Electric conduction-blocking effects of voids and second phases in stabilized zirconia

To elucidate the conduction-blocking process observed in sintered electric ceramics, measurements have been carried out on a weld between two YSZ (Yttria-Stabilized Zirconia) single crystals and on YSZ-Al₂O₃ composites in addition to previous measurements on cracks generated at room temperature in YSZ single crystals. A fraction of the mobile oxide ions appears to be blocked at impermeable parts of the internal surfaces. The surfaces of voids (cracks, pores, and probably parts of the grain boundaries) can generate the same blocking effects as the surface of precipitated second phases.     

Interfacial effects on thermoelectric functions

We investigated the effects of added oxide to FeSi₂ on thermoelectric properties in order to improve the figure of merit. The oxide was not suggested to chemically react with the mother material (FeSi₂), where the oxide materials existed in the grain boundaries. The oxide addition played a role to reduce thermal conductivity and possessed electrical resistivity as well as non oxide-added FeSi₂. Hereafter, the appropriate added amount of Yb₂O₃ showed the largest effect on the figure of merit among other oxides.     

Cobalt Modification of Thin Rutile Films Magnetron-Sputtered in Vacuum

Using X-ray phase analysis, atomic force microscopy, and secondary ion mass-spectrometry, the phase formation and component distribution in a Co–TiO₂ film system have been investigated during magnetron sputtering of the metal on the oxide and subsequent vacuum annealing. It has been found that cobalt diffuses deep into titanium oxide to form complex oxides CoTi₂O₅ and CoTiO₃. A mechanism behind their formation at grain boundaries throughout the thickness of the TiO₂ film is suggested. It assumes the reactive diffusion of cobalt along grain boundaries in the oxide. A quantitative model of reactive interdiffusion in a bilayer polycrystalline metal–oxide film system with limited solubility of components has been developed. The individual diffusion coefficients of cobalt and titanium have been determined in the temperature interval 923–1073 K.

     

Investigation of the thermophysical properties of oxide ceramic materials at liquid-helium temperatures

The main regularities in the transport of thermal phonons in oxide ceramic materials are investigated at liquid-helium temperatures. The dependences of the thermophysical characteristics of ceramic materials on their structural parameters (such as the grain size R, the grain boundary thickness d, and the structure of grain boundaries) are analyzed. It is demonstrated that, in dense coarse-grained ceramic materials with qR?1 (where q is the phonon wave vector), the grain boundaries and the grain size are the main factors responsible for the thermophysical characteristics of the material at liquid-helium temperatures. A comparative analysis of the thermophysical characteristics of optically transparent ceramic materials based on the Y₃Al₅O₁₂ (YAG) and Y₂O₃ cubic oxides synthesized under different technological conditions is performed using the proposed criterion.     

Dependence of the resistance to compression of metalloceramic nickel-aluminum oxide alloys on the grain size of the nickel matrix

A study has been made of the effect of dispersed aluminum oxide in various concentrations on the grain size of nickel in the preparation of Ni-Al₂O₃ alloys and also of the connection between the resistance of the materials to compression and their initial structure. It is shown that the increase in the flow stresses of these alloys as compared with pure nickel is due to the fact that the oxides increase the resistance to movement of the dislocations within the grains, reduce their size, and also change the condition of the grain boundaries.     

Sensors for the nitrogen oxides, NO2, NO and N2O, based on In2O3 and WO3 nanowires

Sensing characteristics of ZnO, In₂O₃ and WO₃ nanowires have been investigated for the three nitrogen oxides, NO₂, NO and N₂O. In₂O₃ nanowires of ∼20 nm diameter prepared by using porous alumina membranes are found to have a sensitivity (defined as the ratio of the sensor resistance in the gas concerned to that in air) of about 60 for 10 ppm of all the three gases at a relatively low temperature of 150 °C. The response and recovery times are around 20 s. The sensitivity of these In₂O₃ nanowires is around 40 for 0.1 ppm of NO₂ and N₂O at 150 °C. WO₃ nanowires of 5–15 nm diameter, prepared by the solvothermal process show a sensitivity of 20–25 for 10 ppm of the three nitrogen oxides at 250 °C. The response and recovery times are 10 s and 60 s, respectively. The sensitivity is around 10 for 0.1 ppm of NO₂ at 250 °C. The sensitivity of In₂O₃ and WO₃ nanowires is not affected by humidity even up to 90% relative humidity. The study also reveals that the sensing mechanism for the three nitrogen oxides have a commonality in that the desorption of oxygen is a crucial step in all the cases. PACS 07.07.Df; 85.35.-p; 82.35.Np     

Reactivity and oxygen diffusion property of resistive barriers for Bi-2223/Ag tapes

Reactivity of several oxide materials (OM) with BSCCO powder and oxygen diffusion through OM layer has been tested at temperature ≈840 °C in air. The OM (e.g.: BaZrO₃, SrCO₃, MgO and ZrO₂) showing the low or no reactivity with BSCCO have been mixed (10 wt.%) with precursor powder and used for single-core tapes. Bi-2223/Ag/OM/Ag single-core tapes with oxide barriers made of BaZrO₃, SrCO₃, ZrO₂ and Al₂O₃ have been also prepared by a standard powder-in-tube technique. The used OM in the direct contact with BSCCO influences the electrical properties of Bi-2223 phase differently. These is because the oxides react with BSCCO during the heat treatment and simultaneously affect the 2212→2223 phase transformation, the Bi-2223 grain growth and so also grain connectivity. SrCO₃ powder has been evaluated as the best material from the point of no destructive effect on 2223 phase transport current property. The oxide barrier controls the oxygen diffusion during the tape heat treatment and simultaneously the HTS phase formation kinetics, its purity and content within the superconducting core. For single-core Bi-2223/Ag/OM/Ag tapes, the highest current density was measured for Al₂O₃ due to only slightly reduced oxygen diffusion through the barrier.     

Oxidation of dodecane on vanadium-molybdenum catalysts

The catalytic oxidation of dodecane with air oxygen on individual and mixed vanadium-molybdenum (1 ? x)V₂O₅ · xMoO₃ oxide is studied over a temperature range of 250 to 400°C. It is shown that oxidation of dodecane to organic acids at 250°C produces undecylic acid C₁₁H₂₆COOH and carbon oxides, as products of the subsequent oxidation of formic acid. The most effective catalyst is a mixed oxide containing 75 mol % MoO₃ and 25 mol % V₂O₅. At 275–300°C, this catalyst provides the maximum yield of acids and a relatively low fraction of complete-oxidation products. Above 250°C, a mixture of acids and carbon oxides is formed, the yield of which increases with the temperature up to 300°C. As the temperature is increased still further, the yield of acids decreases due to their subsequent oxidation. The catalytic oxidation is accompanied by changes in the phase composition, morphology, and degree of crystallinity of the mixed-oxide catalysts. A possible mechanism of the catalytic process is considered.     

Fine crystal structure of porous corundum ceramics

The microstructure of corundum ceramics based on powders with a varying grain size has been investigated. Both commercial alumina powders and those fabricated by denitration of aluminum salts in a high-frequency discharge plasma were used. An increase in the plasma-chemical Al₂O₃ powder content in the sample was found to change the pore structure of the corundum ceramics from a high-porosity ceramic skeleton with a well-developed system of channel-forming pores to ceramics with isolated pores. The change in the pore structure was observed for 50% porosity and caused an increase in the level of crystal lattice microdistortions. An increase in the sintering temperature from 1200 to 1650°C is shown to be responsible for a two-fold increase in the average crystallite size and for annealing of lattice defects along grain boundaries.     

A general and low-cost synthetic route to high-surface area metal oxides through a silica xerogel template

Porous metal oxides with a large surface area are synthesised by means of a procedure based on the templating approach. An inexpensive porous silica xerogel synthesised at moderate temperatures (∼100 °C) in order to preserve the silanol superficial groups was used as template. In a first step, the silica porosity was filled with a concentrated solution containing a metallic salt. Then, the impregnated sample was calcined in air at a temperature of 600 °C. Under these conditions, the metal oxides were synthesised within the confined space provided by the silica pores. Finally, the product was recovered after dissolution of the silica framework in 2 M NaOH solution. The materials obtained by this procedure are made up of aggregates of nanoparticles and/or 3D solid structures containing confined pores. In this work, the synthetic route proposed is illustrated by the preparation of various binary metal oxides (i.e. Fe₂O₃, Cr₂O₃, NiO, CeO₂, Mn₂O₃, Co₂O₃ and Al₂O₃). The BET surface areas measured for these materials are in the range of 100-270 m² g⁻¹. The proposed method is not restricted to the binary metal oxides. It can also be used in the preparation of other inorganic materials such as metal sulphides or mixed metal oxides.     

Effects of oxide coating on the growth of single grain YBCO bulk superconductors

Surface oxide coating and bottom inserting of oxide plates have been conducted to top seeded melt growth (TSMG) processed YBa₂Cu₃O_7−y (Y123) bulk superconductors with an aim of controlling the Y123 nucleation and growth. The coating medium for surfaces was Yb₂O₃ solution and the bottom inserts were Yb₂O₃/Y₂O₃ powder compact. Many vertical cracks were found to develop at the compact/insert interfaces when an Yb₂O₃ insert was used, but the crack evolution was greatly reduced when a (Yb₂O₃ + Y₂O₃) insert was used. The formation of the vertical cracks is ascribed to the difference in thermal expansion between the YBCO compact and bottom insert. Presence of vertical cracks was found to be crucial to the trapped magnetic field and levitation forces of single grain YBCO bulk superconductors. The Y123 nucleation and growth in TSMG-processed YBCO bulk superconductors were successfully controlled by conducting surface coating and bottom plating using a (Yb₂O₃ + Y₂O₃) insert and as a result, the levitation properties were much enhanced.     

Relationship between the structural and catalytic properties of mechanosynthesized lithiated manganese oxides

The relationship between the structural and catalytic properties of lithiated spinel manganese oxides was investigated by means of X-ray diffraction, Infrared and Xanes spectroscopies, thermogravimetric analysis, and by evaluating two catalytic oxidation tests, namely the carbon black combustion and the toluene conversion. Li-Mn-O catalysts were prepared from stoichiometric (Li₂O + MnO₂) mixtures, either by the classical high temperature ceramic method or by mechanochemistry. For both catalytic tests, some spectacular temperature reductions were measured as a function of grinding. A remarkable decrease of 210 °C (from 650 °C to 440 °C) in the carbon black combustion temperature was obtained when using mechanosynthesized Li-Mn-O spinel prepared from a mixture of Li₂O and MnO₂ ground for 3 hours, whereas a 100 % toluene conversion rate was achieved for a temperature lower than 200 °C for the 5 hours milled ceramic LiMn₂O₄ while the as-made ceramic was inactive. The enhancement of the performances (i.e. decrease in carbon black combustion temperature Tc and decrease in toluene conversion temperature T_95%) is due both to an increase in grain boundaries and in specific BET surface area and to the nano-crystallite size nature of the material. Besides, the spinel stoichiometry (both in oxygen or in cations) reflected by the lattice parameter variation plays a significant role in the catalytic reaction mechanism.     

Optical characteristics of highly ordered gallium oxide nano-islands on GaAs

A periodic array of Ga oxide islands was obtained by annealing the highly ordered Ga nano-droplets on GaAs surface at 400°C under an oxygen atmosphere for 7 hours. These Ga oxides are a mixture of α-Ga₂O₃ and β-Ga₂O₃ confirmed by Raman spectroscopy study. Enhanced optical transmission of GaAs with such ordered Ga oxide nano-islands was obtained. Both dielectric and dimensional confinement effects were considered in analysis of the electromagnetic characteristics of the nanostructured materials. Finite-difference time-domain method was used to numerically study the light transmission through the patterned Ga oxide on GaAs surface. Based on the calculated results, the light transmission enhancement is attributed to the formation of the ordered nano Ga oxides.     

TEM observation of oxide scale formed on TiC single crystals with different faces

Cross-sectional observation of the oxide scale formed by oxidation of TiC single crystals with (100), (110) and (111) faces at 1500 °C for 30 min in a mixed gas of Ar/O₂ (P_O2=0.08 kPa) was performed by transmission electron microscope (TEM). The oxide scale was composed of outer (zone 2) and inner (zone 1) subscales. TEM and selected area electron diffraction combined with X-ray energy dispersion analysis showed that the zone 2 consists of rutile and pores and the zone 1 of carbon and titanium oxide, identified as Ti₃O₅ in the oxide scale formed on the (110) face. Zone 1 formed on the (100) and (111) faces showed crisscross patterns, in contrast to the (110) with the wavy lamellar pattern.     

Constituted oxides/nitrides on nitriding 304, 430 and 17-4 PH stainless steel in salt baths over the temperature range 723 to 923 K

The progressively developed oxides and nitrides that form on nitriding 304, 430 and 17-4 PH stainless steel are analysed by X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS) in this study. The experimental results show that the Cr contents and matrix structures (ferrite, austenite and martensite) play an important role in forming FeCr₂O₄, Cr₂O₃ and Fe₂O₃ oxides as well as nitrides. After a short immersion time, oxides of Cr₂O₃ and FeCr₂O₄ form in nitride films on 304 stainless steel samples. Fe₂O₃ oxide will subsequently form following an increasing immersion time. For the 430 stainless steel, Cr₂O₃ predominately forms after a short dipping time which hinders the growth of the nitride layer. As a result, this sample had the thinnest nitride film of the three for a given immersion time. After the formation of oxides, both CrN and Cr₂N were detected near the surface of the nitride films of three samples while Cr₂N phases formed in the deeper zone. The greatest amount of Fe₂O₃ oxide among the three samples was obtained on the nitriding 17-4 PH stainless steel which also had a high intensity count of N 1s.     

Effect of surface roughness on the development of protective Al2O3 on Fe-10Al (at.%) alloys containing 0-10 at.% Cr

The effect of alloy surface roughness, achieved by different degrees of surface polishing, on the development of protective alumina layer on Fe-10 at.% Al alloys containing 0, 5, and 10 at.% Cr was investigated during oxidation at 1000 °C in 0.1 MPa oxygen. For alloys that are not strong Al₂O₃ formers (Fe-10Al and Fe-5Cr-10Al), the rougher surfaces increased Fe incorporation into the overall surface layer. On the Fe-10Al, more iron oxides were formed in a uniform layer of mixed aluminum- and iron-oxides since the layer was thicker. On the Fe-5Cr-10Al, more iron-rich nodules developed on an otherwise thin Al₂O₃ surface layer. These nodules nucleated preferentially along surface scratch marks but not on alloy grain boundaries. For the strong Al₂O₃-forming Fe-10Cr-10Al alloy, protective alumina surface layers were observed regardless of the surface roughness. These results indicate that the formation of a protective Al₂O₃ layer on Fe-Cr-Al surfaces is not dictated by Al diffusion to the surface. More cold-worked surfaces caused an enhanced Fe diffusion, hence produced more Fe-rich oxides during the early stage of oxidation.     

Defect chemistry of grain boundaries in proton-conducting solid oxides

The defect chemistry of charged grain boundaries in an acceptor-doped oxide in equilibrium with water vapour is examined theoretically. The basis of the theoretical approach is that the formation of charged grain boundaries and attendant space-charge zones is governed by differences in the standard chemical potentials of oxygen vacancies and hydroxide ions between bulk and grain-boundary core, that is, by the thermodynamic driving energies for defect redistribution. A one-dimensional continuum treatment is used to predict the space-charge potential and defect concentrations in the grain-boundary core as a function of water partial pressure, temperature and acceptor dopant concentration for various values of the two thermodynamic driving energies. The results are discussed with respect to experimental data in the literature for acceptor-doped perovskite oxides (e.g. BaZrO₃) and fluorite oxides (e.g. CeO₂).     

Solid state interactions in nano-sized oxides

Comparative study of reactivity of nano- and micro-sized alumina and nickel oxide, obtained by the electrical explosion of metal wires in oxidizing atmosphere, was carried out for the reactions NiO + MoO₃, NiO + Al₂O₃, and Al₂O₃ + Bi₂O₃ by coupled anneals of ceramics, measurements of the conductivity of individual oxides and raw oxide mixtures, X-ray diffraction and differential thermal analysis. The total conductivity of nano-structured oxides was found lower than that of micro-structured ceramics. Mixing bismuth oxide with nano-structured alumina leads to stabilization of the low temperature polymorph α-Bi₂O₃ up to 780 °C. The diffusion permeability of NiMoO₄ layer grown at the surface of NiO ceramics, having submicron grains, was found 2 times lower if compared to NiMoO₄ grown at micro-sized NiO ceramics. NiO and Al₂O₃ nano-powders preserve the high reactivity even when heated up to 1000 °C. The results are discussed in terms of size effects on the solid state reactivity of oxides.     

XPS study of vanadium surface oxidation by oxygen ion bombardment

Oxidation of vanadium metal surfaces at room temperature by low-energy oxygen ion beams is investigated by X-ray photoelectron spectroscopy (XPS). It is observed that ion-beam irradiation of clean V results in formation of thin oxide layer containing vanadium in oxidation states corresponding to VO, V₂O₃, VO₂ and V₂O₅ oxides. The composition of the products of ion-beam oxidation depends markedly on oxygen ion fluence. The results of angle-resolved XPS measurements are consistent with a structure of oxide film with the outermost part enriched in V₂O₅ and VO₂ oxides and with V₂O₃ and VO oxides located in the inner region of the oxide layer.     

Formation of three-dimensional arrays of magnetic clusters NiO,Co<Subscript>3</Subscript>O<Subscript>4</Subscript>, and NiCo<Subscript>2</Subscript>O<Subscript>4</Subscript> by the matrix method

A method has been proposed for the formation of three-dimensional arrays of isolated magnetic clusters NiO, Co₃O₄, and NiCo₂O₄ in the sublattice of pores in the matrix of bulk synthetic opals through a single impregnation of the pores with melts of nickel and cobalt nitrate crystal hydrates and their thermal degradation. The method makes it possible to controllably vary the degree of filling of pores in the matrix with oxides within 10–70 vol %. The composition and structure of the synthesized materials, as well as the dependences of their static magnetic susceptibility on the magnetic field strength, have been investigated.