全文获取类型
收费全文 | 5480篇 |
免费 | 141篇 |
国内免费 | 20篇 |
专业分类
化学 | 3768篇 |
晶体学 | 69篇 |
力学 | 76篇 |
数学 | 233篇 |
物理学 | 1495篇 |
出版年
2021年 | 30篇 |
2020年 | 56篇 |
2019年 | 61篇 |
2018年 | 50篇 |
2017年 | 35篇 |
2016年 | 86篇 |
2015年 | 88篇 |
2014年 | 116篇 |
2013年 | 259篇 |
2012年 | 253篇 |
2011年 | 271篇 |
2010年 | 151篇 |
2009年 | 161篇 |
2008年 | 285篇 |
2007年 | 239篇 |
2006年 | 265篇 |
2005年 | 250篇 |
2004年 | 249篇 |
2003年 | 194篇 |
2002年 | 200篇 |
2001年 | 151篇 |
2000年 | 167篇 |
1999年 | 90篇 |
1998年 | 66篇 |
1997年 | 67篇 |
1996年 | 66篇 |
1995年 | 44篇 |
1994年 | 70篇 |
1993年 | 91篇 |
1992年 | 96篇 |
1991年 | 77篇 |
1990年 | 64篇 |
1989年 | 55篇 |
1988年 | 72篇 |
1987年 | 73篇 |
1986年 | 75篇 |
1985年 | 112篇 |
1984年 | 97篇 |
1983年 | 42篇 |
1982年 | 72篇 |
1981年 | 76篇 |
1980年 | 72篇 |
1979年 | 64篇 |
1978年 | 53篇 |
1977年 | 65篇 |
1976年 | 55篇 |
1975年 | 43篇 |
1974年 | 52篇 |
1973年 | 31篇 |
1967年 | 23篇 |
排序方式: 共有5641条查询结果,搜索用时 31 毫秒
11.
Improved holographic particle sizing by using absolute values of the wavelet transform 总被引:3,自引:0,他引:3
A new method for sizing particle from in-line particle holograms by using absolute values of the wavelet transform is proposed in order to improve accuracy in measurements. The proposed method provides direct calculation of the particle size by using spatial frequency information of a chirp signal at minima position of an envelope function. Simulation and experimental results are presented. 相似文献
12.
13.
14.
15.
Lasing conditions for He I in a cold recombining helium plasma are numerically discussed including optical thicknesses. A collisional radiative model is used to calculate population densities. We discuss the effect of self absorptions by the background residual gas on the lasing conditions quantitatively. The optical thickness is essential in the calculation and affects the population inversions between level pairs of the singlet state drastically, while those of the triplet state are not affected very much. In the optically thin plasma, the laser lower levels of the singlet state depopulate mainly through radiative transitions. On the other hand, they depopulate mainly through electron collisional deexcitations in the optically thick plasma. Results of the calculations are verified by the spectroscopic observations of an arc-heated magnetically trapped expanding plasma jet generator which we developed. The requirement for the background pressure in which the lasing is possible is discussed quantitatively. 相似文献
16.
Spacetime correlation functions of the time-varying speckle intensity have been evaluated for the case where speckle patterns are produced in the Fresnel diffraction region by light propagating through a series of moving random-phase screens. The correlation functions are given by a set of recursion formulae which are useful for numerical computations. Three statistical quantities relating to the motion of speckle patterns are derived to examine the relationship between the velocity distribution of the phase screens and the temporal behaviour of resultant speckle patterns. It is found that the velocity differences between the phase screens have a strong effect on both the decorrelation of speckle patterns and the fluctuating speed of the speckle intensity. 相似文献
17.
Takaaki Kakitsuka Shinji Matsuo Seok–Hwan Jeong Toru Segawa Hiroshi Okamoto Yoshihiro Kawaguchi Yasuhiro Kondo Yuzo Yoshikuni Hiroyuki Suzuki 《Optical and Quantum Electronics》2006,38(12-14):1053-1060
We theoretically investigated a digitally tunable laser with a chirped ladder filter and a ring resonator to obtain a wide wavelength tuning range covering the whole C- or L- band. The clear relation between the tuning range and laser structure, especially the ladder filter, is described analytically. The introduction of a chirped structure into a ladder filter is effective in achieving both wide tunability and a stable lasing mode. A numerical simulation based on multimode rate equations shows that a tuning range of over 40 nm and a mode suppression ratio over 40 dB can be achieved by introducing a chirped ladder filter. 相似文献
18.
In this article a stability result for the Falk model system is proven. The Falk model system describes the martensitic phase transitions in shape memory alloys. In our setting, the steady state is a nonlocal elliptic problem. We show the dynamical stability for the linearized stable critical point of the corresponding functional. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
19.
20.
Corrosion-related defects of pure iron were investigated by measuring Doppler broadening energy spectra (DBES) of positron annihilation and positron annihilation lifetime (PAL). Defect profiles of the S-parameter from DBES as a function of positron incident energy up to 30 keV (i.e. ∼1 μm depth) were analyzed. The DBES data show that S-parameter increases as a function of positron incident energy (mean depth) after corrosion, and the increase in the S-parameter is larger near the surface than in the bulk due to corrosion. Furthermore, information on defect size from PAL data as a function of positron incident energy up to 10 keV (i.e. ∼0.2 μm depth) was analyzed. In the two-state trapping model, the lifetime τ2 = 500 ps is ascribed to annihilation of positrons in voids with a size of the order of nanometer. τ1, which decreases with depth from the surface to the bulk, is ascribed to the annihilation of positrons in dislocations and three-dimensional vacancy clusters. The corroded samples show a significant increase in τ1 and the intensity I2, and near the surface the corroded iron introduces both voids and large-size three-dimensional vacancy clusters. The size of vacancy clusters decreases with depth. 相似文献