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排序方式: 共有1319条查询结果,搜索用时 15 毫秒
91.
Seiji Shinohara Jong-Gab Baek Tetsuhiko Isobe Mamoru Senna 《Journal of the American Ceramic Society》2000,83(12):3208-3210
Phase-pure perovskite Pb(Zn x Mg1– x )1/3 Nb2/3 O3 solid solution (PZ x M1– x N) is obtained for x ≦ 0.7 by heating a milled stoichiometric mixture of PbO, Mg(OH)2 , Nb2 O5 , and 2ZnCO3 ·3Zn(OH)2 ·H2 O at 1100°C for 1 h. Percent perovskite ( f P ) with respect to total crystalline phase decreases with increasing temperature of subsequent heating then increases to 900°C for the mixtures where x ≦ 0.8 and milled for 3 h. For mixtures with x = 0.9 and x = 1, f P decreases monotonically. Curie temperature increases almost linearly with increasing x up to x = 0.7. The maximum dielectric constant at 1 kHz is 2×104 and 1.7×104 for the mixture with x = 0.4 and x = 0.7, respectively. The stabilization mechanism of strained perovskite is discussed. 相似文献
92.
We recently described an improved method to synthesize poly(ether-ketones) or PEKs. It utilized an optimized mixture of polyphosphoric acid (PPA) and P2O5 that is not only milder, less corrosive and less expensive than super-acid media, but also can play the multiple roles of solvent, Friedel-Crafts catalyst and dehydrating agent. The as-prepared PEKs from such a highly viscous reaction medium displayed unexpected, thermally induced relaxation exotherms regardless of the amorphous or semi-crystalline nature of polymer. This thermal behavior was not observed for the formally identical polymers [viz. pPEK or poly(oxy-1,4-phenylenecarbonyl-1,4-phenylene] which is normally semi-crystalline) that were separately prepared in a much less viscous mixture of methanesulfonic acid and P2O5 (Eaton's reagent) or N-methyl-2-pyrrolidinone (NMP). Such an observation was first made when both samples of pPEKs were subjected to the same thermal history and the thermal relaxation exotherms were observed only for the pPEK sample that was prepared in PPA/P2O5 medium. Further confirmation of viscosity-and-shear-induced strain stored in the as-synthesized PEKs was provided by a systematic annealing study with differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) analysis and Fourier-transform infrared spectroscopy (FT-IR). Based on these data, pPEK was successfully compression-molded in the temperature range of 230-250 °C which is more than 100 °C below its melting temperature. 相似文献
93.
Lubrication for Micro Forming of Ultra Thin Metal Foil 总被引:1,自引:0,他引:1
The objective of the present investigation is to establish lubrication technique in micro forming operation. Conventional lubricant, in general, cannot be applied directly to micro forming process, since the lubricant thickness is similar to the dimensions of feature size in micro forming operations. In present investigation, the Octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) was chosen as a potential lubricant because it sticks to Si wafer die and can be applied to surface with uniform thickness of several nanometer thick. Friction coefficient and adhesion force were measured for OTS SAM coated wafer. It is shown that OTS SAM coated Si wafer offers substantially lower friction and lower adhesion than bare Si wafer. Micro channel wafer die was coated with OTS SAM and channel forming operation with 1μm thick copper foil was carried out. Experimental results show that OTS SAM coated wafer die improves forming characteristic of ultra thin copper foil substantially. These results show that SAM coating can be used as a lubricant in micro forming successfully. 相似文献
94.
Seok-Heum Baek Seok-Swoo Cho Hyun-Su Kim Won-Sik Joo 《Journal of Mechanical Science and Technology》2006,20(3):366-375
In this paper, it is intended to introduce a method to solve multi-objective optimization problems and to evaluate its performance.
In order to verify the performance of this method it is applied for a vertical roller mill for Portland cement. A design process
is defined with the compromise decision support problem concept and a design process consists of two steps : the design of
experiments and mathematical programming. In this process, a designer decides an object that the objective function is going
to pursuit and a non-linear optimization is performed composing objective constraints with practical constraints. In this
method, response surfaces are used to model objectives (stress, deflection and weight) and the optimization is performed for
each of the objectives while handling the remaining ones as constraints. The response surfaces are constructed using orthogonal
polynomials, and orthogonal array as design of experiment, with analysis of variance for variable selection. In addition,
it establishes the relative influence of the design variables in the objectives variability. The constrained optimization
problems are solved using sequential quadratic programming. From the results, it is found that the method in this paper is
a very effective and powerful for the multi-objective optimization of various practical design problems. It provides, moreover,
a reference of design to judge the amount of excess or shortage from the final object. 相似文献
95.
Semi-blind channel estimation and PAR reduction for MIMO-OFDM system with multiple antennas 总被引:1,自引:0,他引:1
Myung-Sun Baek Mi-Jeong Kim Young-Hwan You Hyoung-Kyu Song 《Broadcasting, IEEE Transactions on》2004,50(4):414-424
A combining of MIMO signal processing with OFDM is regarded as a promising solution of enhancing the performance of next generation wireless system. Therefore, in this paper, an OFDM-based wireless system employing layered space-time architecture is considered for a high-rate transmission. With an emphasis on a preamble design for multi-channel separation, we address a channel estimation based on the time-domain windowing and its imperfectness in MIMO-OFDM system. By properly designing each preamble for multiple antennas to be orthogonal in the time domain, the channel estimation can be executed based on semi-blind processing in the case of more than two transmitting antennas. And also we evaluate the PAR performance in the MIMO-OFDM system using the SLM and PTS approaches. The investigated SLM and PTS schemes for MIMO-OFDM signals select the transmitted sequence with lowest average PAR over all transmitting antennas and retrieve the side information very accurately at the expense of a slight degradation of the PAR performance. The low probability of false side information can improve the overall detection performance of the MIMO-OFDM system with erroneous side information compared to the ordinary SLM and PTS approaches, respectively. Also, we provide closed form of the average BER performance in MIMO-OFDM system using analytic approach. 相似文献
96.
N.S. Baek Y.H. Kim S.‐G. Roh B.K. Kwak H.K. Kim 《Advanced functional materials》2006,16(14):1873-1882
A series of inert and photostable encapsulated lanthanide(III) complexes—based on dendritic anthracene ligands—is shown for the first time to exhibit strong near‐IR emission bands via efficient energy transfer from the excited states of the peripheral antenna to the Ln3+ ions (Er3+, Yb3+, and Nd3+). A significant decrease in the fluorescence of the anthracene ligand is accompanied by a strong increase in the near‐IR emission of the Ln3+ ions. The near‐IR emission intensities of Ln3+ ions in the encapsulated Ln3+–dendrimer complexes are dramatically enhanced on increasing the generation number (n) of dendrons, owing to site‐isolation and light‐harvesting effects. Furthermore, a first attempt is made to distinguish between the site‐isolation and light‐harvesting effects in the present complexes. Photophysical studies indicate the sensitization of Ln3+ luminescence by energy transfer through the excited singlet state of the anthracene ligands, and the energy‐transfer efficiency between the dendritic anthracene ligands and the Ln3+ ion is evaluated to be in the range of 90 to 97 %. Their energy‐transfer efficiency is in good agreement with the result that the biexponential decays contain a radiative decay of anthracene units (< ca. 10 %) and an energy‐transfer component (> ca. 90 %) from the excited state of anthracene ligands to the Ln3+ ions. Time‐resolved luminescence spectra show monoexponential decays with a lifetime of 2 μs for the Er3+ ion 11 μs for the Yb3+ ion and 0.7 μs for the Nd3+ ion in thin films, and calculated intrinsic quantum yields of the Ln3+ ions are in the range of ca. 0.025 to 0.55 %. 相似文献
97.
Estimation of the Longitudinal Dispersion Coefficient Using the Velocity Profile in Natural Streams 总被引:2,自引:0,他引:2
In this study, a theoretical method for predicting the longitudinal dispersion coefficient is developed based on the transverse velocity distribution in natural streams. Equations of the transverse velocity profile for irregular cross sections of the natural streams are analyzed. Among the velocity profile equations tested in this study, the beta distribution equation, which is a probability density function, is considered to be the most appropriate model for explaining the complex behavior of the transverse velocity structure of irregular natural streams. The new equation for the longitudinal dispersion coefficient that is based on the beta function for the transverse velocity profile is developed. A comparison of the proposed equation with existing equations and the observed longitudinal dispersion coefficient reveals that the proposed equation shows better agreement with the observed data compared to other existing equations. 相似文献
98.
Proposed is an all-fibre retro-reflector realised via shaping a fibre end into a right-angled cone for a pump reflector in cladding-pumped fibre lasers. The conical-shaped fibre-optic retro-reflector allows folded, total internal reflections for pump beams that propagate in the inner cladding of a double-clad fibre. The proposed scheme was applied to a cladding-pumped ytterbium-doped fibre laser and readily achieved over 55% of reflection for the unabsorbed pump throughput, thereby generating 29% more signal output power than without the pump retro-reflector 相似文献
99.
Won-Ju Cho Chang-Geun Ahn Kiju Im Jong-Heon Yang Jihun Oh In-Bok Baek Seongjae Lee 《Electron Device Letters, IEEE》2004,25(6):366-368
A plasma-doping technique for fabricating nanoscale silicon-on-insulator (SOI) MOSFETs has been investigated. The source/drain (S/D) extensions of the tri-gate structure SOI n-MOSFETs were formed by using an elevated temperature plasma-doping method. Even though the activation annealing after plasma doping was excluded to minimize the diffusion of dopants, which resulted in a laterally abrupt S/D junction, we obtained a low sheet resistance of 920 /spl Omega///spl square/ by the elevated temperature plasma doping of 527 /spl deg/C. A tri-gate structure silicon-on-insulator n-MOSFET with a gate length of 50 nm was successfully fabricated and revealed suppressed short-channel effects. 相似文献
100.
Young-Hee Cho Jeong-Jun Lee Il-Bum Park Chul-Sung Huh Young-Jin Baek Jiyong Park 《Journal of food science》2005,70(2):E148-E151
ABSTRACT: Two different emulsification methods involving multiple emulsification and heat gelation were used for preparation of whey protein-based microcapsules containing immunoglobulin in yolk (IgY). The residual activity of IgY during the emulsion preparation and the effects of microencapsulation on IgY stability under harsh conditions were investigated. The residual activity of IgY in an emulsion prepared with a membrane emulsifier was higher than for an emulsion using a homogenizer. Microencapsulated IgY showed remarkable stability against both pepsin and acid. Both microencapsulated IgY and nonencapsulated IgY were relatively stable in bile and artificial intestinal juice. Microencapsulated IgY retained 74% of initial activity during heat treatment. There were no significant differences in the residual activities of microencapsulated IgY under storage temperatures of 4, 25, and 37°C. 相似文献