全文获取类型
收费全文 | 199224篇 |
免费 | 2996篇 |
国内免费 | 671篇 |
专业分类
化学 | 113991篇 |
晶体学 | 2430篇 |
力学 | 7744篇 |
综合类 | 8篇 |
数学 | 22671篇 |
物理学 | 56047篇 |
出版年
2021年 | 1486篇 |
2020年 | 1756篇 |
2019年 | 1864篇 |
2018年 | 1983篇 |
2017年 | 1891篇 |
2016年 | 3535篇 |
2015年 | 2436篇 |
2014年 | 3513篇 |
2013年 | 8665篇 |
2012年 | 7441篇 |
2011年 | 9237篇 |
2010年 | 5944篇 |
2009年 | 5651篇 |
2008年 | 8250篇 |
2007年 | 8293篇 |
2006年 | 7932篇 |
2005年 | 7405篇 |
2004年 | 6544篇 |
2003年 | 5544篇 |
2002年 | 5548篇 |
2001年 | 5033篇 |
2000年 | 3930篇 |
1999年 | 3136篇 |
1998年 | 2846篇 |
1997年 | 2773篇 |
1996年 | 2661篇 |
1995年 | 2431篇 |
1994年 | 2424篇 |
1993年 | 2276篇 |
1992年 | 2487篇 |
1991年 | 2519篇 |
1990年 | 2322篇 |
1989年 | 2303篇 |
1988年 | 2282篇 |
1987年 | 2131篇 |
1986年 | 2070篇 |
1985年 | 2793篇 |
1984年 | 2961篇 |
1983年 | 2350篇 |
1982年 | 2653篇 |
1981年 | 2510篇 |
1980年 | 2359篇 |
1979年 | 2464篇 |
1978年 | 2681篇 |
1977年 | 2536篇 |
1976年 | 2487篇 |
1975年 | 2386篇 |
1974年 | 2448篇 |
1973年 | 2399篇 |
1972年 | 1553篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
52.
Thomas Marmin Prof. Yves L. Dory 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(27):6707-6711
Cyclohexane and cyclotri-β-alanyl have been used as scaffolds for the design of new C3-symmetric rings incorporating conjugated alkenes and dienes. All three C3-symmetric lactams share the same triangular shape and their crystal system is trigonal. They all belong to the R3 space group, R3m, R3 and R3c, for the increasingly large 12-, 18- and 24-membered rigid rings, respectively. All lactams stack on top of each other, through H-bonds and van der Waals noncovalent interactions, leading to endless supramolecular cylinders and tubes. The largest member of the family leads to tubes, the central pores of which is wide enough to let water in. A common feature of all the lactams is their very large dipole, of around 9 D, according to DFT calculations. Surprisingly, all the resulting cylinders and tubes pack side by side in the crystals, with all the dipoles pointing to the same direction. As a result, all three crystals are anisotropic and appear to be the first members of a new kind of highly polar crystals. 相似文献
53.
54.
55.
56.
57.
E. A. Andrianova V. D. Davidenko V. F. Tsibulskiy S. V. Tsibulskiy 《Physics of Atomic Nuclei》2015,78(11):1252-1258
Influence of the nuclear energy structure, the conditions of fuel burnup, and accumulation of new fissile isotopes from the raw isotopes on the main parameters of a closed fuel cycle is considered. The effects of the breeding ratio, the cooling time of the spent fuel in the external fuel cycle, and the separation of the breeding area and the fissile isotope burning area on the parameters of the fuel cycle are analyzed. 相似文献
58.
59.
Experimental Mechanics - Insufficient data are available to fully understand the effects of metal additive manufacturing (AM) defects for widespread adoption of the emerging technology.... 相似文献
60.
A model is developed for the formation and propagation of cracks in a material sample that is heated at its top surface, pyrolyses, and then thermally degrades to form char. In this work the sample is heated uniformly over its entire top surface by a hypothetical flame (a heat source). The pyrolysis mechanism is described by a one-step overall reaction that is dependent nonlinearly on the temperature (Arrhenius form). Stresses develop in response to the thermal degradation of the material by means of a shrinkage strain caused by local mass loss during pyrolysis. When the principal stress exceeds a prescribed threshold value, the material forms a local crack. Cracks are found to generally originate at the surface in response to heating, but occasionally they form in the bulk, away from ever-changing material boundaries. The resulting cracks evolve and form patterns whose characteristics are described. Quantities examined in detail are: the crack spacing in the pyrolysis zone; the crack length evolution; the formation and nature of crack loops which are defined as individual cracks that have joined to form loops that are disconnected from the remaining material; the formation of enhanced pyrolysis area; and the impact of all of the former quantities on mass flux. It is determined that the mass flux from the sample can be greatly enhanced over its nominal (non-cracking) counterpart. The mass efflux profile qualitatively resembles those observed in Cone Calorimeter tests. 相似文献