Defects and Diffusion in Ceramics - An Annual Retrospective VII

Volumes 242-244

doi: 10.4028/

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Authors: Masataka Mizuno, Hideki Araki, Yasuharu Shirai, Fumiyasu Oba, Isao Tanaka
Abstract: The formation of Mg vacancy induced by ultra-dilute trivalent impurities in MgO is investigated by a combination of positron lifetime measurements and first-principles calculations. The undoped MgO yields the shortest positron lifetime of 140 ps that is shorter than that of a single crystal sample. The positron lifetime of the doped samples increases with the increase of the Al dopant concentration and is saturated at around 180 ps. This result clearly indicates that the formation of Mg vacancy is induced by Al dopant. The concentration of the other trivalent impurities can be evaluated using the result of component analysis of positron lifetimes. The experimental bulk lifetime of 130 ps, which is obtained by employing trapping model, is well reproduced by the theoretical calculation using the semiconductor model. The calculated defect lifetime is about 20 ps longer than the experimental value. This may be due to the lattice relaxation around Mg vacancy associated with the trapping of positrons.
Authors: Norifumi Ohba, Eri Oikawa, Takuya Hashimoto
Abstract: In order to evaluate potential of La1-xCaxCrO3-δ as a material for interconnector of solid oxide fuel cells, reduction expansion was measured by using high temperature X-ray diffraction under various P(O2). Obtained reduction expansion increased with increasing temperature and Ca content. With combination of obtained lattice constants and oxygen nonstoichiometry data, the effect of oxide ion vacancy on crystal lattice and their thermodynamic behavior have been deduced. It was proposed that expansion behavior by δ of La0.9Ca0.1CrO3-δ can be explained assuming ideal solution model on oxide ion vacancies, however, deviation from ideal solution model was observed in reduction expansion behavior of La0.8Ca0.2CrO3-δ and La0.7Ca0.3CrO3-δ with δ region larger than 0.03 and 0.05, respectively. It was revealed that the crystal system approached to cubic with increase of δ in the region where the deviation from ideal solution model was observed.
Authors: Yasumasa Tomita, Hideyoshi Matsushita, Yasuhisa Maeda, Kenkichiro Kobayashi, Koji Yamada
Abstract: Li3-2xMxInBr6 (M=Mg, Ca, Sr and Ba) and Li3In1-xMxBr6 was synthesized, and thier substitution effect was investigated by means of 7Li and 115In NMR, X-ray diffraction and AC conductivity measurements. Phase transition was observed at 314 K in Li3InBr6 and fast Li+ diffusion was observed in the high temperature phase. Li3InBr6 has high Li+ ion conductivity and showed a little difference in X-ray diffraction patterns between the low-temperature phase and the high-temperature phase. These indicate that the sub-lattice for Li+ ions changed largely at the phase transition point and this change makes Li+ diffusion easily. In the high temperature phase of substituted compounds, the conductivity decreased with the amounts of substitution. and defects produced by the substitution with divalent cation did not contribute to the Li+ ion conduction. In the LT phase for Mg compound, the ionic conductivity increases up to x = 0.4 due to the introduction of the extrinsic vacancies.
Authors: S.N. Savvin, G.N. Mazo, A.K. Ivanov-Schitz
Abstract: The oxygen mobility in La2-xSrxCuO4-δ (x=0.15; 0.6; 1) was studied by the Molecular Dynamics (MD) technique. The parent layered La2CuO4 crystal structure has been shown to give rise to a strong anisotropy of oxygen diffusion coefficient in the lattice. Equatorial oxygen sites in(CuO2) layers were found to provide the paths of the fast oxygen transport in the structure, while the axial ones in (La2O2) blocks were substantially less mobile. The influence of the dopant concentration on structural properties and energetic characteristics of the oxygen migration are discussed. Analysis of the ion trajectories obtained during the simulation allowed explaining the observed dependence of the oxygen diffusion activation energies on the strontium content and provided further insight into the mechanism of oxygen diffusion in the oxides.
Authors: K. Salama, G. Majkic, U.(Balu) Balachandran
Abstract: In this paper we review a number of studies of stress-induced diffusional matter transport in perovskites, with an emphasis on creep studies used as a means of studying defect chemistry on the cation sublattices. Studies of diffusional creep in air or fixed atmospheres are reviewed first, and the common characteristics among these perovskites are identified. Creep studies of several perovskiterelated or perovskite-like structures are reviewed next, and the similarities/dissimilarities to perovskites are outlined. The diffusional creep studies in controlled atmosphere are reviewed next, with the emphasis on defect chemistry modeling from creep data. The paper presents a detailed review of two creep studies in oxygen controlled atmosphere that show particularly interesting and remarkedly different behavior from that predicted by standard defect chemistry models. Defect chemistry modeling from creep data is presented for these two cases. The potential and limitations of using creep experiments for studying diffusional matter transport and cation defect chemistry are discussed.
Authors: J. Molenda, M. Ziemnicki, K. Świerczek, J. Marzec
Abstract: In this paper structural, electrical, electrochemical and thermal (DSC) characterization of series of manganese spinel samples with manganese substituted to different degree (x = 0 – 0.5) with nickel are presented. The conductivity and thermoelectric power measurements were performed in wide temperature range also versus oxygen partial pressure and for deintercalated samples. Electrochemical studies of these cathode materials were conducted in Li / Li+ / LiyNixMn2−xO4 type cells. Substitution of manganese with nickel causes disappearance of the phase transition characteristic of LiMn2O4 spinel. Studies of electrical properties reveal that Ni ions do not participate in charge transport at low temperatures. In the charge curves of Li / Li+/ LiyNixMn2−xO4 cells there are two visible plateaux, separated with distinct potential jump (~0.5V), which position on Li content perfectly matches the Mn3+ content in the doped cathode material. The lower plateau is related to the Mn3+ → Mn4+ oxidation, while the next of higher voltage, of the dopant Ni2+ → Ni4+ oxidation. The schematic diagrams of relative Mn – Ni electronic levels alignment are proposed.
Authors: Vo Van Hoang
Abstract: Static and Dynamic heterogeneities in supercooled SiO2 have been investigated in the models containing 3000 particles obtained by cooling from the melt with the pair interatomic potentials, which have the Morse type part for the short-range interaction. The evolution of structure of the system upon cooling was presented and analyzed in details through the changes in the partial radial distribution functions (PRDFs), coordination number distributions, bond-angle distributions and structural defects. Calculation presented that the temperature dependence of diffusion constant D of components in the system shows an Arrhenius law at low temperatures and it shows a power law, γ ) ( C T T D − ∝ , at high temperatures. The critical temperature Tc is equal to 4200 K and the exponent γ is close to 0.50. In order to study the dynamical heterogeneities in the system, we evaluated the non- Gaussian parameter for the self-part of the van Hove correlation function and luster-size distributions of most mobile or immobile particles in the model. We compared the PRDFs for the 10% most mobile or immobile particles with the corresponding mean ones. We have found that the most mobile and immobile particles form clusters and mean cluster size grows with decreasing temperature.
Authors: M. El-Hofy
Abstract: Four Zinc Molybdenum ceramic samples (S1-S4), have been prepared according to the chemical formula (1-x) ZnO - x (MoO3), where (x = 0.1, 0.2, 0.6, 0.8) mol %. The samples were studied through X-ray Diffraction analysis, SEM, EDAX, I-V characteristics at different temperature up to 200 οC and C-V measurements. X-ray results decleared that Mo contributes to the structure mainly substitution in place of Zn. Entrance of Mo into the structure increases the intensity and shifts the X-ray peaks to higher θ values. At lower additions, Mo is segregated along the grain boundaries in the form of short bars, while at higher additions, circular batches of Mo rich phase are appeared on the surface of the large ZnO grains. The formulation of Schottky barrier is indicated, in case of (x < 0.1) the barrier is attributed to the trapping of electrons by the defects at the grain boundaries, while at higher additions the barrier was attributed to the concentration gradient of Zn vacancies in the grain boundary region. The calculated barrier height and non-linearity coefficient α are (1.15, 1.12, 1.15 and 1.48) eV and (59.4, 22.5, 35 and 87) for the samples S1- S4 respectively. Going from S1 to S4, donor density Nd and density of the interface states Ns decrease from 3.81x 1018 to 0.46x1018 cm-3 and from 6.41x1012 to 2.52x1012 cm-2 respectively, while the width W(cm) of the potential barrier increases from the value 1.68x10-6 cm for S1 to the value 5.5x10-6 cm for S4 . The current processed via electron tunneling through the barrier.

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