全文获取类型
收费全文 | 5612篇 |
免费 | 399篇 |
国内免费 | 46篇 |
出版年
2023年 | 54篇 |
2022年 | 70篇 |
2021年 | 113篇 |
2020年 | 128篇 |
2019年 | 143篇 |
2018年 | 173篇 |
2017年 | 75篇 |
2016年 | 95篇 |
2015年 | 181篇 |
2014年 | 293篇 |
2013年 | 312篇 |
2012年 | 211篇 |
2011年 | 480篇 |
2010年 | 374篇 |
2009年 | 343篇 |
2008年 | 297篇 |
2007年 | 326篇 |
2006年 | 300篇 |
2005年 | 288篇 |
2004年 | 279篇 |
2003年 | 200篇 |
2002年 | 180篇 |
2001年 | 59篇 |
2000年 | 69篇 |
1999年 | 87篇 |
1998年 | 75篇 |
1997年 | 91篇 |
1996年 | 43篇 |
1995年 | 105篇 |
1994年 | 78篇 |
1993年 | 55篇 |
1992年 | 55篇 |
1991年 | 36篇 |
1990年 | 24篇 |
1989年 | 22篇 |
1988年 | 38篇 |
1987年 | 39篇 |
1986年 | 15篇 |
1985年 | 46篇 |
1984年 | 46篇 |
1983年 | 38篇 |
1982年 | 40篇 |
1981年 | 20篇 |
1980年 | 20篇 |
1979年 | 13篇 |
1978年 | 6篇 |
1977年 | 6篇 |
1976年 | 12篇 |
1974年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有6057条查询结果,搜索用时 15 毫秒
1.
2.
Three new drimane-type sesquiterpenoids named xylariaines A–C (1–3), together with one known analogue (4), were isolated from the ethyl acetate extract of cultures of the fungus Xylaria polymorpha (Pers.: Fr.) Grer. Their structures were elucidated unambiguously by NMR and single-crystal X-ray diffraction analysis. Compounds 1 and 2 exhibited weak anti-acetylcholinesterase activities at a concentration of 50 μg/mL with inhibition ratios of 12.4% and 18.0%, respectively. 相似文献
3.
ObjectiveThe purpose of this work is to evaluate the impact of optimization of magnification on performance parameters of the variable resolution X-ray (VRX) CT scanner.MethodsA realistic model based on an actual VRX CT scanner was implemented in the GATE Monte Carlo simulation platform. To evaluate the influence of system magnification, spatial resolution, field-of-view (FOV) and scatter-to-primary ratio of the scanner were estimated for both fixed and optimum object magnification at each detector rotation angle. Comparison and inference between these performance parameters were performed angle by angle to determine appropriate object position at each opening half angle.ResultsOptimization of magnification resulted in a trade-off between spatial resolution and FOV of the scanner at opening half angles of 90°–12°, where the spatial resolution increased up to 50% and the scatter-to-primary ratio decreased from 4.8% to 3.8% at a detector angle of about 90° for the same FOV and X-ray energy spectrum. The disadvantage of magnification optimization at these angles is the significant reduction of the FOV (up to 50%). Moreover, magnification optimization was definitely beneficial for opening half angles below 12° improving the spatial resolution from 7.5 cy/mm to 20 cy/mm. Meanwhile, the FOV increased by more than 50% at these angles.ConclusionIt can be concluded that optimization of magnification is essential for opening half angles below 12°. For opening half angles between 90° and 12°, the VRX CT scanner magnification should be set according to the desired spatial resolution and FOV. 相似文献
4.
The helicase superfamily 2 (SF2) proteins are involved in essentially every step in DNA and RNA metabolism. The radD (yejH) gene, which belongs to SF2, plays an important role in DNA repair. The RadD protein includes all seven conserved SF2 motifs and has shown ATPase activity. Here, we first reported the structure of RadD from Escherichia coli containing two RecA-like domains, a zinc finger motif, and a C-terminal domain. Based on the structure of RadD and other SF2 proteins, we then built a model of the RedD-ATP complex. 相似文献
5.
Samarcandin 1, a natural sesquiterpene-coumarin, was isolated as well as elucidated from F. assa-foetida which has significant effect in Iranian traditional medicine because of its medicinal attitudes. The crystal structure of samarcandin was determined by single-crystal X-ray structure analysis. It is orthorhombic, with unit cell parameters a = 10.8204 (5) Å, b = 12.9894 (7) Å, c = 15.2467 (9) Å, V = 2142.9 (2) Å3, space group P212121 and four symmetry equivalent molecules in the unit cell. Samarcandin was isolated in order to study for its theoretical studies as well as its cellular toxicity as anti-cancer drug against two cancerous cells. In comparison with controls, our microscopic and MTT assay data showed that samarcandin suppresses cancer cell proliferation in a dose-dependent manner with IC50 = 11 μM and 13 for AGS and WEHI-164 cell lines, respectively. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) of the structure was computed by three functional methods and 6-311++G7 standard basis set. The optimized molecular geometry and theoretical analysis agree closely to that obtained from the single crystal X-ray crystallography. To sum up, the good correlations between experimental and theoretical studies by UV, NMR, and IR spectra were found. 相似文献
6.
1 Introduction Molluscan species exhibit a range of morphologies associated with their particular function and habitat. The razor clam (Ensis siliqua) is a bivalve mollusc, usually found in fine sands, that burrows up to depths of 20 fathoms (~36 m)[1]. The species is found in several coastal regions of the British Isles, the Norwegian Sea, the Baltic south, the Iberian Peninsula, the Mediterranean and along the Atlantic coast of Morrocco[2]. The protec- tive outer shell, which is often … 相似文献
7.
8.
I. J. Pickering Graham N. George Verena Van Fleet-Stalder Thomas G. Chasteen Roger C. Prince 《Journal of biological inorganic chemistry》1999,4(6):791-794
Received: 2 April 1999 / Accepted: 17 September 1999 相似文献
9.
Ranganathan Santhanam 《Inorganica chimica acta》2008,361(2):473-478
A novel method of preparation of hexahydroaluminate complexes M3AlH6 (M = Li, Na or K) from the corresponding alkali metal hydride and tetrahydroaluminate has been explored, using dimethyl ether (Me2O) as a solvent at near-ambient temperatures. The results are compared with those obtained using a recently established mechanochemical approach. Characterization of the products by powder X-ray diffraction revealed M3AlH6 to be formed in high yield for M = Li and Na, but not for M = K. The attempted preparation of Li2NaAlH6 and Li2KAlH6 was unsuccessful. 相似文献
10.
Summary Exines fromTypha angustifolia L. pollen were dissolved in hot 2-aminoethanol. The solubilisate was successively fractionated and reaggregated via a dialysis cascade with dialysis tubings of different exclusion volumina. Four fractions of reaggregated material with different molecular mass were obtained. Fraction 1 with a molecular mass above 25,000 Da, fraction 2 with a molecular mass between 10,000–25,000 Da, fraction 3 with a molecular mass between 5,000–10,000 Da, and fraction 4 of a molecular mass lower than 5,000 Da. The fractions were comparatively analysed by scanning and transmission electron microscopy and X-ray microscopy. The material of the fractions with a molecular mass above 10,000 Da exhibit high congruence to the initial material. Analysis of the reaggregated material with the lowest molecular mass revealed special distinct substructures which in form and size showed high similarities to substructures of exines described in literature. In detail, spherical substructures consisting of an electron-dense core surrounded by an electron-transparent corona and in addition elongated substructures with a distinctive surface sculpture were detected.Abbreviations SEM
scanning electron microscopy
- TEM
transmission electron microscopy 相似文献