全文获取类型
收费全文 | 50897篇 |
免费 | 11190篇 |
国内免费 | 2871篇 |
专业分类
化学 | 52235篇 |
晶体学 | 526篇 |
力学 | 1090篇 |
综合类 | 115篇 |
数学 | 4235篇 |
物理学 | 6757篇 |
出版年
2024年 | 30篇 |
2023年 | 272篇 |
2022年 | 500篇 |
2021年 | 707篇 |
2020年 | 1814篇 |
2019年 | 3041篇 |
2018年 | 1444篇 |
2017年 | 1077篇 |
2016年 | 4045篇 |
2015年 | 4101篇 |
2014年 | 4152篇 |
2013年 | 5127篇 |
2012年 | 4393篇 |
2011年 | 3713篇 |
2010年 | 3805篇 |
2009年 | 3715篇 |
2008年 | 3483篇 |
2007年 | 2870篇 |
2006年 | 2407篇 |
2005年 | 2447篇 |
2004年 | 2063篇 |
2003年 | 1703篇 |
2002年 | 2369篇 |
2001年 | 1603篇 |
2000年 | 1495篇 |
1999年 | 603篇 |
1998年 | 237篇 |
1997年 | 199篇 |
1996年 | 174篇 |
1995年 | 186篇 |
1994年 | 160篇 |
1993年 | 131篇 |
1992年 | 116篇 |
1991年 | 99篇 |
1990年 | 101篇 |
1989年 | 56篇 |
1988年 | 63篇 |
1987年 | 59篇 |
1986年 | 50篇 |
1985年 | 58篇 |
1984年 | 43篇 |
1983年 | 22篇 |
1982年 | 28篇 |
1981年 | 21篇 |
1980年 | 19篇 |
1979年 | 17篇 |
1978年 | 18篇 |
1977年 | 13篇 |
1975年 | 13篇 |
1974年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
61.
5‐Amino‐4‐methyl‐2‐phenyl‐6‐substitutedfuro[2,3‐d]pyrimidines ( 2a‐c ) were reacted with 2,5‐dimethoxytetrahydrfuran to afford the pyrrolyl derivatives 3a‐c . Compound 3a was chosen as intermediate for the synthesis of poly fused heterocycles incorporated furopyrimidines moiety 4–11 . Some of the synthesized compounds were screened for their antibacterial and antifungal activities. 相似文献
62.
Complementary regioselective esterification of non-reducing oligosaccharides catalyzed by different hydrolases 总被引:1,自引:0,他引:1
The enzymatic transesterification of several tri- and tetrasaccharides with vinyl laurate is described. The lipases from Candida antarctica B (Novozym 435) and Thermomyces lanuginosus (Lipozyme TL IM) and the alkaline protease from Bacillus licheniformis (subtilisin Carlsberg) have been used with each carbohydrate to obtain different regioisomers. By using the sugars in their amorphous form, complete solubility is achieved in the reaction media (tert-butanol/pyridine mixtures for the lipases and pyridine for the protease) and high isolated yields of the corresponding monoesters are obtained. Good to excellent regioselectivity is observed for all the enzymes, showing a final complementary picture respect to the primary hydroxyls of the oligosaccharides studied. 相似文献
63.
64.
65.
Steven D. Bull Simon W. Epstein Nadeam Mujtaba Edward D. Savory Juan A. Tamayo 《Tetrahedron》2006,62(33):7911-7925
Enolates of (S)-N,N′-bis-(p-methoxybenzyl)-3-iso-propylpiperazine-2,5-dione exhibit high levels of enantiodiscrimination in alkylations with (RS)-1-aryl-1-bromoethanes and (RS)-2-bromoesters, affording substituted diketopiperazines containing two new stereogenic centres in high de. Deprotection and hydrolysis of the resultant substituted diketopiperazines provides a route to the asymmetric synthesis of homochiral methyl 2-amino-3-aryl-butanoates and 3-methyl-aspartates in high de and ee. 相似文献
66.
The structural evolution in amorphous silicon and germanium thin films has been investigated by high-resolution transmission electron microscopy (HRTEM) in conjunction with autocorrelation function (ACF) analysis. The results established that the structure of as-deposited semiconductor films is of a high density of nanocrystallites embedded in the amorphous matrix. In addition, from ACF analysis, the structure of a-Ge is more ordered than that of a-Si. The density of embedded nanocrystallites in amorphous films was found to diminish with annealing temperature first, then to increase. The conclusions also corroborate well with the results of diminished medium-range order in annealed amorphous films determined previously by a variable coherence microscopy method. 相似文献
67.
An optical measurement of vortex shape at a free surface 总被引:1,自引:0,他引:1
We have proposed an optical method of vortex shape measurement based on Fourier transform profilometry (FTP) and verified it by experiment. The results of our experiment proposed in this paper show that FTP can efficiently reconstruct the vortex shape at a free surface and this method is suitable for wide use in studying such problems as liquid shear flow, wake of an object, flow behind a bluff body, and wetting angle. 相似文献
68.
建立了一个四组分一维混合模型,对电子束注入大气产生大尺度等离子体的过程进行了数值模拟.结果表明了能量为140keV、流强为50mA/cm2的注入电子束,可以产生线度为0.5m,密度为1012cm-3量级的大气环境下等离子体.电子束所伴随的空间电荷效应由于等离子体的产生会很快消失,不影响后续的等离子体产生过程.电子束注入流强主要影响产生等离子体的密度,而电子束能量则同时影响其空间线度和密度.
关键词:
电子束
碰撞
电离 相似文献
69.
采用石墨电阻加热的温梯法生长了V:YAG晶体,晶体的不同部位呈现两种不同的颜色:浅绿色和黄褐色.通过对比分析不同颜色V:YAG晶体的室温吸收光谱,推断出石墨发热体高温下扩散出来的C可以起到还原作用,提高晶体中V3+tetra离子的浓度,同时诱导了F心的形成.在1300℃下,对不同颜色的V:YAG晶体进行真空退火处理,发现处于八面体格位中的V3+离子在热激发作用下与近邻的四面体格位Al3+离子存在置换反应,由此产生一定浓度的四面体格位V3+离子.同时,F心在退火过程中被完全消除,释放出来的自由电子被高价态的V离子俘获,可以进一步提高晶体中四面体格位V3+离子的浓度. 相似文献
70.
Wen Zhang Hui Huang Yu Ding Margherita Gavagnin Ernesto Mollo Guido Cimino Yue‐Wei Guo 《Helvetica chimica acta》2006,89(4):813-820
Three new polyoxygenated steroids, muricesteroid ( 1 ), and menellsteroids A ( 2 ) and B ( 3 ), were isolated from two species of the South China Sea gorgonian Muricella flexuosa and Menella verrucosa Brundin , respectively. The structures of these new compounds were elucidated on the basis of extensive spectroscopic analysis, chemical methods and comparison with known related compounds. 相似文献