全文获取类型
收费全文 | 418697篇 |
免费 | 9534篇 |
国内免费 | 5641篇 |
专业分类
化学 | 185226篇 |
晶体学 | 4880篇 |
力学 | 19200篇 |
综合类 | 148篇 |
数学 | 60239篇 |
物理学 | 107033篇 |
无线电 | 57146篇 |
出版年
2021年 | 3318篇 |
2020年 | 3502篇 |
2019年 | 3500篇 |
2018年 | 14719篇 |
2017年 | 14188篇 |
2016年 | 12260篇 |
2015年 | 5326篇 |
2014年 | 6625篇 |
2013年 | 14351篇 |
2012年 | 15512篇 |
2011年 | 24824篇 |
2010年 | 15917篇 |
2009年 | 16140篇 |
2008年 | 19654篇 |
2007年 | 22045篇 |
2006年 | 12685篇 |
2005年 | 13100篇 |
2004年 | 11793篇 |
2003年 | 11079篇 |
2002年 | 9943篇 |
2001年 | 9809篇 |
2000年 | 7928篇 |
1999年 | 6712篇 |
1998年 | 6102篇 |
1997年 | 5866篇 |
1996年 | 5633篇 |
1995年 | 5267篇 |
1994年 | 4950篇 |
1993年 | 4645篇 |
1992年 | 5064篇 |
1991年 | 4986篇 |
1990年 | 4635篇 |
1989年 | 4523篇 |
1988年 | 4318篇 |
1987年 | 3802篇 |
1986年 | 3664篇 |
1985年 | 4640篇 |
1984年 | 4771篇 |
1983年 | 4035篇 |
1982年 | 4411篇 |
1981年 | 4103篇 |
1980年 | 4098篇 |
1979年 | 4155篇 |
1978年 | 4374篇 |
1977年 | 4200篇 |
1976年 | 4254篇 |
1975年 | 4067篇 |
1974年 | 3966篇 |
1973年 | 4230篇 |
1972年 | 2710篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
181.
Accelerated reliability tests on thin oxide capacitors can be affected by series resistance effects at high stress conditions. The purpose of this work is to point out such problems both with measurements and simulations. It is shown that breakdown electric field is overestimated. Due to the resulting nonuniform stress, charge to breakdown density is underestimated if the test structure layout is not accurately designed. In any case the series resistance effects can have an undesirable impact on the reliability evaluation of thin dielectrics. 相似文献
182.
183.
A simple method of alternatively using high-inversion and moderate-inversion erbium-doped fiber amplifiers (EDFA's) in an optical amplifier chain is presented to mitigate the self-filtering effect and equalize both signal power and signal-to-noise ratio (SNR) of multiple wavelength channels in wavelength-division multiplexing (WDM) transmission systems. The performance of the compensated system with alternatively used high- and moderate-inversion amplifiers is compared with the uncompensated ones where only moderate- or high-inversion amplifiers are employed. The result shows that the compensated system has a flatter gain profile, a lesser signal power spread, and SNR degradation 相似文献
184.
Lourtioz J.-M. Chusseau L. Brun E. Hamaide J.-P. Lesterlin D. Leblond F. 《Electronics letters》1992,28(16):1499-1500
Fourier-transform-limited pulses ( Delta tau . Delta nu approximately=0.35) have been obtained from a microwave-modulated DBR laser at 1.56 mu m. The chirp of the gain-switched pulse is compensated for by applying a fraction of the microwave signal to the phase section, thereby creating an instantaneous frequency shift of opposite sign. The generation of short coherent pulses from such a monolithic source can be of great interest for long haul soliton transmission.<> 相似文献
185.
It is shown that the dephasing which suppresses the weak localization correction to the conductivity has the same physical origin than pair-breaking in superconductors and thus-following de Gennes-may be expressed in terms of the correlation function of the operator for time reversal. 相似文献
186.
Many cognitive tasks that are easy for humans to perform are proving difficult to emulate in computer systems. Combining the disciplines of psychology and engineering may offer a solution to some of these problems. A connectionist or neural network model of face recognition by humans which incorporates aspects of a model proposed by cognitive psychologists is presented. A comparative set of experiments has been performed using this simulation and human subjects for familiar face recognition. By employing the same stimuli for both humans and the computer model, it is possible to advance not only our understanding of human cognition but also to develop improved automated systems for face recognition 相似文献
187.
In this paper we deal with the numerical solution of movingboundary problems of two-phase Stefan type. Based on an implicitdiscretization in time and the use of continuous, piecewiselinear finite elements in the space variables with respect tothe weak formulation of the problem, a globally convergent multi-gridalgorithm is developed. That algorithm strongly relies on thevariational characterization of the fully discretized problemas the unconstrained minimization of a subdifferentiable convexobjective functional. Numerical results indicate a significantimprovement in efficiency compared with previous multi-gridapproaches. 相似文献
188.
SiNx/InP/InGaAs doped channel passivated heterojunction insulated gate field effect transistors (HIGFETs) have been fabricated for the first time using an improved In-S interface control layer (ICL). The insulated gate HIGFETs exhibit very low gate leakage (10 nA@VGS =±5 V) and IDS (sat) of 250 mA/mm. The doped channel improves the DC characteristics and the HIGFETs show transconductance of 140-150 mS/mm (Lg=2 μm), ft of 5-6 GHz (Lg=3 μm), and power gain of 14.2 dB at 3 GHz. The ICL HIGFET technology is promising for high frequency applications 相似文献
189.
Sabate J.A. Jovanovic M.M. Lee F.C. Gean R.T. 《Industrial Electronics, IEEE Transactions on》1995,42(1):63-71
The analysis and design of an LCC resonant inverter for a 20 kHz AC distributed power system are presented. Several resonant converter topologies are assessed to determine their suitability for high efficiency power conversion, under resistive and reactive loads. Two LCC-resonant inverter designs were implemented. One with all switches operating with zero voltage switching (ZVS), and another with two switches operating with ZVS and two switches with zero current switching (ZCS). The experimental results are presented along with a performance comparison of the two versions 相似文献
190.
The pyrolysis of tertiarybutylphosphine (TBP) has been studied in the low pressure conditions used for chemical beam epitaxy
(CBE). The pyrolysis studies were carried out in low pressure reactors of two different configurations, one of which is a
cracker cell designed for use in a CBE system. The reaction products were studied using a quadrupole mass spectrometer. The
products observed are accounted for by a reaction mechanism involving homolysis of the parent TBP molecule to produce PH2 and C4H9 radicals. These undergo subsequent reactions to form the stable products C4H8, PH3 and H2, with smaller amounts of P and P2 being produced. The production of the sub-hydride PH2 using this cracker cell design indicates that the use of partially cracked TBP may be a promising technique for reducing
the amount of carbon incorporated into the growing epitaxial layer. 相似文献