全文获取类型
收费全文 | 377824篇 |
免费 | 3844篇 |
国内免费 | 1131篇 |
专业分类
化学 | 164464篇 |
晶体学 | 5374篇 |
力学 | 15531篇 |
综合类 | 7篇 |
数学 | 38467篇 |
物理学 | 109725篇 |
无线电 | 49231篇 |
出版年
2021年 | 3498篇 |
2020年 | 3775篇 |
2019年 | 4241篇 |
2018年 | 5719篇 |
2017年 | 5870篇 |
2016年 | 8060篇 |
2015年 | 4369篇 |
2014年 | 7657篇 |
2013年 | 17088篇 |
2012年 | 13092篇 |
2011年 | 15665篇 |
2010年 | 11962篇 |
2009年 | 12444篇 |
2008年 | 15114篇 |
2007年 | 15630篇 |
2006年 | 14513篇 |
2005年 | 12757篇 |
2004年 | 11907篇 |
2003年 | 10690篇 |
2002年 | 10593篇 |
2001年 | 12296篇 |
2000年 | 9547篇 |
1999年 | 7635篇 |
1998年 | 6594篇 |
1997年 | 6403篇 |
1996年 | 5922篇 |
1995年 | 5220篇 |
1994年 | 5137篇 |
1993年 | 4979篇 |
1992年 | 5405篇 |
1991年 | 5514篇 |
1990年 | 5232篇 |
1989年 | 4959篇 |
1988年 | 4638篇 |
1987年 | 4424篇 |
1986年 | 4111篇 |
1985年 | 5140篇 |
1984年 | 5181篇 |
1983年 | 4311篇 |
1982年 | 4315篇 |
1981年 | 4019篇 |
1980年 | 3907篇 |
1979年 | 4151篇 |
1978年 | 4122篇 |
1977年 | 4114篇 |
1976年 | 4062篇 |
1975年 | 3878篇 |
1974年 | 3776篇 |
1973年 | 3816篇 |
1972年 | 2730篇 |
排序方式: 共有10000条查询结果,搜索用时 390 毫秒
991.
A. P. S. Selvadurai 《Zeitschrift für Angewandte Mathematik und Physik (ZAMP)》1987,38(5):674-688
The paper examines the problem related to the axisymmetric interaction between an external circular crack and a centrally placed penny-shaped rigid inclusion located in the plane of the crack. The interface between the inclusion and the elastic medium exhibits a Mindlin-type imperfect bi-lateral contact. Analytical results presented in the paper illustrate the manner in which the lateral translational stiffness of the inclusion and the stress intensity factor at the boundary of the external circular crack are influenced by the inclusion/crack radii ratio. 相似文献
992.
Canham R.O. Smith S.L. Tyrrell A.M. 《Vision, Image and Signal Processing, IEE Proceedings -》2005,152(6):741-749
Complex and highly distorted line drawings are produced by subjects attempting the Rey Complex Figure test, a clinical test of neuropsychological assessment. However, the marking scheme conventionally employed can be subjective and unreliable. In this paper, the first stages in automating this scoring system are investigated using a robust technique to locate a reduced set of scoring sections and a knowledge-based system that employs spatial metrics and fuzzy approximation techniques. Testing the technique using clinical data produced encouraging results that support the argument that this is a feasible approach for implementing a fully automated system, and that in its current state, can be immediately applied in a semi-automated system. 相似文献
993.
Based on the discrete-structural theory of thin plates and shells, a calculation model for thin-walled elements consisting
of a number of rigid anisotropic layers is put forward. It is assumed that the transverse shear and compression stresses are
equal on the interfaces. Elastic slippage is allowed over the interfaces between adjacent layers. The solution to the problem
is obtained in a geometrically nonlinear statement with account of the influence of transverse shear and compression strains.
The stress-strain state of circular two-layer transversely isotropic plates, both without defects and with a local area of
adhesion failure at their center, is investigated numerically and experimentally. It is found that the kinematic and static
contact conditions on the interfaces of layered thin-walled structural members greatly affect the magnitude of stresses and
strains. With the use of three variants of calculation models, in the cases of perfect and weakened contact conditions between
layers, the calculation results for circular plates are compared. It is revealed that the variant suggested in this paper
adequately reflects the behavior of layered thin-walled structural elements under large deformations.
__________
Translated from Mekhanika Kompozitnykh Materialov, Vol. 41, No. 6, pp. 761–772, November–December, 2005. 相似文献
994.
Papandriopoulos J. Evans J. Dey S. 《Wireless Communications, IEEE Transactions on》2005,4(6):2705-2715
How can we achieve the conflicting goals of reduced transmission power and increased capacity in a wireless network, without attempting to follow the instantaneous state of a fading channel? In this paper, we address this problem by jointly considering power control and multiuser detection (MUD) with outage-probability constraints in a Rayleigh fast-fading environment. The resulting power-control algorithms (PCAs) utilize the statistics of the channel and operate on a much slower timescale than traditional schemes. We propose an optimal iterative solution that is conceptually simple and finds the minimum sum power of all users while meeting their outage targets. Using a derived bound on outage probability, we introduce a mapping from outage to average signal-to-interference ratio (SIR) constraints. This allows us to propose a suboptimal iterative scheme that is a variation of an existing solution to a joint power control and MUD problem involving SIR constraints. We further use a recent result that transforms complex SIR expressions into a compact and decoupled form, to develop a noniterative and computationally inexpensive PCA for large systems of users. Simulation results are presented showing the closeness of the optimal and mapped schemes, speed of convergence, and performance comparisons. 相似文献
995.
We design and implement an efficient on-line approach, FlowMate, for clustering flows (connections) emanating from a busy server, according to shared bottlenecks. Clusters can be periodically input to load balancing, congestion coordination, aggregation, admission control, or pricing modules. FlowMate uses in-band (passive) end-to-end delay measurements to infer shared bottlenecks. Delay information is piggybacked on feedback from the receivers, or, if impossible, TCP or application round-trip time estimates are used. We simulate FlowMate and examine the effects of network load, traffic burstiness, network buffer sizes, and packet drop policies on clustering correctness, evaluated via a novel accuracy metric. We find that coordinated congestion management techniques are more fair when integrated with FlowMate. We also implement FlowMate in the Linux kernel v2.4.17 and evaluate its performance on the Emulab testbed, using both synthetic and tcplib-generated traffic. Our results demonstrate that clustering of medium to long-lived flows is accurate, even with bursty background traffic. Finally, we validate our results on the Internet Planetlab testbed. 相似文献
996.
Ramirez-Angulo J. Sawant M.S. Lopez-Martin A. Carvajal R.G. 《Electronics letters》2005,41(10):570-572
A high-performance compact current mirror implementation with very low input resistance, very high output resistance, high copying accuracy, low input and output voltage supply requirements and high bandwidth is proposed. The circuit characteristics are validated with simulations in 0.5 /spl mu/m CMOS technology and with experimental results. 相似文献
997.
In this paper, space adaptivity is introduced to control the error in the numerical solution of hyperbolic systems of conservation laws. The reference numerical scheme is a new version of the discontinuous Galerkin method, which uses an implicit diffusive term in the direction of the streamlines, for stability purposes. The decision whether to refine or to unrefine the grid in a certain location is taken according to the magnitude of wavelet coefficients, which are indicators of local smoothness of the numerical solution. Numerical solutions of the nonlinear Euler equations illustrate the efficiency of the method. 相似文献
998.
The ferromagnetic shape memory alloy with nominal composition Ni54Fe19Ga27 is investigated by Ac susceptibility and resistivity measurements. The alloy shows long-range ferromagnetic order below 290 K. The anomaly due to the martensitic transition is observed in the susceptibility and resistivity data in the temperature range around 220 K, which is associated with clear thermal hysteresis. Minor hysteresis loop technique was used to investigate the phase coexistence across the martensitic transition, and our analysis indicate that both martensite and austenite phases mutually coexist in the region of hysteresis. 相似文献
999.
The capability of a modern interpretation system for IR spectra has been tested, in which the process of structural assignment
is modelled by methods of fuzzy logic. Figures of merit were generated for different interpretation algorithms. These figures
of merit permit the validation as well as a comparison of independent interpretation systems. The influence of the sample
preparation practice onto the information content of the resulting spectrum has been evaluated as well. Automatically generated
band tables show only restricted reliability.
Received: 13 February 1997 / Revised: 1 April 1997 / Accepted: 3 April 1997 相似文献
1000.
Let Ψ be any adaptive sampling algorithm that can run in real time on a tapeless multichannel electrocardiogram (ECG) Holter system. Simple methods which can significantly improve Ψ's fidelity are described and their results are compared in this paper. It is shown that by adding some simple tests to Ψ, the signals reconstructed by Ψ can be improved as much as 5.45 dB. It is also shown that under the same data rate, a good data compressor with slowly sampled input ECG is preferable to a bad data compressor with highly sampled input ECG 相似文献