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931.
932.
Fe2+/Fe3+ ratio versus sediment depth values have been determined for ten lakes in British Columbia. The values often reflect the local topography of the lakes, the weathering history of the deposited minerals, and the diagenesis of the minerals after deposition in the lake. 相似文献
933.
Room temperature transmission Mössbauer spectra of Fe62Ni16B14Si8 ribbons, annealed in vacuum for time periods ranging from 5 to 60 minutes at the crystallization temperatureT
x=720 K, have been used to investigate the crystallization mechanism by isothermal annealing. Reorientation of the magnetic anisotropy almost normal to the ribbon plane was observed and correlated to the annealing time dependence of the mean hyperfine field (HF) of the amorphous component. Crystallization started at the surface before the bulk and was found to occur in two steps: a metastable equilibrium of the amorphous phase with (FeNi), (FeNi)Si and t-(FeNi)3B, followed by the decomposition of t-(FeNi)3B into t-(FeNi)2B and (FeNi). The increase of the mean magnetic moment at the Fe sites as compared to related iron-based alloys was attributed to compositional small Ni additions. 相似文献
934.
Summary Samples of synthetic leucite and boron-substituted leucite are investigated by infrared spectroscopy and spectrally resolved
thermoluminescence. Evidence is obtained in favour of the assumption that point defects in aluminosilicate lattices are originated
by exchanges of Si+4 and Al+3 ions lying in different cells. 相似文献
935.
S. A. Chernyak O. V. Shekhter N. L. Sergovskaya Yu. S. Tsizin 《Chemistry of Heterocyclic Compounds》1992,28(7):832-834
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 994–996, July, 1992. 相似文献
936.
Brush S.G. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1992,20(6):577-589
Hannes Alfven's research program in solar system physics provides a valuable case study for the historians and philosophers of science. The response of the scientific community to his remarkable contributions is a severe test of widely held views about how science is or should be done. Seven of those contributions are examined, in the light of two doctrines of scientific methodology, those of Karl Popper (1959) and Imre Lakatos (1978). The seven are: application of the magnetic braking concept to the origin of the solar system; magnetohydrodynamic waves; field-aligned (`Birkeland') currents; critical ionization velocity and the existence of planetary rings; jet streams; electrostatic double layers; and partial corotation 相似文献
937.
Dumbrajs O. Nuisnovich G.S. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1992,20(4):452-457
A theory is presented which describes the dependence of the multimode operation of a gyrotron on the modulation and beam voltages. Nonlinear nonstationary processes are considered. An approximate start-up scenario of a gyrotron is determined which makes it possible to achieve single-mode excitation and high-efficiency operation in a desired mode, as well as to switch the tube to a similar operation in other modes 相似文献
938.
C. Caldera N. Caponio C. de Bernardi F. Delpiano G. Destefanis S. Morasca M. Puleo G. Schiavini F. Schiattone A. Stano E. Legros M. Billard 《Optical and Quantum Electronics》1994,26(5):S529-S545
Optical waveguide 3-dB couplers integrated on semicondutors have been studied, designed and fabricated, using both bulk and diluted multi-quantum-well InGaAsP/InP-based materials, grown by MOCVD. The device structure is based on the two-mode interference (TMI) principle and is fully compatible for integration with the optoelectronic and electronic components of a coherent receiver. Bulk material couplers provide an output balanced within 0.05 dB per nanometer and an excess loss of 1.4 dB, compared to a straight guide, while coupling loss to a tapered-lensed single mode fibre is 4 dB.Improved coupling efficiency to single-mode fibres is achieved by use of moderately diluted multi-quantum-well waveguides, which include InGaAsP wells and InP barriers: coupling loss to a tapered-lensed single-mode fibre as low as 0.5 dB and excess loss of 1.8 dB are featured. Couplers fabricated with this waveguide structure have a balance sensitivity of 0.03 to 0.04 dB per nanometer.A moderately diluted multi-quantum-well 3-dB coupler has been permanently pigtailed and butt-coupled to a dual balanced PIN photoreceiver. This hybrid assembly was tested in a coherent transmission system at 155 and 622 Mbits-1 showing sensitivities, for 10-9 BER, of about-38.0 dBm and-28.8 dBm, respectively. 相似文献
939.
J. P. S. Farinha J. M. G. Martinho H. Xu M. A. Winnik R. P. Quirk 《Journal of Polymer Science.Polymer Physics》1994,32(9):1635-1642
Cyclization of a polystyrene chain (Mn = 10,600; Mw/Mn = 1.09) both ends labeled with 4-(1-pyrenyl)butanoamide groups was studied in cyclohexane between 25 and 95°C. The amide groups (peptide bonds) at both ends can form an intrachain hydrogen bond between the amide hydrogen at one chain end and the carbonyl oxygen at the other. The presence of two sets of conformers, random coils, and chains cyclized through hydrogen bonding, complicates the data analysis. The pyrene excimer kinetics of this polymer is well described by a model composed of two monomers (hydrogen bonded and nonbonded chains) and one excimer, in equilibrium. The cyclization rate constant for hydrogen-bonded chains is larger than the one for nonhydrogen-bonded chains. The pyrene excimer binding energy (ca. 1.6 kcal/mol) is lower than the published value for nonhydrogen-bonded chains (~ 9 kcal/mol), suggesting that intrachain hydrogen bonding hinders the stabilization of the excimer. © 1994 John Wiley & Sons, Inc. 相似文献
940.
This note presents a counter-example, to demonstrate that thebasis for the proposed solutions to the nonlinear quadraticregulator problem given in the paper Stabilization andoptimal control for nonlinear systems is not true ingeneral. 相似文献