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RF 스퍼터링 시스템을 이용하여 증착한 비정질 Ga<sub>2</sub>O<sub>3</sub> 박막의 급속 열처리 조건에 따른 결정성과 광학적 특성 변화
김형민,박상빈,홍정수,김경환,Hyungmin Kim,Sangbin Park,Jeongsoo Hong,Kyunghwan Kim 한국전기전자재료학회 2023 전기전자재료학회논문지 Vol.36 No.6
The Ga<sub>2</sub>O<sub>3</sub> thin films were deposited using an RF sputtering system and the effect of crystallographic and optical properties under rapid thermal annealing conditions on Ga<sub>2</sub>O<sub>3</sub> thin film was evaluated. A rapid thermal annealing method can fabricate a crystalline Ga<sub>2</sub>O<sub>3</sub> thin film which is applied to various fields with a low cost and a high efficiency compared with the conventional post-annealing method. In this study, the Ga<sub>2</sub>O<sub>3</sub> treated at 900℃ for 1 min showed the beta and gamma phases in XRD measurement. In optical properties, the crystalline Ga<sub>2</sub>O<sub>3</sub> represented a high transmittance of more than 80% in the visible region and was calculated with a high optical bandgap energy of 4.58 eV. The beta and gamma phases Ga<sub>2</sub>O<sub>3</sub> can be obtained by adjusting the rapid thermal annealing temperatures, and the various properties such as the optical bandgap energy can be controlled. Moreover, it is expected that crystalline Ga<sub>2</sub>O<sub>3</sub> can be applied to various devices by controlling not only temperature but process time.
김형민,함건희,서병태,Kim, Hyungmin,Ham, Geonhee,Seo, Byungtae 한국통계학회 2016 응용통계연구 Vol.29 No.7
혼합회귀모형은 반응 변수와 공변량 사이의 관계를 규명하는 유용한 통계적 모형으로 여러 분야에서 사용되어지고 있다. 하지만 실제로 혼합회귀모형을 이용하여 분석을 하는 과정에서 공변량이 결측값을 포함하는 문제는 흔하게 발생하며, 발생하는 결측의 유형 또한 다양하게 나타난다. 이러한 경우에 있어서 본 논문에서는 최대우도추정량을 구하기 위한 EM 알고리즘을 제안하고자 한다. 제안된 EM 알고리즘의 효용성을 모의실험을 통해 확인하였으며 또한 사례연구를 통해 제시된 방법이 어떻게 사용될수 있는지와 그 효용성을 함께 확인하였다. Finite mixtures of regression models provide an effective tool to explore a hidden functional relationship between a response variable and covariates. However, it is common in practice that data are not fully observed due to several reasons. In this paper, we derived an expectation-maximization (EM) algorithm to obtain the maximum likelihood estimator when some covariates are missing at random in the finite mixture of regression models. We conduct some simulation studies and we also provide some real data examples to show the validity of the derived EM algorithm.
펌프 가압식 추진제 공급유로에서의 오리피스 개도에 따른 동적 수력특성 변화
김형민(Hyungmin Kim),고태호(Taeho Ko),김상민(Sangmin Kim),윤웅섭(Woongsup Yoon) 한국추진공학회 2009 한국추진공학회 학술대회논문집 Vol.2009 No.11
액체추진제로켓엔진의 추진제 공급유로 내 압력보정을 목적으로 설치된 오리피스는 엔진의 시동 및 정지시, 압력파를 발생시키는 원인이 되며 압력파로 인한 저주파 불안정을 막기 위해서는 오리피스의 개도에 따른 동특성의 변화를 살펴볼 필요가 있다. 오리피스의 동특성 변화를 확인하기 위해 오리피스 공급 압력을 급격하게 증가하도록 하였고 오리피스의 개도를 바꿔가며 동적인 압력변화를 측정하였다. 오리피스의 개도가 증가함에 따라 오리피스에 의한 수격현상은 감소하였으며 오리피스 하류의 수력학적 영향이 지배적으로 나타나는 것을 확인하였다. The orifice in the propellent feeding pipe line of a Liquid Rocket Engine(LRE) is used to balance the pressure of the pipe line. When a LRE starts up, pressure at the upstream of the orifice rapidly increases. In this case, pressure waves occuring by resistance of the orifice may induce low frequency instability in the pipe line. For this reason the study of dynamic characteristics of orifices is needed to prevent the instability. A pump is used to build up the pressure, and the pressure is measured upstream and downstream of the orifice when the orifice diameter is changed. With the increase of orifice diameter, water hammer decreases, but the effect of resistance downstream is increases.
분사시기 및 분사각 변화에 따른 HCCI 엔진의 혼합기 분포 및 연소특성에 관한 연구
김형민(Hyungmin Kim),류재덕(Jeaduk Ryu),이기형(Kihyung Lee) 한국자동차공학회 2006 한국 자동차공학회논문집 Vol.14 No.4
Recently, there has been an interest in premixed diesel engines as it has the potential of achieving a more homogeneous and leaner mixture close to TDC compared to conventional diesel engines. Early studies are shown that in a HCCI(Homogeneous Charge Compression Ignition) engine, the fuel injection timing and injection angle affects the mixture formations. Thus the purpose of this study was to investigate relationship of combustion and mixture formations according to injection timing and injection angle in a common rail direct injection type HCCI engine using a early injection method called the PCCI(Premixed Charge Compression Ignition). From this study, we found that the fuel injection timing and injection angle affect the mixture formations and in turn affects combustion in the PCCI engine.
흡기밸브 닫힘시기 지각과 운전조건이 2단 분사 PCCI 엔진에 미치는 영향에 관한 연구
김형민(Hyungmin Kim),이재현(Jaehyeon Lee),김기범(Kibum Kim),이기형(Jaehyeon Lee) 한국자동차공학회 2009 한국자동차공학회 부문종합 학술대회 Vol.2009 No.4
As world attention has focused on global warming and air pollution, high efficiency diesel engines with low CO2 emissions have become more attractive. Premixed diesel engines in particular have the potential to achieve the more homogeneous mixture in the cylinder which results in lower NOx and soot emission. Early studies have shown that the operation conditions such as the EGR, intake conditions, injection conditions and compression ratio are important to reduce emissions in a PCCI (Premixed Charge Compression Ignition) engine. In this study a modified cam was employed to reduce the effective compression ratio. While opening timing of the intake valve was fixed, closing timing of the intake valve was retarded 30°. Although Atkinson cycle with the retarded cam leads to a low in-cylinder pressure in the compression stroke, the engine work can still be increased by advancing injection timing. On that account, we investigated effect of various operating parameters for reduce emission and fuel consumption, as it could be achieved with low NOx at the same level fuel consumption and PM.
조사 분사 방식을 이용한 HCCI 엔진의 연소 및 배기특성에 관한 실험적 연구
김형민(Hyungmin Kim),류재덕(Jeaduk Ryu),이기형(Kihyung Lee),김병수(Byungsu Kim) 한국자동차공학회 2004 한국자동차공학회 춘 추계 학술대회 논문집 Vol.- No.-
The new concept called as Homogeneous Charge Compression Ignition engines are spotlighted because this concept reduced NOx and smoke simultaneously. To accomplish this combustion, many researchers are studying various methods called the PREDIC, MK, NADl, MULDIC, UNIBUS, HCDC, CAI and PCCI. The purpose of this study was to investigate characteristics of combustion and emission in a common rail direct injection type HCCI engine using a early injection method called the PCCI(Premixed Charge Compression Ignition). From this study, we found that the NOx emissions were dramatically reduced compare to conventional diesel combustion. The smoke emissions also showed a slightly reduction, while CO and HC emissions increased substantially. And the operating condition of HCCI engine limited a low loads.
김형민(Hyungmin Kim),이국진(Kukjin Lee),윤웅섭(Woongsup Yoon) 한국추진공학회 2010 한국추진공학회 학술대회논문집 Vol.2010 No.5
액체로켓엔진 시스템의 시동 및 정지 또는 추력 제어와 같은 천이 작동시 동특성을 예측하기 위한 선행 연구로서 추진제 공급 시스템의 구성품에 대한 동특성 모델링을 수행하였다. 연료 공급계통과 산화제 공급 계통의 구성품들은 재생냉각채널을 제외하고 같은 것으로 가정하였다. 동특성 모델링의 대상 구성품은 펌프, 관로, 오리피스, 제어 벨브, 재생냉각채널, 인젝터 등이며 실제 엔진 시스템의 축소 모형에 대한 수력시험을 통해 각 구성품의 동특성 모델링을 검증하였다.