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901.
Nishioka H Kimura A Yamato T Kawatsu T Kakitani T 《The journal of physical chemistry. B》2005,109(5):1978-1987
Electron tunneling routes for the electron transfer from the bacteriopheophytin anion to the primary quinone in the bacterial photosynthetic reaction center of Rhodobactor sphaeroides are investigated by a combined method of molecular dynamics simulations for the protein conformation fluctuation and quantum chemical calculations for the electronic states of the donor, acceptor, and protein medium. The analysis of the tunneling route is made by mapping interatomic electron tunneling currents for each protein conformation. We found that there are two dominant routes mainly passing through Trp(M252) (Trp route) or mainly passing through Met(M218) (Met route). Actual electron tunneling pathways alternate between the two routes, depending on the protein conformation which varies with time. When either the Trp route or the Met route dominates, the electron tunneling matrix element /T(DA)/ becomes large. When both the Trp route and the Met route dominate, /T(DA)/ becomes very small due to the destructive interference of the electron tunneling currents between the two routes. We found that a linear relationship exists between the value of /T(DA)/ and the inverse of the degree of destructive interference Q for a wide range of values (ca. 3-10(3) for Q). A similar relationship was also found previously for electron transfer in ruthenium-modified azurins, suggesting that this relationship holds true in general. From these results, we are led to the conclusion that /T(DA)/ cannot exceed a maximum value at Q = 1, even if much variation of /T(DA)/ happens due to the fluctuation of protein conformation. We also conclude that the property of the electron transfer alternates between constructive and destructive interference, due to the fluctuation of protein conformation. It is impossible to keep a system in either constructive or destructive interference because thermal fluctuation of protein conformation takes place. 相似文献
902.
Carbon nanomaterials show a variety of interesting chemical and physical properties. In this Minireview we focus on the mechanical properties of carbon nanomaterials with emphasis on carbon nanotubes and their composite materials. We introduce some recently developed components made of carbon nanotube composite materials and outline their importance for applications in everyday life. 相似文献
903.
Hajime Kato Masaki Nakashima Yoshihiko Mori Toshiaki Mori Tadashi Hattori Yuichi Murakami 《Research on Chemical Intermediates》1995,21(2):115-126
In CO hydrogenation over Rh/SiO2 catalysts, the effect of additive metal oxides on C-O bond dissociation was studied by using pulse surface reaction rate
analysis (PSRA). The addition of oxides of Al, Ti, Cr, V, and Mn resulted in an increase in the rate constant for the dissociation
according to this sequence, while the oxides of Cu, Zn, and Ag added decreased the rate constant to almost the same extent.
In contrast to these metal oxides, MoO3 and WO3 did not change the dissociation activity. CO adsorption measurement indicated that all of the added metal oxides covered
a considerable portion of the Rh metal surface, although the efficiency of covering was different from one metal oxide to
another. Covering the Rh metal surface with an added metal oxide should decrease the rate constant of C-O bond dissociation,
because ensemble sites, consisting of a group of surface Rh atoms and considered necessary for the dissociation, were destroyed.
The suppression effect resulted from the destruction of the ensemble sites by adding the oxides of Cu, Zn, and Ag. For other
metal oxides, temperature-programmed reduction (TPR) or O2 uptake measurement revealed that the added oxides, especially those existing on the Rh metal surface, were in a partially
reduced state under reaction conditions. Owing to its high affinity for an oxygen atom, the cation in a partially reduced
state participated in the reaction in such a way that the oxygen end of adsorbed CO species was bound to the cation so as
to dissociate the C-O bond, which resulted in promotion of the dissociation. The observed promotion was explained in terms
of the enhancement owing to the high affinity sufficient to overcome the suppression caused by destroying ensemble sites.
Lack of the promotion effect of MoO3 and WO3 might result from a balance between promotion due to the high affinity of the partially reduced Mo or W and suppression caused
by destroying ensemble sites. Excellent correlation was observed between the intrinsic activity increase, from which the suppression
effect was excluded, and the heat of formation of metal oxide including MoO3 and WO3. Since the heat of formation of metal oxide is considered to be a measure of the affinity, this correlation supports the
idea that the high affinity of additive cations for an oxygen atom is of primary importance in the promotion of C-O bond dissociation
in CO hydrogenation. 相似文献
904.
Shoji Kozuka Yukari Yokote Kazuhide Abe Masaru Hayashi Hideki Matsunaga 《Fresenius' Journal of Analytical Chemistry》1995,351(8):801-802
The determination of impurities in SrTiO3 by ICPMS was investigated. The sample was decomposed with hydrochloric and hydrofluoric acids in a PTFE pressure vessel. The internal standard method using Au was selected to eliminate an ion count suppression by the Sr and Ti matrix. Impurities at sub g/ml level in SrTiO3 were determined. The detection limits were in the range of 0.008 to 0.01 g/g. 相似文献
905.
The local environments and dynamics of hydrogen atoms in five samples of protonated forms of ion-exchangeable layered perovskites, Dion-Jacobson-type H[LaNb2O7] and H[LaTa2O7], Ruddlesden-Popper-type H2[SrTa2O7] and H2[La2Ti3O10], and H1.8[(Sr0.8Bi0.2)Ta2O7] derived from an Aurivillius phase, Bi2Sr2Ta2O9, have been investigated by solid-state 1H nuclear magnetic resonance spectroscopy (NMR). Solid-state 1H NMR with a magic-angle spinning technique conducted at room temperature reveals that the mean electron densities around the 1H nuclei in these protonated forms are relatively low, and that they decrease in the following order: H1.8[(Sr0.8Bi0.2)Ta2O7]>H[LaNb2O7]>H2[SrTa2O7]>H[LaTa2O7]>H2[La2Ti3O10]. The temperature-dependent solid-state 1H broad-line NMR spectra measured at 140-400 K reveal a decrease in the signal width for all of these five samples upon heating due to motional narrowing. The NMR spectra of H[LaNb2O7] and H[LaTa2O7] are different from the other three protonated forms due to the weaker dipole-dipole interactions at low temperatures and lower mobility of the hydrogen atoms at high temperatures. 相似文献
906.
Isao Kaetsu Hiroshi Okubo Akihiko Ito Koichiro Hayashi 《Journal of polymer science. Part A, Polymer chemistry》1973,11(6):1149-1156
The radiation-induced polymerization of binary systems consisting of glass-forming monomer and glass-forming solvent in supercooled phase was studied. The initial polymerization rates were markedly affected by Tg (glass transition temperature) and Tv of the system (30–50°C higher than Tg), which turned to be functions of the composition. The composition and temperature dependence of initial polymerization rate in binary glass-forming systems were much affected by homogeneity of the polymerization system and the Tg of the glass-forming solvent. The composition and temperature dependences in the glycidyl methacrylate–triacetin system as a typical homogeneous polymerization system were studied in detail, and the polymerizations of hydroxyethyl methacrylate–triacetin and hydroxyethyl methacrylate–isoamyl acetate systems were studied for the heterogeneous polymerization systems; the former illustrates the combination of lower Tg monomer and higher Tg solvent and the latter typifies a system consisting of higher Tg monomer and lower Tg solvent. All experimental results for the composition and temperature dependence of initial polymerization rate in binary glass-forming systems could be explained by considering the product of the effect of the physical effect relating to Tv and Tg of the system and the effect of composition in normal solution polymerization at higher temperature, which was also the product of a dilution effect and a chemical or physical acceleration effect. 相似文献
907.
Tubiferal A, a novel rearranged triterpenoid lactone, has been isolated from field-collected fruit bodies of the myxomycete, Tubifera dimorphotheca, and its structure elucidated by spectral data. Tubiferal A (1) possesses a 9,10-secocycloartan-16,21-olide skeleton, and this new compound exhibited a reversal effect of vincristine (VCR) resistance (more than 4-fold) against VCR-resistant KB cell lines. Tubiferal B (2), corresponding to the seco acid of 1 was also isolated, but showed no comparable activity. 相似文献
908.
Toshiaki Morimoto Nobuhiro Obara Iku Yoshida Kiyoshi Tanaka Toshiyuki Kan 《Tetrahedron letters》2007,48(17):3093-3095
New axially chiral P,O,N-type ligands bearing both a diphenylphosphino group and 2-(dialkylamino)ethyloxy (or 2-pyridylmethyloxy) groups were designed and prepared along with P,O,O-type ligands by employing an easily available chiral component, (R)-2-hydroxy-2′-diphenylphosphinyl-1,1′-binaphthyl. Among the ligands the simplest P,O,N-type one bearing a 2-(dimethylamino)ethyloxy group was found to be the most efficient in copper-catalyzed enantioselective conjugate addition of diethylzinc to a cyclic enone, 2-cyclohexen-1-one, providing high enantioselectivity up to 99% ee. 相似文献
909.
Shimada T Mukaide K Shinohara A Han JW Hayashi T 《Journal of the American Chemical Society》2002,124(8):1584-1585
Double hydrosilylation of arylacetylenes with trichlorosilane catalyzed first by platinum and second by a chiral monophosphine-palladium complex generated the corresponding 1,2-bis(silyl)-1-arylethanes, the oxidation of which with hydrogen peroxide gave 1-aryl-1,2-diols of high enantiomeric purity (94-98% ee) in high yields. 相似文献
910.
Electrons with an energy of 300 keV or lower were defined as “Soft-electrons”, which showed several advantages over conventional irradiation with gamma-rays or high-energy electrons in decontamination of grains and spices. Energies of electrons necessary to reduce microbial loads to levels lower than 10 CFU/g were 60 keV for brown rice, 75 keV for wheat, 100 keV for white pepper, coriander and basil, 130 keV for buckwheat, 160 keV for rough rice, and 210 keV for black pepper. Electrons with such energies did not significantly influence the quality. 相似文献