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991.
The dielectric and elastic properties of liquid crystals (LCs) generally depend on the molecule structure, such as polar group and carbon chain length. For further investigation of the influence of molecular structure on the dielectric and elastic properties of fluorine-containing tricyclic isothiocyanate LC, the experimental temperature was controlled at 25°C by precision hot stage, and a precision LCR meter was used to measure the capacitance of six LC cells under the voltage from 0.1 to 20 V at 1 kHz. An LC cell capacitance model and a dual-cell model were adopted to obtain the dielectric anisotropy, and the capacitance–voltage curves of six LC materials were plotted. The threshold voltage of Fréedericksz transition was analysed, and a finite difference iterative method was used to attain specific values of three elastic constants. The influence of molecular structure on the dielectric and elastic constants was finally analysed. Experimental results showed that the introduction of meta-difluoro group would increase the dielectric anisotropy and reduce the threshold voltage of LC. As the length of the alkyl carbon chain increased, the dielectric anisotropy would have an odd–even alternation effect, which would lead to changes in the elastic constants of LC. 相似文献
992.
Dr. Stefan van der Vorm Thomas Hansen Dr. Erwin R. van Rijssel Rolf Dekkers Jerre M. Madern Prof. Dr. Herman S. Overkleeft Dr. Dmitri V. Filippov Prof. Dr. Gijsbert A. van der Marel Dr. Jeroen D. C. Codée 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(29):7149-7157
The 3D shape of glycosyl oxocarbenium ions determines their stability and reactivity and the stereochemical course of SN1 reactions taking place on these reactive intermediates is dictated by the conformation of these species. The nature and configuration of functional groups on the carbohydrate ring affect the stability of glycosyl oxocarbenium ions and control the overall shape of the cations. We herein map the stereoelectronic substituent effects of the C2-azide, C2-fluoride and C4-carboxylic acid ester on the stability and reactivity of the complete suite of diastereoisomeric furanoses by using a combined computational and experimental approach. Surprisingly, all furanosyl donors studied react in a highly stereoselective manner to provide the 1,2-cis products, except for the reactions in the xylose series. The 1,2-cis selectivity for the ribo-, arabino- and lyxo-configured furanosides can be traced back to the lowest-energy 3E or E3 conformers of the intermediate oxocarbenium ions. The lack of selectivity for the xylosyl donors is related to the occurrence of oxocarbenium ions adopting other conformations. 相似文献
993.
994.
A new aromatic diamine, 4-(4-trifluoromethyl)phenyl-2,6-bis[4-(4-amino-2-trifluoromethylphenoxy)phenyl]pyridine, was synthesized by a modified Chichibabin reaction of 4-(4-nitro-2-trifluoromethylphenoxy)acetophenone with 4-triflouromethylbenzaldehyde, followed by catalytic reduction. A series of fluorinated pyridine-containing aromatic poly(ether imide)s (PEIs) were prepared from the diamine monomer with various aromatic dianhydrides via conventional two-step thermal imidization method. The resulting PEIs had inherent viscosities values of 0.68–0.90 dL/g, and could be cast and thermally converted into transparent, flexible, and tough polymer films. These PEIs were predominantly amorphous, had good solubility in common solvents such as NMP, DMAc and m-cresol at room temperature, and displayed excellent thermal stability with the glass transition temperatures of 258–315?°C, the temperatures at 5% weight loss of 550–585?°C, and the residue of higher than 55% at 750?°C in nitrogen. Moreover, the PEIs films showed outstanding mechanical properties with tensile strengths of 74.8–103.5?MPa, tensile moduli of 1.08–1.45?GPa, and elongations at break of 10.6–24.4%. These PEIs also exhibited low dielectric constants of 2.81–2.98 (1?MHz) and water uptake 0.39–0.68%, as well as high optical transparency with the UV cutoff wavelength in the 350–378?nm range and the wavelength of 80% transparency in the range of 412–510?nm. 相似文献
995.
In the context of designing an efficient thermoelectric energy-conversion device at nanoscale level, we suggest several important tuning parameters to enhance the performance of thermoelectric converters. We consider a simple molecular junction, which is always helpful to understand the basic mechanisms in a deeper way, where a benzene molecule is coupled to two external baths having unequal temperatures. The key component responsible for achieving better performance is associated with the asymmetric nature of transmission function, and in the present work, we show that it can be implemented in different ways by regulating the physical parameters involving the system. Employing a tight-binding framework we calculate electrical and thermal conductances, thermopower, and figure of merit (FOM) by using Landauer integrals, and thoroughly examine the critical roles played by molecule-to-lead (ML) interface geometry, magnetic field, chemical substituent group, ML coupling, and the direct coupling between the two leads. Our results show that a reasonably large FOM (≫1) can be obtained and lead to a possibility of regulating the efficiency by selectively tuning the physical parameters. We believe that the present analysis will enhance the understanding of designing efficient thermoelectric devices, and can be verified in a laboratory. 相似文献
996.
Reaction Kinetics between Thiobases and Singlet Oxygen Studied by Direct Detection of the 1O2 Luminescence Decay 下载免费PDF全文
Thiobase derivatives have received important investigations due to their wide usage as phototherapeutic agents and their potential carcinogenic side effects as immunosuppressants. The substitution of oxygen atom by the sulfur atom makes the ultraviolet absorption of thiobases redshifted and absorbs UVA light (>300 nm), resulting in unusual high quantum yield of triplet state to generate the singlet oxygen (1O2) through photosensitization. As a type of reactive oxygen species, 1O2 is highly reactive toward thiobases. Herein, we report the measurements of reaction rate constants between di erent thiobases and 1O2 in different solvents through the direct detection of 1O2 luminescence decay kinetics at 1270 nm. The rate constants of thiouracils with 1O2 are five times smaller than that of thioguanine with 1O2, which suggests that thiopurines are more reactive than thiopyrimidines and thus less suitable to be a photosensitive drug on the application of photodynamic therapy. Additionally, the rate constants of thiobases and 1O2 were found to be obviously influenced by the solvent polarity. With the increase of solvent polarity, the rate constants of thiobases and 1O2 decrease. 相似文献
997.
Orfeu Bertolami 《General Relativity and Gravitation》2008,40(9):1891-1898
We propose a Curvature Principle to describe the dynamics of interacting universes in a multi-universe scenario and show,
in the context of a simplified model, how interaction drives the cosmological constant of one of the universes toward a vanishingly
small value. We also conjecture on how the proposed Curvature Principle suggests a solution for the entropy paradox of a universe
where the cosmological constant vanishes.
Essay selected for an honorable mention by the Gravity Research Foundation, 2007. 相似文献
998.
ZHANG Yu-Feng 《理论物理通讯》2008,50(11):1021-1026
A new Lie algebra G of the Lie algebra sl(2) is constructed with complex entries whose structure constants are real and imaginary numbers. A loop algebra G corresponding to the Lie algebra G is constructed, for which it is devoted to generating a soliton hierarchy of evolution equations under the framework of generalized zero curvature equation which is derived from the compatibility of the isospectral problems expressed by Hirota operators. Finally, we decompose the Lie algebra G to obtain the subalgebras G1 and G2. Using the G2 and its one type of loop algebra G2, a Liouville integrable soliton hierarchy is obtained, furthermore, we obtain its bi-Hamiltonian structure by employing the quadratic-form identity. 相似文献
999.
V. Anbazhagan 《Journal of luminescence》2008,128(9):1454-1458
The interaction of 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) with bovine serum albumin (BSA) has been studied using absorption and steady state fluorescence techniques. Fluorescence spectrum of BSA (λexi=280 nm) in the presence of DBO clearly shows that DBO acts as a quencher. The number of binding sites ‘n’ and apparent binding constant ‘K’ were measured by Stern-Volmer equation. Synchronous fluorescence and absorption spectra were used to study protein conformation. The interaction between DBO and BSA is consistent with static quenching and the conformational changes of BSA observed. 相似文献
1000.
S. F. Baranovskii P. A. Bolotin M. P. Evstigneev D. N. Chernyshev 《Journal of Applied Spectroscopy》2008,75(2):251-260
We have used spectrophotometry to study self-association and complexation with DNA by organic heterocyclic compounds in the
acridine and phenothiazine series: proflavin, thionine, and methylene blue. Based on the experimental concentration dependences
of the molar absorption coefficient of the molecules in an aqueous buffer solution (0.01 M NaCl, 0.01 M Na2EDTA, 0.01 M Tris, pH 7.4, T = 298 K), we have determined the equilibrium dimerization constants for the dyes and the DNA
complexation parameters using the Scatchard and McGhee-von Hippel models. The observed increase in the cooperativity parameters
as the dimerization constants of the ligands increase allowed us to hypothesize that the same interactions occur between dye
molecules adsorbed on DNA as in their self-association. The equilibrium DNA-binding constants for the ligands, obtained using
the McGhee-von Hippel cooperative model, are (20.9 ± 2.7)·103 M−1 for proflavin and (33.8 ± 4.1)·103 M−1 for thionine. Using the Scatchard model, taking into account intercalation and “external” binding of ligands with DNA, we
determined the DNA complexation constants for methylene blue: (26.4 ± 4.6)·103 and (96 ± 17)·103 M−1 respectively. Based on analysis of the data obtained, we hypothesized that the predominant type of binding with DNA is intercalation
binding in the case of proflavin and thionine, and “external” binding with the DNA surface in the case of methylene blue.
__________
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 2, pp. 242–249, March–April, 2008. 相似文献