Dokyol Lee

Effect of molybdenum on the microstructure and wear resistance of cobalt-base Stellite hardfacing alloys

Abstract The Stellite 6 hardfacing alloys with different Mo contents have been deposited on AISI 1045-carbon steel using a Plasma Transferred Arc (PTA) welding machine. The effect of Mo on the microstructures and wear resistance properties of the Stellite 6 hardfacing alloys were investigated using optical microscopy, scanning electron microscopy, electron probe microanalysis and X-ray diffraction. With an increase in Mo contents, the M 23 C 6 and M 6 C type carbides were formed instead of Cr-rich M 7 C 3 and M 23 C 6 type carbides observed in the interdenritic region of the Mo-free Stellite 6 hardfacing alloy. The size of Cr-rich carbides in interdendritic region decreased, but that of M 6 C type carbide increased as well as the refinement of Co-rich dendrites. The volume fraction of Cr-rich carbides slightly increased, but that of M 6 C type carbide abruptly increased. This microstructural change was responsible for the improvement of the mechanical properties such as hardness and wear resistance of the Mo-modified Stellite 6 hardfacing alloy.

Effect of co-dopant addition on properties of gadolinia-doped ceria electrolyte

Various trivalent oxides were added as co-dopants to gadolinia-doped ceria (GDC) electrolyte used for solid oxide fuel cells at up to 5 mol%. An examination was made on how they affect the electrical conductivity of the electrolyte and, eventually, the open-circuit voltage (OCV) of a unit cell. Through a comparison of the thermal expansion coefficients (TEC), it was investigated whether or not the co-doped electrolytes are thermomechanically compatible with other cell components. The addition of co-dopants generally improve the electrical properties of the electrolyte by yielding greater OCV values and not changing the TEC significantly (5% at most), except in the case of Pr. Among the electrolytes examined, the one co-doped with Sm (3 mol%) shows the best improvement in performance.

Effect of grain motion on the coarsening of WC grains in the carbon-saturated liquid matrix during liquid phase sintering of WC-Co alloys

The purpose of the present study is to observe the coarsening behavior of WC grains in the WC-Co alloys with various Co contents under the constant carbon potential and thus to clarify the coarsening mechanism of faceted WC grains. Since the coarsening of WC grains is affected sensitively by carbon content in the WC-Co compacts, the carbon content in the liquid Co is controlled at a constant level in all experiments through the alloy compositions having slightly excess carbon content.

Optimization of the GaN-buffer growth on 6HSiC (0001)

Abstract Buffer layers promote lateral growth of films due to a decrease in the interfacial free energy between the film and substrate, and large 2-dimensional nucleation and a smooth surface of the buffer layer are desired. Optimum conditions for GaN-buffer growth on the vicinal surface of 6HSiC (0001) were determined by means of atomic force microscopy (AFM). GaN depositions were carried out in a horizontal MOCVD system using trimethylgallium and NH3. AFM analysis of the GaN nucleation layers led to optimum growth conditions for the GaN-buffer layer and this was confirmed by cross-sectional transmission electron microscopy, Hall measurement and photoluminesncence spectra. The optimum growth condition for a GaN-buffer layer on SiC (0001) was determined to be 1 minute of growth at 550°C.

On the high creep resistant morphology and its formation mechanism in Ni–10 wt.% Cr anodes for molten carbonate fuel cells

Two Ni–10 wt.% Cr anodes with different morphologies of Cr2O3 dispersion are fabricated for molten carbonate fuel cells (MCFCs) through different sintering procedures. The effect of the morphology on the creep resistance of the anode and the mechanism of its formation are investigated. The anode morphology is characterized as a fine oxide dispersion when sintered through a procedure including partial oxidation and reduction, or as a coarse oxide dispersion when sintered conventionally in a reducing atmosphere. The fine oxide dispersion anode exhibits a much higher creep resistance than the coarse oxide dispersion anode. A mechanism is suggested for the formation of the fine oxide dispersion morphology and thus the relatively high creep resistance.

The effect of multiple doping on electrical conductivity of gadolinia-doped ceria electrolyte

Combinations of two or four trivalent oxides were substituted for gadolinia up to 5 mol% in the 20 mol% gadolinia-doped ceria electrolyte as additional dopants. Most of them increased the electrical conductivity of the electrolyte but some did not. Among them (YLa)0.01Gd0.19Ce0.8O1.9 and (YSm)0.03Gd0.17Ce0.8O1.9 showed the highest electrical conductivities, respectively, at 800 ‡C and 600 ‡C. These experimental results were analyzed in terms of activation enthalpy and entropy for ionic conduction. Among the various possible contributions to the activation entropy, only the configurational entropy was taken into account for the explanation.

Simplified and Cost-Effective Sintering Processes for Creep Resistant Ni-10wt.%Cr MCFC Anodes

Two sintering processes were designed for Ni-10wt.%Cr anodes. One included a full oxidation and reduction, referred to as FR hereafter, and the other included a reduction only, referred to as R hereafter. The processes were designed to replace the existing partial oxidation and reduction process, referred to as PR hereafter, which is known to be rather complicated and expensive. The morphologies and creep behaviors of the anodes were also investigated. The anode sintered through a FR process, with an oxidation time of 0.5 hr, showed almost as fine oxide dispersion and high creep resistance as that sintered through a PR process. The creep strain was 3.1% (porosity 60%) after a 100 hr creep test, with no micropores observed in the anode. Conversely, the anode sintered through an R process showed a morphology of coarse oxide dispersion with large Cr2O3 particles. Accordingly, the creep resistance of the anode was not as high as that of the anode sintered through the PR or FR processes.

Synthesis of yttrium barium copper oxide-0.325Ag superconductors via intermediate precursor with overall composition Y:Ba:Cu:0=l:2:3:Y, Y>7 produced by high-energy attrition milling

A highly homogeneous composite precursor containing nano-scale particles was synthesized from the highenergy attrition milling of a mixture of metallic Y, metallic Cu and barium nitrate (Ba(N03)2) containing 5 wt.% silver. The particle size of the 20 hour attrition-milled precursor was in the range of 30~80 nm as estimated by the XRD technique and direct TEM observation. With the heat treatment of the attrition-milled precursor, an intermediate precursor with the overall composition Y:Ba:Cu:O=l:2:3:y, y>7 was synthesized. The analyzed Y:Ba:Cu:Ag:O molar ratio of the intermediate precursor by AES/AAS and iodimetric titration was 1:1.972:3.022:0.323:7.41, which was very close to the estimated composition: 0.5Y2O3+2BaCuO2.5+CuO +0.325Ag. Y123-Ag superconductors in powder or bulk forms were prepared using heat treatment or thermomechanical processing of the intermediate precursor containing BaCu02.5. The high oxygen orthorhombic Y123-Ag phase (y>7) was obtained in quenched samples which were held for 30 min or less at 950°C in ambient air. As the holding time extended to 40 min, the sample was still orthorhombic Y123-Ag (6.7<y <7). The transport critical current density (Jc) at 77 K and zero magnetic field for the quenched sample which was held for 40 min at 950°C was ~1.3xl03 A/cm2.

A Study on In Situ Sintering of Ni–5 wt % Al Anode for Molten Carbonate Fuel Cells

In situ sintering of a molten carbonate fuel cell (MCFC) anode offers simpler procedure and hence lower cost than conventional ex situ sintering. To develop the in situ sintering anode for an MCFC, the insufficient sintering problem of Ni-based anode, due to the lower in situ sintering temperature (at 650°C) than the ex situ sintering temperature (at 1100°C), should be solved. In this study, to improve sinterability at 650°C, liquid-phase sintering using the low melting element of Sn as an additive was adopted for in situ sintering of Ni-5 wt % Al anode and thus in situ sintering of Ni―Al―Sn anodes was attempted. Ni95―A15―Sn30 (weight ratio) anode among these anodes showed an adequate porosity of 65% and a mean pore size of 3.4 μm for an MCFC anode and fairly creep resistance with a creep strain of 4.0% after a 100 h creep test. After a single-cell test, this anode also showed good performance with an open-circuit voltage (OCV) of 1.12 V and a closed-circuit voltage (CCV) of 1.0 V at a current density of 150 mA/cm 2 and a maximum power density of 420 mW/cm 2 . During the 200 h cell operation, the performance was maintained stably with the OCV in the range of 1.11-1.12 V and the CCV in the range of 0.98-1.00 V.

A Study on In-Situ Sintering of Ni-10 wt % Cr Anode for MCFC

In-situ sintering of the Ni-10 wt % Cr MCFC anode is attempted by adding an appropriate amount of Sn, one of those elements with a melting point lower than the operating temperature of an MCFC. The tape-cast green sheet is sintered for 100 h under the same condition as for an MCFC anode in operation and then characterized to determine whether or not it fulfilled the requirements for an MCFC anode. The porosity, pore size distribution, creep resistance and sinter resistance of the anode are judged to be adequate for an MCFC.