Insertion of iron oxide in calciumsilicate gel-derived materials

Samples of the composition of 10Fe₂O₃·10CaO·80SiO₂ were prepared by the sol-gel method and heat-treated in different atmospheres. They were investigated by X-ray diffraction, scanning electron microscopy and Mössbauer spectroscopy. In the heat-treated samples in air iron is present up to 1000 °C in form of hematite and as Fe³⁺ in the tetrahedral sites. A wide range of hematite particle sizes was observed, the average size increased with heating temperature. At 1000 °C wollastonite was observed, at 1200 °C tridymite was formed and all the iron was incorporated in hematite. A heat-treatment at 500 °C under reducing conditions led to poorly crystallized maghemite and at 700 °C to metallic iron and fayalite formation.     

The Iron Behaviour in Aluminosilicate Gel-Derived Materials

Electron spin resonance (ESR) measurements have been carried out on a 10Fe₂O₃ · 10Al₂O₃ · 80SiO₂ gel heat-treated at different temperatures in air and under reducing conditions. ESR spectra were obtained at 300, 50 and 5 K. The effective g value (g = h/H), linewidth (H_pp) and ESR amplitude (A) depend on heat-treatment temperature of the gel-derived samples. ESR spectra exhibit different magnetic characteristics as a function of heat-treatment temperature and atmosphere. X-ray diffraction (XRD), scanning electron microscopy (SEM) and a.c. susceptibility (_a.c.) analyses were used to better understand the ESR results. The results show that in the samples heat-treated in air, up to 700°C, Fe³⁺ ions are incorporated in the glass network in tetrahedral and/or octahedral co-ordinations. In the samples heat-treated between 250 and 700°C was not detected, by ESR, the presence of iron oxide aggregates. However, the formation of hematite particles was observed by XRD and SEM. The presence of iron oxide aggregates was detected (by ESR) in the samples heat-treated at temperatures higher than 700°C. These aggregates are formed, at 1200 and 1300°C, by hematite and magnetite particles as proved by XRD. The ESR spectra and a.c. susceptibility, of the samples heat-treated at 250°C (under reducing conditions), show a behaviour characteristic of small magnetite particles presence. The sample heat-treated at 500°C (under reducing conditios) contains magnetite particles (XRD). In the ESR spectra of the sample heat-treated at 1000°C, under reducing conditions, was not detected any resonance signal.     

Coloration mechanism in silicate gels and gel-glasses containing iron

The present investigation was carried out in order to study the incorporation of iron oxide in several samples with a general composition xFeO-(100-x)SiO₂ (x between 0.1 and 9.5) and 5R_nO-xFeO-(95-x)SiO₂ (R=Li, Na or Ca) prepared by the sol-gel method. Samples were gelated and dried at 60°C and thermally treated between 200 and 1000°C in oxidizing and reducing atmospheres. The color change and structural evolution were studied by VIS and NIR spectrophotometry, TEM and X-ray diffraction.When the temperature increases, the color changes from yellow to brown-reddishness, going through a bright ruby-red, at the same time the absorption edge shifts to higher wavelengths and at elevated temperatures to smaller wavelengths newly. A part of iron oxide is incorporated into the glass network and the remaining as colloidal particles, producing a double mechanism of coloration by optical absorption and dispersion.In the samples heat-treated in oxidizing conditions all iron is present in the ferric state. When the heat-treatment is carried out in reducing conditions the presence of iron (II) was detected at temperatures of nearly 1000°C.     

Characterization and physical properties of Li2O-CaF2-P2O5 glass ceramics with Cr2O3 as a nucleating agent—Physical properties

In this paper, studies on various physical properties, viz., dielectric properties (dielectric constant, loss tan δ, a.c. conductivity σ) over a wide range of frequency and temperature, optical absorption, ESR at liquid nitrogen temperature and magnetic susceptibility at room temperature of Li₂O-CaF₂-P₂O₅: Cr₂O₃ glass ceramics, have been reported. The optical absorption, ESR and magnetic susceptibility studies indicate that the chromium ions exist in Cr⁵⁺, Cr⁴⁺ and Cr⁶⁺ states in addition to Cr³⁺ state in these samples. The dielectric constant and loss variation with the concentration of Cr₂O₃ have been explained on the basis of space charge polarization mechanism. The dielectric relaxation effects exhibited by these samples have been analysed by a graphical method and the spreading of dielectric relaxation has been established. The a.c. conductivity in the high-temperature region seems to be connected both with electronic and ionic movements.     

Electrical conduction nature and phase transition in CaTi1−x FexO3−δ (x = 0.1–0.5)

Transport numbers for oxygen ions and protons are measured by an emf method in the system CaTi_1?x Fe_xO_3?δ (x = 0.1–0.5) in the oxidizing and reducing atmospheres in the temperature interval 973–1173 K. It is shown that the compounds under study are mixed ion-electron conductors at small iron concentrations and electron conductors, at large iron contents. The proton conductivity in the compounds is very poor and does not exceed 0.5% in air. On the basis of the temperature dependences of transport numbers for ions and linear expansion, it is established that the CaTi_0.9Fe_0.1O_3?δ system has a phase transition of a second order in a reducing environment at 1020–1050 K. The total and partial electron conductivities of CaTi_0.9Fe_0.1O_3?δ are studied as a function of the partial pressure of oxygen at 1173 K. The nature of electroconduction in CaTi_1?x Fe_xO_3?δ is discussed.     

A Mössbauer study of oxygen vacancy and cation distribution in 6H-BaTi1−xFexO3−x/2

We have synthesized samples in the system BaTi_1−xFe_xO_3−x/2 with x=0.1−0.6 at temperatures of 1200-1300°C under reducing conditions of oxygen fugacity. After drop quenching, samples were characterized using the electron microprobe, X-ray diffraction and Mössbauer spectroscopy. All samples were hexagonal with a 6H-BaTiO₃ type structure. Mössbauer spectroscopy showed all iron to be present as Fe³⁺, occurring in octahedral and pentahedral sites. Analysis of area ratios indicates that oxygen vacancies are distributed randomly over O1 sites, and that a random distribution of Fe and Ti cations over M1 and M2 sites is consistent with the data. No evidence for ordering of oxygen vacancies was found. Results are consistent with conductivity results, which show generally increasing ionic conductivity with increasing oxygen vacancy concentration.     

Structure and electrical properties of Y,Fe-based perovskite mixed conducting composites fabricated by a modified polymer precursor method

In this work, samples of Y_0.07Sr_0.93Ti_1-xFe_xO_3-δ with 20, 40, 60 and 80 mol% of iron amount were prepared by a low-temperature polymer precursor method. The SEM-EDS analysis proved that analyzed Y_0.07Sr_0.93Ti_1-xFe_xO_3-δ samples were composites of two Ti- and Fe-rich perovskite samples. This kind of composite consists of two phases in which one has a good ionic and the other electronic conductivity, which makes such a composite a potential mixed ionic and electronic conductors (MIECs) material. The total electrical conductivities of analyzed samples were measured in air atmosphere (cathode conditions in Solid Oxide Fuel Cell). The values changed from ∼10⁻³ to 10⁻¹ S cm⁻¹ and depended on the ratio between two observed perovskite phases. The 0.12 S cm⁻¹ conductivity value at 800 °C for sample with the highest amount of Fe-rich perovskite in the structure makes this composite material a candidate for air electrode in electrochemical devices.     

Study of dielectric and ac impedance properties of citrate-gel synthesized Li0.35Zn0.3Fe2.35O4 ferrite

Nano-crystals of Li_0.35Zn_0.3Fe_2.35O₄ ferrite have been synthesized using citrate precursor method. The sample synthesized was sintered at different temperatures in order to vary their crystallite size. The average crystallite size was found in the range 24?C57?nm by varying the temperature from 300 to 1,100?°C. X-ray diffraction measurements confirmed the formation of cubic spinel structure at all the sintering temperatures in this work. The high frequency performance of the ferrite samples were estimated by measuring the frequency dispersion of the dielectric constant, dielectric loss and ac electrical conductivity. The dielectric constant has been observed to show normal behavior with frequency and decreases with the decrease in crystallite size. It is also observed that decrease in dielectric constant depends on sintering temperature because of lithium evaporation at higher temperature. A low value of dielectric constant and dielectric loss has been found, which makes them applicable for high frequency applications by decreasing the skin effect. The impedance spectroscopy technique has been used to study the effect of grain and grain boundary on the electrical properties of Li_0.35Zn_0.3Fe_2.35O₄ ferrite. The analysis of data shows only one semi-circle corresponding to the grain boundary volume suggesting that the conduction mechanism takes place predominantly through grain boundary volume in the prepared samples.     

The Proportion of Fe‐NX,N Doping Species and Fe3C to Oxygen Catalytic Activity in Core‐Shell Fe‐N/C Electrocatalyst

A bifunctional oxygen electrocatalyst composed of iron carbide (Fe₃C) nanoparticles encapsulated by nitrogen doped carbon sheets is reported. X‐ray photoelectron spectroscopy and X‐ray absorption near edge structure revealed the presence of several kinds of active sites (Fe?N_x sites, N doping sites) and the modulated electron structure of nitrogen doped carbon sheets. Fe₃C@N‐CSs shows excellent oxygen evolution and oxygen reduction catalytic activity owing to the modulated electron structure by encapsulated Fe₃C core via biphasic interfaces electron interaction, which can lower the free energy of intermediate, strengthen the bonding strength and enhance conductivity. Meanwhile, the contribution of the Fe?N_x sites, N doping sites and the effect of Fe₃C core for the electrocatalytic oxygen reaction is originally revealed. The Fe₃C@N‐CSs air electrode‐based zinc‐air battery demonstrates a high open circuit potential of 1.47 V, superior charge‐discharge performance and long lifetime, which outperforms the noble metal‐based zinc‐air battery.     

Structural and dielectric characterization of LiNbO3 nano-size powders obtained by Pechini method

Lithium niobate is an artificially synthesized material with wide technological applications, due to its numerous physical properties, such as: ferroelectricity, birefringence and large pyroelectric, piezoelectric, non-linear, acousto-optic, electro-optic and photo-elastic coefficients. It also exhibits very strong bulk photovoltaic and photo refractive effects. In the present work, LiNbO₃ powders were prepared by the Pechini (sol?Cgel) method. The heat-treatment of the base amorphous powders, at 450?°C promotes the formation of LiNbO₃ crystalline phase. The electrical characterization, in function of the heat-treatment time, shows that the dc conductivity depends on the quantity of crystalline phase, which increases with the increase of heat-treatment time. The sample treated for 96?h shows a dielectric constant value of ~35, at low frequencies. A dielectric relaxation phenomenon independent of the temperature of measurement was detected in the samples treated at 48 and 96?h, shifting to higher frequencies with the increase of the heat-treatment time.     

The thermodynamic properties of weakly bound oxygen and electron transport in Sr3Fe2O6+δ under oxidizing conditions

The content of oxygen, electrical conductivity, and thermal electromotive force were measured for ferrite Sr₃Fe₂O_6+δ over the oxygen partial pressure range 10^?4–0.5 atm at 650–950°C. The partial molar thermodynamic functions of weakly bound oxygen in the oxide structure were determined. Labile oxygen ions were characterized as an ensemble of weakly interacting particles. The predominant charge carriers under experimental conditions were electron holes. An analysis of conductivity was performed using the data on the oxygen content. The concentration, activation energy, and mobility of charge carriers were determined. The results can be satisfactorily interpreted using the polaron transfer model of conductivity taking into account the charge disproportionation reaction for iron ions, 2Fe⁴⁺ = Fe³⁺ + Fe⁵⁺.     

pH-dependent magnetic phase transition of iron oxide nanoparticles synthesized by gamma-radiation reduction method

In this study, the influence of pH variation on structural and magnetic phase transition of gamma radiolytic synthesized iron oxide nanoparticles is investigated. The structure and magnetic properties of irradiated samples are characterized using X-ray diffraction, Fourier transfer infrared spectroscopy, transmission electron microscopy, and vibrating sample magnetometer. It was found that, in acidic irradiated solution, Fe³⁺ ions make various complexes with polyvinyl alcohol and water molecules which exhibit a multiphase magnetic property as a mixture of dia and paramagnetic materials. On the other hand, in basic condition, rate of radiation induced reduction of Fe³⁺ ions increased which leads to the formation of superparamagnetic Fe₃O₄ nanoparticles. By increasing pH value, in strong basic condition, the tendency of paramagnetic iron (III) oxy-hydroxide formation was high compared to other phases. This variation in the magnetic properties was explained based on iron ions reduction mechanism and the variation of the ligands’ properties during formation of nanoparticles under irradiation.     

X-ray photoelectron spectroscopy investigation of perovskites La1−x Sr x FeO3−y (0 ≤x < 1.0), prepared via a mechanochemical route

Perovskite-structure oxides La_1?x Sr _x FeO_3?y (x = 0, 0.2, 0.6, 1) were synthesized by the mechanochemical method. In order to refine the stoichiometric composition and the charge state of ions, these samples were studied by X-ray photoelectron spectroscopy (XPS). An investigation of perovskites with x = 0, 0.2, and 0.6 in air at room temperature showed that these samples do not contain oxygen vacancies (y = 0), i.e., they are fully oxidized. Hence, to produce electrical neutrality, these samples should contain iron(4+) cations in an amount proportional to the degree of substitution (x) of strontium(2+) for lanthanum(3+). However, no Fe⁴⁺ cations were found in the oxides. All perovskites contain only Fe³⁺ cations, oxygen ions O^2? along with oxygen ions with reduced electron density (O^?). These data provid evidence of the possible electron density redistribution from oxygen ions to iron cations. The fact that the oxides contain oxygen ions with reduced electron density suggests that weakly bound lattice oxygen in substituted perovskites is represented by O^? ions.     

Preparation of YSZ Coated AISI-Type 316L Stainless Steel by the Sol-Gel Coating Method and Its Corrosion Behavior in Molten Carbonate

The effect of heat-treatment conditions on the corrosion behavior of yttria stabilized zirconia (YSZ) coated AISI-type 316L stainless steels in molten carbonate are investigated. YSZ films on stainless steel were prepared by the sol-gel method. While the sample heat-treated at 800°C for 2 hours in air has an uneven surface microstructure with macrocracks, a YSZ coated sample heat-treated in argon has an even microstructure. The polarization resistance and corrosion rate of YSZ coated sample heat-treated in air were deteriorated from 6.948 cm² and 364.7 mpy (millimeter per year) to 3.291 cm² and 769.8 mpy, respectively, by the corrosion for 100 hours due to its poor surface microstructures. At the same experimental condition, meanwhile, those corrosion parameters of YSZ coated sample heat-treated in argon were 15.43 cm² and 164.2 mpy, respectively, and those were improved to 18.83 cm² and 134.6 mpy after the corrosion for 100 hours. This is attributed to the YSZ film with well developed surface microstructures. The concentration profiles of elements and X-ray diffractograms indicate that the oxide layer of YSZ coated sample heat-treated in argon had a triple layer structure composed of outer YSZ film, in between Fe₂O₃ layer, and inner chromium rich layer during corrosion.     

Electrical properties and initial permeability of Cu–Mg ferrites

A series of polycrystalline spinel ferrites with composition Cu_1−xMg_xFe₂O₄ where 0.0 ≤ x ≤ 1 are prepared by the standard ceramic method. The single-phase cubic spinel structure of all the samples has been confirmed from X-ray diffraction analysis. The lattice constant increases linearly with increasing magnesium content obeying Vegard's law. The electrical properties (ɛ′, and σ) of the prepared samples are measured at different temperatures as a function of applied frequency ranging from 100 kHz up to 5 MHz. The general trend of ɛ′, and σ is decreased with increasing Mg²⁺ and increases with increasing temperature. The observed variation of dielectric properties is explained on the basis of Cu²⁺/Cu¹⁺ ionic concentration as well as the electronic hopping frequency between Fe²⁺ and Fe³⁺ ions in the present samples. The data of initial permeability is also discussed.     

Mg2MTaO6 (M=Nd or La): a group of new pyrochlore oxides

Preparation of two novel mixed metal oxide ceramic materials, namely magnesium neodymium tantalum oxide (Mg₂NdTaO₆) and magnesium lanthanum tantalum oxide (Mg₂LaTaO₆) by conventional solid-state reaction method is reported in this paper. The crystal structure of these new compounds, were studied by indexing the X-ray diffraction patterns, powder pattern calculation and profile fitting. They were found to have a defective cubic pyrochlore structure, with the A site being randomly occupied by Mg and La/Nd, while, Ta and Mg are randomly distributed at the B site. The formula assigned were (MgNd)(MgTa)O₆ and (MgLa)(MgTa)O₆. The variation of dielectric constant, dielectric loss and conductivity of sintered pellets of these materials with applied frequencies in the range of 30 Hz-1 MHz were studied at room temperature. These room temperature studies at 1 MHz gave dielectric constant values of 24.8 and 25.35; conductivity values of 7.75×10⁻⁶ and 8.27×10⁻⁶ S/m as well as dielectric loss values of 0.0055 and 0.006 for Mg₂NdTaO₆ and Mg₂LaTaO₆, respectively. These new pyrochlore compounds were found to have dielectric constant, dielectric loss and conductivity values in the range suitable for possible electronic ceramic applications.     

Effect of ZnNiCrFe<Subscript>2</Subscript>O<Subscript>4</Subscript> content on structural and dielectric properties of polyaniline based nanocomposites,synthesized via in-situ chemical polymerization

Zn_0.5Ni_0.4Cr_0.1Fe₂O₄ nanopowder and its composite with polyaniline were successfully prepared by using wow sol-gel and in situ chemical polymerization respectively. The samples were characterized by X-ray diffraction and field emission scanning electron microscopy. Dielectric properties were investigated as function of frequency by using impedance analyzer. The results showed the presence of the two intended phases. The ac conductivity was found to obey Jonscher’s universal power law. The dielectric constant and loss showed dispersion in low frequency region. Impedance analysis revealed the semiconducting behavior of the investigated samples.     

The Synthesis, Characterization and Microwave Properties of ZnCo-Substituted W-Type Barium Hexaferrite, from a Sol-Gel Precursor

The preparation of BaZn_{2 – x} Co _x Fe₁₆O₂₇ W-type hexaferrites powders by a citrate sol-gel method has been investigated. The samples were characterized by TG-DSC, X-ray diffraction (XRD), scanning electron microscopy (SEM). The complex dielectric constant and complex permeability of hexaferrite-paraffin wax composites were measured by the transmission/reflection coaxial line method in the range from 50 MHz to 3 GHz. The dependence of complex dielectric constant and permeability on annealing temperature, composition and measuring frequency was presented.     

Influence of the thermal treatment conditions and composition of bimetallic catalysts Fe—Pd/SiO2 on the catalytic properties in phenylacetylene hydrogenation

The supported bimetallic Fe—Pd/SiO₂ catalysts with the different Fe (0.025—8 mass.%) and Pd (0.05—3.2 mass.%) loadings were synthesized by the incipient wetness impregnation of support. The samples were heat-treated under different conditions (calcination in air at 240—350 °C or reduction in an H₂ flow at 400 °C). The X-ray phase analysis revealed the formation of Pd⁰, α-Fe₂O₃ and Fe₃O₄ phases after calcination of the samples at 240—260 °C. The reduction of the calcined Fe—Pd samples in an H₂ flow at 400 °C enables the formation of Fe⁰ nanoparticles of size 17—20 nm. The synthesized catalytic systems were studied in the selective hydrogenation of phenylacetylene at room temperature and atmospheric pressure in a solvent (ethanol, propanol). The catalytic properties of the Fe—Pd catalysts depend on the nature of solvent, catalyst composition, and thermal treatment conditions. The application of the Fe—Pd bimetallic catalysts with a low Pd loading of 0.05—0.1 mass.% made it possible to reach the high activity and selectivity to styrene (91%) at the complete conversion of phenylacetylene.     

Dielectric spectroscopic analysis of polyvinyl chloride nanocomposites loaded with Fe2O3 nanocrystals

Iron oxide (α‐Fe₂O₃) nanocrystals powder was successfully synthesized via the sol‐gel method. The microstructural examination of the synthesized nanocrystals confirmed the formation of α‐Fe₂O3 (hematite) structure using X‐ray diffraction and Fourier transform infrared. The synthesized nano‐hematite powder with different weight percentage up to 5 wt% was introduced to polyvinyl chloride (PVC) to fabricate PVC/Fe₂O₃ nanocomposites using the solution‐cast technique. The dielectric spectroscopic analysis for the investigated samples has been studied at room temperature and at different temperatures up to 120°C. The real part of the permittivity (ε^′) exhibited a significant dependence on filler concentrations throughout whole temperature range. However, the dependency of both of the loss tangent (tanδ ) and AC conductivity (σ_ac) on filler concentrations is more pronounced at temperatures higher than room temperature. The obtained values of tan δ for the investigated nancomposites referred to the α‐relaxation around 70°C, which is close to glass transition temperature of the investigated PVC. The dependency of the dielectric strength on Fe₂O₃ nanofiller concentration was observed with enhancement in the dielectric strength reach to 20.5% for PVC/0.7 wt% Fe₂O₃ nanocomposite higher than the recorded value for the pristine PVC.