全文获取类型
收费全文 | 748篇 |
免费 | 135篇 |
国内免费 | 107篇 |
专业分类
化学 | 398篇 |
晶体学 | 12篇 |
力学 | 22篇 |
综合类 | 5篇 |
数学 | 34篇 |
物理学 | 214篇 |
无线电 | 305篇 |
出版年
2024年 | 15篇 |
2023年 | 30篇 |
2022年 | 53篇 |
2021年 | 57篇 |
2020年 | 44篇 |
2019年 | 44篇 |
2018年 | 25篇 |
2017年 | 28篇 |
2016年 | 43篇 |
2015年 | 36篇 |
2014年 | 33篇 |
2013年 | 55篇 |
2012年 | 46篇 |
2011年 | 51篇 |
2010年 | 42篇 |
2009年 | 46篇 |
2008年 | 39篇 |
2007年 | 50篇 |
2006年 | 38篇 |
2005年 | 48篇 |
2004年 | 23篇 |
2003年 | 15篇 |
2002年 | 15篇 |
2001年 | 12篇 |
2000年 | 12篇 |
1999年 | 10篇 |
1998年 | 9篇 |
1997年 | 6篇 |
1996年 | 13篇 |
1995年 | 4篇 |
1994年 | 9篇 |
1993年 | 8篇 |
1992年 | 7篇 |
1991年 | 7篇 |
1990年 | 5篇 |
1989年 | 2篇 |
1987年 | 3篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 1篇 |
排序方式: 共有990条查询结果,搜索用时 0 毫秒
1.
2.
3.
The hydrothermal synthesis and magnetic entropy change for the perovskite manganite La0.5Ca0.3Sr0.2MnO3 have been studied. The La0.5Ca0.3Sr0.2MnO3 can be produced as phase-pure, crystalline powders in one step from solutions of metal salts in aqueous potassium hydroxide solution at a temperature of 513 K in 72 h. Scanning electron microscopy shows that the materials are made up of cuboid-shaped particles in typical dimension of 4.0×2.5×1.6 μm. Heat treatment can improve the magnetocaloric effect for the hydrothermal sample. The maximum magnetic entropy change ΔSM for the as-prepared sample is 0.88 J kg−1 K−1 at 315 K for a magnetic field change of 2.0 T. It increases to 1.52 J kg−1 K−1, near its Curie temperature (317 K) by annealing the sample at 1473 K for 6 h. The hydrothermal synthesis method is a feasible route to prepare high-quality perovskite material for magnetic refrigeration application. 相似文献
4.
介绍了信道编码在激光对潜通信系统中的重要作用,介绍了频域编译RS码的基本方法,并给出了脉冲位置调制信道的RS码频域编译码的计算机模拟方法和结果。 相似文献
5.
Ao Sheng Rong Zhong Liang Sun 《Antennas and Propagation, IEEE Transactions on》1991,39(9):1366-1371
The radiation of millimeter waves from the nonradiative dielectric (NRD) leaky wave antennas with the tapered transition and the tuning aperture is investigated. An accurate theory for the leakage constant and the phase constant of the antennas is presented, which is based on the generalized scattering parameters considering the higher-order mode interactions at the discontinuities. The associated transverse equivalent network involves the characterization of the mode couplings at two air-to-dielectric interfaces, the coupled mode analysis of the tapered transition, and the calculation of the radiation fields at the tuning aperture via the extended spectral domain approach. The interesting behavior of the present modified configuration of NRD leaky wave antennas is illustrated by typical examples. The validity of the theory is confirmed by the comparison of the theoretical results with experimental data for the limiting cases 相似文献
6.
Two novel dithienylethenes with ferrocene units were synthesized by Wittig reaction. The structure of 2a has been established by X‐ray diffraction studies. They could reversibly interconvert between two thermally stable forms, an open and a closed one, using light of different wavelengths as triggers. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
7.
8.
9.
10.