全文获取类型
收费全文 | 67282篇 |
免费 | 12400篇 |
国内免费 | 1529篇 |
专业分类
化学 | 54284篇 |
晶体学 | 551篇 |
力学 | 1141篇 |
数学 | 3974篇 |
物理学 | 7845篇 |
无线电 | 13416篇 |
出版年
2023年 | 327篇 |
2022年 | 347篇 |
2021年 | 945篇 |
2020年 | 2224篇 |
2019年 | 3612篇 |
2018年 | 1892篇 |
2017年 | 1447篇 |
2016年 | 4643篇 |
2015年 | 4569篇 |
2014年 | 4862篇 |
2013年 | 6016篇 |
2012年 | 5093篇 |
2011年 | 4482篇 |
2010年 | 4393篇 |
2009年 | 4434篇 |
2008年 | 4463篇 |
2007年 | 3610篇 |
2006年 | 3297篇 |
2005年 | 3261篇 |
2004年 | 2861篇 |
2003年 | 2584篇 |
2002年 | 3082篇 |
2001年 | 2108篇 |
2000年 | 1961篇 |
1999年 | 905篇 |
1998年 | 444篇 |
1997年 | 396篇 |
1996年 | 393篇 |
1995年 | 272篇 |
1994年 | 261篇 |
1993年 | 272篇 |
1992年 | 215篇 |
1991年 | 186篇 |
1990年 | 170篇 |
1989年 | 135篇 |
1988年 | 104篇 |
1987年 | 92篇 |
1986年 | 61篇 |
1985年 | 91篇 |
1984年 | 64篇 |
1983年 | 58篇 |
1982年 | 71篇 |
1981年 | 72篇 |
1980年 | 39篇 |
1979年 | 50篇 |
1978年 | 49篇 |
1976年 | 42篇 |
1975年 | 39篇 |
1974年 | 38篇 |
1973年 | 36篇 |
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
91.
H.S. Seo Y.G. Choi B.J. Park D.H. Cho K.H. Kim 《Photonics Technology Letters, IEEE》2003,15(9):1198-1200
A flat signal gain over in the entire C- and L-bands by erbium (Er) ions' radiative transition and stimulated Raman scattering in an Er-doped germano-silica fiber can be obtained if proper values of the concentration of Er and background loss in a fiber core are obtained during the fiber fabrication process. The optimized conditions for the flat C- and L-band gain are analyzed as functions of Er concentrations. Even for a low-gain value provided by a germano-silica core fiber with a low Er concentration and an optimum fiber length, a relatively low pump is required to obtain the flat gain band. 相似文献
92.
M. Ganschow C. Hellriegel E. Kneuper M. Wark C. Thiel G. Schulz‐Ekloff C. Bruchle D. Whrle 《Advanced functional materials》2004,14(3):269-276
Dye‐loaded AlPO4‐5 single crystals were prepared by microwave‐assisted hydrothermal synthesis from a batch, containing a mixture of three chromophores (Coumarin 40, Rhodamine BE50, and Oxazine 1) differing in their absorption range, molecular dimensions, and solubilities. Confocal fluorescence images reveal a spatial separation effect of the dye molecules, where the slimmer, more‐soluble dye molecule (Coumarin 40) is uniformly distributed in the body of the single crystal, and the bulky and/or less‐soluble ones (Rhodamine BE50, Oxazine 1) are situated in distinct domains. Visible spectra show good panchromatic absorption of visible light. Fluorescence lifetime measurements indicate the presence of an energy transfer cascade of the entirely fixed dye molecules from Coumarin 40 to Oxazine 1. The transfer mechanism is predominantly radiative. 相似文献
93.
This paper presents a low‐cost RF parameter estimation technique using a new RF built‐in self‐test (BIST) circuit and efficient DC measurement for 4.5 to 5.5 GHz low noise amplifiers (LNAs). The BIST circuit measures gain, noise figure, input impedance, and input return loss for an LNA. The BIST circuit is designed using 0.18 μm SiGe technology. The test technique utilizes input impedance matching and output DC voltage measurements. The technique is simple and inexpensive. 相似文献
94.
Bong Keun Kim Wan Kyun Chung 《Industrial Electronics, IEEE Transactions on》2003,50(6):1207-1216
Disturbance-observer (DOB)-based controller design is one of the most popular methods in the field of motion control. In this paper, the generalized disturbance compensation framework, named the robust internal-loop compensator (RIC) is introduced and an advanced design method of a DOB is proposed based on the RIC. The mixed sensitivity optimization problem, which is the main issue of DOB design, is also solved through the parametrization of the DOB in the RIC framework. Differently from conventional methods, the Q-filter is separated from the mixed sensitivity optimization problem and a systematic design law for the DOB is proposed. This guarantees the robustness and optimality of the DOB and enables the design for unstable plants. 相似文献
95.
96.
An antenna array for wideband operation (up to 70%) is presented. The structure has low windloading area and consists of parallel printed circuit boards (PCB) with microstrip dipoles, feed network and metal fences placed between the PCBs. The low profile, low weight antenna array forms the main beam and three difference patterns for sidelobe cancellation. Experimental results are compared with calculations for both microstrip dipole and array 相似文献
97.
Kang-Ngee Chia Hea Joung Kim Lansing S. Mangione-Smith W.H. Villasensor J. 《Very Large Scale Integration (VLSI) Systems, IEEE Transactions on》1998,6(3):364-371
Under the Mojave configurable computing project, we have developed a system for achieving high performance on an automatic target recognition (ATR) application through the use of configurable computing technology. The ATR system studied here involves real-time image acquisition from a synthetic aperture radar (SAR). SAR images exhibit statistical properties which can be used to improve system performance. In this paper, the Mojave configurable computing system uses field programmable gate arrays (FPGA's) to implement highly specialized circuits while retaining the flexibility of programmable components. A controller sequences through a set of specialized circuits in response to real-time events. Computer-aided design (CAD) tools have been developed to support the automatic generation of these specialized circuits. The resulting configurable computing system achieves a significant performance advantage over the existing solution, which is based on application specific integrated circuit (ASIC) technology 相似文献
98.
Uhm H.S. Lee P.H. Kim Y.I. Kim J.H. Chang H.Y. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1995,23(4):628-635
A theory is developed for the density profile of low temperature plasmas confined by applied magnetic field and an experiment of the electron-cyclotron-resonance (ECR) plasma is conducted to compare the theoretical prediction and experimental measurements. Due to a large electron mobility along the magnetic field, electrons move quickly out of the system, leaving ions behind and building a space charge potential, which leads to the ambipolar diffusion of ions. In a steady-state condition, the plasma generation by ionization of neutral molecules is in balance with plasma loss due to the diffusion, leading to the electron temperature equation, which is expressed in terms of the plasma size, chamber pressure, and the ionization energy and cross section of neutrals. The power balance condition leads to the plasma density equation, which is also expressed in terms of the electron temperature, the input microwave power and the chamber pressure. It is shown that the plasma density increases, reaches its peak and decreases, as the chamber pressure increases from a small value (0.1 mTorr). These simple expressions of electron temperature and density provide a scaling law of ECR plasma in terms of system parameters. After carrying out an experimental observation, it is concluded that the theoretical predictions of the electron temperature and plasma density agree remarkably well with experimental data 相似文献
99.
Zhengmao Ye Campbell J.C. Zhonghui Chen Eui-Tae Kim Madhukar A. 《Quantum Electronics, IEEE Journal of》2002,38(9):1234-1237
An InAs/AlGaAs quantum-dot infrared photodetector based on bound-to-bound intraband transitions in undoped InAs quantum dots is reported. AlGaAs blocking layers were employed to achieve low dark current. The photoresponse peaked at 6.2 /spl mu/m. At 77 K and -0.7 V bias, the responsivity was 14 mA/W and the detectivity, D*, was 10/sup 10/ cm/spl middot/Hz/sup 1/2//W. 相似文献
100.
Taesung Kim Sung Ho Wang Beomsup Kim 《Electronics letters》2002,38(17):950-951
A delay-locked loop (DLL) architecture capable of incorporating fast locking and low jitter features simultaneously is reported. A test chip was fabricated in a 0.6 μm CMOS process to prove its functionality. The proposed DLL can align the internal clock to the external reference clock within two cycles and maintain its locking state with the aid of feedback operation 相似文献