全文获取类型
收费全文 | 140539篇 |
免费 | 2469篇 |
国内免费 | 405篇 |
专业分类
化学 | 64857篇 |
晶体学 | 1111篇 |
力学 | 8370篇 |
数学 | 41297篇 |
物理学 | 27778篇 |
出版年
2023年 | 484篇 |
2022年 | 403篇 |
2021年 | 657篇 |
2020年 | 918篇 |
2019年 | 894篇 |
2018年 | 11290篇 |
2017年 | 10909篇 |
2016年 | 7937篇 |
2015年 | 2660篇 |
2014年 | 2189篇 |
2013年 | 4369篇 |
2012年 | 8020篇 |
2011年 | 14369篇 |
2010年 | 8022篇 |
2009年 | 8010篇 |
2008年 | 10089篇 |
2007年 | 12206篇 |
2006年 | 3696篇 |
2005年 | 4564篇 |
2004年 | 4413篇 |
2003年 | 4239篇 |
2002年 | 3008篇 |
2001年 | 1562篇 |
2000年 | 1414篇 |
1999年 | 974篇 |
1998年 | 791篇 |
1997年 | 699篇 |
1996年 | 906篇 |
1995年 | 589篇 |
1994年 | 630篇 |
1993年 | 640篇 |
1992年 | 600篇 |
1991年 | 543篇 |
1990年 | 509篇 |
1989年 | 500篇 |
1988年 | 430篇 |
1987年 | 425篇 |
1986年 | 391篇 |
1985年 | 596篇 |
1984年 | 561篇 |
1983年 | 384篇 |
1982年 | 485篇 |
1981年 | 476篇 |
1980年 | 451篇 |
1979年 | 404篇 |
1978年 | 379篇 |
1977年 | 329篇 |
1976年 | 355篇 |
1975年 | 338篇 |
1973年 | 331篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
J. Szonert B. Bieniak M. Głódź M. Piechota 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1995,33(3):177-180
We report measurements of lifetimes of the Rb(n
2
F)(n=6, 7, 8) states performed in a vapour cell. TheF-states were excited in a two-step sequence of an electric-dipole transition followed by an electric-quadrupole transition. Single photon counting was used for detection. The results are: (62
F)=171(4), (72
F)=262(15), (82
F)=387(12) (in ns). These lifetimes, have been compared with theoretical values given by different authors. An estimate of the cross sections for quenching due to collisions with ground-state Rb atoms is also given.This work was supported by the Polish Committee for Scientific Research under grant No. 2 2341 92 03 相似文献
992.
The possibilities of amperometric enzyme electrodes in reversed micellar systems for the determination of phenol, 4-chloro-3-methylphenol and 2,4-dimethylphenol are illustrated. The used enzymatic reaction consisted of the oxidation of the phenolic compounds by oxygen, catalysed by tyrosinase. The reduction of the liberated quinones was amperometrically detected. The concentration of the components of the reversed micelles, as well as the potential applied to the tyrosinase electrode have been optimized. The stability of the enzyme electrode with time was also evaluated. The effect of the analyte solubility in water upon the analytical performance of the electrode was explored. Advantages of amperometric biosensors in reversed micelles are shown with respect to aqueous media and organic phase enzyme electrodes. 相似文献
993.
Cantat T Jaroschik F Nief F Ricard L Mézailles N Le Floch P 《Chemical communications (Cambridge, England)》2005,(41):5178-5180
New samarium carbene complexes have been synthesized and characterized by X-ray diffraction analysis; the carbenic nature was assessed by reactivity studies. 相似文献
994.
Gunnlaugsson T Leonard JP Sénéchal K Harte AJ 《Chemical communications (Cambridge, England)》2004,(7):782-783
The cationic cyclen based Eu(III)-phen conjugated 1.Eu was synthesised as a chemosensor for Cu(II), where the recognition in water at pH 7.4 gave rise to quenching of the Eu(III) luminescence and the formation of tetranuclear polymetallic Cu(II)-Eu(III) macrocyclic complexes in solution where Cu(II) was bound by three 1.Eu conjugates. 相似文献
995.
Guryca V Pacáková V Tlust'áková M Stulík K Michálek J 《Journal of separation science》2004,27(13):1121-1129
Topography and thickness of hydrophilic polymer coatings of fused-silica capillaries for capillary electrophoresis (CE) were investigated using atomic force microscopy (AFM), scanning electron microscopy (SEM), and profilometry. Three hydrogels, poly(2-hydroxyethyl methacrylate) [poly(HEMA)], poly(diethylene glycol monomethacrylate) [poly(DEGMA)], and poly(triethylene glycol monomethacrylate) [poly(TEGMA)], were deposited using two procedures, either by simple physical sorption of the polymers, or by derivatization of the capillary wall surface with glycidyl methacrylate (EPMA) followed by polymerization of the appropriate monomers. The performance of the modified capillaries was tested under CE conditions (decrease in the electroosmotic flow, EOF dependence on pH, separation of milk and standard proteins). It has been found that the most important property of the polymer coating is its thickness, whereas its topography and the degree of its hydrophobicity are less significant. Film deposition by physical adsorption is preferable to polymerization on the derivatized surface. 相似文献
996.
Quassinoids from Picrasma crenata 总被引:1,自引:0,他引:1
From woods of Picrasma crenata, a new stereoisomer dihydronorneoquassin was obtained together with others well knowns dihydronorneoquassin, parain, alpha-neoquassin, beta-neoquassin and quassin. The structures were determined by spectroscopic data and chemical evidence. 相似文献
997.
R. Acevedo T. Meruane E. Cortés S. O. Vasquez C. D. Flint 《Theoretical chemistry accounts》1994,88(2):99-110
Summary A theoretical model to calculate the vibronic intensities induced by the odd vibrational modes in centrosymmetric lanthanide complexes is developed and applied to octahedral complex ions, LnX
6
3–
, such as occur in the hexachloroelpasolites Cs2NaLnCl6. Both the crystal field and the ligand polarisation contributions are evaluated using a standard set of symmetry coordinates. For the crystal field term a truncated expansion of the intermediates states is employed rather than the more conventional closure approximation. Special care is necessary to ensure that the phases of the contributions are correctly determined since the cross-term between the ligand polarisation and crystal field contributions is signed. General equations applicable to anyf
n
complex ion are derived and an example of their application to the PrCl
6
3–
ion is given The agreement with experiment is satisfactory. 相似文献
998.
Freedberg DI 《Journal of the American Chemical Society》2002,124(10):2358-2362
A simple solution NMR method is presented for pucker determination of five-membered rings using only residual dipolar couplings obtained in a single liquid crystalline medium, DMPC/DHPC bicelles (DMPC = dimyristoylphosphatidylcholine; DHPC = dihexanoylphosphatidylcholine). The method was applied to determine the pucker of the fructofuranosyl ring of sucrose. The results indicate a fructofuranosyl pucker phase in the 20 degrees - 70 degrees range. The pucker phases are in agreement with those from previous NMR and optical spectroscopic studies and, importantly, do not rely on empirically parametrized Karplus curves. Furthermore, the analysis implies more than one stable pucker phase and rapid ring interconversion in this range. The present results suggest that using residual dipolar couplings alone can reveal multiple conformations present in solution. Hence, when a sufficient number of residual dipolar coupling constants is measured, the outcome is a robust, reliable, and independent route for determining carbohydrate and nucleic acid structure by NMR spectroscopy. 相似文献
999.
Báthori M Pongrácz Z Tóth G Simon A Kandra L Kele Z Ohmacht R 《Journal of chromatographic science》2002,40(7):409-415
A new ecdysteroid glycoside, 2-deoxy-20-hydroxyecdysone 22-O-beta-D-glucopyranoside, is isolated from the herb Silene italica ssp. nemoralis (Waldst. and Kit.) Nyman. The compound is purified with multistep chromatography, such as classical column chromatography on alumina and droplet countercurrent distribution. Also, it is expanded using twice low-pressure reversed-phase liquid column chromatography. Chromatography in four steps results in the purified 2-deoxy-20-hydroxyecdysone 22-O-beta-D-glucopyranoside. Two other ecdysteroids have also been separated, including the formerly identified integristerone A and 24(28)-dehydromakisterone A. 相似文献
1000.
Yu. A. Simonov N. V. Gérbéléu P. N. Bourosh V. B. Arion A. D. Garnovskii A. N. Sobolev V. A. Alekseenko 《Journal of Structural Chemistry》1990,31(2):365-369
Institute for Applied Physics, Academy of Sciences of the Moldavian SSR. Institute of Chemistry, Academy of Sciences of the Moldavian SSR. Nat. Research Institute for Physics and Organic Chemistry, Rostov State University. Translated from Zhurnal Strukturnoi Khimii, Vol. 31, No. 2, pp. 199–202, 1990. 相似文献