Jai Young Lee

Electrochemical properties of LiCoO2-Coated LiMn2O4 prepared by solution-based chemical process

In order to enhance both the cycle stability at elevated temperatures and the rate capability of LiMn 2 O 4 , the surface of LiMn 2 O 4 was covered with fine LiCoO 2 particles prepared by a solution-based chemical process. The amount of LiCoO 2 measured with inductively coupled plasma was about 7 mol %. LiCoO 2 -coated LiMn 2 O 4 had an excellent cycle stability at 65°C compared to pure LiMn 2 O 4 . This can be explained by suppression of the Mn dissolution. The rate capability of LiCoO 2 -coated LiMn 2 O 4 improved significantly. The improved rate capability may be attributed to a decrease in the passivation film that acts as an electronic insulating layer and interparticle contact resistance. Consequently, it is proposed that the surface encapsulation of LiMn 2 O 4 with LiCoO 2 improves its rate capability and cycling stability at high temperatures.

The Characterization of an Alkaline Fuel Cell That Uses Hydrogen Storage Alloys

The characterization of new type of alkaline fuel cell based on oxidation of chemical hydride has been studied. The chemical hydride can he used as a new fuel source in a fuel cell system. As a result, we have discovered that the electrochemical reaction rate is higher at a normal temperature compared with cells containing other hydrogen fuels where a hydrogen-releasing agent, NaBH 4 , is added to an aqueous alkaline solution of electrolyte as hydrogen fuel. That is, the fuel can be supplied very simply for the cell. If air is supplied to the oxygen cathode made of highly dispersed platinum particles supported in high-surface-area carbon paper, and the hydrogen releasing agent is fed to the alkaline solution of electrolyte at the side of metal hydride anode (ZnCr 0.8 Ni 1.2 alloy), the cell can produce electric current continuously. Also it can be operated at a normal temperature and produce a large amount of energy due to its high energy density of 6,000 Ah/kg or more (for NaBH 4 or KBH 4 ). Therefore, the developed cell has higher electrochemical reaction rate and energy density than the conventional fuel cells using other hydrogen sources.

Improvement on the electrochemical characteristics of graphite anodes by coating of the pyrolytic carbon using tumbling chemical vapor deposition

Abstract The electrochemical characteristics of graphite coated with pyrolytic carbon materials using tumbling chemical vapor deposition (CVD) process have been studied for the active material of anodes in lithium ion secondary batteries. Coating of pyrolytic carbons on the surface of graphite particles, which tumble in a rotating reactor tube, was performed through the pyrolysis of liquid propane gas (LPG). The surface morphology of these graphite particles coated with pyrolytic carbon has been observed with scanning electron microscopy (SEM). The surface of graphite particles can well be covered with pyrolytic carbon by tumbling CVD. High-resolution transmission electron microscopy (HRTEM) image of these carbon particles shows that the core part is highly ordered carbon, while the shell part is disordered carbon. We have found that the new-type carbon obtained from tumbling CVD has a uniform core (graphite)-shell (pyrolytic carbon) structure. The electrochemical property of the new-type carbons has been examined using a charge–discharge cycler. The coating of pyrolytic carbon on the surface of graphite can effectively reduce the initial irreversible capacity by 47.5%. Cyclability and rate-capability of theses carbons with the core-shell structure are much better than those of bare graphite. From electrochemical impedance spectroscopy (EIS) spectra, it is found that the coating of pyrolytic carbon on the surface of graphite causes the decrease of the contact resistance in the carbon electrodes, which means the formation of solid electrolyte interface (SEI) layer is suppressed. We suggest that coating of pyrolytic carbon by the tumbling CVD is an effective method in improving the electrochemical properties of graphite electrodes for lithium ion secondary batteries.

Hydrogen trapping phenomena in carbon steel

Hydrogen trapping phenomena in carbon steel with different amounts of trapping sites were investigated by thermal analysis and permeation experiments. In thermal analysis, the relative amount of trapped hydrogen and the activation energy for evolution from various lattice defects were calculated by monitoring the pressure change caused by the release of hydrogen from hydrogen-charged specimens heated at a uniform rate. Hydrogen release peaks were observed at 116, 205 and 387° C, respectively, when the hydrogen-charged specimens with various defects were heated at a constant heating rate of 2.6° C min−1. Analysis suggested that the peak at 116° C corresponded to release from ferrite-cementite interfaces and the peak at 205° C corresponded to release from dislocations. The activation energy for evolution of trapped hydrogen determined experimentally from the measured peak temperature at different heating rates was found to be 18.4 kJ mol−1 in the ferrite-cementite interface. The hydrogen energy level around the trapping site was suggested from the trap activation energy and expected saddle-point energy. It was observed that most of the hydrogen is trapped in dislocations in spheroidized 0.49 wt% carbon steel.

THE MORPHOLOGY CHANGES IN DIAMOND SYNTHESIZED BY HOT-FILAMENT CHEMICAL VAPOR-DEPOSITION

Hot‐filament‐assisted chemical vapor deposition has been used to study the growth morphology of synthetic diamond deposited on silicon substrate in a dilute (1 volu2009%) CH3COCH3/H2 at high substrate temperature (about 777u2009°C). Scanning electron microscope pictures of the diamond particles show that the surfaces of synthetic diamond consist of rough‐octahedral (111) faces and smooth‐cubic (100) faces, which is cubo‐octahedron. And also the (110) facets on the octahedral face are observed. The relative growth rate of (111) faces to that of (100) faces in the cubo‐octahedron is double that derived from the calculated specific surface energy. So the apparent growth rate of the octahedral face must be explained by the growths of two constituent crystallographic planes of (100) and (110). The observed roughness of (111) faces arises from the competing growths of (100) and (110) planes. The (110) faces separate the (111) faces into three (110) planes. For the study of diamond crystal growth during deposition, it i...

The elevated temperature performance of LiMn2O4 coated with LiNi1−XCoXO2 (X = 0.2 and 1)

Abstract The surface coating of LiMn 2 O 4 using a gel precursor of LiNi 1− X Co X O 2 ( X =0.2 and 1) prepared from a solution-based chemical process was attempted in order to enhance the electrochemical performances of LiMn 2 O 4 at elevated temperature. After the surface of LiMn 2 O 4 was coated with LiNi 1− X Co X O 2 ( X =0.2 and 1) coating solution and heated at 750xa0°C, the surface of LiMn 2 O 4 was covered with fine LiNi 1− X Co X O 2 ( X =0.2 and 1) particles. LiNi 1− X Co X O 2 ( X =0.2 and 1)-coated LiMn 2 O 4 showed an excellent capacity retention at 65xa0°C compared to pure LiMn 2 O 4 . While pure LiMn 2 O 4 retained 81% of the initial capacity after storage in the discharged state at 65xa0°C for 300xa0h, LiCoO 2 -coated LiMn 2 O 4 showed no capacity loss. The improvement of storage performance at 65xa0°C is attributed to the suppression of electrolyte decomposition and the reduction of Mn dissolution resulting from encapsulating the surface of LiMn 2 O 4 with LiCoO 2 . The surface coating with LiNi 0.8 Co 0.2 O 2 also enhanced the high temperature cycle performance of LiMn 2 O 4 . Consequently, It is proposed that the surface encapsulation of LiMn 2 O 4 with fine LiNi 1− X Co X O 2 ( X =0.2 and 1) particles improve its high temperature performance.

A study of the wetting, microstructure and bond strength in brazing SiC by Cu-X(X = Ti,V,Nb,Cr) alloys

In the active brazing of SiC by copper-based alloys, the effects of various active elements such as titanium, vanadium, niobium and chromium on the wetting, microstructure and bond strength are investigated. In wetting, Cu-Cr alloys have the lowest wetting angles on SiC of 10°–20° depending on the chromium content. SiC is decomposed on contact with alloy melts during brazing. Carbon and silicon released from this decomposition of SiC react with active elements to produce their carbides and suicides at the interface. The reacted layers have different microstructures depending on the brazing alloys, but Cu-Ti and Cu-Cr alloys show similar microstructure, as do Cu-V and Cu-Nb alloys. In the four-point bend tests, the brazed joints of Cu-5 at % Ti, Cu-5 at % V and Cu-5 at % Nb alloys have similar bend strengths of 86.9, 80.3 and 92.4 MPa, respectively. The brazed joints of Cu-2 at % Nb alloys show a high bend strength of 154 MPa, although the wetting angle is a little higher, at about 60°. Niobium is found as a new active element of copper-based alloys to braze SiC. Cu-Nb alloys are promising for substitution for Cu-Ti alloys.

Effect of reaction pressure on the nucleation behaviour of diamond synthesized by hot-filament chemical vapour deposition

Synthetic diamond particles were deposited on a Si (1 0 0) substrate using a hot-filament chemical-vapour-deposition method in order to study the effect of the reaction pressure on the nucleation behaviour. The reaction pressure was controlled, as an experimental variable, from 2 to 50 torr under the following conditions: a filament temperature of 2200 °C, a substrate temperature of 850 °C, a total flow rate of 200 s.c.c.m. and a methane concentration of 0.8 vol%. Diamond deposits on the Si wafer were characterized by micro-Raman spectroscopy, scanning electron microscopy (SEM) and optical microscopy.The maximum nucleation density of diamond particles on the unscratched Si substrate is shown at the reaction pressure of 5 torr. These phenomena can be explained by the competition effect between β-SiC formation, which increases the diamond nucleation density, and atomic-hydrogen etching which decreases the nucleation sites.A new fabrication method for a high-quality diamond film without any surface pretreatments is introduced using a combination process between diamond nucleation at low pressure (5 torr) and growth at high pressure (30 torr).

Effect of Cu powder as an additive material on the inner pressure of a sealed-type Ni–MH rechargeable battery using a Zr-based alloy as an anode

Abstract Extensive and systematic work on improving the surface catalytic activity of electrodes has been carried out to decrease the internal pressure of a sealed Ni–MH battery in which Zr 0.9 Ti 0.1 (Mn 0.7 V 0.5 Ni 1.2 ) 0.92 alloys are used in the anode. In order to improve the surface catalytic activity of a Zr–Ti–Mn–V–Ni alloy which are closely related to the inner pressure behavior in a sealed cell, the electrode was fabricated by mixing the alloy with Cu powder. By replacing 50% of carbon black with Cu powder the inner cell pressure rarely increased with cycles. This was due to the active gas recombination reaction. After measuring the surface area of the electrode and the surface catalytic activity, it was found that the surface catalytic activity for the oxygen recombination reaction was much more improved by the addition of Cu powder. This phenomenon is due to the thin Cu layer on the Zr–Ti–Mn–V–Ni alloy surface which is formed by alternate dissolution and precipitation reactions of mixed Cu powder during the charge–discharge cycle. This thin Cu layer has the role of preventing the alloy surface from oxidation and enhancing the possibility of the presence of metallic Ni, which is believed to have a high catalytic activity for the oxygen recombination reaction, on the MH surface. The inner pressure of the cell with the Zr–Ti–Mn–V–Ni alloy was lowered to a level equaling that of the commercial AB 5 -type alloys by the addition of Cu powder.

Prediction of microstructure evolution during high temperature blade forging of a Ni−Fe based superalloy, Alloy 718

The mechanical properties of the Ni−Fe-based Alloy 718 depend very much on grain size, as well as the strengthening phases, γ’ and γ. The grain structure of the superalloy components is mainly controlled during thermo-mechanical processes by the dynamic, meta-dynamic recrystallization and grain growth. In this investigation, the evolution of the grain structure in the process of two-step blade forging was experimentally and numerically dealt with. The evolution of the grain structure in Alloy 718 during blade forging was predicted using a 2-DFE simulator with implemented constitutive models on dynamic recrystallization and grain growth. The comparison of the simulated microstructure with the actual grain structure of the forged parts validated the prediction of the grain structure evolution. The effect of dynamic recrystallization on the evolution of grain structure is highlighted in this article.