全文获取类型
收费全文 | 927篇 |
免费 | 28篇 |
国内免费 | 4篇 |
专业分类
化学 | 649篇 |
晶体学 | 7篇 |
力学 | 21篇 |
数学 | 134篇 |
物理学 | 148篇 |
出版年
2022年 | 10篇 |
2021年 | 11篇 |
2020年 | 10篇 |
2019年 | 13篇 |
2017年 | 8篇 |
2016年 | 12篇 |
2015年 | 19篇 |
2014年 | 17篇 |
2013年 | 32篇 |
2012年 | 42篇 |
2011年 | 57篇 |
2010年 | 33篇 |
2009年 | 23篇 |
2008年 | 42篇 |
2007年 | 38篇 |
2006年 | 47篇 |
2005年 | 40篇 |
2004年 | 39篇 |
2003年 | 33篇 |
2002年 | 34篇 |
2001年 | 24篇 |
2000年 | 20篇 |
1999年 | 16篇 |
1998年 | 12篇 |
1997年 | 13篇 |
1996年 | 17篇 |
1994年 | 10篇 |
1993年 | 18篇 |
1992年 | 23篇 |
1991年 | 9篇 |
1990年 | 13篇 |
1989年 | 11篇 |
1988年 | 11篇 |
1987年 | 10篇 |
1986年 | 9篇 |
1985年 | 8篇 |
1984年 | 9篇 |
1983年 | 9篇 |
1982年 | 8篇 |
1981年 | 16篇 |
1980年 | 11篇 |
1979年 | 8篇 |
1978年 | 9篇 |
1977年 | 6篇 |
1976年 | 9篇 |
1975年 | 8篇 |
1974年 | 7篇 |
1973年 | 14篇 |
1969年 | 8篇 |
1934年 | 4篇 |
排序方式: 共有959条查询结果,搜索用时 15 毫秒
1.
2.
Douglas R. White Jason Owen-Smith James Moody Walter W. Powell 《Computational & Mathematical Organization Theory》2004,10(1):95-117
Social action is situated in fields that are simultaneously composed of interpersonal ties and relations among organizations, which are both usefully characterized as social networks. We introduce a novel approach to distinguishing different network macro-structures in terms of cohesive subsets and their overlaps. We develop a vocabulary that relates different forms of network cohesion to field properties as opposed to organizational constraints on ties and structures. We illustrate differences in probabilistic attachment processes in network evolution that link on the one hand to organizational constraints versus field properties and to cohesive network topologies on the other. This allows us to identify a set of important new micro-macro linkages between local behavior in networks and global network properties. The analytic strategy thus puts in place a methodology for Predictive Social Cohesion theory to be developed and tested in the context of informal and formal organizations and organizational fields. We also show how organizations and fields combine at different scales of cohesive depth and cohesive breadth. Operational measures and results are illustrated for three organizational examples, and analysis of these cases suggests that different structures of cohesive subsets and overlaps may be predictive in organizational contexts and similarly for the larger fields in which they are embedded. Useful predictions may also be based on feedback from level of cohesion in the larger field back to organizations, conditioned on the level of multiconnectivity to the field. 相似文献
3.
D. Hugh Powell Andr E. Merbach Gabriel Gonzlez Ern Brücher Kroly Micskei M. Francesca Ottaviani Klaus Khler Alex Von Zelewsky Oleg Ya. Grinberg Yakob S. Lebedev 《Helvetica chimica acta》1993,76(5):2129-2146
EPR Spectra have been measured for aqueous solutions of a series of Gd3+ complexes at variable temperature and a range of magnetic fields; S-band (0.14 T), X-band (0.34 T), Q-band (1.2 T), and 2-mm-band (5.0 T). The major contribution to the observed line widths is magnetic-field-dependent and is interpreted as being due to the modulation of the zero-field splitting produced by distortion of the complexes from perfect symmetry. The transverse and longitudinal relaxation matrices for an 8S ion with such an interaction have been calculated using Redfield theory with vector-coupling methods, and diagonalised numerically to obtain relaxation rates and intensities for the degenerate transitions which contribute to the multiplet. The observed line width, which is inversely proportional to the magnetic field at low temperatures, is best described by the intensity-weighted mean transverse relaxation time for the four transitions with non-zero intensity. A least-squares fit of the data yields the square of the zero-field splitting tensor, Δ2, and a correlation time, τv, with activation energy, Ev. The physical significance of these parameters and the extent of validity of the theoretical approach are considered. The parameters are used to predict the magnetic-field dependence of the longitudinal and transverse electronic relaxation times, which are discussed in the context of their relevance to 1H-NMR relaxivity. 相似文献
4.
In previous work the authors consider the dynamic assignment problem, which involves solving sequences of assignment problems over time in the presence of uncertain information about the future. The algorithm proposed by the authors provides generally high-quality but non-optimal solutions. In this work, though, the authors prove that if the optimal solution to a dynamic assignment problem in one of two problem classes is unique, then the optimal solution is a fixed point under the algorithm. 相似文献
5.
We report on further developments in the three-dimensional tracking of a particle deep within the tumbling ball charge of
an experimental mill. The experimental X-ray program employing the use of bi-planar X-ray angiography now includes the tracking
of a typical 6 mm bulk charge particle in three dimensions with a spatial resolution that is accurate to within 0.15 mm. The
improved experimental tracking techniques presented were developed for the purpose of generating accurate three-dimensional
particle trajectory data against which to validate a numerical method for the simulation of discrete media, namely the discrete
element method (DEM). These improvements are complimented with techniques for comparing charge profiles between numerical
DEM simulations and three-dimensional experimental trajectory data. 相似文献
6.
7.
8.
C. Linda Powell 《Tetrahedron letters》2007,48(7):1251-1254
1-(2-Nitrophenyl)-1,2-dicarba-closo-dodecaborane(12) shows C-H?O intramolecular hydrogen bonding in chloroform. The reaction of isomeric 1-(nitrophenyl)-1,2-dicarba-closo-dodecaborane(12)s and of 1-(4-fluorophenyl)-1,2-dicarba-closo-dodecaborane(12) with wet DMSO causes the removal of 3-B or 6-B, leading cleanly to nido-carboranes. The rank order of rates of these deboronations is consistent with developing negative charge in the rate-determining step. 相似文献
9.
10.
The first 3-D open-framework TiGaPO complex, constructed from Ti(III)O(6), Ti(IV)O(6), GaO(4), and PO(4) polyhedra, contains pyridinium cations in a 1-D pore network and can be oxidized in air at 543 K with retention of the original framework structure. 相似文献