Increasing V2O5 Content Research Articles

EPR and optical studies of microwave synthesized glasses containing VO2+ ions: Meta and pyrophosphate regime

Glasses with compositions xV2O5 - (100 - x)[0.8NaPO3 - 0.2ZnO] (where 0.2 ≤ x ≤ 10 in mol%) were prepared by a new microwave heating method. The Electron Paramagnetic Resonance (EPR) spectra of vanadyl ions in the glasses exhibit a hyperfine structure. From EPR spectra the spin Hamiltonian parameters g∥, g⊥, A∥, A⊥, dipolar hyperfine coupling parameter (P), Fermi contact interaction parameter (K), Pβ2∗2 K and PK/γ have been evaluated. An increase in the ratio of Δg‖/Δg⊥ with an increase in V2O5 content shows that the tetragonal nature of the V4+O6 complex is enhanced. The increase in A'|| and P indicates that the 3dxy orbit contracts with V2O5 content. The molecular orbital coefficients α2, γ2 have been determined by correlating the EPR and optical absorption data. The values of α2 increase while γ2 decreases with an increase in V2O5 content, indicating the σ - bonding between the vanadium atom and the equatorial ligands decreases and the effect of π - bonding with the vanadyl oxygen increases. The bond ionicity and covalency of the glass network has been calculated using Pauling's electronegativity support the experimental findings. Observed variations in the optical band gap, refractive index, molar refraction, metallization criterion, electronic polarizability of the oxide ion and optical basicity are correlated with the glass structure. The photoluminescence (PL) excitation spectra reveal two individual peaks at 338 and 468 nm which are ascribed to the charge transfer reaction in [VO4]3− groups. An emission peak occurs at 531 nm and concentration quenching is seen beyond 0.5 mol% of V2O5.

Study of polaron transport mechanisms in two transition metal ions doped borophosphate glasses

Borophosphate glasses in the compositions, (B2O3)0.2 · (P2O5)0.3 · (V2O5) x · (CoO)0.5x , where x = 0.05, 0.1, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50 were synthesized at 1500 K by standard melt quenching method. Non-crystalline nature of the samples was confirmed by XRD studies. Room temperature density and dc electrical conductivity in the temperature range from 350 K to 625 K have been measured. Density decreased up to about 0.25 mole fractions of V2O5 and increased thereafter. Conductivity was almost constant up to 0.25 mole fractions of V2O5 and increased for higher amount of V2O5. Temperature variation of conductivity data has been analyzed using Mott’s small polaron hopping (SPH) model and, activation energy and Debye’s temperature were determined. Activation energy decreased with increase of V2O5 content. The data deviated from the Mott’s SPH model has been analyzed in view of variable range hopping models of Mott and Greaves and the density of states at Fermi level was determined. It is for the first time that borophosphate glasses doped with V2O5 and CoO were synthesized to study conductivity variation with temperature. The data analyzed using small polaron hopping and variable range hopping models.

Investigation on Physical and Optical Properties of Vanadium Oxide in Soda Lime Borate Glasses

In this work, the physical and optical properties of soda-lime-borate glasses doped with V2O5 concentration of 0.0 to 2.5 in mol% have been investigated. The glass samples were prepared by the normal melt-quench technique. The results show that the density and refractive index decreased with additional 0.5 mol% content of V2O5. Then both results increased with increasing V2O5 content from 1.0-2.5 mol%. The optical absorption spectra of glass samples were measured by UV-visible spectrophotometer in the wavelength range 300-1100 nm and color coordinate in CIEL*a*b* system. The absorption peaks locate around 605 and 960 nm for all V2O5 concentration that are associated to V4+ ions and produce the brown color.

Microstructure of Porous V<sub>2</sub>O-CeO<sub>2</sub>-SiO<sub>2 </sub>Catalyst for SCR of NO with NH<sub>3 </sub>at Low Temperature

Selective catalytic reduction (SCR) is a well-proven method to reduce NO emission. However, to choose the right catalyst that provides a surface for reaction between NO and ammonia at low temperatures is a challenging task for a catalysts developers. In an earlier study, we prepared V2O5-CeO2-SiO2 catalyst with increasing V2O5 content by sol-gel route and found that the catalytic activity improved with increasing the V2O5 loading up to 0.5%. The catalytic activity, however, dropped when V2O5 loading was about 1% and increased back when the loading of V2O5 was about 5%. In this study, we looked into the microstructural relationship to explain these findings. The microstructures of the catalysts before and after exposure to NO gas revealed that the catalysts with 0.2% and 0.5% V2O5 were more porous after the reduction process possibly due to improved breakdown of (NH4)HCO3 to NH3 by the possible interaction with the V2O5 and CeO2-containing catalysts which consequently resulted in a more efficient NO reduction to N2 and H2O at low temperature. The microstructure of the catalyst with 1% V2O5 content to 5%, improved back the efficiency although clogging by CeVO4 phase still possible due to its presence based on XRD. The well-ordered micropores before exposure to NO and the more efficient breakdown of (NH4)HCO3 could have contributed to increase back the catalytic activity at low temperature.

The Microstructures and Mechanical Properties of V2O5-Doped Al2O3/TiAl In-Situ Composites by Reactive Hot Pressing Process

Al2O3/TiAl composites are successfully fabricated by the in-situ hot pressing method from the elemental powders of Ti, Al, TiO2, and V2O5. The effect of V2O5 addition on the microstructure and mechanical properties of the Al2O3/TiAl in-situ composites is investigated in detail. It is found that the as-synthesized composites mainly consist of V-dissolved γ-TiAl, α2-Ti3Al, and Al2O3 particles along with a small amount of V3Al phase, and the in-situ-formed fine Al2O3 particles tend to disperse on the grain boundaries of TiAl matrix. With increasing V2O5 content, the density and Vickers hardness of the resulting composites gradually increase, whereas the fracture toughness and flexural strength first increase and then decrease with the increase of V2O5 content. The composite with 3.5 wt.% V2O5 has the maximum value of 9.35 MPa m1/2 and 713.36 MPa for the fracture toughness and flexural strength, respectively. The toughening mechanism is also discussed in detail.

Effect of vanadium pentoxide on the electrical, dielectric, and optical properties of poly(vinyl alcohol)/vanadium pentoxide nanocomposites

In this study, the effect of vanadium pentoxide (V2O5) on the electrical, dielectric, and optical properties of poly(vinyl alcohol) (PVA)/V2O5 nanocomposites was examined. The PVA/V2O5 nanocomposites were prepared by solution mixing, followed by film casting. The as-prepared films were characterized by X-ray diffraction, thermogravimetric analysis, as well as impedance spectroscopy. The obtained results indicated that, with increasing V2O5 content, the PVA composite exhibits considerable vestige of crystallization and the PVA peaks become broader. The electrical conductivity, σ dc, increases as the temperature and the dopant level of V2O5 increase. The frequency dependence of alternating current conductivity was governed by a critical frequency, ω p. The values of ω p are found to be thermally activated. The loss tangent (tan δ) shows a peak with the frequency, which indicates a dielectric relaxation in these composites with relaxation time decreases with both temperature and V2O5 content increase. Optical absorption edge and direct and indirect energies for all mentioned samples were determined and discussed.

Study of electrical conductivity and memory switching in the zinc–vanadium–phosphate glasses

Vanadium zinc phosphate glasses were prepared by the conventional melt quenching technique and effect of V2O5 concentration on d.c. conductivity of prepared samples were investigated. X-ray diffraction patterns confirmed the glassy character of the samples. The d.c. conductivity increased with increase in V2O5 content. Results showed that activation energy has a single value in the investigated range of temperature, which can be explained in accordance with Mott small pollaron hopping model. I–V characteristics at high electric field showed that switching in these glasses was memory type. The threshold field of switching was found to decrease with increase in V2O5 content. Non-linear behavior and switching phenomenon was explained by Pool–Frenkel effect and thermal model.

Effect of V2O5 on electrical and microstructural properties of ZnOceramics

ZnO ceramics with small amounts of V2O5 as a varistor-former ranging from 1–5mol% were prepared by conventional powder processing route and sintered at 700, 800 and 900°C for 2h. It was observed that the grain growth behavior in ZnO ceramics was strongly influenced by doping with V2O5. Morever, lower temperatures about 900°C could be used for sintering. The microstructure of the samples consists of ZnO grains as a main phase and Zn3(VO4)2 as a secondary phase. All of the prepared ceramics showed characteristic of non-ohmic current–voltage behavior. Non-linear coefficient increased with increase in V2O5 content and sintering temperature. Activation energy of the ceramics lies in the range of 0.071–0.402eV which decreased with increase in V2O5 content and sintering temperature. The origin of the non-linear electrical behavior was explained by considering the solid state reactions and formation of the potential barriers at grain boundaries.

Effect of vanadium pentoxide on the mechanical, thermal, and electrical properties of poly(vinyl alcohol)/vanadium pentoxide nanocomposites

AbstractIn this study, we examined the effect of vanadium pentoxide (V2O5) on the mechanical, thermal, and morphological properties of poly(vinyl alcohol) (PVA)/V2O5 nanocomposites. The PVA/V2O5 nanocomposites were prepared by solution mixing, followed by film casting. The results show that the Young's moduli of the resulting nanocomposites films were higher than the pure PVA modulus with increasing V2O5 content, and it reached a maximum point at about 0.4 wt % V2O5 content at 8.55 GPa. The tensile strength and stress at break increased with increasing V2O5 content. The addition of V2O5 did not affect the melting temperature. The crystallization temperatures of PVA were significantly changed with increasing V2O5 content. The 5% weight loss degradation temperature of the nanocomposites was measured by thermogravimetric analysis. The degradation temperatures of the V2O5 nanocomposites increased with increasing filler content and were higher than the degradation temperature of pure PVA; this showed a lower thermal stability compared to those of the nanocomposites. The results show that the thermal stability increased with the incorporation of V2O5 nanoparticles. The dielectric constant of PVA had a tendency to improve when the dispersion of particles was effective. The morphology of the surfaces the nanocomposites was examined by scanning electron microscopy. We observed that the dispersion of the V2O5 nanoparticles was relatively good; only few aggregations existed after the addition of the V2O5 nanoparticles at greater than 0.4 wt %. In perspective, the addition of 0.4 wt % V2O5 nanoparticles into PVA maximized the mechanical, thermal, and electrical properties. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Optical and other structural properties of some zinc vanadate semiconducting glasses

In this paper we have reported the optical and other structural properties for different compositions of xV2O5–(1−x)ZnO (x = 0.5–0.8) semiconducting glasses. It has been observed that the density and the glass-transition temperature increase, while the molar volume decreases with the decrease of the V2O5 content in the compositions. It has also been observed from the analysis of FT-IR spectra that the network structure of these glasses consists of VO4-polyhedra. The fundamental absorption edge for the present glasses has been analyzed using the theory proposed by Davis and Mott. It has been observed that the fundamental absorption edge and cut-off wavelength shift towards red with the increase in V2O5 content. The composition dependence of the optical gap is consistent with that of the activation energy for the dc electrical conductivity.

Effect of vanadium doping on sintering and dielectric properties of strontium barium niobate ceramics

Strontium barium niobate, Sr0.5Ba0.5Nb2O6 (SBN50) ceramics doping up to 3 wt% V2O5 were fabricated by solid state reaction route, starting from raw materials (oxides and carbonates) of analytical grade. The phase composition, microstructure and dielectric properties were investigated by X-ray diffraction, scanning electric microscope and impedance analyzer. The results show that the addition of V2O5 improves sintering densification of SBN ceramic samples. The relative density of the samples firstly increases and then slightly decreases with increasing amounts of V2O5 and sintering temperature. With the help of the additive of 1 wt% V2O5, the relative density of the sample sintered at 1,280 °C for 3 h can reached 97.2%. Only single tetragonal tungsten bronze phase SBN exists in all the doped samples. With increase in V2O5 content, the dielectric constant of SBN ceramics at both room temperature and in the vicinity of the phase transition temperature increases significantly and the Curie temperatures (Tc) obviously shifts to low temperature as well as the dielectric loss remains below 0.06. The diffuseness in the phase transition is found to increase with increase in vanadium doping level. The addition of V2O5 results in an increased grain size associated with rod-like grain growth.

Study of the hyperfine structure, thermal stability and electric–dielectric properties of vanadium iron phosphate glasses

In the present study the thermal analysis, hyperfine structure and electric transport properties were investigated for the (60−X) P2O5 (20+X) V2O5 20Fe2O3 [PVF] (X=10, 15, 20, 30 and 40 mol%) glasses. The glassy state of the samples was characterized using DTA and Mössbauer spectroscopy. Two glass transition temperatures Tg1 and Tg2 were detected at the DTA traces of the investigated system. The glassy sample with X=20 mol% (PVF3) is of the highest glass thermal stability where ΔT=177±2 K. Also, it has the highest value of crystallization activation energy for the first and the second crystallization peaks (305 and 316±3 kJ/mol%) among the other samples. The obtained ME spectra showed the presence of Fe3+ alone located in the tetrahedral and octahedral sites. Increasing V2O5 content, the dc conductivity increases while the activation energy decreases. The room temperature dc conductivity is typically 2.9×10−7–2.5×10−5 with an activation energy 0.60–0.37±0.011 eV. The power law exponent s was found to be temperature dependent and exhibited a minimum, for PVF3 and PVF4. The dielectric constant ε1(ω) increases with increasing V2O5 content which was attributed to the increase in the deformation of glass network.

Preparation, characterisation and catalytic evaluation of MgF2 supported V2O5 catalysts for ammoxidation of 3-picoline

Non-conventional MgF2 supported V2O5 catalysts with different vanadia contents (2–15 wt.%) were prepared by impregnation (using NH4VO3), characterised and catalytically evaluated for selective ammoxidation of 3-picoline to nicotinonitrile. Oxygen and ammonia chemisorption uptakes increased continuously from 60 to over 600 μmol g−1 and 275 to >750 μmol g−1, respectively, with rise in V2O5 proportion indicating that the redox as well as acidic sites are increasing with increase in V2O5 content. Thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) analysis revealed endothermic as well as exothermic thermal effects mainly due to liberation of water and ammonia, and also due to structural changes. XRD patterns showed the formation of crystalline V2O5 in the fresh solids having 8 wt.% V2O5 and above and NH4VO3 phase in the spent samples. The conversion of 3-picoline is observed to increase continuously with increase in V2O5 loading. However, the selectivity of nicotinonitrile is found to be independent on conversion of 3-picoline. The catalyst with the highest V2O5 loading (15.7 wt.%) displayed the best activity (X > 90%) and selectivity (S > 95%) compared to all other catalysts of this series. The 3-picoline conversion of 10% at 548 K is increased to almost 100% with rise in temperature to 663 K. Increase in 3-picoline feed rate and NH3: 3-picoline ratio exhibited an inhibiting effect on the conversion, while an increase in air: 3-picoline ratio has no significant influence on the performance.

Elastic and structural properties of vanadium–lithium–borate glasses

The ternary xV2O5–(40 − x)Li2O–60B2O3 glass system, where x = 1, 2, 3, 4 and 6 mol%, was prepared by normal quenching. Ultrasonic velocities and attenuation were measured at room temperature using a pulse-echo technique. Various parameters, such as elastic moduli, micro-hardness, Poisson's ratio and Debye temperature, were determined from the measured densities and velocities. The composition dependence of these parameters, in addition to the glass-transition temperature, suggested that vanadium ions were incorporated into these glasses as a network modifier, resulting in the reconversion of BO4 tetrahedra to BO3 triangles by the breaking of B–O–B linkages and the formation of nonbridging oxygens (NBOs). The outcome was a reduction in network connectivity and rigidity with increasing V2O5 content. The results are explained quantitatively in terms of fractal bond connectivity, average atomic volume, network dimensionality, packing density, number of network bonds per unit volume, cross-link density and atomic ring size. The Makishima and Mackenzie model appears to be valid for the studied glasses when the fate of BO4 tetrahedra and creation of NBOs are taken into account.

The Characteristics of Ferroelectric Sr0.4Ba0.6Nb2-xVxO6 Dielectrics

In this study, it would be interesting to investigate ceramic material based on Sr0.4Ba0.6Nb2O6 composition, V2O5 was used to substitute Nb2O5 to form the compositions of Sr0.4Ba0.6Nb2-xVxO6, where 0 ≤ x ≤ 0.3. The prepared Sr0.4Ba0.6Nb2-xVxO6 ceramics were sintered from 1060oC to 1300oC, and the needed sintering temperatures were dependent on V2O5 content. The SEM micrographs showed that even 1220oC was used as the sintering temperature of Sr0.4Ba0.6Nb2O6 ceramics, the pores were still observed. The crystal intensities of some (h,k,l) planes, for example (3,2,0) and (5,5,0) planes, apparently increased with the increase of V2O5 content and slightly decreased with the increase of sintering temperatures. The temperature-dependent dielectric characteristics were measured from 25oC to 300oC at 1MHz as a function of sintering temperatures and V2O5 content. The studied results would show that the sintering temperatures and V2O5 content also had large influence on the maximum dielectric constants (revealed at Curie temperatures) and the temperature-dielectric curves of Sr0.4Ba0.6Nb2-xVxO6 ceramics.

Optical and other physical properties of semiconducting cadmium vanadate glasses

The optical and other physical properties of semiconducting xV2O5−(1−x)CdO glasses have been reported in this article. The changes in both density and molar volume indicate structural modification of crystalline V2O5 due to addition of CdO. The glass transition temperatures are observed to decrease with an increase in V2O5 content in the compositions. It has been observed from the analysis of Fourier-transform infrared spectra that the network structure strongly depends upon the V2O5 content in the glass compositions. Introduction of CdO into the V2O5 matrix changes the two-dimensional layer structure of the crystalline V2O5 into a more complicated three-dimensional structure. The fundamental absorption edge for all the glasses is analyzed in terms of the theory proposed by Davis and Mott [E. A. Davis and N. F. Mott, Philos. Mag. 22, 903 (1970)]. The position of absorption edge and, hence, the value of the optical band gap are found to depend on the V2O5 content in the glass compositions.

IR and Raman investigation on the structure of (100-x)[0.33B2O3–0.67ZnO]–xV2O5 glasses

Glasses with molar composition of (100-x)[0.33B2O3–0.67ZnO]–xV2O5, x=0, 5, 10, 15 and 20, were prepared, and the effect of V2O5 on the structure of the glass was investigated by IR and Raman spectroscopy, TEM and DTA. This investigation shows that [BO3], [BO4] and [VO4] structural units are the predominant coordination polyhedra in the borovanadate glass. From the vanadium-free zinc borate glass, meta-, di-, pyro- and orthoborate groups were observed. Adding V2O5 leads to the random substitution of [BO3] or [BO4] units in these borate groups by the [VO4] units, forming corresponding borovanadate groups. With increasing V2O5 content, the transition temperature of the glass decreases, that comes from the decrease of the interconnection of the structure units and the substitution of O–B–O by weaker O–V–O linkages.

Structure, magnetic and positron lifetime studies on CaO–ZrO2–SiO2 glass system doped with vanadium oxide

The structure of CaO–ZrO2–SiO2 glasses doped with V2O5 (0.1–2 mole%) was studied using density and molar volume measurements. It was observed that the molar volume increased while the density decreased with increasing V2O5 content. Magnetic susceptibility measurements showed the diamagnetic behaviour of this glass system. The free volume was probed using ortho-positronium pick-off annihilation lifetime parameters. The mean free volume and fractional free volume have also been calculated from the positron annihilation lifetime data. The overall measured parameters from the structural and/or positron annihilation spectra indicated that the majority of the vanadium ions in the range 0–0.5 mole% of V2O5 are in the V5+ valence state which act as a glass former. On the other hand, the majority of the vanadium ions in the range 0.7–2 mole% of V2O5 are the V3+ and V4+ states acting as a glass modifier.

Low-temperature firing and microwave dielectric properties of MgTiO3 ceramics with Bi2O3–V2O5

The effects of Bi2O3–V2O5 additive on the microstructures, the phase formation and the microwave dielectric properties of MgTiO3 Ceramics were investigated. The Bi2O3–V2O5 addition lowered the sintering temperature of MgTiO3 ceramics effectively from 1400 to 875 °C due to the liquid-phase effect. The microwave dielectric properties were found to strongly correlate with the amount of Bi2O3–V2O5 addition. The saturated dielectric constant decreased and the maximum Qf values increased with the increasing V2O5 content, which is attributed to the variation of the second phase including Bi2Ti2O7, Bi4V1.5Ti0.5O10.85 and BiVO4. At 875 °C, MgTiO3 ceramics with 5.0 mol% Bi2O3–7 mol% V2O5 gave excellent microwave dielectric properties: εr=20.6,Qf=10420 GHz (6.3 GHz).

Study of grain boundary contribution and enhancement of magnetoresistance in La0.67Ca0.33MnO3/V2O5 composites

Electrical transport (90–300 K) and magnetic (5–375 K) properties of the La0.67Ca0.33MnO3 + x% V2O5 (x = 0, 2, 5, 15 and 30) type magnetic–nonmagnetic composite system show appreciable grain boundary contribution. These composites, except for the one with x = 30%, show metal–insulator transitions at T = Tp which decrease with increasing V2O5 concentration. The high temperature (T > Tp) semiconducting part of the resistivity (ρ) data follow a small polaron hopping conduction mechanism. The magnetoresistance (MR) in this system increases with increasing V2O5 content, showing a maximum value (25% at 93 K and field H = 1.15 T) for the sample with x = 15%. Spin polarization is considered to be responsible for such an increase of MR. Neither thermoelectric power (between 90 and 300 K) nor magnetization data indicate any noticeable change of the Curie temperature (TC) with change of x, suggesting that V2O5 remains at the grain boundary. The complex impedance (Z*) of the composite system, measured over a frequency range of 0.01 kHz–13 MHz, at 300 K shows strong frequency dependence and the impedance plane plots (Z′–Z″) are semicircular for different V2O5 content. An equivalent resistor–capacitor circuit (Rg (Rgb Cgb)) has been proposed to explain the impedance results in this composite system.