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51.
The steady-shear viscosity, dynamic viscoelasticity, and stress relaxation behavior were measured for suspensions of silica nanoparticles dispersed in aqueous solutions of poly(ethylene oxide) (PEO). The suspensions of silica with diameters of 8-25 nm show striking shear-thickening profiles in steady shear and highly elastic responses under large strains in oscillatory shear. Since the silica particles are much smaller than the polymer coils, one molecule can extend through several particles by intrachain bridging. Each polymer coil may remain isolated as a floc unit and the silica particles hardly connect two flocs. Therefore, the flow of suspensions is Newtonian with low viscosity at low shear rates. When the polymer coils containing several nanoparticles are subjected to high shear fields, three-dimensional network is developed over the system. The shear-thickening flow may arise from the elastic forces of extended bridges. But, the polymer chain is easily detached from particle surface by thermal energy because of large curvature of particles. As a result, the network structures are reversibly broken down in a quiescent state and the suspensions behaves as viscoelastic fluids with the zero-shear viscosity. 相似文献
52.
Nakamura T Araki Y Ito O Takimiya K Otsubo T 《The journal of physical chemistry. A》2008,112(6):1125-1132
Photoinduced excitation energy transport dynamics in oligothiophene-fullerene linked dyads, nT-C60 (n = 4, 8, and 12), have been investigated by femtosecond fluorescence up-conversion. In 8T-C60 and 12T-C60, each time profile of the fluorescence due to the 1nT* moiety consists of two components. The sub-picosecond component and a few picosecond components were experimentally evaluated depending on the lengths of oligothiophenes (n =8 and 12) and on the analyzing wavelength of the fluorescence. However, the time trace of the fluorescence due to 14T*-C60 decayed with a single short component in approximately 300 fs due to direct excited energy transfer (EET) from the 14T* moiety to the C60 moiety. On the basis of the kinetic models considering the short and long locally pi-conjugative thiophene segments in 8T-C60 and 12T-C60, the rate parameters of the elemental processes were evaluated. Sub-picosecond time constants of nT-C60 were found to be EET from the thiophene segment vicinal to the C60 moiety and intrachain energy transfer. Slower picosecond dynamics mainly corresponds to EET from the thiophene segments apart from the C60 moiety. 相似文献
53.
Associating polymers are hydrophilic long-chain molecules containing a small amount of hydrophobic groups. The aqueous solutions
show viscoelastic responses above some critical concentrations because a three-dimensional structure is formed by association
of hydrophobic groups. When the associating polymers are added to silica suspensions at low concentrations, the flocculation
is induced by bridging mechanisms, and the flow of suspensions become shear-thinning. For suspensions prepared with polymer
solutions in which the associating network is developed, the viscosity decreases, shows a minimum, and then increases with
increasing particle concentration. The viscosity decrease may arise from the breakdown of associating network due to adsorption
of polymer chains onto the silica surfaces. As the particle concentration is increased, the polymer concentration in solution
is decreased, and finally, all polymer chains are adsorbed on the surfaces. Beyond this point, the partial coverage of particle
surfaces takes place and strong interactions are generated between particles by polymer bridging. Since the stable suspensions
are converted to highly flocculated systems, the viscosity is increased and the flow becomes shear-thinning. The concentration
effect of silica particles on the viscosity behavior of suspensions can be explained by a combination of viscosity decrease
in solution due to polymer adsorption and viscosity increase due to flocculation. 相似文献
54.
Casado J Takimiya K Otsubo T Ramírez FJ Quirante JJ Ponce Ortiz R González SR Moreno Oliva M López Navarrete JT 《Journal of the American Chemical Society》2008,130(43):14028-14029
The vibrational Raman spectra of a decathiophene are provided in three relevant oxidations states: for the radical cation, its class III mixed valence system and its "frozen, -170 degrees C" class II MV analogue; for the dication, its singlet biradical pi-dimer and its "hot, +70 degrees C" magnetically active triplet excited state. Everything is compatible with interpentathiophene charge delocalization occurring at distances of 4-5 A similar to those found in the crystals of pi-stacked oligothiophenes. This stresses the interest of this spectroscopic tool for the analyses of electronic processes in crystals or in thin films of conjugated organic molecules. 相似文献
55.
Ken‐ichi Otake Dr. Kazuya Otsubo Dr. Kunihisa Sugimoto Prof. Akihiko Fujiwara Prof. Hiroshi Kitagawa 《Angewandte Chemie (International ed. in English)》2016,55(22):6448-6451
We report the structural design and control of electronic states of a new series of ultrafine metal–organic right square prism‐shaped nanowires. These nanowires have a very small inner diameter of about 2.0 Å, which is larger than hydrogen and similar to xenon atomic diameters. The electronic states of nanowires can be widely controlled by substitution of structural components. Moreover, the platinum homometallic nanowire shows a 100 times higher proton conductivity than a palladium/platinum heterometallic one depending on the electronic states. 相似文献
56.
The mixed catalyst system, Pd(CH3CN)2Cl2/P(t-Bu)3/[Ru(2,2'-bipyridine)3].2PF6, promotes the copper-free Sonogashira coupling reaction of aryl bromides at room temperature under irradiation of visible light. 相似文献
57.
Kazuo Takimiya Akinobu Oharuda Atsushi Morikami Yoshio Aso Tetsuo Otsubo 《Angewandte Chemie (International ed. in English)》1998,37(5):619-622
Recrystallization of the isomeric mixtures of 1 a and 1 b obtained by synthesis gave single crystals of the cis-trans and cis-cis isomers, respectively. The unique stacked structures of these compounds were elucidated by X-ray structure analysis. 相似文献
58.
Masahisa Wada Laurent Heux Yoshiharu Nishiyama Paul Langan 《Cellulose (London, England)》2009,16(6):943-957
X-ray crystallographic and cross-polarization/magic angle spinning 13C nuclear magnetic resonance techniques have been used to study an ethylenediamine (EDA)-cellulose I complex, a transient structure in the cellulose I to cellulose IIII conversion. The crystal structure (space group P2 1 ; a = 4.546 Å, b = 11.330 Å, c = 10.368 Å and γ = 94.017°) corresponds to a one-chain unit cell with one glucosyl residue in the asymmetric unit, a gt conformation for the hydroxymethyl group, and one EDA molecule per glucosyl residue. Unusually, there are no O–H···O hydrogen bonds between the cellulose chains; the chains are arranged in hydrophobic stacks, stabilized by hydrogen bonds to the amine groups of bridging EDA molecules. This new structure is an example of a complex in which the cellulose chains are isolated from each other, and provides a number of insights into the structural pathway followed during the conversion of cellulose I to cellulose IIII through EDA treatment. 相似文献
59.
Natsumi Tanaka Takahiro Suzuki Dr. Takehiko Matsumura Yosuke Hosoya Masahisa Nakada Prof. Dr. 《Angewandte Chemie (International ed. in English)》2009,48(14):2580-2583
Intelligent design : The total synthesis of the cytotoxic (?)‐FR182877 relies on 1) tandem Diels–Alder reactions to close rings A–D (see scheme), 2) a palladium‐mediated 7‐exo‐trig reaction, and 3) an iridium‐mediated isomerization followed by epimerization and stereoselective reduction.
60.
We extend Geiger’s size-biased tree method to the multitype case to give another proof of Zubkov’s result that the asymptotic relative generation distribution of the closest common ancestor of the multitype Galton–Watson branching process conditioned on non-extinction is uniform. 相似文献