全文获取类型
收费全文 | 4567篇 |
免费 | 290篇 |
国内免费 | 21篇 |
专业分类
化学 | 3459篇 |
晶体学 | 60篇 |
力学 | 123篇 |
数学 | 367篇 |
物理学 | 869篇 |
出版年
2024年 | 2篇 |
2023年 | 32篇 |
2022年 | 40篇 |
2021年 | 127篇 |
2020年 | 100篇 |
2019年 | 123篇 |
2018年 | 101篇 |
2017年 | 78篇 |
2016年 | 191篇 |
2015年 | 170篇 |
2014年 | 209篇 |
2013年 | 351篇 |
2012年 | 393篇 |
2011年 | 468篇 |
2010年 | 262篇 |
2009年 | 241篇 |
2008年 | 345篇 |
2007年 | 275篇 |
2006年 | 269篇 |
2005年 | 233篇 |
2004年 | 177篇 |
2003年 | 151篇 |
2002年 | 138篇 |
2001年 | 73篇 |
2000年 | 78篇 |
1999年 | 47篇 |
1998年 | 39篇 |
1997年 | 28篇 |
1996年 | 25篇 |
1995年 | 19篇 |
1994年 | 18篇 |
1993年 | 17篇 |
1992年 | 10篇 |
1991年 | 12篇 |
1990年 | 14篇 |
1989年 | 7篇 |
1988年 | 5篇 |
1987年 | 5篇 |
1986年 | 2篇 |
1984年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
排序方式: 共有4878条查询结果,搜索用时 234 毫秒
981.
Based on time-domain laser modeling, we have systematically evaluated multi-mode excitation and mode competition in directly modulated Fabry–Perot laser diodes biased with different current levels. In our simulation, random noise fluctuations depending on the bias current are included since they play an important role in determining the number of lasing modes. Multi-mode excitation and mode competition occurred at the biased current below the threshold current can be explained with the aid of frequency chirp at turn-on and gain difference in between '0' and '1' bits. The positive frequency chirp (25 GHz for lasers biased below the threshold current and 15 GHz for lasers biased above the threshold current) affects to enhance the probability of multi-mode excitation in low-frequency region during turn-on delay. 相似文献
982.
Brandon Carter Richard A. Battye Jean–Philippe Uzan 《Communications in Mathematical Physics》2003,235(2):289-311
The computation of long range linear self-interaction forces in string and higher dimensional brane models requires the evaluation
of the gradients of regularised values of divergent self-interaction potentials. It is shown that the appropriately regularised
gradient in directions orthogonal to the brane surface will always be obtainable simply by multiplying the regularised potential
components by just half the trace of the second fundamental tensor, except in the hypermembrane case for which the method
fails. Whatever the dimension of the background this result is valid provided the codimension is two (the hyperstring case)
or more, so it can be used for investigating brane-world scenarios with more than one extra space dimension.
Received: 15 April 2002 / Accepted: 23 October 2002 Published online: 25 February 2003
Communicated by H. Nicolai 相似文献
983.
The marker surface method and the adaptive grid refinement technique have been applied to the three‐dimensional (3‐D) finite element analysis of the filling stage in the die‐casting process. Especially, the marker surface plugging technique and the marker surface regeneration technique incorporated in the marker surface method have been proposed for the efficient analysis of 3‐D practical problems. Through the marker surface plugging technique, new parts of marker surface are effective lycreated in order to eliminate the gaps between the parts of marker surface or between the edge of marker surface and cavity wall. By using the marker surface regeneration technique, the marker surface including a great number of marker elements is recreated on the basis of its original shape in order to decrease the number of marker elements and computational time. A3‐D example used as the benchmark test and a typical industrial problem of the die‐casting process have been analysed. The numerical results have been in good agreement with the experimental results and the efficiency of the adaptive grid refinement technique has been verified. It has been shown that the proposed techniques incorporated in the marker surface method and the adaptive grid refinement technique can be effectively applied to general industrial problems. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
984.
Correction of a coherent image during KrF excimer laser ablation using a mask projection 总被引:1,自引:1,他引:0
Dong Sig Shin Jae Hoon Lee Jeong Suh To Hoon Kim 《Optics and Lasers in Engineering》2006,44(6):615-622
Using masks for laser ablation has proven useful in the fabrication of prototypes for the manufacturing of micro-fluidic devices. In this work, an excimer laser was used to engrave microscopic channels on the surface of polyethylene terephthalate (PET), which showed a high absorption ratio for an excimer laser beam with a wavelength of 248 nm. When 50 μm wide rectangular microscopic channels were made using a 500×500 μm square mask and a magnification ratio of 1/10, ditch-shaped defects were found at both corners. The calculation of the laser beam intensity showed that a coherent image in the PET specimen caused the defects. An analysis based on the Fourier diffraction theory enabled the prediction of a coherent shape at the image plane, as well as a diffracted beam between the mask and the image plane. The analysis also showed that the diameter of the aperture was a predominant factor toward the elimination of ditch-shaped defects in the rectangular microscopic channels on the PET produced by an excimer laser ablation. 相似文献
985.
T.G. Kim J.H. Lee J.H. Song K.H. Chae S.W. Shin H.M. Hwang J. Lee K. Jeong C.N. Whang J.-S. Lee K.-B. Lee 《Journal of magnetism and magnetic materials》2006
The irradiation effect of 1 MeV C+ on the interface and magnetic anisotropy of epitaxial Cu/Ni system with a perpendicular magnetic anisotropy was investigated by using magneto-optical Kerr effects, grazing incident diffraction and X-ray reflectivity. The magnetic easy-axis was altered from the direction along the surface normal to in-plane and the strain in the Ni layer was relaxed after ion irradiation. Though the interface between the top Cu layer and the Ni layer becomes rough, the contrast of electron densities of Cu and Ni layer increases and the grain-growth occurs during ion irradiation. These phenomena arise from thermo-chemical driving force, i.e. heat of formation, which may be a crucial factor in determining the interface shape in the case of indirect energy transfer mechanism. Therefore, the change of the magnetic anisotropy of the Ni/Cu system after ion irradiation is not due to the formation of the intermixed layer at the interface. The ion irradiation effects on the grain-growth and enhancement of the electronic contrast between Ni and Cu are explained by the interfacial atomic movement caused by thermo-chemical driving force. 相似文献
986.
Hyun Sook Kim Yu-Mi Song Jin Seok Choi Jung Woon Yang Hogyu Han 《Tetrahedron》2004,60(52):12051-12057
The silica gel-supported bis-cinchona alkaloid 1a was prepared and found to be an efficient heterogeneous chiral organocatalyst with high catalytic activities, enantioselectivities (up to 92% ee), and recyclability for the asymmetric desymmetrization of meso-cyclic anhydrides with alcoholysis. 相似文献
987.
Jeong Won Kang Ki Ryang Byun Ju Yul Lee Sun Cheol Kong Young Wan Choi Ho Jung Hwang 《Physica E: Low-dimensional Systems and Nanostructures》2004,24(3-4):349-354
We investigated potassium ion transport in the (5, 5) carbon nanotube by using classical molecular dynamics simulations under applying external force fields. This can be applied to nanoscale ionic-field-effect devices. As the applying external force field increases, the potassium ions rapidly go through the nanochannel. Under the low external force fields, the thermal fluctuation of the nanochannel affected on the tunneling of the potassium ions, whereas the effects of the thermal fluctuation were negligible under the high external force fields. 相似文献
988.
The diffusion of copper nanocluster in carbon nanotube was investigated using a classical molecular dynamics simulation and three empirical potential functions. The results indicated a growth mechanism of the copper-filled ultra-thin carbon nanotubes: the copper nanoclusters inserted into carbon nanotubes swiftly migrate along the tube axis, and then the copper nanowires grow in the ultra-thin carbon nanotubes. Periodic energy barriers in the carbon nanotubes induced the directional movement of copper nanoclusters in the carbon nanotubes. The diffusion speeds of copper nanocluster in the carbon nanotube showed the Arrherius relation. 相似文献
989.
Mohammad Rezaul Karim Jeong Hyun Yeum 《Journal of Polymer Science.Polymer Physics》2008,46(21):2279-2285
Conducting polypyrrole (PPy)‐montmorillonite (MMT) clay nanocomposites have been synthesized by the in situ intercalative polymerization method. The PPy‐MMT nanocomposites are characterized by field‐emission scanning electron microscopy (FE‐SEM), transmission electron microscopy (TEM), X‐ray diffraction (XRD), ultraviolet–visible (UV–vis) spectroscopy, thermogravimetric analysis (TGA), and Fourier‐transform infrared (FTIR) spectroscopy. XRD patterns show that after polymerization by the in situ intercalative method with ammonium persulfate and 1 M HCl, an increase in the basal spacing from 1.2 to 1.9 nm was observed, signifying that PPy is synthesized between the interlayer spaces of MMT. TEM and SEM micrographs suggest that the coexistence of intercalated MMT layers with the PPy macromolecules. FTIR reveals that there might be possible interfacial interactions present between the MMT clay and PPy matrix. The study also shows that the introduction of MMT clay results in thermal stability improvement of the PPy. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2279–2285, 2008 相似文献
990.