B. W. Chung

Development and Characterization of a High Performance Thin-Film Planar SOFC Stack

A planar solid oxide fuel cell (SOFC) was fabricated using a tape-cast Ni/yttria-stabilized zirconia (YSZ) anode support, a YSZ thin-film electrolyte, and a composite cathode of YSZ and (La 0 . 8 5 Sr 0 . 1 4 ) 0 . 9 8 MnO 3 (LSM). Using pure hydrogen as the fuel gas, a three-cell stack with a cross-flow design and external manifolds produced peak power densities of 0.85 and 0.41 W/cm 2 at 800 and 700°C, respectively. Using wet methane as the fuel gas, the stack produced a peak power density of 0.22 W/cm 2 at 700°C. Individual cells in the stack showed identical current-voltage characteristics. Stack lifetime was limited because of degradation of the cells from oxidation products coming from the metallic interconnect used.

On the electronic configuration in Pu: spectroscopy and theory

Photoelectron spectroscopy, synchrotron-radiation-based x-ray absorption, electron energy loss spectroscopy, and density-functional calculations within the mixed-level and magnetic models, together with canonical band theory, have been used to study the electron configuration in Pu. These methods suggest a 5fn occupation for Pu of 5≤n<6, with , contrary to what has recently been suggested in several publications. We show that the n = 6 picture is inconsistent with the usual interpretation of photoemission, x-ray absorption, and electron energy loss spectra. Instead, these spectra support the traditional conjecture of a 5f5 occupation in Pu as is obtained by density-functional theory. We further argue, based on 5f-band filling, that an n = 6 hypothesis is incompatible with the position of Pu in the actinide series and its monoclinic ground-state phase.

Evidence of dynamical spin shielding in Ce from spin-resolved photoelectron spectroscopy

Using Fano effect measurements upon polycrystalline Ce, we have observed a phase reversal between the spectral structure at the Fermi edge and the other 4f derived feature near a binding energy of 2 eV. The Fano effect is the observation of spin polarized photoelectron emission from nonmagnetic materials, under chirally selective excitation, such as circularly polarized photons. The observation of phase reversal between the two peaks is a direct experimental proof of Kondo shielding in Ce, confirming the predictions of Gunnarsson and Shoenhammer, albeit with a small modification.

Influence of Electrode Configuration on the Performance of Electrode-Supported Solid Oxide Fuel Cells

The effect of the cathode-to-anode area ratio on the normalized power density for solid oxide fuel cells was investigated. A geometrically symmetric cell having a screen printed cathode produced a power density of 0.45 W/cm 2 at 800°C. However, using a cathode with a geometric area significantly less than that of the anode can enhance the normalized power density by a factor of two. Furthermore, using a current collector with a geometric area less than that of the cathode contributes an additional enhancement that leads to an apparent power density of 1.2 W/cm 2 . This difference in cell performance is interpreted as an artifact of normalization. When the performance of the cathode is improved using optimized processing technology and careful control of electrode microstructure, the effect of the area ratio on the normalized power density enhancement is less pronounced. Only testing in a symmetric cell configuration can result in power densities that are also achievable in stacks.

An instrument for the investigation of actinides with spin resolved photoelectron spectroscopy and bremsstrahlung isochromat spectroscopy.

A new system for spin resolved photoelectron spectroscopy and bremsstrahlung isochromat spectroscopy has been built and commissioned at Lawrence Livermore National Laboratory for the investigation of the electronic structure of the actinides. Actinide materials are very toxic and radioactive and therefore cannot be brought to most general user facilities for spectroscopic studies. The technical details of the new system and preliminary data obtained therein will be presented and discussed.

Narrowing the range of possible solutions to the Pu electronic structure problem: Developing a new Bremstrahlung Isochromat Spectroscopy capability

Bremstrahlung Isochromat Spectroscopy (BIS) has a proven record for the probing of unoccupied electronic structure, including feasibility studies of actinides such as U and Th. A description of the BIS or Inverse Photoelectron Spectroscopy (IPES) process and of our reasoning concerning the potential importance of extending the application of BIS related techniques to the actinides, will be provided. To put this into the proper context, our past successes, using soft x-ray techniques to interrogate the electronic structure of Pu, will be briefly reviewed. Finally, the initial results of the calibration of the BIS device, including preliminary spectra from of the surrogate system CeOxide, will be presented.

Splittings, Satellites and Fine Structure in the Soft X-ray Spectroscopy of the Actinides

Perhaps the most demanding and powerful actinide spectroscopy is that using soft X-ray and VUV photons. Because of the relatively low energy and fairly small sampling depths of these photons and the corresponding electrons, it is necessary to use un-encapsulated samples with highly cleaned and well-prepared surfaces. This causes a myriad of sample containment problems for these radioactive materials. Despite these hindrances and difficulties, the soft-X-ray and ultra-violet spectroscopy of the actinides can provide an amazing level of detailed information, particularly having to do with 5f electronic structure. In this paper, the splittings, satellites and fine structure of the following actinide soft X-ray spectroscopies will be discussed: X-ray photoelectron spectroscopy; X-ray absorption spectroscopy; and inverse photoelectron spectroscopy, including Bremstrahlung isochromat spectroscopy and resonant inverse photoelectron spectroscopy.

Direct comparison of the x-ray emission and absorption of cerium oxidea)

X-ray emission spectroscopy and x-ray absorption spectroscopy have been used to investigate the photon emission and absorption associated with the Ce 3d5/2 and Ce 3d3/2 core levels in Ce oxide. A comparison of the two processes and their spectra will be made.

Direct comparison of spectroscopic data with cluster calculations of plutonium dioxide and uranium dioxide

The density of states predicted by cluster calculations of PuO2 and UO2 is directly compared to the results from soft x-ray spectroscopy. Remarkably good agreement between the experimentally measured spectroscopic peaks and the calculated density of states of the central part of the cluster is observed.

Probing Radiation Damage in Plutonium Alloys with Multiple Measurement Techniques

A material subjected to radiation damage will usually experience changes in its physical properties. Measuring these changes in the physical properties provides a basis to study radiation damage in a material which is important for a variety of real world applications from reactor materials to semiconducting devices. When investigating radiation damage, the relative sensitivity of any given property can vary considerably based on the concentration and type of damage present as well as external parameters such as the temperature and starting material composition. By measuring multiple physical properties, these differing sensitivities can be leveraged to provide greater insight into the different aspects of radiation damage accumulation, thereby providing a broader understanding of the mechanisms involved. In this report, self-damage from {alpha}-particle decay in Pu is investigated by measuring two different properties: magnetic susceptibility and resistivity. The results suggest that while the first annealing stage obeys second order chemical kinetics, the primary mechanism is not the recombination of vacancy-interstitial close pairs.