全文获取类型
收费全文 | 236316篇 |
免费 | 2532篇 |
国内免费 | 653篇 |
专业分类
化学 | 110828篇 |
晶体学 | 3652篇 |
力学 | 8012篇 |
综合类 | 11篇 |
数学 | 19217篇 |
物理学 | 63449篇 |
无线电 | 34332篇 |
出版年
2020年 | 1954篇 |
2019年 | 2116篇 |
2018年 | 2489篇 |
2017年 | 2572篇 |
2016年 | 3855篇 |
2015年 | 2343篇 |
2014年 | 3887篇 |
2013年 | 9823篇 |
2012年 | 6914篇 |
2011年 | 8479篇 |
2010年 | 6269篇 |
2009年 | 6577篇 |
2008年 | 8290篇 |
2007年 | 8522篇 |
2006年 | 8200篇 |
2005年 | 7542篇 |
2004年 | 6940篇 |
2003年 | 6517篇 |
2002年 | 6191篇 |
2001年 | 7540篇 |
2000年 | 5921篇 |
1999年 | 4711篇 |
1998年 | 3891篇 |
1997年 | 3881篇 |
1996年 | 3710篇 |
1995年 | 3490篇 |
1994年 | 3380篇 |
1993年 | 3220篇 |
1992年 | 3898篇 |
1991年 | 3791篇 |
1990年 | 3668篇 |
1989年 | 3538篇 |
1988年 | 3463篇 |
1987年 | 3158篇 |
1986年 | 2992篇 |
1985年 | 3695篇 |
1984年 | 3719篇 |
1983年 | 2951篇 |
1982年 | 3021篇 |
1981年 | 3032篇 |
1980年 | 2827篇 |
1979年 | 3081篇 |
1978年 | 3079篇 |
1977年 | 3147篇 |
1976年 | 2992篇 |
1975年 | 2686篇 |
1974年 | 2625篇 |
1973年 | 2584篇 |
1972年 | 1807篇 |
1968年 | 1780篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
We consider a space-time coded (STC) orthogonal frequency-division multiplexing (OFDM) system with multiple transmitter and receiver antennas over correlated frequency- and time-selective fading channels. It is shown that the product of the time-selectivity order and the frequency-selectivity order is a key parameter to characterize the outage capacity of the correlated fading channel. It is also observed that STCs with large effective lengths and ideal built-in interleavers are more effective in exploiting the natural diversity in multiple-antenna correlated fading channels. We then propose a low-density parity-check (LDPC)-code-based STC-OFDM system. Compared with the conventional space-time trellis code (STTC), the LDPC-based STC can significantly improve the system performance by exploiting both the spatial diversity and the selective-fading diversity in wireless channels. Compared with the previously proposed turbo-code-based STC scheme, LDPC-based STC exhibits lower receiver complexity and more flexible scalability. We also consider receiver design for LDPC-based STC-OFDM systems in unknown fast fading channels and propose a novel turbo receiver employing a maximum a posteriori expectation-maximization (MAP-EM) demodulator and a soft LDPC decoder, which can significantly reduce the error floor in fast fading channels with a modest computational complexity. With such a turbo receiver, the proposed LDPC-based STC-OFDM system is a promising solution to highly efficient data transmission over selective-fading mobile wireless channels 相似文献
142.
摩托罗拉微控制器(MCU)具有编程语言简单、外围设备齐全、存储器模型用户友好、选择广及供应多、性能价格比高等优点,被设计者评为最容易使用的产品之一。在全球顶级的原始设备制造厂商(OEM)的无数嵌入式系统和用户最终产品中都可找到摩托罗拉的MCU,包括键盘、传呼机、电子游戏机、洗衣机、安全系统及汽车等。 相似文献
143.
I. L. Davies K. E. Markides M. L. Lee M. W. Raynor K. D. Bartle 《Journal of separation science》1989,12(4):193-207
Although coupled liquid chromatographygas chromatography (LC-GC) was first demonstrated ten years ago, only in the last few years has there been a sudden surge of interest in the technique. Approximately 70% of the total number of LC-GC applications have been published in the last two years (1987–88) alone. This review categorizes LC-GC publications into four main application areas: fossil fuels, foods, environmental samples, biologiical/pharmaceutical samples, and miscellaneous samples. Multidimensional separations carried out using other coupled-column chromatographic techniques (such as supercritical fluid chromatography (SFC) with GC, and on-line trace enrichment-GC) have also been included in this review. 相似文献
144.
145.
T. Kumita Y. Kamiya M. Babzien I. Ben-Zvi K. Kusche I. V. Pavlishin I. V. Pogorelsky D. P. Siddons V. Yakimenko T. Hirose T. Omori J. Urakawa K. Yokoya D. Cline F. Zhou 《Laser Physics》2006,16(2):267-271
Thomson scattering of high-power laser and electron beams is a good test of electrodynamics in the high-field region. We demonstrated production of high-intensity X-rays in the head-on collision of a CO2 laser and 60-MeV electron beams at Brookhaven National Laboratory, Accelerator Test Facility. The energy of an X-ray photon was limited at 6.5 keV in the linear (lowest order) Thomson scattering, but the nonlinear (higher order) process produces higher energy X-rays. We measured the angular distribution of the high-energy X-rays and confirmed that it agrees with theoretical predictions. 相似文献
146.
Debendra K. Mohapatra Dhananjoy Mondal Mukund S. Chorghade Mukund K. Gurjar 《Tetrahedron letters》2006,47(52):9215-9219
The non-proteinogenic amino acid 3-hydroxy-4-methylproline (HMP) is an active constituent of some potent antimicrobials including echinocandins, nostopeptins, pneumocandins, sporiofungin and mulundocandins. A synthesis has been achieved in 10 steps with 29% overall yield; the Evans’ aldol reaction using Crimmins’ modified method was pivotal to the success of the strategy. 相似文献
147.
S. N. Solov’ev K. A. Minasyan A. A. Korunov 《Russian Journal of Physical Chemistry A, Focus on Chemistry》2006,80(4):663-664
The enthalpies of reactions of La2CoO4(cr) and CoCl2(cr) with hydrochloric acid were measured with an isothermal-jacket calorimeter. The results obtained and the available literature data were used to calculate the standard enthalpy of formation of La2CoO4(cr) at 298.15 K, Δf H o = ?2179 ± 7 kJ/mol. 相似文献
148.
An algorithm is proposed which allows one to construct higher symmetries of arbitrary order for some special classes of hyperbolic
systems possessing integrals. The Pohlmeyer-Lund-Regge system and the open two-dimensional Toda lattices are shown to belong
to the class of systems where our algorithm is applicable.
__________
Translated from Fundamental’naya i Prikladnaya Matematika (Fundamental and Applied Mathematics), Vol. 10, No. 1, Geometry
of Integrable Models, 2004. 相似文献
149.
Thin films of, N-N′diphenyl 1-4phenylene-diamineane are prepared using vacuum sublimation technique. The electrical conductivity from room temperature down to 127 K is studied. It is found that the conduction of charge carriers obeys T−1/2 dependence on temperature. The average hopping distance, hopping energy, density of states and their variation due to post-deposition heat treatment are studied. Schottky diodes are fabricated with gold as ohmic contact and aluminium as Schottky contact. From the observed current voltage characteristics the saturation current density, diode ideality factor and the barrier height are determined. Their variation with air annealing is also investigated. 相似文献
150.
A two-dimensional, nonlinear, compressible, diabatic, nonhydrostatic photochemical-dynamical gravity wave model has been advanced. The model includes diabetic process produced by photochemistry and the effect of gravity wave on atmospheric chemical species. In the horizontal direction, the pseudospectral method is used. The finite difference approximations are used in vertical direction z and time t. The FICE method is used to solve the model. The model results on small amplitude fluctuation are very close to those of linear theory, which demonstrates the correctness of the model.
相似文献