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N.J. Mellors X. Zhao L.M. Simmons C.J. Quinn S.H. Kilcoyne 《Journal of magnetism and magnetic materials》2012
Ribbons of Fe100−xGax (x=15, 17.5, 19.5 and 22.5) were prepared by rapid solidification from the melt. 57Fe Mössbauer spectroscopy and high resolution neutron diffraction have revealed that Fe100−xGax alloys with x=15 and 17.5 have the disordered bcc (A2) structure even after annealing, but the alloy with x=19.5 developed the short-range ordered D03 phase when annealed. The x=22.5 alloys showed mainly D03 phase with a fraction of bcc phase. A fraction of the bcc phase transformed into D03 phase and the long-range ordering of D03 phase was improved after annealing. 57Fe Mössbauer spectra showed no observable L12 phase in any samples even though less than 1% volume of L12 phases has been found in the annealed samples by neutron diffraction. The additional absorption at hyperfine field of 25 T in x=22.5 samples was regarded as a result of imperfect D03 structure, rather than L12 phase. 相似文献
34.
This article reviews the current state of research involving semiconductor quantum dots, provides a brief review of the theory behind their unique properties, and an introduction explaining the importance of quantum dot research. The characteristic shifting of the band gap energy with quantum dot size, as predicted from the density of states for low-dimensional structures, allows experimental measurements to determine the extent to which quantum confinement effects play a role in the resulting properties. A few of the current techniques used to measure the presence and physical characteristics of quantum dots and their energy levels is reviewed, including transmission electron microscopy, optical transmission, and Raman and photoluminescence spectroscopy. Finally, some of the more exciting applications for quantum dots currently being researched for use in the field of optoelectronics are reviewed, including quantum dot infrared photodetectors, quantum dot lasers, and quantum dot solar cells. Comments are made on the current progress and the future prospects of quantum dot research and device applications. 相似文献
35.
J A Simmons E G Freedman S B Stevenson L Chen T J Wohlgenant 《The Journal of the Acoustical Society of America》1989,86(4):1318-1332
The ability of the echolocating bat, Eptesicus fuscus, to detect a sonar target is affected by the presence of other targets along the same axis at slightly different ranges. If echoes from one target arrive at about the same delay as echoes from another target, clutter interference occurs and one set of echoes masks the other. Although the bat's sonar emissions and the echoes themselves are 2 to 5 ms long, echoes (of approximately equal sensation levels--around 15 dB SL) only interfere with each other if they arrive within 200 to 400 microseconds of the same arrival time. This figure is an estimate of the integration time of the bat's sonar receiver for echoes. The fine structure of the clutter-interference data reflects the reinforcement and cancellation of echoes according to their time separation. When clutter interference first occurs, the waveforms of test and cluttering echoes already overlap for much of their duration. The masking effect underlying clutter interference appears specifically due to overlap, not between raw echo waveforms, but between the patterns of mechanical excitation created when echoes pass through bandpass filters equivalent to auditory-nerve tuning curves. While the time scale of clutter interference is substantially shorter than the duration of echo waveforms, it still is much longer than the eventual width of a target's range-axis image expressed in terms of echo delay. 相似文献
36.
A method for inverting the transforms of the terms in generalized ray series representations for disturbances in layered media is presented. It differs from the Cagniard reduction in that the solution of algebraic equations depending upon position x and time t is not required. This step is, in effect, replaced by contour integration of relatively simple functions. The method is applicable to anisotropic layers but it simplifies when applied to isotropic layers, for which any term in the ray series is represented as a single contour integral, around a fixed contour, of the product of a function that embodies material properties and a simple explicit function of x and t. The ‘material function’ can be tabulated and used repeatedly when the integral is evaluated for a range of values of x and t, so that the procedure is computationally quite efficient. It is illustrated by a computation of Green's function for an isotropic half-space, either free or overlaid by a fluid. Wave-front singularities are obtained explicitly from the representation and are given in an appendix. 相似文献
37.
A five-step total synthesis of the antibiotic marinopyrrole A (1) is described. The developed synthetic technology enabled the synthesis of several marinopyrrole A analogs whose antibacterial properties against methicillin-resistant Staphylococcus aureus TCH1516 were evaluated. 相似文献
38.
We show that arylpalladium(II) reagents linked to biotin and indocyanine dye residues can be prepared by decarboxylative palladation of appropriately substituted electron-rich benzoic acid derivatives. When prepared under the conditions described, these organometallic intermediates are tolerant of air and water, can be stored for several months in solution in dimethyl sulfoxide, and permit biotin- and indocyanine dye-labeling of functionally complex olefinic substrates in water by Heck-type coupling reactions. 相似文献
39.
Simmons CJ Stratemeier H Hitchman MA Reinen D Masters VM Riley MJ 《Inorganic chemistry》2011,50(11):4900-4916
The crystal structures of trans-diaquabis(methoxyacetato)copper(II) and the isostructural nickel(II) complex have been determined over a wide temperature range. In conjunction with the reported behavior of the g-values, the structural data suggest that the copper(II) compound exhibits a thermal equilibrium between three structural forms, two having orthorhombically distorted, tetragonally elongated geometries but with the long and intermediate bonds to different atoms, and the third with a tetragonally compressed geometry. This is apparently the first reported example of a copper(II) complex undergoing an equilibrium between tetragonally elongated and compressed forms. The optical spectrum of single crystals of the copper(II) compound is used to obtain metal-ligand bonding parameters which yield the g-values of the compressed form of the complex and hence the proportions of the complex in each structural form at every temperature. When combined with estimates of the Jahn-Teller distortions of the different forms, the latter produce excellent agreement with the observed temperature dependence of the bond lengths. The behavior of an infrared combination band is consistent with such a thermal equilibrium, as is the temperature dependence of the thermal ellipsoid parameters and the XAFS. The potential surfaces of the different forms of the copper(II) complex have been calculated by a model based upon Jahn-Teller coupling. It is suggested that cooperative effects may cause the development of the population of tetragonally compressed complexes, and the crystal packing is consistent with this hypothesis, though the present model may oversimplify the diversity of structural forms present at high temperature. 相似文献
40.
Stepwise, selective DIBAL reduction of the acetonide diester derived from tartaric acid followed by the Horner-Emmons reaction effectively provided desymmetrized hydroxy mono-olefination products in a one-pot operation. 相似文献