全文获取类型
收费全文 | 24307篇 |
免费 | 2999篇 |
国内免费 | 2594篇 |
专业分类
化学 | 11584篇 |
晶体学 | 306篇 |
力学 | 830篇 |
综合类 | 238篇 |
数学 | 2058篇 |
物理学 | 5898篇 |
无线电 | 8986篇 |
出版年
2024年 | 86篇 |
2023年 | 348篇 |
2022年 | 612篇 |
2021年 | 615篇 |
2020年 | 602篇 |
2019年 | 635篇 |
2018年 | 540篇 |
2017年 | 646篇 |
2016年 | 709篇 |
2015年 | 906篇 |
2014年 | 1133篇 |
2013年 | 1482篇 |
2012年 | 1672篇 |
2011年 | 1803篇 |
2010年 | 1420篇 |
2009年 | 1535篇 |
2008年 | 1671篇 |
2007年 | 1592篇 |
2006年 | 1535篇 |
2005年 | 1318篇 |
2004年 | 1186篇 |
2003年 | 971篇 |
2002年 | 892篇 |
2001年 | 801篇 |
2000年 | 744篇 |
1999年 | 472篇 |
1998年 | 331篇 |
1997年 | 294篇 |
1996年 | 327篇 |
1995年 | 263篇 |
1994年 | 289篇 |
1993年 | 262篇 |
1992年 | 254篇 |
1991年 | 195篇 |
1990年 | 214篇 |
1989年 | 163篇 |
1988年 | 175篇 |
1987年 | 110篇 |
1986年 | 99篇 |
1985年 | 135篇 |
1984年 | 99篇 |
1983年 | 87篇 |
1982年 | 71篇 |
1981年 | 68篇 |
1980年 | 68篇 |
1979年 | 50篇 |
1977年 | 45篇 |
1976年 | 45篇 |
1974年 | 54篇 |
1973年 | 57篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
介绍了一台10mm口径两级双程离轴放大系统,实现了对5mm×5mm口径光束的激光放大,耦合系统采用高功率LDA紧密侧面直接抽运棒状Nd:YLF方式。分析并实验研究了在不同抽运电流、放大脉冲与放大器LDA抽运时刻的不同延时及不同注入能量条件下,放大系统及光束每次放大时放大特性的规律。实验得到:在放大系统5mm×5mm软光阑处注入1.58mJ能量时,放大系统可输出129.2mJ能量,能量提取效率达到19.5%,满足该系统的设计指标。 相似文献
32.
在喷气Z pinch内爆等离子体研究中,雪铲模型是一种常用的、比较简单的物理模型。根据实验中提供的电流波形,负载线质量和初始半径,可以通过雪铲模型来估算内爆到心的时刻。根据一维运动方程和不同构形下的解析解以及部分实验结果相结合,讨论了雪铲模型的适用范围。数值计算的内爆时间和实验(Gamble II, Double EAGLE, BLACKJACK 5)测量值符合得较好。结果表明,雪铲模型在喷气Z pinch实验的负载优化设计研究中是很有参考价值的方法。 相似文献
33.
提出了一种新的高功率宽带放大器,它由速调管和自由电子脉塞组成,用永久周期磁铁聚焦。研究了这种器件的电子束聚焦、群聚和辐射。在速调管提供一阶调制系数为30%,波导管的半径1.51cm,摇摆器的磁场振幅0.16T,周期7.92cm,电子束的电压490kV,电流50A和半径0.368cm的条件下,预估这种器件的辐射波频率为11.4GHz,输出辐射功率为18MW,输出辐射带宽为44%。 相似文献
34.
Let Ψ be any adaptive sampling algorithm that can run in real time on a tapeless multichannel electrocardiogram (ECG) Holter system. Simple methods which can significantly improve Ψ's fidelity are described and their results are compared in this paper. It is shown that by adding some simple tests to Ψ, the signals reconstructed by Ψ can be improved as much as 5.45 dB. It is also shown that under the same data rate, a good data compressor with slowly sampled input ECG is preferable to a bad data compressor with highly sampled input ECG 相似文献
35.
Jenghwa Chang Graber H.L. Ping Chen Koo Aronson R. Barbour S.-L.S. Barbour R.L. 《IEEE transactions on medical imaging》1997,16(1):68-77
Presents a model suitable for computing images of absorption cross sections of thick tissue structures illuminated at near infrared (NIR) wavelengths from tomographic projection data. Image reconstruction is accomplished by solving a system of linear equations derived from transport theory. Reconstruction results using different algebraic solvers are shown for anatomical maps of the breast, derived from magnetic resonance imaging data, containing two simulated pathologies, in which case qualitatively good reconstructions were obtained. Evaluation of magnetic resonance (MR) data to optimize NIR optical tomographic imaging methods and to assess the feasibility of a combined MR-optical measurement scheme is discussed 相似文献
36.
Wen-Chau Liu Lih-Wen Laih Shiou-Ying Cheng Wen-Lung Chang Wei-Chou Wang Jing-Yuh Chen Po-Hung Lin 《Electron Devices, IEEE Transactions on》1998,45(2):373-379
In this paper, a new multiple negative-differential-resistance (MNDR) device based on a metal-insulator-semiconductor-insulator-metal (MISIM)-like structure with step-compositional InxGa1-x As quantum wells has been fabricated and demonstrated. The interesting MNDR phenomena are found in the current-voltage (I-V) characteristics of this device. At room temperature, the triple switching behaviours and quadruple stable operation states are obtained. In addition, the sixfold switching behaviors and a staircase-shaped I-V characteristic are observed at -105°C. A sequential carrier accumulation at InGaAs subwells and the potential lowering process are used to qualitatively explain the interesting MNDR phenomena. From the experimental results, it is shown that the studied device has good potential in multiple-valued logic applications 相似文献
37.
GIS技术与微机制图 总被引:1,自引:0,他引:1
地理信息系统(GIS)随着信息社会的发展愈发显示出强大的生命力。本文简要叙述GIS的重要作用及其发展进程中涌出出来的四个设计模型,总结了运用GIS国产软件平台MAPGIS制作遥感地质构造解译图的工作步骤和技术方法。 相似文献
38.
J.S. Tsay H.W. Chang Y.L. Chiou K.T. Huang W.Y. Chan Y.D. Yao 《Journal of magnetism and magnetic materials》2006
Influences of oxygen exposure on the magnetic properties of Co/Ge(1 1 1) ultrathin films have been investigated by surface magneto-optic Kerr effect technique. As the oxygen exposure increases on Co/Ge(1 1 1) films, their magnetic properties could be modified. As an example for 15 ML Co/Ge(1 1 1) films, the coercivity increases from 730 to 920 Oe and the remanence Kerr intensity is reduced for 500 Langmuir (L) of oxygen exposure. Corresponding compositions analyzed by Auger electron spectroscopy measurement shows that the amount of oxygen on the surface layers increases with increasing the oxygen exposure time. Oxygen distributes on the topmost layers of the film. The adsorbed oxygen influences the electronic density of states of Co and results in the changes of the magnetic properties. Besides, the appearance of O/Co/Ge interface could modify the stress anisotropy, and as a result the coercivity of ultrathin Co/Ge(1 1 1) film is enhanced. 相似文献
39.
40.
Jui-Chieh Chiu Chieh-Pin Chang Mau-Phon Houng Yeong-Her Wang 《Microwave and Wireless Components Letters, IEEE》2006,16(1):19-21
A novel configuration of balanced frequency InGaAs pseudomorphic high electron mobility transistor (PHEMT) monolithic microwave integrated circuit (MMIC) tripler is proposed. A resonant LC filter is used to eliminate the fundamental frequency and a phase delay line is employed to suppress the second harmonic. The separation of the independent phase shifters makes the tripler more compact and flexible. The conversion loss of the tripler operating from 12 to 36GHz is less than 9.4dB at 9-dBm input power. As compared to the third-harmonic frequency, the fundamental frequency is suppressed more than 21.4dB while for the second harmonic is more than 22.3dB at 36GHz. 相似文献