全文获取类型
收费全文 | 38005篇 |
免费 | 6623篇 |
国内免费 | 4309篇 |
专业分类
化学 | 26081篇 |
晶体学 | 519篇 |
力学 | 2467篇 |
综合类 | 285篇 |
数学 | 4424篇 |
物理学 | 15161篇 |
出版年
2024年 | 141篇 |
2023年 | 777篇 |
2022年 | 1284篇 |
2021年 | 1292篇 |
2020年 | 1485篇 |
2019年 | 1413篇 |
2018年 | 1255篇 |
2017年 | 1174篇 |
2016年 | 1773篇 |
2015年 | 1760篇 |
2014年 | 2137篇 |
2013年 | 2692篇 |
2012年 | 3263篇 |
2011年 | 3397篇 |
2010年 | 2322篇 |
2009年 | 2287篇 |
2008年 | 2484篇 |
2007年 | 2234篇 |
2006年 | 2137篇 |
2005年 | 1650篇 |
2004年 | 1328篇 |
2003年 | 1120篇 |
2002年 | 1038篇 |
2001年 | 832篇 |
2000年 | 870篇 |
1999年 | 821篇 |
1998年 | 731篇 |
1997年 | 683篇 |
1996年 | 691篇 |
1995年 | 616篇 |
1994年 | 540篇 |
1993年 | 430篇 |
1992年 | 425篇 |
1991年 | 329篇 |
1990年 | 302篇 |
1989年 | 239篇 |
1988年 | 204篇 |
1987年 | 172篇 |
1986年 | 153篇 |
1985年 | 152篇 |
1984年 | 104篇 |
1983年 | 71篇 |
1982年 | 60篇 |
1981年 | 36篇 |
1980年 | 16篇 |
1979年 | 10篇 |
1977年 | 2篇 |
1971年 | 1篇 |
1959年 | 1篇 |
1957年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
251.
252.
253.
254.
255.
HL-1装置中LHCD和等离子体参数的关系 总被引:1,自引:1,他引:0
本文研究了在HL-1托卡马克的不同放电阶段的低混杂波驱动特性。给出了驱动电流及驱动效率和等离子体参数,如电子平均密度ne、等离子体电流Ip及纵向磁场的关系。也给出和分析了波驱动和入射波功率的关系。在放电平段,对正反向驱动效率进行了研究和比较。 相似文献
256.
Three-dimensional astigmatic resonators, typical examples of which are the resonators bounded by cylindrical-spherical mirrors and cylindrical-cylindrical mirrors oriented at an arbitrary crossed angle, are investigated in detail by using a complex curvature tensor concept and generalized tensor ABCD law. Computerized numerical calculations illustrate some interesting characteristics of these astigmatic resonators. 相似文献
257.
Magnetization reversal processes by a switching field in single-domain nano-sized magnetic particles in the presence of a small transverse non-static bias field are studied. Applying an oscillating bias field instead of a static field, the reversal time becomes much shorter when the switching field is slightly stronger than the effective anisotropy field. A pulsed bias field of a suitably chosen duration in the nanosecond scale is found to induce a rapid switching, even when the switching field is smaller than the anisotropy field. The dependence of the reversal time on the frequency of an oscillating bias field and the duration of a pulsed bias field are studied. The present work thus complement the earlier studies on switching in the presence of a static bias field. 相似文献
258.
J. P. S. Farinha J. M. G. Martinho H. Xu M. A. Winnik R. P. Quirk 《Journal of Polymer Science.Polymer Physics》1994,32(9):1635-1642
Cyclization of a polystyrene chain (Mn = 10,600; Mw/Mn = 1.09) both ends labeled with 4-(1-pyrenyl)butanoamide groups was studied in cyclohexane between 25 and 95°C. The amide groups (peptide bonds) at both ends can form an intrachain hydrogen bond between the amide hydrogen at one chain end and the carbonyl oxygen at the other. The presence of two sets of conformers, random coils, and chains cyclized through hydrogen bonding, complicates the data analysis. The pyrene excimer kinetics of this polymer is well described by a model composed of two monomers (hydrogen bonded and nonbonded chains) and one excimer, in equilibrium. The cyclization rate constant for hydrogen-bonded chains is larger than the one for nonhydrogen-bonded chains. The pyrene excimer binding energy (ca. 1.6 kcal/mol) is lower than the published value for nonhydrogen-bonded chains (~ 9 kcal/mol), suggesting that intrachain hydrogen bonding hinders the stabilization of the excimer. © 1994 John Wiley & Sons, Inc. 相似文献
259.
Yuxin Leng Lihuang Lin Wenyao Wang Yunhua Jiang Bin Tang Zhizhan Xu 《Optics & Laser Technology》2003,35(6):425-429
A birefringent crystal quartz plate of known thickness has been used as a spectral filter for spectral shaping in a Ti:sapphire regenerative amplifier. The spectral profile of the amplified pulse ejected from the regenerative amplifier was observed while adjusting the birefringent crystal plate in the cavity. By altering the gain spectrum, the bandwidth of the regeneratively amplified pulse was increased from 18 to 35 nm by using a 0.34-mm thick birefringent plate. The output pulse spectrum from the regenerative amplifier neared the bandwidth of the seed pulse. As a comparison, we used a coated filter outside the regenerative amplifier cavity, and the bandwidth of the regeneratively amplified pulse was stretched to 28 nm. When the bandwidth was stretched to 35 nm, the pulse was compressed to 35 fs. 相似文献
260.
Low-threshold, high-efficiency, high-repetition-rate optical parametric generator based on periodically poled LiNbO3 下载免费PDF全文
We report a high-repetition-rate optical parametric generator (OPG) with a periodically poled lithium niobate (PPLN) crystal pumped by an acousto-optically Q-switched CW-diode-end-pumped Nd:YVO_4 laser. For the maximum 1064nm pump power of 970mW, the maximum conversion efficiency is 32.9% under the conditions of 250℃, 1064nm pulse repetition rate of 22.6kHz and pulse width of 12ns, and the PPLN OPG threshold in the collinear case is less than 23.7μJ. The output power increases with the increase of the crystal temperature. The 1485-1553nm signal wave and 3383-3754nm idler wave are obtained by changing the temperature and the angle of the PPLN crystal. 相似文献