全文获取类型
收费全文 | 84717篇 |
免费 | 20047篇 |
国内免费 | 346篇 |
专业分类
化学 | 76791篇 |
晶体学 | 375篇 |
力学 | 3990篇 |
综合类 | 12篇 |
数学 | 13371篇 |
物理学 | 10571篇 |
出版年
2024年 | 468篇 |
2023年 | 4786篇 |
2022年 | 1676篇 |
2021年 | 2901篇 |
2020年 | 5270篇 |
2019年 | 2680篇 |
2018年 | 3876篇 |
2017年 | 2234篇 |
2016年 | 7022篇 |
2015年 | 6818篇 |
2014年 | 6148篇 |
2013年 | 6807篇 |
2012年 | 7036篇 |
2011年 | 3987篇 |
2010年 | 5800篇 |
2009年 | 5454篇 |
2008年 | 2200篇 |
2007年 | 1956篇 |
2006年 | 1207篇 |
2005年 | 4986篇 |
2004年 | 4318篇 |
2003年 | 2685篇 |
2002年 | 785篇 |
2001年 | 471篇 |
2000年 | 244篇 |
1999年 | 314篇 |
1997年 | 225篇 |
1996年 | 253篇 |
1995年 | 320篇 |
1994年 | 238篇 |
1993年 | 339篇 |
1992年 | 368篇 |
1991年 | 295篇 |
1990年 | 247篇 |
1989年 | 232篇 |
1988年 | 263篇 |
1981年 | 231篇 |
1980年 | 271篇 |
1979年 | 278篇 |
1978年 | 283篇 |
1977年 | 390篇 |
1976年 | 519篇 |
1975年 | 563篇 |
1974年 | 570篇 |
1973年 | 400篇 |
1972年 | 509篇 |
1971年 | 451篇 |
1970年 | 665篇 |
1969年 | 498篇 |
1968年 | 544篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
We consider the one-dimensional steady-state semiconductor deviceequations modelling a pnpn device. There are two relevant scalingsof the equations corresponding to small and large applied voltages.In both scalings, the semiconductor equations can be consideredas singularly perturbed. It turns out that the small-voltagescaling breaks down for current values between two saturationcurrents. In that interval, the large-voltage scaling has tobe employed. For both scalings, we derive the first-order termsof an asymptotic expansion and show that the reduced problemhas a solution. An example verifies that the current-voltagecurves obtained have the expected qualitative structure. 相似文献
62.
63.
64.
Rose Vogel Dr. 《ZDM》2005,37(5):445-449
Taking advantage of patterns is typical of our everyday experience as well as our mathematical thinking and learning. For example a working day or a morning at school displays a certain structure, which can be described in terms of patterns. On the one hand regular structures give us the feeling of permanence and enable us to make predictions. On the other hand they also provide a chance to be creative and to vary common procedures. School students usually encounter patterns in math classes either as number patterns or geometric patterns. There are also patterns that teachers can find in analyzing the errors students make during their calculations (error patterns) as well as patterns that are inherent to mathematical problems. One could even go so far as to say that identifying and describing patterns is elementary for mathematics (cf. Devlin 2003). Practising good interacting with patterns supports not only the active learning of mathematics but also a deeper understanding of the world in general. Patterns can be explored, identified, extended, reproduced, compared, varied, represented, described and created. This paper provides some examples of pattern utilization and detailed analyses thereof. These ideas serve as “hooks” to encourage the good use of patterns to facilitate active learning processes in mathematics. 相似文献
65.
66.
Dr. Konrad Natterer 《Monatshefte für Chemie / Chemical Monthly》1885,6(1):519-522
Ohne Zusammenfassung 相似文献
67.
Could simple intraannular-arm macrocyclic systems exist in enantiopure stable forms? The effective synthesis of two representative compounds of such a class, their resolution into enantiomers, and experiments justifying their stability toward racemization are presented. 相似文献
68.
Akira Yanagisawa Prof. Dr. Takahiro Suzuki Takanori Koide Shogo Okitsu Takayoshi Arai Prof. Dr. 《化学:亚洲杂志》2008,3(10):1793-1800
A Barbier‐type regioselective propargylation of aldehydes and ketones with (3‐bromobut‐1‐ynyl)trimethylsilane has been achieved using reactive barium as a low‐valent metal in THF. Especially in the case of ketones, the corresponding homopropargylic alcohols form almost exclusively. In the reaction of α,β‐unsaturated carbonyl compounds, only 1,2‐adducts have been observed. This method is also applicable to propargylation of imines, and the corresponding homopropargylic amines are obtained regiospecifically in good yields with diastereomeric ratios of up to 87:13. 相似文献
69.
Silica@copper (SiO2@Cu) core–shell nanoparticles were synthesized and well characterized by XRD, TEM, AFM, XPS, UV/Vis, TGA–MS, and ICP–AES techniques. The synthesized SiO2@Cu core–shell nanoparticles were employed as catalysts for the conjugate addition of amines to α,β‐unsaturated compounds in water to obtain β‐amino carbonyl compounds in excellent yields in shorter reaction times. Furthermore, the catalyst works well for hetero‐Michael addition reactions of heteroatom nucleophiles such as thiols to α,β‐unsaturated compounds. As the reaction is performed in water, it allows for easy recycling of the catalyst with consistent activity. 相似文献
70.