Stoichiometric Regime Research Articles

Ionic conductivity of undoped BaTiO 3− δ with electron transfer suppressed

Ionic conductivity has been directly measured on “undoped” BaTiO 3− δ over a temperature range of 550 to 1000 °C with the aid of a modified electron-blocking cell. The modified cell could eliminate or greatly suppress or circumvent the haunting annoying artifacts associated with gas leak, reaction of a sample with an electron-blocking electrode or sealing glass, and large interfacial resistances in a conventional two-probe electron-blocking cell. The ionic conductivity of undoped BaTiO 3− δ in the stoichiometric regime ( δ≈0) may best be represented depending on temperature range as: σ ion / Scm − 1 = ( 6.6 × 10 5 / T ) exp ( − 1.58 eV / k ⁢ T ) for T ≥ 1073 K ; σ ion / Scm − 1 = ( 1.20 × 10 7 / T ) exp ( − 1.83 eV / k ⁢ T ) for T < 1073 K . It is indicated that association (with an association enthalpy of 0.25 eV) occurs between carrier oxygen vacancies and the charge compensating acceptor species, whether extrinsic or intrinsic, at temperatures below 1073 K. The higher temperature result is in good agreement with those obtained indirectly from the identical quality specimens. The chemical diffusivity has been determined from the temporal change of the cell voltage under a galvanostatic condition and is also in satisfactory agreement with the earlier study.

Statistical design of experiments for evaluation of Y–Zr–Ti oxides as anode materials in solid oxide fuel cells

AbstractMixed conducting anode materials for solid oxide fuel cells are desirable in order to extend the electron transfer reaction zone for fuel gas conversion and to minimise the nickel content for achieving a redox stable anode. Partial substitution by titania in yttria stabilised zirconia (YSZ) is known to increase the electronic conductivity in reducing atmospheres. Nine different compositions were selected from the quasi ternary phase diagram according to principles used in statistical design of experiments covering the whole stoichiometric regime relevant for ionic applications. The dc electrical conductivity values increase strongly with high Ti contents under reducing (Ar–4%H2) conditions, whereas they decrease continuously with increasing Ti content under oxidising conditions (air). The results clearly show that the chosen screening process for materials selection can considerably reduce the number of samples. For solid oxide fuel cell anodes, the compositions in the YO1.5–ZrO2–TiO2 system shoul...

P-Type Partial Conductivity of Donor(La)-Doped BaTiO3

P-type partial conductivity has been determined on donor (La Ba • )-doped BaTiO3 in full thermodynamic equilibrium state at a fixed temperature of 1200°C: For the nominal compositions of Ba0.99La0.01Ti0.9975O3, Ba0.99La0.01TiO3 and Ba0.985La0.01TiO3, the p-type conductivity is found to vary with oxygen activity as σp = (σm/2)(a O 2/a O 2*)+1/4 with σm ≈ 0.01 S cm−1 and a O 2* ≈ 32, 120, 310, respectively, in the a O 2 region where conventionally the electronic conductivity varies as σ ∝ aO 2 −1/4 and hence, the doped donors are believed to be compensated by cation vacancies (say, [LaBa •] ≈ 4[V″Prime;Ti]). This experimental fact supports that in the vicinity of the stoichiometric composition of the system which falls approximately at a O 2 = a O 2*, while cation vacancy concentration is fixed by the donor concentration, oxygen vacancy concentration in the minority is also essentially fixed, thus, keeping the activity of TiO2 (or BaO) fixed. It is consequently suggested that donor-doped BaTiO3 contains a second phase even in its stoichiometric regime.

Growth process monitoring and crystalline quality assessment of CuInS(Se) 2 based solar cells by Raman spectroscopy

This work reviews the use of Raman scattering for the crystalline quality assessment of CuInS(Se) 2 thin film chalcopyrite materials for photovoltaic applications. The Raman spectra of these semiconductors show a dominant A 1 mode with spectral features sensitive to the microcrystalline quality of the layers. The spectra are also characterised by additional modes, which have been interpreted and modelled as related to the existence of polymorphic domains with Cu–Au ordering. This interpretation is supported by the detailed microstructural analysis of various specimens, including epi- and polycrystalline layers obtained under different stoichiometric regimes leading to significant differences in final solar cell efficiency. The relative intensity of these additional modes in the spectra is proposed for the monitoring and quality assessment of the absorber layers for photovoltaic devices. Besides, identification of the main vibrational modes related to secondary phases characteristic of the different thin film growth processes allows the use of the technique for process monitoring. Moreover, thanks to its non-destructive character, its implementation can be in principle designed at both in situ and ex situ levels.

Chemical diffusion in complex oxides with an emphasis on BaTiO3

Chemical diffusion has long been studied mostly on binary oxides, but practical interest in it is increasing with more complex oxides these days, e.g., perovskite oxides that are finding a variety of electroceramic and electrochemical applications. In this paper, we compile the chemical diffusivity data of oxygen as measured on a prototype perovskite BaTiO3 and review the connotation of their variation with oxygen activity in the light of the chemical or ambipolar diffusion theory. The chemical diffusivity of BaTiO3 is compared with those of other perovskite oxides of mostly electrochemical concern and of typical binary transition metal oxides. The chemical diffusivity is further compared with those of the compounds such as Ag2S, Ag2Te, YSZ and Fe3O4, that have dissimilar crystal structures, but with a similar defect structure to BaTiO3, in order to get a further insight into the inner workings of chemical diffusivity particularly in the stoichiometric regime of complex oxides. Finally, possible complication with nonstoichiometry re-equilibration kinetics due to cation mobility is pointed out with reference to what has been observed from LiNbO3 under an oxygen activity gradient.

Reaction Kinetics and Chemical Equilibrium of Homogeneously and Heterogeneously Catalyzed Acetic Acid Esterification with Methanol and Methyl Acetate Hydrolysis

The reaction kinetics and chemical equilibrium of the reversible esterification of methanol with acetic acid were investigated. This system is of major importance as a model reaction for reactive distillation. The reaction has been catalyzed both homogeneously by acetic acid itself and heterogeneously by an acidic ion-exchange resin (Amberlyst 15). The chemical equilibrium composition was measured for various temperatures and starting compositions of the reactants and products. Kinetic information was obtained at temperatures between 303.15 and 343.15 K at various starting compositions covering concentration ranges from the stoichiometric regime to the dilute regions. Both the esterification and the hydrolysis reaction were investigated to yield a model which is applicable for any starting composition. The homogeneous reaction has been described with a simple power-law model. The use of activities in the kinetic model instead of mole fractions results in a much smaller residual error. To compare pseudohom...

The reversible magnetization of oxygen deficient YBa 2Cu 3O 7−δ

The reversible magnetization M rev of polycrystalline YBa 2Cu 3O 7−δ (0< δ<0.5) was measured. Due to a small grain size a nd low critical current densities in oxygen deficient specimens M rev could be measured over most of the superconducting regime from 10 K to T c up to a magnetic field of 5.5 T. The magnetization data is in excellent agreement with calculated values obtained from Ginzburg-Landau (GL) theory. Values for the thermodynamic critical field H c and the GL parameter κ are obtained. High values of κ ≈ 170–300 (for H∥ c) are necessary to fit the magnetization data. H c( T = 0) decreases strongly for increasing oxygen deficiencies up to δ ≈ 0.15, and then remains nearly constant. Specific heat jumps Δ c at T c were calculated from the data and the fit. Quantitative agreement with data in the literature is obtained. Just like H c, the specific heat jump is strongly dependent on the oxygen deficiency in the near stoichiometric regime.

Morphological defects arising during MBE growth of GaAs

The growth by molecular beam epitaxy of GaAs and GaAlAs is reviewed, with special attention given to morphological defects which can occur. Although the allowed stoichiometric growth regime for this system is quite wide, more restricted conditions are required when overall perfection is important. The paper characterizes the growth process within the framework of achieving material suitable for GaAs–GaAlAs double heterostructure lasers. This includes substrate preparation, growth initiation, and growth evolution. Specific morphological defects which are found to be observed quite generally are characterized and some explanations will be ventured. Achieving featureless growths is important for integrated optic devices and this is possible over large area substrates.