全文获取类型
收费全文 | 10537篇 |
免费 | 728篇 |
国内免费 | 599篇 |
专业分类
化学 | 8070篇 |
晶体学 | 34篇 |
力学 | 232篇 |
综合类 | 61篇 |
数学 | 1794篇 |
物理学 | 1673篇 |
出版年
2023年 | 78篇 |
2022年 | 133篇 |
2021年 | 176篇 |
2020年 | 200篇 |
2019年 | 270篇 |
2018年 | 210篇 |
2017年 | 232篇 |
2016年 | 277篇 |
2015年 | 241篇 |
2014年 | 432篇 |
2013年 | 671篇 |
2012年 | 456篇 |
2011年 | 660篇 |
2010年 | 578篇 |
2009年 | 739篇 |
2008年 | 665篇 |
2007年 | 615篇 |
2006年 | 564篇 |
2005年 | 418篇 |
2004年 | 354篇 |
2003年 | 335篇 |
2002年 | 270篇 |
2001年 | 156篇 |
2000年 | 155篇 |
1999年 | 134篇 |
1998年 | 113篇 |
1997年 | 91篇 |
1996年 | 93篇 |
1995年 | 67篇 |
1994年 | 66篇 |
1993年 | 56篇 |
1992年 | 58篇 |
1991年 | 46篇 |
1990年 | 28篇 |
1989年 | 26篇 |
1988年 | 22篇 |
1986年 | 17篇 |
1985年 | 417篇 |
1984年 | 283篇 |
1983年 | 238篇 |
1982年 | 213篇 |
1981年 | 152篇 |
1980年 | 117篇 |
1979年 | 131篇 |
1978年 | 75篇 |
1977年 | 86篇 |
1976年 | 87篇 |
1975年 | 89篇 |
1974年 | 115篇 |
1973年 | 130篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Trithiaazapentalene derivatives were prepared by the reaction of 2-alkylidene-4-oxothiazolidines with Lawesson's reagent. They are classified as two structurally different trithiaazapentalene compounds that have different contributions of monocyclic 1,2-dithiole and 1,2,4-dithiazole structures and degrees of aromaticity of the bicyclic trithiaazapentalene system. The electron-donating ability of substituents at the C(5) position of the trithiaazapentalene system is recognized as the main cause for changes in π-electron distribution. This is the first complete study of substituent effects on the structure of trithiapentalenes. 相似文献
992.
993.
Norma A. Cortez‐Lemus Rodolfo Salgado‐Rodríguez Angel Licea‐Claveríe 《Journal of polymer science. Part A, Polymer chemistry》2010,48(14):3033-3051
The design, synthesis, and use of two new, stable, functionalized chain transfer agents (CTA's) containing OH and amine end groups for the RAFT polymerization is reported: 2‐hydroxyethoxy‐carbonylphenylmethyl dithiobenzoate and 2‐(2‐(tert‐butoxycarbonyl)ethylamino)‐2‐oxo‐1‐phenylethyl benzodithioate, respectively. The RAFT polymerization of n‐hexyl acrylate (HA) using those CTA's, were compared to several other functionalized dithiobenzoate esters reported in the literature containing COOH and Ester groups. The performances of the dithiobenzoates were compared in terms of kinetics and molecular weight distribution control. Good control in polymerization of n‐hexyl acrylate with a linear increase of Mn with conversion mantaining polydispersity indices (PDI) below 1.1 was obtained by use of the new functionalized CTA's developed and also by use of some other CTA's tested, to produce well‐defined linear polymers with one specific chain‐end functionality: ? OH, ? COOH or Amine. Using a postpolymerization reaction with functionalized azocompounds in a 5 to 1 ratio, α,ω‐telechelic polymers, with ? OH or ? COOH as functional group at the second end were obtained. By using this synthetic strategy α,ω‐homotelechelic and heterotelechelic polymers were readily prepared. The chemical availability of functional end‐groups in the telechelics was demonstrated by reaction with methacrylic anhydride. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3033–3051, 2010 相似文献
994.
Dongxia Wu Xiaohui Song Tao Tang Hanying Zhao 《Journal of polymer science. Part A, Polymer chemistry》2010,48(2):443-453
Well‐defined macromolecular brushes with poly(N‐isopropyl acrylamide) (PNIPAM) side chains on random copolymer backbones were synthesized by “grafting from” approach based on click chemistry and reversible addition‐fragmentation chain transfer (RAFT) polymerization. To prepare macromolecular brushes, two linear random copolymers of 2‐(trimethylsilyloxy)ethyl methacrylate (HEMA‐TMS) and methyl methacrylate (MMA) (poly(MMA‐co‐HEMA‐TMS)) were synthesized by atom transfer radical polymerization and were subsequently derivated to azide‐containing polymers. Novel alkyne‐terminated RAFT chain transfer agent (CTA) was grafted to polymer backbones by copper‐catalyzed 1,3‐dipolar cycloaddition (azide‐alkyne click chemistry), and macro‐RAFT CTAs were obtained. PNIPAM side chains were prepared by RAFT polymerization. The macromolecular brushes have well‐defined structures, controlled molecular weights, and molecular weight distributions (Mw/Mn ≦ 1.23). The RAFT polymerization of NIPAM exhibited pseudo‐first‐order kinetics and a linear molecular weight dependence on monomer conversion, and no detectable termination was observed in the polymerization. The macromolecular brushes can self‐assemble into micelles in aqueous solution. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 443–453, 2010 相似文献
995.
The halogen/lithium (X/Li) exchange’s usefulness is challenged by the extreme conditions employed during the conversion of a bromo or an iodo substituent to an organolithium intermediate. Our preliminary disclosure reveals that simple X/Li exchanges can be accomplished at ambient temperature and in doped hydrocarbon media using n-BuLi. Use of a continuous reactor featuring brief reaction times (?1 s) and large product throughputs further facilitates the production of a product resulting from a sequential X/Li exchange and nucleophilic addition. 相似文献
996.
Safaa L. Diab 《Optics & Laser Technology》2011,43(4):838-847
Image compression is one of the important fields that has useful applications in data storage and transmission. In this research a new algorithm is developed and tested for multiple-image compression and enhancement. The algorithm, in addition, is applied to multiple noisy images. Also, the effect of compression ratio on the peak signal to noise ratio (PSNR) is explored by applying different compression ratios. The developed algorithm gives good compression and noise immunity. It can be used for storage/transmission of encrypted and compressed information. 相似文献
997.
Asymmetric 1,4‐conjugation addition of dialkylzinc (diethylzinc and dimethylzinc) to cyclic enones, chalcone and nitroalkenes was achieved by a 25 mol% (R)‐6,6′‐Br2‐BINOL( 1f ), 25 mol% CuSPh and 100 mol% dicyclohexylmethylamin(Cy2NMe) catalyst system. The Cu(I) catalyst system enables the cyclic enone, chalcone and nitroalkene generality with high enantioselectivity (up to 84% ee) and isolated yield (up to 94%) under mild reaction conditions. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
998.
A copper‐catalyzed tandem intramolecular cyclization‐addition reaction of N‐(o‐alkynylphenyl)imines is disclosed. This strategy offers a simple and promising method for accessing ring‐fused indoles. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
999.
1000.