Ilwon Kim

Critical thickness for transformation of epitaxially stabilized cubic AlN in superlattices

The epitaxial stabilization and transformation of cubic AlN layers in AlN/VN and AlN/TiN superlattices, grown by reactive sputtering on MgO (001), is described. In AlN/VN, the critical AlN thickness lC for transformation from cubic to hexagonal increased from ≈3.0 to >4 nm when the VN superlattice layer thickness was increased from 2.0 to 6.0 nm. The effect of lattice mismatch was observed by comparing AlN/VN (mismatch=1.46%) and AlN/TiN (mismatch=3.84%). The lC values were smaller, 2–2.5 nm, for the larger mismatch AlN/TiN system. The dependence of lC on the lattice mismatch and stabilizing layer thickness is discussed based on models of epitaxial stabilization.

Structures of AlN/VN superlattices with different AlN layer thicknesses

AIN/VN superlattices with different periods were studied using x-ray diffraction and transmission electron microscopy (TEM). A phase transformation of the AIN from an epitaxially stabilized rock-salt structure to a hexagonal wurtzite structure was observed for an AIN layer thickness greater than 4 nm. A structural model is proposed on the basis of TEM results for the orientation of the transformed phase. The VN layer grown on top of the hexagonal AlN was observed to be reoriented compared to that in the stabilized Bl-AlN/VN. The VN nucleated by taking the w-AlN(002) plane as its (111) plane instead of the (002) plane.

Solid oxide fuel cell with oxide anode-side support

Solid oxide fuel cells (SOFCs) with Sr 0.8 La 0.2 TiO 3 anode-side supports, along with Ni-Y 2 O 4 -stabilized ZrO 2 (YSZ) anode, YSZ electrolyte, and LSM-YSZ (LSM = La 0.8 Sr 0.2 MnO 3 ) cathode, were prepared. Button cells yielded a power density up to 1.0 W/cm 2 in humidified H 2 and air at 800°C. The cells showed much-improved stability against coking in methane compared with conventional Ni-YSZ anode-supported SOFCs. A detailed model was used to fit the SOFC electrical results and to show that the good stability in methane was a result of the high 0-to-C ratio in the Ni-YSZ anode due to the Sr 0.8 La 0.2 TiO 3 support.

Stabilization of zinc-blende cubic AlN in AlN/W superlattices

AlN/W superlattices with bilayer periods of 3.5–7 nm were grown on MgO (001) by magnetron sputtering. For AlN thicknesses lAlN⩽1.5 nm, a cubic phase of AlN was observed using high-resolution cross-sectional transmission electron microscopy and x-ray diffraction (XRD). Pole-figure XRD scans showed a reflection that matched the theoretically predicted interplanar spacing of zinc-blende phase (Zb-AlN), and that could not be explained by either the wurtzite or rocksalt structures. The stabilization of zb–AlN is explained as a result of good interfacial matching between W(100) and zb–AlN(011). When lAlN was increased above 1.5 nm, XRD scans showed a rapid decrease in satellite peak intensities, indicating a degradation of the layered structure, and the appearance of a wurtzite structure.

New approach to depositing yttria-stabilized zirconia buffer layers for coated conductors

Abstract : A new approach for the production of yttria-stabilized zirconia (YSZ) oxide buffer layers directly on metal rolling-assisted biaxially textured substrates (RABiTS) is described in this paper. This represents a significant advance over existing techniques and avoids the need for complicated steps to avoid substrate oxidation during direct deposition of oxides. Current densities of about 1 MA/cm2 have been achieved for YBa2Cu3O7-delta layers on the YSZ buffer, with an intermediate CeO2 layer. The process consists of reactive sputtering of a YxZr1-xN film directly on the RABiTS, which adopts its biaxial texture. This nitride film is then converted to YSZ via a thermal oxidation step. The YSZ films retain the texture of the nitride film (and of the RABiTS) through local syntaxy. In many cases, YSZ films exhibit improved biaxial texture over that of the RABiTS substrate. Nitrides can be sputter deposited at much higher rates relative to oxides, making the approach industrially scalable and economical.

Liquid-Hydrocarbon Internal Reforming In Catalyst-Assisted SOFCs

Direct internal reforming of hydrocarbon fuels such as gasoline and diesel has been problematic due to coking on Ni-based SOFC anodes. This paper describes stable direct internal reforming SOFCs (Ni-YSZ/YSZ/LSM-YSZ) using anode reforming catalysts. Button cells were tested with a separate catalyst piece placed against the anode. Button cell power densities of ~ 0.5W/cm2 at ~ 800oC have been demonstrated with n-dodecane and n-tridecane mixed with H2O-CO2-air. Life tests with n-dodecane showed stable operation for > 140 h. Flattened-tube segmented-in- series modules with 14 cells on each side were tested with a catalyst layer coating on the fuel side of the zirconia support tube. For iso-octane mixed with either air or H2O-CO2-air, a stable maximum power density of ~ 0.6W/cm2 was obtained at ~ 800oC.