全文获取类型
收费全文 | 5867篇 |
免费 | 296篇 |
国内免费 | 32篇 |
专业分类
化学 | 4091篇 |
晶体学 | 14篇 |
力学 | 181篇 |
数学 | 744篇 |
物理学 | 1165篇 |
出版年
2023年 | 49篇 |
2022年 | 79篇 |
2021年 | 100篇 |
2020年 | 114篇 |
2019年 | 115篇 |
2018年 | 91篇 |
2017年 | 74篇 |
2016年 | 173篇 |
2015年 | 158篇 |
2014年 | 201篇 |
2013年 | 276篇 |
2012年 | 427篇 |
2011年 | 467篇 |
2010年 | 222篇 |
2009年 | 200篇 |
2008年 | 376篇 |
2007年 | 364篇 |
2006年 | 345篇 |
2005年 | 326篇 |
2004年 | 272篇 |
2003年 | 164篇 |
2002年 | 174篇 |
2001年 | 68篇 |
2000年 | 62篇 |
1999年 | 53篇 |
1998年 | 33篇 |
1997年 | 45篇 |
1996年 | 63篇 |
1995年 | 52篇 |
1994年 | 63篇 |
1993年 | 53篇 |
1992年 | 54篇 |
1991年 | 33篇 |
1990年 | 35篇 |
1989年 | 31篇 |
1988年 | 26篇 |
1986年 | 37篇 |
1985年 | 53篇 |
1984年 | 57篇 |
1983年 | 33篇 |
1982年 | 46篇 |
1981年 | 59篇 |
1980年 | 37篇 |
1979年 | 44篇 |
1978年 | 45篇 |
1977年 | 43篇 |
1976年 | 46篇 |
1975年 | 43篇 |
1974年 | 32篇 |
1973年 | 31篇 |
排序方式: 共有6195条查询结果,搜索用时 15 毫秒
21.
A. Ross M. Czisch G.C. King 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1997,124(2):355-365
A theoretical approach to calculate the time evolution of magnetization during a CPMG pulse sequence of arbitrary parameter settings is developed and verified by experiment. The analysis reveals that off-resonance effects can cause systematic reductions in measured peak amplitudes that commonly lie in the range 5–25%, reaching 50% in unfavorable circumstances. These errors, which are finely dependent upon frequency offset and CPMG parameter settings, are subsequently transferred into erroneousT2values obtained by curve fitting, where they are reduced or amplified depending upon the magnitude of the relaxation time. Subsequent transfer to Lipari–Szabo model analysis can produce significant errors in derived motional parameters, with τeinternal correlation times being affected somewhat more thanS2order parameters. A hazard of this off-resonance phenomenon is its oscillatory nature, so that strongly affected and unaffected signals can be found at various frequencies within a CPMG spectrum. Methods for the reduction of the systematic error are discussed. Relaxation studies on biomolecules, especially at high field strengths, should take account of potential off-resonance contributions. 相似文献
22.
23.
24.
Vincent Bailliez 《Tetrahedron》2004,60(5):1079-1085
Polynitrogenated analogues of glucopyranoses were synthesised from levoglucosan. These compounds are useful intermediates for the synthesis of new aminoglycoside mimetics. 相似文献
25.
Christophe Rochais Vincent Lisowski Patrick Dallemagne Sylvain Rault 《Tetrahedron letters》2004,45(33):6353-6355
Novel arylpyrrolo- and pyrazolopyrrolizinones were prepared in three or four steps starting from aminoarylpyrrole and pyrazole carboxylates through the cyclisation of a Vilsmeier-Haack intermediate. This synthesis was enhanced by diverse N-protections of the aza-heterocycle and furnish new series with potential biological interest. 相似文献
26.
Ross W. Mair David G. Cory Sharon Peled Ching-Hua Tseng Samuel Patz Ronald L. Walsworth 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1998,135(2):478-486
Pulsed-field-gradient NMR techniques are demonstrated for measurements of time-dependent gas diffusion. The standard PGSE technique and variants, applied to a free gas mixture of thermally polarized xenon and O2, are found to provide a reproducible measure of the xenon diffusion coefficient (5.71 × 10−6m2s−1for 1 atm of pure xenon), in excellent agreement with previous, non-NMR measurements. The utility of pulsed-field-gradient NMR techniques is demonstrated by the first measurement of time-dependent (i.e., restricted) gas diffusion inside a porous medium (a random pack of glass beads), with results that agree well with theory. Two modified NMR pulse sequences derived from the PGSE technique (named the Pulsed Gradient Echo, or PGE, and the Pulsed Gradient Multiple Spin Echo, or PGMSE) are also applied to measurements of time dependent diffusion of laser polarized xenon gas, with results in good agreement with previous measurements on thermally polarized gas. The PGMSE technique is found to be superior to the PGE method, and to standard PGSE techniques and variants, for efficiently measuring laser polarized noble gas diffusion over a wide range of diffusion times. 相似文献
27.
28.
J. Walz S. B. Ross C. Zimmermann L. Ricci M. Prevedelli T. W. Hänsch 《Hyperfine Interactions》1996,100(1):133-144
Experiments with electrons and ions in a combined trap are reported. The unique capability to confine particles with opposite charge and very different mass simultaneously in the same spatial region makes the combined trap a promising device for future synthesis of antihydrogen. 相似文献
29.
30.
COSEARCH: A Parallel Cooperative Metaheuristic 总被引:1,自引:0,他引:1
In order to design a well-balanced metaheuristic for robustness, we propose the COSEARCH approach which manages the cooperation
of complementary heuristic methods via an adaptive memory which contains a history of the search already done. In this paper,
we present the idiosyncrasies of the COSEARCH approach and its application for solving large scale instances of the quadratic
assignment problem (QAP). We propose an original design of the adaptive memory in order to focus on high quality regions of
the search and avoid attractive but deceptive areas. For the QAP, we have hybridized three heuristic agents of complementary
behaviours: a Tabu Search is used as the main search algorithm, a Genetic Algorithm is in charge of the diversification and
a Kick Operator is applied to intensify the search. The evaluations have been executed on large scale network of workstations
via a parallel environment which supports fault tolerance and adaptive dynamic scheduling of tasks. 相似文献