全文获取类型
收费全文 | 11982篇 |
免费 | 2645篇 |
国内免费 | 1241篇 |
专业分类
化学 | 2675篇 |
晶体学 | 169篇 |
力学 | 1288篇 |
综合类 | 159篇 |
数学 | 2101篇 |
物理学 | 9476篇 |
出版年
2024年 | 18篇 |
2023年 | 128篇 |
2022年 | 261篇 |
2021年 | 315篇 |
2020年 | 377篇 |
2019年 | 296篇 |
2018年 | 293篇 |
2017年 | 426篇 |
2016年 | 498篇 |
2015年 | 448篇 |
2014年 | 691篇 |
2013年 | 884篇 |
2012年 | 778篇 |
2011年 | 836篇 |
2010年 | 788篇 |
2009年 | 811篇 |
2008年 | 823篇 |
2007年 | 751篇 |
2006年 | 761篇 |
2005年 | 692篇 |
2004年 | 718篇 |
2003年 | 595篇 |
2002年 | 546篇 |
2001年 | 441篇 |
2000年 | 428篇 |
1999年 | 366篇 |
1998年 | 315篇 |
1997年 | 250篇 |
1996年 | 222篇 |
1995年 | 158篇 |
1994年 | 143篇 |
1993年 | 110篇 |
1992年 | 93篇 |
1991年 | 81篇 |
1990年 | 77篇 |
1989年 | 87篇 |
1988年 | 61篇 |
1987年 | 46篇 |
1986年 | 55篇 |
1985年 | 38篇 |
1984年 | 27篇 |
1983年 | 13篇 |
1982年 | 23篇 |
1981年 | 20篇 |
1980年 | 17篇 |
1979年 | 12篇 |
1978年 | 13篇 |
1977年 | 8篇 |
1976年 | 7篇 |
1957年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
991.
为解决光导开关耐受场强的提高问题,研制了2种体结构光导开关,并进行了实验研究。两种开关均由半绝缘GaAs材料制成,一种尺寸为10.0 mm×10.0 mm×0.6 mm,电极位于10.0 mm×10.0 mm面上相对位置,电极直径6 mm;另一种尺寸为15.0 mm×15.0 mm×3.0 mm,8 mm直径电极位于15.0 mm×15.0 mm面上相对位置。测试了第1种开关在不同半高宽脉冲加载电压下的击穿电压,结果表明其最大耐受电压达7.6 kV,击穿电场127 kV/cm。对第2种结构测试了开关在直流加载条件下的暗态伏安特性并进行了触发实验,结果表明在15 kV工作电压下,其放电最大电流超过3.5 kA。 相似文献
992.
建立了微波脉冲在建筑物内传播、反射及透射过程的3维仿真模型,提取了空间电场时域最大值进行统计分析。分析表明:不同入射角窄带调制方波脉冲激励下,场增强区域大小与微波通过窗户和门能直接照射到的区域大小呈正比,窗户的大小对建筑物内空间场强增强区域的大小有显著影响;同时在微波脉冲的传播方向上,窗沿后的区域场强幅值明显减小;脉冲宽度对建筑物内空间场强增强区域的大小及空间场强最大值影响很小;无上升下降沿的窄带调制方波脉冲激励下,空间电场叠加增强效应更强。测量了微波脉冲辐照下,建筑物内空间功率密度分布,验证了仿真结果。 相似文献
993.
994.
995.
996.
针对微槽式芯片冷却器,建立三维仿真模型,分析其稳态工况下的传热情况,获得其温度和热应力分布.分析了五种不同结构形式芯片冷却器工作性能的影响,比较了纯铜和氮化硅复合物两种材质对散热性能的影响.结果表明,铜质冷却器散热性能较好,但内部热应力却高于复合材质冷却器.通过将原冷却通道分割成两个相同的小通道,可提高冷却性能,且应将... 相似文献
997.
998.
Merlet D Béguin L Courtieu J Giraud N 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2011,209(2):315-322
The application of the G-SERFph pulse sequence is presented on enantiomeric mixtures dissolved in a chiral liquid crystal. It aims at editing, within one single 2D spectrum, every proton coupling which is experienced by a given proton site in the molecule, and leads to real phased T-edited spectroscopy (T=J+2D). This NMR experiment is based on the combination of homonuclear semi-selective refocusing techniques with a spatial frequency encoding of the sample. This approach, which consists in handling selectively each coupling in separate cross sections of the sample, is applied to the visualization of enantiomers dissolved in a chiral liquid crystalline phase. Advantages and limits of this methodology are widely discussed. 相似文献
999.
The frequency, field distributions and filling factors of a DR/TE??? probe, consisting of two cylindrical dielectric resonators (DR1 and DR2) in a rectangular TE??? cavity, are simulated and analyzed by finite element methods. The TE(+++) mode formed by the in-phase coupling of the TE??(δ)(DR1), TE??(δ)(DR2) and TE??? basic modes, is the most appropriate mode for X-band EPR experiments. The corresponding simulated B(+++) fields of the TE(+++) mode have significant amplitudes at DR1, DR2 and the cavity's iris resulting in efficient coupling between the DR/TE??? probe and the microwave bridge. At the experimental configuration, B(+++) in the vicinity of DR2 is much larger than that around DR1 indicating that DR1 mainly acts as a frequency tuner. In contrast to a simple microwave shield, the resonant cavity is an essential component of the probe that affects its frequency. The two dielectric resonators are always coupled and this is enhanced by the cavity. When DR1 and DR2 are close to the cavity walls, the TE(+++) frequency and B(+++) distribution are very similar to that of the empty TE??? cavity. When all the experimental details are taken into account, the agreement between the experimental and simulated TE(+++) frequencies is excellent. This confirms that the resonating mode of the spectrometer's DR/TE??? probe is the TE(+++) mode. Additional proof is obtained from B?(x), which is the calculated maximum x component of B(+++). It is predominantly due to DR2 and is approximately 4.4 G. The B?(x) maximum value of the DR/TE??? probe is found to be slightly larger than that for a single resonator in a cavity because DR1 further concentrates the cavity's magnetic field along its x axis. Even though DR1 slightly enhances the performance of the DR/TE??? probe its main benefit is to act as a frequency tuner. A waveguide iris can be used to over-couple the DR/TE??? probe and lower its Q to ≈150. Under these conditions, the probe has a short dead time and a large bandwidth. The DR/TE??? probe's calculated conversion factor is approximately three times that of a regular cavity making it a good candidate for pulsed EPR experiments. 相似文献
1000.
We provide scientific background information and personal accounts relating to our publication of “Microstructural and Physiological Features of Tissues Elucidated by Quantitative-Diffusion-Tensor MRI” in the Journal of Magnetic Resonance B. This paper provided a framework for measuring and mapping intrinsic features of diffusion anisotropy obtained from diffusion tensor MRI (DTI) data. 相似文献