全文获取类型
收费全文 | 312806篇 |
免费 | 4329篇 |
国内免费 | 2257篇 |
专业分类
化学 | 149829篇 |
晶体学 | 4659篇 |
力学 | 16594篇 |
综合类 | 104篇 |
数学 | 46583篇 |
物理学 | 101623篇 |
出版年
2021年 | 1567篇 |
2020年 | 1771篇 |
2019年 | 1756篇 |
2018年 | 10933篇 |
2017年 | 11841篇 |
2016年 | 7458篇 |
2015年 | 3580篇 |
2014年 | 3761篇 |
2013年 | 10001篇 |
2012年 | 12630篇 |
2011年 | 22768篇 |
2010年 | 13933篇 |
2009年 | 14123篇 |
2008年 | 19876篇 |
2007年 | 23307篇 |
2006年 | 8962篇 |
2005年 | 15211篇 |
2004年 | 10815篇 |
2003年 | 9626篇 |
2002年 | 7481篇 |
2001年 | 7550篇 |
2000年 | 5934篇 |
1999年 | 4488篇 |
1998年 | 3515篇 |
1997年 | 3406篇 |
1996年 | 3446篇 |
1995年 | 3013篇 |
1994年 | 2869篇 |
1993年 | 2608篇 |
1992年 | 2960篇 |
1991年 | 2920篇 |
1990年 | 2630篇 |
1989年 | 2526篇 |
1988年 | 2602篇 |
1987年 | 2519篇 |
1986年 | 2371篇 |
1985年 | 3334篇 |
1984年 | 3331篇 |
1983年 | 2648篇 |
1982年 | 2863篇 |
1981年 | 2739篇 |
1980年 | 2633篇 |
1979年 | 2669篇 |
1978年 | 2722篇 |
1977年 | 2620篇 |
1976年 | 2576篇 |
1975年 | 2499篇 |
1974年 | 2377篇 |
1973年 | 2526篇 |
1972年 | 1510篇 |
排序方式: 共有10000条查询结果,搜索用时 948 毫秒
981.
U. Täuber C.E.M. Carvalho R.F. dos Santos C.G. Carvalhaes C.E. Fellows 《Applied physics. B, Lasers and optics》2007,87(3):475-481
A six level rate equation system was used to investigate the impact of photodepletion to amplified spontaneous emission (ASE)
of intramolecular proton-transfer (IPT) dyes incorporated into polymeric hosts. The model includes the most important transitions
for the normal and tautomer form of the molecule as well as intersystem crossing and triplet–triplet transitions. The experimentally
observed pulse shape as well as photodepletion phenomena, i.e. a first order exponential decay of the ASE intensity and a
shortening in ASE pulse width, have been simulated successfully. Additionally, the model was used to propose an explanation
of the unexpected high photodepletion of proton-transfer dyes in solids. The results show that the emission cross section
and reabsorption cross section of the tautomer form of the molecule are the most important parameter not only for efficiency
but also for photodepletion. The model was tested by comparison with experimental results of 2-(2’-hydroxyphenyl)benzimidazole
in polymethylmethacrylate (PMMA), polystyrene (PS) and a 1:1 alternating copolymer matrix PS-co-PMMA.
PACS 78.45.th; 42.55.Mv; 42.70.Jk 相似文献
982.
N C Proudlove S Black A Fletcher 《The Journal of the Operational Research Society》2007,58(2):145-158
This paper considers efforts to improve in-patient flows, a particularly urgent issue in the National Health Service (NHS). The context is described and related to reasons why OR has been making relatively little contribution. The paper argues that large complex models may often be unnecessary and even get in the way of providing clear insight and guidance for problem owners. The importance of understanding the generic working of systems to lead to improvement, and the limitations of simply describing them, is stressed. It is demonstrated that some very simple models can be of significant practical value in understanding and managing complex systems, changing mindsets and driving collection and use of operationally valuable data. Recommendations for more effective engagement with the NHS are offered. 相似文献
983.
Joseph M. Ling 《Discrete and Computational Geometry》2007,37(3):455-469
In this paper we prove four new (infinite) lists of quadratic inequalities, and four cubic inequalities, for the flag f-vectors
of 4-polytopes. These extend and supplement the only four currently known non-linear inequalities, which were proved by Bayer
in 1987. The new lists of inequalities for flag f-vectors yield new lists of inequalities for
f-vectors of 4-polytopes. Using the latter, we managed to improve an estimate discovered by Hoppner and Ziegler concerning
upper bounds of f1 in terms of f0 and f3. 相似文献
984.
Lee Stephen; Harrison Martin C.; Robinson Carol L. 《Teaching Mathematics and its Applications》2007,26(3):111-118
In the past 6 years changes have occurred in GCE A-levels. Inparticular, there have been several major changes in A-levelMathematics courses. As engineering students are usually requiredto have studied A-level Mathematics, or its equivalent, thesechanges have had an effect on their prior mathematical knowledge.Moreover, engineering students traditionally obtained a goodgrounding in mechanics as part of their A-level Mathematicsqualification. However, mechanics, which was once included inthe core syllabus, is now optional. This article investigatesthe current availability and uptake of mechanics modules withinA-level Mathematics courses in schools. Comparisons are drawnbetween these results and results of a survey of schools in2004 and surveys of first year engineering students conductedin 2004 and 2005. It is found that there is a decline in theavailability of mechanics modules and the uptake of more thanone mechanics module has also decreased. The implications ofthese findings for engineering educators are discussed. 相似文献
985.
H. Lapp F. E. Bartelle H. J. C. Tendeloo 《Analytical and bioanalytical chemistry》1938,112(11-12):420-421
986.
987.
E. Schild und C. Enders 《Fresenius' Journal of Analytical Chemistry》1938,115(7-8):272
Ohne Zusammenfassung 相似文献
988.
Fr. Hernler R. Pfeningberger F. Pregl K. Fuchs C. Mai H. Hurts 《Analytical and bioanalytical chemistry》1938,112(11-12):421-425
989.
C. L. Wilson 《Fresenius' Journal of Analytical Chemistry》1940,120(5-6):200-202
Ohne Zusammenfassung 相似文献
990.