全文获取类型
收费全文 | 370894篇 |
免费 | 4411篇 |
国内免费 | 1208篇 |
专业分类
化学 | 171969篇 |
晶体学 | 4471篇 |
力学 | 13060篇 |
综合类 | 7篇 |
数学 | 36434篇 |
物理学 | 95042篇 |
无线电 | 55530篇 |
出版年
2020年 | 2171篇 |
2019年 | 2150篇 |
2018年 | 2184篇 |
2017年 | 2078篇 |
2016年 | 4256篇 |
2015年 | 3494篇 |
2014年 | 4884篇 |
2013年 | 14874篇 |
2012年 | 11405篇 |
2011年 | 14207篇 |
2010年 | 9135篇 |
2009年 | 9753篇 |
2008年 | 14388篇 |
2007年 | 15124篇 |
2006年 | 14671篇 |
2005年 | 13728篇 |
2004年 | 12424篇 |
2003年 | 11073篇 |
2002年 | 10915篇 |
2001年 | 12333篇 |
2000年 | 9809篇 |
1999年 | 8021篇 |
1998年 | 6825篇 |
1997年 | 6593篇 |
1996年 | 6579篇 |
1995年 | 6158篇 |
1994年 | 5874篇 |
1993年 | 5683篇 |
1992年 | 6110篇 |
1991年 | 6001篇 |
1990年 | 5533篇 |
1989年 | 5187篇 |
1988年 | 5386篇 |
1987年 | 4427篇 |
1986年 | 4292篇 |
1985年 | 5959篇 |
1984年 | 6030篇 |
1983年 | 4968篇 |
1982年 | 5312篇 |
1981年 | 5289篇 |
1980年 | 5030篇 |
1979年 | 5074篇 |
1978年 | 5076篇 |
1977年 | 5045篇 |
1976年 | 4972篇 |
1975年 | 4857篇 |
1974年 | 4721篇 |
1973年 | 4857篇 |
1972年 | 2823篇 |
1971年 | 2132篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
111.
A short-pulse 1.444-μm laser based on Nd:YAG technology has been demonstrated. The 1.444-μm is eye-safe. With the cavity-dump technique, a pulse of 50 m× and 14 ns was obtained. The beam quality was excellent with an M2 of 1.6 by the use of a telescopic resonator. Silicon-window polarizers were used to suppress the 1.06-μm radiation but showed 1.444-μm absorption as well 相似文献
112.
Polycarpou A.C. Balanis C.A. Aberle J.T. Birtcher C. 《Antennas and Propagation, IEEE Transactions on》1998,46(9):1297-1306
A three-dimensional finite-element method hybridized with the spectral/spatial domain method of moments is presented for the analysis of ferrite-tuned cavity-backed slot antennas. The cavity, which is partially filled with magnetized ferrite layers, is flush mounted on an infinite ground plane with possible dielectric or magnetic overlay. The antenna operates primarily in the ultrahigh-frequency band. The finite-element method is used to solve for the electric-field distribution inside the cavity, whereas the spectral-domain approach is used to solve for the exterior region. An asymptotic extraction of the exponential behavior of the Green's function followed by a spatial evaluation of the resulting integral is used to improve computational speed. Radar cross section, input impedance, return loss, gain, and efficiency of ferrite-tuned cavity-backed slots (CBS) are calculated for various biasing conditions. Numerical results are compared with experimental data 相似文献
113.
Kang W.P. Wisitsora-at A. Davidson J.L. Kerns D.V. 《Electron Device Letters, IEEE》1998,19(10):379-381
A boron-doped diamond field emitter diode with ultralow turn-on voltage and high emission current is reported. The diamond field emitter diode structure with a built-in cap was fabricated using molds and electrostatic bonding techniques. The emission current versus anode voltage of the capped diamond emitter diode with boron doping, sp2 content, and vacuum thermal electric (VTE) treatment shows a very low turn-on voltage of 2 V. A high emission current of 1 μA at an anode voltage of less than 10 V can be obtained from a single diamond tip. The turn-on voltage is significantly lower than comparable silicon field emitters 相似文献
114.
Sampled-data techniques are used to examine the dynamic characteristics and closed-loop performance of the quasi-square-wave boost converter. Direct duty-ratio control and current-mode control are considered. With direct duty-ratio control, the system poles remain well damped under all load conditions, and there is no zero in the control-to-output transfer function. Good closed-loop performance is therefore achieved. Under current-mode control, the requirement for a stabilizing ramp is seen to depend on load conditions, and the achievable voltage control-loop bandwidth is smaller than that using direct duty-ratio control; however, there is a significant reduction in the magnitude of the DC source to output-voltage frequency response 相似文献
115.
116.
The microwave subsidiary absorption threshold in tangentially magnetized yttrium-iron-garnet (YIG) films was measured as a function of the static magnetic field, film thickness and decrease in the 0.3–3 μm range is observed. The effect is explained quantitatively by using modified Damon-Eshbach dispersion relations which take into account exchange interactions.
The effect of temperature on hcrit, the smallest threshold field amplitude, in the 250<T< 450 K temperature range can be expressed by an empirical relationship hcrit Ms(T)-1.6 where Ms is the saturation magnetization. This hcrit vs Ms relationship is similar to that obtained for the magnetization dependence of parallel pump thresholds in Li-Ti ferrite compounds. 相似文献
117.
For multiuser packet communications with unpredictable user demands (e.g., in a local or metropolitan area network), coordination and control of access to the frequency-division multiplexing (FDM) channels are difficult. B. Glance (J. Lightwave Technol., vol.10, pp.1323-1328, Sep 1992) proposed using a simple protection-against-collision (PAC) circuit to solve this media access problem and achieve full optical connectivity. The PAC system has the potential to interconnect hundreds of ports, each transmitting at several gigabits per second. Performance aspects of the PAC optical packet network are discussed here. The delay-throughout performance of this network is analyzed for uniform traffic patterns. The results show that in geographically distributed applications the maximum achievable throughput (normalized to the transmission rate) is typically between 0.4 and 0.5 per channel. In a centralized switch the (normalized) maximum achievable throughput can approach 0.8 per channel 相似文献
118.
A numerical study of confined jets in a cylindrical duct is carried out to examine the performance of two recently proposed turbulence models: an RNG-based K-? model and a realizable Reynolds stress algebraic equation model. The former is of the same form as the standard K-? model but has different model coefficients. The latter uses an explicit quadratic stress-strain relationship to model the turbulent stresses and is capable of ensuring the positivity of each turbulent normal stress. The flow considered involves recirculation with unfixed separation and reatachment points and severe adverse pressure gradients, thereby providing a valuable test of the predictive capability of the models for complex flows. Calculations are performed with a finite volume procedure. Numerical credibility of the solutions is ensured by using second-order-accurate differencing schemes and sufficiently fine grids. Calculations with the standard K-? model are also made for comparison. Detailed comparisons with experiments show that the realizable Reynolds stress algebraic equation model consistently works better than does the standard K-? model in capturing the essential flow features, while the RNG-based K-? model does not seem to give improvements over the standard K-? model under the flow conditions considered. 相似文献
119.
Punt J.B. Sparreboom D. Brouwer F. Prasad R. 《Vehicular Technology, IEEE Transactions on》1998,47(4):1302-1313
Dynamic channel selection (DCS) is an algorithm for flexible resource sharing in mobile radio systems. The digital enhanced cordless telecommunications (DECT) standard implements a version of DCS based on time as well as frequency multiplexing. In this paper, mathematical models are developed to evaluate the probabilities of channel availability, desired carrier power, and the carrier-to-interference ratio (CIR) for a constant traffic load. These results can be used to compute the call setup blocking probability. The models reported in this study are based on a decentralized DCS according to the DECT standard specifications. The results show that blocking due to availability of a channel (resource blocking), not interference blocking, generally is the most important factor in overall call blocking. Furthermore, it is seen that resource blocking is sensitive to the implementation of portable sets. Also, with homogeneous static traffic, this type of DCS ran be as good as fixed channel allocation (FCA) with respect to resource blocking 相似文献
120.
Hartung J. Jacquin A. Pawlyk J. Rosenberg J. Okada H. Crouch P.E. 《Selected Areas in Communications, IEEE Journal on》1998,16(1):42-55
This paper describes a prototype video coding platform meant for the conception and testing of multimedia products such as next-generation videophones. The platform is largely based on ITU-T Recommendation H.263, with a number of additional object-oriented quality enhancement features which make it especially well suited for very low bit-rate coding of “head-and-shoulders” video material typical of real-time multimedia applications, video teleconferencing, and video telephony. These features consist of: (1) segmentation into objects of interest, (2) segmentation-based prefiltering, (3) model-assisted rate control, (4) adaptive vector quantization, and finally (5) segmentation-based postfiltering. In the spirit of Recommendation H.263, these enhancements are modular and can be selectively turned on or off, thereby enabling a wide variety of coding modes 相似文献