High Nonstoichiometry Research Articles

Non-stoichiometry and electrochemical properties of lithiated iron hydroxysulfides.

LiFeOHS is a material with Li2(OH)2 layers intercalated between Fe2S2 planes. Its hydrothermal synthesis in various concentrations of LiOH yields materials with a high non-stoichiometry of the Li/Fe ratio which can be explained by partial substitution of Li+ for Fe2+ in the Li2(OH)2 layers. Thermogravimetry, X-ray diffraction and Mössbauer studies indicate that the charge balance is obtained by substitution of hydroxyl ions OH- by oxide ions O2-. This material has been tested as an electrode for lithium-ion batteries against lithium metal. Specific capacities above 200 mA h g-1 at C/10 are achieved, involving 1 lithium per chemical formula when cycled between 1 V and 3 V vs. lithium. The first irreversible discharge leads to the insertion of one lithium atom and the evolution of hydrogen gas while iron remains in its +2-oxidation state. An original Li2OFeS oxysulfide is formed. The following reversible oxidation/reduction cycles involve the Fe3+/Fe2+ redox couple between the two limiting compositions: Li2OFeIIS and LiOFeIIIS.

Non-stoichiometry, Electrowetting and Contact Interaction of Zirconia, Titania or Hafnia with Metal Melts

Transition metal oxides, in particular titania, zirconia and hafnia, due to high anion mobility, can lose oxygen with the formation of non-stoichiometric phases and also passage of electric current at high temperature. This influences the properties of the materials, in particular the contact interaction with metals. This paper studies the wetting of zirconia, hafnia and titania with inert metal melts (Cu, Ni, Pt, Cu-Ga, Cu-Ge, etc.) in conditions where high non-stoichiometry of oxides was provided and where current passed through the interfaces. Sessile drop wetting in vacuum, microstructure analysis and x-ray studies were conducted. In wetting experiments with high non-stoichiometry, oxide samples contacted at the same time with active metal melts (Ti, Cu-Ti, Ni-Ti) to provide the oxygen deficit in substrates and with inert metal melts (Cu, Ni, Pt, Cu-Ga, Cu-Ge, etc.). In experiments with current, metal drops were situated between ceramic or ceramic and metal plates connected to current leads. When inert and active metals contacted the ceramic simultaneously, inert melts wet the zirconia and the hafnia but not the titania. This effect was explained by dissolution of surplus zirconium and hafnium in melts. Titania in contact with active metal was reduced to a lower oxide, and permanent oxygen removal from the sample volume was not provided. Dissolution of zirconium was confirmed for the nickel/zirconia/Ni-Ti samples through microstructure investigation and thermodynamic calculations. When a positive electrode was connected to a ceramic, wetting significantly improves for zirconia, titania and hafnia. It was also explained by oxygen depletion under the current and subsequent dissolution of surplus titanium, zirconium or hafnium in the melt. The results obtained were used to braze zirconia ceramics. Thus the non-stoichiometry caused by high anion mobility was found to influence the titania-, zirconia- and hafnia-to-metal interaction.

Tunable Internal and Surface Structures of the Bifunctional Oxygen Perovskite Catalysts

Perovskite oxide ceramics attracts significant attention as a strong candidate of bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrocatalyst for the metal‐air batteries. Numerous approaches to the viability of bifunctional perovskite electrocatalyst represent that the electro­chemical performance is highly correlated with defect chemistry, surface structure, and overall polycrystalline perovskite structure. By making use of the intrinsic flexibility of internal structure and high nonstoichiometry in perovskite oxide, the heat treatment effect of the complex Ba0.5Sr0.5CoxFe1‐xO3‐δ (x = 0.2 and 0.8) perovskites in argon atmosphere at 950 °C (Ar‐BSCF5582 and Ar‐BSCF5528) on the surface structure/defect chemistry and electrocatalytic performance is intensively investigated. Upon heat‐treatment in argon atmosphere, the amorphous thickness layer increases from ≈20 to 180–200 nm in BSCF5582, while there is little change in BSCF5528 with ≈20 nm. The electrocatalytic performance of BSCF5582 catalyst both in ORR and OER deteriorates seriously, while Ar‐BSCF5528 demonstrates a significant increase of electro­chemical performance in ORR. This study demonstrates that the electrochemical performances of a perovskite catalyst can be significantly determined by the simultaneous modification of both surface structure and internal defect chemistry, which are explained with transmission electron microscopy and atomic‐selective X‐ray absorption fine structure analyses, respectively.

Precursor Dependent Microstructure Evolution and Nonstoichiometry in Nanostructured Cerium Oxide Coatings Using the Solution Precursor Plasma Spray Technique

A solution precursor plasma spray (SPPS) technique has been used for direct deposition of cerium oxide nanoparticles (CNPs) from various cerium salt solutions as precursors. Solution precursors were injected into the hot zone of a plasma plume to deposit CNP coatings. A numerical study of the droplet injection model has been employed for microstructure development during SPPS. The decomposition of each precursor to cerium oxide was analyzed by thermogravimetric‐differential thermal analysis and validated by thermodynamic calculations. The presence of the cerium oxide phase in the coatings was confirmed by X‐ray diffraction studies. Transmission electron microscopy studies confirmed nanocrystalline (grain size <14 nm) characteristic of the coatings. X‐ray photoelectron spectroscopy studies indicated the presence of a high concentration of Ce3+ (up to 0.32) in the coating prepared by SPPS. The processing and microstructure evolution of cerium oxide coatings with high nonstoichiometry are reported.

Compositional Stresses in Polycrystalline Ceria Films

Nanocrystalline ceria generally has significantly higher defect concentrations and more oxygen nonstoichiometry than coarse grained material that is equilibrated at the same temperature and oxygen partial pressure. The volume changes associated with such high nonstoichiometry can lead to significant stresses in constrained ceria films. This may have significant implications for applications such as solid electrolytes, where stresses can lead to mechanical failures. To systematically study these compositionally induced stresses, ceria films with varying film thickness were grown by MOCVD and in situ stress measurements were made by monitoring changes in the curvature. The results show that nanocrystalline films exhibit significantly higher compositional stresses than coarse grained films. The film thickness also appears to have a measurable effect on the observed compositional stresses.

Effect of double exchange on thermoelectric power of Cu xGa yCr zSe 4

A correlation between double exchange magnetic interaction and the thermoelectric power is considered in the ferromagnetically ordered Cu x Ga y Cr z Se 4 nonstoichiometric p-type spinel conductors in the temperature range from 5 to 400 K between y = 0.036 and 0.394. The magnetic and thermopower investigations provide evidence that: (1) the carrier diffusion dominates in the most stoichiometric single crystals and the intensity of diffusion thermopower is five times larger than the intensities of the phonon and magnon excitations, (2) for samples with low gallium content the strong ferromagnetic coupling connected mainly with double exchange mechanism makes easier the spin wave excitations at higher temperatures on one side and the high nonstoichiometry hinders the spin wave excitations on the other, and (3) for samples with higher gallium content, which are more stoichiometric, the ferromagnetic coupling connected here with the weaker double exchange interaction causes a shift of the magnon drag peak into lower temperatures.

In situ characterization of Zircaloy-4 oxidation at 500 °C in dry air

The in situ oxidation of Zircaloy-4 at 500 °C in dry air was investigated by thermogravimetric analysis (TGA) and electrochemical impedance spectroscopy (EIS). The coating of the alloy by a platinum film as electrode material was observed as not to modify the oxidation kinetic properties. After an initial cubic rate law, a transition to a quasi-linear curve occurs. The independence of the oxidation behavior to the Pt coupling is compatible with oxygen diffusion as the rate-determining step. During the pre-transition step, the rest potential of the cell Pt/oxide/Zy-4, the color of the oxide and the modulus of the single EIS signature indicate the high non-stoichiometry of the oxide. The kinetic transition was proposed to be correlated to the degradation of the film into a partially porous layer. This alteration of the oxide is associated to the appearance of a 1.2 V constant rest potential and the modification of the impedance diagrams in two high modulus contributions. The Cole–Cole representation has been used to demonstrate that the time variation of impedance spectra is related to the oxide growth. An equivalent circuit including two RC loops in series, whose capacitances are frequency dispersed, was proposed to be related to the film structure. Fitted data show that the thickness of the assumed protective layer of the film, close to the metal–oxide interface, is time independent in agreement with a constant oxidation rate. Finally, electrical properties of this inner layer were found to be quite different in pre- and post-transition stage.

Diffusional growth of multiphase scales with variable composition

The paper presents a quantitative theory for the growth of multilayered scales formed of phases with variable compositions, molar volumes and diffusivities. The theory enables the determination of moving-boundary state growth kinetics for the scales formed of phases showing high non-stoichiometry. It completely describes the multilayered (multiphase) planar scale growing on pure metal and allows for the calculation of the total as well as partial reaction rates, the ratio of thickness of the phases formed and the metal concentration distribution in the scale.

ChemInform Abstract: ANNEALED SINGLE‐CRYSTAL CADMIUM SELENIDE ELECTRODES IN LIQUID JUNCTION SOLAR CELLS

AbstractStrom‐Spannungs‐Charakteristiken, die Spannungsabhängigkeit der Quantenausbeute und die spektrale Empfindlichkeit werden für unter verschiede‐ nen Bedingungen geglühte (600°C im Vakuum oder in Gegenwart von Cd oder Se) CdSe‐Photoanoden verglichen.

Annealed Single‐Crystal Cadmium Selenide Electrodes in Liquid Junction Solar Cells

I‐V characteristics, voltage dependence of the quantum efficiency, and spectral response were compared for annealed single‐crystal photoanodes. Annealing in cadmium atmosphere improved the overall solar response considerably, while annealing under vacuum revealed a poor response for photon energies larger than 1.8 eV and a high quantum efficiency for near‐bandgap energies. This behavior may be attributed to electron‐hole pair generation from interbandgap states and a large density of minority carrier recombination centers near the crystal surface, owing to a high nonstoichiometry and a selenium layer at the surface. Annealing in selenium atmosphere resulted in very poor solar response caused by compensation. Simultaneous illumination of the electrodes with a He‐Ne laser strongly enhanced the quantum efficiency for vacuum‐annealed crystals for near‐bandgap photons. We attribute this to electron trapping in the selenium‐rich surface, with a resulting increase in depletion‐layer width in the cadmium selenide.

Defects in metal-deficient cobalt-wüstites, (Co, Fe) 1−yO

By a simple statistical thermodynamic approach an equation describing the variation of nonstoichiometry with oxygen pressure is derived for metal-deficient oxides assuming that the predominant defect is of the type: 4V″ Fe, Fe … i. Comparing experimental data obtained by thermogravimetric equilibration measurements with data calculated from this equation shows good agreement both for pure Fe 1− y O (wüstite) throughout the whole nonstoichiometry range and for (Co, Fe) 1− y O at low and moderate degree of nonstoichiometry. For the latter oxides, however, deviations are observed at high nonstoichiometry indicating that ordering probably takes place in this system at high defect concentrations.

Defect Equilibria in Transition Metal Oxides

This paper assesses the importance of vacancy aggregates in transition metal oxides. It presents the predictions of quantitative atomistic calculations and discusses them in relation to observed structural, diffusion, thermodynamic, and other data. The results indicate that vacancy complexes constitute the major defect species at high nonstoichiometry. The commonly discussed vacancy‐hole complexes appear to be unimportant except, perhaps, over a very limited composition range.

Effects of high nonstoichiometry on EuO properties

Electric, magnetic, and electron paramagnetic resonance (EPR) measurements have been performed on thin EuO films prepared either by reactive evaporation of Eu at 400°C or by oxidation of a Eu metal thin film at 200–250°C. The films obtained by reactive evaporation have electrical and magnetic properties very similar to those reported for EuO single crystals of various degrees of nonstoichiometry. The films prepared by oxidation of Eu films at 200–250°C are characterized by a much higher-valued Curie temperature (Tc≃148 K), while their resistivity is in the range 10−2−1 Ω cm. The EPR signal can be attributed to Eu spins, which shows that in these films there exist fluctuations of Eu doping due to nonstoichiometry. We analyzed the high concentration of oxygen vacancies in the bound magnetic polaron (BMP) model. A high level of defects and oxygen vacancies, inhomogeneously distributed in the film, can account for both their electrical and magnetic properties.