全文获取类型
收费全文 | 4650篇 |
免费 | 139篇 |
国内免费 | 17篇 |
专业分类
化学 | 2925篇 |
晶体学 | 14篇 |
力学 | 239篇 |
数学 | 826篇 |
物理学 | 802篇 |
出版年
2023年 | 40篇 |
2022年 | 97篇 |
2021年 | 135篇 |
2020年 | 109篇 |
2019年 | 101篇 |
2018年 | 85篇 |
2017年 | 92篇 |
2016年 | 186篇 |
2015年 | 146篇 |
2014年 | 139篇 |
2013年 | 252篇 |
2012年 | 267篇 |
2011年 | 298篇 |
2010年 | 177篇 |
2009年 | 152篇 |
2008年 | 287篇 |
2007年 | 282篇 |
2006年 | 230篇 |
2005年 | 239篇 |
2004年 | 167篇 |
2003年 | 160篇 |
2002年 | 118篇 |
2001年 | 51篇 |
2000年 | 50篇 |
1999年 | 49篇 |
1998年 | 34篇 |
1997年 | 33篇 |
1996年 | 55篇 |
1995年 | 45篇 |
1994年 | 39篇 |
1993年 | 38篇 |
1992年 | 31篇 |
1991年 | 29篇 |
1990年 | 30篇 |
1989年 | 30篇 |
1988年 | 31篇 |
1987年 | 41篇 |
1986年 | 24篇 |
1985年 | 46篇 |
1984年 | 39篇 |
1983年 | 23篇 |
1982年 | 25篇 |
1981年 | 26篇 |
1980年 | 27篇 |
1979年 | 20篇 |
1978年 | 16篇 |
1977年 | 29篇 |
1975年 | 15篇 |
1974年 | 15篇 |
1858年 | 11篇 |
排序方式: 共有4806条查询结果,搜索用时 46 毫秒
991.
Meccanica - The internal resonance of a two degree-of-freedom mechanical system with all typical (quadratic and cubic) geometric nonlinearities is studied, limiting to the case of free dynamics.... 相似文献
992.
In this note, with a purely geometric approach, the covering radius of the group is determined. Also, a new proof establishing the covering radii of and is provided. 相似文献
993.
Francesco Noto Giuseppe Costa Luigi Celona Francesco Chines Santo Gammino Ornella Leonardi 《辐射效应与固体损伤》2019,174(11-12):1076-1088
ABSTRACTThe ion sources for accelerators devoted to medical applications must provide intense ion beams, with high reproducibility, stability and brightness. AISHa (Advanced Ion Source for Hadron therapy) is a compact ECRIS whose hybrid magnetic system consists of a permanent Halbach-type hexapole magnet and a set of independently energized superconducting coils. These coils will be enclosed in a compact cryostat with two cryocoolers for LHe-free operation. The AISHa ion source has been designed by taking into account the typical requirements of hospital-based facilities, where the minimization of the mean time between failures (MTBF) is a key point together with the maintenance operations which should be fast and easy. It is a multipurpose device, operating at 18?GHz, in order to achieve higher plasma densities able to provide enough versatility for future needs of the hadron therapy, including the ability to run at larger microwave power to produce different species and highly charged ion beams. In this paper, the innovative solutions, used for the plasma containment chamber and for the permanent magnet hexapole holder, are presented to solve the insulation and structural issues. The choice of the different materials used is hereinafter discussed together with all the involved processes (spinning, curing and machining). The glass fibers and carbon fibers are used to reinforce polymer matrices and give rise to structural composites and composites by molding. The paper shows also some results of ion source commissioning along with next developments. Innovative active coupling techniques are planned to be tested to optimize the first pass wave absorption, which plays an important role in the coupling optimization of the new-generation ECRIS. 相似文献
994.
Michele D'Ambrosio Antonio Guerriero Ccile Debitus Olivier Ribes Bertrand Richer De Forges Francesco Pietra 《Helvetica chimica acta》1989,72(7):1451-1454
It is shown that the demosponge Corallistes sp. (Tetractinomorpha, Lithistida, Corallistidae) collected in the Coral Sea, contains corallistin A ( 1 ), the second example, of a free porphyrin from a living organism. The compound proved to be active against the Kb cell line. In contrast with the geoporphyrins which do not bear any O-atom corallistin A ( 1 ) carries two carboxylic groups. 相似文献
995.
996.
Summary
In this paper order-isomorphisms between subnormal structures of subsoluble groups are considered, and the images of generalized nilpotent groups in such isomorphisms are studied. A result of Pazderski about the Fitting subgroup of finite soluble groups is also extended to the upper Baer series of subsoluble groups, and an extension to infinite groups of a theorem of Heineken about isomorphisms between lattices of subnormal subgroups of finite groups is given.
Lavoro eseguito nell'ambito del G.N.S.A.G.A. del C.N.R. 相似文献
Lavoro eseguito nell'ambito del G.N.S.A.G.A. del C.N.R. 相似文献
997.
The effect of substituents at C4, 5, 6 and 7 on solvolysis rates and products of 2-exo- and 2-endo-norbornyl tosylates confirms that differential carbon participation is responsible for varying exo/endo rate and product ratios. 相似文献
998.
We introduce a model suggested by disordered anharmonic quantum crystals. We then investigate in detail the ergodic properties
exhibited by such a model.
Received: 14 January 2002 / Accepted: 14 October 2002 Published online: 10 February 2003
Communicated by J. L. Lebowitz 相似文献
999.
1000.
Ba5−xLaxNb4−xTixO15 solid solutions were prepared by solid state reaction method. Structural analysis for the stoichiometric phases was performed for x=0, 1, 2 and 3 by Rietveld analysis of neutron powder diffraction data. The x=0, 1 and 2 members could be refined in the space group P-3m1 (stacking sequence chhcc, polytypoid 5 H). There is a decrease in cell volume as x increases. La3+ occupies preferentially the A2 site (Wyckoff site 2d) and Ti4+ the B2 site (Wyckoff site 2c). As x increases there is an increase of the global instability index (GII) (which is a measure of the extent to which the BVS rule is violated over the whole structure) indicating the presence of intrinsic strains large enough to cause instability at room temperature. This strain is responsible for a structural change for the member with x=3, which could be refined in the space group P-3c1 (stacking sequence (chhcc)2, polytypoid 2×5H=10H). This change in space group is associated with a cooperative rotation of (Nb/Ti)O6 octahedra around the c-axis, necessary to accommodate the smaller La3+ ion in the cuboctahedral cavity. 相似文献