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Hwang, Wontae,Pang, Changhyun,Chae, Heeyeop IOP 2016 Nanotechnology Vol.27 No.43
<P>Aligned nanofiber arrays and mats were fabricated with an electrospinning process by manipulating the electric field. The electric field was modified by insulating blocks (IBs) that were installed between the nozzle and the substrate as guiding elements to control the trajectory of the electrospinning jet flow. Simulation results showed that the electric field was deformed near the IBs, resulting in confinement of the electrospinning jet between the blocks. The balance of the electric field in the vertical direction and the repulsive force by space charges in the confined electrified jet stream was attributed to the aligned motion of the jet. Aligned arrays of 200 nm thick polyethylene oxide nanofibers were obtained, exhibiting wave-shaped and cross patterns as well as rectilinear patterns. In addition, 40 <I>μ</I>m thick quasi-aligned carbon-nanofiber mats with anisotropic electrical property were also attained by this method.</P>
Electrospray deposition of polymer thin films for organic light-emitting diodes
Hwang, Wontae,Xin, Guoqing,Cho, Minjun,Cho, Sung Min,Chae, Heeyeop Springer 2012 Nanoscale research letters Vol.7 No.1
<P>Electrospray process was developed for organic layer deposition onto polymer organic light-emitting diode [PLED] devices in this work. An electrospray can be used to produce nanometer-scale thin films by electric repulsion of microscale fine droplets. PLED devices made by an electrospray process were compared with spin-coated ones. The PLED device fabricated by the electrospray process showed maximum current efficiency of 24 cd/A, which was comparable with that of the spin-coating process. The electrospray process required a higher concentration of hole and electron transport materials in the inks than spin-coating processes to achieve PLED maximum performance. Photoluminescence [PL] at 407 nm was observed using electrosprayed poly(<I>N</I>-vinyl carbazole) films, whereas a peak at 410 nm was observed with the spin-coated ones. Similar difference in peak position was observed between aromatic and nonaromatic solvents in the spin-coating process. PLED devices made by the electrospray process showed lower current density than that of spin-coated ones. The PL peak shift and reduced current of electrosprayed films can therefore be attributed to the conformation of the polymer.</P>
황원태(Wontae Hwang),김동승(Dongseung Kim) 한국정보과학회 2005 한국정보과학회 학술발표논문집 Vol.32 No.1
본 논문은 전체 데이터베이스에서 일부 추출된 샘플 데이터에서 빈발항목 집합을 찾는 연관규칙 마이닝 알고리즘을 기술한다. 샘플링기술을 이용하면 마이닝과정에서 필요한 데이터베이스의 접근 양을 줄이므로써 실행시간을 단축시킬 수 있다는 장점이 있지만, 전체데이터베이스를 이용한 마이닝보다 정확도가 떨어진다는 단점이 함께 존재한다. 이전의 Chen의 FAST알고리즘은 샘플링을 이용한 마이닝과정에서 거리오차함수를 이용한 트리밍과정을 통해 빈발 1항목집합에 대한 정확도를 개선시켰다. 이후 IFAST 알고리즘은 트리밍과정에서 빈발2-항목집합까지 고려하여 빈발2-항목집합 이상의 빈발항목집합에서도 정확도를 개선시켰다. 본 논문에서는 트리밍과정에서 사용될 추정데이터를 여러 개의 샘플데이터를 이용하여 얻으므로써 오류항목집합(false itemset)의 수를 줄이고 전체적인 정확도를 향상시키는 새로운 알고리즘을 소개한다.
황원태(Wontae Hwang) 한국추진공학회 2016 한국추진공학회 학술대회논문집 Vol.2016 No.12
비행기 제트 엔진과 발전용 가스터빈의 효율 향상을 위해서는 firing temperature 가 계속 상승하고 있다. 연소기 및 고압 터빈과 같은 고온 부품들의 정확한 수명 예측을 위해서는 표면 온도장 및 냉각 유동장을 파악하는 것이 중요하다. Full speed engine condition 으로 회전하는 고압터빈 블레이드와 full load 로 작동하는 연소기의 liner 내부 표면 온도장을 적외선 카메라들로 관측하는 기술을 소개하고, 아울러 고압 터빈 블레이드의 복잡한 내부 냉각 유동을 의료용 MRI scanner 로 3차원 계측하는 기술을 소개하고자 한다. The firing temperature of aircraft jet engines and power generation gas turbines continues to increase in order to obtain higher efficiency. It is important to know the surface temperature and cooling flow fields for accurate life prediction of hot parts such as the combustor and high-pressure turbine. I will first introduce a measurement technique that utilizes infrared (IR) cameras to obtain temperature fields of high-pressure turbine blades rotating at full speed engine condition, and the interior surface of a combustor liner operating at full load. Next I will introduce a 3D flow measurement technique that uses a healthcare MRI scanner to measure the complex internal cooling flow field of a high-pressure turbine blade.