Effect of rare-earth oxides on fracture properties of ceria ceramics

The influences of the sintering additive content of rare-earth oxide (Y₂O₃, Gd₂O₃, Sm₂O₃) on microstructure and mechanical properties of ceria ceramics were investigated by scanning electron microscopy and small specimen technique. A small punch testing method was employed to determine the elastic modulus and biaxial fracture stress of the ceria-based ceramics, and the fracture toughness was estimated by Vickers indentation method. Grain growth in the rare-earth oxides doped ceria ceramics was significantly suppressed, compared to the pure ceria ceramics. However, the elastic modulus, fracture stress and fracture toughness were decreased significantly with increasing additive content of the rare-earth oxides, possibly due to the oxygen vacancies induced by the rare earth oxides doping. The experimental results suggest that the change in the mechanical properties should be taken into account in the use of ceria-based ceramics for solid oxide fuel cells, in addition to the improvement of oxygen ion conductivity.     

Microstructure and electrical properties of rare-earth oxides doped ZnO-based linear resistance ceramics

The ZnO-based linear resistance ceramics doped with rare-earth oxides (Y₂O₃, La₂O₃) have been prepared by the conventional ceramics method. The effect of the different doping amounts of rare-earth oxides on the microstructure, electrical properties and aging behavior were investigated in detail. The electrical properties analysis shows that the La₂O₃ addition on ZnO-based linear resistance ceramics has more obviously influence than the addition of Y₂O₃. The samples with the La₂O₃ content of 0.25 mol% sintered at 1,320 °C exhibits excellent electrical properties with the resistivity of 253.5 Ω cm and the resistance temperature coefficient of 3.62 × 10^?4/ °C, which are improved by 60 and 90 %, respectively. The nonlinear coefficient of voltage decreases to 1.16, decreases by 4.3 % and the stability of ?1.2 % in the variation rate of the resistivity for aging temperature.     

Magnetism of rare-earth elements, alloys, and compounds

The magnetic properties of the rare-earth metals and a number of the alloy and compound systems formed with rare earths are discussed. Emphasis is placed on current experimental results of bulk magnetic measurements and related transport phenomena and their bearing on theoretical studies. Topics covered include exchange interactions, magnetization processes, crystal field effects and anisotropy, magnetostriction, spin structures and their field dependence, specific heat, optical effects, resistivity, magnetoresistivity, and other transport properties affected by the magnetic order. Examples are drawn from a number of material systems including ionic compounds, solid solution compounds (e.g., rare-earth magnetic semiconductors), intermetallic compounds (permanent magnet materials, etc.), and bubble domain materials in addition to the rare-earth elements and intra-rare-earth alloys. Tables are presented giving the spin structures, ordering temperatures, saturation and effective moments, and other properties of the elements and specific compounds.     

Phase relationships in systems based on Y,Gd, Mn,Ti, and U oxides with addition of Ca,Al, and Fe oxides

Samples of titanate-based ceramics with Y, Gd, Mn, U, Ca, Al, and Fe oxides, which are of interest as hosts for immobilization of radioactive waste containing lanthanides, actinides, and corrosion products, were prepared by cold pressing, sintering, and fusion. The major components of the ceramics are phases of the structure derived from the fluorite structure (pyrochlore, “murataite,” cubic oxide with fluorite lattice type), and also brannerite. Introduction of Al and Fe oxides (corosion products) leads to the formation of additional phases of the perovskite and/or crichtonite structure.     

Study of phase composition of compact ceramics based on titanium nitride

A method of studying the phase composition of compact ceramics based on titanium nitride is described and the results of the X-ray diffraction analysis of the material obtained are presented. It is shown that, in the process of nitridation, the phase composition of the surface of the specimen changes and every step of the process is characterized by an individual and definite set of phase states.     

Phase composition and microwave dielectric properties of Mg-excess MgTiO3 ceramics

The phase composition and microwave dielectric properties of Mg-excess MgTiO₃ (Mg/Ti = 1, 1.02, 1.04, 1.05, 1.07) ceramics prepared via the conventional solid-state reaction route were investigated. A slight deviation from stoichiometry does not practically affect the relative permittivity and temperature coefficient of resonant frequency of the specimen. However, the Q _f value is very sensitive to the composition and it shows a non-linear variation corresponding to a relative amount of Mg. A very high Q _f can be achieved for specimen with single MgTiO₃ phase, which can be obtained within the suitable composition. As the increasing of Mg content, the MgTi₂O₅ phase which was derived from Mg/Ti = 1 was disappeared, and when it exceeded 1.02, the phase of Mg₂TiO₄ emerged. Along with the augmentation of Mg/Ti, the bulk density and Q _f showed an initial increase, followed by a decrease, but ε_r had been declining. The specimen with single MgTiO₃ phase was obtained at the level of Mg/Ti = 1.02. A high Q _f of 357,400 GHz (at 10 GHz) together with an ε_r = 17 and τ _f  = ?51 ppm/°C for MgTiO₃ ceramics (Mg/Ti = 1.02) were obtained at 1,390 °C sintered for 4 h.     

掺杂稀土氧化物的ZnO-Bi2O3系压敏陶瓷中的"软心"

研究了掺杂Ce2O3和Gd2O3对ZnO—Bi2O3系压敏陶瓷电气性能的影响，发现大尺寸试样的内层电位梯度明显低于表层，即表现出“软心”特征．“软心”导致试样平均电位梯度随着试样几何尺寸的增大而下降．沿轴向晶粒尺寸没有明显的差异．根据沿轴向截取的等厚薄片的电流与温度的关系，发现表层的晶界势垒高度比内层的高．这种晶界势垒高度的非均匀分布可能与添加稀土氧化物后造成的氧元素分布不均匀有关．     

Silicon switching structures with fluorides of rare-earth elements

Results are presented of an investigation of the photoelectric characteristics of silicon metal-insulator-metal switching structures with a thin-film insulating layer of rare-earth fluoride. Studies of the steady-state and kinetic characteristics of the photocurrent revealed that in the low-resistivity state the metal-tunnel insulator-semiconductor model can be applied to the structures. Pis’ma Zh. Tekh. Fiz. 24, 91–95 (August 26, 1998)     

Kinetics of redox processes in manganese oxides

The redox processes in ceramic and powder samples of α-Mn₂O₃ (kurnakite) and β-Mn₃O₄ (hausmannite) have been studied in the temperature range 20–1000°C. The results demonstrate that the conversion of α-Mn₂O₃ to β-Mn₃O₄ in the binary system Mn-O is essentially irreversible, even if there are nucleation centers in the form of the higher oxide α-Mn₂O₃ during the oxidation of hausmannite. It is shown that the high reactivity of hausmannite ground in a WC mill is not due to a mechanochemical effect. Grinding-induced contamination has a significant effect on the oxidation behavior of the system: the impurities incorporated into the sample during grinding in a mill favor complete (WC contamination) or partial (Fe contamination) conversion of hausmannite to kurnakite. Hausmannite contaminated with tungsten carbide oxidizes by a catalytic mechanism. In the case of Fe impurities, oxidation follows a solid-state mechanism, through the formation of restricted Fe_xMn_2?x O₃ solid solutions.     

Chemical vapour deposition of titanium oxides in the composition range TiO1.60–TiO1.75

Ti₃O₅ and Ti₄O₇ were grown by chemical vapour deposition in a hot-wall reactor at 1015 °C and a total pressure of 50 Torr. The reaction gas mixture contained TiCl₄, CO₂ and H₂. For growth of single-phase coatings the vapour composition has to be controlled accurately. Ti₃O₅ was grown at near-equilibrium conditions, while Ti₄O₇ could only be grown under conditions far from equilibrium. The coatings grew at a rate of about 0.3 microm min^?1 and consisted of well-shaped columnar crystals.     

Phase composition and optical properties of thin films based on lanthanum and tungsten oxides

We have studied the phase composition and optical properties of 150-to 500-nm-thick films based on lanthanum and tungsten oxides, produced by magnetron sputtering followed by heat treatment at 773 K in vacuum and flowing oxygen. The oxide films have been found to have high transmission in the range 250 to 900 nm and an absorption band between 190 and 250 nm. Analysis of their absorption edge indicates that its complex structure is due to the presence of several phases. The energies of direct transitions evaluated from the spectra of the films coincide with the band gaps of the phases present in the films.     

Phase composition and pore structure of nanoparticulate tin oxides prepared by AC electrochemical synthesis

The effect of current density and sodium chloride concentration in solution on the phase composition and pore structure of the products of ac electrolysis of metallic Sn has been studied using X-ray diffraction, thermogravimetry, differential scanning calorimetry, electron microscopy, and low-temperature nitrogen adsorption measurements. The results demonstrate that the synthesis products consist of tin(II) and tin(IV) oxides and hydroxides, whose percentages depend on electrolysis conditions, and have a large specific surface area and mesoporous structure. The average particle size ranges from 10 to 30 nm.     

Thermodynamic modeling to analyse composition of carbonaceous coatings of MnO and other oxides of manganese grown by MOCVD

Equilibrium thermodynamic analysis has been applied to the low-pressure MOCVD process using manganese acetylacetonate as the precursor. “CVD phase stability diagrams” have been constructed separately for the processes carried out in argon and oxygen ambient, depicting the compositions of the resulting films as functions of CVD parameters. For the process conduced in argon ambient, the analysis predicts the simultaneous deposition of MnO and elemental carbon in 1:3 molar proportion, over a range of temperatures. The analysis predicts also that, if CVD is carried out in oxygen ambient, even a very low flow of oxygen leads to the complete absence of carbon in the film deposited oxygen, with greater oxygen flow resulting in the simultaneous deposition of two different manganese oxides under certain conditions. The results of thermodynamic modeling have been verified quantitatively for low-pressure CVD conducted in argon ambient. Indeed, the large excess of carbon in the deposit is found to constitute a MnO/C nanocomposite, the associated cauliflower-like morphology making it a promising candidate for electrode material in supercapacitors. CVD carried out in oxygen flow, under specific conditions, leads to the deposition of more than one manganese oxide, as expected from thermodynamic analysis (and forming an oxide–oxide nanocomposite). These results together demonstrate that thermodynamic analysis of the MOCVD process can be employed to synthesize thin films in a predictive manner, thus avoiding the inefficient trial-and-error method usually associated with MOCVD process development. The prospect of developing thin films of novel compositions and characteristics in a predictive manner, through the appropriate choice of CVD precursors and process conditions, emerges from the present work.     

Overview of the effects of rare-earth elements used as additive materials in molten carbonate fuel cell systems

From the viewpoint of materials issues, there are some problems in molten carbonate fuel cell (MCFC) systems due to the corrosive and evaporative electrolytes and the high pressure caused by a stack in temperature of 650°C. The rare earth metals (RE) in as material additives primarily improve the creep resistance, corrosion resistance and high temperature resistance of materials. However, efforts to enhance the properties of MCFC materials using RE have not yielded the marked effects associated with their use in solid oxide fuel cells (SOFC). Therefore, we have conducted this review in order to describe and discuss the effects of RE as additive materials in the context of MCFC. This review also provides information regarding the development of MCFC materials using RE. The incorporation of low concentrations of RE into previously RE-free materials may improve the stability of these materials to some degree, and also effect an increase in the cell efficiency of MCFC. La₂O₃-added cathode materials have primarily been applied as alternative materials, for the reduction of the dissolution of conventional NiO cathodes. Ce and Dy have both been theorized to possibly enhance the stability of anode electrode materials. Ce and La can both be employed as additives which enhance the stability of reforming catalysts. The addition of La₂O₃ to electrolytes has been previously shown to reduce the degree of dissolution in cathodes. Ce-based ceramics are thought to be promising coating materials, and it is believed that they may help to prevent the corrosion of the separator. However, future research into materials which exhibit long-term stability and low electrical conductivity is clearly warranted, as the field is in its infancy.     

Simultaneous synthesis of high crystalline manganese oxides with layered and tunnel structures

High crystalline manganese oxides with birnessite-type (H-BirMO) and large 2 × 4 tunnel-type (Na-2 × 4) were synthesized simultaneously using a low crystalline Na-form birnessite (NaBir) as a precursor in a 2 M NaOH solution at low temperature (150 °C) and autogenous pressure. The weight ratio of the obtained Na-2 × 4 to H-BirMO was about 50. Elemental analyses indicated that Na-2 × 4 had a formula in one unit cell of Na₈Mn₂₇O₅₃·9H₂O, and the manganese average oxidation state was 3.63. H-BirMO had a formula of Na_0.31Mn_2.04O₄·0.66H₂O and the manganese average oxidation state of 3.77. H-BirMO contained a set of basal reflections with d values that correspond to a minimum periodicity along c equal to 7.06 Å. H-BirMO belonged to a monoclinic system, and the lattice parameters were a = 5.18 Å, b = 2.85 Å, c = 7.35 Å, and β = 103.3°, respectively. Na-2 × 4 showed that the first peak appeared at a d spacing of 12.07 Å in the [0 0 2] direction, and the second peak had a similar d spacing of 7.16 Å in the [2 0 0] direction with Na-birnessite in the [1 0 0] direction. FE-SEM image of H-BirMO showed a very large plate-like morphology, while SEM photograph of Na-2 × 4 consisted of nanofibers with a thickness of about 80 nm and lengths ranging between 6 and 10 μm.     

X-ray photoelectron and x-ray absorption spectroscopic study of rare-earth molybdenum oxides

The chemical state of atoms in rare-earth molybdenum oxides has been studied by x-ray photoelectron and x-ray absorption (XANES) spectroscopies. The photoelectron spectra has been analyzed in the Mo 3d, Mo 3p, O 1s, Ce 3d, Y 3d, Er 4d, and Yb 4d regions. The results indicate that the oxidation state of the molybdenum in the compounds studied is 6+. The measured Mo K-edge spectra and XANES calculations are used to develop a model of a molybdenum-oxygen cluster in the rare-earth molybdenum oxides.     

The flux growth of some rare-earth and iron group complex oxides

The flux growth of crystals of the following rare-earth complex oxides is reported here for the first time: (Nd, Pb)MnO₃, (Eu, Pb)MnO₃. Tm₂Ge₂O₇ and Pr₂MoO₆. Starting compositions for the flux growth of RBO₃ (R=Nd, Pr, La), (La, Pb)MnO₃, Pb₃Mn₇O₁₅, R₂Ti₂O₇ (R=Tm to Pr), R₂SiO₅ (R=Er, Ho, Dy) and LiCoPO₄ are also given; these compositions have yielded larger crystals than previously reported. In many cases, only a small number of crystals was obtained by spontaneous nucleation.     

Magnetic orientation and magnetic anisotropy in paramagnetic layered oxides containing rare-earth ions

The magnetic anisotropies and easy axes of magnetization at room temperature were determined, and the effects of rare-earth (RE) ions were clarified for RE-based cuprates, RE-doped bismuth-based cuprates and RE-doped Bi-based cobaltite regarding the grain orientation by magnetic field. The easy axis, determined from the powder orientation in a static field of 10 T, depended qualitatively on the type of RE ion for all three systems. On the other hand, the magnetization measurement of the c-axis oriented powders, aligned in static or rotating fields, revealed that the type of RE ion strongly affected not only the directions of the easy axis but also the absolute value of magnetic anisotropy, and an appropriate choice of RE ion is required to minimize the magnetic field used for grain orientation. We also studied the possibility of triaxial grain orientation in high-critical-temperature superconductors by a modulated oval magnetic field. In particular, triaxial orientation was attempted in a high-oxygen-pressure phase of orthorhombic RE-based cuprates Y₂Ba₄Cu₇O_y. Although the experiment was performed in epoxy resin, which is not practical, in-plane alignment within 3° was achieved.