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Hung T.-C.,Roc Military Academy | Huang Y.-X.,National Chung Cheng University | Sheu T.-S.,Roc Military Academy | Yan W.-M.,Research Center for Energy Technology and Strategy
Numerical Heat Transfer; Part A: Applications | Year: 2014

The present study deals with the optimal geometric parameters design of a microchannel heat sink (MCHS) filled with sintered porous medium through an optimization procedure. This procedure integrates the simplified conjugate-gradient method (SCGM) and a three-dimensional heat sink model as an optimizer. The optimal design variables such as the number of channel N, channel-width ratio β, and channel aspect ratio α that minimize the overall thermal resistance of the microchannel heat sink are obtained under various pumping powers and porous conditions. Optimization results show that, for a given pumping power, the optimal design variables are N = 108, β = 0.90, and α = 8.15, with a corresponding minimum overall thermal resistance of 0.070 K W-1, and the overall thermal resistance is decreased by 40% over that of the initial guess (N = 56, β = 0.4, α = 4.8, and R T = 0.115 K W-1). The predicted results also reveal that the optimal thermal resistance and all the corresponding optimal values of N, β, and α decrease with the increase of porosity. Moreover, as the pumping power increases, the optimal thermal resistance decreases, both the corresponding optimal values of N and α increase, where as β reaches a maximum limitation. The proposed combined approach is effective in optimizing the geometric parameters for a porous-microchannel heat sink. © 2014 Copyright Taylor and Francis Group, LLC.


Chen C.-Y.,National Taiwan University | Chen C.-Y.,Georgia Institute of Technology | Huang J.-H.,Research Center for Energy Technology and Strategy | Huang J.-H.,National Cheng Kung University | And 9 more authors.
ACS Nano | Year: 2011

We studied the dependence of the output of the piezoelectric nanogenerator (NG) on the inclining orientation of the ZnO nanowire arrays (NWAs). The oblique-aligned NWAs were grown by combing a modified oblique-angle sputtering technique for preparing the seed layer and hydrothermal growth. The piezoelectric output of the NWAs was studied by scanning the tip of an atomic force microscope along four different directions in reference to the inclining direction of the NWs. The statistical outputs were analyzed in reference to the theoretically calculated piezopotential distribution in the NWs. Our study provides in-depth understanding about the performance of NGs. © 2011 American Chemical Society.


Huang M.R.S.,National Cheng Kung University | Erni R.,Empa - Swiss Federal Laboratories for Materials Science and Technology | Liu C.-P.,National Cheng Kung University | Liu C.-P.,Research Center for Energy Technology and Strategy | Liu C.-P.,Advanced Optoelectronic Technology Center
Applied Physics Letters | Year: 2013

The influence of surface oxidation on the low-loss spectrum of aluminum nitride (AlN) is investigated in electron energy-loss spectroscopy with scanning transmission electron microscopy. Contrary to intrinsic bulk AlN, oxidized AlN exhibits considerable spectral broadening both in the full width at half maximum of bulk plasmon and the subsidiary features. The modification in the low-loss lineshapes due to oxidation significantly complicates the determination of the dielectric function intrinsic to AlN. Simulations based on dielectric theory qualitatively consist with the experimental results while incorporating thick overlayers, further suggesting that the surface oxide of AlN can be rough and porous in nature. © 2013 American Institute of Physics.


Corani A.,Lund University | Li M.-H.,National Cheng Kung University | Shen P.-S.,National Cheng Kung University | Chen P.,National Cheng Kung University | And 10 more authors.
Journal of Physical Chemistry Letters | Year: 2016

There is a mounting effort to use nickel oxide (NiO) as p-type selective electrode for organometal halide perovskite-based solar cells. Recently, an overall power conversion efficiency using this hole acceptor has reached 18%. However, ultrafast spectroscopic investigations on the mechanism of charge injection as well as recombination dynamics have yet to be studied and understood. Using time-resolved terahertz spectroscopy, we show that hole transfer is complete on the subpicosecond time scale, driven by the favorable band alignment between the valence bands of perovskite and NiO nanoparticles (NiO(np)). Recombination time between holes injected into NiO(np) and mobile electrons in the perovskite material is shown to be hundreds of picoseconds to a few nanoseconds. Because of the low conductivity of NiO(np), holes are pinned at the interface, and it is electrons that determine the recombination rate. This recombination competes with charge collection and therefore must be minimized. Doping NiO to promote higher mobility of holes is desirable in order to prevent back recombination. © 2016 American Chemical Society.


Hung T.-C.,ROC Military Academy | Yan W.-M.,National University of Tainan | Yan W.-M.,Research Center for Energy Technology and Strategy
International Journal of Heat and Mass Transfer | Year: 2012

A three-dimensional analysis aimed at enhancing the thermal performance of a double-layered microchannel heat sink by using a nanofluid and varying the geometric parameters has been conducted. A system of fully elliptic equations that govern the flow and thermal fields are solved using the finite volume method. The analysis indicates that the dominant factors determining the thermal resistance of the channel include the type of nanofluid; particle volume fraction; geometric parameters of the channel, such as the channel number, channel width ratio, channel aspect ratio; and pumping power. The results indicate that the greatest enhancement in channel cooling can be expected when an Al 2O 3-water nanofluid is used. The thermal resistance of the channel can be minimized by properly adjusting the particle volume fraction under various pumping powers; the minimum thermal resistance depends on the geometric parameters. The study also reveals that the relationship between the thermal resistance and channel number, channel width ratio, or channel aspect ratio exhibits a decrease followed by an increase. The thermal performance of the channel can usually be improved by decreasing the channel number or channel aspect ratio, or increasing the channel width ratio. Finally, increasing the pumping power reduces the overall thermal resistance. An Al 2O 3 (1%)-water nanofluid shows an average improvement in thermal performance of 26% over that of pure water for a given pumping power. However, the design's effectiveness declines significantly under high pumping power. In particular, the thermal resistance obtained by employing nanofluids was not necessarily lower than that of water under all pumping powers, but it can be reduced by properly adjusting the geometric parameters under optimal conditions. © 2012 Elsevier Ltd. All rights reserved.


Hung T.-C.,Military Academy | Yan W.-M.,National University of Tainan | Yan W.-M.,Research Center for Energy Technology and Strategy | Wang X.-D.,North China Electrical Power University | Huang Y.-X.,National Cheng Kung University
International Journal of Heat and Mass Transfer | Year: 2012

This work uses an optimization procedure consisting of a simplified conjugate-gradient method and a three-dimensional fluid flow and heat transfer model to investigate the optimal geometric parameters of a double-layered microchannel heat sink (DL-MCHS). The overall thermal resistance R T is the objective function to be minimized, and the number of channels N, channel width ratio β, lower channel aspect ratio α l, and upper channel aspect ratio α u are the search variables. For a given bottom area (10 × 10 mm) and heat flux (100 W/cm 2), the optimal (minimum) thermal resistance of the double-layered microchannel heat sink is about R T = 0.12°C/m 2W. The corresponding optimal geometric parameters are N = 73, β = 0.50, α l = 3.52, and, α u = 7.21 under a total pumping power of 0.1 W. These parameters reduce the overall thermal resistance by 52.8% compared to that yielded by an initial guess (N = 112, β = 0.37, α l = 10.32, and α u = 10.93). Furthermore, the optimal thermal resistance decreases rapidly with the pumping power and then tends to approach an constant value. As the pumping power increases, the optimal values of N, α l, and α u increase, whereas the optimal β value decreases. However, increasing the pumping power further is not always cost-effective for practical heat sink designs. © 2012 Elsevier Ltd. All rights reserved.


Wang S.-H.,Research Center for Energy Technology and Strategy | Lin Y.-Y.,Research Center for Energy Technology and Strategy | Teng C.-Y.,Research Center for Energy Technology and Strategy | Chen Y.-M.,Research Center for Energy Technology and Strategy | And 5 more authors.
ACS Applied Materials and Interfaces | Year: 2016

This study reports on a high ionic-conductivity gel polymer electrolyte (GPE), which is supported by a TiO2 nanoparticle-decorated polymer framework comprising poly(acrylonitrile-co-vinyl acetate) blended with poly(methyl methacrylate), i.e., PAVM:TiO2. High conductivity GPE-PAVM:TiO2 is achieved by causing the PAVM:TiO2 polymer framework to swell in 1 M LiPF6 in carbonate solvent. Raman analysis results demonstrate that the poly(acrylonitrile) (PAN) segments and TiO2 nanoparticles strongly adsorb PF6 - anions, thereby generating 3D percolative space-charge pathways surrounding the polymer framework for Li+-ion transport. The ionic conductivity of GPE-PAVM:TiO2 is nearly 1 order of magnitude higher than that of commercial separator-supported liquid electrolyte (SLE). GPE-PAVM:TiO2 has a high Li+ transference number (0.7), indicating that most of the PF6 - anions are stationary, which suppresses PF6 - decomposition and substantially enlarges the voltage that can be applied to GPE-PAVM:TiO2 (to 6.5 V vs Li/Li+). Immobilization of PF6 - anions also leads to the formation of stable solid-electrolyte interface (SEI) layers in a full-cell graphite|electrolyte|LiFePO4 battery, which exhibits low SEI and overall resistances. The graphite|electrolyte|LiFePO4 battery delivers high capacity of 84 mAh g-1 even at 20 C and presents 90% and 71% capacity retention after 100 and 1000 charge-discharge cycles, respectively. This study demonstrates a GPE architecture comprising 3D space charge pathways for Li+ ions and suppresses anion decomposition to improve the stability and lifespan of the resulting LIBs. © 2016 American Chemical Society.


Fung K.-Z.,Research Center for Energy Technology and Strategy | Fung K.-Z.,National Cheng Kung University | Tsai S.-Y.,Research Center for Energy Technology and Strategy | Tsai S.-Y.,National Cheng Kung University | And 2 more authors.
ECS Transactions | Year: 2013

Metallic interconnects are less expensive, more conductive and able to form complex shapes than ceramic interconnects for SOFC applications. Most metallic interconnects use alloys containing chromium. At elevated temperatures, chromia scales react with cathode and degrade its performance. Manganese cobalt oxide ((Mn,Co)3O4, (MCO), with spinel structure is found to be a good material to form a protective coating for metallic interconnects. In this study, the high-temperature ferritic stainless steel Crofer 22H and nickel-chromium alloy Inconel 625 were thermally sprayed with MCO, and the oxidation, scale structure, and resistance of metallic substrates were investigated using XRD, TEM and SEM. © The Electrochemical Society.


Bangalee M.Z.I.,University of Dhaka | Bangalee M.Z.I.,National Cheng Kung University | Miau J.J.,National Cheng Kung University | Miau J.J.,Research Center for Energy Technology and Strategy | Lin S.Y.,National Cheng Kung University
International Journal of Heat and Mass Transfer | Year: 2013

For pure buoyancy driven flow in an open cavity, the boundary phenomena at the openings of the original model are self-induced, so the flow cannot be captured by imposing fixed boundary conditions. A computational fluid dynamics (CFD) method is developed to solve the buoyancy-driven flow accurately when imposing the boundary conditions at the openings of the original model is avoided. A k-ω based shear stress transport (SST) turbulence model is chosen to capture the turbulence of the flow. Two experimental cases from literature are chosen and simulated numerically, in order to validate the method used in this study. Buoyancy-driven natural ventilation through a heating passage (HP) and a window of a building is studied. The power input and the inclination angle of the HP are varied, in order to check the sensitivity of the ventilation performance to those factors. A proportional relation between the flow rate and the input power is demonstrated. The system is capable of ventilating air sufficiently and the building only contributes to the increase in the frictional force. The ventilation rate is a maximum when the HP is attached at 45. The best fitted relations between Ra, Nu, Re and Pr are also reported. © 2013 Elsevier Ltd. All rights reserved.


Lin H.-H.,National Cheng Kung University | Chen W.-H.,National Cheng Kung University | Hong F.C.-N.,National Cheng Kung University | Hong F.C.-N.,Research Center for Energy Technology and Strategy
Journal of Vacuum Science and Technology B:Nanotechnology and Microelectronics | Year: 2013

The creation of nanostructures on polycrystalline silicon wafer surface to reduce the solar reflection can enhance the solar absorption and thus increase the solar-electricity conversion efficiency of solar cells. The self-masking reactive ion etching (RIE) was studied to directly fabricate nanostructures on silicon surface without using a masking process for antireflection purpose. Reactive gases comprising chlorine (Cl2), sulfur hexafluoride (SF6), and oxygen (O2) were activated by radio-frequency plasma in an RIE system at a typical pressure of 120-130 mTorr to fabricate the nanoscale pyramids. Poly-Si wafers were etched directly without masking for 6-10 min to create surface nanostructures by varying the compositions of SF 6, Cl2, and O2 gas mixtures in the etching process. The wafers were then treated with acid (KOH:H2O = 1:1) for 1 min to remove the damage layer (100 nm) induced by dry etching. The damage layer significantly reduced the solar cell efficiencies by affecting the electrical properties of the surface layer. The light reflectivity from the surface after acid treatment could be significantly reduced to <10% for the wavelengths between 500 and 900 nm. The effects of RIE and surface treatment conditions on the surface nanostructures and the optical performance as well as the efficiencies of solar cells will be presented and discussed. The authors have successfully fabricated large-area (156 × 156 mm2) subwavelength antireflection structure on poly-Si substrates, which could improve the solar cell efficiency reproducibly up to 16.27%, higher than 15.56% using wet etching. © 2013 American Vacuum Society.

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