全文获取类型
收费全文 | 2512篇 |
免费 | 369篇 |
国内免费 | 194篇 |
专业分类
化学 | 1346篇 |
晶体学 | 25篇 |
力学 | 141篇 |
综合类 | 13篇 |
数学 | 190篇 |
物理学 | 719篇 |
无线电 | 641篇 |
出版年
2024年 | 6篇 |
2023年 | 51篇 |
2022年 | 71篇 |
2021年 | 109篇 |
2020年 | 84篇 |
2019年 | 75篇 |
2018年 | 88篇 |
2017年 | 87篇 |
2016年 | 108篇 |
2015年 | 136篇 |
2014年 | 155篇 |
2013年 | 180篇 |
2012年 | 228篇 |
2011年 | 228篇 |
2010年 | 145篇 |
2009年 | 152篇 |
2008年 | 156篇 |
2007年 | 147篇 |
2006年 | 124篇 |
2005年 | 102篇 |
2004年 | 54篇 |
2003年 | 54篇 |
2002年 | 54篇 |
2001年 | 47篇 |
2000年 | 52篇 |
1999年 | 54篇 |
1998年 | 48篇 |
1997年 | 37篇 |
1996年 | 43篇 |
1995年 | 25篇 |
1994年 | 27篇 |
1993年 | 19篇 |
1992年 | 9篇 |
1991年 | 20篇 |
1990年 | 27篇 |
1989年 | 16篇 |
1988年 | 12篇 |
1987年 | 13篇 |
1986年 | 9篇 |
1985年 | 2篇 |
1981年 | 2篇 |
1976年 | 2篇 |
1957年 | 1篇 |
1942年 | 1篇 |
1941年 | 1篇 |
1936年 | 4篇 |
1930年 | 2篇 |
1929年 | 1篇 |
1927年 | 1篇 |
1923年 | 1篇 |
排序方式: 共有3075条查询结果,搜索用时 599 毫秒
911.
Tiefeng Xu Feifei Chen Shixun Dai Xiang Shen Xunsi Wang Qiuhua Nie Chao Liu Kai Xu Jong Heo 《Journal of Non》2011,357(11-13):2219-2222
Tellurite glasses from TeO2–Bi2O3–BaO pseudo-ternary system were prepared using a conventional melt-quenching method and its glass-forming region was determined. A series of glasses were selected and their third-order optical nonlinearities (TONL) were measured by employing the Z-scan method at a wavelength of 800 nm with femtosecond laser pulses. The results showed that glass former Te4+ ions exhibited positive influences on the TONL and glass modifiers Ba2+ ions behaved similarly; low concentrated Bi3+ ions as glass modifiers weakened the nonlinearities, but an excess amount of Bi3+ behaved oppositely. FTIR measurements demonstrated that chemical bonds especially Te–Oeq vibrated at a high energy level remarkably promoted the TONL susceptibility χ(3), and the glass sample with the highest Bi2O3 content exhibited the largest χ(3) value which was due to the presence of BiO3 polyhedra. 相似文献
912.
913.
Yan-Hong Bi Wen-Ying Du Zhao-Yu Wang Xiao-Ming Chen Ling-Hong Nie Min-Hua Zong 《Applied biochemistry and biotechnology》2014,174(2):556-563
The effect of 2′-differing substituent groups (such as the H, F, Cl, Br, OCH3) as well as various chain lengths of the acyl substrates (C6, C10, and C14) on the performance of Thermomyces lanuginosus lipase for the regioselective acylation of pyrimidine nucleoside analogs was investigated systematically for the first time. The results evidently demonstrated that changing substituents in nucleosides had a significant influence on the substrate specificity for the enzymatic reaction, possibly due to the differences in physicochemical properties of the substituents and the special shape of the substrate binding site in enzyme active region. With respect to the chain-length selectivity, the enzyme exhibited the highest 5′-regioselectivity toward the longer chain (C14) as compared to short (C6 and C10) ones. The reason might be attributed to the presence of the interaction between the acyl chain and the large hydrophobic substrate-binding pocket. 相似文献
914.
pH‐Regulated Selectivity in Supramolecular Polymerizations: Switching between Co‐ and Homopolymers
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Hendrik Frisch Dr. Yan Nie Prof. Dr. Stefan Raunser Dr. Pol Besenius 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(8):3304-3309
A strategy is presented to regulate the selectivity in aqueous supramolecular polymerizations by changes in pH. In neutral buffered conditions, oppositely charged phenylalanine‐based dendritic peptide amphiphiles self‐assemble into (A?B)n alternating copolymers of low polydispersity when mixed in a 1:1 comonomer feed ratio. Via pH switch of the glutamic acid and lysine side chains, attractive Coulomb interactions in the coassembled materials are screened and selective polymerization occurs to form (A)n homopolymers of the acidic comonomer at low pH and (B)n homopolymers of the basic comonomer at high pH, while the complementary comonomer is released during the transition. Reversible switching is demonstrated between these three different polymeric states, which were characterized by CD and fluorescence spectroscopy, using a peptide based minimalistic fluorophore/quencher pair, and transmission electron microscopy. 相似文献
915.
A simple, highly efficient, and economical biphasic cell-free system was developed for biocatalytic reduction of prochiral aromatic ketones to furnish enantiopure alcohols. This system is characterized by using endogenous enzymes of the cell-free extract to form enzyme-coupled NADPH recycling system. Besides, it offered much higher productivity than whole cells and greatly simplified the preparation process of biocatalysts in comparison with isolated enzymes. Various prochiral aromatic ketones, especially α-substituted acetophenone derivatives, were reduced to chiral alcohols with excellent enantiomeric excess (ee) and moderate to good yield by this cell-free system. 相似文献
916.
917.
918.
919.
A single population growth model with stage‐structured and state‐dependent impulsive control is proposed. By using the Poincar'e map and the analogue of Poincaré's criterion, we prove the existence and the stability of positive order‐1 or order‐2 periodic solution. Moreover, we show that there is no periodic solution with order greater than or equal to three. Numerical results are carried out to illustrate the feasibility of our main results and the superiority of state feedback control strategy is also discussed. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
920.
Liang-Deng Nie Xiao-Xin Shi Na Quan Fei-Feng Wang Xia Lu 《Tetrahedron: Asymmetry》2011,22(16-17):1692-1699
A novel asymmetric synthesis of oseltamivir phosphate 1 from the naturally abundant (?)-shikimic acid via 3,4-cyclic sulfite intermediate 3 (Scheme 1) is described. Target compound 1 was obtained in 39% overall yield from this nine-step synthesis, and the characteristic step of the synthesis is the regio- and stereospecific nucleophilic substitution with sodium azide at the allylic (C-3) position of 3,4-cyclic sulfite 3. Since the yield of the direct-aziridine-formation from the unprotected dihydroxyl azide 4 was not satisfactory, two improved preparations of the established compound 7 via protected 3,4-cyclic sulfites 10 and 13 (Scheme 2) have been developed. In these two improved preparations, compound 7 was obtained from 3,4-cyclic sulfite 3 in 7-steps in 64% or 62% overall yield, respectively. 相似文献