全文获取类型
收费全文 | 1623篇 |
免费 | 23篇 |
国内免费 | 6篇 |
专业分类
化学 | 952篇 |
晶体学 | 65篇 |
力学 | 31篇 |
数学 | 105篇 |
物理学 | 499篇 |
出版年
2022年 | 24篇 |
2021年 | 30篇 |
2020年 | 22篇 |
2019年 | 28篇 |
2018年 | 23篇 |
2017年 | 26篇 |
2016年 | 37篇 |
2015年 | 27篇 |
2014年 | 42篇 |
2013年 | 125篇 |
2012年 | 81篇 |
2011年 | 97篇 |
2010年 | 52篇 |
2009年 | 50篇 |
2008年 | 61篇 |
2007年 | 63篇 |
2006年 | 76篇 |
2005年 | 52篇 |
2004年 | 57篇 |
2003年 | 30篇 |
2002年 | 41篇 |
2001年 | 24篇 |
2000年 | 19篇 |
1999年 | 15篇 |
1998年 | 12篇 |
1997年 | 13篇 |
1996年 | 12篇 |
1995年 | 15篇 |
1994年 | 19篇 |
1993年 | 18篇 |
1992年 | 28篇 |
1991年 | 18篇 |
1990年 | 20篇 |
1989年 | 19篇 |
1988年 | 17篇 |
1987年 | 12篇 |
1986年 | 19篇 |
1985年 | 25篇 |
1984年 | 22篇 |
1983年 | 25篇 |
1982年 | 18篇 |
1981年 | 19篇 |
1980年 | 29篇 |
1979年 | 25篇 |
1978年 | 20篇 |
1977年 | 19篇 |
1976年 | 11篇 |
1975年 | 15篇 |
1973年 | 12篇 |
1971年 | 9篇 |
排序方式: 共有1652条查询结果,搜索用时 15 毫秒
991.
The study of preferential addition of heterocyclic amines onto halo-substituted arylalkynes over N-arylation under various catalytic conditions is described. The present work supports and confirms the mechanistic pathway of our recent work on the tandem synthesis of indolo- and pyrrolo-[2,1-a]isoquinolines via hydroamination followed by oxidative addition and not vice versa. 相似文献
992.
S Koch RK Joshi M Noyong J Timper JJ Schneider U Simon 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(37):11614-11620
The formation of stochastically oriented carbon-nanotube networks on top of an array of free-standing chromium-capped silicon nanopillars is reported. The combination of nanosphere lithography and chemical vapor deposition enables the construction of nanostructures that exhibit a hierarchical sequence of structural sizes. Metallic chromium serves as an etching mask for Si-pillar formation and as a nucleation site for the formation of carbon nanotubes through the chemical vapor deposition of ethene, ethanol, and methane, respectively, thereby bridging individual pillars from top to top. Iron and cobalt were applied onto the chromium caps as catalysts for CNT growth and the influence of different carbon sources and different gas-flow rates were investigated. The carbon nanotubes were structurally characterized and their DC electrical properties were studied by in situ local- and ex situ macroscopic measurements, both of which reveal their semiconductor properties. This process demonstrates how carbon nanotubes can be integrated into Si-based semiconductors and, thus, this process may be used to form high-surface-area sensors or new porous catalyst supports with enhanced gas-permeation properties. 相似文献
993.
The electronic, optical and structural properties of ZnxCd1−xSySe1−y quaternary alloys lattice matched to GaAs and InP are studied. The electronic band structure and density of states are computed using empirical pseudopotential method. The disorder effects are included via modified virtual crystal approximation. The bandgap computed from band structures are utilized to evaluate refractive indices, dielectric constants and ionicity factors for the alloys. Among structural properties elastic constants and bulk moduli are computed by combining the EPM with Harrison bond orbital model. All possible semiconductors from the ZnCdSSe system are found to have direct bandgap. The lattice matched alloys have larger band gap and more ionic character than the lattice matched compounds. 相似文献
994.
Samanta D Shanmugaraju S Joshi SA Patil YP Nethaji M Mukherjee PS 《Chemical communications (Cambridge, England)》2012,48(17):2298-2300
Three-component self-assembly of a cis-blocked 90° Pd(II) acceptor with a mixture of a tetraimidazole and a linear dipyridyl donor self-discriminated into unusual Pd(8) molecular swing (1) and Pd(6) molecular boat (2), which are characterized by single-crystal X-ray diffraction analysis; their ability to bind C(60) in solution is established by fluorescence titration. 相似文献
995.
In the present study we have investigated the effect of methyl substitution at various positions on the spectral and decay behavior of some quinolinium ions. The spectra reveal marked differences when methyl group is present at 7th and 8th positions. The non-radiative deactivation is also considerably affected by the substituent position. The results are explained by considering the modulation (decrease as compared to the unsubstituted quinolinium) in nonradiative rate (intersystem crossing) due to substitution. 相似文献
996.
The essential oil of the leaves of Feronia elephantum Corr. was analyzed by gas chromatography and gas chromatography/mass spectrometry. The main constituents were beta-pinene (28.4%), Z-anethole (22.1%), methyl chavicol (12.0%) and E-anethole (8.1%), among thirty-three identified compounds, which represented 92.6% of the total oil. The antimicrobial activity was tested against five Gram-positive and eight Gram-negative bacteria, and four fungi. The oil was active against Micrococcus luteus (Gram-positive bacterium), Proteus mirabilis (Gram-negative bacterium), Penicillium chrysogenum and Aspergillus niger (fungi) with MIC values of 0.31 +/- 0.06, 0.52 +/- 0.10, 0.20 +/- 0.50 and 0.26 +/- 0.52 mg/mL, respectively. 相似文献
997.
Ramin Aghababaei Shailendra P. Joshi 《International Journal of Solids and Structures》2011,48(18):2585-2594
Metal matrix composites (MMCs) comprising nano/microcrystalline matrices and reinforcements exhibit impressive mechanical behaviors derived by exploiting the size effects due to development of geometrically necessary dislocations. In such nanostructured MMCs intricate interactions between the grain size dg and inclusion size di may exist in their overall response, but are difficult to isolate in experiments and are also not accounted for in the size-dependent homogenized models. In this paper, we computationally investigate the grain size–inclusion size interaction in model MMCs architectures wherein the grains and inclusions are explicitly resolved. A mechanism-based slip-gradient crystal plasticity formulation (Han et al., 2005a) is implemented in a finite element framework to model polycrystalline mass as an aggregate of randomly oriented single crystals that host elastic inclusions. The slip gradients that develop across grain boundaries and at inclusion–grain interfaces during deformation result in length-scale dependent responses that depend on both dg and di, for a fixed inclusion volume fraction f. For a given di and f, the overall hardening exhibits a nonlinear dependence on grain size for dg ? di indicating that interaction effects become important at those length-scales. Systematic computational simulations on bare polycrystalline and MMC architectures are performed in order to isolate the contributions due to grain size, inclusion size and the interaction thereof. Based on these results, an analytical model developed for the interaction hardening exhibits a Hall–Petch type dependence on these microstructural sizes that can be incorporated into homogenized approaches. 相似文献
998.
The regio- and stereoselective addition of N-heterocycles to alkynes using KOH is reported. Formation of (Z)-isomers and their conversion to (E)-products were found to be dependent upon time as well as the choice of base. Selective attack of N-heterocycles on a more electrophilic alkynyl carbon was supported by DFT calculations, and the stereochemistry of the products was established by X-ray crystallographic studies and intramolecular cyclization of ortho-haloalkynes in indolo-[2,1-a]isoquinolines. 相似文献
999.
Aneeta Kharkwal Dr. Melepurath Deepa Dr. Amish G. Joshi Dr. Avanish Kumar Srivastava 《Chemphyschem》2011,12(6):1176-1188
Poly(3,4‐ethylenedioxypyrrole) (PEDOP)–Ag and PEDOP–Au nanocomposite films have been synthesized for the first time by electropolymerization of the conducting‐polymer precursor in a waterproof ionic liquid, 1‐butyl‐1‐methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, followed by Ag/Au nanoparticle incorporation. That the Ag/Au nanoparticles are not adventitious entities in the film is confirmed by a) X‐ray photoelectron spectroscopy, which provides evidence of Ag/Au–PEDOP interactions through chemical shifts of the Ag/Au core levels and new signals due to Ag–N(H) and Au–N(H) components, and b) electron microscopy, which reveals Au nanoparticles with a face‐centered‐cubic crystalline structure associated with the amorphous polymer. Spectroelectrochemistry of electrochromic devices based on PEDOP–Au show a large coloring efficiency (ηmax=270 cm2 C?1, λ=458 nm) in the visible region, for an orange/red to blue reversible transition, followed by a second, remarkably high ηmax of 490 cm2 C?1 (λ=1000 nm) in the near‐infrared region as compared to the much lower values achieved for the neat PEDOP analogue. Electrochemical impedance spectroscopy studies reveal that the metal nanoparticles lower charge‐transfer resistance and facilitate ion intercalation–deintercalation, which manifests in enhanced performance characteristics. In addition, significantly faster color–bleach kinetics (five times of that of neat PEDOP!) and a larger electrochemical ion insertion capacity unambiguously demonstrate the potential such conducting‐polymer nanocomposites have for smart window applications. 相似文献
1000.
Joshi P Shewale V Pandey R Shanker V Hussain S Karna SP 《Physical chemistry chemical physics : PCCP》2011,13(2):476-479
First principles density functional theory calculations are performed on tryptophan-ZnO nanoparticles complex in order to study site specific interactions between tryptophan and ZnO. The calculated results find the salt bridge structure involving the -COOH group and ZnO cluster to be energetically more favorable than other interacting sites, such as indole and amine groups in tryptophan. The interaction between tryptophan and ZnO appears to be mediated by both ionic and hydrogen bonds. The calculated molecular orbital energy levels and charge distributions suggest non-radiative energy transfer from an excited state of tryptophan to states associated with ZnO, which may lead to a reduction in the emission intensity assigned to the π-π* transition of the indole functional group of tryptophan. 相似文献