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저온 분자선에피탁시 방법으로 성장시킨 GaMnAs의 planar Hall 효과
김경현,박종훈,김병두,김도진,김효진,임영언,김창수,Kim, Gyeong-Hyeon,Park, Jong-Hun,Kim, Byeong-Du,Kim, Do-Jin,Kim, Hyo-Jin,Im, Yeong-Eon,Kim, Chang-Su 한국재료학회 2002 한국재료학회지 Vol.12 No.3
Planar Hall effect of ferromagnetic GaMnAs thin films was investigated for the first time. The films were grown in an optimized growth condition via molecular beam epitaxy at low temperatures. For the optimization of the growth conditions, we used reflection high-energy electron diffraction, electrical conductivity, double crystal x-ray diffraction, and superconducting quantum interference device measurements techniques. We observed that the difference between the longitudinal resistance and the transverse resistance matches the planar Hall resistance. The ratio of the planar Hall resistance at saturation magnetic field to that at zero reached above 500%.
Al-Cu-Mu 주조합금의 피로성질에 미치는 Sn 첨가의 영향
김경현,김정대,김인배,Kim, Kyung-Hyun,Kim, Jeung-Dae,Kim, In-Bae 한국재료학회 2002 한국재료학회지 Vol.12 No.4
Effect of Sn addition on the fatigue properties of Al-Cu-Mn cast alloy was investigated by low and high cycle fatigue tests. Fatigue life showed the maximum value of 5450cycles in the Al-Cu-Mn alloy containing 0.10%Sn, but decreased rapidly beyond 0.20% of Sn additions. It was found that the fatigue strength was 132MPa and fatigue ratio was 0.31 in the alloy containing 0.10%Sn. Metallographic observation revealed that the fatigue crack initiated at the surface and propagated along the grain boundary. This propagation path was attributed to the presence of PFZ along the grain boundary. The tensile strength increased from 330MPa in 7he Sn-free Al-Cu-Mn cast alloy to 429MPa in the alloy containing 0.10%Sn. But above 0.20%Sn additions, tensile strength was decreased by the segregation of Sn.
암모니아를 이용하여 분자선에피탁시 방법으로 AIN/Si 기판에 성장시킨 GaN의 구조적,광학적 특성
김경현,홍성의,강석준,이상현,김창수,김도진,한기평,백문철,Kim, Gyeong-Hyeon,Hong, Seong-Ui,Gang, Seok-Jun,Lee, Sang-Hyeon,Kim, Chang-Su,Kim, Do-Jin,Han, Gi-Pyeong,Baek, Mun-Cheol 한국재료학회 2002 한국재료학회지 Vol.12 No.5
A new approach of using double buffer layers of AlN and GaN for growth of GaN films on Si has been undertaken via molecular beam epitaxy using ammonia. The first buffers layer of AlN was grown using $N_2$plasma and the second of GaN was grown using ammonia. The surface roughness of the grown films was investigated by atomic force microscope and was compared with the normally grown films on sapphire. Double crystal x-ray rocking curve and low temperature photoluminescence techniques were employed for structural and optical properties examination. Donor bound exciton peak at 3.481 eV with full width half maximum of 41 meV was observed at 13K.
Al-Cu-Mn 주조합금의 피로성질에 미치는 Cd 첨가의 영향
김경현,이병훈,김인배,Kim, Gyeong-Hyeon,Lee, Byeong-Hun,Kim, In-Bae 한국재료학회 2001 한국재료학회지 Vol.11 No.4
Al-Cu-Mn 주조합금의 피로성질에 미치는 Cd 첨가의 영향을 저주기 및 고주기 피로시험을 통하여 조사하였다. Cd 첨가량이 증가함에 따라 피로수명이 증가하였으며, 인장강도도 증가하였다. 고주기 피로시험결과 피로강도는 115MPa이었으며 피로비는 0.31이었다. 피로시험 결과 균열이 표면에서 발생하여 입계를 따라 전파되었는데 이러한 입계파괴는 입계를 따라 존재하는 무석출대의 영향으로 생각된다. 인장강도값은 Cd이 첨가되지 않았을 경우 330MPa이었으나 0.15%의 Cd이 첨가됨으로써 401MPa 까지 증가되었다. Effect of Cd addition on the fatigue properties of Al-Cu-Mn cast alloy was investigated by low and high cycle fatigue tests. With increasing Cd content, fatigue life and tensile strength were increased. It was found that the fatigue strength was 115MPa and the fatigue ratio was 0.31. Metallographic observation revealed that the fatigue crack initiated at the surface and propagated along the grain boundary. This propagation path was attributed to the presence of PFZ along the grain boundary. The tensile strength increased from 330MPa in the Cd-free Al-Cu- Mn cast alloy to 401MPa in the 0.15%Cd-containing alloy.
김경현,홍성의,백문철,조경익,최상식,양전욱,심규환,Kim, Kyung-Hyun,Hong, Sung-Ui,Paek, Moon-Cheol,Cho, Kyung-Ik,Choi, Sang-Sik,Yang, Jeon-Wook,Shim, Kyu-Hwan 한국전기전자재료학회 2006 전기전자재료학회논문지 Vol.19 No.7
We have investigated $Al_{0.25}Ga_{0.75}As/In_{0.2}Ga_{0.8}As$ structures for pseudomorphic high electron mobility transistor(pHEMT), which were grown by molecular beam epitaxy(MBE) and consequently annealed by rapid thermal anneal(RTA), using Hall measurement, photoluminescence, and transmission electron microscopy (TEM). According to intensity and full-width at half maximum maintained stable at the same energy level, the quantized energy level in $Al_{0.25}Ga_{0.75}As/In_{0.2}Ga_{0.8}As$ quantum wells was independent of the RTA conditions. However, the Hall mobility was decreased from $6,326cm^2/V.s\;to\;2,790cm^2/V.s\;and\;2,078cm^2/V.s$ after heat treatment respectively at $500^{\circ}C\;and\;600^{\circ}C$. The heat treatment which is indispensable during the fabrication procedure would cause catastrophic degradation in electrical transport properties. TEM observation revealed atomically non-uniform interfaces, but no dislocations were generated or propagated. From theoretical consideration about the mobility changes owing to inter-diffusion, the degraded mobility could be directly correlated to the interface scattering as long as samples were annealed below $600^{\circ}C$ lot 1 min.
Al-10%Si-0.3%Mg 주조재의 보수용접에 따른 내구특성 연구
김경현,이정무 ( Kyung Hyun Kim,Jung Moo Lee ) 한국주조공학회 1994 한국주조공학회지 Vol.14 No.6
N/A The effects of weld repair on the durability in Al-10%Si-0.3%Mg castings were evaluated. The strength and fracture toughness in fusion zone were higher than those in the base metal by rapid cooling of fusion zone after welding. There were no significant differences between fatigue properties in castings and weld repaired specimen as the results of low cycle fatigue and rotating bending fatigue test. Therefore it was concluded that weld repairing did not have any significant effect on the mechanical properties of castings.
대형Back-Up roll에서 차등열처리가 기계적 성질 및 조직변화에 미치는 영향
김경현,강석봉,전의진,장윤석,Kim, Gyeong-Hyeon,Gang, Seok-Bong,Jeon, Ui-Jin,Jang, Yun-Seok 한국기계연구원 1986 기계연구원소보 Vol.16 No.-
The first specimens were sampled across the depth of roll products processed by rapid heating and cooling of the roll, namely, differential heat treatment. The second samples were taken from the non-heat treated roll at different depths. The samples were heat treated following the same temperature history as that at each corresponding location in the roll where the samples were taken. Consequently, both specimens showed the similar microstructure and mechanical properties (tensile, impact and fatigue strength etc.)
진공성형을 이용한 삼엽식 고분자 심장판막의 제작과 혈류역학적 성능평가
김경현,황창모,정기석,안치범,김범수,이정주,남경원,선경,Kim, K.H.,Hwang, C.M.,Jeong, G.S.,Ahn, C.B.,Kim, B.S.,Lee, J.J.,Nam, K.W.,Sun, K. 대한의용생체공학회 2006 의공학회지 Vol.27 No.6
In the artificial heart application, productivity and hemodynamic properties of artificial heart valves are crucial in successiful application to long term in vivo trials. This paper is about manufacture and assessment of trileaflet polymer heart valves using vacuum forming process(VFP). The VFP has many advantages such as reduced fabrication time, reproducibility due to relatively easy and simple process for manufacturing. Prior to VFP of trileaflet polymer heart valves, polyurethane(Pellethane 2363 80AE, Dow Chemical) sheet was prepared by extrusion. The sheets were heated and formed to mold shape by vacuum pressure. The vacuum formed trileaflet polymer heart valves fabrication is composed of two step method, first, leaflet forming and second, conduit forming. This two-step forming process made the leaflet-conduit bonding stable with any organic solvents. Hydrodynamic properties and hemocompatibility of the vacuum formed trileaflet polymer heart valves was compared with sorin bicarbon bileaflet heart valve. The percent effective orifice area of vacuum formed trileaflet polymer heart valves was inferior to bileaflet heart valve, but the increase of plasma free hemoglobin level which reflect blood damage was superior in vacuum formed trileaflet polymer heart valves Vacuum formed trileaflet polymer heart valves has high productivity, and superior hemodynamic property than bileaflet heart valves. Low manufacturing cost and blood compatible trileaflet polymer heart valves shows the advantages of vacuum forming process, and these results give feasibility in in vivo animal trials in near future, and the clinical artificial heart development program.