全文获取类型
收费全文 | 3933篇 |
免费 | 161篇 |
国内免费 | 23篇 |
专业分类
化学 | 2617篇 |
晶体学 | 62篇 |
力学 | 133篇 |
数学 | 211篇 |
物理学 | 760篇 |
无线电 | 334篇 |
出版年
2023年 | 43篇 |
2022年 | 100篇 |
2021年 | 94篇 |
2020年 | 103篇 |
2019年 | 136篇 |
2018年 | 107篇 |
2017年 | 88篇 |
2016年 | 157篇 |
2015年 | 96篇 |
2014年 | 148篇 |
2013年 | 307篇 |
2012年 | 242篇 |
2011年 | 283篇 |
2010年 | 162篇 |
2009年 | 122篇 |
2008年 | 205篇 |
2007年 | 151篇 |
2006年 | 146篇 |
2005年 | 120篇 |
2004年 | 121篇 |
2003年 | 89篇 |
2002年 | 107篇 |
2001年 | 46篇 |
2000年 | 44篇 |
1997年 | 28篇 |
1996年 | 37篇 |
1995年 | 22篇 |
1994年 | 26篇 |
1993年 | 21篇 |
1992年 | 25篇 |
1991年 | 21篇 |
1988年 | 28篇 |
1987年 | 26篇 |
1986年 | 31篇 |
1985年 | 36篇 |
1984年 | 33篇 |
1983年 | 29篇 |
1981年 | 29篇 |
1980年 | 31篇 |
1979年 | 23篇 |
1978年 | 30篇 |
1977年 | 21篇 |
1976年 | 19篇 |
1975年 | 20篇 |
1966年 | 27篇 |
1965年 | 19篇 |
1964年 | 18篇 |
1963年 | 23篇 |
1962年 | 51篇 |
1961年 | 23篇 |
排序方式: 共有4117条查询结果,搜索用时 15 毫秒
111.
A high-performance thin layer chromatographic (HPTLC) method for the rapid and simple quantification of the four major anthraquinone derivatives i.e. physcion, chrysophanol, emodin and chrysophanol glycoside in Rheum emodi is described. HPTLC of anthraquinone derivatives was performed on pre-coated RP-18 F254S HPTLC plates. For achieving good separation, the mobile phase of methanol-water-formic acid (80:19:1, v/v/v) was used. The densitometric determination of anthraquinone derivatives was carried out at 445 nm in reflection/absorption mode. The calibration curves were linear in the range of 20-100 ng for physcion, 80-400 ng for chrysophanol and emodin, and 200-1000 ng for chrysophanol glycoside. The method was found to be reproducible and convenient for quantitative analysis of anthraquinone derivatives in the methanolic extract of rhizomes of R. emodi collected from three different locations of Western Himalaya, India. 相似文献
112.
M. Prakash Y. R. Waghmare I. Mehrotra 《Zeitschrift für Physik A Hadrons and Nuclei》1976,278(2):155-164
Low-lying spectra of6Li,18F,18O,42Sc,42Ca,58Ni and92Zr are studied with Sussex matrix elements (SME) and their central, spin-orbit and tensor components. It is observed that major contribution to level energies comes from the central part, while the tensor part provides the finer details of spectra, particularly forT=0 levels. The spin-orbit part does not make any appreciable contribution to level energies. A phenomenological renormalization of the SME is carried out to improve the agreement with the experimental results. It turns out that some of the low-lyingT=0 levels can be satisfactorily described if the SME in the3S1 relative state are made (1+α) times their bare interaction value, whereα is a constant to be determined from a comparison with experimental level energies. Similarly, forT=1 levels, better agreement with the experimental results is obtained if aδ-function-plus-quadrupole interaction is added to the SME. 相似文献
113.
114.
P. Ravi Girish M. Gore Surya P. Tewari Arun K. Sikder 《Journal of heterocyclic chemistry》2013,50(6):1322-1327
We report herein a facile, rapid, and environmentally friendly synthesis of nitropyrazoles in good yields using silica‐bismuth nitrate and silica‐sulfuric acid‐bismuth nitrate at room temperature for the first time. The relatively non‐toxic nature, ease of handling, easy availability, and low cost make the present procedure attractive for the nitration of a wide variety of diazoles in the drug and pharmaceutical industries. 相似文献
115.
T. Arun Kumar V.S.S. Sastry Ken Ishikawa Hideo Takezoe N.V. Madhusudana 《Liquid crystals》2013,40(8):971-979
We report experimental studies on defects in a nematic liquid crystal with negative dielectric anisotropy mounted in a cell with perfluoropolymer-coated surfaces. The sample exhibits a discontinuous anchoring transition from planar to homeotropic on cooling at zero or a small electric field, and above a cross-over voltage a continuous ‘inverse Freedericksz transition’, at which the director starts tilting in opposite directions at the two surfaces. Defects of strength ±1/2 are either annihilated or expelled when the director tilts. On the other hand, disclination lines of ±1 which end in partial point defects (boojums) at the surfaces in the planar alignment regime acquire point defects of strength ±1 at the midplane of the cell when the director tilts. At a low enough temperature, the homeotropic anchoring becomes strong, and an electric field above the Freedericksz threshold generates the usual umbilic defects, which follow the dynamic scaling laws found in earlier studies. 相似文献
116.
Jinesh M. Kuthanapillil A. Nijamudheen Nayana Joseph Praveen Prakash E. Suresh Ayan Datta K.V. Radhakrishnan 《Tetrahedron》2013
A detailed investigation on the effect of solvent polarity, temperature, and microwave irradiation on periselectivity in cycloaddition reactions of pentafulvenes with 3-oxidopyrylium betaine is described. The base catalyzed generation of 3-oxidopyrylium betaine in CHCl3 resulted in the exclusive formation of [6+3] adducts. With increase in solvent polarity and temperature, mixtures of [6+3] and [3+2] adducts were formed, where as under microwave irradiation, [3+2] adducts were formed exclusively. The experimental results have been rationalized on the basis of theoretical calculations. 相似文献
117.
Pyrazolopyrimidine and pyrimidopyrimidine derivatives have shown a wide range of biological activities such as acting as A1 adenosine receptors, kinase insert domain receptor (KDR), Rous sarcoma oncogene (Src), epidermal growth factor receptor (EGFR), antiproliferative, dihydrofolate reductase (DHFR), antimicrobial, antifungal, and lipid peroxidation. Because of this wide range of activities, we have synthesized pyrazolo[3,4‐d]pyrimidines and pyrimido[4,5‐d]pyrimidin‐4‐one derivatives. 相似文献
118.
119.
120.