全文获取类型
收费全文 | 92054篇 |
免费 | 18357篇 |
国内免费 | 6749篇 |
专业分类
化学 | 66483篇 |
晶体学 | 753篇 |
力学 | 2645篇 |
综合类 | 357篇 |
数学 | 6810篇 |
物理学 | 16657篇 |
无线电 | 23455篇 |
出版年
2024年 | 199篇 |
2023年 | 1163篇 |
2022年 | 1547篇 |
2021年 | 2067篇 |
2020年 | 3381篇 |
2019年 | 4641篇 |
2018年 | 2844篇 |
2017年 | 2484篇 |
2016年 | 5757篇 |
2015年 | 6042篇 |
2014年 | 6410篇 |
2013年 | 7732篇 |
2012年 | 7277篇 |
2011年 | 6791篇 |
2010年 | 6141篇 |
2009年 | 6220篇 |
2008年 | 5952篇 |
2007年 | 5103篇 |
2006年 | 4521篇 |
2005年 | 4280篇 |
2004年 | 3479篇 |
2003年 | 3084篇 |
2002年 | 3761篇 |
2001年 | 2836篇 |
2000年 | 2577篇 |
1999年 | 1581篇 |
1998年 | 1138篇 |
1997年 | 1023篇 |
1996年 | 995篇 |
1995年 | 884篇 |
1994年 | 803篇 |
1993年 | 704篇 |
1992年 | 612篇 |
1991年 | 507篇 |
1990年 | 450篇 |
1989年 | 335篇 |
1988年 | 274篇 |
1987年 | 198篇 |
1986年 | 208篇 |
1985年 | 211篇 |
1984年 | 139篇 |
1983年 | 102篇 |
1982年 | 99篇 |
1981年 | 73篇 |
1980年 | 70篇 |
1979年 | 67篇 |
1978年 | 53篇 |
1976年 | 47篇 |
1974年 | 51篇 |
1973年 | 48篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
Min-Shiang Hwang Chih-Wei Lin Cheng-Chi Lee 《Electronics letters》2002,38(23):1429-1431
An authenticated multiple-key agreement protocol is proposed. The protocol is not only secure against the unknown-key attack but also more efficient than other protocols. 相似文献
132.
无线局域网安全问题研究 总被引:10,自引:0,他引:10
为了提供相当于有线局域网的数据安全,IEEE802.11定义了有线等价保密(WEP)协议。然而,最近的研究发现WEP存在严重的缺陷。介绍了无线局域网存在的安全隐患,分析了WEP的结构以及WEP协议的缺陷以及可能遭受的攻击,并探讨了改进方案。 相似文献
133.
Using AuGeNiCr multilayered metals as the wafer bonding medium, long-wavelength GaInAsP/InP vertical cavity surface emitting lasers employing Al-oxide/Si as the upper and lower distributed Bragg reflectors were fabricated on Si substrate with the bonding interface formed outside the vertical cavity surface emitting laser cavity. Laser emission at 1.545 μm was measured under pulsed operations near room temperature. The low-temperature metallic bonding process demonstrates a great potential in device fabrication 相似文献
134.
135.
Coplanar waveguide-fed uniplanar bow-tie antenna 总被引:3,自引:0,他引:3
Yu-De Lin Syh-Nan Tsai 《Antennas and Propagation, IEEE Transactions on》1997,45(2):305-306
The design of coplanar waveguide (CPW)-fed bow-tie antenna for the 2.4-GHz ISM band is described. A coplanar waveguide-to-coplanar strips (CPW-to-CPS) balun is used to obtain the balanced feed line for the printed bow-tie. An analysis method based on the mixed potential integral equation method is used to characterize the input characteristics of the bow-tie antenna. The numerical results obtained are in good agreement with the experimental data. Through experiments with bow-tie antennas of various extended angles, the bow-tie antenna with a 90° extended angle exhibits the widest bandwidth in the desired frequency band which has a bandwidth of 17% for a VSWR <1.5:1 相似文献
136.
Color image retrieval based on hidden Markov models 总被引:1,自引:0,他引:1
In this correspondence, a new approach to retrieving images from a color image database is proposed. Each image in the database is represented by a two-dimensional pseudo-hidden Markov model (2-D PHMM), which characterizes the chromatic and spatial information about the image. In addition, a flexible pictorial querying method is used, by which users can paint the rough content of the desired images in a query picture. Image matching is achieved by comparing the query picture with each 2-D PHMM in the database. Experimental results show that the proposed approach is indeed effective. 相似文献
137.
Control of peak sidelobe level in adaptive arrays 总被引:1,自引:0,他引:1
Renbiao Wu Zheng Bao Yuanliang Ma 《Antennas and Propagation, IEEE Transactions on》1996,44(10):1341-1347
Adaptive beamforming techniques are now widely used to reject interference (jammer/clutter) signals in radar, communication, and sonar applications. In adaptive arrays using the sample matrix inversion (SMI) algorithm, inadequate estimation of the covariance matrix results in adaptive antenna patterns with high sidelobes and distorted mainbeams. In this paper, a method is proposed to precisely control the peak (rather than average) sidelobe level of adaptive array patterns. The proposed method is also generalized to adaptive array antennas with moderate bandwidth and large random amplitude and phase errors. Theoretical analysis and simulation results are provided to illustrate the performance of the method proposed 相似文献
138.
Horng-Dar Lin 《Circuits and Devices Magazine, IEEE》1995,11(4):25-31
Identifies the basic technological components of multimedia communications. Among these technologies, multimedia processing and transmission already benefit heavily from VLSI advances. in fact, these two technologies could not have matured without special-purpose VLSI chips. We have examined basic processing required in these technologies and some VLSI architectures. The focus has been on standard-compliant VLSI chips, because the eventual goal of communications is to allow everyone to reach others without constraints. There are still a number of evolving standards, which means that we might witness yet another wave of VLSI chips for multimedia communications 相似文献
139.
Yoshiaki Murata Cheng-Huang Lin Totaro Imasaka 《Fresenius' Journal of Analytical Chemistry》1993,346(6-9):543-544
Summary A supersonic jet spectrum of 9,10-dichloroanthracene is measured by stimulated-emission-pumping fluorescence dip spectrometry and conventional fluorescence spectrometry. The performance obtained is compared for these spectrometric methods, providing same information concerned with the energy level of the ground state. The former is more preferential for measurement of a high-resolution spectrum, since the spectral resolution is determined by the linewidth of the dumping laser. On the other hand, the latter is more preferential for measurement with better sensitivity at the expense of the spectral resolution, since the fluorescence throughput can be improved by increasing the slit width of the monochromator.Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday 相似文献
140.
L-Quebrachitol (1) isolated from the root of Elaeagnus formosana Nakai (Elaeagnaceae) has not been reported before for any species of the genus of Elaeasnus. Its structure was established from spectral data and was completely characterized by single-crystal X-ray analysis. L-Quebrachitol crystallizes in the monoclinic system, space group P21, with cell parameters are a = 6.702(4), b = 7.207(4), c = 8.758(5) Å, β = 90.24(5)° and Z = 2. 相似文献