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951.
Thermoelectric power (TEP) of high-T
C superconductors has been investigated in a wide range of temperature (T
C < T < 700 K) for La2–x
Sr
x
CuO4. TEP of La2–x
Sr
x
CuO4 shows different temperature dependences in three temperature regions. In the low-temperature region, a positive broad TEP peak is observed near T
p, which shifts to lower temperature upon doping. As temperature increases, TEP decreases linearly at intermediate temperature. In the high-temperature region, TEP deviates from the linear temperature dependence at a certain temperature, T
h showing a saturation behavior. As the doping concentration increases, the characteristic temperatures, T
C, T
p, and T
h, show systematic changes. In comparison with pseudogap temperature estimated from other experiments, the large pseudogap behavior in TEP at high temperature has been discussed and distinguished from the small pseudogap observed at lower temperature. A possibility of bound pairs formation in the normal state opening the pseudogap at high temperature is discussed briefly. The coexistence of bound pairs and the normal independent carriers for T
C < T < Th could be the origin of the intrinsic inhomogeneity. 相似文献
952.
Hak Joon Lee Sung Hong Hahn Eui Jung Kim Yong Zoo You 《Journal of Materials Science》2004,39(11):3683-3688
We prepared TiO2-SiO2 thin films with various TiO2/SiO2 ratios by sol-gel dip coating method and explored the dependence of their structural and optical properties on calcination temperature. The absorption peaks relevant to Si—O, Si—O—Ti and Ti—O bonds appeared in the FTIR spectra. With increasing TiO2 content, the intensity of Si—O bond peaks decreases and that of Ti—O bond peaks increases. The XRD results show that the temperature of transformation from amorphous to anatase phase is lowered as TiO2 content increases. The crystallite size of anatase phase in composite thin films increases with increasing TiO2 content and calcination temperature. At 1000°C, the mixed phase of anatase and rutile appears in the pure TiO2 thin films. The rutile films are denser than the anatase films. The increase in refractive index of composite thin films with calcination temperature is related to the decreased thickness and increased density as a result of evaporation of water and organic matters below 400°C. On the other hand, it is related to the change in the crystal phase and crystallite size of the films over 400°C. 相似文献
953.
Yun-jae Kim Ki-hyun Chung Jin-su Kim Young-jin Kim 《International Journal of Fracture》2004,130(4):803-825
Based on detailed two-dimensional (2-D) and three-dimensional (3-D) finite element (FE) analyses, this paper attempts to quantify in-plane and out-of-plane constraint effects on elastic-plastic J and crack tip stresses for a plate with a through-thickness crack and semi-elliptical surface crack under positive biaxial loading. For the plate with a through-thickness crack, plate thickness and relative crack length are systematically varied, whereas for the plate with a semi-elliptical surface crack, the relative crack depth and aspect ratio of the semi-elliptical crack are systematically varied. It is found that the reference stress based approach for uniaxial loading can be applied to estimate J under biaxial loading, provided that the limit load specific to biaxial loading is used, implying that quantification of the biaxiality effect on the limit load is important. Investigation on the effect of biaxiality on the limit load suggests that for relatively thin plates with small cracks, in particular with semi-elliptical surface cracks, the effect of biaxiality on the limit load can be neglected for positive biaxial loading, and thus elastic-plastic J for a biaxially loaded plate could be estimated, assuming that such plate is subject to uniaxial load. Regarding the effect of biaxiality on crack tip stress triaxiality, it is found that such effect is more pronounced for a thicker plate. For plates with semi-elliptical surface cracks, the crack aspect ratio is found to be more important than the relative crack depth, and the effect of biaxiality on crack tip stress triaxiality is found to be more pronounced near the surface points along the crack front. 相似文献
954.
Hyo-Uk Kim Chung Yi Shi-Woo Rhee 《Journal of Materials Science: Materials in Electronics》2004,15(1):37-41
The effect of O2 plasma pretreatment on the SiO2/Si interface property was studied using direct plasma varying the plasma power, He or Ar/O2 ratio and the pretreatment time. The decrease of the pretreatment plasma power decreased the plasma damage and improved the interface property. The addition of He in O2 glow discharge improved the electrical and the interface properties and there was an optimum He/O2 ratio. The improvement of the interface property by Ar/O2-plasma pretreatment was better than that by He/O2, which is believed to be due to the lower oxidation rate of the Si surface. C–V analysis showed that the Pb center defect density was influenced by plasma pretreatment process parameters. To investigate the oxidation states near to and at the SiO2/Si interface, X-ray photoelectron spectroscopy depth analysis was used and the gas phase in the glow discharge was investigated using optical emission spectroscopy analysis at various experimental conditions. 相似文献
955.
Won-Jun Lee Sun-Jae Kim Wonhee Lee Yoon-Jik Lee Youn-Seoung Lee Sa-Kyun Rha Chong Ook Park 《Journal of Materials Science: Materials in Electronics》2004,15(1):9-13
Ti films prepared by ionized physical vapor deposition (I-PVD) and TiN films prepared by metalorganic chemical vapor deposition (MOCVD) were examined as the underlayers of the Al interconnect films. The crystallographic texture of the Al films and the sheet resistance of the thin-film stacks were investigated at various thicknesses of the Ti or TiN thin film. The sheet resistance of the thin-film stacks was also measured after annealing at 400 °C in an N2 ambient. For the I-PVD Ti underlayer, the excellent texture of the Al (1 1 1) was obtained even on a 5-nm thick Ti film. However, the sheet resistance of the multilayer structure increased after the annealing due to the reaction between Al and Ti. MOCVD TiN layers between the Ti film and the Al film could suppress the Al–Ti reaction without severe degradation of the Al (1 1 1) texture. Excellent texture of the Al film was obtained with thin MOCVD TiN films below 5 nm. 相似文献
956.
957.
Dae-Gon Kim Chang-Youl Lee Seung-Boo Jung 《Journal of Materials Science: Materials in Electronics》2004,15(2):95-98
The growth kinetics of intermetallic compound layers formed between pure indium solder and bare Cu substrate by solid-state isothermal aging were examined at temperatures between 343 and 393 K for 0–4×106 s. A quantitative analysis of the intermetallic compound layer thickness as a function of time and temperature was performed. Experimental results showed that the Cu11In9 intermetallic compound was observed for bare copper substrate. Additionally, the thickness of the Cu11In9 intermetallic compound was increased with the aging temperature and time. The layer growth of the intermetallic compound in the couple of the In/Cu system followed a parabolic law over the given temperature range. As a whole, because the values of time exponent (n) were approximately 0.5, the layer growth of the intermetallic compound was mainly controlled by a diffusion mechanism over the temperature range studied. The apparent activation energy of Cu11In9 intermetallic compound in the couple of the In/Cu was 34.16 kJ mol–1. 相似文献
958.
Maria Q. Feng Doo Kie Kim Jin-Hak Yi Yangbo Chen 《Canadian Metallurgical Quarterly》2004,130(5):562-569
A baseline model is essential for long-term structural performance monitoring and evaluation. This study represents the first effort in applying a neural network-based system identification technique to establish and update a baseline finite element model of an instrumented highway bridge based on the measurement of its traffic-induced vibrations. The neural network approach is particularly effective in dealing with measurement of a large-scale structure by a limited number of sensors. In this study, sensor systems were installed on two highway bridges and extensive vibration data were collected, based on which modal parameters including natural frequencies and mode shapes of the bridges were extracted using the frequency domain decomposition method as well as the conventional peak picking method. Then an innovative neural network is designed with the input being the modal parameters and the output being the structural parameters of a three-dimensional finite element model of the bridge such as the mass and stiffness elements. After extensively training and testing through finite element analysis, the neural network became capable to identify, with a high level of accuracy, the structural parameter values based on the measured modal parameters, and thus the finite element model of the bridge was successfully updated to a baseline. The neural network developed in this study can be used for future baseline updates as the bridge being monitored periodically over its lifetime. 相似文献
959.
Wave Propagation and Mode Localization in Simply Supported Multispan Beams with Couplers on Supports
Dong-Ok Kim Byoung-Wan Kim Jong-Heon Lee In-Won Lee 《Canadian Metallurgical Quarterly》2004,130(7):747-752
The influences of couplers on wave propagation and mode localization in simply supported multispan beams with couplers consisting of lumped rotational stiffness and mass on supports are studied. A transfer matrix equation governing the vibrational wave propagation in the simply supported multispan beams with couplers is newly derived and simplified. The eigenvalue of the simplified transfer matrix shows that the larger stiffness or the larger mass of the coupler makes the internal coupling between spans weaker and so it makes the system more sensitive to mode localization. As the wave frequency or the eigenvalues of the system increases, the mass effect is increased while the stiffness effect is decreased. In a case considering the large stiffness and mass at the same time, there is a region with relatively wider passbands and narrower stop bands having small attenuation rates and the normal modes in it become delocalized ones. As an example structure, a simply supported two span beam with a coupler at the midspan is considered. 相似文献
960.
Ho-Kyung Kim Masanobu Shinozuka Sung-Pil Chang 《Canadian Metallurgical Quarterly》2004,130(7):848-857
A geometrically nonlinear buffeting analysis of a cable-stayed bridge in the time domain is described. The bridge structure is modeled with three-dimensional thin-walled beam elements and three-dimensional elastic catenary cable elements. Spatially correlated wind velocity fluctuations are modeled and simulated using an algorithm for generating sample functions of a stationary, multivariate stochastic process according to its prescribed cross-spectral density matrix. Aerodynamic damping and aerodynamic stiffness are formulated based on experimentally determined flutter derivatives. The focus of this paper is on the effect of fluctuating components of the spatially correlated wind velocity on the geometrically nonlinear buffeting response for an 870 m cable-stayed bridge. 相似文献