Browsing by Author "Chen, SF"

Now showing 1 - 14 of 14

A numerical study of heat transfer of a porous block with the random porosity model in a channel flow
(2002-08-01)
Fu, WS
;
Chen, SF
;
機械工程學系
;
Department of Mechanical Engineering
In this paper, heat transfer of a hot plate with a porous block in a channel flow is numerically investigated. A porous block is simulated as a fin type heat sink. The random/artificial porosity models are used to generate the distribution of porosity. In fact, the distribution of porosity in porous medium is irregular, thus the random porosity model is more realistic than the constant or variable porosity model to describe the phenomena happening in porous medium. Therefore, the distribution of porosity of porous block obeys the random porosity model, and the factors of mean porosity and standard deviation are taken into consideration. The variations of the porosity and the velocity in porous block are no longer smooth. For obtaining more heat transfer rate, the artificial porosity model is proposed. The heat transfer rates of the several cases derived by the artificial porosity model are better than those of the random porosity model. The thermal performance of porous block is larger than that of solid block as the mean porosity is larger than 0.5.
A numerical study of the effects of a moving operator on particles in a cleanroom with a curtain
(2002-02-01)
Fu, WS
;
Chen, SF
;
Yang, SJ
;
機械工程學系
;
Department of Mechanical Engineering
The motion of airflow and particles induced by a moving operator in a cleanroom installed with a solid worktable and a vertical isolating curtain were studied numerically. This situation is classified as a kind of moving boundary problem, and a Galerkin finite element formulation with an arbitrary Lagrangian-Eulerian kinematic description method is adopted to analyze this problem. Two different moving speeds of the operator under Reynolds number Re = 500 and Schmidt number Sc = 1.0 are taken into consideration. The results show that recirculation zones, which are disadvantageous to remove the particles, are observed around the operator and near the worktable. The behavior of the operator usually prevents the particles from being removed, and the higher moving speed of the operator demonstrates that the removal of particles becomes more difficult. These phenomena are quite different from those that regard the moving operator as stationary in the cleanroom.
An explicit model for a quantum channel in 2DEG
(2002-05-01)
Fedirko, VA
;
Chen, MH
;
Chiu, CC
;
Chen, SF
;
Wu, KH
A two-parametric model for a channel in a two-dimensional electron gas (2DEG) is proposed which allows for the explicit analytical solution for the problem of quantum electron transport through the constriction. Conductance step smearing appears naturally and a simple criterion for the step occurrence in terms of channel parameters is given. (C) 2002 Elsevier Science Ltd. All rights reserved.
Characteristics of photogenerated bipolar terahertz radiation in biased photoconductive switches
(2002-10-15)
Lin, PI
;
Chen, SF
;
Wu, KH
;
Juang, JY
;
Uhn, TM
The characteristics of the optically induced bipolar terahertz (THz) radiation from biased semi-insulating GaAs photoconductive switches were investigated using a free-space electrooptic sampling technique, The emitted radiation shows a nearly symmetrical waveform with a broad-band frequency spectrum spanning over 0.1-3 THz, which displays essentially no dependence on the optical excitation fluence or strength of biased field. However, it does slightly depend on the file emitter gap spacing. The dynamics of the emitted THz transient is in agreement with the optically induced ultrafast charge transport process driven by the biased field.
Effect of moving distance of a moving block on heat transfer in a channel flow
(2001-04-01)
Fu, WS
;
Chen, SF
;
Wang, KN
;
機械工程學系
;
Department of Mechanical Engineering
A numerical investigation of heat transfer rate of an insulated block moving on a heated surface in a channel was studied. The study mainly investigated the effect of block moving distance on the heat transfer rate of heated surface. This subject belongs to a kind of moving boundary problems, and the modified Arbitrary Lagrangian Eulerian method is suitable for solving this subject. The results show that the block moving distance affects the flow and thermal fields remarkably. The heat transfer rate of heated surface increases proportionally to the increment of the block moving distance, when the block moving distance is larger than a critical value. (C) 2001 Elsevier Science Ltd.
Efficient strategy for the occasionally proportional feedback method in controlling chaos
(1999-05-01)
Yang, TH
;
Chen, SF
;
Gou, YS
;
電子物理學系
;
光電工程研究所
In this work, the generic mechanism of the occasionally proportional feedback (OPF) technique in controlling chaos has been explored extensively. Except for stabilizing the unstable states that are embedded in the chaotic attractors, the OFF method is also found to generate a great number of new states during the control processes. The forms and characteristics of these new states have been addressed. Moreover, we clarify the roles of the parameters in the OFF method and this clarification leads to a practical and systematic approach in adjusting the parameters for control. To demonstrate the validity, an analogous electronic circuit of the resistively shunted Josephson junction oscillator is employed in addition to a numerical illustration of the logistic mapping. [S1063-651X(99)15805-7].
Growth and transport properties of (110)-oriented La2/3Sr1/3MnO3 thin films on TiO2-Buffered Si (100) substrates
(2003-03-15)
Liu, SJ
;
Chen, SF
;
Juang, JY
;
Lin, JY
;
Wu, KH
The growth and transport properties of (110)-oriented La2/3Sr1/3MnO3 thin films on TiO2-buffered silicon substrates by pulsed-laser deposition are reported. An insulator-metal transition associated with a ferromagnetic transition was observed at about 360 K. Magnetic measurements showed that in-plane coercive fields are about 60 Oe and 300 Oe at 300 K and 5 K, respectively. A magnetoresistance with a Deltarho/rho(H = 0) ratio of -20% in a magnetic field of 5 T was observed not only near the insulator-metal transition temperature but also below 30 K. Moreover, a T-2- to T-3-dependence crossover of resistivity was observed around 60 K, indicating that an unconventional one-magnon scattering may have occurred.
Growth kinetics of homoepitaxial strontium titanate films by interrupted pulsed laser deposition
(2001-08-01)
Lee, JY
;
Wang, TC
;
Chen, SF
;
Juang, JY
;
Lin, JY
The effects of the initial surface state on the evolution kinetics of the surface morphology of strontium titanate (STO) films was investigated by in-situ monitoring the intensity variations of reflection high-energy electron diffraction (RHEED). In order to create various surface states prior to subsequent depositions, we intentionally interrupted the pulsed laser deposition (PLD) at various points during the whole deposition course to perform in-situ annealing over various periods of time. The depth of the initial drop of RHEED intensity, which is a direct indication of changes in surface step densities, shows two distinct time scales. The results suggest that the evolution of a growing surface, and hence the ultimate film surface morphology, may be manipulated by controlling the number density of the as-deposited growing islands through interrupted annealing.
Magnetoresistance of La0.7Ca0.3MnO3 thin film biepitaxial step junctions
(2003-08-01)
Chen, SF
;
Chang, WJ
;
Liu, SJ
;
Juang, JY
;
Lin, JY
The biepitaxial La0.7Ca0.3MnO3 (LCMO) thin films grown on SrTiO3 substrates using a buffer layer of anatase TiO2 were fabricated. The magnetoresistance (MR) of biepitaxial step junction (BSJ) across the boundary layer of biepitaxial LCMO (0 0 1) and LCMO (I 10) film was investigated. The temperature and field dependence of MR for BSJ are qualitatively similar to those obtained in other type of artificial grain boundaries with a comparable MR ratio at low temperatures. However, the observed linear current-voltage characteristic across BSJ is in sharp contrary to the commonly reported non-ohmic characteristics. The results are consistent with features predicted by the model of spin-dependent transport across a depressed magnetic ordering and metallic-like junction layer. (C) 2003 Elsevier Science B.V. All rights reserved.
Metallic percolation in La0.67Ca0.33MnO3 thin films
(2003-02-24)
Chen, SF
;
Lin, PI
;
Juang, JY
;
Uen, TM
;
Wu, KH
Phase separation in La0.67Ca0.33MnO3 thin films was investigated by scanning tunneling microscopy. The correlation between the grain structure and the spatial distribution of the coexisting metallic and insulating phases was evidently established. At temperatures not far below the metal-insulator transition, the spatial variation of the coexisting metallic and insulating phases is susceptible to magnetic field in an irreversible manner. The irreversibility suggests that the metallic percolation paths can be affected randomly by magnetic field. However, the variation becomes insensitive to magnetic field at lower temperatures. (C) 2003 American Institute of Physics.
Metalorganic chemical vapor deposition of tungsten carbonitride films by bis(tertbutylimido)bis(diethylamido)tungsten.
(2002-04-07)
Chuang, SH
;
Chiu, HT
;
Chou, YH
;
Chang, YH
;
Chen, SF
Numerical study of variations of airflow induced by a moving automatic guided vehicle in a cleanroom
(2002-01-01)
Yang, SJ
;
Chen, SF
;
Fu, WS
;
機械工程學系
;
Department of Mechanical Engineering
The variations of airflow induced by a moving automatic guided vehicle (AGV) in a vertical laminar flow cleanroom are studied numerically. From a viewpoint of fluid mechanics, the characteristic of the variations of the airflow induced by a moving object is dynamic and is classified as a moving boundary problem. A Galerkin finite element formulation with an arbitrary Lagrangian-Eulerian (ALE) kinematic description method is adopted to analyze this problem. Three different moving velocities of the AGV under Reynolds number Re=500 and two different positions of the wafer cassette are considered. The results show that the formation of recirculation zones, which are disadvantageous for removing contaminants, is remarkably dependent on the moving velocity of the AGV and the position of the wafer cassette.
On the origin of photogenerated terahertz radiation from current-biased superconducting YBa2Cu3O7-delta thin films
(2004-06-15)
Lin, PI
;
Luo, CW
;
Liu, HS
;
Chen, SF
;
Wu, KH
The origin of photogenerated terahertz radiation pulse emitted from current-biased superconducting YBa2Cu3O7-delta thin films excited by femtosecond optical laser pulses is delineated. By investigating the performance of the transient terahertz radiation generated under different operating parameters, pulse reshaping in the measured terahertz electric field caused by the kinetic inductance of the superconducting charge carriers is identified. After recovering the original wave forms of the emitted terahertz pulses, the transient supercurrent density directly correlated to the optically excited quasiparticle dynamics is obtained. A fast decreasing component of about 1.0 ps and a slower recovery process with a value of 2.5 ps are unambiguously delineated in the optically induced supercurrent modulation. The radiation mechanism of the transient terahertz pulse related to nonequilibrium superconductivity is discussed. (C) 2004 American Institute of Physics.
Superconducting NdBa2Cu3O7-delta thin films grown on bare (1(1)over-bar02) sapphire by pulsed laser deposition
(2003-05-01)
Cheng, TY
;
Lin, PI
;
Chen, SF
;
Liu, SJ
;
Juang, JY
Superconducting NdBa2Cu3O7-delta (NBCO) thin films with T-c=89 K have been. successfully grown on bare (1 (1) over bar 02) sapphire substrates by pulsed laser deposition (PLD). The X-ray diffraction results show that the as-grown NBCO films are all c-axis oriented with no observable second phase. The c-axis parameter decreases monotonically with increasing the deposition temperature (T-s), suggesting that the corresponding degradation of T-c for T-s>790degreesC might not originate from oxygen deficiency. It is conceived that the film-substrate reaction that occurring at the interface may cause Ba-deficiency and hence enhance Nd-Ba antisite substitutions.