全文获取类型
收费全文 | 179190篇 |
免费 | 2189篇 |
国内免费 | 543篇 |
专业分类
化学 | 100907篇 |
晶体学 | 2239篇 |
力学 | 7169篇 |
综合类 | 7篇 |
数学 | 18821篇 |
物理学 | 52779篇 |
出版年
2021年 | 1343篇 |
2020年 | 1583篇 |
2019年 | 1643篇 |
2018年 | 1784篇 |
2017年 | 1716篇 |
2016年 | 3190篇 |
2015年 | 2182篇 |
2014年 | 3159篇 |
2013年 | 7575篇 |
2012年 | 6537篇 |
2011年 | 8061篇 |
2010年 | 5324篇 |
2009年 | 5171篇 |
2008年 | 7214篇 |
2007年 | 7182篇 |
2006年 | 6828篇 |
2005年 | 6298篇 |
2004年 | 5547篇 |
2003年 | 4708篇 |
2002年 | 4785篇 |
2001年 | 4749篇 |
2000年 | 3700篇 |
1999年 | 2951篇 |
1998年 | 2618篇 |
1997年 | 2539篇 |
1996年 | 2349篇 |
1995年 | 2191篇 |
1994年 | 2210篇 |
1993年 | 2044篇 |
1992年 | 2263篇 |
1991年 | 2330篇 |
1990年 | 2140篇 |
1989年 | 2115篇 |
1988年 | 2070篇 |
1987年 | 1938篇 |
1986年 | 1858篇 |
1985年 | 2490篇 |
1984年 | 2603篇 |
1983年 | 2090篇 |
1982年 | 2294篇 |
1981年 | 2167篇 |
1980年 | 2095篇 |
1979年 | 2213篇 |
1978年 | 2418篇 |
1977年 | 2251篇 |
1976年 | 2256篇 |
1975年 | 2167篇 |
1974年 | 2230篇 |
1973年 | 2195篇 |
1972年 | 1440篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Under idealized conditions, electroconvection in a nematic liquid crystal sets in at a well-defined threshold of the driving ac voltage. Fluctuating convection rolls of a small amplitude below that threshold have been observed recently. The measurement of the amplitude of these fluctuations is described in detail in this paper. It is based on averaging the light intensity signals using the structure function and a quantitative analysis of the light deflection. 相似文献
42.
43.
44.
The singular-perturbation expansion of the plasma cold-fluid equations for crossed fields in a planar geometry is presented. The general expansion is carried out to third order. Various instabilities that occur in the first, second, and third orders are discussed. 相似文献
45.
William F. Bowlin 《The Journal of the Operational Research Society》1987,38(2):127-135
The objective of this research is to evaluate a new methodology, data envelopment analysis (DEA), as a tool for measuring and evaluating the operational efficiency of US Air Force organizations. This study involves the application of DEA to locate possible inefficiencies in the performance of US Air Force real-property maintenance activities. Dynamic behaviour, as exhibited via window analyses, is reviewed along with static counterparts. The testing was done in close coordination with Air Force officials, who reviewed the results for accuracy, validity and relevance. We conclude that this type of efficiency analysis does have value for the Air Force, where it can serve as a guide to auditors, budget programmers, managers and others in measuring, evaluating and enhancing operational efficiency. 相似文献
46.
F. G. Gerasimato 《Journal of Mathematical Sciences》1991,57(5):3381-3383
We propose a simplified method of determining the integral characteristics of the opening of a parachute which makes it possible to compute the maximal stress on a parachute and the trajectory parameters of the motion of the load in the region of the opening of the parachute, and to determine the size of the portion of the canopy at the time of maximal stress.Translated fromDinamicheskie Sistemy, No. 6, 1987, pp. 38–41. 相似文献
47.
Vijaya Gracias Alan F. Gasiecki Joel D. Moore Irini Akritopoulou-Zanze Stevan W. Djuric 《Tetrahedron letters》2006,47(50):8977-8980
A general method for the preparation of diaza-spirocycles is reported. This method used an olefin metathesis in order to construct the desired spirocyclic framework. Beginning with commercially available protected amino ketones, this strategy ultimately produced pharmacologically relevant diaza-scaffolds in an efficient and high-yielding process. 相似文献
48.
Kurenkov V. F. Zaitseva O. A. Kirdyashova G. N. 《Russian Journal of Applied Chemistry》2002,75(4):641-644
The kinetics of radical copolymerization of acrylamide with lithium, sodium, and potassium 2-acrylamido-2-methylpropanesulfonates in concentrated aqueous solutions in the presence of potassium persulfate as initiating agent at pH 9 and 50°C was studied by dilatometry. 相似文献
49.
Let (\gnm)n,m ? \Zst(\gnm)_{n,m\in\Zst} be a Gabor frame for \LtR\LtR for given window gg. We show that the window \ho = \SQI g\ho=\SQI g that generates the canonically associated tight Gabor frame minimizes ||g-h||\|g-h\| among all windows hh generating a normalized tight Gabor frame. We present and prove versions of this result in the time domain, the frequency domain, the time-frequency domain, and the Zak transform domain, where in each domain the canonical \ho\ho is expressed using functional calculus for Gabor frame operators. Furthermore, we derive a Wiener--Levy type theorem for rationally oversampled Gabor frames. Finally, a Newton-type method for a fast numerical calculation of \ho\ho is presented. We analyze the convergence behavior of this method and demonstrate the efficiency of the proposed algorithm by some numerical examples. 相似文献
50.
We consider the problem of contact of two elastic wedges and assume that only the vertices of the wedges touch before loading. After loading, the edges of both wedges come in contact near their common vertex. We reduce the constructed system of dual integral equations to a Fredholm integral equation of the second kind with difference kernel given on the semiaxis. We analytically solve the Fredholm equation by reducing it to the boundary-value Riemann problem for analytic functions. We obtain an analytic expression for contact stresses. 相似文献