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Now showing items 1 - 16 of 37317

  • Femtosecond laser sintering of copper nanoparticles

    Cheng, C. W.   Chen, J. K.  

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  • Ablation of copper by a scanning Gaussian beam of a femtosecond laser pulse

    Cheng, C. W.  

    This study investigated ablation from a copper metal surface using a scanning femtosecond laser beam with a Gaussian beam profile. A method was developed herein to calculate the ablation profile with experimentally identified parameters (e.g., effective focused Gaussian incident beam radius, ablation threshold fluence, effective energy penetration depth). The results show the relationship between the maximum ablation depth and maximum ablation width. The calculated ablation profile agrees well with the experimental measurements.
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  • Gallium Nitride on Silicon for Consumer & Scalable Photonics

    Bayram, C.   Shiu, K. T.   Zhu, Y.   Cheng, C. W.   Sadana, D. K.  

    Gallium Nitride (GaN) is a unique material system that has been heavily exploited for photonic devices thanks to ultraviolet-to-terahertz spectral tunability. However, without a cost effective approach, GaN technology is limited to laboratory demonstrations and niche applications. In this investigation, integration of GaN on Silicon (100) substrates is attempted to enable widespread application of GaN based optoelectronics. Controlled local epitaxy of wurtzite phase GaN on on-axis Si(100) substrates is demonstrated via metal organic chemical vapor deposition (MOCVD). CMOS-compatible fabrication scheme is used to realize [SiO2-Si{111}-Si{100}] groove structures on conventional 200-mm Si(100) substrates. MOCVD growth (surface treatment, nucleation, initiation) conditions are studied to achieve controlled GaN epitaxy on such grooved Si(100) substrates. Scanning electron microscopy and transmission electron microscopy techniques are used to determine uniformity and defectivity of the GaN. Our results show that aforementioned groove structures along with optimized MOCVD growth conditions can be used to achieve controlled local epitaxy of wurtzite phase GaN on on-axis Si(100) substrates.
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  • Micro/nano-structuring of medical stainless steel using femtosecond laser pulses

    Lin, C. Y.   Cheng, C. W.   Ou, K. L.  

    The medical stainless steel (SUS 304) surface is irradiated by femtosecond laser pulses with linear or circular polarization to form nanostructure-covered conical microstructures. The mean spacing of the conical microstructures and the type of the nanostructure can be controlled by the laser-processing parameters. The liquid test (water and normal-saline solution) demonstrates that the process provides a fast single-step structuring method to generate hydrophobic-enhanced metal parts. The biocompatibility test demonstrated that the femtosecond laser micro/nano-structuring surfaces have excellent biocompatibility properties compared to an untreated surface. (C) 2012 Published by Elsevier B.V. Selection and/or review under responsibility of Bayerisches Laserzentrum GmbH
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  • Economic feasibility of solar-powered led roadway lighting

    Wu, M. S.   Huang, H. H.   Huang, B. J.   Tang, C. W.   Cheng, C. W.  

    The optical efficacy of light emitting diode (LED) has exceeded 72 lm/W in 2006. This implies that energy can be saved about 75%, as compared to mercury lamps widely used in roadway lighting. In some remote areas where the grid power cannot reach, independent solar-powered lighting using high-power LED provides a promising solution. However, the cost of solar photovoltaic device may cause the application of solar-powered LED roadway lighting to be not economically feasible. The present study investigates the design of the solar-powered LED roadway lighting using high-power LED luminaire (100 W) and estimates the installation cost for a 10 km highway with 2 lanes. LED luminaries are installed on both side of the road with staggered arrangement. The pole distance is 30 m. The cost comparison of LED lighting using grid and solar power with the conventional mercury lamps was carried out. It shows that the installation cost is 22 million USD for LED powered by grid power and 26 million USD for solar-powered. The total installation cost of conventional mercury lighting is 18 million USD. The excess cost of LED mainly comes from the cost of LED lamp and solar PV. But, the cost of power generation and electrical transmission line can be greatly reduced since about 75% energy was saved for LED. This permits the use of smaller copper wire and shorter line length for solar-powered system which in turn saves installation cost. The payback time for the excess investment of LED is 2.2 years for LED using grid power and 3.3 years for LED using solar power. (C) 2008 Elsevier Ltd. All rights reserved.
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    Cheng, C. W.   Bian, Z. X.  

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  • Fabrication of micro/nano crystalline ITO structures by femtosecond laser pulses

    Cheng, C. W.   Shen, W. C.   Lin, C. Y.   Lee, Y. J.   Chen, J. S.  

    A method is proposed for the fabrication of micro/nano crystalline indium tin oxide (c-ITO) structures using a Ti:Sapphire laser with a repetition rate of 1 kHz and a wavelength of 800 nm. In the proposed approach, an amorphous ITO (a-ITO) thin film is transformed into a c-ITO micro/nano structure over a predetermined area via laser beam irradiation, and the residual a-ITO thin film is then removed using an etchant solution. The fabricated c-ITO structures are observed using scanning electron microscopy (SEM) and cross-sectional transmission electron microscopy (TEM). The observation results show that the use of a low repetition rate laser induces a high thermal cycling effect within the ITO film and therefore prompts the formation of micro-cracks in the c-ITO structure. In addition, it is shown that as the laser power approaches the ablation threshold of the a-ITO thin film, nanogratings and disordered nanostructures are formed along the center lines of the c-ITO patterns formed using linearly polarized and circularly polarized laser beam irradiation, respectively. The nanogratings are found to have a period of approximately 200 nm (i.e. one-quarter of the irradiation wavelength), while the nanostructures have an average diameter of approximately 100-160 nm.
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  • Performance optimization for a variable throat ejector in a solar refrigeration system

    Yen, R. H.   Huang, B. J.   Chen, C. Y.   Shiu, T. Y.   Cheng, C. W.   Chen, S. S.   Shestopalav, K.  

    In a solar vapor ejector refrigeration system, the solar heat supply may vary because of variations in solar irradiation intensity, making it difficult to maintain a steady generator temperature. To improve ejector performance, this study proposes a variable throat ejector (VTEJ) and analyzes its performance using CFD simulations. The following conclusions can be drawn. An ejector with a greater throat area and larger solar collector allows a wider operating range of generator temperatures, but may be overdesigned and expensive. Conversely, decreasing the throat area limits the operating range of generator temperatures. Thus the ejector with a fixed throat area may be unsuitable to use solar energy as a heat source. For a VTEJ, this study derives a curve-fitting relationship between the optimum throat area ratio and the operating temperatures. Using this relationship to adjust the throat area ratio, the ejector can consistently achieve optimal and stable performances under a varying solar heat supply. (C) 2013 Elsevier Ltd and IIR. All rights reserved.
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  • Predicting the spatiotemporal dynamics of hair follicle patterns in the developing mouse

    Cheng, C. W.   Niu, B.   Warren, M.   Pevny, L. H.   Lovell-Badge, R.   Hwa, T.   Cheah, K. S. E.  

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  • Feedrate control for non-uniform rational B-spline motion command generation

    Cheng, C. W.   Tsai, M. C.   Maciejowski, J.  

    This paper presents a real-time feedrate-controlled interpolator (FCI) for accurate feedrate control of computer numerical controlled (CNC) machine tools along non-uniform rational B-spline (NURBS) curves for precision machining. Unlike most of the existing interpolators that are developed based on the Taylor's expansion, the proposed method uses a predictor-corrector algorithm instead. In the predictor stage, an efficient algorithm is used to estimate the servo command of the next sampling time, and in the corrector stage, the errors arising from the prediction can be corrected. This study conducts an extensive mathematical analysis of the FCI, where the criterion for selecting the sampling period, i.e. sampling period bound, is also established. The experimental results are presented to demonstrate the feasibility of the proposed FCI for machining the NURBS curves.
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  • Surface plasmon enhanced band edge luminescence of ZnO nanorods by capping Au nanoparticles

    Cheng, C. W.   Sie, E. J.   Liu, B.   Huan, C. H. A.   Sum, T. C.   Sun, H. D.   Fan, H. J.  

    The author observe sixfold enhancement in the near band gap emission of ZnO nanorods by employing surface plasmon of Au nanoparticles, while the defect-related emission is completely suppressed. Time-resolved photoluminescence indicates that the decay process becomes much faster by Au capping. The remarkable enhancement of the ultraviolet emission intensities and transition rates is ascribed to the charge transfer and efficient coupling between ZnO nanorods and Au surface plasmons. The suppression of the green emission might be due to a combined effect of Au surface plasmon and passivation of the ZnO nanorod surface traps.
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  • Improvement of protective coating on Nd–Fe–B magnet by pulse nickel plating

    Cheng, C. W.   Cheng, F. T.   Man, H. C.  

    Sintered Nd-Fe-B magnets were coated by pulse nickel plating at different plating conditions. Optimal pulse plating condition was established (average current density=1 A/dm 2, peak current density=6 A/dm 2 with T on:T off=1:2). In order to make a comparison, magnets with similar nickel coating thickness plated by dc were also prepared. The corrosion resistance of the coated magnets was evaluated by (i) Normal Salt Spray Test (5% NaCl, 35 degC) and (ii) potentiodynamic polarization measurement (3.5% NaCl solution). It was found that the corrosion resistance of the pulse nickel plated magnet was significantly improved as compared with that of the conventional dc plated ones, with negligible deterioration in magnetism. The microstructure of the coating was examined by optical microscopy and scanning electron microscopy. It was found that the porosity was much lower, and the grains much finer in the pulse-plated layer as compared with the dc plated ones
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  • Merging buoyant discharges in an ambient current

    Cheng, C. W.   Davidson, M. J.   Wood, I. R.  

    The results of a preliminary experimental investigation into the merging of buoyant discharges in an ambient current are presented. Trajectory and dilution data from the merging buoyant flows are reported. The investigation shows that the conditions at the point of merging have an important effect on the behaviour of such flows. It is also shown that if there is an ambient current there may be some advantage in designing an outfall such that the buoyant flows merge.
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  • Randomly packed n-SnO(2) nanorods/p-SiC heterojunction light-emitting diodes RID A-3885-2010 RID A-2662-2010

    Yang, H. Y.   Yu, S. F.   Cheng, C. W.   Tsang, S. H.   Liang, H. K.   Fan, H. J.  

    A layer of randomly packed n-SnO(2) nanorods is grown by vapor transport method on the p-SiC(4H) substrate to realize heterojunction light-emitting diodes. Diodelike rectifying current-voltage characteristics, with a turn-on voltage of similar to 4.5 V and reverse leakage current density of < 0.25 A/m(2), are obtained at room temperature. Furthermore, electroluminescent spectra with emission peaks at around 395, 434, and 497 nm are observed from the heterojunction under forward bias. This is due to the relaxation of electrons in the conduction band of SnO(2) to the surface defect states and subsequent radiative recombination with holes injected from the p-SiC substrate.
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  • High-temperature lasing characteristics of randomly assembled SnO(2) backbone nanowires coated with ZnO nanofins

    Yang, H. Y.   Yu, S. F.   Liang, H. K.   Mote, Rakesh G.   Cheng, C. W.   Fan, H. J.   Sun, T.   Hng, H. H.  

    Lasing characteristics of randomly assembled SnO(2) backbone nanowires coated with ZnO nanofins are investigated. It is shown that the hierarchical nanostructures can sustain ultraviolet random lasing action even at substrate temperature higher than 700 K and the corresponding characteristic temperature is found to be about 390 K. This is because the presence of ZnO nanofins improves heat transfer from the SnO(2) backbone nanowires to the surrounding. Hence, some portion of the hierarchical nanostructures can be cooled down and the corresponding optical gain can be maintained even at high substrate temperature.
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    Mok, C. C.   Lo, L. Y. T.   Poon, K. S.   Cheng, C. W.  

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