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21.
Derivation of the elementary Fowler-Nordheim equation is based on several strong physical assumptions (e.g. smooth flat surface and uniform work function across emitting surface, constant emission area and a uniform distribution of the applied field over it). A real emitter, however, definitely does not fulfill these assumptions. In spite of it the total emission current follows the FN equation and is frequently used for an estimation of the "average" work-function. The physical basis of this procedure and of the terms "average" work function and "emission area" are analyzed from the experimentalist's point of view. Thus some of the older - more or less intuitive - conclusions are substantiated. 相似文献
22.
Acousto-optic (AO) imaging is a promising technique that is able to reveal optical properties in the millimeter range inside scattering media by tagging the photon paths with an ultrasonic beam. To increase both the contrast and the resolution of the AO images, we have explored the possibility of using the nonlinear response of the speckle modulation. Variation of the second-harmonic signal as the square of the ultrasonic amplitude has been found, and strong reduction of the tagged zone size has been demonstrated. 相似文献
23.
A comparison and evaluation of the various methods for reducing spectroscopic data to spectroscopic constants or term values is made with the aid of an analysis of a number of “synthetic” Σ-Σ bands generated from fixed sets of constants with random noise superimposed on the line positions. It is shown that the strong correlations that exist between the upper-state constants B′, D′ and the lower-state constants B0, D0 can be effectively broken up by using the difference constants ΔB = B′ - B0 and ΔD = D′ - D0, along with ν0, for representing the upper states. The lower state constants B0 and D0 and their standard errors calculated from the combination differences Δ2F″(J) are shown to be as good as those obtained from direct polynomial fits. If data for a number of bands originating in the same lower state are available, a considerable increase in accuracy of estimating the lower-state constants can be attained by analyzing the bands simultaneously, e.g., using combination differences, provided the data are free from systematic errors. The dependence of the accuracy of determining the constants B0, D0, ν0, ΔB, and ΔD on the extent of the band analyzed was investigated by varying the minimum and maximum J-values. The plots showing this dependence for both the actual errors and standard errors can be used e.g., to assess the band size necessary to attain a desired accuracy for a given constant.Åslund's term-value method is cast in a form which permits simple derivation of explicit formulas for the correlation coefficients connecting all the upper- and lower-state term values, and of explicit relations between the term values and the combination differences. Modifications of the term-value method suitable for the case where one or more transitions originate in an unperturbed state are explored. It is shown that the “difference term values” referred to the lower (unperturbed) state with the same value of J, Ti(J)-T0(J), are essentially uncorrelated to the lower state constants B0 and D0. Since these quantities can be expressed directly as the eigenvalues of the energy matrix for a given J in the presence of perturbations, their use for representing the energies of perturbed rovibronic states is recommended. 相似文献
24.
Martinelli LA Gat JR de Camargo PB Lara LL Ometto JP 《Isotopes in environmental and health studies》2004,40(1):45-56
The stable isotope content of samples of precipitation and of the river water throughout the Piracicaba basin in Brazil was measured over a two-year period. The isotope values of precipitation follow a consistent pattern of relatively depleted values of both deuterium and oxygen 18 during the rainy summers and enriched ones during the dry winters, with all values aligned slightly above the Global Meteoric Water Line. The isotopic composition of the river water throughout the basin shows a remarkable spatial coherence and much smaller scatter of data than those of the precipitation. The isotope composition of river water is close to that of the precipitation in the rainy season, however, with a consistent lower d-excess value by 1/1000-2/1000. This is attributed to evaporative water loss in the basin, in part an expression of the recycling of water due to the anthropogenic activity in the region. The more divergent values are recorded during high-water stages in the rivers. In many cases, the floods during the beginning of the rainy season are characterized by an enrichment of the heavy isotopes and lower d-excess values when compared to the precipitation, with the opposite situation later in the rainy season. This is interpreted as resulting from the watershed/riverflow interaction pattern, and it thus suggests that the isotope composition can monitor the hydrologic situation in the basin and its changes. 相似文献
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E. Cherbuliez Pl. Plattner S. Ariel R. Wahl Muhlemann E. Abderhalden P. Karrer W. Karrer H. Thomann E. Horlacher W. Mäder S. L. Jodidi V. Henriques S. P. L. Sörensen J. Tillmans P. Hirsch F. Stoppel L. Michaelis H. Bauer E. Strauss C. J. Weber J. Froideveaux 《Analytical and bioanalytical chemistry》1932,87(3-4):142-147
28.
D. O'Sullivan H. Oestermann P. S. Macmahon L. N. Srivastava J. R. Stubbs N. Mainsbrecq Vivario Stainier M. Schnetka Th. Forgács M. F. Bengen E. Bohm W. Plücker M. Klimmer H. Haupt F. Borchers A. I. Burstein F. S. Frum Orla-Jensen M. O. Winther H. Damen H. A. Sirks Th. v. Fellenberg 《Analytical and bioanalytical chemistry》1936,104(11-12):453-458
29.
Plínio C. Sathler Charles S. Craik Toshihiko Takeuchi Russolina B. Zingali Helena C. Castro 《Applied biochemistry and biotechnology》2010,160(8):2355-2365
Ecotin is a bidentate, fold-specific inhibitor of mammalian serine-proteases produced by Escherichia coli. This molecule may be engineered to increase and/or change its affinity and specificity providing significant biotechnological
potential. Since ecotin binds tightly to serine proteases of the trypsin fold, it may help to identify the role of these enzymes
in different biological processes. In this work, we tested ecotin variants as an affinity purification reagent for identifying
enzymes in samples of tumor progression and mammary gland involution. Initially, we used a commercial source of urokinase-type
plasminogen activator (u-PA) that remained fully active after elution from an affinity column of the ecotin variant (M84R,
M85R). We then successfully identified u-PA from more complex mixtures including lysates from a prostate cancer cell line
and involuting mouse mammary glands. Interestingly, a membrane-type serine protease 1 was isolated from the Triton X-100-solubilized
PC-3 cell lysates, and surprisingly, haptoglobin, a serine-protease homolog protein, was also identified in mammary gland
lysates and in blood. Haptoglobin does not prevent ecotin inhibition of u-PA, but it may act as a carrier within blood when
ecotin is used in vivo. Finally, this affinity purification matrix was also able to identify a thrombin-like enzyme from snake venom using an ecotin
variant directed against thrombin. Overall, the ecotin variants acted as robust tools for the isolation and characterization
of proteins with a trypsin fold. Thus, they may assist in the understanding of the role of these serine proteases and homologous
proteins in different biological processes. 相似文献
30.
Jakub Plášil Elena Buixaderas Jiří Čejka Jiří Sejkora Jan Jehlička Milan Novák 《Analytical and bioanalytical chemistry》2010,397(7):2703-2715
The uranyl sulphate mineral zippeite was studied by Raman spectroscopy. The phase purity of the sample was initially checked
by X-ray powder diffraction and its chemical composition was defined by electron microprobe (wavelength dispersive spectroscopy,
WDS) analysis. The Raman spectroscopy research focused on the low wavenumber and uranyl stretching vibration regions. Vibration
bands down to 50 cm–1 were tentatively assigned. The U–O bond lengths were calculated based on empirical relations. Inferred values are consistent
with those obtained from the crystal structure analysis of synthetic zippeite. Number of bands was interpreted on the basis
of factor group analysis. 相似文献