全文获取类型
收费全文 | 83247篇 |
免费 | 11799篇 |
国内免费 | 8592篇 |
专业分类
化学 | 23621篇 |
晶体学 | 1229篇 |
力学 | 15532篇 |
综合类 | 1252篇 |
数学 | 27907篇 |
物理学 | 34097篇 |
出版年
2024年 | 193篇 |
2023年 | 879篇 |
2022年 | 2115篇 |
2021年 | 2193篇 |
2020年 | 2620篇 |
2019年 | 2301篇 |
2018年 | 2237篇 |
2017年 | 3057篇 |
2016年 | 3502篇 |
2015年 | 2695篇 |
2014年 | 4368篇 |
2013年 | 6326篇 |
2012年 | 4956篇 |
2011年 | 5673篇 |
2010年 | 4867篇 |
2009年 | 5474篇 |
2008年 | 5307篇 |
2007年 | 5258篇 |
2006年 | 4802篇 |
2005年 | 4281篇 |
2004年 | 3840篇 |
2003年 | 3367篇 |
2002年 | 3042篇 |
2001年 | 2591篇 |
2000年 | 2435篇 |
1999年 | 2089篇 |
1998年 | 1958篇 |
1997年 | 1604篇 |
1996年 | 1382篇 |
1995年 | 1183篇 |
1994年 | 1088篇 |
1993年 | 895篇 |
1992年 | 872篇 |
1991年 | 670篇 |
1990年 | 547篇 |
1989年 | 428篇 |
1988年 | 398篇 |
1987年 | 336篇 |
1986年 | 243篇 |
1985年 | 288篇 |
1984年 | 275篇 |
1983年 | 145篇 |
1982年 | 205篇 |
1981年 | 139篇 |
1980年 | 77篇 |
1979年 | 99篇 |
1978年 | 81篇 |
1977年 | 74篇 |
1976年 | 34篇 |
1973年 | 43篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
3.
Alexander Zeller 《Journal of organometallic chemistry》2006,691(21):4379-4385
The ethylene polymerization reaction of a neutral nickel catalyst was studied by DFT calculations at the Becke3LYP/6-31G(d) level of theory. As in related cases a β-agostic bond stabilizes the nickel alkyl ground states. Transition states for the insertion of the olefin show a distinct α-agostic interaction, which has not been observed for late metal polymerization catalysts before. An ethylene-alkyl complex was identified as the resting state of the reaction. The overall barrier height of the reaction amounts to 17.54 kcal/mol, which slightly increases to 17.60 kcal/mol for the polymerization of deuterated ethylene. Therefore, a small positive kinetic isotope effect (kH/kD = 1.09) can be calculated, which is caused by the α-agostic interaction in the transition state. A comparison to other late metal based polymerization systems reveals that the ethylene coordination step of highly active catalysts is significantly lower in energy compared to catalysts which are only moderately active. 相似文献
4.
A. Foussats C. Repetto O. P. Zandron O. S. Zandron 《International Journal of Theoretical Physics》2002,41(6):1053-1082
In order to describe the dynamics of the t–J model, two different families of first-order Lagrangians in terms of the generators of the Hubbard algebra are found. Such families correspond to different dynamical second-class constrained systems. The quantization is carried out by using the path-integral formalism. In this context the introduction of proper ghost fields is needed to render the model renormalizable. In each case the standard Feynman diagrammatics is obtained and the renormalized physical quantities are computed and analyzed. In the first case a nonperturbative large-N expansion is considered with the purpose of studying the generalized Hubbard model describing N-fold-degenerate correlated bands. In this case the 1/N correction to the renormalized boson propagator is computed. In the second case the perturbative Lagrangian formalism is developed and it is shown how propagators and vertices can be renormalized to each order. In particular, the renormalized ferromagnetic magnon propagator coming from our formalism is studied in details. As an example the thermal softening of the magnon frequency is computed. The antiferromagnetic case is also analyzed, and the results are confronted with previous one obtained by means of the spin-polaron theories. 相似文献
5.
M. Georgelin A. Pocheau 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(2):169-174
The polycrystalline perovskitelike manganese oxides La1-xAxMnO (A = Na, and K, ) have been fabricated by sol-gel technique. For all the compositions explored in this work, the average manganese oxidation
state is practically constant, at for A = Na, and for A = K, respectively. A close relationship is confirmed to hold between the Curie temperature (Tc) and the bond distance of Mn-O. Results of magnetic measurements show that these materials can be utilized as suitable candidates
for magnetic refrigerants with wide applied temperature span, for their significant entropy change and the easily tuned Curie
temperature.
Received: 12 September 1997 / Revised: 18 December 1997 / Accepted: 21 January 1998 相似文献
6.
7.
Rafael López 《Annals of Global Analysis and Geometry》1997,15(3):201-210
In this paper we study constant mean curvature compact surfaces with two Jordan curves in parallel planes as boundary and we investigate the point at which the surface inherits the symmetries of its boundary. 相似文献
8.
Summary Experiments and simulations prove that correlation chromatography can greatly reduce the disadvantage of a non-linear response of the chromatographic column. A factor that has been accepted as being an important source of error in correlation or multiplex chromatography, has been shown not to be. Separations affected, improve dramatically when correlation chromatography is used, and a substantial amount of correlation noise only arises when there is a large difference in separation between a conventional chromatogram and a correlogram.A model has been developed for simulating these nonlinearities. It is shown that, especially for correlation chromatography, the simulation results match the practical measurements very well. 相似文献
9.
The theory of tree-growing (RECPAM approach) is developed for outcome variables which are distributed as the canonical exponential family. The general RECPAM approach (consisting of three steps: recursive partition, pruning and amalgamation), is reviewed. This is seen as constructing a partition with maximal information content about a parameter to be predicted, followed by simplification by the elimination of ‘negligible’ information. The measure of information is defined for an exponential family outcome as a deviance difference, and appropriate modifications of pruning and amalgamation rules are discussed. It is further shown how the proposed approach makes it possible to develop tree-growing for situations usually treated by generalized linear models (GLIM). In particular, Poisson and logistic regression can be tree-structured. Moreover, censored survival data can be treated, as in GLIM, by observing a formal equivalence of the likelihood under random censoring and an appropriate Poisson model. Three examples are given of application to Poisson, binary and censored survival data. 相似文献
10.
J. C. Fabris S. V. B. Gonçalves P. E. de Souza 《General Relativity and Gravitation》2002,34(12):2111-2126
The mass power spectrum for a Universe dominated by the Chaplygin gas is evaluated numerically from scales of the order of the Hubble horizon to 100 Mpc. The results are compared with a pure baryonic Universe and a cosmological constant model. In all three cases, the spectrum increases with k, the wavenumber of the perturbations. The slope of the spectrum is higher for the baryonic model and smaller for the cosmological constant model, the Chaplygin gas interpolating these two models. The results are analyzed in terms of the sound velocity of the Chaplygin gas and the moment the Universe begins to accelerate. 相似文献