Long-term performance testes by CRIEPI (Central Research Institute for Electric Power Industry) on six industrial stacks have revealed an interesting correlation between cathode polarization loss and ohmic loss. To make clear the physicochemical meaning of this correlation, detailed analyses were made on the conductivity degradation of YSZ electrolyte in button cells and then on the ohmic losses in the industrial cells in terms of time constants which are determined from speed of the tetragonal transformation through the Y diffusion from the cubic phase to the tetragonal phase. In some cases, shorter time constants (faster degradations) were detected than those expected from the two-time-constant (with and without NiO reduction effects) model, suggesting that additional ohmic losses after subtracting the contribution from the tetragonal transformation must be caused from other sources such as cathode-degradation inducing effects. Main cathode degradations can be ascribed to sulfur poisoning due to contamination in air in the CRIEPI test site. An important feature was extracted as this cathode degradations became more severe when the gadolinium-doped ceria (GDC) interlayers were fabricated into dense film. Plausible mechanisms for cathode degradations were proposed based on the Sr/Co depletion on surface of lanthanum strontium cobalt ferrite (LSFC) in the active area. Peculiar cathode degradations found in stacks are interpreted in term of changes in surface concentration by reactions with sulfur oxide, electrochemical side reactions for water vapor emission or Sr volatilization, and diffusion of Sr/Co from inside LSCF.

We evaluated for the first time the thickness dependence of the critical current density (Jc) of micrometre thick YBCO films on CeO2-buffered sapphire. YBCO films were successfully grown in microcrack-free form up to a thickness of by large-area pulsed laser deposition. Jc was found to decrease exponentially with YBCO thickness. Results suggest that the reduction in Jc with film thickness can be attributed to an evolving film microstructure as a function of thickness, as well as a corresponding change in the defect structures responsible for flux pinning. It was observed that film porosity and roughness increased with film thickness due to the growth and encroachment of the BaY2O4 phase. To clarify the flux pinning mechanism, we measured the angular dependence of Jc for films of different thicknesses and correlated this with the defect structure as revealed from the etch pit method and atomic force microscopy (AFM) observations. An unusually prominent Jc peak was observed when , which is due to correlated extended defects parallel to the c-axis of YBCO. Examination of the film microstructure revealed two defect types that give rise to the Jc peak: linear defects in the form of screw and edge dislocations, and planar defects possibly in the form of stacking faults. The density of linear defects decreased with film thickness whereas that of the planar defects increased considerably. From the behaviour of Jc with film thickness, these results suggest that linear defects may be more effective pinning centres than planar defects, or that an overabundance of planar defects may offset the increase of Jc for thick films.

Performance degradation of SOFC cathodes has been partly attributed to sulfur dioxide (SO2), which is one of the major impurities naturally present in air. In this work, the sulfur poisoning behavior of (LaSr)(CoFe)O3 (LSCF) cathodes exposed to 1 ppm of SO2 in air was investigated as a function of time under various operation temperatures of 923 K, 973 K, and 1073 K. Severe performance degradation was observed at low temperatures of 923 K and 973 K. For these cases, only the polarization resistance (Rp) was increased, additionally no sulfur component was detected in the LSCF cathode. At 1073 K, both Rp and ohmic resistance (Rohmic) increased during the progress of degradation and SrSO4 was observed in cathode, especially near the LSCF/GDC interface. Plausible sulfur poisoning mechanisms were proposed based on the observed experimental phenomena.

Experimental evidence of c-axis correlated vortex pinning in superconducting Y Ba2Cu3O7−δ (YBCO) thin films deposited on annealed CeO2-buffered sapphire was obtained. YBCO films were grown by pulsed laser deposition on CeO2-buffered () sapphire substrates. Prior to the superconducting film deposition, a self-assembly process was initiated where high-temperature (1025 °C) O2 annealing induced surface reconstruction of CeO2 on sapphire substrates. The result was an atomically flat CeO2 layer, self-assembled formation of nanodots on top of the CeO2 wetting layer, and high critical current density of the YBCO films deposited on such buffered sapphire substrates. The magnetic field dependence of Jc at various temperatures and the dependence of Jc on the orientation of the magnetic field were investigated. YBCO films with an annealed CeO2 buffer layer showed a high Jc peak when the applied field was directed along the c-axis of the YBCO. The surface morphology and microstructure of YBCO films on sapphire with as-grown and annealed CeO2 buffer layers were investigated using transmission electron microscopy. The results suggest that the Jc peaks in YBCO with an annealed CeO2 buffer layer are caused by c-axis correlated pinning sites, such as threading dislocations possibly induced by CeO2 nanodots and a high density of egg-shaped nanometre-sized precipitates that are elongated along the c-axis of YBCO.

Flux pinning properties of pinning centres having correlation along the c-axis in epitaxial YBCO films were investigated by measuring the magnetic-field angle ψ-dependence of the critical current density JC and the E–J-characteristics. YBCO films were prepared by using the pulsed-laser-deposition method on four different substrates at three different target-to-substrate distances D. The ψ-dependence of JC showed large peaks when magnetic field B was applied parallel to the c-axis (), and we observed two types of JC-peak: that is, a broad peak for the films deposited at small D (50–60 mm), and a narrow peak for the films deposited at large D (112 and 142 mm). The E–J-characteristics followed the power law, , and the ψ-dependence of the n-value also showed broad peaks around for the films deposited at D = 50–60 mm, and narrow peaks for the film deposited at D = 112 mm. Based on these results and our previous microstructural observations by AFM and TEM, we confirm that the broad-angle flux pinning effect around may be attributed to a high density of elongated precipitates, and the narrow-angle pinning effect around may be attributed to dense planar defects parallel to the c-axis.