全文获取类型
收费全文 | 9711篇 |
免费 | 552篇 |
国内免费 | 42篇 |
专业分类
化学 | 7244篇 |
晶体学 | 32篇 |
力学 | 159篇 |
数学 | 1502篇 |
物理学 | 1368篇 |
出版年
2023年 | 75篇 |
2022年 | 79篇 |
2021年 | 154篇 |
2020年 | 229篇 |
2019年 | 199篇 |
2018年 | 146篇 |
2017年 | 120篇 |
2016年 | 415篇 |
2015年 | 378篇 |
2014年 | 401篇 |
2013年 | 476篇 |
2012年 | 661篇 |
2011年 | 704篇 |
2010年 | 503篇 |
2009年 | 410篇 |
2008年 | 652篇 |
2007年 | 576篇 |
2006年 | 541篇 |
2005年 | 554篇 |
2004年 | 472篇 |
2003年 | 338篇 |
2002年 | 311篇 |
2001年 | 156篇 |
2000年 | 129篇 |
1999年 | 149篇 |
1998年 | 115篇 |
1997年 | 158篇 |
1996年 | 109篇 |
1995年 | 109篇 |
1994年 | 105篇 |
1993年 | 92篇 |
1992年 | 88篇 |
1991年 | 58篇 |
1990年 | 42篇 |
1989年 | 68篇 |
1988年 | 54篇 |
1987年 | 31篇 |
1986年 | 31篇 |
1985年 | 45篇 |
1984年 | 36篇 |
1983年 | 22篇 |
1982年 | 45篇 |
1981年 | 30篇 |
1980年 | 25篇 |
1979年 | 23篇 |
1978年 | 20篇 |
1977年 | 18篇 |
1975年 | 21篇 |
1973年 | 15篇 |
1968年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 375 毫秒
931.
932.
Jeremy Drapeau Manon Rossano Didier Touraud Ulla Obermayr Martin Geier Andreas Rose Werner Kunz 《Comptes Rendus Chimie》2011,14(7-8):629-635
A simple and efficient method was developed for the synthesis of para-Menthane-3,8-diol (PMD), a well-known repellent active against mosquitoes, from Eucalyptus citriodora essential oil by a treatment of citric acid in a biphasic medium (H2O/essential oil). The E. citriodora contains as main component (+)-citronellal (74% in the present case), which cyclises (Prins reaction) to form cis/trans PMD isomers. As an example, an emulsion containing an aqueous solution of 7% citric acid and E. citriodora oil at 50°, conducted after 15 h stirring to 82% conversion of (+)-citronellal with a selectivity of 80% with the remaining presence of monoterpenes and sesquiterpenes in the medium. Investigations of lasting protection on human volunteers are carried out using a cage test bioassay protocol and Aedes aegypti mosquitoes. At 20% in iPrOH, the new reaction mixture showed a complete protection of 303 min compared to 22 min with the pure essential oil. The modified oil was compared with N,N-diethyl-m-methylbenzamide (DEET), the most popular active used in repellent formulations. Thermogravimetric analyses (TGA) of E. citriodora, the modified oil, PMD, DEET, and (+)-citronellal were performed and showed a slow evaporation rate for PMD and DEET as well as for the modified oil, which may explain their long lasting protection action. 相似文献
933.
A new assay for protein kinase CK2 activity determination based on the quantification of a phosphorylated substrate was developed. The common CK2 substrate peptide RRRDDDSDDD, conjugated with the fluorophore 5‐[(2‐aminoethyl)amino]naphthalene‐1‐sulfonic acid at the C‐terminus served as the analyte. By means of CZE using 2 mol/L acetic acid as electrolyte and UV detection at 214 nm, the non‐phosphorylated and the phosphorylated peptide variants could be resolved within 6 min from a complex assay mixture. By this means, activity of human CK2 could be monitored by a kinetic, as well as an endpoint, method. Inhibition of human recombinant CK2 holoenzyme by 6‐methyl‐1,3,8‐trihydroxyanthraquinone and 4,5,6,7‐tetrabromobenzotriazole resulted in IC50 values of 1.33 and 0.27 μM, respectively, which were similar to those obtained with the standard radiometric assay. These results suggest that the CE/UV strategy described here is a straightforward assay for CK2 inhibitor testing. 相似文献
934.
935.
936.
937.
938.
Gabriel J. Stylianides Andreas J. Stylianides 《Mathematical Thinking and Learning》2008,10(2):103-133
There are currently increased efforts to make proof central to school mathematics throughout the grades. Yet, realizing this goal is challenging because it requires that students master several abilities. In this article we focus on one such ability, namely, the ability for deductive reasoning, and we review psychological research to enhance what is currently known in mathematics education research about this ability in the context of proof and to identify important directions for future research. We first offer a conceptualization of proof, which we use to delineate our focus on deductive reasoning. We then review psychological research on the development of students' ability for deductive reasoning to see what can be said about the ages at which students become able to engage in certain forms of deductive reasoning. Finally, we review two psychological theories of deductive reasoning to offer insights into cognitively guided ways to enhance students' ability for deductive reasoning in the context of proof. 相似文献
939.
940.