Structural studies of some V2O5-P2O5-B2O3-Fe2O3 glass systems

X-ray diffraction and infrared measurements were performed on vanadium borophosphate glass containing different amounts of iron ranging from 0–7.5 mol % and heat treated at 300 °C for various times. The structure and phase separation could be determined for each glass composition. V₂O₅ was the main precipitated phase in all heat-treated samples, and its amount was dependent on the heat-treatment time and Fe₂O₃ content. Also FeP was detected in samples heat treated for 24 h. The infrared measurements showed the presence of both V⁴⁺ and V⁵⁺. The symmetry of V₂O ₇ ⁴⁻ and VO ₄ ³⁻ groups was found to increase with increasing Fe₂O₃ content. It was also found that some PO₄ changed to BO₃, forming a non-bridging oxygen.     

An infrared study of the Na2O-V2O5-Fe2O3 glass system

Structural studies of Na₂O-V₂O₅-Fe₂O₃ glasses have been made from their IR spectra which show that vibrational bands characteristic of the vanadium-oxygen bonds in V₂O₅ are maintained in these glasses, but the addition of Na₂O to these glasses results in a shifting of the higher frequency peaks towards lower wave number due to structural changes produced in V₂O₅. It is inferred that Na⁺ ions make bonds interstitially with isolated V=O bonds and VO₅ polyhedra are destroyed, resulting in the formation of VO₄ polyhedra through intermediate complexes. The variation of Fe₂O₃, however, produces an insignificant structural change in these glasses. The IR spectra of samples heated to their temperature of crystallization confirm the formation of a series of complexes with several isolated V=O bonds.     

A further investigation on the MnO2-Fe2O3 system roasted under CO-CO2 atmosphere

Investigations on the MnO₂-Fe₂O₃ system roasted in air has been reported in our previous work. This study further investigated the MnO₂-Fe₂O₃ system roasted under CO-CO₂ atmosphere. Extensive investigations were concentrated on the reduction of simplex iron oxides or manganese oxides, and little attention were paid on the reduction of MnO₂-Fe₂O₃ system regarding to interactive reactions between them. In this work, it was found that spinel-type Mn_xFe_3?xO₄ with high magnetism formed easily under CO-CO₂ atmosphere. The reduction and thermodynamic analyses of pure MnFe₂O₄ were also researched to better understand the reduction behaviors of MnO₂-Fe₂O₃ system. Phase study showed that a series of Mn-Fe composite oxides, including Mn_xFe_3?xO₄ and (MnO)_y(FeO)_1?y, generated during the reduction of MnO₂-Fe₂O₃ system. Mn_xFe_3?xO₄ was readily generated under CO content of 2.5–25?vol% at 1000?°C. With further increase of CO content, Mn_xFe_3?xO₄ was reduced to (MnO)_y(FeO)_1?y and then to MnO and metallic iron. Reduction of manganese oxides, iron oxides and manganese ferrites happened concurrently during the reduction of MnO₂-Fe₂O₃ system. And the reduction of MnO₂, Fe₂O₃, Fe₃O₄ and MnFe₂O₄ were compared by TG and thermodynamic analyses. In addition, the morphology evolution and magnetism change of the MnO₂-Fe₂O₃ system reduced under different CO contents were also studied.     

Facile synthesis of capped γ-Fe2O3 and Fe3O4 nanoparticles

Facile methods for the selective preparation of capped iron oxide nanoparticles (γ-Fe₂O₃, Fe₃O₄) are described. The magnetic oxides are obtained via oxidative transformation of an iron hydroxide gel using H₂O₂ or (NH₄)₂S₂O₈ solutions as oxidants. Capping with oleic or other aliphatic acids is established simultaneously in one step by adding a toluene solution of the capping agent and refluxing the resulting biphase system. The method is simple, soft and affords nanoparticles of γ-Fe₂O₃ or Fe₃O₄ of controlled size depending on the reaction conditions. The capped nanoparticles are readily soluble in organic or aqueous media according to the nature of the sheath surrounding the surface of the particles, providing stable and high concentration ferrofluids.     

Electron spin resonance studies of evaporated V2O5 and co-evaporated V2O5/B2O3 thin films

Electron spin resonance studies of evaporated V₂O₅ and co-evaporated V₂O₅/B₂O₃ amorphous thin films have been made. For lower molar contents of B₂O₃, the co-evaporated V₂O₅/B₂O₃ films show poorly resolved hyperfine structure, whereas for higher content of B₂O₃ the hyperfine spectra are well resolved. This behaviour of films is attributed to the increase in the lifetime of a particular V⁴⁺ ion due to Anderson localization of charge, as the degree of disorder increases with increase in the molar content of B₂O₃. The unpaired electron at a given time is localized on a single ⁵¹V nucleus. The low intensity of ESR signal for higher concentration of V₂O₅ in the co-evaporated V₂O₅/B₂O₃ films has been related to the less effective concentration of V⁴⁺ ions due to antiferromagnetic coupling of the V⁴⁺ ions.     

Magnetic properties of the phases Fe2V4O13, FeVMoO7 and Fe4V2Mo3O20

The Curie constant,C, Weiss constant, , effective magnetic moment, _eff, and spectroscopic splitting factor,g, were determined for the Fe³⁺ ions in Fe₂V₄O₁₃, FeVMoO₇ and Fe₄V₂Mo₃O₂₀ at 76–300 K based on measurements of magnetic susceptibility of the phases. The Neel temperature,T _N, of interest was based on the temperature dependence of magnetization of the phases. It was shown that a local antiferromagnetic arrangement of the Fe³⁺ ions in Fe₂V₄O₁₃, FeVMoO₇ and Fe₄V₂Mo₃O₂₀ is already involved at a temperature much higher than the Neel temperature, resulting from the cation-anion-cation superexchange between the Fe³⁺ ions with ad ⁵ configuration.     

Hydrothermal synthesis of Fe3O4 and α-Fe2O3 nanocrystals as anode electrode materials for rechargeable Li-ion batteries

This paper describes a facile, economical and environment-friendly hydrothermal method of fabricating Fe₃O₄ and α-Fe₂O₃ nanoparticles at 180 °C for 12 h, respectively. The as-obtained products were characterized in detail. X-ray powder diffraction and transmission electron microscopy were used to investigate the products’ properties of crystal form, size, and morphology. The results showed the Fe₃O₄ and α-Fe₂O₃ nanocrystals’ diameter were about 5 and 20 nm, respectively. Moreover, the electrochemical performances of the Fe₃O₄ and α-Fe₂O₃ nanoparticles as anode materials for Li-ion batteries were also evaluated. The first-discharge capacities of Fe₃O₄ and α-Fe₂O₃ nanocrystals were 1,380 and 1,280 mAh g^?1, and stabled about 96 and 75 mAh g^?1 after 20 cycles, respectively. These materials offer substantial promise for developing alternative, high capacity negative electrodes for safer lithium batteries as energy storage and conversion materials.     

Effect of the addition of Fe2O3 and heat treatment duration on the magnetic susceptibility of V2O5-P2O5-B2O3 glass system

The effect of the addition of Fe₂O₃ and heat treatment duration on the magnetic susceptibility of vanadium borophosphate glass were studied. The magnetic susceptibility of glass samples was found to increase with increasing Fe₂O₃ content, which may be explained by the formation of the FeO₆ group and the change of Fe²⁺ to Fe³⁺ which has higher paramagnetic properties. No detectable changes in the magnetic susceptibility with heat treatment for the samples containing 0.0, 0.5 and 1.0 mol% Fe₂O₃ was observed. The magnetic susceptibility for the heat treated samples containing 2.5, 5.0 and 7.5 mol% Fe₂O₃ decreases sharply with increasing duration of heat treatment up to 6 h and then remains almost constant. The sharp decrease in magnetic susceptibility of 2.5 mol% Fe₂O₃ is attributed to the increase in the number of ferrous ions. The sharp decrease for samples containing 5.0 and 7.5 mol% Fe₂O₃ is attributed to the increase in the number of Fe³⁺ in tetrahedral co-ordination. The rate of crystallization owing to the heat treatment was calculated and was found to increase with increasing iron oxide content. The geometry of crystallization was found to be in three-, two-and one-dimension(s) for samples containing 2.5, 5.0 and 7.5 mol% Fe₂O₃, respectively.     

Sintering behaviour of lithium ferrites containing Nb2O5 and V2O5

Lithium ferrite toroids with various compositions and with various amounts of Nb₂O₅ or V₂O₅, have been prepared by a powder compacting method, and their sintering behaviours, microstructures and weight-loss characteristics have been investigated. The rate of sintering is higher in an Fe₂O₃-deficient lithium ferrite than in an Fe₂O₃-excess lithium ferrite due to the presence of excess oxygen vacancy in the Fe₂O₃-deficient lithium ferrite during sintering. The presence Of LiNbO₃ or LiVO₃, which is formed by Nb₂O₅ or V₂O₅, on the surface of lithium ferrite grains causes an increase in oxygen activity of the lithium ferrite and a decrease in the rate Of lithium evaporation during sintering, resulting in a strong enhancement in the sintering rate. The enhancing effect of V₂O₅ in the sintering Of lithium ferrite is stronger than that of Nb₂O₅ due to the possible formation of liquid V₂O, before the formation of LiVO₃.     

A simple route to V2O5·xH2O bundle-like nanostructures

V₂O₅·xH₂O bundle-like nanostructures composed of nanobelts have been synthesized by a simple hydrothermal method with the aid of Co²⁺. The widths, thickness and lengths of V₂O₅·xH₂O nanobelts are 80–100 nm, 10–20 nm, and several micrometers, respectively. The morphologies and the crystallographic structures of the V₂O₅·xH₂O bundle-like nanostructures were characterized by X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and electron diffraction (ED). The effects of the inorganic ions and reaction temperature on the morphologies of the resulting products have been investigated.     

Optical absorption spectra of evaporated V2O5 and co-evaporated V2O5/B2O3 thin films

The optical absorption spectra of evaporated V₂O₅ and co-evaporated V₂O₅/B₂O₃ thin films have been studied. For higher photon energies, the absorption is found to be due to a direct forbidden electronic transition process from the oxygen 2p band to the vanadium 3d band in a similar way to that observed in crystalline V₂O₅. The exponential behaviour of absorption edge for lower photon energies is attributed to the electronic transitions between the tailed-off d-d states corresponding to V⁴⁺ ions. For co-evaporated V₂O₅/B₂O₃ films the optical energy gap is observed to increase with the increase in V₂O₅ content of the composite films.     

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.     

X-ray photoelectron spectroscopic studies of evaporated V2O2and co-evaporated V2O5/B2O3 films

X-ray photoelectron spectra of evaporated V₂O₅ and co-evaporated V₂O₅/B₂O₃ thin films have been investigated. The photoelectron spectrum of a simple V₂O₅ film shows the splitting of the V 2p level in accordance with the spins. The values of binding energies corresponding to V 2p and O1s are comparable with those reported previously. For co-evaporated V₂O₅/B₂O₃ films a chemical shift in the O 1s level has been observed which has been attributed to the changed chemical environment of oxygen as a result of the presence of boron and vanadium atoms. The values of binding energies for V 2p_3/2 and O 1s corresponding to simple evaporated V₂O₅ and co-evaporated V₂O₅/B₂O₃ show the presence of V₂O₄ species in the films.     

Effect of heat treatment and irradiation on electrical conductivity and crystallization in the BaO-B2O3-Fe2O3 glass system

A glass system was prepared according to the formula 75 mol % B₂O₃-(25 –x) mol % BaO –x mol % Fe₂O₃, wherex = 0, 1, 2.5, 5, 7.5 and 10. The glasses were subjected to heat treatment at 550° C for 2, 6, 12, 18 and 24 h. The glasses were also irradiated using-rays at a dose of 4.805 × 10⁴ rad h^–1 for 12, 18 and 24 h. An X-ray diffraction technique was used to identify the separated crystalline phases. The electrical conductivity and activation energy of untreated, heat-treated and irradiated samples were measured and calculated. The rate and the dimensions of crystallization were also calculated by using the Avrami equation. It was found that-Fe₂O₃ is the separated phase when a sample containing 7.5 mol% Fe₂O₃ is heat treated for 24h;-Fe₂O₃ and Fe₂O₃ are the separated phases when the sample containing 10 mol% Fe₂O₃ is heat treated for 6, 12 and 18 h, with the addition of BaO when the sample is heat treated for 24 h. A miminum value for the electrical conductivity of glass samples was found to occur around an Fe₂O₃/BaO ratio of 0.425. The rate of crystallization in the sample containing 10 mol% Fe₂O₃ is 1.30607 × 10^–3 and the geometry of crystallizationn is 1.2238, which indicates that the crystallization was in one dimension.     

Some electrical and magnetic properties of γ-Fe2O3

γ-Fe₂O₃ synthesized from FeC₄H₄O₄·4H₂O has been studied using various techniques. The phase transformation observed by electrical conductivity measurements agrees well with the initial magnetization measurement. The magnetic hysteresis values compare with those ofγ-Fe₂O₃ samples synthesized using established procedures.γ-Fe₂O₃ particles obtained were circular in shape showing a well resolved six narrow bands in Mössbauer spectrum. The presence of hydrogen ferrite phase was also confirmed by electrical and magnetic measurements.     

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.     

Synthesis and characterization of γ-Fe2O3 — a magnetic tape material

Acicular FeC₂O₄ · 2H₂O was precipitated from glycerol and starch media. Thermal decomposition of this oxalate in dry and moist nitrogen yielded primarily FeO and Fe₃O₄ respectively. Characterization was attempted through DTA, TG, x-ray diffraction, TEM and magnetization studies. It was found that the oxalate can be completely decomposed to Fe₃O₄ in moist nitrogen (P_{H 2}O ∼ 35 torr) at 775 K and then oxidised by dry air to acicular γ-Fe₂O₃ at 575 K. The resulting material has saturation magnetization (∼ 70 emu/g), coercive field (∼ 300 Oe) and squareness ratio (∼ 0·60–0·65), which values art comparable with those of the commercial samples.     

Preparation and characterisation of γ-Fe2O3 as tape recording material

Optimum conditions for the preparation of tape recording quality γ-Fe₂O₃ by the thermal decomposition of ferrous oxalate dihydrate have been established. Formation of the intermediate Fe₃O₄ which is most important in forming γ-Fe₂O₃ takes place only in the presence of water vapour. Various stages of decomposition have been characterised by DTA, TG, DTG, and x-ray powder diffraction. The method for the preparation of acicular γ-Fe₂O₃ that matches very well with the commercial tape recording material has been developed.     

New understanding on formation mechanism of CaFe2O4 in Fe2O3 -Fe3O4-CaO-SiO2 system during sintering Process: Phase transformation and morphologies evolution

The complex calcium ferrite is the main binder phase of high basicity, high iron and low silicon sinter used in blast furnace (BF), and its formation amount and structure are important factors affecting sinter quality. This work research on the phase transformation and morphologies evolution of Fe₂O₃ -Fe₃O₄-CaO-SiO₂ systems roasted 900 °C ? 1200 °C in air atmosphere to understand the formation process of calcium ferrite. In CaO-Fe₃O₄ system, the prolonged roasting time and higher temperature promotes that CaFe₅O₇, CaFe₃O₅, and Ca₂Fe₂O₅ gradually transformed into the stable existence of CaFe₂O₄. In CaO-Fe₃O₄ system, the higher temperature promotes that the combination of Fe₃O₄ and CaO formed the stable CaFe₃O₅ with orthorhombic structure. The replacement reaction between the newly formed CaFe₃O₅ phase and the unreacted CaO phase occur to form Ca₂Fe₂O₅. In CaO-Fe₃O₄-SiO₂ system, FeSiO₃ can be combined with Fe₃O₄ to form Fe₂SiO₄ and Fe₂O₃. Under the action of high temperature, FeSiO₃ and CaO undergo displacement reaction to form the unstable CaSiO₃, the new formation of CaSiO₃ can be easily combined with CaO to form the stable Ca₂SiO₄. With the further increase of temperature, the complex calcium ferrite is formed in calcium silicate layer. The final product complex calcium ferrite and calcium silicate exist at the same time. The formation of calcium silicate has an unfavorable effect on formation of complex calcium ferrite in the sintering process.     

Preparation and characterization of V2O5 macro-plates

V₂O₅ macro-plates have been prepared using water/ethanol media. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The as-prepared V₂O₅ plates appear to be an orthorhombic structure, and they seem to be composed of strip-like V₂O₅ nanocrystals which appear to be parallel or perpendicular to each other. The band gap of the V₂O₅ macro-plates calculated from the optical absorption edge is about 3.4 eV, showing a strong blue shift compared to bulk V₂O₅. It is also found that the final shape of the V₂O₅ macro-plates strongly depends on the reaction conditions. The possible formation mechanism of the V₂O₅ macro-plates is discussed.