全文获取类型
收费全文 | 67831篇 |
免费 | 12401篇 |
国内免费 | 1531篇 |
专业分类
化学 | 54638篇 |
晶体学 | 563篇 |
力学 | 1132篇 |
综合类 | 1篇 |
数学 | 4068篇 |
物理学 | 7870篇 |
无线电 | 13491篇 |
出版年
2024年 | 49篇 |
2023年 | 337篇 |
2022年 | 445篇 |
2021年 | 934篇 |
2020年 | 2220篇 |
2019年 | 3612篇 |
2018年 | 1888篇 |
2017年 | 1439篇 |
2016年 | 4641篇 |
2015年 | 4561篇 |
2014年 | 4849篇 |
2013年 | 6027篇 |
2012年 | 5103篇 |
2011年 | 4512篇 |
2010年 | 4423篇 |
2009年 | 4428篇 |
2008年 | 4503篇 |
2007年 | 3627篇 |
2006年 | 3305篇 |
2005年 | 3301篇 |
2004年 | 2867篇 |
2003年 | 2585篇 |
2002年 | 3081篇 |
2001年 | 2116篇 |
2000年 | 1992篇 |
1999年 | 908篇 |
1998年 | 455篇 |
1997年 | 404篇 |
1996年 | 410篇 |
1995年 | 286篇 |
1994年 | 270篇 |
1993年 | 281篇 |
1992年 | 220篇 |
1991年 | 198篇 |
1990年 | 184篇 |
1989年 | 143篇 |
1988年 | 118篇 |
1987年 | 96篇 |
1986年 | 61篇 |
1985年 | 97篇 |
1984年 | 68篇 |
1983年 | 63篇 |
1982年 | 76篇 |
1981年 | 82篇 |
1979年 | 59篇 |
1978年 | 53篇 |
1977年 | 46篇 |
1976年 | 49篇 |
1975年 | 43篇 |
1974年 | 45篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
131.
A graph G is N2‐locally connected if for every vertex ν in G, the edges not incident with ν but having at least one end adjacent to ν in G induce a connected graph. In 1990, Ryjá?ek conjectured that every 3‐connected N2‐locally connected claw‐free graph is Hamiltonian. This conjecture is proved in this note. © 2004 Wiley Periodicals, Inc. J Graph Theory 48: 142–146, 2005 相似文献
132.
A new pulse stabilisation scheme for a harmonically frequency modulated (FM) modelocked erbium fibre ring laser using a distributed feedback laser diode (DFBLD) is presented. More than 65 dB supermode noise suppression in the RF spectrum was obtained with the proposed method. Effective reduction of supermode noise can be accomplished as the DFBLD in the fibre laser acts not only as a fast-gain saturated medium but also as an optical bandpass filter for suppressing supermode noise in a harmonically modelocked laser system. 相似文献
133.
Junho Lee Hyungsoo Kim Joungho Kim 《Microwave and Wireless Components Letters, IEEE》2005,15(8):505-507
We experimentally demonstrated the great advantages of a high dielectric constant thin film electromagnetic bandgap (EBG) power distribution network (PDN) for the suppression of power/ground noises and radiated emissions in high-performance multilayer digital printed circuit boards (PCBs). Five-layer test PCBs were fabricated and their scattering parameters measured. The power plane noise and radiated emissions were measured, investigated and related to the PDN impedance. This successfully demonstrated that the bandgap of the EBG was extended more than three times, covering a range of hundreds of MHz using a 1-cm /spl times/ 1-cm EBG cell, the SSN was reduced from 170 mV to 10 mV and the radiated emission was suppressed by 22 dB because of the high dielectric constant thin film EBG power/ground network. 相似文献
134.
Sei‐ichiro Kawase 《International Journal of Satellite Communications and Networking》2005,23(1):67-76
This paper proposes a technique for monitoring the longitudinal motion of geosynchronous satellites. A two‐antenna interferometer is used, with the baseline orientation adjusted to a particular horizontal azimuth. The interferometer then becomes insensitive to the north–south motion of the satellite, and this enables direct measurement of the satellite's longitudinal motion. Longitudes are monitored continuously, with no delay when orbital manoeuvres occur. The proposed technique is particularly useful when two or more satellites are operating in close proximity and their longitudes must be precisely monitored for safety control. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
135.
A W-band divide-by-3 frequency divider with wide bandwidth and low power dissipation is presented using harmonic injection-locking technique. A cascode FET is employed for a self-oscillating second-harmonic mixer which is injection-locked by third-harmonic input to obtain the division order of three. The fabricated frequency divider using 0.1 /spl mu/m GaAs metamorphic HEMT technology shows superior performance such as large bandwidth of 6.1 GHz around 83.1 GHz (7.3%) under small DC power consumption of 12 mW. 相似文献
136.
Ho Nam Kwon Man Geun Kim Il-Han Hwang Young Kab Yun Seon-Ju Kim Jong-Hyun Lee 《Photonics Technology Letters, IEEE》2005,17(4):774-776
We developed a micromachined X-type 2/spl times/4 optical add-drop module (OADM) featuring no difference in propagation length. Four pairs of lensed fibers are aligned in "X" position, and four micromirrors are located between the pairs of optical fibers. The OADM was fabricated utilizing a silicon-on-insulator process. Electrostatic comb actuators can be driven up to 90 /spl mu/m to change the light path within 1 ms. The insertion loss and the on-off ratio were less than 3 and 70 dB, respectively. The loss uniformity in every channel was 1.5 dB. 相似文献
137.
Ching‐Yuan Cheng Kuan‐Jiuh Lin 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(8):m363-m365
In the title compound, catena‐poly[lithium‐μ3‐ethylenediphosphonato], [Li(C2H7O6P2)]n, the supramolecular monoclinic (C2/c) structure consists of one‐dimensional lithium chains [Li⋯Li = 2.7036 (8) Å] that are embedded within ethylenediphosphonate anions linked by strong symmetric hydrogen bonds [O⋯O = 2.473 (3) Å]. The Li atoms and the H atom in the symmetric hydrogen bond reside on twofold rotation axes and there is an inversion center at the mid‐point of the C—C bond of the ethylenediphosphonate ligand. 相似文献
138.
Seungsoo Kim Hyunchol Shin 《Microwave and Wireless Components Letters, IEEE》2008,18(10):701-703
A wideband complementary metal oxide semiconductor (CMOS) semidynamic frequency divide-by-3 covering more than two octave bandwidths is presented. The wideband operation without requiring a quadrature signal source is realized by employing a three-stage RC polyphase filter. The transfer function analysis on Type-II two- and three-stage polyphase filters is performed to provide analytic solutions of the peak phase error and peak attenuation. Implemented in 0.18 mum CMOS, the divide-by-3 operates over the input frequency range between 0.6 and 2.7 GHz while dissipating 15 mA from a 1.8 V supply. 相似文献
139.
The inhomogeneous Dirichlet problems concerning divergence form elliptic equations are studied. Optimal regularity requirements on the coefficients and domains for the W1,p theory, 1 < p < ∞, are obtained. The principal coefficients are supposed to be in the John‐Nirenberg space with small BMO seminorms. The domain is a Reifenberg domain. These conditions for the W1,p theory not only weaken the requirements on the coefficients but also lead to a more general geometric condition on the domains. In fact, these domains might have fractal dimensions. © 2004 Wiley Periodicals, Inc. 相似文献
140.
Steensgaard J. Zhiqing Zhang Wenhuan Yu Sarhegyi A. Lucchese L. Dae-Ik Kim Temes G.C. 《IEEE transactions on circuits and systems. I, Regular papers》2008,55(5):1289-1296
Incremental data converters (IDCs) are useful in instrumentation and measurement applications, where low-frequency analog signals need to be converted into digital form with high accuracy and low power dissipation. They are particularly well suited for applications where a single analog-digital converter is multiplexed between many channels. This paper proposes an exact design methodology for IDCs, which optimizes the signal-to-noise ratio of the converter under practical design constraints. The process also allows the designer to apportion the noise budget in an arbitrary manner between thermal and quantization noise. The design process is illustrated by an example which describes the optimization of a third-order multiplexed IDC. 相似文献