全文获取类型
收费全文 | 380879篇 |
免费 | 3620篇 |
国内免费 | 1078篇 |
专业分类
化学 | 201132篇 |
晶体学 | 6438篇 |
力学 | 16573篇 |
综合类 | 15篇 |
数学 | 41365篇 |
物理学 | 120054篇 |
出版年
2021年 | 2969篇 |
2020年 | 3368篇 |
2019年 | 3777篇 |
2018年 | 4697篇 |
2017年 | 4674篇 |
2016年 | 6892篇 |
2015年 | 4326篇 |
2014年 | 6834篇 |
2013年 | 17249篇 |
2012年 | 12980篇 |
2011年 | 15740篇 |
2010年 | 11008篇 |
2009年 | 10908篇 |
2008年 | 14108篇 |
2007年 | 14108篇 |
2006年 | 13289篇 |
2005年 | 12015篇 |
2004年 | 11108篇 |
2003年 | 9974篇 |
2002年 | 9640篇 |
2001年 | 11418篇 |
2000年 | 8765篇 |
1999年 | 6776篇 |
1998年 | 5346篇 |
1997年 | 5365篇 |
1996年 | 5051篇 |
1995年 | 4694篇 |
1994年 | 4481篇 |
1993年 | 4224篇 |
1992年 | 5139篇 |
1991年 | 5006篇 |
1990年 | 4923篇 |
1989年 | 4737篇 |
1988年 | 4808篇 |
1987年 | 4843篇 |
1986年 | 4539篇 |
1985年 | 5912篇 |
1984年 | 6123篇 |
1983年 | 4921篇 |
1982年 | 5135篇 |
1981年 | 5189篇 |
1980年 | 4871篇 |
1979年 | 5237篇 |
1978年 | 5347篇 |
1977年 | 5443篇 |
1976年 | 5313篇 |
1975年 | 4905篇 |
1974年 | 4836篇 |
1973年 | 4942篇 |
1972年 | 3263篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
161.
Songxin Tan Ram M. Narayanan Sunil K. Shetty 《International Journal of Infrared and Millimeter Waves》2005,26(8):1175-1194
There is growing interest in the use of lidar for remote sensing of vegetation owing to the emergence of reliable and rugged lasers and highly sensitive detectors. Lidar remote sensing has a distinct advantage over conventional techniques in vegetation remote sensing due to its capability for three-dimensional characterization of vegetative targets. The Multiwavelength Airborne Polarimetric Lidar (MAPL) system was developed primarily for vegetation remote sensing applications from an airborne platform of up to 1,000 -m altitude. The lidar system has full waveform capture and polarimetric measurement capability at two wavelengths in the near-infrared (1064 nm) and the green (532 nm) spectral regions. This study presents preliminary ground-based lidar reflectance measurements on a variety of deciduous and coniferous trees under fully foliated conditions with a view towards tree species discrimination. Variations in the reflectance characteristics of selected deciduous trees under unfoliated and fully foliated conditions were also investigated. Our study reveals distinct differences in the reflectance characteristics of various trees. 相似文献
162.
The anisotropic g-values of defects in hydrogenated microcrystalline silicon prepared by hot-wire chemical vapour deposition have been measured as a function of crystalline volume fraction at room temperature. The defect has been identified as a silicon-dangling bond existing on the surface of crystalline grain. Their anisotropic g-values are discussed in the light of theoretical calculations by Ishii et al. and Ishii and Shimizu. The defect density is also discussed as a function of crystalline volume fraction. 相似文献
163.
Electrohydrodynamic instability in homeotropically oriented nematic samples of 4'-n-octyl-4-cyano-biphenyl and 4'n-alkyloxy-4-cyanobiphenyl, (n = 8.9) have been studied in an a.c. electric field. The domain patterns during the instability in these compounds in a very low frequency a.c. field are very similar to those in a d.c. field. The domain patterns observed at higher frequencies have been identified as 'maltese crosses' or 'crossed isogyres'. The electro-convective 'isotropic' flows near the electrode play an important role in the observed instability. 相似文献
164.
Erich Miersemann Hans D. Mittelmann W. Trnig 《Mathematical Methods in the Applied Sciences》1989,11(1):95-104
In this paper we generalize recent theoretical results on the local continuation of parameter-dependent non-linear variational inequalities. The variational inequalities are rather general and describe, for example, the buckling of beams, plates or shells subject to obstacles. Under a technical hypothesis that is satisfied by the simply supported beam, we obtain the existence of a continuation of both the solution and the eigenvalue with respect to a local parameter. A numerical continuation method is presented that easily overcomes turning points. Numerical results are presented for the non-linear beam. 相似文献
165.
166.
Using the DFT/B3LYP method with a 6-31G(d) basis set, the structure, normal vibration frequencies, and the absolute band intensities in the IR spectra of the chlorin molecule and its four symmetric isotopomers have been calculated. Scaling of the force field by the Pulay method in independent and natural coordinates has been carried out. A method for obtaining effective force fields without using experimental data on the fundamental vibration frequencies is proposed. By comparing the vibration modes and constructing special matrices, complete assignment of the fundamental frequencies of porphin and chlorin has been carried out. It has been shown that the majority of porphin macroring vibrations upon pyrrolenine ring hydrogenation are frequency-characteristic and only 12 vibrations change considerably. A frequency correlation with regard for the mode transition between chlorin and all its isotopomers under consideration has been established. Comparative analysis of the force fields of porphin and chlorin in dependent natural coordinates has revealed the unique nonlocal character of the change in force constants of the macroring upon hydrogenation of one pyrrolenine ring. Modeling of the IR spectra of chlorin and its isotopomers has been performed. Assignment and interpretation of the normal vibrations of the molecules under consideration have been carried out. 相似文献
167.
Quasi-wavelets (QWs) are a representation of turbulence consisting of self-similar, eddy-like structures with random orientations and positions in space. They are used in this paper to calculate the scattering, due to turbulent velocity fluctuations, of sound behind noise barriers as a function of the size and spatial location of the eddies. The sound scattering cross-section for QWs of an individual size class (eddy size) is derived and shown to reproduce results for the von Kármán spectrum when the scattered energies from a continuous distribution of QW sizes are combined. A Bragg resonance condition is derived for the eddy size that scatters most strongly for a given acoustic wavenumber and scattering angle. Results for scattering over barriers show that, for typical barrier conditions, most of the scattered energy originates from eddies in the size range of approximately one-half to twice the size of the eddies responsible for maximum scattering. The results also suggest that scattering over the barrier due to eddies with a line of sight to both the source and receiver is generally significant only for frequencies above several kilohertz, for sources and receivers no more than a few meters below the top of the barrier, and for very turbulent atmospheric conditions. 相似文献
168.
In different regimes of exposure of metals to pulsed laser radiation, we show the role played by the liquid-drop phase formed from the target material in a laser-induced erosion plume due to bulk vaporization, in the dynamics of this plume. For some metals, the ranges of power densities of the acting laser radiation, at which the condensed-phase particles influence the passage of laser radiation to the target surface, have been determined experimentally. The mechanism of realization of a low-threshold breakdown has been revealed. 相似文献
169.
F. Grunewald B. Kunyavskii D. Nikolova E. Plotkin 《Journal of Mathematical Sciences》2003,116(1):2972-2981
We study two-variable Engel-like relations and identities characterizing finite-dimensional solvable Lie algebras and, conjecturally, finite solvable groups and introduce some invariants of finite groups associated with such relations. Bibliography: 29 titles. 相似文献
170.
F. Trani G. Cantele D. Ninno G. Iadonisi 《Physica E: Low-dimensional Systems and Nanostructures》2004,22(4):808-814
In this paper we report on tight-binding calculations of lowest unoccupied molecular orbitals states for silicon ellipsoidal nanocrystals. The electronic structure has been calculated for different nanocrystal shapes either keeping constant or varying the number of silicon atoms. We have found that changing the ellipsoid aspect ratio a non-obvious energy level structure is obtained. The implications for the infrared optical transitions and their relationship with the polarization of the radiation involved are discussed. 相似文献