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.     

Magnetic susceptibility of lithium borosilicate glasses containing metal oxide

Lithium borosilicate glasses with different chromium, manganese and cobalt contents were prepared and heat treated at 480, 580 and 680° C for 5 h. The magnetic susceptibilities of untreated and heat-treated samples were measured. The magnetic susceptibility was found to increase as the Cr₂O₃, MnO or CoO content increased. The values for magnetic susceptibility were co-related to the relative proportion of valence states of transition elements. The magnetic susceptibilities were found to have random behaviour with heat-treatment temperatures. Samples containing Cr₂O₃ showed transformations from diamagnetic to paramagnetic behaviour with the increase in Cr₂O₃ content, which may be due to the change of chromium ions from the high valence to the low valence state.     

Structural and other physical properties of barium vanadate glasses

XRD, IR spectra, DTA, density, oxygen molar volume and dc conductivity of barium vanadate glasses of compositions xBaO(100 − x)V₂O₅, where x = 30, 35, 40, 45 and 50 mol%, are reported. The IR studies of the glasses suggest the glass network is built up of mainly VO₄ polyhedra. The glass transition temperatures are observed to increase with an increase of BaO content in the compositions. The cross-linking density decrease with increasing BaO content in the compositions. Introduction of BaO into the V₂O₅ matrix changes the 2D layer structure of the crystalline V₂O₅ into a more complicated 3D structure. 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.     

Electron spin resonance and electrical conductivity of vanadate glasses

Electron spin resonance (ESR) and d.c. conductivity were measured for a series of vanadium borophosphate glasses before and after heat treatment. The ESR spectra showed the presence of vanadium in the V⁴⁺ state in all untreated and heat-treated samples free from iron. The variable temperature ESR and d.c. conductivity results obtained on the sample free from iron showed an inflexion at about 140°C. The electrical conductivity was found to decrease on substitution of 1 mol.% V₂O₅ by 1 mol.% Fe₂O₃ which may be due to a decrease in the V⁴⁺/V ratio. However, the electrical conductivity was found to increase on addition of more than 1 mol.% Fe₂O₃ which may be due to possible hopping conduction between Fe²⁺−Fe³⁺, V⁴⁺−Fe³⁺ and Fe²⁺−V⁵⁺. The increase in conductivity in the sample heat treated at 350°C relative to those heat treated at 300°C and 400°C may be due to the variation in the V⁴⁺/V total ratio. The activation energy values for untreated and heat-treated samples were calculated and were found to depend on the variation in the V⁴⁺/V ratio and the microstructure.     

Structural study of semiconducting and superionic conducting silver vanadate glasses

A structural study of semiconducting and superionic conducting silver vanadate glasses, containing 40 mol% Agl and 1 mol %⁵⁷Fe₂O₃, is performed by means of Mössbauer spectroscopy. Distinct composition dependency of Mössbauer parameters ( and ) of Fe³⁺ ions, having a minimum when Ag₂O content is 30 mol %, suggests that increasing Ag₂O content results in a gradual change of the glass matrix from a two-dimensional layer structure composed of VO₅ tetragonal pyramids to a chain structure composed of VO₄ tetrahedra. An increase in the Mössbauer parameters observed when the Ag₂O content is higher than 30 mol %, corresponding to a metavanadate structure (Ag₂O/V₂O₅=1), indicates a reverse change of the glass matrix into a complicated two- or three-dimensional network structure composed of VO₄ tetrahedra. These conclusions are in good agreement with those obtained from a distinct composition dependency of glass transition temperatures (T _g). Composition dependency of electrical conductivity at room temperature suggests that the structural change of glass matrix primarily affects the semiconductivity caused by a step-by-step electron hopping from V⁴⁺ to V⁵⁺ ions.     

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.     

Semiconducting properties of barium vanadate glasses doped with barium chloride

Semiconducting barium vanadate glasses doped with BaCl₂ from 0 to 15 mol% were studied. Electrical d.c. conductivity measurements and an electron spin resonance investigation in the X-band were made. The concentration dependence of d.c. conductivity exhibits a minimum at 7 mol% BaCl₂. The activation energy increases with increasing BaCl₂ content up to 7% and decreases at higher concentrations of BaCl₂. The temperature dependence of the d.c. conductivity shows the effect of BaCl₂ as an oxidizing agent which controls the electrical conductivity, as a result of modification of the reduced vanadium valence ratio and a change in concentration of the paramagnetic ions. The plots of log versus 1 000/T are linear for all the BaCl₂-doped glasses. The data of the d.c. investigations were analyzed in terms of small polaron theory, and this confirms the applicability of the polaronic hopping model of electrical transport. The influence of BaCl₂ content on density indicates that in this ternary system the additivity role is not obeyed. The minimum occurs at the same concentration of 7 mol% BaCl₂ as noted for the electrical conductivity.     

Magnetic Barkhausen emission study in heat-treated Fe-Nb-Cu-Si-B alloy

Magnetic hysteresis and magnetic Barkhausen emission (MBE) parameters have been studied for as-received and annealed Fe/sub 72/Nb/sub 4.5/Cu/sub 1/Si/sub 13.5/B/sub 9/ alloys. The coercivity rapidly decreased at the initial stage of annealing, in contrast to the slow change of root-mean-square voltage of Barkhausen emissions. The amplitude of the Barkhausen emission signal reduced almost to the background noise level at an intermediate annealing temperature, at which the material exhibited superior soft magnetic properties. Pulse height distribution of the MBE signal showed the existence of a large number of small amplitude pulses at the intermediate range of annealing, an indication that the magnetization process of the system is dominated by the rotation of the magnetization vector within a small volume of nanocrystalline particles. We explain the results by a random anisotropy model, assuming the exchange coupling between the Fe/sub 80/Si/sub 20/ nanograins of higher magnetic moment takes place through the magnetically weaker amorphous matrix.     

Optical absorption near the fundamental absorption edge in some vanadate glasses

The fundamental absorption edges of some samples of V₂O₅-P₂O₅ and V₂O₅-P₂O₅-TeO₂ glasses were measured in the short wavelength part of the visible region, and it was found that the fundamental absorption of these glasses is dependent on composition and arises from direct forbidden transitions and occurs at a photon energy in the range 1.9 to 2.6 eV, depending on composition.     

Magnetic annealing of amorphous alloys

Amorphous alloys with nominal composition of Ni40Fe40P14B6are shown to respond to annealing in a magnetic field. Coercive forces are reduced by a factor of 10 to 50 during annealing of straight ribbons to values of 0.003 Oe, as low as ever reported for potentially useful materials. Concurrently the ratio of the magnetization in 1 Oe applied field, to saturation, increases from about 0.5 to 0.95. These changes during annealing correlate with measured stress relief changes. It thus appears that most of the strain-magnetostriction contribution to the anisotropy is removed during annealing. Magnetic annealing at temperatures as low as 100°C results in noticeable changes in properties. From measurements transverse to the magneticaliy induced anisotropy axis, the induced anisotropy is calculated to be about 800 ergs/cm³, considerably smaller than obtained in crystalline Ni50Fe50. This field-induced anisotropy is reversible in direction and magnitude by reheating the sample to its Curie temperature and then cooling in a field. Annealing of 1.5 cm diameter toroids, made from 50 μm thick tapes, increases the initial permeability by more than a factor of 10 and decreases losses by more than a factor of 10. Losses and permeabilities after heat treatment compare favorably to the Permalloys with similar saturation magnetizations.     

Magnetic properties of amorphous ribbon

The magnetic properties of transition-metal amorphous alloys are discussed, with emphasis on methods to increase the value of the saturation induction at room temperature, and on the factors that control the low-field magnetic properties that are important in most engineering applications.     

Some optical properties of Se-Ge-As amorphous chalcogenide glasses

The effects of composition, film thickness, substrate temperature, and annealing of amorphous thin films of Se₇₅Ge_25−x As _x (5⩽x⩽20) on their optical properties have been investigated. X-ray diffraction revealed the formation of amorphous films. The absorbance and transmission of vacuum-evaporated thin films were used to determine the band gap and refractive index. Optical absorption measurements showed that the fundamental absorption edge is a function of glass composition and the optical absorption is due to indirect transition. The energy gap increases linearly with increasing arsenic content. The optical band gap,E _opt, was found to be almost thickness independent. The shapes of the absorption edge of annealed samples displayed roughly the same characteristic as those of the unannealed films, but were shifted towards shorter wavelengths; as a result,E _opt increased andE _e, the width of the band tails, decreases. The increase inE _opt is believed to be associated with void removal and microstructural re-arrangement during annealing. The influence of substrate temperature on the optical parameters is discussed.     

Study of rigidity of semiconducting vanadate glasses and its importance in use of coatings

The elastic moduli of some multicomponent vanadate based glasses were analysed in terms of the bond compression model by some physical parameters such as, the density, average stretching force constant and average atomic ring size. These parameters were calculated for all the glass series and for all the glass composition to estimate the rigidity of these glasses. The results showed that the average force constant and the elastic moduli of these glasses are sensitive to the decrease in PbO content. This behaviour was attributed to the increase in the molar volume and the role of different modifiers. These parameters along with the coordination number of the glasses affect the glass transition temperature. The correlation between the elastic moduli and thermal properties of these samples showed that 0·25MoO₃–0.25PbO–0·5V₂O₅ glass is the most rigid and has an applicable glass transition temperature for coating.     

Effect of microstructure and fusion temperature on the electrical and optical properties of vanadate glasses

The electrical and optical properties of binary semiconducting oxide glasses containing 45 mol % V₂O₅ and 55 mol % GeO₂ fused and equilibrated at various temperatures (T ) in air were measured.T was varied over the range from 1000 to 1350° C. Their electrical and optical properties are shown to be sensitive to microstructure and melt temperature. We suggest that the change inT caused progressive microstructure changes of these glasses, which dramatically affected the electronic conductivity and the activation enthalpy for conduction.     

Magnetic properties of nickel-strontium-borate oxide glasses

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.