全文获取类型
收费全文 | 1746篇 |
免费 | 47篇 |
专业分类
化学 | 1261篇 |
晶体学 | 19篇 |
力学 | 31篇 |
数学 | 156篇 |
物理学 | 326篇 |
出版年
2022年 | 20篇 |
2021年 | 21篇 |
2020年 | 19篇 |
2019年 | 35篇 |
2018年 | 25篇 |
2017年 | 14篇 |
2016年 | 37篇 |
2015年 | 43篇 |
2014年 | 25篇 |
2013年 | 89篇 |
2012年 | 78篇 |
2011年 | 100篇 |
2010年 | 57篇 |
2009年 | 53篇 |
2008年 | 94篇 |
2007年 | 99篇 |
2006年 | 97篇 |
2005年 | 101篇 |
2004年 | 87篇 |
2003年 | 63篇 |
2002年 | 57篇 |
2001年 | 27篇 |
2000年 | 31篇 |
1999年 | 30篇 |
1998年 | 13篇 |
1997年 | 14篇 |
1996年 | 22篇 |
1995年 | 22篇 |
1994年 | 19篇 |
1993年 | 29篇 |
1992年 | 27篇 |
1991年 | 23篇 |
1990年 | 18篇 |
1989年 | 23篇 |
1988年 | 24篇 |
1987年 | 13篇 |
1986年 | 12篇 |
1985年 | 19篇 |
1984年 | 26篇 |
1983年 | 8篇 |
1982年 | 15篇 |
1981年 | 11篇 |
1980年 | 23篇 |
1979年 | 8篇 |
1978年 | 16篇 |
1977年 | 15篇 |
1976年 | 10篇 |
1975年 | 12篇 |
1974年 | 9篇 |
1973年 | 10篇 |
排序方式: 共有1793条查询结果,搜索用时 15 毫秒
81.
Abstract Three homologous series of 4-(5-alkyl-2-pyridinyl)phenyl alkanoates have been prepared. The synthesis and the liquid crystal transition temperatures of these esters are reported. Many members of these three ester series exhibit enantiotropic, wide range smectic F mesophases and some narrow range smectic C mesophases. However, in admixture with a chiral smectic C base mixture, the esters often induce a substantial increase in the chiral smectic C–smectic A transition temperature and, at the same time, decrease the temperature of crystallization. Thus, the temperature range of the chiral smectic C mesophase is substantially extended at both high and low temperatures. Ordered smectic mesophases are only observed at very low temperatures, if at all. These chiral smectic C mixtures are characterized by short switching times in surface-stabilized ferroelectric liquid crystal displays (SSFLCD). These novel esters are of especial interest for short-pitch chiral smectic C mixtures for short pitch bistable ferroelectric liquid crystal displays (SBFLCD) with short response times. 相似文献
82.
William E. Butler Paula N. Kelly Andy G. Harry Rachel Tiedt Blanaid White Rosaleen Devery Peter T. M. Kenny 《应用有机金属化学》2013,27(6):361-365
A series of N‐(ferrocenylmethyl amino acid) fluorinated benzene‐carboxamide derivatives 4b , 4c , 4d , 4e , 4f , 4g , 4h , 4i and 5b , 5c , 5d , 5e , 5f , 5g , 5h , 5i have been synthesized by coupling ferrocenylmethyl amine 3 with various substituted N‐(fluorobenzoyl) amino acid derivatives using the standard N‐(3‐dimethylaminopropyl)‐N′‐ethylcarbodiimide hydrochloride, 1‐hydroxybenzotriazole protocol. The amino acids employed in this study were glycine and L‐alanine. All of the compounds were fully characterized using a combination of 1H NMR, 13C NMR, 19F NMR, distortionless enhancement by polarization transfer (DEPT)‐135, 1H–1H correlation spectroscopy (COSY) and 1H–13C COSY (heteronuclear multiple‐quantum correlation) spectroscopy. The compounds were biologically evaluated on the oestrogen‐positive MCF‐7 breast cancer cell line. Compounds 4g , 4i , 5h and 5i exhibited cytotoxic effects on the MCF‐7 breast cancer cell line. N‐(Ferrocenylmethyl‐L‐alanine)‐3,4,5‐trifluorobenzene‐carboxamide ( 5h ) was the most active compound, with an IC50 value of 2.84 μm . Compounds 4i , 5h and 5i had lower IC50 values than that found for the clinically employed anticancer drug cisplatin (IC50 = 16.3 μm against MCF‐7). Guanine oxidation studies confirmed that 5h was capable of generating oxidative damage via a reactive oxygen species‐mediated mechanism. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
83.
The morphological evolution of pyramids, developed on bombarded copper surfaces, as a function of consecutive doses of 12 keV Kr—ions from 2.4 × 1016 up to 3 × 1016 ions/cm2, has been studied. In disagreement with many previous studies, apparently based on a limited choice of doses which were sometimes not in sequence, we found that the pyramids disappeared at high enough doses. Pyramids are therefore not an equilibrium structure. 相似文献
84.
85.
The understanding of the electronic effects of the diastereoselective addition of a nucleophile to a polar substituted aldehyde or ketone is not complete, with several theories competing to explain the data. For numerous hydride reductions of 3‐X‐2‐butanones (X = F, Cl, Br), the selectivity for the major syn isomer is significantly and consistently higher for X = Br than for X = F. This result is rationalized as a shift in mechanism from Cornforth (X = F) to Felkin–Anh (X = Br). The experimental data is well modeled by ab initio calculations for the addition to these ketones by BH3, but not by other nucleophiles such as LiH or LiAlH4. The energetic ordering of the BH3 transition states largely follows the trends for the ground state ketones. Here, consistent with electrostatic arguments, the anti orientation of the C―X and C?O bonds is always lower in energy than the syn arrangement. The gauche conformer is intermediate between these two, becoming gradually lower in energy as X increases in size. The hyperconjugative interaction invoked by the Felkin–Anh model provides only a modest stabilization of the relevant transition states as judged by NBO analysis. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
86.
87.
88.
Miklos Guttman David D. Weis John R. Engen Kelly K. Lee 《Journal of the American Society for Mass Spectrometry》2013,24(12):1906-1912
Noisy and overlapped mass spectrometry data hinder the sequence coverage that can be obtained from hydrogen deuterium exchange analysis, and places a limit on the complexity of the samples that can be studied by this technique. Advances in instrumentation have addressed these limits, but as the complexity of the biological samples under investigation increases, these problems are re-encountered. Here we describe the use of binomial distribution fitting with asymmetric linear squares regression for calculating the accurate deuterium content for mass envelopes of low signal or that contain significant overlap. The approach is demonstrated with a test data set of HIV Env gp140 wherein inclusion of the new analysis regime resulted in obtaining exchange data for 42 additional peptides, improving the sequence coverage by 11 %. At the same time, the precision of deuterium uptake measurements was improved for nearly every peptide examined. The improved processing algorithms also provide an efficient method for deconvolution of bimodal mass envelopes and EX1 kinetic signatures. All these functions and visualization tools have been implemented in the new version of the freely available software, HX-Express v2. Figure
? 相似文献
89.
The impact of Cu leveling additives on electrodeposited Cu topography and subsequent planarization behaviour was studied on both the pattern and wafer scales. The leveling agent significantly reduces as-deposited Cu topography, especially “mounding”. The reduction in topography results in a higher effective Cu removal rate during subsequent Cu planarization, both at the pattern and wafer scales. On the wafer scale, this effect is more evident for lower overburdens as the topography must be eliminated in a shorter total polish time. For Cu electrodeposited from leveler additive-free chemistries, significant pattern-scale topography persists throughout almost the entire planarization process, whereas for Cu deposited using a leveling agent only very wide features (~ > 100 μm) show any significant topography evolution during Cu polish. It is shown that excess electrodeposited Cu topography can lead to poor in-plane Cu wiring leakage performance. 相似文献
90.
Leonid N. Koikov Mingming Han Dawn M. Wellman Jim A. Kelly Irina P. Smoliakova 《合成通讯》2013,43(18):3451-3464
tert-Butyl vinyl ether (1) reacts with p-TolSCl to give 2-tert-butoxy-2-chloroethyl p-tolyl sulfide (2). In the presence of SnCl4, 2 reacts with silyl enol ethers, allyltrimethylsilane, and vinyl ethers to form a C-C bond. In the case of vinyl ethers, the reaction proceeds through the formation of the 5-membered sulfonium salt intermediate which in turn can react with H2O, TMSCN, allyltrimethylsilane, and Grignard reagents. 相似文献