全文获取类型
收费全文 | 1750篇 |
免费 | 25篇 |
国内免费 | 6篇 |
专业分类
化学 | 1243篇 |
晶体学 | 5篇 |
力学 | 44篇 |
数学 | 264篇 |
物理学 | 225篇 |
出版年
2022年 | 15篇 |
2021年 | 13篇 |
2020年 | 21篇 |
2017年 | 20篇 |
2016年 | 23篇 |
2015年 | 16篇 |
2014年 | 20篇 |
2013年 | 32篇 |
2012年 | 50篇 |
2011年 | 74篇 |
2010年 | 38篇 |
2009年 | 33篇 |
2008年 | 61篇 |
2007年 | 64篇 |
2006年 | 51篇 |
2005年 | 51篇 |
2004年 | 57篇 |
2003年 | 41篇 |
2002年 | 51篇 |
2001年 | 29篇 |
2000年 | 17篇 |
1999年 | 23篇 |
1998年 | 16篇 |
1997年 | 22篇 |
1996年 | 27篇 |
1995年 | 17篇 |
1994年 | 19篇 |
1993年 | 12篇 |
1992年 | 16篇 |
1990年 | 13篇 |
1988年 | 12篇 |
1985年 | 14篇 |
1982年 | 19篇 |
1981年 | 12篇 |
1980年 | 12篇 |
1979年 | 16篇 |
1977年 | 19篇 |
1976年 | 16篇 |
1974年 | 17篇 |
1973年 | 12篇 |
1936年 | 12篇 |
1930年 | 18篇 |
1927年 | 15篇 |
1909年 | 14篇 |
1906年 | 16篇 |
1904年 | 13篇 |
1885年 | 11篇 |
1884年 | 19篇 |
1883年 | 11篇 |
1881年 | 14篇 |
排序方式: 共有1781条查询结果,搜索用时 15 毫秒
101.
The problem of multidimensional scaling with city-block distances in the embedding space is reduced to a two level optimization
problem consisting of a combinatorial problem at the upper level and a quadratic programming problem at the lower level. A
hybrid method is proposed combining randomized search for the upper level problem with a standard quadratic programming algorithm
for the lower level problem. Several algorithms for the combinatorial problem have been tested and an evolutionary global
search algorithm has been proved most suitable. An experimental code of the proposed hybrid multidimensional scaling algorithm
is developed and tested using several test problems of two- and three-dimensional scaling. 相似文献
102.
103.
104.
105.
106.
Julius Meyer 《欧洲无机化学杂志》1902,35(4):3952-3957
107.
108.
Summary Birefringence and orientation angle of solutions of Polyox WSR 301 in the concentration range of 50 to 1350 ppm have been measured up to 8000 s–1. A few marked differences with ordinary flexible polymers have been evidenced: (i) a very low value of the extinction angle at high shear rate, which could be attributed to an unusually high molecular weight; (ii) the independence of this angle on the concentration, which is due to the variation of the flexibility with the concentration, as shown by the study of the concentration dependence of the segmental optical anisotropy; (iii) the concentration dependence of the elongation shows a plateau value forc 150 ppm, which may come from a concentration dependent and reversible association process, as shown from reduced viscosity measurements.
With 5 figures 相似文献
Zusammenfassung Es wurden die Doppelbrechung und der Orientierungswinkel von Polyox-WSR 301-Lösungen im Konzentrationsbereich 50–1350 ppm bis zu Schergeschwindigkeiten von 8000 s–1 gemessen. Dabei wurden einige ausgeprägte Unterschiede gegenüber normalen flexiblen Polymeren gefunden: (i) ein sehr niedriger Wert des Auslöschwinkels, was auf ein ungewöhnlich hohes Molekulargewicht schließen läßt: (ii) die Unabhängigkeit dieses Winkels von der Konzentration, was von einer Änderung der Flexibilität mit der Konzentration herrührt, wie sie durch die Untersuchung der Konzentrationsabhängigkeit der optischen Anisotropie der Segmente aufgezeigt wird; (iii) die Existenz eines Plateaus in der Konzentrationsabhängigkeit der Streckung beic 150 ppm, welches auf einen konzentrationsabhängigen reversiblen Assoziationsvorgang hindeutet, der auch durch Messung der reduzierten Viskosität nahegelegt wird.
With 5 figures 相似文献
109.
As part of an EEC Science Stimulation programme on extensional viscosity two major conferences were organised on the subject. The second of these was devoted to the results obtained on a standard fluid, M 1. The data obtained in shear flow was remarkably consistent from laboratory to laboratory. Extensional flow results presented quite a different picture. Using a series of nonequilibrium techniques such as the spinline rheometer, opposing jet, falling drop and converging flow, extensional viscosity results were obtained which differed by as much as two to three orders of magnitude. Nevertheless, it was apparent that consistancy did exist between similar techniques. It is in the context of this information that the measurements described below have been made.The shear and extensional flow properties of partially ionised polyacrylamide in solution at concentrations ranging from 5 ppm were measured. The method of solution preparation was found to have a profound effect on the behaviour of the solutions in shear flow. The influence of salt concentration and pH was investigated and is discussed in the context of molecular shape in solution.Extensional flow measurements, using the spinline rheometer, show that the solutions are strongly strain thickening even at concentrations as low as 5 ppm. These results are considered in the light of polymer entanglement and association in the strong flow field.Delivered as a Keynote Lecture at the Golden Jubilee Conference of the British Society of Rheology and Third European Rheology Conference, Edinburgh, 3–7 September, 1990. 相似文献
110.
Thomas Wolff Helmut Görner 《Journal of photochemistry and photobiology. A, Chemistry》2010,209(2-3):219-223
The cleavage of four coumarin dimers, the syn-head-to-tail (ht) dimer of parent coumarin (syn-ht-CC1), the anti- and syn-hh dimers of 6-methylcoumarin (anti-hh-CC2 and syn-hh-CC2, respectively) and the anti-hh dimer of 6-dodecylcoumarin (anti-hh-CC3), was studied by UV–vis and IR spectroscopy and HPLC upon direct 254 nm irradiation as well as sensitized excitation. The quantum yield of dimer splitting is Φsp = 0.1–0.3 in various solvents and the effects of structure and solvent polarity are small. In certain solvents some of the dimers produced CO2 along with the monomers in the splitting reaction. Electron transfer from dimers to the triplet state of sensitizers, such as benzophenone or 9,10-anthraquinone, was observed in acetonitrile. 相似文献