Oxygen gas-sensing behaviour of V2O5–SnO–TeO2 glass

The d.c. conductivity, σ, and the oxygen gas-sensing behaviour of V₂O₅–SnO–TeO₂ glass prepared by press-quenching were studied in argon and oxygen gas atmospheres at temperatures ranging from 303–473 K. The glass of 50V₂O₅·20SnO·30TeO₂ (mol %) was n-type semiconducting. The high-temperature conductivity was lower in oxygen and higher in argon than that in air. This was explained by the V⁴⁺ ions in the glass being oxidized by oxygen which had diffused into the glass, resulting in an increase in V⁵⁺ with time. The experimental relationship between σ and oxygen partial pressure, P _O2, agreed quantitatively with the theoretical relation σ ∝ P _O2 ^-1/4 . Changes in conductivity by switching the atmospheres between oxygen and argon gases were found to be reproducible. From the data of these dynamic changes, the oxygen gas sensitivity, S, at 473 K was obtained to be 1.3 in oxygen atmosphere. The dynamic changes could be quantitatively explained by an oxygen diffusion model. Throughout these discussions, the present tellurite glass was found to possess a potential applicability as an oxygen gas sensor. This revised version was published online in November 2006 with corrections to the Cover Date.     

Electrical properties of Sb2O3–CaO–V2O5 glasses and glass-ceramics

The d.c. conductivity (σ) of (a) glasses prepared by the press-quenching method and (b) glass-ceramics (crystallized glass) produced by post-heat treatment was investigated in the system Sb₂O₃–CaO–V₂O₅ and their conduction mechanism was studied. The glasses were n-type semiconductors with σ = 2.6 × 10^-6 ∼ 2.8 × 10^-5 S cm^-1 at 333 K for varying glass compositions. The conduction was attributed to small polaron hopping in the adiabatic regime. The estimated carrier density was 1.7 ∼ 3.8 × 10²¹ cm^-3 for V₂O₅ = 70 ∼ 80 mol% and the mobility was 3.5 × 10^-9 to 6.9 × 10^-8 cm² V^-1 s^-1. Crystallization raised the conductivity by a factor of 10³. The crystalline product in the glass-ceramics was Ca_0.17V₂O₅. The glass-ceramics were n-type semiconductors, and the conduction was interpreted by a superposition of the small polaron hopping in the crystalline and glassy phases. This revised version was published online in November 2006 with corrections to the Cover Date.     

Non-adiabatic polaron hopping conduction in semiconducting V2O5-Bi2O3 oxide glasses doped with BaTiO3

BaTiO₃-doped (5–40 wt %) 90V₂O₅-10Bi₂O₃ (VB) glasses have been prepared by a quick quenching technique. The d.c. electrical conductivities, _d.c., of these glasses have been reported in the temperature range 80–450 K. The electrical conductivity of these glasses, which arises due to the presence of V⁴⁺ and V⁵⁺ ions, has been analysed in the light of the small-polaron hopping conduction mechanism. The adiabatic hopping conduction valid for the undoped VB glasses (with 80–95 mol % V₂O₅), in the high-temperature region, is changed to a non-adiabatic hopping mechanism in the BaTiO₃-doped VB glasses. At lower temperatures, however, a variable range hopping (VRH) mechanism dominates the conduction mechanism in both the glass systems. Such a change-over from adiabatic to non-adiabatic conduction mechanism is a new feature in transition metal oxide glasses. Various parameters, such as density of states at the Fermi level N(E_F), electron wave-function decay constant, , polaron radius, r _p, and its effective mass, m _p ^* , etc., have been obtained for all the glass samples from a critical analysis of the electrical conductivity data satisfying the theory of polaron hopping conduction.     

Structural investigation of xFe2O3 · (100 − x)[P2O5 · TeO2] glass system by FT-IR study and EPR spectroscopy

Glasses from xFe₂O₃ · (100 − x)[P₂O₅ · TeO₂] system, with 0 ≤ x ≤ 50 mol%, were investigated by X-ray diffraction, FT-IR and EPR spectroscopies. The XRD patterns show a vitreous state of studied samples for x ≤ 35 mol% Fe₂O₃. The FT-IR spectrum of the P₂O₅ · TeO₂ glass matrix reveals a structure formed from PO₄, TeO₄ and TeO₃ units. The addition and the increasing of Fe₂O₃ content modify progressively the structure of the glass matrix. The local structure in the investigated glasses was revealed by means of EPR using Fe³⁺ (3d⁵; ⁶S_5/2) ions as paramagnetic probes. The EPR spectra present two resonance absorption lines characteristic to Fe³⁺ ions centred at g_eff ≈ 2.0, for 0.5 ≤ x ≤ 35 mol% and g_eff ≈ 4.3, for 0.5 ≤ x ≤ 5 mol%. The variation of the EPR parameters, the intensity and line-width of these absorption lines, with iron ions composition has been followed.     

Small polaron transport in V2O5–NiO–TeO2 glasses

Semiconducting glasses of the V₂O₅–NiO–TeO₂ system were prepared by the press-quenching method and their d.c. conductivities in the temperature range 300–450 K were measured. The d.c. conductivities at 395 K for the present glasses were determined to be 10^–7 to 10^–1 S m^–1, indicating that the conductivity increased with increasing V₂O₅ concentration. A glass of composition 67.5V₂O₅–2.5NiO–30TeO₂ (mol %) having a conductivity of 2.47×10^–2 S m^–1 at a temperature of 395 K was found to be the most conductive glass among the vanadium-tellurite glasses. From the conductivity–temperature relation, it was found that a small polaron hopping model was applicable at the temperature above _D/2 (_D: the Debye temperature); the electrical conduction at T>_D/2 was due to adiabatic small polaron hopping of electrons between vanadium ions. The polaron bandwidth ranged from 0.06 to 0.21 eV. The hopping carrier mobility varied from 1.1×10^–7 to 5.48×10^–5 cm² V^–1 s^–1 at 400 K. The carrier density is evaluated to be 1.85×10¹⁹–5.50×10¹⁹ cm^–3. The conductivity of the present glasses was primarily determined by hopping carrier mobility. In the low-temperature (below _D/2) regime, however, both Mott's variable-range hopping and Greaves intermediate range hopping models are found to be applicable.     

Gadolinium doping of vanadate-tellurate glasses and glass ceramics

In order to further elucidate the local structure of ternary xGd₂O₃(100 − x)[0.7TeO₂ · 0.3V₂O₅] glasses with x = 0, 5, 10, 15, 20 mol%, FTIR spectroscopy, XRD diffraction and density measurement were performed. FTIR and density data show that by increasing the gadolinium ions content of the samples the excess of oxygen may be accommodated by the inter-conversion of some [VO₄] into [VO₅] structural units and of [TeO₃] into [TeO₄] units. The composition of the heat-treated glasses was found to consist mainly of the Te₂V₂O₉ crystalline phase. Varying x between 15 and 20 mol% Gd₂O₃ produces structural modification having as result an increase of the glass network polymerization degree. Accordingly, the gadolinium ions play a particular role related to the improvement of the homogeneity of the glasses and in accommodating the glass network with the excess of oxygen.     

Structure and electrical conductivity relaxation studies of Nb2O5-doped B2O3–Bi2O3–LiF glasses

Glasses with composition (70 − x) B₂O₃·15Bi₂O₃·15LiF·xNb₂O₅ with x = 0–1.0 mol% were prepared by conventional glass-melting technique. The molar volume V _m values decrease and the glass transition temperatures T _g increase with increase of Nb₂O₅ content up to 0.2 mol%, which indicates that Nb⁵⁺ ions act as a glass former. Beyond 0.2 mol% Nb₂O₅ the V _m increases and the T _g decreases, which suggests that Nb⁵⁺ ions act as a glass modifier. The FTIR spectra suggest that Nb⁵⁺ ions are incorporated into the glass network as NbO₆ octahedra, substituting BO₄ groups. The temperature dependence of the dc conductivity follows the Greaves variable range hopping model below 454 K, and follows the small polaron hopping model at temperatures >454 K. σ _dc, σ _ac conductivity and dielectric constant (ε) decrease and activation energy for dc conduction ΔE _dc which increases with increasing Nb₂O₅ content up to 0.2 mol%, whereas σ _dc, σ _ac and (ε) increase and ΔE _dc decreases with increasing Nb₂O₅ content beyond 0.2 mol%. The impedance spectroscopy shows a single semicircle or arcs which indicate only the ionic conduction mechanism. The electric modulus formalism indicates that the conductivity relaxation is occurring at different frequencies exhibit temperature-independent dynamical process. The (FWHM) of the normalized modulus increases with increase in Nb₂O₅ content suggesting that the distribution of relaxation times is associated with the charge carriers Li⁺ or F⁻ ions in the glass network.     

Seebeck coefficient of V2O5-R2O3-TeO2 (R = Sb or Bi) glasses

The thermoelectric power of glasses in the systems V₂O₅-Sb₂O₃-TeO₂ and V₂O₅-Bi₂O₃-TeO₂ was measured at temperatures in the range 373–473 K. The glasses in both systems were found to be n-type semiconductors. The Seebeck coefficient, Q, at 473 K was determined as –192 to –151 VK^–1 for V₂O₅-Sb₂O₃-TeO₂ glasses, and –391 to –202 VK^–1 for V₂O₅-Bi₂O₃-TeO₂ glasses. For these glasses in both systems, Heikes' formula was satisfied adequately for the relationship between Q and In [C _v/(1-C_v)] (C _v = V⁴⁺/V_total, C _v is the ratio of the concentration of reduced vanadium ions), and discussions confirmed small polaron hopping conduction of the glasses in both systems. Mackenzie's formula relating to Q and V⁵⁺/V⁴⁺ was also applicable to the glasses in both systems, and it was concluded that the dominant factor determining Q was C _v.     

Electrical properties of V<Subscript>2</Subscript>O<Subscript>5</Subscript>-CaO-P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses exhibiting majority charge carrier reversal

The thermoelectric power and d.c electrical conductivity of x V₂O₅⋅40CaO⋅(60−x)P₂O₅ (10 ≤ x ≤ 30) glasses were measured. The Seebeck coefficient (Q) varied from +88 μ V K⁻¹ to −93 μV K⁻¹ as a function of V₂O₅ mol%. Glasses with 10 and 15 mol% V₂O₅ exhibited p-type conduction and glasses with 25 and 30 mol% V₂O₅ exhibited n-type conduction. The majority charge carrier reversal occurred at x = 20 mol% V₂O₅. The variation of Q was interpreted in terms of the variation in vanadium ion ratio (V^{5 +}/V^{4 +}). d.c electrical conduction in x V₂O₅⋅40CaO⋅(60−x)P₂O₅ (10 ≤ x ≤ 30) glasses was studied in the temperature range of 150 to 480 K. All the glass compositions exhibited a cross over from small polaron hopping (SPH) to variable range hopping (VRH) conduction mechanism. Mott parameter analysis of the low temperature data gave values for the density of states at Fermi level N (E_F) between 1.7 × 10²⁶ and 3.9 × 10²⁶ m⁻³ eV⁻¹ at 230 K and hopping distance for VRH (R_VRH) between 3.8 × 10⁻⁹m to 3.4 × 10⁻⁹ m. The disorder energy was found to vary between 0.02 and 0.03 eV. N (E_F) and R_VRH exhibit an interesting composition dependence.     

Electrical transport studies in alkali iron phosphate glasses

The electrical properties of xFe₂O₃−(100 −x) Na₂P₂O₅ glasses with x = 0, 6, 12, 18 and 24 mol% have been studied in the temperature range from 323 to 573 K. The dc conductivity was found to decrease as the iron content increases while the activation energy increases with increasing iron content in the glasses. In the high—temperature regime above θ_D/2 (θ_D is the Debye temperature), the Mott model of small polaron hopping (SPH) between nearest neighbors is consistent with the conductivity data. The electron—phonon interaction coefficient γ_P was very large (66.74–97.60). The electrical conduction of the glasses was confirmed to be non-adiabatic small polaron hopping. The physical parameters obtained by fitting the experimental results to these models are consistent with glass compositions.     

Effect of sulfur addition and nanocrystallization on the transport properties of lithium–vanadium–phosphate glasses

The glasses defined by the formula 37.5Li₂O–25V₂O₅–37.5P₂O₅ mol% containing different sulfur (0, 10, 50 and 100 mol%) content were studied before and after nanocrystallization. X-ray diffraction and transmission electron micrograph of the heat treated samples indicated nanocrystals embedded in the glass matrix. The average crystallite size was found between 18 and 37 nm. Sulfur (S) behaved as a reducing agent for redox reaction during preparation of glass and affected the conductivity, i.e., the V⁴⁺–V⁵⁺ or V³⁺–V⁴⁺ion pairs increased with increasing S content and led to increasing conductivity of glasses. After creation of the nanocrystalline phase, S-free glass–ceramic nanocomposite exhibited improvement in electrical conductivity around three orders of magnitude than initial glass. This great improvement of electrical conductivity is related to increase in a concentration of V⁴⁺–V⁵⁺or V³⁺–V⁴⁺ ion pairs and also, forming of defective and well-conducting regions along the crystallite/glass interfaces. The decrease in electrical conductivity in the 50S glass–ceramic nanocomposite, which possessed the highest crystallite size, could be related to the increase of grain boundaries scattering because of the increasing crystallite size. The conduction was attributed to non-adiabatic small polaron hopping and mostly determined by hopping carrier mobility.     

Preparation and characterization of binary V2O5-Bi2O3 glasses

Characterization of the binary V₂O₅-Bi₂O₃ glasses prepared by rapidly quenching the melt has been made from the studies of X-ray diffraction, scanning electron microscopy, infrared absorption, differential thermal analysis, electron paramagnetic resonance, chemical analysis, density and electrical properties. Stable glasses are obtained for 95 to 75 mol % V₂O₅ by quenching on a stainless steel substrate, while quenching on a copper substrate extends the glass formation range from 95 to 70 mol % V₂O₅. The V-O bond vibration in the glasses occurs at 1020 cm^–1 and the V^5% ion exists in six-fold coordination as in crystalline V₂O₅. All the glasses appear to be in single phase. The spin concentration in the glasses is found to be independent of temperature. A second heat-treatment at 255° C develops crystalline phase in the glasses. Unlike infrared absorption, electron paramagnetic resonance, density and chemical compositions, the electrical and thermal (DTA) properties are found to be slightly sensitive to the thermal history of preparation of the glasses. The high-temperature (300 to 500 K) conduction in the glasses seems to be due to adiabatic hopping of polarons. The thermopower is observed to be independent of temperature and provides evidence for small polaron formation in the glasses.     

Novel structural properties of the lead–vanadate–tellurate glass ceramics

In this paper, we have examined and analyzed the effects of systematic intercalation of the lead ions on vanadate–tellurate glass ceramics with interesting results. The structural properties of the lead–vanadate–tellurate glass ceramics of compositions xPbO·(100 − x)[6TeO₂·4V₂O₅], x = 0 − 100 mol%, are reported for the first time. It has been shown by X-ray diffraction that single-phase homogeneous glasses with a random network structure can be obtained in this system. Among these unconventional lead–vanadate–tellurate glass ceramics, we found that network formers are good host material for lead ions and are capable to intercalate a variety of species such as Te₂V₂ ⁵⁺O₉, Pb₃(V⁵⁺O₄)₂, Pb₂V₂ ⁵⁺O₇, and V₂O₅-rich amorphous phase. On the other hand, these glass ceramics contain V⁴⁺ and V⁵⁺ ions necessary for the electrical conduction. Based on these experimental results, we propose that the V⁴⁺=O bonds are created by two different mechanisms: the first of reduction of V⁵⁺ ions to V⁴⁺ ions and thus of creation of V⁴⁺=O bonds.     

A TEM study and non-isothermal crystallization kinetic of tellurite glass-ceramics

The glass transition temperature was studied via differential thermal analysis of glasses in the system (100 − x)TeO₂–5Bi₂O₃–xZnO and (100 − x)TeO₂–10Bi₂O₃–xZnO where x = 15, 20, 25 in mol%. The crystallization behavior and microstructure development of the 0.7TeO₂/0.1Bi₂O₃/0.2ZnO glass during annealing were investigated by non-isothermal differential thermal analysis (DTA), X-ray diffractometry, and transmission electron microscopy. The glass transition temperature, crystallization temperature, and the nature of crystalline phases formed were determined. From the heating rate dependence of the glass transition temperature, the glass transition activation energy was derived. From variation of DTA peak maximum temperature with heating rate, the activation energies of crystallization were calculated to be 305.8 and 197 kJ mol⁻¹ for first and second crystallization exotherms, respectively. Moreover, synthesized crystalline Bi_3.2Te_0.8O_6.4, Bi₂Te₄O₁₁, and Zn₂Te₃O₈ were investigated. In addition, the change in particle size with increasing annealing time was observed by high-polarized optical microscope.     

Effect of iron on the electrical properties of lead–bismuth glasses

Semiconducting oxide glasses of the system (80 − x)Bi₂O₃–20PbO–xFe₂O₃, where x = 5, 10 and 15 mol.%, were prepared and investigated for dielectric properties in the frequency range 120–100 KHz and temperature range 300–550 K. Analysis of the electrical properties has been made in the light of small polaron hopping model. The parameters obtained from the fits of the experimental data to this model are reasonable and consistent with glass composition. The conduction is attributed to non-adiabatic hopping of small polaron. The ac conductivity results suggest that the correlated barrier hopping (CBH) is dominant in ac conductivity.     

Non-linear magnetic susceptibility of some vanadate glasses

The first measurements of the anomalous variation of d.c. magnetic susceptibility of the (50 – x) P₂O₅-xM-50V₂O₅ (M = Bi₂O₃ and Sb₂O₃, and x=0 to 40 mol% M) oxide glasses around 20 to 30 mol% M are reported here. Similar anomalous behaviour was also observed in the electrical and other physical properties of the glasses (reported in the previous paper). Like electrical and dielectric properties, this anomaly in the magnetic properties was also found to be mostly associated with the anomalous variation of the V⁴⁺/V⁵⁺ ratio (around 20 to 30 mol% M) with the concentration of M. From the temperature (80 to 300 K)-dependent magnetic susceptibility data the V⁴⁺ ion concentrations have also been calculated, which agree quite well with those obtained from chemical analysis, and the small discrepancy is attributed to the presence of V³⁺ and/or V²⁺ ions in the glass.     

Non-linear concentration-dependent electrical properties of some semiconducting vanadate glasses

Characterizations of (50 – x) P₂O₅-x M-50V₂O₅ (M = Bi₂O₃, Sb₂O₃, and GeO₂ and x=0 to 45 mol% M) and P₂O₅-Bi₂O₃ semiconducting oxide glasses have been made from studies of electrical conductivities (both a.c. and d.c.) in the temperature range 77 to 400 K. All these glasses showed some interesting non-linear variation of d.c. and a.c. conductivity, together with other properties for particular values of M (between 20 and 30 mol% M). Because the non-vanadate (1–x) P₂O₅-x Bi₂O₃ glasses also showed similar conductivity anomaly (minimum) around 25 mol% Bi₂O₃ with a corresponding maximum in the activation energy (W), it is concluded (in contradiction to earlier suggestions) that not only the ratio (= V⁵⁺/V⁴⁺) but also the network-former ions in the vanadate glasses make a substantial contribution to the anomalous concentration variation of the physical properties of these glasses. The electrical conduction in these glasses is found to be mainly due to hopping of polarons in the adiabatic approximation. At low temperature, the d.c. conductivity obeys Mott's T ^–1/4 behaviour. The a.c. conductivity obeying the general ^s law (exponent s lying between 0.85 and 0.98) supports the theory based on the hopping over the barrier model.     

Nanocrystallization as a method of improvement of electrical properties of Fe2O3–PbO2–TeO2 glasses

Selected glasses of Fe₂O₃–PbO₂–TeO₂ system have been transformed into nanomaterials by annealing at a temperature close to the crystallization temperature (T_c). The effects of the annealing of the present samples on the structural and electrical properties were studied by transmission electron micrograph (TEM), X-ray diffraction (XRD), differential scanning calorimeter, density (d) and dc conductivity (σ). TEM and XRD of glass–ceramic naocrystals indicated nanocrystals embedded in the glassy matrix with average particle size of 20–35 nm. The glass–ceramic naocrystals obtained by annealing at T_c exhibit improvement of electrical conductivity up to four orders of magnitude than the starting glasses. This considerable improvement of electrical conductivity after nanocrystallization is attributed to formation of extensive and dense network of electronic conduction paths which are situated between Fe₂O₃ nanocrystals and on their surface. The conduction is attributed to non-adiabatic hopping of small polaron.     

Surface degradation behaviour of sodium borophosphate glass in aqueous media: Some studies

The degradation behaviour of phosphate glass with nominal composition, 40Na₂O-10BaO-xB₂O₃-(50-x)P₂O₅, where 0 ≤ x ≤ 20 mol%, was studied in water, HCl and NaOH solutions at room temperature to 60°C for different periods extending up to 300 h. These glasses were synthesized by conventional melt-quench technique. Dissolution rates were found to increase with B₂O₃ content in the glass. The dissolution rates for the glass having 10 mol% B₂O₃ were found to be 0·002 g/cm² and 0·015 g/cm² in distilled water and 5% NaOH solution, respectively, at room temperature after 225 h of total immersion period, whereas it increased considerably to 0·32 g/cm² in 5% NaOH at 60°C after 225 h. However, glass samples with x = 15 and 20 mol% B₂O₃ were dissolved in 5% HCl solution after 5 h immersion. The degradation behaviour has been correlated with the structural features present in the glass. The optical microscopy of the corroded surface revealed that the corrosion mechanism were different in acid and alkali media.     

A study of electron spin resonance of copper phosphate glasses containing tellurium oxide

Electron paramagnetic resonance in glasses of compositions (P₂O₅)_100–x-(CuO)_x and (P₂O₅)₅₀-(TeO₂)_50–x-(CuO)_x where x varied from 20 to 40 mol% has been investigated. The distribution of the copper between the two main valency states is determined from measurements of e.s.r. arising from the Cu²⁺ centres. The study of e.s.r. enables the distribution of copper in the glasses between the different valency states to be determined and thus indirectly provides evidence for hopping conduction in the glasses. From the results of e.s.r. and the chemical analysis of the samples it is found that the reduced valency ratioC, i.e. the ratio of the concentration of Cu⁺ ions to that of total copper in the glass, increases with increasing CuO content.