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161.
Lesson study (LS) is a form of professional development, with a strong foundation in mathematics education, based on teachers collaborating to design lesso 相似文献
162.
DFT calculations have been performed on the adsorption of NH3 on Ni(1 1 1) to obtain information on the structure of the absorbed species, the nature of the chemical interactions between the adsorbate and the surface and the structure of multilayers formed at high coverage. A cluster model, using localized basis functions as well as an approach based on plane waves and periodic boundary conditions have been considered. The two approaches lead to similar results for the relative stabilities of investigated adsorption sites (atop > fcc > hcp) with an adsorption energy of about 15-24 kcal/mol (depending on the coverage). On the atop site, α-ammonia adsorbs molecularly with an equilibrium distance between the nitrogen atom and the surface of 2.03 ± 0.02 Å and a geometry close to the one of the molecule in the gas phase. The good agreement between the two DFT approaches clearly underlines the local nature of the adsorption reaction. The vibrational frequencies computed for NH3 adsorbed in this site are in good agreement with experimental values. They show that the interaction with the surface leads to a weakening of the strength of the N-H bond while the angular stretching is stronger. Both orbital and topological analyses were used to investigate chemical interactions between the cluster and the molecule. The results strongly suggest an electrostatic (non-covalent) interaction between the substrate and the molecule. Calculations with NH3 coverages above 0.25 confirm that saturation occurs at a coverage of 0.25. Above the saturation coverage, ammonia molecules in excess form multiple layers (β and γ ammonia) bonded to the first layer by intermolecular hydrogen bonds. N 1s core level shifts calculations performed for the several investigated coverages are also in good agreement with experimental XPS data. It is shown that the H-bond network more than the bond to the surface allows to understand the N 1s core level variations. 相似文献
163.
Single crystals of a new compound, FeV2O6H0.5, have been obtained by hydrothermal synthesis at 650°C and 2 kbar. An electron microprobe analysis indicated that the chemical formula is FeV2O6. This compound has an orthorhombic symmetry, space group P212121 with Z = 4. The unit cell dimensions areThese parameters are related to a′, b′, c′ of the diaspore-type VO2 by the relations: . The structure, based on single crystal data, has been determined from Patterson and Fourier syntheses. A structural refinement gave a final R-factor of 2.4%. The new structure can be deduced from that of the diaspore-type VO2, by displacement of one third of the cations from an octahedral site to an adjacent unoccupied tetrahedral site, which is located in a channel parallel to the c axis. The calculation of the cation and oxygen valences indicated that some O2? were in fact OH?. This conjecture was supported by thermogravimetric analysis. The chemical formula was indeed Fe3+2V3+V4+V5+2O11(OH). Some intensities of the powder diffraction lines changed strongly when the preparation temperature was reduced from 650°C to 300°C. This was explained by an increase of the hydrogen ratio in the formula FeV2O6Hx, which implies a structural change. The different phases FeV2O6Hx can be considered as solid solutions between two extreme phases with different structure: one with the present one and the other with a diaspore-type structure. 相似文献
164.
The present work addresses the question of the range and amplitude of bridging attraction that is induced between surfactant micelles functionalized with complexing groups in the presence of coordination centers. An alkylethoxylated ester phosphate (AEP) is synthesized from a non‐ionic surfactant and anchored into DTAB micelles. In the absence of any coordination center, functionalized micelles repel each other. Phase behavior, dynamic light scattering and small angle neutrons scattering (SANS) experiments show that this repulsive interaction is switched to attractive by the addition of coordination centers such as aluminum cations. The extent of the composition range of coexisting phase depends on the concentration of coordination center and on the pH. Analysis of the structure factor obtained from SANS shows that the range of attraction is determined by the molecular dimension of the chelating surfactant, while the depth can be tuned with the concentration of coordination center and the pH. The strong influence of the pH is interpreted as arising from the condensation of aluminium cations that lead to high functional polynuclear complexes. 相似文献
165.
The influence of substituent nature and position on the unimolecular decomposition of nitroaromatic compounds was investigated using the density functional theory at a PBE0/6-31+G(d,p) level. As the starting point, the two main reaction paths for the decomposition of nitrobenzene were analyzed: the direct carbon nitrogen dissociation (C6H5 + NO2) and a two step mechanism leading to the formation of phenoxyl and nitro radicals (C6H5O + NO). The dissociation energy of the former reaction was calculated to be 7.5 kcal/mol lower than the activation energy of the second reaction. Then the Gibbs free energies were computed for 15 nitrobenzene derivatives characterized by different substituents (nitro, methyl, amino, carboxylic acid, and hydroxyl) in the ortho, meta, and para positions. In meta position, no significant changes appeared in the reaction energy profiles whereas ortho and para substitutions led to significant deviations in energies on the decomposition mechanisms due to the resonance effect of the nitro group without changing the competition between these mechanisms. In the case of para and meta substitutions, the carbon-nitro bond dissociation energy has been directly related to the Hammett constant as an indicator of the electron donor-acceptor effect of substituents. 相似文献
166.
Na0.4Y0.6F2.2:Er3+ (NYF:Er) crystals with an erbium concentration as high as 100 at. % (Na0.4Er0.6F2.2) were grown by the Bridgman-Stockbarger method. The optical spectra were investigated at low (6 K) and room temperatures. It is shown that the absorption spectrum of NYF:Er crystals contains wide bands (790–801 and 965–980 nm) corresponding to the emission range of laser diodes. The peak absorption cross section σa for the band peaked at λ=970.4 nm is 0.15×10?20 cm2. On the basis of the analysis of the absorption and luminescence spectra at low (6 and 12 K) temperatures, the structure of the Stark splitting of erbium levels was determined as a structure of quasi-centers for which Stark components are inhomogeneously broadened. The oscillator strengths of the transitions from the ground state of erbium to excited multiplets were calculated from the absorption spectra measured at T=300 K, and the intensity parameters Ωt were determined by the Judd-Ofelt method: Ω2=1.65×10?20 cm2, Ω4=0.56× 10?20 cm2, and Ω6=1.01×10?20 cm2. These values of the intensity parameters were used to calculate the probabilities of radiative transitions and the branching ratios. The rates of multiphonon nonradiative transitions in NYF: Er were estimated. The luminescence decay kinetics for radiative levels of erbium ions upon their selective excitation by nanosecond laser pulses was studied. The intracenter lifetimes of radiative levels of erbium ions were determined from the luminescence kinetics upon selective ion excitation by low-intensity light in a sample with a low erbium concentration (0.5%). It is demonstrated that, with an increase in temperature from 6 to 300 K, luminescence from the 4 G 11/2, 2 G(H)9/2, and 4 F 9/2 levels is quenched as a result of multiphonon nonradiative transitions. Luminescence from the 4 I 9/2 level is quenched only insignificantly with increasing temperature, and no quenching of luminescence from the 4 I 11/2 and 4 I 13/2 levels is observed. The spectra of steady-state luminescence of NYF:Er(0.5–15%) crystals were investigated upon broadband excitation by UV and UV-visible lamp light and selective time-resolved laser excitation. It is shown that low-lying levels of erbium ions separated by an energy gap smaller than 2500 cm?1 are populated via cascade mechanisms. On the basis of the results obtained, it is concluded that NYF:Er 3+ crystals are promising candidates for active media of tunable diode-pumped lasers. 相似文献
167.
Danielle van der Westhuizen Dr. Cathryn A. Slabber Dr. Manuel A. Fernandes Daniël F. Joubert Dr. George Kleinhans Dr. C. Johan van der Westhuizen Dr. André Stander Prof. Orde Q. Munro Dr. Daniela I. Bezuidenhout 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(32):8295-8307
The syntheses of bis(triazolium)carbazole precursors and their corresponding coinage metal (Au, Ag) complexes are reported. For alkylated triazolium salts, di- or tetranuclear complexes with bridging ligands were isolated, while the bis(aryl) analogue afforded a bis(carbene) AuI-CNC pincer complex suitable for oxidation to the redox-stable [AuIII(CNC)Cl]+ cation. Although the ligand salt and the [AuIII(CNC)Cl]+ complex were both notably cytotoxic toward the breast cancer cell line MDA-MB-231, the AuIII complex was somewhat more selective. Electrophoresis, viscometry, UV-vis, CD and LD spectroscopy suggest the cytotoxic [AuIII(CNC)Cl]+ complex behaves as a partial DNA intercalator. In silico screening indicated that the [AuIII(CNC)Cl]+ complex can target DNA three-way junctions with good specificity, several other regular B-DNA forms, and Z-DNA. Multiple hydrophobic π-type interactions involving T and A bases appear to be important for B-form DNA binding, while phosphate O⋅⋅⋅Au interactions evidently underpin Z-DNA binding. The CNC ligand effectively stabilizes the AuIII ion, preventing reduction in the presence of glutathione. Both the redox stability and DNA affinity of the hit compound might be key factors underpinning its cytotoxicity in vitro. 相似文献
168.
C. Boudaya L. Laroussp E. Dhahrp J. C. Joubert A. Cheikh-Rouhou 《Phase Transitions》2013,86(4):455-465
We investigate structural features (cell distortions), magnetic and electric properties versus temperature for La1-x K x MnO3 (0.05 < x < 0.2) perovskites. All the phases crystallize in a rhomboedral symmetry (R3c) and are ferromagnetic. Curie temperatures are strongly dependent of x, with a maximum around 308 K for La0.8K0.2MnO3. A sharp decrease of the resistivity is observed just below the Curie temperature, accompanied by a very steep increase of the spontaneous magnetization, characteristic of a conventional Brillouin type curve. The average magnetic moment of Mn is about 88 e.m.u/g at 5 K for the composition La0.8K0.2MnO3. The transition from ferromagnetic-metallic to paramagnetic-semiconducting states is explained from the suggested distribution of the cations (La3+ 1-x K+ x )A(Mn4+ 1-2x Mn4+ 2x )BO3 by the double exchange of Mn3+-Mn4+ pairs at the B-sublattice. Preliminary magnetoresistance measurements on bulk ceramic samples display an MR effect of the same amplitude as in the case of the alkaline earth substituted La manganites. 相似文献
169.
A systematic study of the structural, magnetic and electrical properties of the manganites Pr1-x Sr x MnO3(0≤ x ≤ 0.5) has been carried out. X-ray diffraction investigation shows a structural change with composition, from orthorhombic (0 ≤ x ≤ 0.2) to rhombohedral (0.25 ≤ x ≤ 0.5). The magnetic properties of Pr1-x Sr x MnO3 samples could be explained on the basis of a double exchange mechanism between pairs of Mn3+ and Mn4+ ions. These properties are strongly dependent on the ratio of Mn3+/Mn4+. The maximum of the ferromagnetic transition temperature Tc is reached at x ≈ 0.35 corresponding to a value 1.85 of this ratio. The investigation of the electrical properties shows a semiconductor to metal transition as a function of temperature (0.25≤x≤0.4) with a metallic-like behaviour above a critical temperature Tp . A semiconducting-like one is observed for all the range of temperature (50–300 K) for (0 ≤ x ≤ 0.2 and x = 0.5). The evolution of activated energies with the carrier concentration has been investigated. 相似文献
170.
Bryan's effect is the phenomenon of the rotation of the vibration pattern within the body of a rotating, vibrating body. In this paper we generalise Rayleigh's dissipation function whereby we include isotropic, nonlinear damping in the Euler–Lagrange equations. We then derive the equations of motion of a slowly rotating, vibrating, symmetric body. In so doing we analyse the effect that such damping has on Bryan's effect. Using a combination of linear and nonlinear damping in the equations of motion of a slowly rotating symmetric body, a numerical experiment indicates that underdamping, critical damping and overdamping scenarios appear to exist. For the underdamped case we compare the effect on amplitude of vibration made by linear, quadratic and a combination of these types of damping. Our final result for light, isotropic, nonlinear damping, mimicks a known result for light, isotropic, linear damping, namely for a slowly rotating symmetric body, Bryan's effect is invariant and the rate of rotation of the damped pattern is the same as it would be for the pattern of an ideal slowly rotating symmetric body. 相似文献