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911.
Arkadiy S. Baltenkov Steven T. Manson Alfred Z. Msezane 《Central European Journal of Physics》2011,9(5):1209-1220
A comprehensive study is undertaken of angular distributions of electron knock-out from atomic targets by fast electrons with
a small transfer of momentum. The general expressions for the parameters of the triple differential cross-section of impact
ionization in the optical limit are derived. The calculated parameters are compared with those of the angular distribution
of electrons ejected from an atom in the process of photoionization. In these processes, when the multipole transitions are
involved, the one-to-one correspondence between the photoionization and impact ionization parameters disappears. The nondipole
transitions lead to the backward/forward asymmetry of the angular distribution of ejected electrons that is absent in the
dipole approximation for ionization by both fast electrons and photons. Using the He atom as an example, the character of
the asymmetry for these two processes is qualitatively different and the backward/forward asymmetry results in macroscopic
directed motion of secondary electrons accompanying the passing of a fast electron beam through gas or plasma. The general
formulas for this drag current are derived and applied to gaseous He. 相似文献
912.
Fei Qi Anna Moiseev Joachim Deubener Alfred Weber 《Journal of nanoparticle research》2011,13(3):1325-1334
Anatase is the low-temperature (300–550 °C) crystalline polymorph of TiO2 and it transforms to rutile upon heating. For applications utilizing the photocatalytic properties of nanoscale anatase at
elevated temperatures (over 600 °C) the issue of phase stabilisation is of major interest. In this study, binary TiO2/SiO2 particles were synthesized by a flame aerosol process with TiCl4 and SiCl4 as precursors. The theoretical Si/Ti ratio was varied in the range of 0.7–1.3 mol/mol. The synthesized TiO2/SiO2 samples were heat treated at 900 and 1,000 °C for 3 h to determine the thermostability of anatase. Pyrogenic TiO2 P25 (from Evonik/Degussa, Germany) widely applied as photocatalyst was used as non-thermostabilized reference material for
comparison of photocatalytic activity of powders. Both the non-calcinated and calcinated powders were characterized by means
of XRD, TEM and BET. Photocatalytic activity was examined with dichloroacetic acid (DCA) chosen as a model compound. It was
found that SiO2 stabilized the material retarding the collapse of catalyst surface area during calcination. The weighted anatase content
of 85% remains completely unchanged even after calcination at 1,000 °C. The presence of SiO2 layer/bridge as spacer between TiO2 particles freezes the grain growth: the average crystallite size increased negligibly from 17 to 18 nm even during the calcination
at 1,000 °C. Due to the stabilizing effect of SiO2 the titania nanoparticles calcinated at 900 and 1,000 °C show significant photocatalytic activity. Furthermore, the increase
in photocatalytic activity with calcination temperature indicates that the titania surface becomes more accessible either
due to intensified cracking of the SiO2 layer or due to enhanced transport of SiO2 into the necks thus releasing additional titania surface. 相似文献
913.
Iglev H Fischer MK Gliserin A Laubereau A 《Journal of the American Chemical Society》2011,133(4):790-795
The photodetachment of aqueous hydroxide (OH(?)(aq) and OD(?)(aq)) is studied using femtosecond pump?probe and pump?repump?probe spectroscopy. The electron is detached after excitation of the hydroxide ion to a charge-transfer-to-solvent (CTTS) state at 202 nm. An early intermediate is observed that builds up within 160 fs and is assigned to nonequilibrated OH?electron pairs. The subsequent dynamics are governed by thermalization, partial recombination, and dissociation of the pairs, yielding the final hydrated electrons and hydroxyl radicals. An additional pulse at 810 nm is used for secondary excitation of the intermediate species so that more insight is gained into the recombination process(es). Using this technique we observe a novel geminate recombination channel of OH with adjacent hydrated electrons. This channel leads to ultrafast quenching (700 fs) of almost half the initial number of radicals. The fast mechanism displays an isotope effect of 1.4 (for OD(?)(aq) quantum yield 35%, time constant 1.0 ps). This process was not observed in similar experiments on aqueous bromide and seems to be related to the special properties of the hydroxide ion and its local H-bonding environment. Our findings underline the high reactivity of the prehydrated electron. 相似文献
914.
Yulian Pang Shuheng Fan Qunying Wang Dennis Oprych Alfred Feilen Knut Reiner Dietmar Keil Yuriy L. Slominsky Sergey Popov Yingquan Zou Bernd Strehmel 《Angewandte Chemie (International ed. in English)》2020,59(28):11440-11447
Cyanines comprising either a benzo[e]‐ or benzo[c,d]indolium core facilitate initiation of radical photopolymerization combined with high power NIR‐LED prototypes emitting at 805 nm, 860 nm, or 870 nm, while different oxime esters function as radical coinitiators. Radical photopolymerization followed an initiation mechanism based on the participation of excited states, requiring additional thermal energy to overcome an existing intrinsic activation barrier. Heat released by nonradiative deactivation of the sensitizer favored the system, even under conditions where a thermally activated photoinduced electron transfer controls the reaction protocol. The heat generated internally by the NIR sensitizer promotes generation of the initiating reactive radicals. Sensitizers with a barbiturate group at the meso‐position preferred to bleach directly, while sensitizers carrying a cyclopentene moiety unexpectedly initiated the photosensitized mechanism. 相似文献
915.
Ramona Samba Kai Fuchsberger Ilona Matiychyn Sebastian Epple Lydia Kiesel Alfred Stett Wolfgang Schuhmann Martin Stelzle 《Electroanalysis》2014,26(3):548-555
In this work, composite microelectrodes from poly(3,4‐ethylenedioxythiophene) (PEDOT) and carbon nanotubes (CNT) are characterized as electrochemical sensing material for neurotransmitters. Dopamine can be detected using square wave voltammetry at these microelectrodes. The CNTs improve the sensitivity by a factor of two. In addition, the selectivity towards dopamine in the presence of ascorbic acid and uric acid was examined. While both electrodes, PEDOT and PEDOT‐CNT are able to detect all measured concentrations of dopamine in the presence of uric acid, small concentrations of dopamine and ascorbic acid are only distinguishable at PEDOT‐CNT electrodes. Changing the pH has a strong influence on the selectivity. Moreover, it is possible to detect concentrations as low as 1 µM dopamine in complex cell culture medium. Finally, other catecholamines like serotonin, epinephrine, norepinephrine and L ‐dopa are also electrochemically detectable at PEDOT‐CNT microelectrodes. 相似文献
916.
Dr. Markus Kalmutzki Dr. Markus Ströbele Dipl.‐Chem. Frank Wackenhut Prof. Dr. Alfred J. Meixner Prof. Dr. H.‐Jürgen Meyer 《Angewandte Chemie (International ed. in English)》2014,53(51):14260-14263
Calcium cyanurate is synthesized by reacting calcium chloride with potassium cyanate following a solid‐state reaction. The formation of the new compound Ca3(O3C3N3)2 (CCY), which occurs by the cyclotrimerization of cyanate ions, was examined thermoanalytically and the crystal structure was determined by single‐crystal structure analysis. The structure of CCY is closely related to the structure of the well‐known oxoborate β‐BaB2O4 (BBO). Second harmonic generation (SHG) measurements on crystal powders show a higher SHG efficiency for CCY than for BBO by about one order of magnitude. 相似文献
917.
Previous studies of calculated diffraction patterns for cellulose crystallites suggest that distortions that arise once models have been subjected to molecular dynamics (MD) simulation are the result of both microfibril twisting and changes in unit cell dimensions induced by the empirical force field; to date, it has not been possible to separate the individual contributions of these effects. To provide a better understanding of how twisting manifests in diffraction data, the present study demonstrates a method for generating twisted and linear cellulose structures that can be compared without the bias of dimensional changes, allowing assessment of the impact of twisting alone. Analysis of unit cell dimensions, microfibril volume, hydrogen bond patterns, glycosidic torsion angles, and hydroxymethyl group orientations confirmed that the twisted and linear structures collected with this method were internally consistent, and theoretical powder diffraction patterns for the two were shown to be effectively indistinguishable. These results indicate that differences between calculated patterns for the crystal coordinates and twisted structures from MD simulation can result entirely from changes in unit cell dimensions, and not from microfibril twisting. Although powder diffraction patterns for models in the 81-chain size regime were shown to be unaffected by twisting, suggesting that a modest degree of twist is not inconsistent with available crystallographic data, it may be that other diffraction techniques are capable of detecting this structural difference. Until such time as definitive experimental evidence comes to light, the results of this study suggest that both twisted and linear microfibrils may represent an appropriate model for cellulose Iβ. 相似文献
918.
Thomas Rosenau Antje Potthast Karin Krainz Hubert Hettegger Ute Henniges Yuko Yoneda Christian Rohrer Alfred D. French 《Cellulose (London, England)》2014,21(4):2271-2283
In the present work, bacterial cellulose (BC) was analyzed for its chromophore content with the chromophore release and identification (CRI) method. In aged BC, seven chromophores were unambiguously identified, despite their very low (ppb) presence. The compounds contain 2-hydroxy-[1,4]benzoquinone, 2-hydroxyacetophenone and 5,8-dihydroxy-naphthoquinone moieties. These three classes of key chromophores had also been found in other celluloses such as bleached pulp, fiber and derivatives. The detection of chromophores in lignin-free BC confirms that the chromophores are formed from oxidized and degraded carbohydrate material rather than lignin fragments. Samples that were insufficiently purified before the aging step also yielded three N-heterocyclic compounds. They originated from tryptophan moieties in residual adhering protein and protein degradation products. N-Heteropentacycles 9 and 10 showed strong adsorption onto cellulosic surfaces, and that adsorption aided oxidation of cyclohexadiene derivative 9 into the corresponding aromatic and intensely yellow system 10, consistent with the strain-induced bond localization theory. In purified fresh BC, no chromophores were detectable by the CRI method. The present contribution is the first report on defined chromophoric structures isolated from BC. 相似文献
919.
Keith B. Male Mahsa HamzehJohnny Montes Alfred C.W. LeungJohn H.T. Luong 《Analytica chimica acta》2013
Titanium dioxide (TiO2) nanoparticles (NPs) with different sizes and structures were probed for plausible cytotoxicity using electric cell-substrate impedance sensing (ECIS), a non-invasive and on-line procedure for continuous monitoring of cytotoxicity. For insect cells (Spodoptera frugiperda Sf9), the ECIS50 values, i.e., the concentration required to achieve 50% inhibition of the response, differed depending on the size and shape of the TiO2 nanostructure. The lowest ECIS50 value (158 ppm) was observed for the needle shaped rutile TiO2 (10 nm × 40 nm, 15.5 nm nominal particle size), followed by 211 ppm for P-25 (34.1 nm, 80% anatase and 20% rutile), 302 ppm for MTI5 (5.9 nm, 99% anatase) and 417 ppm for Hombitan LW-S bulk TiO2 (169.5 nm, 99% anatase). Exposure of TiO2 NPs to UV light at 254 nm or 365 nm exhibited no significant effect on the ECIS50 value due to the aggregation of TiO2 NPs with diminishing photocatalytic activities. Chinese hamster lung fibroblast V79 cells, exhibited no significant cytotoxicity/inhibition up to 400 ppm with P25, MTI5 and bulk TiO2. However, a noticeable inhibitory effect was observed (ECIS50 value of 251 ppm) with rutile TiO2 as cell spreading on the electrode surface was prevented 相似文献
920.
When heptachlor (la) (I-exo-3,7,8,9,10,10-heptachloro-endo-tricyclo-(5,2,1,0,2,6)-deca-4,8 diene) in benzene is treated with an alumina/sulfuric acid solid matrix up to 90% of 3-exo-phenylchlordene (1b) is obtained as the only isolable product. This easy and efficient route to substituted chlordenes suggest the possibility of incorporating photochemically degradable residues onto the heptachlor skeleton. 相似文献