₄ and TiOSO₄ solutions into the pores of polycarbonate membrane. For comparison purposes, pure V₂O₅ nanorod arrays were prepared through the similar template-based method with V₂O₅ sol and the sol was synthesized through the V₂O₅-H₂O₂ route. The nanorods covered completely a large area and projected from the surface of ITO substrate. The addition of TiO₂ to V₂O₅ has demonstrated to greatly affect the Li⁺ intercalation capacity of V₂O₅. For example, V₂O₅-TiO₂ nanorod array with molar ratio V/Ti=75/25 delivered 1.5 times discharge capacity of V₂O₅ nanorods at a current density of 92 mA/g. Such improvement in the intercalation properties was ascribed to the change of crystallinity and possible modification in lattice structure and interaction forces between adjacent layers in V₂O₅. PACS 81.05.Je; 82.45.Yz; 81.10.Dn; 81.20.Fw; 82.47.Aa     

Charge-ordering signatures in the optical properties of beta-Na0.33V2O5

Temperature dependent optical spectra are reported for beta-Na0.33V2O5. The sodium ordering transition at T(Na)=240 K and, in particular, the charge ordering transition at T(MI)=136 K strongly influence the optical spectra. The metal-insulator transition at T(MI) leads to the opening of a pseudogap ( variant Planck's over 2pi omega=1700 cm(-1)) and to the appearance of a large number of optical phonons. These observations and the presence of a midinfrared band (typical for low dimensional metals) strongly suggest that the charge carriers in beta-Na0.33V2O5 are small polarons.