X-ray,electrical conductivity,and infrared studies of oxide spinels containing cobalt,titanium and manganese

X-ray, electrical conductivity and infrared studies of the system Co_2–x Ti_1–x Mn_2x O₄ were carried out with a view to investigate the cation distribution. Compounds 0 x 0.6 showed cubic symmetry, compounds 0.8 x 1 are tetragonal. X-ray intensity calculation indicated the presence Of Co²⁺ and Mn³⁺ at both octahedral and tetrahedral sites. A plot of activation energy against composition shows a break where crystal symmetry changes. The electrical conductivity-temperature behaviour obeys the Raschhinrichsen law. The mobility of holes calculated from infrared and conductivity data is of the order Of 10^–9 cm² V^–1 sec^–1. X-ray intensity calculations and conductivity measurements suggest the ionic configuration of the system to be Co _1–x ²⁺ Mn _x ³⁺ [Co _2–2x ²⁺ Mn _x ³⁺ Ti _x ⁴⁺ ]O ₄ ^2– .     

Mesoporous tin-doped indium oxide thin films: effect of mesostructure on electrical conductivity

We present a versatile method for the preparation of mesoporous tin-doped indium oxide (ITO) thin films via dip-coating. Two poly(isobutylene)-b-poly(ethyleneoxide) (PIB-PEO) copolymers of significantly different molecular weight (denoted as PIB-PEO 3000 and PIB-PEO 20000) are used as templates and are compared with non-templated films to clarify the effect of the template size on the crystallization and, thus, on the electrochemical properties of mesoporous ITO films. Transparent, mesoporous, conductive coatings are obtained after annealing at 500 °C; these coatings have a specific resistance of 0.5 Ω cm at a thickness of about 100 nm. Electrical conductivity is improved by one order of magnitude by annealing under a reducing atmosphere. The two types of PIB-PEO block copolymers create mesopores with in-plane diameters of 20–25 and 35–45 nm, the latter also possessing correspondingly thicker pore walls. Impedance measurements reveal that the conductivity is significantly higher for films prepared with the template generating larger mesopores. Because of the same size of the primary nanoparticles, the enhanced conductivity is attributed to a higher conduction path cross section. Prussian blue was deposited electrochemically within the films, thus confirming the accessibility of their pores and their functionality as electrode material.     

Mesoporous tin-doped indium oxide thin films: effect of mesostructure on electrical conductivity

Abstract

We present a versatile method for the preparation of mesoporous tin-doped indium oxide (ITO) thin films via dip-coating. Two poly(isobutylene)-b-poly(ethyleneoxide) (PIB-PEO) copolymers of significantly different molecular weight (denoted as PIB-PEO 3000 and PIB-PEO 20000) are used as templates and are compared with non-templated films to clarify the effect of the template size on the crystallization and, thus, on the electrochemical properties of mesoporous ITO films. Transparent, mesoporous, conductive coatings are obtained after annealing at 500 °C; these coatings have a specific resistance of 0.5 Ω cm at a thickness of about 100 nm. Electrical conductivity is improved by one order of magnitude by annealing under a reducing atmosphere. The two types of PIB-PEO block copolymers create mesopores with in-plane diameters of 20–25 and 35–45 nm, the latter also possessing correspondingly thicker pore walls. Impedance measurements reveal that the conductivity is significantly higher for films prepared with the template generating larger mesopores. Because of the same size of the primary nanoparticles, the enhanced conductivity is attributed to a higher conduction path cross section. Prussian blue was deposited electrochemically within the films, thus confirming the accessibility of their pores and their functionality as electrode material.     

Lanthanum oxide doping effect on dc electrical conductivity of ferroelectrics sodium vanadate and rubidium vanadate

The temperature dependence of dc electrical conductivity was measured by two-probe technique in the vicinity of phase transition point for ferroelectrics sodium vanadate and rubidium vanadate doped with different concentrations of La₂O₃. These materials show a sharp change in conductivity at their phase transition temperatures. The results were found to obey the conventional exponential law and the activation energies were calculated for ferroelectric and paraelectric states. It was found that activation energy in ferroelectric phase is smaller than in the paraelectric phase. The activation energy increases slowly with increase in doping concentration of La₂O₃ up to 0·1 mol%, however, it decreases with further increase in doping concentration, in both ferro and para states. The dc electrical conductivity below the Curie temperature is of mixed type (ionic-electronic) while it is electronic type above the Curie temperature.     

The electrical conductivity of some azine compounds

Correlation of the molecular structure and thermal study of the electrical conductivity of benzaldazine (BA) and its NO₂-derivatives were carried out. The values of the activation energies, obtained from the electrical conductivity measurements, as well as the energy gaps calculated from ultra violet (u.v.) and visible spectra obtained either in liquid or in solid forms indicate that all compounds behave like a semiconducting material. Both n*s and *s were found to be the main sources participatiog in the cooduction processes.     

The electrical conductivity of doped and undoped uranium oxide

The conductivity of undoped and doped uranium dioxide was investigated over the temperature range of approximately 300–1600 K. Activation energies calculated for the undoped samples and those doped with Cr₂O₃ and Gd₂O₃ are in agreement with previously published values. The values calculated for TiO₂- and La₂O₃-doped samples were nearly twice that for the undoped UO₂. The addition of Nb₂O₅ resulted in the extension of the intrinsic region of conductivity to lower temperatures than had been previously reported. The dependence of the extrinsic conductivity of Gd₂O₃- and Cr₂O₃-doped samples on the dopant level supports an impurity compensation model of conduction.     

Direct current electrical conductivity of gamma ferric oxide

The study of the direct current electrical conductivity, , of freshly prepared -Fe₂O₃ and that of a sample stored for seven days in static air suggests that -Fe₂O₃ adsorbs oxygen and water from the atmosphere. From infra-red spectra it is deduced that the absorbed water in -Fe₂O₃ is present as the physically adsorbed water and as lattice water. The adsorbed oxygen and physically adsorbed water are removed by heating to 100 C, while the lattice water remains in -Fe₂O₃ even up to 280 C. The removal of lattice water is associated with a decomposition during which some of the hydrogen formed occupies the vacancy sites. This suggested formation of the hydrogen ferrite phase is based on the kink in the log against T ^–1 curve observed at 177 C. This kink is very well resolved for a sample equilibrated at 100 C in normal atmosphere, and the measurements of above 100 C of this sample are done in an N₂ atmosphere. The suggestion that the hydrogen ferrite phase is formed has been substantiated by comparison of the X-ray diffraction patterns of -Fe₂O₃ heated under the different atmospheres. From the log against T ^–1 plot for a sample heated under a nitrogen atmosphere the activation energy is small (< 0.05 eV) up to 215 C, and it is comparatively large (0.95 eV) above 215 C. These results suggest a hopping mechanism for the direct current electrical conductivity of -Fe₂O₃. This suggestion has been substantiated by data of the temperature variation of Seebeck voltage.     

Influence of praseodymium oxide/cobalt oxide ratio on microstructure and electrical properties of zinc oxide varistor ceramics

The microstructure and electrical properties of varistor ceramics, which are composed of (99.5–x–y)ZnO+xPr₆O₁₁+yCoO+0.5Dy₂O₃ system, were investigated with Pr₆O₁₁/CoO mole ratio (x/y=0.5/0.5, 0.5/1.0, 1.0/0.5, 1.0/1.0) and sintering temperature. The density of varistor ceramics with Pr₆O₁₁=1.0 was almost constant with sintering temperature, whereas it was increased noticeably in Pr₆O₁₁=0.5. Increasing Pr₆O₁₁ content enhanced the densification for any CoO content and the density was greatly affected not by CoO content but by Pr₆O₁₁ content. The varistor ceramics with Pr₆O₁₁/CoO=0.5/1.0 exhibited a higher nonlinearity than any other composition ratios. In particular, the varistor ceramics sintered at 1350 °C. exhibited the best electrical properties, with nonlinear exponent of 37.8, leakage current of 7.6 μA, and dissipation factor of 0.059. It was found that Pr₆O₁₁/CoO ratio greatly affects various characteristics of varistor ceramics.     

Thermal decomposition and electrical conductivity of oxide cathode emission materials

Thermal decomposition and electrical conductivity of oxide cathode emission materials used for cathode ray tubes (CRTs) have been studied under different heat treatment conditions for commercial sprayed cathode systems based on barium-strontium carbonate precursors. Conversion of the carbonate precursor commenced at temperatures above approximately 700 K in vacuum, evidenced by increases in conductivity, however, the rate of the conversion reaction increased dramatically as the temperature was increased. The corresponding chemical and microstructural changes have also been investigated by thermo-gravimetric analysis (TGA) and scanning electron microscopy (SEM), with multiple decomposition stages identified corresponding to the conversion of the carbonate precursor and separate activation steps associated with the reaction of barium oxide with the Mg and Al activating agents in the nickel cathode substrate. © 2000 Kluwer Academic Publishers     

Air effect on electrical conductivity of oriented doped polyacetylene

Journal of Materials Science Letters -     

Impact of the memory effect on the catalytic activity of Li-Al hydrotalcite-like compounds for the cyanoethylation reaction

The target of this work is to investigate structural and textural modifications of Li-Al hydrotalcite-like compounds by memory effect and their impact on the catalytic activity of these solids for the cyanoethylation reaction. The samples have been characterized by XRD, FT-IR, TG-DTG, surface area and porous structure. The catalytic test results showed that the reconstruction of the hydrotalcite leads to a decrease of the catalytic activity compared to the parent hydrotalcite. This fact may be related either to the smaller specific surface area of the reconstructed form or to the leaching of Li⁺ during reconstruction. It has been also noticed that the calcination of Li-Al hydrotalcite leads to the formation of a α-LiAlO₂ stable phase.     

The electric conductivity of nanodimensional cobalt oxide in a porous glass

Cobalt(II) oxide was introduced in an amount of 3.5–56.4 mass % into a porous glass by repeated cycles of impregnation with an aqueous solution of cobalt(II) nitrate followed by dehydration and thermal decomposition of the salt. The results of conductivity measurements, in combination with the data on the cobalt oxide volume and the specific area of the porous carrier, reflect the formation of sequential cobalt oxide monolayers on the surface of pore walls.     

Large-scale preparation of needle-like zinc oxide with high electrical conductivity

Needle-like zinc oxide with high electrical conductivity has been successfully prepared in large-scale from calcining the need-like precursor synthesized by a simple co-precipitation approach with ZnCl₂ as Zinc source, GaCl₃ as Gallium source and NH₄HCO₃ as precipitant under an optimized conditions (45 °C and pH = 7.4–7.5). The as-fabricated products were characterized by means of TEM, SEM, XRD, EDS and XPS. Their electrical conductivities were also studied, showing that the volume resistivity of the needle-like zinc oxide with 2.2 mol% Ga³⁺ dopant was lower than 20 Ω·cm.     

Synthesis, characterization and catalytic activity of chromium substituted cobalt ferrospinels

Chromium substituted cobalt ferrospinels were prepared by soft citrate gel method. The synthesized material was characterized by various physico-chemical methods. All the samples showed a single-phase cubic structure. Lattice constant varies from 8.389 to 8.323 Å. Transmission electron microscopic study indicated the nanostructure of the catalysts while homogenous grain distribution was presented by scanning electron microscopic studies. The catalytic activity of the samples was investigated towards acetylation of phenols. The presence of active centers on the surface of the material was confirmed through pyridine adsorption studies. The surface acidity of the catalyst is responsible for better catalytic performance. The material was found to serve as a promising catalyst for acylation and benzoylation of phenols under solvent free condition. These catalysts are ∼100% selective towards o-acylation of phenols, a promising reaction for perfumery intermediates. The catalysts were seen to be reusable without any further treatment. Catalytic activities of cobalt, chromium and iron oxides were also investigated for comparison. The cobalt ferrospinel was found to have better catalytic activity as compared to the Cr-substituted ferrospinels and the pure oxides. Cobalt ferrite catalyst offers high yields in a short reaction time under solvent-free conditions.     

Synthesis, magnetic properties and catalytic activity of hierarchical cobalt microflowers

A simple one pot synthesis method for the silver catalyzed growth of pure hexagonal close packed cobalt by the reduction of cobalt salt using hydrazine hydrate in the presence of triethanolamine (TEA), diethanolamine (DEA) and ethylene glycol (EG) as capping agents at 90 degrees C within 10 min has been reported. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the product prepared in the presence of capping agents show the formation of the well defined porous flowery architecture originating from the interlinked 2D wavy nanoflakes. When the same reaction is performed in the absence of any capping agent, the agglomeration of the flakes of cobalt with irregular spherical morphology is observed. The effect of the reaction conditions on the size and the shape of the products have also been studied. Vibration sample magnetometer (VSM) shows, that the products are ferromagnetic in nature irrespective of the capping agents used and possess high value of coercivity, when prepared in the absence of any capping agent. These cobalt microflowers have also been proved as an alternative to the other available expensive catalysts (Au, Ag, Pt) in the room temperature production of p-aminophenol for its applications in pharmaceutical, photographic and plastic industries.     

Enhancement of catalytic activity of Raney nickel by cobalt addition

The effects of cobalt as a promoter of the catalytic activity of Raney nickel for various manufacturing esses have been studied. The variation of particle size distribution, specific surface area and residual aluminium content can also affect the catalytic properties of Raney nickel. The characteristics of the catalyst are considerably influenced by the relative nickel and aluminium contents. When the catalysts are prepared from 60 wt.% nickel and 40 wt.% aluminium, the electrode performance shows the highest current density of 450 mA/cm². With the addition of cobalt, the particle size is diminished from 13.92 to 11.70 μm and the decrease of the particle size contributes to a slight enlargement of the specific surface area. The mass activity of a hydrogen electrode manufactured with cobalt Raney nickel in alkaline solution is increased from 3.74 to 4.03 A/g when 120 mg/cm² of catalyst are loaded in the electrode.     

Effect of anions on the textural and catalytic activity of titania-silica mixed oxide

An attempt has been made to review and study the effect of anions (phosphate, sulfate and fluoride) on titania-silica samples. A comparative study has been made on structural characteristics and physico-chemical properties between the unmodified and anion modified titania-silica mixed oxides. The stabilisations of phase, porosity and sulphate ion at higher activation temperatures are discussed. The increase and decrease of specific surface area, acid strength and surface acid sites with respect to anions are emphasised. The effect of preparation method and conditions with specific surface area, porosity and surface acid sites are also discussed. The generation of new catalytic active sites and effect of porosity are reported towards esterification of acetic acid and mono-nitration of toluene, respectively.