全文获取类型
收费全文 | 382802篇 |
免费 | 3262篇 |
国内免费 | 1349篇 |
专业分类
化学 | 188239篇 |
晶体学 | 5487篇 |
力学 | 19351篇 |
综合类 | 3篇 |
数学 | 65072篇 |
物理学 | 109261篇 |
出版年
2021年 | 2389篇 |
2020年 | 2640篇 |
2019年 | 3054篇 |
2018年 | 13854篇 |
2017年 | 13740篇 |
2016年 | 11575篇 |
2015年 | 4310篇 |
2014年 | 5670篇 |
2013年 | 14233篇 |
2012年 | 14082篇 |
2011年 | 22677篇 |
2010年 | 14186篇 |
2009年 | 14459篇 |
2008年 | 17744篇 |
2007年 | 19758篇 |
2006年 | 10886篇 |
2005年 | 10990篇 |
2004年 | 10254篇 |
2003年 | 9792篇 |
2002年 | 8869篇 |
2001年 | 9258篇 |
2000年 | 6959篇 |
1999年 | 5388篇 |
1998年 | 4657篇 |
1997年 | 4572篇 |
1996年 | 4246篇 |
1995年 | 3924篇 |
1994年 | 3713篇 |
1993年 | 3788篇 |
1992年 | 4096篇 |
1991年 | 4195篇 |
1990年 | 3950篇 |
1989年 | 3919篇 |
1988年 | 3931篇 |
1987年 | 3740篇 |
1986年 | 3613篇 |
1985年 | 4772篇 |
1984年 | 4993篇 |
1983年 | 4147篇 |
1982年 | 4496篇 |
1981年 | 4309篇 |
1980年 | 4154篇 |
1979年 | 4257篇 |
1978年 | 4556篇 |
1977年 | 4376篇 |
1976年 | 4520篇 |
1975年 | 4071篇 |
1974年 | 4210篇 |
1973年 | 4498篇 |
1972年 | 2836篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
M. Castagnino R. Id Betan R. Laura 《International Journal of Theoretical Physics》2003,42(10):2423-2439
By extending the notion of mixed states to functionals acting on the space of observables with diagonal singularity we obtain a well-defined complex spectral decomposition of the time evolution for a quantum decaying system. In this formalism, generalized Gamow states are obtained with well-defined physical properties. 相似文献
32.
33.
The disturbance of sleep by traffic noise is a major problem area in noise pollution. Extensive laboratory tests using multichannel electroencephalograms (EEG) have been carried out by many workers to determine the general response of people when exposed to noise during sleep. An experimental technique for obtaining results in people's homes has been developed using a simplified one-channel EEG. The responses of six people in the age range 19-24 were measured. A significant correlation was found between sleep disturbance and traffic noise when the noise is expressed in EPNdB. In the home experiments the subjects appeared to be approximately 10 dB less sensitive to noises than laboratory subjects for similar noise exposure. There also appeared to be some adaptation to the noise exposure; however, only one subject was tested for a period of 20 days and was insufficient to give definite results on adaptation. 相似文献
34.
R. Salomon 《Zeitschrift für Physik B Condensed Matter》1989,73(4):519-527
The diffusion constant and the diagonal conductivity for non-interacting electrons in a two-dimensional, disordered system are studied. A homogeneous magnetic field perpendicular to the electron system is assumed. For weak short-range random potentials and high fields the Landau quantum numbern can be used as expansion parameter. In the limit of high Landau levels the system shows metallic behaviour. Corrections for finiten decrease the conductivity and indicate localized states in the whole energy band. A breakdown of the expansion and stronger localization are observed only for the lowest Landau levels if the typical experimental length scale of the quantized Hall effect is used. 相似文献
35.
H E Cline W E Lorensen R J Herfkens G A Johnson G H Glover 《Magnetic resonance imaging》1989,7(1):45-54
A three-dimensional examination of blood vessels is provided using MR data from seven cases. The vascular surfaces are constructed with an algorithm that automatically follows the selected artery or vein and generates a projected three-dimensional gradient shaded image. Fast 3DFT pulse sequences were optimized to enhance the time-of-flight contrast of the intravascular region. By increasing the surface threshold value in a three-dimensional head study, the flesh of a patient's face was peeled away to demonstrate the superfacial temporal artery. Gated cardiac images show the great vessels and cardiac chambers. A three-dimensional view of the aorta shows an irregular surface in the vicinity of an adrenal tumor. 3D MR exams provide a non-invasive technique for assessing vascular morphology in a clinical setting. 相似文献
36.
We consider the optimal service control of a multiclass M/G/1 queueing system in which customers are served nonpreemptively and the system cost rate is additive across classes and increasing convex in the numbers present in each class. Following Whittle's approach to a class of restless bandit problems, we develop a Langrangian relaxation of the service control problem which serves to motivate the development of a class of index heuristics. The index for a particular customer class is characterised as a fair charge for service of that class. The paper develops these indices and reports an extensive numerical investigation which exhibits strong performance of the index heuristics for both discounted and average costs. 相似文献
37.
Porous road surfaces offer an effective means of reducing the generation and propagation of noise from road traffic. However, the porosity of these surfaces can deteriorate over time, leading to a reduction in their noise reducing properties. Efficient methods are therefore required for monitoring this performance. Existing techniques for performing in-situ measurements of acoustic absorption are unsuitable for use within the traffic stream. Static measurements using time domain Maximum Length Sequence (MLS) based techniques have been demonstrated to offer advantages over traditional techniques, presenting the opportunity for measurements under dynamic conditions. This paper describes the design of a system for carrying out dynamic MLS-based measurements. Results are presented which demonstrate that stable dynamic measurements can be carried out at speeds of up to 30 km/h. 相似文献
38.
Andrew Barton 《The Journal of the Operational Research Society》1987,38(10):965-974
This paper describes some work carried out in the Scientific Research and Development Branch (SRDB) of the Home Office, intended to contribute to an overall aim of building up internal expertise in the field of expert systems. This was done by carrying out a number of pilot and demonstrator projects, two of which are described in some detail. The plan, which proved successful, was to build as far as possible on the relevant skills already possessed by an OR group in the branch. The lessons learned from these are summarized in the hope that they will be of use to other groups who wish to become involved in this important area. 相似文献
39.
40.