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91.
Vishal Sipani Alec B. Scranton 《Journal of polymer science. Part A, Polymer chemistry》2003,41(13):2064-2072
We have characterized the effective rate constants for termination/trapping (kt/t) and propagation (kp) for solvent‐free cationic photopolymerizations of phenyl glycidyl ether for conversions up to 50%. We have performed dark‐cure experiments in which active centers are produced photochemically for a specified period of time until the initiating light is shuttered off, and then the polymerization rate is monitored in the dark. This method is especially well suited for characterizing cationic polymerizations because of the long active center lifetimes. Our analysis provides profiles of the instantaneous kinetic rate constants as functions of conversion (or time). For photopolymerizations of phenyl glycidyl ether initiated with iodonium photoinitiators, kt/t and kp remain essentially invariant for conversions up to 50%. For the photoinitiator (tolycumyl) iodonium tetrakis (pentafluorophenyl) borate (IPB), the values of kt/t at 50 and 60 °C are 0.027 and 0.033 min?1, respectively. The corresponding values of kt/t for diaryliodonium hexafluoroantimonate (IHA) are 0.041 and 0.068 min?1. The values of kp at 50 °C for IPB and IHA are 0.6 and 0.4 L mol?1 s?1, respectively. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2064–2072, 2003 相似文献
92.
93.
Whited MT Djurovich PI Roberts ST Durrell AC Schlenker CW Bradforth SE Thompson ME 《Journal of the American Chemical Society》2011,133(1):88-96
Multichromophoric arrays provide one strategy for assembling molecules with intense absorptions across the visible spectrum but are generally focused on systems that efficiently produce and manipulate singlet excitations and therefore are burdened by the restrictions of (a) unidirectional energy transfer and (b) limited tunability of the lowest molecular excited state. In contrast, we present here a multichromophoric array based on four boron dipyrrins (BODIPY) bound to a platinum benzoporphyrin scaffold that exhibits intense panchromatic absorption and efficiently generates triplets. The spectral complementarity of the BODIPY and porphryin units allows the direct observation of fast bidirectional singlet and triplet energy transfer processes (k(ST)((1)BDP→(1)Por) = 7.8 × 10(11) s(-1), k(TT)((3)Por→(3)BDP) = 1.0 × 10(10) s(-1), k(TT)((3)BDP→(3)Por) = 1.6 × 10(10) s(-1)), leading to a long-lived equilibrated [(3)BDP][Por]?[BDP][(3)Por] state. This equilibrated state contains approximately isoenergetic porphyrin and BODIPY triplets and exhibits efficient near-infrared phosphorescence (λ(em) = 772 nm, Φ = 0.26). Taken together, these studies show that appropriately designed triplet-utilizing arrays may overcome fundamental limitations typically associated with core-shell chromophores by tunable redistribution of energy from the core back onto the antennae. 相似文献
94.
Brockway AJ González-López M Fettinger JC Shaw JT 《The Journal of organic chemistry》2011,76(9):3515-3518
The direct displacement of alkoxy groups from the β position of aromatic and unsaturated esters and ketones is described. The reaction is chemo- and regioselective, displaying wide substrate scope. 相似文献
95.
Nonlinear metamaterials have been predicted to support new and exciting domains in the manipulation of light, including novel phase-matching schemes for wave mixing. Most notable is the so-called nonlinear-optical mirror, in which a nonlinear negative-index medium emits the generated frequency towards the source of the pump. In this Letter, we experimentally demonstrate the nonlinear-optical mirror effect in a bulk negative-index nonlinear metamaterial, along with two other novel phase-matching configurations, utilizing periodic poling to switch between the three phase-matching domains. 相似文献
96.
We present our winning entry for the EVA2017 challenge on spatiotemporal prediction of extreme precipitation. The aim of the competition is to predict extreme rainfall quantiles that score as low as possible on the competition error metric. Good or bad predictions are defined only by the metric used. Our methodology was simple and produced accurate predictions under this metric. This outcome emphasizes the importance of cross-validation and identifying model over-fitting. 相似文献
97.
EFFECTS OF ACRIDINE PLUS NEAR ULTRAVIOLET LIGHT ON ESCHERICHIA COLI MEMBRANES AND DNA IN VIVO 总被引:1,自引:0,他引:1
Stephen Wagner William D. Taylor Alec Keith Wallace Snipes 《Photochemistry and photobiology》1980,32(6):771-779
Results from a variety of experiments indicate that photodynamic damage to E. coli treated with the hydrophobic photosensitizer acridine plus near-UV light involves both cell membranes and DNA. Split-dose survival experiments with various E. coli mutants reveal that cells defective in rec A, uvr A, or pol A functions are all capable of recovery from photodynamic damage. Alkaline sucrose gradient analysis of DNA from control and treated cells revealed that acridine plus near-UV light treatment converts normal DNA into a more slowly sedimenting form. However, the normal DNA sedimentation properties are not restored under conditions where split-dose recovery is effective. Several lines of evidence suggest that membrane damage may be important in the inactivation of cells by acridine plus near-UV light. These include (a) a strong dependence of sensitivity on the fatty acid composition of the membranes; (b) a strong dependence of sensitivity on the osmolarity of the external medium; and (c) the extreme sensitivity of an E. coli mutant having a defect in its outer membrane barrier properties. Direct evidence that acridine plus near-UV light damages cell membranes was provided by the observations that (a) the plasma membrane becomes permeable to o-nitrophenyl-ß-D-galactopyranoside and (b) the outer membrane becomes permeable to lysozyme after treatment. A notable result was that cells previously sensitized to lysozyme by exposure to acridine plus near-UV light lose that sensitivity upon subsequent incubation. This strongly suggests that E. coli cells are capable of repairing damage localized in the outer membrane. 相似文献
98.
In this Communication, we report the direct, catalytic, asymmetric aldol addition of methyl ynones using our dinuclear zinc catalyst. A spontaneous reversal in the sense of enantioinduction was observed for these reactions; formation of the (S)-enantiomer is favored in the early stages (69% ee after 5 min), whereas the (R)-enantiomer is isolated as the major product after prolonged reaction times (97% ee after 22 h). It could be shown that this reversal in enantioselectivity is due to formation of a new catalytic species which incorporates the aldol product. 相似文献
99.
The Extremal Types Theorem identifies three distinct types of extremal behaviour. Two different strategies for statistical
inference for extreme values have been developed to exploit this asymptotic representation. One strategy uses a model for
which the three types are combined into a unified parametric family with the shape parameter of the family determining the
type: positive (Fréchet), zero (Gumbel), and negative (negative Weibull). This form of approach never selects the Gumbel type
as that type is reduced to a single point in a continuous parameter space. The other strategy first selects the extremal type,
based on hypothesis tests, and then estimates the best fitting model within the selected type. Such approaches ignore the
uncertainty of the choice of extremal type on the subsequent inference. We overcome these deficiencies by applying the Bayesian
inferential framework to an extended model which explicitly allocates a non-zero probability to the Gumbel type. Application
of our procedure suggests that the effect of incorporating the knowledge of the Extremal Types Theorem into the inference
for extreme values is to reduce uncertainty, with the degree of reduction depending on the shape parameter of the true extremal
distribution and the prior weight given to the Gumbel type. 相似文献
100.
Sirjani DB Salt AN Gill RM Hale SA 《The Journal of the Acoustical Society of America》2004,115(3):1219-1229
Distortion generated by the cochlea can provide a valuable indicator of its functional state. In the present study, the dependence of distortion on the operating point of the cochlear transducer and its relevance to endolymph volume disturbances has been investigated. Calculations have suggested that as the operating point moves away from zero, second harmonic distortion would increase. Cochlear microphonic waveforms were analyzed to derive the cochlear transducer operating point and to quantify harmonic distortions. Changes in operating point and distortion were measured during endolymph manipulations that included 200-Hz tone exposures at 115-dB SPL, injections of artificial endolymph into scala media at 80, 200, or 400 nl/min, and treatment with furosemide given intravenously or locally into the cochlea. Results were compared with other functional changes that included action potential thresholds at 2.8 or 8 kHz, summating potential, endocochlear potential, and the 2 f1-f2 and f2-f1 acoustic emissions. The results demonstrated that volume disturbances caused changes in the operating point that resulted in predictable changes in distortion. Understanding the factors influencing operating point is important in the interpretation of distortion measurements and may lead to tests that can detect abnormal endolymph volume states. 相似文献