全文获取类型
收费全文 | 30232篇 |
免费 | 2092篇 |
国内免费 | 907篇 |
专业分类
化学 | 14818篇 |
晶体学 | 211篇 |
力学 | 790篇 |
综合类 | 51篇 |
数学 | 2302篇 |
物理学 | 5936篇 |
无线电 | 9123篇 |
出版年
2024年 | 60篇 |
2023年 | 422篇 |
2022年 | 533篇 |
2021年 | 729篇 |
2020年 | 654篇 |
2019年 | 706篇 |
2018年 | 576篇 |
2017年 | 598篇 |
2016年 | 947篇 |
2015年 | 925篇 |
2014年 | 1131篇 |
2013年 | 1728篇 |
2012年 | 1996篇 |
2011年 | 2205篇 |
2010年 | 1446篇 |
2009年 | 1519篇 |
2008年 | 1830篇 |
2007年 | 1780篇 |
2006年 | 1613篇 |
2005年 | 1449篇 |
2004年 | 1309篇 |
2003年 | 1162篇 |
2002年 | 1095篇 |
2001年 | 853篇 |
2000年 | 735篇 |
1999年 | 522篇 |
1998年 | 414篇 |
1997年 | 388篇 |
1996年 | 422篇 |
1995年 | 338篇 |
1994年 | 339篇 |
1993年 | 293篇 |
1992年 | 284篇 |
1991年 | 229篇 |
1990年 | 240篇 |
1989年 | 183篇 |
1988年 | 180篇 |
1987年 | 108篇 |
1986年 | 100篇 |
1985年 | 147篇 |
1984年 | 112篇 |
1983年 | 102篇 |
1982年 | 87篇 |
1981年 | 75篇 |
1980年 | 78篇 |
1979年 | 62篇 |
1977年 | 58篇 |
1976年 | 60篇 |
1974年 | 56篇 |
1973年 | 66篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Novel high voltage pumping circuits for low supply voltages are proposed. Utilising a small pumping circuit and the new substrate-connected techniques can enhance charge transfer efficiency and eliminate the body effect at low supply voltages. Furthermore, a diode-connected transistor technique can improve the reverse charge sharing phenomenon when the output has a load current. With this technique high boosted voltages can be obtained at low supply voltages 相似文献
72.
73.
This paper reports a new, highly integrated modular design approach for pulsewidth-modulation AC-AC converters based on a modular phase-bank structure. Novel high-power 3-in-1 integrated bi-directional power modules (IBPM) rated at 1200-V AC and 150 A and the 3-to-1 phase bank circuits have been successfully developed, fabricated and tested. This enables the modularity design of the multiphase converter systems and reduces the critical parasitic inductance. A theoretical analysis of the IBPM's silicon utilization for this new breed of direct power converters has been outlined. A lab prototype at medium power level has been successfully designed, implemented, and tested with good results. A nearly 460-VRMS out voltage at a 1:0.955 voltage transfer ratio, which, perhaps, is the highest performance reported so far in the literature, has been achieved by the novel two-side modulation control system. The converter-fed AC motor system is able to operate over the 0-240 Hz range with inherent regenerative capability and four-quadrant operation. Potential industrial applications are also briefly highlighted in this paper 相似文献
74.
Park Y.C. Lee K.Y. Youn D.H. Kim N.H. Kim W.K. Park S.H. 《IEEE transactions on bio-medical engineering》1992,39(8):868-871
An efficient algorithm detecting the presence of a fetal QRS complex is presented. The proposed fetal QRS detection method computes the averaged magnitude of the difference between the fetal ECG signal and the reference signal to detect the fetal QRS event. The detected fetal QRS complexes are exponentially averaged to generate the template signal which can track the slowly varying shape of the fetal ECG signal. As an effort to obtain improved detection performances, two approaches of normalizing the fetal ECG signal and the template are considered. 相似文献
75.
Theoretical studies point to significant improvements in the performance of semiconductor laser amplifiers by injecting carriers with pulsed electric currents of sub-nanosecond duration. A pulsed Fabry-Perot amplifier (FPA) is most sensitive to input lightwave at the instant the carrier density is crossing the critical region, and gives a sharply pulsed sampling effect on the input lightwave signal. Compared with a FPA operating at subcritical electron density, the pulsed amplifier gives much higher gain, peak power, and bandwidth. In fact, pulsed operation of a FPA is also expected to give significantly higher gain and about the same peak output power as a traveling wave amplifier. Pulsed operation also improves the performance of a traveling wave amplifier by attenuating its internally reflected waves 相似文献
76.
77.
Hyunkook Park Machiko Tsukiji Kazuaki Wagatsuma SangChun Lee 《Analytical sciences》2007,23(9):1133-1136
The emission characteristics of several Cu lines emitted from a Ne-Ar mixed gas glow discharge plasma were investigated. The addition of small amounts of Ar to a Ne plasma increases the sputtering rate of a Cu sample because Ar ions, which work as the impinging ions for cathode sputtering, are predominantly produced through Penning ionization collisions between Ne metastables and Ar atoms. Ar addition also elevates the number density of electrons in the plasma. These changes occurring in the Ne-Ar mixed gas plasma result in enhanced emission intensities of the Cu lines. The Cu II 270.10-nm and the Cu II 224.70-nm lines yield different intensity dependence on the Ar partial pressure added. This phenomenon is because these Cu II lines are excited principally through different charge transfer processes: collisions with Ne ions for the Cu II 270.10-nm line and collisions with Ar ions for the Cu II 224.70-nm line. The shape of sputtered craters in the Ne-Ar glow discharge plasma was measured. The depth resolution was improved when Ar was added to a Ne plasma because the crater bottoms were flatter with larger Ar partial pressures. 相似文献
78.
Highly ordered composite nanowires with multilayer Ni/Cu and NiFe/Cu have been fabricated by pulsed electrodeposition into
nanoporous alumina membrane. The diameter of wires can be easily varied by pore size of alumina, ranging from 30 to 100 nm.
The applied potential and the duration of each potential square pulse determine the thickness of the metal layers. The nanowires
have been characterized by transmission electron microscopy (TEM), magnetic force microscopy (MFM), and vibrating sample magnetometer
(VSM) measurements. The MFM images indicate that every ferromagnetic layer separated by Cu layer was present as single isolated
domain-like magnet. This technique has potential use in the measurement and application of magnetic nanodevices. 相似文献
79.
The electromagnetic properties of electrically thick rectangular microstrip antennas were investigated experimentally. Antennas were fabricated with different patch sizes and with electrical thicknesses ranging from 0.03 to 0.23 wavelengths in the dielectric substrate. The resonant frequencies were measured and compared to existing formulas. The bandwidth was calculated as a function of electrical thickness and the antenna radiation patterns were measured. 相似文献
80.
A flow visualization of the two-dimensional rigid fling-clap motions of the flat-plate wing is performed to get the knowledge
of fling-clapping mechanism that might be employed by insects during flight. In this numerical visualization, the time-dependent
Navier-Stokes equations are solved for two types of wing motion; ‘fling followed by clap and pause motion’ and ‘cyclic fling-clapping
motion’. The result is observed regarding the main flow features such as the sequential development of the two families of
separation vortex pairs and their movement. For the ‘fling followed by clap and pause motion’, a strong separation vortex
pair of counterrotation develops in the opening between the wings in the fling phase and they then move out from the opening
in the following clap phase. For ‘the cyclic fling-clapping motion’, the separation vortex pair developed in the outside space
in the clap phase move into the opening in the following fling phase. The separation vortex pair in the opening developed
in the fling phase of the cyclic motion is observed to be stronger than those of the ‘fling followed by clap and pause motion’.
Regarding the strong fling separation vortex and the weak clap separation vortex above it in the opening, the flow pattern
of the fling phase of the cyclic fling and clap motion is different to that of the fling phase of the first cycle. The flow
pattern of the third cycle of the cyclic fling-clapping motion is observed to be almost same as that of the second cycle.
Therefore, a periodicity of the flow pattern is established after the second cycle. 相似文献