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301.
Approximate expressions are discussed which are applicable for acoustic quantities in the vicinity of a plane piston source of ultrasound which radiates into an absorbing medium. A particularly useful approach for nearfield calculations combines an expression valid near the axis with another, given by Pierce [Acoustics, An Introduction to Its Physical Principles and Applications (McGraw-Hill, New York, 1981), Chap. 5], which is valid elsewhere. This approach gives reasonable accuracy at relatively low computational cost. Computed plots are presented, showing spatial distributions of the square of the pressure amplitude. Most of the plots are for a source diameter of 1.2 cm, a frequency of 3 MHz, and an absorption coefficient of 0.15 Np/cm; these are representative of conditions for medical applications of ultrasound. 相似文献
302.
Abstract In the case of small pelagic fish, it seems reasonable to consider harvest functions depending nonlinearly on fishing effort and fish stock. Indeed, empirical evidence about these fish species suggests that marginal catch does not necessarily react in a linear way neither to changes in fishing effort nor in fish stock levels. This is in contradiction with traditional fishery economic models where catch‐to‐input marginal productivities are normally assumed to be constant. While allowing for nonlinearities in both catch‐to‐effort and catch‐to‐stock parameters, this paper extends the traditional single‐stock harvesting economic model by focusing on the dependence of the stationary solutions upon the nonlinear catch‐to‐stock parameter. Thus, we analyze equilibrium responses to changes in this parameter, which in turn may be triggered either by climatic or technological change. Given the focus in this study on the case of small pelagic fish, the analysis considers positive but small values for the catch‐to‐stock parameter. 相似文献
303.
304.
A.C. conductivity measurements on a number of lithium salt-poly(ethylene oxide) (PEO) complex polymer electrolytes have been correlated with the results of D.S.C. analysis. An enhancement in the conductivity of compositions with O:Li ratios of greater than 6:1 on annealing at above 150°C was attributed to the melting of the polymer crystalline phase and the retention of an amorphous polymer structure in the electrolyte on cooling down to the crystallisation temperature of the pure PEO phase. 相似文献
305.
Unsuccessful attempts were made to prepare LixMo6S7.7 by direct ionic exchange of Cu2Mo6S7.7 using aqueous, acetontrille, and molten salt solvents. Li2.3Mo6S7.7 was eventually prepared by lithiation of Mo6S8 using n-butyl lithium reagent. The mass transport properties of hot-pressed samples were measured using a four-point dc technique and the following values at 415°C for the lithium ionic conductivity, .4×10−4 (ohms cm−1) and lithium chemical diffusion coefficient 3.5×10−7 cm2/s, were derived. A possible explanation for the relatively low lithium ion mobility in Li2.3Mo6S7.7 is discussed. 相似文献
306.
E. Bagan M. R. Ahmady V. Elias T. G. Steele 《Zeitschrift fur Physik C Particles and Fields》1994,61(1):157-170
We compare plane-wave, coordinate-space and moment methods for evaluating operator-product expansion (OPE) coefficients of the light-quark and gluon condensates. Equivalence of these methods for quark condensate contributions is proven to all orders in the quark mass parameterm. The three methods are also shown to yield equivalent gluon condensate contributions to two-current correlation functions, regardless of the gauge chosen for external gluon fields in the coordinate space approach. An improved method for evaluating quarkcondensate OPE coefficients is presented for several (two-current) correlation functions. Gauge-dependent Green functions are also discussed. It is shown that contradictory expressions for the gluon-condensate contribution to the quark propagator occurring from the plane-wave and coordinate-space approaches yield identical relations between the heavy-quark and gluon condensates, as anticipated from the gauge invariance of the heavy-quark expansion. 相似文献
307.
Laser desorption-atmospheric pressure chemical ionization-mass spectrometry (LD-APCI-MS) is presented for the atmospheric pressure (AP) sampling of tryptic peptides directly from a polyacrylamide gel. In contrast to other gel sampling mass spectrometric approaches, this technique does not require the addition of any exogenous matrices to the gel to assist with ionization. In this arrangement, a CO(2) laser at 10.6 micro m is used to desorb intact neutral peptide molecules from the gel, followed by ionization in the gas-phase with APCI. The ions are then sampled via a heated capillary inlet and transferred to a quadrupole ion trap mass spectrometer for mass analysis. Preliminary results suggest the polyacrylamide gel electrophoresis-LD-APCI-MS technique provides several advantages that could translate into a more convenient, robust methodology for the rapid identification and characterization of proteins. Finally, strategies regarding the further development of the method are presented. 相似文献
308.
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310.
Aggarwal VK Steele RM Ritmaleni R Barrell JK Grayson I 《The Journal of organic chemistry》2003,68(10):4087-4090
Ketene dithioacetals undergo a Sharpless-type asymmetric oxidation using (+)-DET, Ti(O(i)()Pr)(4), and cumene hydroperoxide to give the trans bis-sulfoxides 4a-f with essentially complete control of enantioselectivity and diastereoselectivity. The high enantioselectivity is a consequence of carrying out two asymmetric processes on the same substrate. However, this should lead to the formation of a small amount of the meso isomer but none was isolated. From monitoring the enantioselectivity of the monoxide over time, it was concluded that small amounts of the meso isomer must be formed. The inability to isolate this compound could be because it acted as a ligand on titanium and remained tightly bound even upon workup. 相似文献