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221.
We obtain a formula for the density \(f(\theta , t)\) of the winding number of a planar Brownian motion \(Z_t\) around the origin. From this formula, we deduce an expansion for \(f(\log (\sqrt{t})\,\theta ,\,t)\) in inverse powers of \(\log \sqrt{t}\) and \((1+\theta ^2)^{1/2}\) which in particular yields the corrections of any order to Spitzer’s asymptotic law (in Spitzer, Trans. Am. Math. Soc. 87:187–197, 1958). We also obtain an expansion for \(f(\theta ,t)\) in inverse powers of \(\log \sqrt{t}\) , which yields precise asymptotics as \(t \rightarrow \infty \) for a local limit theorem for the windings. 相似文献
222.
223.
Tandem mass spectrometry has been applied to differentiate three sets of o-, m- and p-methyl, -methoxy and -nitro-substituted-6-phenyl-dibenzo(d,f)(1,3)dioxepines. Collision-induced dissociation (CID) experiments have been carried out on 2-phenylbenzo[b]furan fragment ions, which originate from the decomposition of the molecular ions after their EI-induced isomerization to spirocyclic structures. With the exception of m- and p-methylphenylbenzo[b]furan isomers, which display identical CID mass spectra, the three isomeric methoxy- and nitrophenylbenzo[b]furan fragment ions display very characteristic CID behavior which allows unequivocal differentiation of the 6-phenyl-dibenzo(d,f)(1,3)dioxepine isomers. 6-(o-nitrophenyl)-dibenzo(d,f)(1,3)dioxepine isomer, does not form a 2-(o-nitrophenyl)benzo[b]furan ion and, therefore, it can be differentiated from the m- and p- isomers based on the mere EI mass spectra. Furthermore, it shows a characteristic ion most likely due to an ortho effect between the nitro group and the dioxepine ring. Multiple stage mass spectrometric techniques (MSn), labeled derivatives and reference compounds were used in order to gain additional information on the structures of product ion from the CID fragmentation. 相似文献
224.
Silvana Halac Ernesto García-Mendoza Anastazia T. Banaszak 《Photochemistry and photobiology》2009,85(3):807-815
This study demonstrates that UV radiation (UVR) reduces the photoprotective capacity of the diatom Phaeodactylum tricornutum by affecting xanthophyll cycle (XC) activity. The short‐term reduction of photosystem II (PSII) maximum efficiency of charge separation (Fv/Fm) in cells exposed to UVR could be explained mainly by a reduced photoprotective capacity under this condition. Phaeodactylum tricornutum cells acclimated to two different photosynthetically active radiation (PAR) intensities, high light (HL, 200 μmol quanta m?2 s?1) and low light (LL, 50 μmol quanta m?2 s?1), were exposed to saturating irradiance (1100 μmol quanta m?2 s?1) in the presence (PAR + UVR) and absence of UVR (PAR). HL cells exhibited a greater reduction in Fv/Fm in PAR + UVR when compared with the PAR treatment that was related to a reduction in the de‐epoxidation of XC pigments. In contrast, in LL cells, UVR did not considerably affect XC de‐epoxidation even though the reduction in Fv/Fm was greater than in HL cells. The negative effect of UVR on photoprotection was more pronounced in HL cells because they synthesized more XC pigments than LL cells. This was confirmed when XC activity was blocked with dithiothreitol and when PSII repair was inhibited with chloramphenicol (CAP). The differential reduction of Fv/Fm between PAR + UVR and PAR treatments disappeared when XC was blocked in HL cells. A higher reduction and an incomplete recovery of Fv/Fm were observed in cells incubated with CAP in the presence of UVR. Such responses confirm that UVR had a negative effect on photoprotective mechanisms causing an enhancement of damage by PAR, especially in HL‐acclimated cells in which heat dissipation is important for PSII regulation. 相似文献
225.
Lisia M.G. dos Santos Rennan G.O. Araujo Silvana do C. Jacob Helmut Becker-Ross 《Talanta》2009,78(2):577-583
Cadmium and iron are antagonistic elements in the sense that they produce different effects in the human body. Both elements have to be determined routinely in grain products, cadmium because of its toxicity, and iron because all grain products, according to Brazilian law, have to contain a minimum of 42 mg kg−1 Fe to combat anemia. A routine screening method has been developed for the quasi simultaneous determination of cadmium and iron using high-resolution continuum source electrothermal atomic absorption spectrometry and direct solid sampling. The primary absorption line at 228.802 nm has been used for Cd, and an adjacent secondary line at 228.726 nm for the determination of Fe. Various chemical modifiers have been investigated, and a mixture of tungsten and iridium, applied as a permanent modifier, showed the best performance; it stabilized Cd up to a pyrolysis temperature of 700 °C and did not over-stabilize Fe. Two atomization temperatures were used sequentially, 1700 °C for Cd and 2600 °C for Fe, because of their significantly different volatilities. The characteristic masses obtained were 0.9 pg for Cd and 1.2 ng for Fe. The limits of detection (3σ, n = 10) were 0.6 μg kg−1 for Cd and 0.5 mg kg−1 for Fe. The relative standard deviation ranged from 3 to 7% for Cd and from 4 to 13% for Fe, which is satisfactory for the purpose. The accuracy of the method was confirmed by the analysis of three certified reference materials; the results were in agreement with the certified values at a 95% confidence interval. The Cd content in the investigated grain products was between 0.9 and 10.5 μg kg−1, but most of them did not contain the required minimum amount of iron. 相似文献
226.
Silvana Bernasconi Giancarlo Jommi Roberto Pagliarin Paolo Tavecchia 《Journal of heterocyclic chemistry》1988,25(3):725-729
Eight compounds containing N- or N′-alkylated imidazole rings have been synthesized and their structures have been chemically and spectroscopically demonstrated. These molecules, submitted to biological tests in order to evaluate their activity as juvenile hormones, provoke morphological changes in the pupae of Tenebrio Molitor. 相似文献
227.
Riassunto In questo lavoro sono studiati gli isomorfismi tra reticoli di sottogruppi normali di gruppi nilpotenti senza torsione. Si
prova che ogni gruppo iperciclico con la struttura normale di gruppo nilpotente senza torsione è un gruppo nilpotente sanza
torsione.
Summary Lattice isomorphisms between the normal structures of torsion-free nilpotent groups are studied in this paper. We prove that every hypercyclic group with the normal structure of a torsion-free nilpotent group is a torsion-free nilpotent group.相似文献
228.
Silvana Modena Pierangelo Calini Guglielmo Gregorio Giovanni Moggi 《Journal of fluorine chemistry》1988,40(2-3):349-357
Liquid phase fluorination of diethyleneglycol dimethyl ether tetraethyleneglycol dimethyl ether, 2[1,2-dichloro-1,2-difluoroethyl] tetrahydrofuran and 2[1,2-dichloro-1,2-difluoroethyl] dioxane 1,4 produce the corresponding perfluoroethers in 56, 43, 67 and 74 % yields respectively.
The by-products are partially fluorinated compounds and no fragmentation is observed.
The simplicity of the reactions and method of preparation make this approach to fluorinated fluids and oils industrially attractive. 相似文献
229.
Silvia Bradamante Silvana Colombo Giorgio A. Pagani Stefano Roelens 《Helvetica chimica acta》1981,64(2):568-571
Phenylglycine esters react with pyruvic acid to give α-methylsuccinic amides 9 instead of the expected Schiff bases 8 , analogously to p-anisidine but unlike aniline. 相似文献
230.
Silvia Bradamante Silvana Colombo Giorgio A. Pagani Stefano Roelens 《Helvetica chimica acta》1981,64(8):2524-2527
N-Substituted 9-fluorenimines 1 are easily prepared from primary amines or aminoesters and 9-fluorenimine: their conjugate carbanions 2 are formed by the action of alkoxides and are reprotonated to 1 or to the tautomeric 9-alkylidenamino-fluorenes 3 depending upon substituents. 相似文献