M.-B. Choi

Electrical Conductivity and Thermoelectric Power of La2NiO4+δ

The electrical conductivity, Seebeck coefficient, and thermal chemical expansion properties of La 2 NiO 4+δ were measured as a function of temperature and oxygen partial pressure (pO 2 ). The electronic conductivity was analyzed in relation to the thermoelectric power (thermopower) data to elucidate the positive deviation of the activity coefficient of hole on the basis of the delocalized electron model by using the Joyce-Dixon approximation of the Fermi-Dirac integral from the partition function in the quasi-free-particle approximation with the regular solution model. The electrical conductivity and thermopower vs oxygen activity were all explained consistently in the logical frame of the degenerate p-type conductor across the entire experimental range investigated. Like other K 2 NiF 4 -type oxides, the chemical diffusion coefficient was slightly higher than the surface exchange coefficient, suggesting the need for further work to enhance the sluggish surface exchange kinetics of oxygen. The best-estimated mobility values were in reasonable agreement with the reported values and the very weak temperature dependence of hole mobility suggested a band conduction.

Determination of Oxygen Chemical Diffusivity from Chemical Expansion Relaxation for BaCo0.7Fe0.22Nb0.08O3 − δ

The thermal and chemical expansion properties of BaCo 0.7 Fe 0.22 Nb 0.08 O 3-δ , a potential candidate for oxygen permeable membranes and cathodes in solid oxide fuel cells, are reported here. The average thermal expansion coefficient is about 14 × 10- 6 K -1 below 500°C, about 19 × 10- 6 K -1 between 600 and 900°C, and about 29 × 10- 6 K -1 between 900 and 1000°C, which consists of the previous weight change in a thermal gravimetric analysis experiment. The transient behavior in the thermochemical expansion can be best fitted with Ficks second law for a small displacement from the equilibrium. The chemical diffusivity of oxygen and the surface exchange kinetics are successfully extracted.

Partial Conductivities and Chemical Diffusivities of Mixed Protonic–Electronic Conducting CaZr0.9Y0.1O3 − δ

The electrical properties of CaZr 0.9 Y 0.1 O 3―δ (CZY) were studied as a function of both oxygen partial pressure (P O2 ) and water vapor partial pressure (P H2O ) in the temperature range of 500―950°C and the partial conductivities of protons, holes, and oxygen vacancies were calculated from the defect model. In a wet atmosphere, CZY was a mixed conductor of protons, holes, and oxygen ions. No conduction transition from protons to holes and/or oxygen ions occurred even as temperature was increased up to 950°C. The calculated activation energy of proton transport was 1.13 eV. The standard solution enthalpy for water dissolution was best fitted with a slope of ―413 kJ/mol. The twofold conductivity relaxation behavior that was observed at 750°C during hydration/ dehydration was interpreted in terms of decoupled ambipolar diffusion of oxygen vacancies and protons by holes. The best-estimated chemical diffusivities of oxygen and hydrogen were several orders of magnitude lower than that reported for other proton-conducting perovskite oxides due to the relatively low ion conductivity of CZY. .

Polyol Synthesis of Pd/Ag Alloy Nanocrystalline

A solid solution of Pd-Ag nanoparticles was synthesized by the conventional polyol process. Silver nitrate (AgNO 3 , Aldrich), palladium(II) nitrate hydrate [Pd(NO 3 ) 2 ·xH 2 O, Aldrich], and ethylene glycol were used as the starting materials. Ethylene glycol was used as the surfactant for the solvent and reductant. Water was used to ensure that the reduction potentials of the two metals remained very similar in the aqueous solution. The obtained alloy nanopowder sample was characterized by X-ray diffraction (XRD). The obtained XRD pattern was refined using the Full-Prof program according to the Rietveld method and the mean crystallite sizes were estimated using the Scherrer equation. The quantitative atomic percentage across the nanoparticles was determined by using scanning transmission electron microscopy images to confirm the formation of the Pd/Ag alloy nanopowder.