全文获取类型
收费全文 | 4602篇 |
免费 | 247篇 |
国内免费 | 24篇 |
专业分类
化学 | 3429篇 |
晶体学 | 38篇 |
力学 | 92篇 |
数学 | 602篇 |
物理学 | 712篇 |
出版年
2023年 | 29篇 |
2022年 | 60篇 |
2021年 | 89篇 |
2020年 | 103篇 |
2019年 | 107篇 |
2018年 | 67篇 |
2017年 | 69篇 |
2016年 | 154篇 |
2015年 | 121篇 |
2014年 | 150篇 |
2013年 | 233篇 |
2012年 | 330篇 |
2011年 | 379篇 |
2010年 | 171篇 |
2009年 | 149篇 |
2008年 | 319篇 |
2007年 | 279篇 |
2006年 | 288篇 |
2005年 | 265篇 |
2004年 | 198篇 |
2003年 | 163篇 |
2002年 | 158篇 |
2001年 | 63篇 |
2000年 | 68篇 |
1999年 | 42篇 |
1998年 | 44篇 |
1997年 | 43篇 |
1996年 | 38篇 |
1995年 | 49篇 |
1994年 | 35篇 |
1993年 | 37篇 |
1992年 | 39篇 |
1991年 | 38篇 |
1990年 | 28篇 |
1989年 | 29篇 |
1988年 | 23篇 |
1987年 | 27篇 |
1986年 | 15篇 |
1985年 | 47篇 |
1984年 | 43篇 |
1983年 | 22篇 |
1982年 | 39篇 |
1981年 | 29篇 |
1980年 | 27篇 |
1979年 | 28篇 |
1978年 | 25篇 |
1977年 | 26篇 |
1976年 | 20篇 |
1975年 | 13篇 |
1973年 | 10篇 |
排序方式: 共有4873条查询结果,搜索用时 15 毫秒
991.
A single isomer of a monofunctionalised C70 fullerene is isolated and characterised using variable temperature NMR spectroscopy. Pendant methylene bromide groups allow for further cycloaddition reactions and a C60-C70 asymmetric fullerene dimer is thus prepared. 相似文献
992.
Arthur J. Carty Shaun F. Malone Nicholas J. Taylor 《Journal of organometallic chemistry》1979,172(2):201-211
The synthesis of selenium bound selenourea complexes of methylmercury, [CH3HgSeC(NH2)2]X (X=Cl, Br, NO3, ClO4) are described. A single crystal X-Ray analysis of the nitrate salt has provided for the first time bond length data pertinent to the biologically important mercury-selenium interaction. Crystals of [CH3HgSeC(NH2)2] NO3 belong to the space group PnZic with a = 7.524(1), b = 11.204(2), c = 9.738(2)Å, and z = 4. The structure was solved and refined using 561 observed reflections measured on a Syntex P21 diffractometer to a final R value of 0.037. The mercury atom is linearly coordinated to the methyl group and the selenium atom of the selenourea with Hg-Se of 2.477(3)Å. Strong Hg-Se bonding is indicated by the X-Ray data and by the 1H - 199Hg nmr coupling constants. A comparison of spectroscopic data for analogous thio and selenourea complexes is presented. 相似文献
993.
Eric W. Bohannan Xuerong Gao Kimberly R. Gaston Christopher D. Doss Chariklia Sotiriou-Leventis Nicholas Leventis 《Journal of Sol-Gel Science and Technology》2002,23(3):235-245
This study shows that conventional photolithography can be applied for patterning native or organic dye-doped silica films (0.5 m thick) obtained via a base-catalyzed sol-gel process. Photoresist was spin-coated onto high optical quality xerogel films, soft-baked, exposed to UV irradiation through a photomask, and developed with a commercial photoresist developing solution. Etching away of the photoresist-unprotected areas of the silica films was carried out with a dilute HF solution, while the remaining unexposed photoresist was removed with acetone. Interdigitated array patterns with features as small as 0.5 mm show a smooth surface and extremely sharp interfaces. Densification of the films at 550°C for 2 h decreases the film thickness by 11%, increases the refractive index from 1.420 to 1.456, and allows for well-defined patterning down to length scales of 10 m. Since the densification conditions are incompatible with organic dopants, it is demonstrated that sol-gel films can be doped after pattering (post-doping) by adsorption of cationic dyes from solution. Scanning electron microscopy reveals that the microstructure of patterned sol-gel films is similar to that of bulk monoliths, indicating that the photolithographic procedure is not harmful to the film quality. All patterned films demonstrate highly regular light diffraction patterns. 相似文献
994.
Pirrung MC Liu Y Deng L Halstead DK Li Z May JF Wedel M Austin DA Webster NJ 《Journal of the American Chemical Society》2005,127(13):4609-4624
The principle of methyl scanning is proposed for determination of the sites of interaction between biologically active small molecules and their macromolecular target(s). It involves the systematic preparation of a family of methylated derivatives of a compound and their biological testing. As a functional assay, the method can identify the regions of a molecule that are important (and unimportant) for biological activity against even unknown targets, and thus provides an excellent complement to structural biology. Methyl scanning was applied to demethylasterriquinone B1, a small-molecule mimetic of insulin. A new, optimal total synthesis of this natural product was developed that enables the family of methyl scan derivatives to be concisely prepared for evaluation in a cellular assay. The results of this experiment were used to design a biotin-demethylasterriquinone conjugate for use as an affinity reagent. This compound was prepared in tens of milligram quantities in a four-step synthesis. 相似文献
995.
Nicholas E. Leadbeater Sharon J. Pillsbury Erik Shanahan Victoria A. Williams 《Tetrahedron》2005,61(14):3565-3585
In the light of the controversy in the area and since reports using simultaneous cooling in conjunction with microwave heating are appearing in the literature, we were keen to assess the methodology, looking at temperature measurement issues as well as the use of the technique in three classes of reaction, namely a Heck coupling, a Diels-Alder cycloaddition and a Michael addition. We present our initial findings here. 相似文献
996.
997.
A. Bonesi M. Asteazaran M. S. Moreno G. Zampieri S. Bengio W. Triaca A. M. Castro Luna 《Journal of Solid State Electrochemistry》2013,17(7):1823-1829
Supported PtSnIr/C, PtSn/C, and IrSn/C catalysts with potential application in a direct alcohol fuel cell were prepared by chemical reduction employing Pechini methodology. The catalyst particles were characterized by high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy (XPS). Linear sweep voltammetry (LV), chronoamperometry, and electrochemical impedance spectroscopy (EIS) measurements were performed by using a glassy carbon working electrode covered with the catalyst in a 1 M ethanol?+?0.5 M H2SO4 solution at 60 °C. It was demonstrated through XPS that PtSnIr/C and IrSn/C contain both IrO2 and SnO2. LV and chronoamperometry show a better catalytic behavior for ethanol oxidation on PtSnIr/C in the low-potential region and the improvement is attributed to the presence of both Sn and Ir oxides. The EIS accurately established that PtSnIr/C improved ethanol oxidation at lower potentials than PtSn/C. 相似文献
998.
In this paper, we show that Goodwillie calculus, as applied to functors from stable homotopy to itself, interacts in striking ways with chromatic aspects of the stable category. Localized at a fixed prime p, let T(n) be the telescope of a vn self map of a finite S–module of type n. The Periodicity Theorem of Hopkins and Smith implies that the Bousfield localization functor associated to T(n)* is independent of choices. Goodwillies general theory says that to any homotopy functor F from S–modules to S–modules, there is an associated tower under F, {PdF}, such that FPdF is the universal arrow to a d–excisive functor. Our first main theorem says that PdFPd-1F always admits a homotopy section after localization with respect to T(n)* (and so also after localization with respect to Morava K–theory K(n)*). Thus, after periodic localization, polynomial functors split as the product of their homogeneous factors. This theorem follows from our second main theorem which is equivalent to the following: for any finite group G, the Tate spectrum
is weakly contractible. This strengthens and extends previous theorems of Greenlees–Sadofsky, Hovey–Sadofsky, and Mahowald–Shick. The Periodicity Theorem is used in an essential way in our proof. The connection between the two theorems is via a reformulation of a result of McCarthy on dual calculus. Mathematics Subject Classification (2000) 55P65, 55N22, 55P60, 55P91 相似文献
999.
This paper investigates the application of the steady-state master equation (SSME) to complex unimolecular reactions. It is shown how the steady-state approximation and the Boltzmann reservoir approximation can be applied, both together or separately, to improve the efficiency and the numerical robustness of simple master equation calculations. The case of a two-well isomerization is considered in detail, and two versions of the SSME are derived for it, one of which shows very clearly and analytically the relationship between the rate coefficients and the flux coefficients. It is shown for a reversible second order recombination reaction that the second order phenomenological equation can be regained in either version of the SSME. It is also shown how, under steady state conditions, the SSME may be used to determine rate coefficients for each reaction in a multiple well scheme. 相似文献
1000.
Wijnands F. Pendry J. B. Garcia-Vidal F. J. Bell P. M. Roberts P. J. Moreno L. Marti´n 《Optical and Quantum Electronics》1997,29(2):199-216
We present a new formalism for calculating the Green's function for Maxwell's equations. As our aim is to apply our formalism
to light scattering at surfaces of arbitrary materials, we derive the Green's function in a surface representation. The only
requirement on the material is that it should have periodicity parallel to the surface. We calculate this Green's function
for light of a specific frequency and a specific incident direction and distance with respect to the surface. The material
properties entering the Green's function are the reflection coefficients for plane waves at the surface. Using the close relationship
between the Green's function and the density of states (DOS), we apply our method to calculate the spontaneous emission rate
as a function of the distance to a material surface. The spontaneous emission rate can be calculated using Fermi's Golden
Rule, which can be expressed in terms of the DOS of the optical modes available to the emitted photon. We present calculations
for a finite slab of cylindrical rods, embedded in air on a square lattice. It is shown that the enhancement or suppression
of spontaneous emission strongly depends on the frequency of the light.
This revised version was published online in November 2006 with corrections to the Cover Date. 相似文献