Jong-Ku Park

Characterization of Thin-Film YSZ Deposited via EB-PVD Technique in Anode-supported SOFCs

An anode-supported solid oxide fuel cell (SOFC) based on 8 mol % yttria-stabilized zirconia (8YSZ) electrolyte that was deposited via an electron-beam physical vapor deposition (EB-PVD) technique, was investigated. A NiO-8YSZ plate was used as both an anode substrate and as a substrate for the deposition of 8YSZ. An additional buffering layer was coated on the anode substrate via a screen printing before the deposition of YSZ film in order to enhance the surface smoothness for the film deposition. In this study, we controlled various experimental variables for the film deposition and subsequent heat treatment process to obtain highly dense and impervious electrolyte film. Consequently, we were able to obtain a large area (12 X 12 cm) and dense YSZ film with a uniform thickness of ∼ 8 μm. The surface and cross-sectional microstructure and the corresponding phase of deposited film were thoroughly analyzed by scanning electron microscopy and X-ray diffraction. The electrochemical performance of the EB-PVD unit cell was investigated using dc current-voltage load testing, a dc current interruption technique, and an ac-impedance spectroscopy. Peculiar microstructural features of the deposited film, such as the columnar structure of grains along the growing axis, and the films microstructural effect on unit cell performance, were addressed.

Photoluminescence behavior of Al 3+ , Pr 3+ doped perovskite La 2/3 TiO 3 and pyrochlore La 2 Ti 2 O 7

The purpose of this study is to develop an understanding of the photoluminescence properties of Al 3+ , Pr 3+ doped perovskite-type La 2/3 TiO 3 and pyrochlore-type La 2 Ti 2 O 7 phosphor, which is characterized by the red emission ( 1 D 2 → 3 H 4 transition) of the Pr 3+ ion. The explanation for the energy transfer and the corresponding critical distance is proposed on the basis of the role of Al 3+ ions in the perovskite-type La 2/3 TiO 3 :Pr phosphor. To clarify the distinction of photoluminescence properties between the perovskite-type La 2/3 TiO 3 and the pyrochlore-type La 2 Ti 2 O 7 , the trap-involved process and the charge transfer band have been investigated, respectively.

Effect of compact structures on the phase transition, subsequent densification and microstructure evolution during sintering of ultrafine gamma alumina powder

Abstract The effect of compact structures on the phase transition from gamma alumina (γ-Al2O3) to alpha alumina (α-Al2O3) and densification during sintering of the compacts of ultrafine gamma alumina powder (n-Al2O3) has been studied. The onset temperature of gamma→alpha phase transition during the sintering of n-Al2O3 compacts is affected significantly by the compact density as well as by the population of α-Al2O3 seed particles in the γ-Al2O3 matrix. The nucleation and growth of alpha phase is considered to be facilitated by the mass transfer around the particle contacts and by the imbedded α-Al2O3 particles. The gamma→alpha phase transition is proved to be detrimental to full densification of the n-Al2O3 compacts of gamma phase. The densification of high density compacts seems to be affected detrimentally by the phase transition in the range of 1050°C to 1150°C but finally reaches about 99% of theoretical density (TD) with fine grain structure. The unseeded compacts made by slurry casting are not densified above 73% TD even at 1400°C, whereas the α-Al2O3-seeded compacts are densified up to 90% TD at 1400°C and they have very large grain size of about 1 μm in diameter. High density compaction technology is considered a useful method required to achieve fully dense sintered compacts with fine grains below 100nm even in the sintering of ultrafine powder compacts with a metastable phase.

Photoregulation of Ion Permeation through a Polyelectrolyte Multilayer Membrane by Manipulating the Chromophore Orientation

Toward the realization of nanoscale device control, we report a novel method for photoregulation of ion flux through a polyelectrolyte multilayer membrane by chromophore orientation that is adjusted either by illumination at normal incidence or by slantwise irradiation at an angle of 10 degrees with respect to the surface. Our results indicate that the chromophore reorientation caused by the slantwise irradiation controls the effective pore size and, consequently, the transport behavior on the nanoscale. The slantwise illumination, which includes six EZE photoisomerization cycles generated by alternately irradiating with ultraviolet (lambda = 360 nm) and visible (lambda = 450 nm) light, reversibly switches the orientation of E-azobenzene in the membrane between 53 +/- 2 degrees (high tilt) and 17 +/- 5 degrees (low tilt) with respect to the surface. The novel feature of this light-gated valve system is its extremely long-lived open-switch state; this behavior stands in contrast to that of other systems based on labile photoisomers, which tend to instantly return to the thermodynamically stable state.

Ion-Permselective Polyelectrolyte Multilayer Membrane Installed with a pH-Sensitive Oxazine Switch

This paper describes a pH-responsive multilayer film composed of two layered components, namely poly(allylamine hydrochloride) (PAH) and a copolymer of acrylic acid and [1,3]oxazine-modified acrylate (POA). The oxazine ring is an acidochromic chromophore and opens to form either cationic 3H-indolium or anionic hemiaminal in a pH-dependent manner. This structural transition was used to generate a net positive or negative charge on the membrane for selective ion permeation. Interestingly, the reversible oxazine ring opening and closing proceeded smoothly without significant steric hindrance in the multilayer film comprising 10 PAH/POA bilayers. The pH-responsive permselectivity for cationic and anionic probe molecules was demonstrated using a POA monolayer film adsorbed electrostatically onto an amino-functionalized ITO electrode. The origin of the excellent ion-transport selectivity in the 1 nm ultrathin POA membrane is discussed in terms of alternating charges of the aromatic amphoteric group, oxazine, in the polyeletrolyte membrane.

Effect of compact structure on phase transformation kinetics from anatase phase to rutile phase and microstructure evolution during sintering of ultrafine titania powder compacts

The effect of compact structure both on phase transition kinetics and on the densification during sintering of n-TiO2 compacts has been investigated. The compact structures varied from loose powder pack to very high density using different compaction pressures. The compact structure is verified to significantly affect phase transition kinetics and densification during sintering. The onset temperature of anatase -> rutile phase transition decreased with the increase of the compact density. The highest density compact of 86.9% showed phase transition at the lowest temperature and no detrimental effect on densification. The compact structure, i.e., the coordination number of particles in the compacts, is considered to significantly affect both the phase transition kinetics and the subsequent densification during the sintering of n-TiO2 compacts.

White Light Generation from Single Gallium Oxide Nanoparticles co-doped with Rare-Earth Metals

The synthesis of pure and rare-earth doped gallium oxide (β-Ga₂O₃) nanoparticles is reported. The synthesized nanoparticles are characterized with XRD, TEM, and PL analyses. Strong blue emission is observed from un-doped gallium oxide nanoparticles, while nanoparticles doped with Eu 3+ and Tb 3+ give strong red and green emissions, respectively. When doped with Eu 3+ and Tb 3+ together, gallium oxide nanoparticles emit white light. The CIE co-ordinate of the emitted light was found to be (0.33, 0.33), which is well within the white light region.

Synthesis and characterization of single-crystal Cu(In,Ga)Se2 nanowires: high Ga contents and growth behaviour

Precise control over the defect density, a high Ga content, and uniform stoichiometry are critical for controlling the physical and optical properties of Cu(In,Ga)Se2 (CIGS) nanowires (NWs). In this study, we investigated the synthesis of epitaxially grown, single-crystal CIGS NWs by a vapour-phase transport method using multiple sources of Ga2Se3, In2Se3, and Cu2Se as the precursors. No catalysts were employed, and r-cut Al2O3 substrates were used for the fabrication of the NWs. The synthesized CIGS NWs had a uniform composition along their length, and the NWs with the highest Ga/(In + Ga) content ratio (0.8) had a chalcopyrite structure. The bandgap energy of the CIGS NWs was higher than that of typical CIGS thin films grown by co-evaporation methods because of the high Ga content ratio. These single-crystal CIGS NWs offer an attractive platform for exploring various concepts related to hierarchical nanostructures and devices based on fully epitaxial semiconductor structures.

Growth of helical carbon nanofibers in the presence of Ni−Cu catalysts

Carbon nanofibers (CNFs) are synthesized by the dissociation of ethylene gas in the presence of Ni−Cu alloy catalysts; helical CNFs are favored by Ni−Cu alloy catalysts. The role of alloy compositions and reaction time in the morphology of CNFs were determined by varying both the Cu content in the alloy catalyst and the holding time at a particular synthesis temperature. The chemical composition, the size, and the shape of the catalyst particles were shown to affect the morphology of CNFs. Two kinds of growth mechanism are suggested to explain the growth of helical and straight CNFs.

Fabrication of recyclable catalyst supports for synthesis of carbon nanofibers

Carbon nanofiber (CNF) has been synthesized by thermal dissociation of ethylene gas in the presence of nickel catalyst. Catalyst particles of nickel were immobilized on the surface of sintered yttria-stabilized zirconia supports (YSZ support) via either direct sintering of composite compacts or chemical doping into presintered porous YSZ compacts. The effect of support microstructures on the CNF morphology was investigated. In addition, the feasibility of a recyclable support for economical synthesis of CNF was assessed.