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.     

Energy band alignment at interfaces of semiconducting oxides: A review of experimental determination using photoelectron spectroscopy and comparison with theoretical predictions by the electron affinity rule, charge neutrality levels, and the common anion rule

The energy band alignment at interfaces of semiconducting oxides is of direct relevance for the electrical function of electronic devices made with such materials. The most important quantities of the interface determined by band alignment are the barrier heights for charge transport given by the Fermi level position at the interface and the band discontinuities. Different models for predicting energy band alignment are available in literature. These include the vacuum level alignment (electron affinity rule), branch point or charge neutrality level alignment governed by induced gap states, and an alignment based on the orbital contributions to the density of states (common anion rule). The energy band alignment at interfaces of conducting oxides, which have been experimentally determined using photoelectron spectroscopy with in situ sample preparation, are presented. The materials considered include transparent conducting oxides like In₂O₃, SnO₂, ZnO, and Cu₂O, dielectric and ferroelectric perovskites like (Ba,Sr)TiO₃ and Pb(Zr,Ti)O₃, and insulators like Al₂O₃. Interface formation with different contact partners including metals, conducting and insulating oxides are addressed. The discussion focuses on the energy band alignment between different oxides. A good estimate of the band alignment is derived by considering the density of states of the materials involved.     

Microstructure and phase transformation of zirconia-based ternary oxides for thermal barrier coating applications

Five dopant oxides, Sc₂O₃, Yb₂O₃, CeO₂, Ta₂O₅, and Nb₂O₅, were incorporated into 7YSZ to create ternary zirconia-based oxides with varying oxygen vacancies and substitutional defects. These ternary oxides were consolidated using a high-temperature sintering process. The resulting bulk oxides were subjected to microstructural study using scanning electron microscopy (SEM), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). The results show that the microstructures of the ternary zirconia-based oxides are determined by the amount of oxygen vacancies in the system, the dopant cation radius, and atomic mass. Increasing the number of oxygen vacancies in the lattice by the addition of trivalent dopant as well as the use of larger cations promotes the stabilization of the high-temperature cubic phase. The tetravalent cation, on the other hand, has the effect of retaining tetragonal phase to room temperature without the influence of oxygen vacancy. The addition of pentavalent oxide leads to the formation of monoclinic phase upon cooling.     

Interface Modulation of MoS2/Metal Oxide Heterostructures for Efficient Hydrogen Evolution Electrocatalysis

Developing efficient earth‐abundant MoS₂ based hydrogen evolution reaction (HER) electrocatalysts is important but challenging due to the sluggish kinetics in alkaline media. Herein, a strategy to fabricate a high‐performance MoS₂ based HER electrocatalyst by modulating interface electronic structure via metal oxides is developed. All the heterostructure catalysts present significant improvement of HER electrocatalytic activities, demonstrating a positive role of metal oxides decoration in promoting the rate‐limited water dissociation step for the HER mechanism in alkaline media. The as‐obtained MoS₂/Ni₂O₃H catalyst exhibits a low overpotential of 84 mV at 10 mA cm^?2 and small charge‐transfer resistance of 1.5 Ω in 1 m KOH solution. The current density (217 mA cm^?2) at the overpotential of 200 mV is about 2 and 24 times higher than that of commercial Pt/C and bare MoS₂, respectively. Additionally, these MoS₂/metal oxides heterostructure catalysts show outstanding long‐term stability under a harsh chronopotentiometry test. Theoretical calculations reveal the varied sensitivity of 3d‐band in different transition oxides, in which Ni‐3d of Ni₂O₃H is evidently activated to achieve fast electron transfer for HER as the electron‐depletion center. Both electronic properties and energetic reaction trends confirm the high electroactivity of MoS₂/Ni₂O₃H in the adsorption and dissociation of H₂O for highly efficient HER in alkaline media.     

Electrical characteristics and reheat-treatment effects in a ZnO varistor fabricated by two-stage heat-treatment

Conventional ZnO varistors are generally fabricated by sintering ZnO powder mixed with additives such as Bi₂O₃, Sb₂O₃, Cr₂O₃, Co₂O₃, and MnO₂. To reduce abnormal grain growth and change in electrical characteristics in the conventional ZnO varistors caused by volatilization of Bi₂O₃, the ZnO powder with all additive oxides except Bi₂O₃ was pressed into disc form and sintered. The disc was then painted with metal oxide paste containing Bi₂O₃ and again fired. The ZnO varistor fabricated by this process, i.e. a two-stage heat-treatment process, showed typical non-linearI-V characteristics with higher breakdown voltage exceeding 800 V mm^–1. It was also observed that the non-linearI-V coefficient change rate, , in the ZnO varistor due to reheat-treatment is almost linearly proportional to the sintered density.     

Effects of MO (M = Mg, Ca, Ba) on crystallization and flexural strength of semi-transparent lithium disilicate glass-ceramics

In this study, semitransparent lithium disilicate glass-ceramics in the Li₂O-K₂O-Al₂O₃-Y₂O₃-La₂O₃-SiO₂ system was investigated by incorporation of P₂O₅ as nucleation agent and alkaline earth oxides as additive. The influence of alkaline earth oxides on the structure of glasses network, crystalline phases, microstructure and mechanical properties were investigated by means of Raman spectra, differential scanning calorimetry (DSC), X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The mechanical strength was measured corresponding to norm ISO 6872. The Raman spectra predominantly showed that small additions of alkaline earth oxides not only form asymmetrical vibrations of the M-Si-O bonds, but also enhance the intensities of symmetrical vibrations of the P-O bonds, making the glass network more stable. And the small additions of CaO or BaO has more influence on the crystallization behaviour, crystalline phase, microstructure and mechanical properties of the glass-ceramics than the addition of MgO. The additions of alkaline earth oxides enhanced the first exothermic peak temperature but decreased the flexural strength of lithium disilicate glass-ceramics.     

Spontaneous oxide reduction in metal stacks

Aluminum and copper interconnects are widely used for microelectronic applications. A problem can arise when interfacial oxides are present. Such oxides can significantly degrade device performance by increasing electrical resistance. This paper describes analyses of interfacial oxide layers found in Al/Ta and Ta/Cu metal stacks. The analyses were performed through transmission electron microscopy (TEM). The data indicated that the interfacial oxides resulted from spontaneous reductions; that is, Al spontaneously reduced Ta₂O₅ to form Al₂O₃, while Ta spontaneously reduced Cu oxide to form Ta₂O₅.     

Infrared spectra, Raman laser, XRD, DSC/TGA and SEM investigations on the preparations of selenium metal, (Sb2O3, Ga2O3, SnO and HgO) oxides and lead carbonate with pure grade using acetamide precursors

Ga₂O₃, Se metal, SnO, Sb₂O₃, HgO and PbCO₃ are formed upon the reaction of acetamide aqueous solutions with Ga(NO₃)₃, SeO₂, SnCl₂, SbCl₃, HgCl₂ and Pb(NO₃)₂, respectively, at 90°C. Different amorphous or crystalline phases can be obtained depending upon the experimental conditions (molar ratios, metal salts and temperature). The chemical mechanisms for the formations of this metal, oxides or carbonate are discussed and the X-ray diffraction, scanning electron microscopy (SEM) and atomic force microscope (AFM) are described. The type of metal ions plays an important role in the decomposition of acetamide, leading to the formation of solid stable (metal, oxides or carbonate), soluble and gases species. These new precursors are more stable preventing the rapid precipitation of metal, oxides or carbonate. Furthermore, this route allows the formation of pure compounds in solutions.     

High-temperature interaction of components of spent uranium nuclear fuel with alkali metal carbonates in the course of voloxidation in the Carbex process

Interaction of UO₂, U₃O₈, ZrO₂, Nb₂O₅, CeO₂, Ce₂O₃, Pr₆O₁₁, Eu₂O₃, Yb₂O₃, Tb₇O₁₂, and Re metal with lithium, sodium, and potassium carbonates in the temperature interval 300–900°С was studied using methods of thermal analysis and X-ray diffraction. The major products of the high-temperature reactions are alkali metal uranates and diuranates, and in the case of ZrO₂, Nb₂O₅, and lanthanide oxides, oxides of metals in higher oxidation states and salts of the corresponding metal-containing acids, formed by reactions of the oxides with sodium carbonate.     

Facile synthesis of magnetic metal (Mn, Co, Fe, and Ni) oxide nanosheets

This paper demonstrated a simple, reproducible, and low-cost strategy of controlled synthesis of 2D magnetic Mn₂O₃, Co₃O₄, Fe₂O₃ and NiO nanosheets. The products were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and high-resolution transmission electron microscopy (HRTEM). A kinetically controlled growth mechanism via a dissolution-recrystallization process has also been proposed. This method proves the advantage of synthesizing metal oxide nanosheets without employing catalysts, surfactants, templates, complex metal ligands or fatty-acids to assist growth. By using generic reactants and one solvent, this route yields high-purity oxides providing a baseline for precise characterization and property analysis.     

Doping and hydrogen in wide gap oxides

The general conditions for doping are considered. Doping by hydrogen is then considered, and whether a single rule separates those oxides in which hydrogen forms only deep states and those in which it acts as a donor. Hydrogen is calculated to act as a shallow donor in oxides ZrO₂, HfO₂, SnO₂, La₂O₃, Y₂O₃, TiO₂, SrTiO₃ and LaAlO₃ but it is deep in the oxides SiO₂, Al₂O₃, ZrSiO₄, HfSiO₄ and SrZrO₃.     

Cation interdiffusion between thick film of YBa2Cu3O7-δ and ceramic substrate

Thick films of superconducting oxides, YBa₂Cu₃O_7-, were successfully made by conventional screen-printing technology on Al₂O₃, MgO, and ZrO₂ substrates. Interdiffusion between the superconductive film and substrate was investigated using analytical electron microscopy. The results indicate that MgO and ZrO₂ are superior to Al₂O₃ for substrate materials.     

Studies of Beryllium Chromite and Other Beryllia Compounds With R2O3 Oxides

Reactions between BeO and R₂O₃ oxides at high temperatures were studied. Compound formation was observed between BeO and the following oxides: B₂O₃, Al₂O₃, Ga₂O₃, Y₂O₃, La₂O₃, and Cr₂O₃. No reaction was observed with Sc₂O₃, ln₂O₃, and Fe₂O₃. Detailed studies were made of BeO·Cr₂O₃ which is isostructural with BeO·Al₂O₃. BeO·Cr₂O₃ is a semiconductor. Optical and X-ray data are given for all reaction products.     

A comparative study of thin film transistors using rare earth oxides as gates

A comparative study of CdS/Nd₂O₃, CdSe/Nd₂O₃, CdS/Pr₆O₁₁-Yb₂O₃, CdSe/Pr₆O₁₁-Yb₂O₃, CdS-CdSe/Nd₂O₃, CdS-CdSe/Pr₆O₁₁ and CdS-CdSe/ Yb₂O₃ thin film transistors is presented. Rare earth oxides were used as the gate insulators. Various parameters, such as transconductance, output resistance, amplification factor, gain-bandwith product, mobility, concentration of traps, critical donor density and grain size, were determined and are presented. Comparison of these devices shows that Cdse/Nd₂O₃ and CdS-CdSe/Nd₂O₃ transistors are superior in performance.     

Phase Equilibria in Systems Involving the Rare-Earth Oxides. Part I. Polymorphism of the Oxides of the Trivalent Rare-Earth Ions

The polymorphic relationships of the pure rare-earth oxides have been reinvestigated using X-ray diffraction methods for identification of phases. The oxides of the trivalent rare earth ions crystallize in three different types: A, B, and C. Each oxide has only one truly stable polymorph: La₂O₃, Ce₂O₃, Pr₂O₃, and Nd₂O₃ belong to the A type; Sm₂O₃, Eu₂O₃, and Gd₂O₃ to the B type; Tb₂O₃, Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, and Lu₂O₃ to the C type. In addition Nd₂O₃, Sm₂O₃, Eu₂O₃, and Gd₂O₃ have low-temperature, apparently metastable, C-type polymorphs. The low-temperature form inverts irreversibly to the stable form at increasingly higher temperatures for decreasing cation radius.     

Measurement of Young’s modulus of oxides at high temperature related to the oxidation study

Abstract

α-Al₂O₃, Cr₂O₃, and α-Fe₂O₃ specimens were prepared by a sintering process. A 400 – 1000-Hz sine wave was applied to the specimen at 290 – 1273 K. The applied and respond waves were monitored by using force and acceleration sensors. The intensity ratio and phase shift between the applied and respond waves were analysed, and the anti-resonance frequency was obtained. Young’s moduli of α-Al₂O₃, Cr₂O₃, and α-Fe₂O₃ are estimated to be 386, 286, and 220 GPa at 298 K, respectively. The temperature dependence values of these oxides are estimated to be 54.3, 46.9, and 42.0 MPa K^-1, respectively. The temperature dependence of Young’s modulus can be classified on the basis of the crystal structure of solids. The estimation of Young’s modulus at 1273K is possible with an error range of 10 – 30 GPa for a crystalline solid if the crystal structure of the solid is known. It is found that the temperature dependence of Young’s modulus depends on the density of the oxides, and an experiment in which well-characterized crystalline solids are used must be conducted to minimize the error range.     

Low-temperature sintering and electrical properties of Co-doped ZnO varistors

ZnO–Bi₂O₃–B₂O₃-based varistors doped with each kind of cobalt oxides were prepared by conventional ceramic processing. The effects of CoO, Co₂O₃ and Co₃O₄ on the microstructure and the electrical characteristics of varistor samples sintered at 880 °C were investigated separately. Analysis of microstructure indicated the cobalt cations were distributed both in grain regions and grain boundary regions and no crystalline phases containing cobalt were detected in XRD patterns for the samples with various cobalt oxides. All these cobalt oxides could effectively enhance the varistor performance by effectively increasing the nonlinear coefficient and lowing the leakage current, while the breakdown voltage fields increased slightly. Capacitance–voltage characteristics showed the potential barriers of varistor samples increased with the addition of each cobalt oxide. It was found that the addition of same amount of cobalt cations in various cobalt oxides had a different effect on the varistor samples. Best electrical properties were obtained for the varistor sample containing Co₃O₄, in which the nonlinearity coefficient is 28.5, the leakage current density is 3.4 μA and the breakdown voltage field is as low as 260 V/mm.     

Corrosion properties of plasma-sprayed Al2O3-TiO2 coatings on Ti metal

Three different oxides of CrO₂-TiO₂, Al₂O₃ and Al₂O₃-TiO₂ were plasma-sprayed on Ti substrate to evaluate the crystal structure and the corrosion properties of the coatings. No phase change of the coatings after corrosion test in 0.5 M H₂SO₄ solution at 25°C was found regardless of the presence of the NiCoCrAlY bond layer. Electrochemical measurements and SEM results revealed that the single coatings without the bond layer were always effective against corrosion resistance due to lower current density within the passive region. Pitting corrosion of the surface was observed for the Al₂O₃ coating. It can be concluded that the Al₂O₃-TiO₂ coating without the bond layer may be the best oxide among the oxides investigated due to low porosity (5.4%), smooth surface roughness (4.5 μm), low current density (6.3 × 10^‒8 A/cm²) in the passive region, low corrosion potential (E_corr, ‒0.55 V) and no pitting corrosion.     

Simple synthesis of nano-sized refractory metal carbides by combustion process

Combustion reactions of transition metal oxides (WO₃, MoO₃, Ta₂O₅, Nb₂O₅, ZrO₂, and TiO₂), magnesium, carbon, and sodium fluoride produce a range of nanostructured transition metal carbides (W₂C, Mo₂C, TaC, NbC, ZrC, and TiC) with low amounts of free carbon. Sodium fluoride improves unfavorable combustion regimes and facilitates the carburization of metal carbides. The average particle size of carbides prepared in this study was below 100 nm in pure phase. The carbides were characterized by X-ray diffraction, scanning electron microscopy, specific surface area analysis, and carbon analysis.     

Phase separation in tellurite glass-forming systems containing B2O3,GeO2,Fe2O3,MnO,CoO,NiO and CdO

The boundaries between the regions of single-phase and two-phase glasses were established in tellurite glass-forming systems containing B₂O₃ and one of the following oxides: GeO₂, Fe₂O₃,CoO, NiO, MnO and CdO. The character of the microstructures inside and outside the regions of stable phase separation were determined by electron microscopy. It was shown that the existing microheterogeneities may either result from incomplete liquid immiscibility during melting and supercooling or be due to typical metastable separation.