全文获取类型
收费全文 | 1296篇 |
免费 | 201篇 |
国内免费 | 247篇 |
专业分类
化学 | 1103篇 |
晶体学 | 22篇 |
力学 | 27篇 |
综合类 | 14篇 |
数学 | 255篇 |
物理学 | 323篇 |
出版年
2024年 | 2篇 |
2023年 | 12篇 |
2022年 | 22篇 |
2021年 | 30篇 |
2020年 | 44篇 |
2019年 | 56篇 |
2018年 | 34篇 |
2017年 | 29篇 |
2016年 | 59篇 |
2015年 | 53篇 |
2014年 | 67篇 |
2013年 | 84篇 |
2012年 | 80篇 |
2011年 | 90篇 |
2010年 | 98篇 |
2009年 | 80篇 |
2008年 | 90篇 |
2007年 | 95篇 |
2006年 | 68篇 |
2005年 | 53篇 |
2004年 | 62篇 |
2003年 | 70篇 |
2002年 | 141篇 |
2001年 | 118篇 |
2000年 | 71篇 |
1999年 | 35篇 |
1998年 | 18篇 |
1997年 | 14篇 |
1996年 | 18篇 |
1995年 | 9篇 |
1994年 | 8篇 |
1993年 | 7篇 |
1992年 | 3篇 |
1991年 | 8篇 |
1990年 | 3篇 |
1988年 | 4篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1981年 | 1篇 |
1936年 | 2篇 |
排序方式: 共有1744条查询结果,搜索用时 31 毫秒
991.
As one of the most adopted sequential data assimilation methods in many areas, especially those involving complex nonlinear dynamics, the ensemble Kalman filter (EnKF) has been under extensive investigation regarding its properties and efficiency. Compared to other variants of the Kalman filter (KF), EnKF is straightforward to implement, as it employs random ensembles to represent solution states. This, however, introduces sampling errors that affect the accuracy of EnKF in a negative manner. Though sampling errors can be easily reduced by using a large number of samples, in practice this is undesirable as each ensemble member is a solution of the system of state equations and can be time consuming to compute for large-scale problems. In this paper we present an efficient EnKF implementation via generalized polynomial chaos (gPC) expansion. The key ingredients of the proposed approach involve (1) solving the system of stochastic state equations via the gPC methodology to gain efficiency; and (2) sampling the gPC approximation of the stochastic solution with an arbitrarily large number of samples, at virtually no additional computational cost, to drastically reduce the sampling errors. The resulting algorithm thus achieves a high accuracy at reduced computational cost, compared to the classical implementations of EnKF. Numerical examples are provided to verify the convergence property and accuracy improvement of the new algorithm. We also prove that for linear systems with Gaussian noise, the first-order gPC Kalman filter method is equivalent to the exact Kalman filter. 相似文献
992.
Effect of surface tension on a liquid-jet produced by the collapse of a laser-induced bubble against a rigid boundary 总被引:1,自引:0,他引:1
The effect of surface tension on the behavior of a liquid-jet is investigated experimentally by means of a fiber-coupled optical beam deflection (OBD) technique. It is found that a target under water is impacted in turn by a laser-plasma ablation force and by a high-speed liquid-jet impulse induced by bubble collapse in the vicinity of a rigid boundary. The liquid-jet impact is found to be the main damage mechanism in cavitation erosion. Furthermore, the liquid-jet increases monotonously with surface tension, so cavitation erosion rises sharply with increasing surface tension. Surface tension also reduces bubble collapse duration. From the experimental results and the modified Rayleigh theory, the maximum bubble radius is obtained and it is found to reduce with increasing surface tension. 相似文献
993.
Z. Zhang R. Zhang Z.L. Xie B. Liu M. Li D.Y. Fu H.N. Fang X.Q. Xiu H. Lu Y.D. Zheng Y.H. Chen C.G. Tang Z.G. Wang 《Solid State Communications》2009,149(25-26):1004-1007
A sizable spin-dependent photocurrent related to the interband transition in InN films is observed. The surface charge accumulation layer is suggested to be the origin of the circular photogalvanic current, which is consistent with the result of uniaxial strain experiments and the comparison of front and back incidence. The homogeneous photocurrent demonstrates the existence of spin splitting in the InN surface layer, and the structure inversion asymmetry (SIA)-dominant mechanism indicates a great possibility for the manipulation of spin splitting, which would undoubtedly benefit further research and applications of spintronics. 相似文献
994.
995.
996.
Z. Zhang R. Zhang B. Liu Z.L. Xie X.Q. Xiu P. Han H. Lu Y.D. Zheng Y.H. Chen C.G. Tang Z.G. Wang 《Solid State Communications》2008,145(4):159-162
The circular photogalvanic effect (CPGE) is observed in InN at inter-band excitation. The function of the CPGE induced current on laser helicity is experimentally demonstrated and illustrated with the microscopic model. A spin-dependent current obtained in InN is one order larger than in the AlGaN/GaN heterostructures at inter-band excitation. The dependence of CPGE current amplitude on light power and incident angle can be well evaluated with phenomenological theory. This sizeable spin-dependent current not only provides an opportunity to realize spin polarized current at room temperature, but also can be utilized as a reliable tool of spin splitting investigation in semiconductors. 相似文献
997.
激光二极管侧抽运双棒串接准连续Nd:YAG高功率绿光激光器 总被引:2,自引:0,他引:2
研制一台激光二极管(LD)侧面抽运双棒串接准连续Nd:YAG折叠腔高功率绿光激光器,理论分析了热致双折射效应对系统的影响,并对用石英旋转片补偿前后的情况进行模拟对比.在考虑了补偿后的情况下设计了热稳定谐振腔.实验中采用两个串接的由30个20 W的LD阵列侧面抽运的Nd: YAG棒和Ⅱ类临界相位匹配HGTR-KTP晶体.在抽运电流均为21.6 A,重复频率为27.2 kHz时.获得了最大平均输出功率为164 W.脉冲宽度为130 ns的532 nm绿光输出,光-光转换效率为13.7%,测得光束质量因子为M2x=9.52,M2x=9.86,不稳定度为2.3%.实验结果显示.经补偿后的激光系统能在宽的稳区范围内稳定运转. 相似文献
998.
Simultaneous Generation of Multi-Wavelength Laser from Nd:YAG Crystals in a Cruciform Cavity
下载免费PDF全文
![点击此处可从《中国物理快报》网站下载免费的PDF全文](/ch/ext_images/free.gif)
A cruciform cavity is presented for multi-wavelength laser generation. On the basis of considering the optimal power ratio and good spatial overlap of the two fundamental beams, the maximum output power of 589 nm laser reaches 3.5 W when the pumping power of Nd:YAG A and Nd:YAG B are 311.5 W and 261.8 W, respectively. At the same time, the other wavelength lasers are also obtained with the output power distribution of 2.5 W at 66Onto, 15 W at 532nm, lOOmW at 1319nm and 240mW at 1064nm. The corresponding beam quality factors are M^2 x = 4.93, M^2 y = 5.01 at 589nm, M^2z = 4.51, M^2 y = 4.85 at 660hm, and M^2 x = 4.12, M^2 y = 3.96 at 532nm, respectively. The instabilities of the three visible lights are measured, which are also less than 2% within three hours. 相似文献
999.
对氢原子的径向Schrodinger方程,完全用因式分解方法,导出关于主量子数n和角量子数l的另外两类升降算子.用它们构造出第三类、第四类升降算子.并通过计算给出了四类升降算子的归一化系数. 相似文献
1000.
建筑节能镀膜玻璃热传递系数的研究 总被引:4,自引:0,他引:4
建筑节能镀膜玻璃的热传递系数k是反映节能薄膜性能的重要指标,本文提出其热传递系数的计算和测量方法,并介绍部分测量结果,研究结果表明节能薄膜确能较大地降建筑物的能耗。 相似文献