全文获取类型
收费全文 | 398352篇 |
免费 | 4069篇 |
国内免费 | 1910篇 |
专业分类
化学 | 218823篇 |
晶体学 | 5824篇 |
力学 | 16259篇 |
综合类 | 67篇 |
数学 | 45756篇 |
物理学 | 117602篇 |
出版年
2020年 | 3405篇 |
2019年 | 3772篇 |
2018年 | 5086篇 |
2017年 | 5043篇 |
2016年 | 7176篇 |
2015年 | 4434篇 |
2014年 | 6766篇 |
2013年 | 16750篇 |
2012年 | 13332篇 |
2011年 | 16253篇 |
2010年 | 11600篇 |
2009年 | 11329篇 |
2008年 | 15383篇 |
2007年 | 15561篇 |
2006年 | 14578篇 |
2005年 | 13285篇 |
2004年 | 11855篇 |
2003年 | 10607篇 |
2002年 | 10415篇 |
2001年 | 11293篇 |
2000年 | 8824篇 |
1999年 | 6505篇 |
1998年 | 5480篇 |
1997年 | 5513篇 |
1996年 | 5335篇 |
1995年 | 4554篇 |
1994年 | 4701篇 |
1993年 | 4583篇 |
1992年 | 4979篇 |
1991年 | 5063篇 |
1990年 | 4807篇 |
1989年 | 4724篇 |
1988年 | 4581篇 |
1987年 | 4518篇 |
1986年 | 4478篇 |
1985年 | 5831篇 |
1984年 | 6031篇 |
1983年 | 4932篇 |
1982年 | 5373篇 |
1981年 | 5089篇 |
1980年 | 4711篇 |
1979年 | 5083篇 |
1978年 | 5345篇 |
1977年 | 5393篇 |
1976年 | 5409篇 |
1975年 | 5016篇 |
1974年 | 5151篇 |
1973年 | 5295篇 |
1972年 | 3896篇 |
1967年 | 3223篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
We have studied by ESR and ENDOR spectroscopy the free radicals produced in γ-irradiated inclusion compound formed between the ketone 10-nonadecanone and urea. Only one type of long lived radical is formed by the removal of an α-proton from the ketone. The hyperfine (hf) coupling constants of the α- and β-protons of the radicals have been measured by ESR at different temperatures in the range 110–292 K and at different orientations of the crystals. The hf coupling of the γ-protons of the radical and of the urea protons have been studied by ENDOR. The temperature and angular dependences of the coupling constants have been analyzed in terms of the internal and overall motions of the radical inside the hexagonal channels formed by the urea molecules. It has been found that the radical cannot perform complete reorientations around the long molecular axis, but it undergoes restricted rotational diffusion. This process is explained by assuming a coupling between the rotational and translational degrees of freedom of the radical inside the urea channels. 相似文献
992.
993.
F. Seiffert R. Schwengner G. Winter L. Funke W. Lieberz R. Reinhardt K. P. Schmittgen D. Weil R. Wrzal K. O. Zell P. von Brentano 《Zeitschrift für Physik A Hadrons and Nuclei》1991,340(2):141-153
Excited states of73Se have been investigated up to spin, 21/2 using techniques of in-beamγ-ray spectroscopy in connection with the70Ge(α, n) reaction. Mean lifetimes of 12 levels have been determined applying Doppler-shift andγ-RF-methods. Five different bands have been identified that reflect a variety of different excitation modes. The decoupled 9/2+ band is likely to correspond to an oblate deformation while the 5/2+ band is interpreted as a strongly coupled prolate band built on the Nilsson configuration [422] 5/2+. The 3/2? band is a strongly coupled band built on the [301] 3/2} configuration.Nuclear reactions:70Ge(α,n),E=14, 16, 18, 19, 20MeV; measuredE γ,I γ,σ(E γ,θ),γγ-coin, linear polarization, DSA,γ(t).75Se deduced levels,I, π, τ, δ(E2/M1), B(σλ). Enriched targets, Ge detectors. 相似文献
994.
We review a little-used but powerful method of solving the scalar wave equation. It uses a modification of the well-known Airy functions, which are easily calculated on desktop computers. The techniques are reminiscent of the WKBJ methodology, but the solution, although approximate, is much more useful than the traditional WKBJ solution and can be used with almost as much ease. Re method is extremely powerful but, to our knowledge, is not used in the optics community. It is useful in analyzing integrated optical waveguide components. 相似文献
995.
P. Dawson J. W. Haas III K. B. Alexander J. Thompson T. L. Ferrell 《Surface science》1991,250(1-3):L383-L388
The intensity of surface enhanced Raman scattering from benzoic acid derivatives on mildly roughened, thermally evaporated Ag films shows a remarkably strong dependence on metal grain size. Large grained (slowly deposited) films give a superior response, by up to a factor of 10, to small grained (quickly deposited) films, with films of intermediate grain size yielding intermediate results. The optical field amplification underlying the enhancement mechanism is due to the excitation of surface plasmon polaritons (SPPs). Since surface roughness characteristics, as determined by STM, remain relatively constant as a function of deposition rate, it is argued that the contrast in Raman scattering is due to differences in elastic grain boundary scattering of SPPs (leading to different degrees of internal SPP damping), rather than differences in the interaction of SPPs with surface inhomogeneities. 相似文献
996.
The detection of serial changes in magnetic resonance (MR) signal intensity of the heart following acute myocardial infarction may provide a useful method of characterizing tissue healing. Fourteen patients with acute Q-wave infarction underwent T2-weighted, spin-echo cardiac imaging during hospitalization, followed by one or more additional MR studies (total 31) over a 6- to 27-wk period (mean: 3 mo). Visual assessment of the images demonstrated a gradual reduction in signal intensity and localization of the bright signal to the subendocardium of the infarction region over the three-mo study period. A quantitative measurement of signal intensity (infarction/normal myocardium) fell from 1.81 +/- 0.42 on the initial study to 1.34 +/- 0.37 (p less than 0.05) at a mean of 14 wk. Two patients had an increase in signal intensity on the follow-up study and both patients had been readmitted with acute coronary syndromes. In summary, characterization of changes in signal intensity may provide a useful method of assessing myocardial healing following acute myocardial infarction. Further studies are indicated to determine the prognostic significance of these parameters. 相似文献
997.
998.
E. Possenriede P. Jacobs H. Kröse O. F. Schirmer 《Applied Physics A: Materials Science & Processing》1992,55(1):73-81
On the basis of a previous identification of paramagnetic defects in nominally undoped as grown BaTiO3 single crystals, we have investigated the changes of the concentrations of these centers and their optical absorptions under illumination with light of varying wavelengths. The most pronounced charge transfers occur by hole ionization of Fe4+ and — to a lesser extent — of Cr5+ and Cr4+. At low temperatures the created holes are trapped in the form of O–-ions next to Al3+ or unknown acceptor defects. Corresponding Fe4+ and O– absorptions have been identified. 相似文献
999.
Summary We provide an alternate proof of McMullen's theorem on contractive properties of the Poincaré series operator in the special case of the universal covering. This case includes in particular Kra's Theta Conjecture.Oblatum 16-X-1991 & 14-IV-1992First author supported in part by a grant from the National Science Foundation 相似文献
1000.
Pure adaptive search in global optimization 总被引:10,自引:0,他引:10
Pure adaptive seach iteratively constructs a sequence of interior points uniformly distributed within the corresponding sequence of nested improving regions of the feasible space. That is, at any iteration, the next point in the sequence is uniformly distributed over the region of feasible space containing all points that are strictly superior in value to the previous points in the sequence. The complexity of this algorithm is measured by the expected number of iterations required to achieve a given accuracy of solution. We show that for global mathematical programs satisfying the Lipschitz condition, its complexity increases at mostlinearly in the dimension of the problem.This work was supported in part by NATO grant 0119/89. 相似文献