全文获取类型
收费全文 | 1606296篇 |
免费 | 32328篇 |
国内免费 | 8439篇 |
专业分类
化学 | 714770篇 |
晶体学 | 20755篇 |
力学 | 75983篇 |
综合类 | 104篇 |
数学 | 246238篇 |
物理学 | 389620篇 |
无线电 | 199593篇 |
出版年
2021年 | 14863篇 |
2020年 | 17473篇 |
2019年 | 17619篇 |
2018年 | 15945篇 |
2017年 | 14058篇 |
2016年 | 30805篇 |
2015年 | 22157篇 |
2014年 | 32956篇 |
2013年 | 78980篇 |
2012年 | 44237篇 |
2011年 | 44219篇 |
2010年 | 42512篇 |
2009年 | 46766篇 |
2008年 | 46567篇 |
2007年 | 44532篇 |
2006年 | 48683篇 |
2005年 | 42163篇 |
2004年 | 41964篇 |
2003年 | 38866篇 |
2002年 | 39199篇 |
2001年 | 38806篇 |
2000年 | 33860篇 |
1999年 | 30068篇 |
1998年 | 27871篇 |
1997年 | 27735篇 |
1996年 | 27150篇 |
1995年 | 24909篇 |
1994年 | 24381篇 |
1993年 | 23795篇 |
1992年 | 23853篇 |
1991年 | 24070篇 |
1990年 | 22828篇 |
1989年 | 22399篇 |
1988年 | 21566篇 |
1987年 | 20218篇 |
1986年 | 19079篇 |
1985年 | 25590篇 |
1984年 | 26627篇 |
1983年 | 22475篇 |
1982年 | 23840篇 |
1981年 | 22996篇 |
1980年 | 22289篇 |
1979年 | 22329篇 |
1978年 | 23461篇 |
1977年 | 23086篇 |
1976年 | 22802篇 |
1975年 | 21485篇 |
1974年 | 21150篇 |
1973年 | 21725篇 |
1972年 | 15847篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
This paper presents the evolution of low-frequency, high-power transcontinental transmission, starting from the first successful transatlantic link from Poldhu up to the Italian transcontinental station of Coltano. The trend toward higher and higher transmitting power, and the dead end into which wireless was heading if it were not for research in the higher frequency ranges, are pointed out. 相似文献
972.
In order to complement subjective evaluation of the quality of segmentation masks, this paper introduces a procedure for automatically assessing this quality. Algorithmically computed figures of merit are proposed. Assuming the existence of a perfect reference mask (ground truth), generated manually or with a reliable procedure over a test set, these figures of merit take into account visually desirable properties of a segmentation mask in order to provide the user with metrics that best quantify the spatial and temporal accuracy of the segmentation masks. For the sake of easy interpretation, results are presented on a peaked signal-to-noise ratio-like logarithmic scale. 相似文献
973.
Reducible rank codes and their applications to cryptography 总被引:2,自引:0,他引:2
Gabidulin E.M. Ourivski A.V. Honary B. Ammar B. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2003,49(12):3289-3293
We present a new family of so-called reducible rank codes which are a generalization of rank product codes . This family includes maximal rank distance (MRD) codes for lengths n>N in the field F/sub N/. We give methods for encoding and decoding reducible rank codes. A public key cryptosystem based on these codes and on the idea of a column scrambler is proposed. The column scrambler "mixes" columns of a generator (parity-check) matrix of a code. It makes the system more resistant to structural attacks such as Gibson's attacks. Possible attacks on the system are thoroughly studied. The system is found to be secure against known attacks for public keys of about 16 kbits and greater. 相似文献
974.
This article presents a graph-theoretic method for constructing low-density parity-check (LDPC) codes from connected graphs without the requirement of large girth. This method is based on finding a set of paths in a connected graph, which satisfies the constraint that any two paths in the set are either disjoint or cross each other at one and only one vertex. Two trellis-based algorithms for finding these paths are devised. Good LDPC codes of practical lengths are constructed and they perform well with iterative decoding. 相似文献
975.
976.
Lopera J.M. Prieto M.J. Pernia A.M. Nuno F. 《Power Electronics, IEEE Transactions on》2003,18(3):896-906
This paper presents a method to obtain an electric model for transformers and inductors, including both frequency and geometry effects in the windings, which can be linked with existing core models. One-dimensional distributions for magnetic and electric fields are assumed, and from Maxwell's equations an equivalent electric circuit is easily obtained. This equivalent circuit has been included in analog simulators (Spice, AnalogWorkBench, Saber ...), and comparisons between measured and simulated results are shown, both in time domain and in AC sweep, which verify the model accuracy. The model described in this paper allows designers to deal with key issues in the design of high-frequency magnetic components (copper losses, leakage inductance, skin and proximity effects) by using analog simulators, which are usually more familiar to them than finite-element analysis tools. 相似文献
977.
Feng-Wen Sun Yimin Jiang Baras J.S. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2003,49(1):180-190
Many issues in signal processing involve the inverses of Toeplitz matrices. One widely used technique is to replace Toeplitz matrices with their associated circulant matrices, based on the well-known fact that Toeplitz matrices asymptotically converge to their associated circulant matrices in the weak sense. This often leads to considerable simplification. However, it is well known that such a weak convergence cannot be strengthened into strong convergence. It is this fact that severely limits the usefulness of the close relation between Toeplitz matrices and circulant matrices. Observing that communication receiver design often needs to seek optimality in regard to a data sequence transmitted within finite duration, we define the finite-term strong convergence regarding two families of matrices. We present a condition under which the inverses of a Toeplitz matrix converges in the strong sense to a circulant matrix for finite-term quadratic forms. This builds a critical link in the application of the convergence theorems for the inverses of Toeplitz matrices since the weak convergence generally finds its usefulness in issues associated with minimum mean squared error and the finite-term strong convergence is useful in issues associated with the maximum-likelihood or maximum a posteriori principles. 相似文献
978.
Paul S. Wesson 《General Relativity and Gravitation》2003,35(1):111-119
Results from 5D induced-matter and membrane theory with null paths are extended to show that a particle obeys the 4D Klein-Gordon equation but with a variable mass. The Dirac equation also follows, but raises concerns about 4D quantization in the two natural 5D gauges, and reopens the question of a Regge-like trajectory for the spin angular momenta and squared masses of gravitationally-dominated systems. 相似文献
979.
In this work the results of the statistical topometric analysis of fracture surfaces of soda-lime-silica glass with and without ionic exchange treatment are reported. In this case, the mechanism of substitution is K+-Na+. atomic force microscopy (AFM) was employed to record the topometric data from the fracture surface. The roughness exponent (ζ) and the correlation length (ξ) were calculated by the variable bandwidth method. The analysis for both glasses (subjected and non-subjected to ionic exchange) for ζ shows a value ∼0.8, this value agrees well with that reported in the literature for rapid crack propagation in a variety of materials. The correlation length shows different values for each condition. These results, along with those of microhardness indentations suggest that the self-affine correlation length is influenced by the complex interactions of the stress field of microcracks with that resulting from the collective behavior of the point defects introduced by the strengthening mechanism of ionic exchange. 相似文献
980.
In this paper, we describe a method for increasing the external efficiency of polymer light‐emitting diodes (LEDs) by coupling out waveguided light with Bragg gratings. We numerically model the waveguide modes in a typical LED structure and demonstrate how optimizing layer thicknesses and reducing waveguide absorption can enhance the grating outcoupling. The gratings were created by a soft‐lithography technique that minimizes changes to the conventional LED structure. Using one‐dimensional and two‐dimensional gratings, we were able to increase the forward‐directed emission by 47 % and 70 %, respectively, and the external quantum efficiency by 15 % and 25 %. 相似文献