全文获取类型
收费全文 | 329984篇 |
免费 | 3045篇 |
国内免费 | 934篇 |
专业分类
化学 | 164797篇 |
晶体学 | 5374篇 |
力学 | 15531篇 |
综合类 | 7篇 |
数学 | 38470篇 |
物理学 | 109784篇 |
出版年
2021年 | 3132篇 |
2020年 | 3331篇 |
2019年 | 3763篇 |
2018年 | 5199篇 |
2017年 | 5439篇 |
2016年 | 7370篇 |
2015年 | 4139篇 |
2014年 | 7031篇 |
2013年 | 15909篇 |
2012年 | 12375篇 |
2011年 | 14826篇 |
2010年 | 11012篇 |
2009年 | 10916篇 |
2008年 | 13235篇 |
2007年 | 13191篇 |
2006年 | 12138篇 |
2005年 | 10522篇 |
2004年 | 9833篇 |
2003年 | 8687篇 |
2002年 | 8677篇 |
2001年 | 10399篇 |
2000年 | 7658篇 |
1999年 | 5968篇 |
1998年 | 4976篇 |
1997年 | 4814篇 |
1996年 | 4480篇 |
1995年 | 3865篇 |
1994年 | 3820篇 |
1993年 | 3703篇 |
1992年 | 4097篇 |
1991年 | 4308篇 |
1990年 | 4097篇 |
1989年 | 4011篇 |
1988年 | 3779篇 |
1987年 | 3917篇 |
1986年 | 3683篇 |
1985年 | 4630篇 |
1984年 | 4683篇 |
1983年 | 3886篇 |
1982年 | 3975篇 |
1981年 | 3697篇 |
1980年 | 3649篇 |
1979年 | 3913篇 |
1978年 | 3901篇 |
1977年 | 3907篇 |
1976年 | 3879篇 |
1975年 | 3675篇 |
1974年 | 3617篇 |
1973年 | 3650篇 |
1972年 | 2604篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
J. Maul I. Strachnov S. Karpuk P. Bernhard A. Oelsner G. Schönhense G. Huber 《Applied Physics A: Materials Science & Processing》2006,82(1):43-47
We report morphologic changes of metallic surfaces at the onset of ablation, starting from gentle ablation to the emergence of ablation craters. The evolution of both observed melting zones and of ablation craters therein are investigated in dependence of the ablation laser fluence for nanosecond ultraviolet laser pulses. Further, consequences of crater formation for cluster synthesis within the released atomic vapor are pointed out. PACS 52.38.Mf; 79.20.Ds; 65.40.De 相似文献
22.
We give a coring version for the duality theorem for actions and coactions of a finitely generated projective Hopf algebra.
We also provide a coring analogue for a theorem of H.-J. Schneider, which generalizes and unifies the duality theorem for
finite Hopf algebras and its refinements.
This paper was written while the first author visited the Mathematics Departments of Syracuse University and California State
University Dominguez Hills. He would like to thank both departments for their hospitality. 相似文献
23.
J. C. Fabris S. V. B. Gonçalves P. E. de Souza 《General Relativity and Gravitation》2002,34(12):2111-2126
The mass power spectrum for a Universe dominated by the Chaplygin gas is evaluated numerically from scales of the order of the Hubble horizon to 100 Mpc. The results are compared with a pure baryonic Universe and a cosmological constant model. In all three cases, the spectrum increases with k, the wavenumber of the perturbations. The slope of the spectrum is higher for the baryonic model and smaller for the cosmological constant model, the Chaplygin gas interpolating these two models. The results are analyzed in terms of the sound velocity of the Chaplygin gas and the moment the Universe begins to accelerate. 相似文献
24.
In the study of the behaviors of barriers in an enclosed field, one should take into account such phenomena as sound energy reflection, absorption, scattering and diffraction. Therefore, the study is much more difficult than that in free field. In this paper, sound barriers are classified into four kinds according to their size, number and shape. Each kind of barriers is modelled by a corresponding method based on a computer program—SOFIS. The program combines the ray-tracing technique and statistical method. The impulse response and some acoustical parameters such as sound pressure level at different positions can be calculated by the program, no matter there are a certain kind of barriers in the field or the field is empty. The ray-tracing program and the algorithms for various barriers are validated by the comparison between measurement and prediction of the reverberation room and the anechoic room of the Northwestern Polytechnic University. 相似文献
25.
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 54, No. 5, pp. 815–819, May, 1991. 相似文献
26.
27.
A. S. Merkur'ev 《Journal of Mathematical Sciences》1991,57(6):3489-3497
This article considers the splitting properties of finite-dimensional division rings over universal splitting fields of quadratic forms. An example of a field with u-invariant equal to 6 is constructed, which contradicts Kaplansky's conjecture concerning u-invariants.Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im, V. A. Steklova Akademii Nauk SSSR, Vol. 175, pp. 75–89, 1989. 相似文献
28.
The dynamics of multiphoton transitions in a two-level spin system excited by transverse microwave and longitudinal RF fields with the frequencies ωmw and ωrf, respectively, is analyzed. The effective time-independent Hamiltonian describing the “dressed” spin states of the “spin + bichromatic field” system is obtained by using the Krylov-Bogoliubov-Mitropolsky averaging method. The direct detection of the time behavior of the spin system by the method of nonstationary nutations makes it possible to identify the multiphoton transitions for resonances ω0 = ωmw + rωrf (ω0 is the central frequency of the EPR line, r = 1, 2), to measure the amplitudes of the effective fields of these transitions, and to determine the features generated by the inhomogeneous broadening of the EPR line. It is shown that the Bloch-Siegert shifts for multiphoton resonances at the inhomogeneous broadening of spectral lines reduce only the nutation amplitude but do not change their frequencies. 相似文献
29.
The structural properties of polycrystalline silicon films, prepared by plasma enhanced chemical vapor deposition system, with different flow rates of SiH4/SiF4 mixtures at 300 °C were investigated. This study indicates that the low hydrogen coverage on the growing surface, under optimum fluorine radicals, will be leaded to an improvement of crystallized area as compared with case of high hydrogen coverage surface. Moreover, the studies of the role of SiH4 and SiF4 radicals show that the SiH4 radicals are important in the nucleation and growth of grains. However, SiF4 radicals are effective in the structural change of grain boundaries regions and by this way, in the present system, establish the growth of grains under the dominant 〈1 1 0〉 direction. The stress investigation indicates that addition of high flow rate of SiF4 in amorphous film, results in the nearly stress free films. Finally, we found that the changes in g-value reflect the changes in the intrinsic compressive and tensile stress in the both polycrystalline and amorphous silicon films. 相似文献
30.