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261.
Gagliardi G Borri S Tamassia F Capasso F Gmachl C Sivco DL Baillargeon JN Hutchinson AL Cho AY 《Isotopes in environmental and health studies》2005,41(4):313-321
We report the development of a novel laser spectrometer for high-sensitivity detection of methane and nitrous oxide. The system relies on a quantum-cascade laser source emitting wavelength of around 8.06 microm, where strong fundamental absorption bands occur for the considered species and their isotopomers. The detection technique is based on audio-frequency and radio-frequency modulation of laser radiation. First experimental tests have been performed to estimate the achievable detection limits and the signal reproducibility levels in view of possible measurements of (13)C/(12)C, (18)O/(16)O, (17)O/(16)O and (15)N/(14)N isotope ratios. 相似文献
262.
Mitchell DG Quine RW Tseitlin M Meyer V Eaton SS Eaton GR 《Radiation measurements》2011,46(9):993-996
The E' defect in irradiated fused quartz has spin lattice relaxation times (T(1)) about 100 to 300 μs and spin-spin relaxation times (T(2)) up to about 200 μs, depending on the concentration of defects and other species in the sample. These long relaxation times make it difficult to record an unsaturated continuous wave (CW) electron paramagnetic resonance (EPR) signal that is free of passage effects. Signals measured at X-band (~9.5 GHz) by three EPR methods: conventional slow-scan field modulated EPR, rapid scan EPR, and pulsed EPR, were compared. To acquire spectra with comparable signal-to-noise, both pulsed and rapid scan EPR require less time than conventional CW EPR. Rapid scan spectroscopy does not require the high power amplifiers that are needed for pulsed EPR. The pulsed spectra, and rapid scan spectra obtained by deconvolution of the experimental data, are free of passage effects. 相似文献
263.
Timothy J. Bunning Deborah L. Vezie Pamela F. Lloyd Peter D. Haaland Edwin L. Thomas W. Wade Adams 《Liquid crystals》1994,16(5):769-781
Bright-field image contrast of thermotropic cholesteric liquid crystalline materials in the transmission electron microscope (TEM) is investigated. Possible sources of contrast for these systems are discussed in terms of their molecular anisotropy. A cholesteric side-chain liquid crystalline compound was investigated with TEM, low voltage high resolution scanning electron microscopy (LVHRSEM), and atomic force microscopy (AFM) to determine the origin of the strong contrast observed in these systems using bright-field transmission electron microscopy. Initial contrast of thin microtomed sections, as viewed with TEM low dose techniques and an image intensifier, was much weaker than observed under normal viewing conditions. The periodic steady state contrast typically observed for these materials is the result of beam damage. Furthermore, the surface of microtomed samples (parallel to the cholesteric helical axis) is corrugated with a periodicity of 1/2 the pitch due to a preferred fracture path in the glassy cholesteric state. AFM profile analysis shows an average peak to valley height of approximately 20-25 nm. AFM of free surfaces from aligned films also indicates a corrugation with a periodicity equal to 1/2 the pitch with substantially smaller average corrugation depths. TEM indicates a series of +1/2 and -1/2 disclination lines at the surface due to a rotation of the preferred helix direction parallel to the surface, consistent with previously reported models. 相似文献
264.
Deborah M. Sarzotti Joo B. P. Soares Alexander Penlidis 《Journal of Polymer Science.Polymer Physics》2002,40(23):2595-2611
A series of poly(ethylene‐co‐1‐hexene) samples made with rac‐ethylene bis(indenyl)zirconium dichloride/methylaluminoxane were analyzed by crystallization analysis fractionation (CRYSTAF). The nine samples had comonomer contents of 0–4.2 mol % 1‐hexene with a narrow range of molecular weights (34,000–39,000 g/mol). Because all the copolymer samples had narrow, unimodal chemical composition distributions, they were ideal as calibration standards for CRYSTAF. A linear calibration curve was constructed relating the peak crystallization temperature from CRYSTAF operated at a cooling rate of 0.1 °C/min and the comonomer content as determined by 13C NMR. Reactivity ratios for ethylene and 1‐hexene were estimated by the fitting of reactant liquid‐phase compositional data to the Mayo–Lewis equation. It was found that a value of the 1‐hexene reactivity ratio could not be unequivocally determined from the set of samples analyzed because the range of comonomer incorporation was too narrow. Stockmayer's bivariate distribution was used to model the fractionation process in CRYSTAF, and although a good fit to experimental CRYSTAF profiles was attained, the model did not fully describe the underlying crystallization phenomena. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2595–2611, 2002 相似文献
265.
Peter Jackson Geoffrey Dent David Carter Deborah J. Schofield John M. Chalmers Tom Visser Marjo Vredenbregt 《Journal of separation science》1993,16(9):515-521
As part of a detailed investigation into the application of GC-FTIR in industrial and environmental analysis, representative sets of samples have been analyzed in parallel using commercial high-sensitivity instruments. Two of the instruments utilize low temperature storage of the GC eluate to extend the time available for FTIR analysis, yielding greater sensitivity than that possible by conventional ‘light-pipe’ GC-FTIR. In certain circumstances, instruments using both types of sample storage give rise to spectra exhibiting features characteristic of the interface used. Chromatographic resolution was found not to be significantly degraded by use of either sample storage interface. Particular advantages were found in having parallel flame ionization detection and mass spectrometry; this enabled the location of smaller components and gave greater certainty of identification. 相似文献
266.
Carlo Sirtori Jérôme Faist Federico Capasso Deborah L. Sivco Albert L. Hutchinson Alfred Y. Cho 《Superlattices and Microstructures》1996,19(4):357-363
The continuous wave (cw) operation of a quantum cascade laser at wavelengths ∼8 μm is reported. The structures, grown by molecular beam epitaxy in the AlInAs/GaInAs material system, are based on a vertical intersubband transition scheme and use a plasmon-enhanced waveguide geometry to reduce the losses and increase the confinement factor. The single mode optical power from one facet is 2 mW at a maximum operating temperature of 110 K. In pulsed operation the highest temperature is 210 K and the threshold shows a weak temperature dependence typical of this class of lasers, with aTo=110 K. 相似文献
267.
Jérôme Faist Federico Capasso Carlo Sirtori Deborah L. Sivco Albert L. Hutchinson Sung-Nee G. Chu Alfred Y. Cho 《Superlattices and Microstructures》1996,19(4):337-345
Continuous wave operation of a quantum cascade laser at λ=4.6 μm is reported above liquid nitrogen temperature. Optical powers of 15 mW at 50 K and 2 mW at 85 K are reported. The single mode spectrum is temperature tunable over 1.8 cm−1. A linewidth of 115 Mhz is measured with a Fabry–Perot on those free running devices. Gain measurements show evidence for ultra low linewidth enhancement factor α<0.1. These devices also operated in pulse mode with 20 mW peak power at 200 K. 相似文献
268.
Thomas Deborah A. Bloor John E. Bartmess John E. 《Journal of the American Society for Mass Spectrometry》1990,1(4):295-300
The 60-u anion observed in the ion/molecule chemistry of methyl nitrite is shown to have the structure O=NCH2O?, and not that of the expected deprotonation product ?CH2ONO, by the use of mass-analyzed ion kinetic energy spectrometry, ion/molecule reactivity, and ab initio calculations.
相似文献269.
270.
O'Neill BM Ratto JE Good KL Tahmassebi DC Helquist SA Morales JC Kool ET 《The Journal of organic chemistry》2002,67(17):5869-5875
We describe the preparation and structure of the deoxyribonucleoside of 4-fluoro-6-methylbenzimidazole, abbreviated dH (8), which acts as a close shape mimic of the nucleoside deoxyguanosine. The nucleoside is prepared from 2-fluoro-4-methylaniline in seven steps. The X-ray crystal structure reveals a (-sc) glycosidic orientation, an S conformation for the deoxyribose moiety, and quite close shape mimicry of guanine by the substituted benzimidazole. Conformational studies by (1)H NMR and (1)H-(1)H ROESY experiments reveal an S-type conformation and an anti glycosidic orientation in solution (D(2)O), essentially the same as that of deoxyguanosine. Base-stacking studies in a "dangling end" context reveal that the benzimidazole base mimic stacks more strongly than all four natural bases, and more strongly than its counterpart guanine by 1.1 kcal/mol. Base-pairing studies in a 12mer DNA duplex show that, like other nonpolar nucleoside isosteres, H is destabilizing and nonselective when paired opposite natural bases. However, when paired opposite another nonpolar isostere, difluorotoluene (F), a mimic of thymine, the pair exhibits stability approaching that of its natural analogue, a G-T (wobble) base pair. The nucleoside analogue dH will be useful in studies of protein-DNA interactions, and the H-F base pair will serve as a structurally and thermodynamically close mimic of G-T in studies of DNA mismatch repair enzymes. 相似文献