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51.
52.
Shing Horng Lee Christina F. Jou Chien Ping Lee Cheng Chi Hu Jin Jei Wu 《International Journal of Infrared and Millimeter Waves》1998,19(1):149-163
A frequency tunable active leaky-wave scanning antenna using Gunn-diode voltage control oscillator (VCO) as source is developed. The frequency tuning controlled by changing either the varactor diode dc bias or the Gunn diode dc bias is demonstrated. The measured scanning angle of active antenna is close to 15 degree as the Gunn VCO frequency tuned from 12.58GHz to 12.98GHz. To excite the first higher order mode of the microstrip leaky-wave antenna is fed asymmetrically. The dominant mode excitation has been successfully suppressed by adding a sequence of covered wire in the middle line of the microstrip leaky wave antenna. This is a prototype of frequency scanning antenna using two terminal device, which can be easily scaled up to millimeter wave frequency region. 相似文献
53.
A red-emitting Y2(WO4)3: Eu3+ phosphor (orthorhombic high temperature phase, anhydride) is prepared by two different methods: the firing of mixtures of constituent oxides and that of precipitates from aqueous solutions. After optimizing preparation conditions, the cathodoluminescence brightness reaches 56% that of Y2O2S: Eu3+, a commercial red phosphor for color TV. Formation of a high temperature phase below the reported transition temperature is noted in the fired precipitates. This phase occurrence is shown to depend on the treatment of the precipitates to be fired. Reflection difference measurement of Eu-doped and undoped samples assigns an excitation band of about 245 nm to the Eu-O charge transfer band. Different by-products in the two preparation methods are identified by measuring emission spectra under selective excitation. Reversible hydration-dehydration of the phosphor is demonstrated by successively measuring photoluminescence first in vacuum and then in air at various temperatures. No deterioration of luminescence efficiency is observed after repeating this reversible structural change. 相似文献
54.
55.
Interaction between the nonionic surfactant Tergitol 15-S-7 and hydrophobically modified 2-hydroxyethyl cellulose (HMHEC) was studied rheologically in a semidilute regime of HMHEC. The low-shear viscosity of HMHEC was increased with addition of surfactant from 25 to 250 ppm, in which the critical micelle concentration of surfactant was near 39 ppm, and then decreased to a value smaller than that of pure HMHEC with further addition of surfactant to 1000 ppm. An interesting shear-induced phenomenon was observed. The steady-state shear measurements show that there exist crossovers between viscosity-shear rate curves of HMHEC solutions with and without surfactant added, whereas it was not observed in the HEC-surfactant systems. Moreover, added Tergitol 15-S-7 reversed the temperature effect on the viscosity of the HMHEC solution. That is, increasing temperature to or near the cloud point raises the viscosity of the HMHEC-surfactant aggregates, in contrast to the viscosity decrease in the pure HMHEC solutions. A possible mechanism based on the necklace model and the clouding phenomenon is conjecturally introduced to explain such phenomena. 相似文献
56.
Total energy calculations based on density functional theory (DFT) with generalized gradient approximation (GGA) and ultrasoft pseudopotential approximation and an analysis tool of atom‐resolved density of states (ADOS) have been used to investigate (1) the energetic profiles for the possible initial dissociative adsorption of XH4 (X?Si and Ge) onto the Si(100)? (2 × 2) surface to evaluate their reactivity and (2) the effect of surface electronic states of Si(100)? (2 × 2) on gaseous molecular precursors XH4 (X?Si and Ge) during initial dissociative adsorption to understand the factors governing their reactivity. Our calculated lower‐energy barrier for initial dissociative adsorption of GeH4 is due to the forming of stronger bond of Si? H between H within GeH4 and buckled‐down Si atom on the Si(100)? (2 × 2) surface accompanying the larger extent of unbuckling of the buckled Si?Si dimer on the Si(100)? (2 × 2) surface at the transition state. Our evaluated better reactivity for GeH4 than SiH4 (a factor of around 14.6) is slightly larger than observed higher reactivity for GeH4 than SiH4 (a factor of between 2 and 5 depending on the incident kinetic energy) employed supersonic molecular bean techniques. Finally, our calculated ADOS indicate that the surface electronic states of buckled Si?Si dimer on the Si(100)? (2 × 2) surface energetically favorably participate in the transition state during GeH4 initial dissociative adsorption to reduce the energy barrier, i.e., enhance its reactivity, in comparison with SiH4 initial dissociative adsorption onto the Si(100)? (2 × 2) surface under the same reaction conditions. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004 相似文献
57.
Wong MS Xia PF Zhang XL Lo PK Cheng YK Yeung KT Guo X Shuang S 《The Journal of organic chemistry》2005,70(7):2816-2819
[reaction: see text] A facile and efficient protocol for the synthesis of oligophenylene OPP(n)-substituted calix[4]arenes (with n up to 4) via iodo-substituted oligoarylcalix[4]arenes has been developed. The cooperation effect of the proximate fluoroionophores in hexylsulfanyl end-capped OPP(n)-substituted calix[4]arene assemblies leads to metal ion binding enhancement. 相似文献
58.
Tyrosinase was immobilized on Chitosan (CTS) beads to produce 3,4‐dihydroxy‐L‐phenylalanine (L ‐DOPA) from L ‐tyrosine. Epichlorohydrin (ECH), ethylene glycol diglycidyl ether (EGDE), and glutaraldehyde (GLU) were used as coupling agents, respectively. Ultraviolet/visible measurements on CTS films showed that the reaction intermediate (L ‐dopaquinone) attacked the amino groups on CTS, so the amine residues on chitosan were capped by acetic acid anhydride (Ac) or formaldehyde (Fm) to avoid the deactivation of the immobilized tyrosinase. The pH and temperature of the maximal rate to produce L‐DOPA were investigated. GLU (coupling agent) and Ac (capping agent) were selected for practical utility. A 7.5% (w/v) concentration of GLU was found to attain maximal activity of the immobilized enzyme. The thermal stability of tyrosinase immobilized on CTS‐GLU‐Ac, and after treatment with sodium borohydride, was enhanced to a great extent. The L ‐DOPA converting efficiency in the environmental conditions of this study decreased from 45.1% to 39.9% (between 1st and 30th batch). This immobilized tyrosinase can be used practically in the production of L‐DOPA from L‐tyrosine. 相似文献
59.
Two equations of linear type (Eqs. 10 and 17 in the text) have been derived to analyze the IR data to determine the dimerization constant consistently. Equation 10 is to be used to fit the integrated absorbances of the monomer band to obtain the molar monomer absorptivity, ?m, and dimerization constant, K; Eq. 17 is to be used to fit the integrated absorbances of the dimer bands to obtain the molar dimer absorptivity, ?d, and dimerization constant, K. Thus the same dimerization constant can be independently determined either from the monomer band or from the dimer band. The discrepancy between the two determined values provides an assessment of the consistency of determination. The monomer‐dimer self‐association of 2,2‐dimethyl‐3‐ethyl‐3‐pentanol in the solvent of carbon tetrachloride was chosen to demonstrate the utility of these two equations. 相似文献
60.
Chen SB 《The Journal of chemical physics》2005,123(7):074702
Off-lattice Monte Carlo simulations are employed to study the behavior of chain molecules confined in a long cylindrical pore under the condition of hard-body interaction. The emphasis is placed on the chain and bead distributions as well as the location dependence of the chain conformation and anisotropy. The simulation results show that the chains very near the pore surface tend to wrap around the surface in various configurations. This behavior is qualitatively similar to that of the chains near but outside a cylindrical rod. Moreover, the bead concentration near the pore surface increases with increasing surface curvature. 相似文献