Sang Hoon Hyun

A nanocomposite material for highly durable solid oxide fuel cell cathodes

We have synthesized well-engineered nanocomposite particles for the achievement of highly durable solid oxide fuel cell cathodes. The use of a dual-composite approach in which both LSM and YSZ phases are placed on a YSZ grain allows for the development of an ideal cathode microstructure with improved phase contiguity and interfacial coherence. Microstructural observations in conjunction with impedance analysis during accelerated lifetime tests clearly reveal that three-dimensionally interconnected percolative networks of both electronic and ionic conduction give cells based on the dual-composite cathode better long-term stability. A cell with such a well-controlled cathode maintains nearly constant power density over 500 h. In contrast, a cathode prepared by mechanical mixing undergoes significant degradation during the stability test due to thermochemically and electrochemically driven coarsening and shrinkage of the LSM phase.

Fabrication and Characterization of Cu-Ni- YSZ SOFC Anodes for Direct Utilization of Methane via Cu pulse plating

The Cu-Ni-YSZ cermet anodes for direct use of methane in solid oxide fuel cells have been fabricated by electroplating Cu into the porous Ni-YSZ cermet anode. The uniform distribution of Cu in the Ni-YSZ anode could be obtained via pulse electroplating in the aqueous solution mixture of CuSO₄ㆍ5H₂O and H₂SO₄ for 30 min with 0.05 A of average applied current. The power density (0.17W㎝?²) of a single cell with a Cu-Ni-YSZ anode was shown to be slightly lower in methane at 700℃, compared with the power density (0.28W㎝?²) of a single cell with a Ni-YSZ anode. However, the performance of the Ni-YSZ anode-supported single cell was abruptly degraded over 21 h because of carbon deposition, whereas the Cu-Ni-YSZ anode-supported single cell showed the enhanced durability upto 52 h.

Surface Modification of Ag Coated Cu Conductive Metal Powder for Conductive Silicone Sealant Gasket Paste

Conductive pastes have drawn significant interest due to their advantageous physical properties as well as their potential in electronic material industries. Conductive pastes consist of conductive fillers, organic binders, solvents and additives. Metal powders, such as Au, Ag, Ni, Cu, etc. are used for conductive filler. Filler dispersion is a very important and yet a difficult process in conductive paste manufacturing. Meanwhile, there are some metal powders such as copper, nickel etc that are used for pastes which have serious surface corrosion problems. This problem leads to change of the color and decrease in conductivity and affect storage stability of conductive pastes. To overcome these problems, we adopted two kinds of surface modification method. One is a method involving coating of silane coupling agent to conductive metal powders to overcome the corrosion stability and the other is a method involving addition of a dispersing agent to increase the degree of dispersion of metal powders. A coupling agent and dispersing agent are used to reduce the interfacial energy between filler and organic binder to enhance dispersion and its stability. By using silane coupling agent and dispersion agent, we can ensure both the corrosion stability and long term storage stability, and enhance the high performance electrical and mechanical properties of EMI shielding silicone sealant.

Mechanochemical Synthesis of Nano-Sized CeO2 and its Application for CMP Slurry

Nano-crystalline CeO2 was synthesized by the mechanical milling and subsequent heat-treatment from the mixture of Ce(OH)4 as precursor, and NaCl as diluent. The diluent provided diffusion barrier during milling and heat-treatment, which was easily dissolved out by deionized water. The size of crystallite and the strain variance of CeO2 were depended on the temperature and heat-treatment time: increased with the temperature (400~700oC) and time (1~24 hours) increasing, and saturated near at 20nm in size owing to the densification of diluent. The synthesized nano-crystalline CeO2 powder was applied as an abrasive in CMP (Chemical Mechanical Planarization) slurry. When blanket-type SiO2 and Si3N4 wafers were polished with the slurries, the removal rates (RR) of SiO2 and Si3N4 wafers and selectivities (RRSiO2/RRSi3N4) were influenced by synthetic condition of abrasive, the suspension stability and the pHs of slurries.

Corn-cob like nanofibres as cathode catalysts for an effective microstructure design in solid oxide fuel cells

An efficient cathode for solid oxide fuel cells (SOFC) is mainly determined by the oxygen reduction reaction (ORR) activity of mixed materials. We demonstrate a new microstructure design through a nanofibrous electrode based on a unique corn-cob structure. A one-step process to produce corn-cob ceramic nanofibers of La0.8Sr0.2MnO3 (LSM) and Y2O3-stabilized ZrO2 (YSZ) is introduced using an electrospinning system equipped with a coaxial nozzle. From the microscope analysis, perfect corn-cob nanofibers are finely produced with a diameter of 350 nm for the core and nanoparticles (30–40 nm) stacked on the surface similar to a core–shell structure. The cathode fabricated using nanofibers with LSM outside and YSZ inside (YSZ@LSM) shows the best maximum power density of 1.15 W cm−2 at 800 °C with low polarization resistance, which is higher than that of the reverse core and shell positions (LSM@YSZ) and even the commercial LSM–YSZ. This better outcome is more prominent at elevated temperatures due to its accelerated catalytic activity. Therefore, insight into the key factors that enhance ORR activity and single cell performance is obtained in terms of not only the nanofibrous core@shell structure but also more reaction active sites from the optimum catalyst position at the designed corn-cob nanofiber based cathodes.

Fabrication of YSZ Thin-Film for SOFC Applied by Electron Beam PVD

Thin film of yttria stabilized zirconia (YSZ) was prepared on alumina substrates by applying an electron beam physical vapor deposition (EB-PVD) method. The morphology and deposition behavior of the coating layer were investigated using atomic force microscope, secondary electron microscope, and X-ray diffractometer. The electrical conductivity of the YSZ coating layer was also evaluated. The fracture microstructure of YSZ electrolyte film, which was deposited with thickness of 10 μm, has showed a columnar structure. Also, the activation energy of the coating layer was similar to that of bulk YSZ.

Deposition Behavior of YSZ Nano-Coating Layer by EB-PVD

This paper presents the work on the development of ceramic coating processing. Nano-structured zirconia coating has been developed with functions; substrate temperature and oxygen gas change in chamber by electron beam physical vapor deposition (EB-PVD). The microstructure of the coating layer has been characterized with FE-SEM, and SEM. The crystalline phase of the coating layer has been also characterized with XRD. The zirconia coating by EB-PVD had not monoclinic zirconia phase as shown in XRD pattern and Raman spectra and the thickness of coating were quite homogeneous. The fracture microstructure of the coating layer for a thickness of ~15 μm showed columnar or non-columnar structure and had nano-structure with nano scaled grain as shown in micrograph by FE-SEM.

Fabrication of Nanoabrasive Grinding Wheels and Their Application to Grinding Silicon Wafers

In the surface machining of brittle materials, there exists a transition from brittle to ductile modes when the depth of cut is reduced below a critical size using ultrafine abrasive grains. Vitrified grinding wheels containing ultrafine abrasives in the sub-micrometer to nanometer range were fabricated by mechanochemically milling nanoabrasive particles and subsequent viscous sintering of abrasive-binder composites. The grinding characteristics of the nanoabrasive grinding wheels were evaluated for the fine grinding of silicon wafers in terms of a variety of variables. Preliminary wafer grinding results are presented on material removal rate and surface quality of silicon wafers.