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Xin, Guoqing,Hwang, Wontae,Kim, Namhun,Cho, Sung M,Chae, Heeyeop IOP Pub 2010 Nanotechnology Vol.21 No.40
<P>High quality graphene was obtained though microwave irradiated expansion following a solution process. This method is facile, inexpensive, and produces usable results. Ultrathin, uniform graphene films were fabricated at room temperature by a vacuum filtration method. Combining carbon nanotubes (CNTs) as bridges between graphene flakes allowed the fabrication of high-performance conductive films for flexible applications, with conductivities and optical properties comparable to commercial ITO: 181 Ω sq<SUP> − 1</SUP> at 82.2% transmittance after chemical treatment and doping. With future work, this versatile material could well provide an appropriate transparent electrode for flexible optical electronics. </P>
Numerical Investigations of Bump Effects on the Performance of Double-Ended Airfoils and ABC Rotors
Xin Yuan,Guoqing Zhao,Qijun Zhao 한국항공우주학회 2024 International Journal of Aeronautical and Space Sc Vol.25 No.2
Numerical simulations based on RANS have been performed to investigate the aerodynamic characteristics of the double- ended airfoil deformed by local bumps on its lower surface, both in airfoil and coaxial rotor environments. The two-dimensional results show that the bump can reduce the magnitude of negative lift but increase the drag at large negative AoAs and high Mach numbers, and can increase the stability of the airfoil at positive AoAs in forward flow and at large negative AoAs in reverse flow. A bump has a more significant effect on airfoil performance if it is closer to the leading edge and has a larger area. A coaxial rotor with different double-ended airfoils deformed by bumps located inboard was predicted under a 0.5 advance ratio with a lift-offset of 0.3. Bumps under such flight conditions influence the root region on the advancing side more than on the retreating side. The root region at 14 ~ 33% span produces about 6% of the total thrust, so bump’s effects on the whole performance are insignificant. Through the flow details around inboard sections, it suggests that bumps have the potential to improve ABC rotor performance, and flight conditions and blade geometry should be considered in conjunction with the application of bumps for effectiveness. Although the double-ended airfoil only covers the blade range of 0.14 ~ 0.33R, the total rotor thrust is slightly increased by adding bumps, and a higher bump has a greater influence.
김남훈,Guoqing Xin,조성민,방창현,채희엽 한국물리학회 2015 Current Applied Physics Vol.15 No.9
Microwave-assisted reduced graphene oxide (MR-GO) layer was applied to hole extraction layer (HEL) of polymer solar cells (PSCs) and was compared with the widely used poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) in bulk hetero-junction (BHJ) solar cells. The power conversion efficiency (PCE) of 3.57% was achieved with the MR-GO layer, which is 21% higher than that of PSCs with the conventional PEDOT:PSS HEL material. This enhancement of PCE is mainly attributed to the increase of short-circuit current density originated from the hydrophobic surface of the MR-GO layer. The hydrophobic graphene oxide surface is believed to improve wetting property and physical contact of active blends. In addition, the MR-GO interfacial layer is found to show the excellent device stability in atmospheric condition. The PCE of conventional PEDOT:PSS based PSCs showed total degradation when the device was exposed to atmospheric condition for 1000 h without any encapsulation, while that of MR-GO based PSC showed over 85% of PCE.
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>
Sequence Impedance Modeling and Frequency Characteristic Analysis of MMC-based HVDC
Tong Huang,Xin Chen,Guanghui Li,Guoqing He 전력전자학회 2019 ICPE(ISPE)논문집 Vol.2019 No.5
Modular multilevel converter (MMC) has emerged as a preferred technology for high-voltage dc (HVDC) transmission, while many HVDC-MMC projects have been reported resonance problems. The existing researches show that stability of a HVDC-grid system is decided by both the sequence impedance characteristic of HVDC-MMC and power grid, and due to coupling effect, the power grid impedance will also impact the HVDCMMC impedance characteristic. Multi-harmonic linearization method will be adopted in this paper to develop a HVDC-MMC analytical sequence impedance model. Then the developed model is used to illustrate the effect of the power control loop as well as the bandwidth of other controllers on HVDC-MMC impedance characteristic. Finally, based on the sequence impedance model considering coupling effect, the influence of HVDC-MMC bandwidth on the system stability will be analyzed as well.
Electrospray deposition of carbon nanotube thin films for flexible transparent electrodes.
Meng, Yinan,Xin, Guoqing,Nam, Jaewook,Cho, Sung Min,Chae, Heeyeop American Scientific Publishers 2013 Journal of Nanoscience and Nanotechnology Vol.13 No.9
<P>Flexible transparent carbon nanotube (CNT) electrodes were fabricated by electrospray deposition, a large-area scalable and cost-effective process. The carbon nanotubes were dispersed in N,N-dimethylformamide (DMF) and deposited on polyethylene terephthalate (PET) substrates by electrospray deposition process at room temperature and atmospheric pressure. Major process variables were characterized and optimized for the electrospray process development such as electric field between nozzle and substrates, CNT solution flowrate, gap between nozzle and substrates, solution concentration, solvent properties and surface temperature. The sheet resistance of the electrospray deposited CNT films were reduced by HNO3 doping process. 169 Omega/sq sheet resistance and 86% optical transmittance was achieved with low surface roughness of 1.2 nm. The films showed high flexibility and transparency, making them potential replacements of ITO or ZnO in such as solid state lighting, touch panels, and solar cells. Electrospray process is a scalable process and we believe that this process can be applied for large area carbon nanotube film formation.</P>
Xiangqiang Wu,Tong Huang,Xin Chen,Haibing Hu,Guoqing He 전력전자학회 2019 ICPE(ISPE)논문집 Vol.2019 No.5
Decoupled double synchronous reference frame phase locked loop (DDSRF-PLL) can effectively eliminate negative-sequence fundamental frequency component, which means the PLL control loop will have an effect on the frequency characteristics and stability of gridconnected inverters. Based on the three-phase LCL-type grid-connected inverter, the positive- and negative-sequence output impedance model of the grid-connected inverter and the frequency characteristics of DDSRF-PLL were firstly derived. Meanwhile, the regulator parameters of DDSRFPLL were designed and its effect on impedance stability of inverter was analyzed. Then, the frequency characteristics of DDSRF-PLL and the output impedance model of the grid-connected inverter were verified by the simulation and experimental results. Finally, the stability of the gridconnected with different PLL bandwidths was analyzed and verified by experimental results.