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Structural, optical and photoelectrochemical studies on the nanodispersed titania
Gi-Won Lee,So-Yeon Bang,Chaehyeon Lee,Won-Mok Kim,김동환,김경곤,박남규 한국물리학회 2009 Current Applied Physics Vol.9 No.5
Nanodispersion of aggregated TiO2 powders has been performed by microbead milling and its effect on photovoltaic performance has been investigated with dye-sensitized solar cell. Plasma-treated 30 um diameter zirconia beads are used to disperse the aggregated nanocrystalline TiO2 powders in ethanolic medium. Particle size distribution, surface area, film morphology, porosity, transmittance and haze are investigated with different milling speed. Microbead milling leads to a reduction of particle size, narrow size distribution and increase of surface area. A slight crystal phase transformation from anatase to rutile is also observed after microbead milling. Optical property is found to be influenced by microbead milling speed, where transmittance increases and haze decreases with increasing milling speed. Compared with photovoltaic performance of dye-sensitized solar cells based on titania before and after microbead milling, overall conversion efficiency is substantially improved from 4.46% to 6.31% after microbead milling at 2490 rpm for 90 min, corresponding to 42% increment, which is mainly due to a noticeable increase in photocurrent density, associated with highly dispersed characteristics. According to the photocurrent and photovoltage transient spectroscopic study, time constant for electron transport is hardly affected, while that for recombination is slightly decreased due to the increased surface area by nanodispersion.
Ad hoc 네트워크에서 제어메시지 부하를 감소시키는 클러스터 유지 방법
왕기철(Gi-Cheol Wang),방상원(Sang-Won Bang),조기환(Gi-Hwan Cho) 한국정보과학회 2004 정보과학회논문지 : 정보통신 Vol.31 No.1
클러스터 구조는 Ad hoc 네트워크내의 전체 호스트로 데이타를 방송하는 경우에, 재전송되는 메시지의 수를 감소시킨다. 이러한 클러스터 구조의 이점을 보존하기 위해 클러스터 유지방법이 이용된다. 그러나 기존의 클러스터 유지방법들은 이웃정보 파악을 위한 제어메시지 외에도 클러스터 재구성을 위한 추가적인 메시지 교환을 필요로 한다. 이로 인해 클러스터 구조의 유지에 따른 이점은 크게 약화된다. 본 논문에서는 클러스터 구조의 중첩성을 이용하여 Hello시간에 클러스터 헤드들만 제어메시지를 broadcast 전송하고 일부 멤버 호스트들은 제어메시지의 unicast전송을 통해 분리된 게이트웨이를 파악하는 클러스터 유지방법을 제안한다. 제안하는 방법은 클러스터 재구성이 필요할 때에도, 각 호스트간에 전송되는 제어메시지를 최소한으로 줄이기 위한 전략을 사용한다. 제안된 방법은 이 과정에서 2홉 클러스터의 정의를 파괴하지 않으며, 클러스터를 완전히 분산된 방법으로 생성한다. 본 논문에서 제안한 방법은 실험결과에 의해 LCC[1]보다 우수한 것으로 평가된다. The cluster structure reduces the number of retransmission messages, when a broadcast to all hosts in ad hoc network is needed. A cluster maintenance scheme is employed to preserve this advantage from time to time. However, most of the cluster maintenance schemes require additional control messages for cluster reformation as well as control messages for acquiring neighbor information. This mitigates the advantages of employing cluster structure in ad hoc network. In this paper, a cluster maintenance scheme which forces only clusterheads to broadcast control messages during hello time is proposed. When the cluster reformation is needed, the proposed scheme employs a strategy to reduce the control messages to a minimum. In these processes, the proposed scheme doesn't violate the definition of 2-cluster and produces the clusters in fully distributed method. The simulation results prove that our scheme is better than LCC[1].
Structural, optical and photoelectrochemical studies on the nanodispersed titania
Lee, Gi-Won,Bang, So-Yeon,Lee, Chaehyeon,Kim, Won-Mok,Kim, Donghwan,Kim, Kyungkon,Park, Nam-Gyu Elsevier 2009 Current Applied Physics Vol.9 No.5
<P><B>Abstract</B></P><P>Nanodispersion of aggregated TiO<SUB>2</SUB> powders has been performed by microbead milling and its effect on photovoltaic performance has been investigated with dye-sensitized solar cell. Plasma-treated 30 μm-diameter zirconia beads are used to disperse the aggregated nanocrystalline TiO<SUB>2</SUB> powders in ethanolic medium. Particle size distribution, surface area, film morphology, porosity, transmittance and haze are investigated with different milling speed. Microbead milling leads to a reduction of particle size, narrow size distribution and increase of surface area. A slight crystal phase transformation from anatase to rutile is also observed after microbead milling. Optical property is found to be influenced by microbead milling speed, where transmittance increases and haze decreases with increasing milling speed. Compared with photovoltaic performance of dye-sensitized solar cells based on titania before and after microbead milling, overall conversion efficiency is substantially improved from 4.46% to 6.31% after microbead milling at 2490rpm for 90min, corresponding to 42% increment, which is mainly due to a noticeable increase in photocurrent density, associated with highly dispersed characteristics. According to the photocurrent and photovoltage transient spectroscopic study, time constant for electron transport is hardly affected, while that for recombination is slightly decreased due to the increased surface area by nanodispersion.</P>