全文获取类型
收费全文 | 3945篇 |
免费 | 689篇 |
国内免费 | 340篇 |
专业分类
化学 | 2609篇 |
晶体学 | 54篇 |
力学 | 300篇 |
综合类 | 18篇 |
数学 | 401篇 |
物理学 | 1592篇 |
出版年
2024年 | 10篇 |
2023年 | 107篇 |
2022年 | 118篇 |
2021年 | 161篇 |
2020年 | 163篇 |
2019年 | 157篇 |
2018年 | 148篇 |
2017年 | 120篇 |
2016年 | 211篇 |
2015年 | 189篇 |
2014年 | 219篇 |
2013年 | 295篇 |
2012年 | 373篇 |
2011年 | 372篇 |
2010年 | 251篇 |
2009年 | 221篇 |
2008年 | 240篇 |
2007年 | 226篇 |
2006年 | 216篇 |
2005年 | 178篇 |
2004年 | 133篇 |
2003年 | 105篇 |
2002年 | 82篇 |
2001年 | 73篇 |
2000年 | 86篇 |
1999年 | 71篇 |
1998年 | 64篇 |
1997年 | 52篇 |
1996年 | 59篇 |
1995年 | 52篇 |
1994年 | 33篇 |
1993年 | 40篇 |
1992年 | 40篇 |
1991年 | 23篇 |
1990年 | 21篇 |
1989年 | 15篇 |
1988年 | 11篇 |
1987年 | 7篇 |
1986年 | 6篇 |
1985年 | 12篇 |
1984年 | 1篇 |
1983年 | 6篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有4974条查询结果,搜索用时 31 毫秒
901.
以α-氨基酸与邻苯二胺在微波辐射下反应合成了α-氨基取代苯并咪唑(1~5).1-(1H-苯并[d]咪唑-2-基)乙胺(1)与溴丁烷反应可形成单丁基、二丁基、三丁基取代产物1a~1d,1的氨基经Boc保护,N-烷基化后制备咪唑环上的N-烷基化产物1i~1g.制备的氨基取代咪唑与Ru(II)化合物原位组成催化体系,考察了其在取代苯乙酮的氢转移反应中的催化活性.结果表明RuCl2(PPh3)3与各配体组成的催化剂均有较好的催化活性,含有NH2基团的α-氨基取代苯并咪唑化合物参与的催化体系催化活性最好,TOF(Turnover frequency)可达到40200 h-1. 相似文献
902.
Chen Xu Zhen Li Lu-Meng Duan Zhi-Qiang Wang Wei-Jun Fu Xin-Qi Hao Mao-Ping Song 《Transition Metal Chemistry》2012,37(4):373-378
N-4,6-dimethyl-2-pyrimidinylimidazole 1 and its hydroxyethyl derivative 1-(2-hydroxyethyl)-3-(4,6-dimethyl-2-pyrimidinyl)imidazolium chloride 2 have been synthesized and characterized. The attempted synthesis of bis(N-heterocyclic carbene)palladium complexes via the direct reaction of 2 with Pd(OAc)2 results in the unexpected formation of a bis(N-arylimidazole) palladium complex 3. Additionally, the analogous bis(N-methylimidazole) palladium complex 4 has also been synthesized by the above method. Compounds 1–4 were characterized by elemental analysis, IR, and 1H NMR. Additionally, their crystal structures have been determined by X-ray diffraction. Complexes 3 and 4 were found to be efficient catalysts for the Suzuki reaction. 相似文献
903.
根据云高仪工作原理,从米散射激光雷达方程出发,推导出一种低层大气中气溶胶消光系数边界值的获取方法,并在此基础上利用Fernald方法反演得到了大气消光系数的垂直分布;通过垂直路径上的消光系数反演得到垂直路径的平均能见度,结果表明,该平均能见度更接近垂直方向真实的能见度。用该算法处理了半导体激光云高仪实际测量的数据,并与Vaisala云高仪测量的垂直能见度进行了对比,结果表明:该算法在反演大气垂直能见度时具有较高的准确性和可靠性。 相似文献
904.
905.
906.
We demonstrated continuous-wave (CW) and Q-switched operation of a Tm,Ho:YAP ring laser at 77 K. The maximum CW output power of 2 W at 2130.7 nm was obtained under the incident pump power of 12 W, corresponding to a slope efficiency of 23% and an optical-to-optical efficiency of 16.7%. For the Q-switched regime the maximum output energy of 5 mJ with the pulse width of 160 ns at the repetition rate of 100 Hz was achieved, corresponding to a peak power of 31.25 kW. 相似文献
907.
Breakdown voltage analysis of Al0.25Ga0.75N/GaN high electron mobility transistors with partial silicon doping in the AlGaN layer
下载免费PDF全文
![点击此处可从《中国物理 B》网站下载免费的PDF全文](/ch/ext_images/free.gif)
In this paper,two-dimensional electron gas(2DEG) regions in AlGaN/GaN high electron mobility transistors(HEMTs) are realized by doping partial silicon into the AlGaN layer for the first time.A new electric field peak is introduced along the interface between the AlGaN and GaN buffer by the electric field modulation effect due to partial silicon positive charge.The high electric field near the gate for the complete silicon doping structure is effectively decreased,which makes the surface electric field uniform.The high electric field peak near the drain results from the potential difference between the surface and the depletion regions.Simulated breakdown curves that are the same as the test results are obtained for the first time by introducing an acceptor-like trap into the N-type GaN buffer.The proposed structure with partial silicon doping is better than the structure with complete silicon doping and conventional structures with the electric field plate near the drain.The breakdown voltage is improved from 296 V for the conventional structure to 400 V for the proposed one resulting from the uniform surface electric field. 相似文献
908.
Xueqiang Zhang Yongqing Huang Xiaofeng Duan Xinye Fan Qi Wang Xia Zhang Bing Shen Wei Wang Xiaomin Ren 《Optics Communications》2012,285(21-22):4338-4343
To meet the requirement of the dense wavelength-division-multiplexed (DWDM) system and optoelectronic integrated circuit, we design a Si-Based InGaAs photodetector, which is fabricated by bonding InGaAs/InP photodetector on a Si-Based dielectric film Fabry–Perot filter. The photodetector can exhibit an extremely good flat-top and steep-edge spectral response through designing the structure of Si-Based dielectric film Fabry–Perot filter. The simulation result of phtodetector demonstrates that the spectral response linewidth, ?at-top and steep-edge characteristics of these photodetectors are suitable to be used in 50 GHz, 100 GHz, 200 GHz DWDM system. 相似文献
909.
Potassium lithium tantalate niobate single crystals doped with erbium ions are grown by top-seeded solution growth method. The crystals are characterized by X-ray diffraction and differential thermal analysis. The refractive indices of the crystal are measured using ellipsometry method and fitted by Sellmeier equation. The as-grown crystals are tetragonal phase tungsten bronze-type structure with Curie temperature of 271.3 °C. Characteristic Er3 + absorption bands are observed from 350 to 1100 nm in ultraviolet–visible-near infrared absorption spectra. These crystals emit brightly green and red upconversion fluorescence under 975 nm LD laser excitation, and the steady state upconversion spectra are obtained at room temperature. The red emission intensity increases as the erbium ions concentration increases in crystals. Processes of excited state absorption and energy transfer are responsible for upconversion luminescence. The emission intensities are quadratic dependences on pump power from pump power dependence analyses and deduction of transition rate equation model. 相似文献
910.
Transparent phosphate glass ceramics co-doped with Er3+ and Yb3+ in the system P2O5Li2OCaF2TiO2 were successfully synthesized by melt-quenching and subsequent heating. Formation of the nanocrystals was confirmed by X-ray powder diffraction. Judd–Ofelt analyses of Er3+ ions in the precursor glasses and glass ceramics were performed to evaluate the intensity parameters Ω2,4,6. Under 975 nm excitation, intense upconversion (UC) and infrared emission (1545 nm) were observed in the glass ceramics by efficient energy transfer from Yb3+ to Er3+. The luminescence processes were explained and the emission cross section was calculated by Fuchtbauer–Ladenburg (F–L) formula. The results confirm the potential applications of Er3+/Yb3+ co-doped glass ceramics as laser and fiber amplifier media. 相似文献