全文获取类型
收费全文 | 2371篇 |
免费 | 77篇 |
国内免费 | 19篇 |
专业分类
化学 | 1745篇 |
晶体学 | 19篇 |
力学 | 44篇 |
数学 | 295篇 |
物理学 | 364篇 |
出版年
2022年 | 8篇 |
2021年 | 20篇 |
2020年 | 45篇 |
2019年 | 26篇 |
2018年 | 24篇 |
2017年 | 15篇 |
2016年 | 44篇 |
2015年 | 31篇 |
2014年 | 37篇 |
2013年 | 104篇 |
2012年 | 132篇 |
2011年 | 154篇 |
2010年 | 64篇 |
2009年 | 43篇 |
2008年 | 111篇 |
2007年 | 130篇 |
2006年 | 124篇 |
2005年 | 164篇 |
2004年 | 127篇 |
2003年 | 138篇 |
2002年 | 139篇 |
2001年 | 38篇 |
2000年 | 30篇 |
1999年 | 24篇 |
1998年 | 20篇 |
1997年 | 34篇 |
1996年 | 44篇 |
1995年 | 32篇 |
1994年 | 19篇 |
1993年 | 31篇 |
1992年 | 8篇 |
1991年 | 19篇 |
1990年 | 19篇 |
1989年 | 29篇 |
1988年 | 16篇 |
1987年 | 13篇 |
1986年 | 10篇 |
1985年 | 32篇 |
1984年 | 25篇 |
1983年 | 22篇 |
1982年 | 29篇 |
1981年 | 34篇 |
1980年 | 36篇 |
1979年 | 22篇 |
1978年 | 41篇 |
1977年 | 36篇 |
1976年 | 18篇 |
1975年 | 23篇 |
1974年 | 9篇 |
1973年 | 24篇 |
排序方式: 共有2467条查询结果,搜索用时 359 毫秒
971.
This paper investigates how well different kinds of fMRI functional connectivity analysis reflect the underlying interregional neural interactions. This is hard to evaluate using real experimental data where such relationships are unknown. Rather, we use a biologically realistic neural model to simulate both neuronal activities and multiregional fMRI data from a blocked design. Because we know how every element in the model is related to every other element, we can compare functional connectivity measurements across different spatial and temporal scales. We focus on (1) psycho-physiological interaction (PPI) analysis, which is a simple brain connectivity method that characterizes the activity in one brain region by the interaction between another region's activity and a psychological factor, and (2) interregional correlation analysis. We investigated the neurobiological underpinnings of PPI using simulated neural activities and fMRI signals generated by a large-scale neural model that performs a visual delayed match-to-sample task. Simulated fMRI data are generated by convolving integrated synaptic activities (ISAs) with a hemodynamic response function. The simulation was done under three task conditions: high-attention, low-attention and a control task ('passive viewing'). We investigated how biological and scanning parameters affect PPI and compared these with functional connectivity measures obtained using correlation analysis. We performed correlational and PPI analyses with three types of time-series data: ISA, fMRI and deconvolved fMRI (which yields estimated neural signals) obtained using a deconvolution algorithm. The simulated ISA can be considered as the 'gold standard' because it represents the underlying neural activity. Our main findings show (1) that evaluating the change in an interregional functional connection using the difference in regression coefficients (as is essentially done in the PPI method) produces results that better reflect the underlying changes in neural interrelationships than does evaluating the functional connectivity difference as a change in correlation coefficient; (2) that using fMRI and deconvolved fMRI data led to similar conclusions in the PPI-based functional connectivity results, and these generally agreed with the nature of the underlying neural interactions; and (3) the functional connectivity correlation measures often led to different conclusions regarding significance for different scanning and hemodynamic parameters, but the significances of the PPI regression parameters were relatively robust. These results highlight the way in which neural modeling can be used to help validate the inferences one can make about functional connectivity based on fMRI data. 相似文献
972.
David J. Crouch Peter J. Skabara Martin Heeney Iain McCulloch David Sparrowe Simon J. Coles Michael B. Hursthouse 《Macromolecular rapid communications》2008,29(22):1839-1843
A series of selenophene oligomers incorporating conjugated fluorinated phenylene units have been synthesised as potential semiconductor materials for organic field‐effect transistors (OFETs). X‐ray crystallography shows that the molecules are held in close proximity by several short intermolecular contacts, making them ideal candidates for OFET applications.
973.
Reaction of ethyl (2S*,3R*)‐3‐chloro‐2‐(dialkylamino)‐3‐phenylpropanoates ( 4 ) with hydrazine monohydrate gave pyrazolidin‐3‐ones ( 5 ) via selective opening of the in situ generated aziridinium ions, followed by intramolecular amidation. (1Z,4S*,5S*)‐1‐Arylmethylidene‐4‐(dialkylamino)‐3‐oxo‐5‐phenylpyrazolidinium‐1‐ide ( 6 ), prepared from pyrazolidin‐3‐ones ( 5 ) and aromatic aldehydes under acid catalysis, reacted with a variety of 1,3‐dipolarophiles to afford good yields of pure cycloadducts 7 – 11 . The cycloaddition regio‐ and/or stereoselectivities, in all relevant cases, were also high. 相似文献
974.
Rudolph D. Deanin Carol M. Barry Somsak Woramongconchai Sona C. Parikh 《Macromolecular Symposia》1998,135(1):55-62
Mixed plastics from junked autos were homogenized by milling or extrusion, modified by addition of low-molecular-weight low-melt-viscosity polymers, and processed by compression or injection molding. Properties were comparable with high-density polyethylene and common building panel materials. 相似文献
975.
Barry A. A. L. van Setten Paola Russo Sytse J. Jelles Michiel Makkee Paolo Ciambelli Jacob A. Moulijn 《Reaction Kinetics and Catalysis Letters》1999,67(1):3-7
Diesel soot is combusted simultaneously by two reactions: combustion with NO2 and combustion with O2 with the aid of a molten salt catalyst. Both reaction pathways should always be considered to avoid misinterpretation of
experimental data. 相似文献
976.
977.
Warren G. Lewis Luke G. Green Flavio Grynszpan Zoran Radi Paul R. Carlier Palmer Taylor M. G. Finn K. Barry Sharpless 《Angewandte Chemie (International ed. in English)》2002,41(6):875-875
The cover picture shows the electric eel, Electrophorus electricus, a source for commercially available acetylcholinesterase. In an experiment described by K. B. Sharpless and M. G. Finn and co‐workers on pp. 1053–1057, a femtomolar inhibitor was assembled by the enzyme from a collection of building blocks containing azide and alkyne functional groups, shown floating in solution. The templated 1,3‐dipolar cycloaddition reaction, producing the inhibitor, is represented by the flare of light at the center of the image. 相似文献
978.
Francisco Prieto Emma Hill Barry A. Coles Richard G. Compton John H. Atherton 《Journal of Solid State Electrochemistry》1999,3(4):187-192
A high-speed channel flow cell is used to study the reaction of solid p-chloranil with basic aqueous solutions. Dissolution of the solid is shown to be induced by reaction of OH− ions at the solid/liquid interface and appropriate kinetic parameters are reported.
Received: 1 September 1998 / Accepted: 2 November 1998 相似文献
979.
Spatial Regression Models for Extremes 总被引:2,自引:0,他引:2
Meteorological data are often recorded at a number of spatial locations. This gives rise to the possibility of pooling data through a spatial model to overcome some of the limitations imposed on an extreme value analysis by a lack of information. In this paper we develop a spatial model for extremes based on a standard representation for site-wise extremal behavior, combined with a spatial latent process for parameter variation over the region. A smooth, but possibly non-linear, spatial structure is an intrinsic feature of the model, and difficulties in computation are solved using Markov chain Monte Carlo inference. A simulation study is carried out to illustrate the potential gain in efficiency achieved by the spatial model. Finally, the model is applied to data generated from a climatological model in order to characterize the hurricane climate of the Gulf and Atlantic coasts of the United States. 相似文献
980.
F. Javier Del Campo B. A. Coles F. Marken R. G. Compton E. Cordemans 《Ultrasonics sonochemistry》1999,6(4):H63
The use of high frequency ultrasound in electrochemical systems is of major interest for the optimisation of electrosynthetic and electroanalytical procedures, especially when the strong mechanical effects of 20 kHz ultrasound are detrimental. The characterisation of a 500 kHz ultrasound reactor for sonoelectrochemical experiments by voltammetric and potentiometric measurements revealed the presence of considerable thermal, as well as mass transport, effects depending on geometric parameters and the material used for the construction of the working electrode. Micromixing and cavitation processes govern the mass transport to and from the electrode surface and are shown by atomic force microscopy (AFM) to cause erosion on the electrode surface. Electrochemically active films of Prussian blue are shown to be gradually removed by cavitation erosion. Degassing the solution prior to sonication increases the efficiency of cavitation processes. 相似文献