全文获取类型
收费全文 | 399256篇 |
免费 | 4743篇 |
国内免费 | 1454篇 |
专业分类
化学 | 218395篇 |
晶体学 | 5823篇 |
力学 | 16556篇 |
综合类 | 12篇 |
数学 | 46200篇 |
物理学 | 118467篇 |
出版年
2020年 | 2462篇 |
2019年 | 2405篇 |
2018年 | 2421篇 |
2016年 | 4758篇 |
2015年 | 3954篇 |
2014年 | 5375篇 |
2013年 | 17282篇 |
2012年 | 12929篇 |
2011年 | 16218篇 |
2010年 | 10060篇 |
2009年 | 9907篇 |
2008年 | 14893篇 |
2007年 | 15153篇 |
2006年 | 14834篇 |
2005年 | 13732篇 |
2004年 | 12309篇 |
2003年 | 10928篇 |
2002年 | 10689篇 |
2001年 | 12167篇 |
2000年 | 9378篇 |
1999年 | 7504篇 |
1998年 | 6153篇 |
1997年 | 5975篇 |
1996年 | 5987篇 |
1995年 | 5550篇 |
1994年 | 5165篇 |
1993年 | 4983篇 |
1992年 | 5593篇 |
1991年 | 5438篇 |
1990年 | 5089篇 |
1989年 | 4932篇 |
1988年 | 5192篇 |
1987年 | 4918篇 |
1986年 | 4727篇 |
1985年 | 6800篇 |
1984年 | 6926篇 |
1983年 | 5671篇 |
1982年 | 6305篇 |
1981年 | 6205篇 |
1980年 | 5980篇 |
1979年 | 6053篇 |
1978年 | 6201篇 |
1977年 | 6115篇 |
1976年 | 6084篇 |
1975年 | 5913篇 |
1974年 | 5647篇 |
1973年 | 5918篇 |
1972年 | 3496篇 |
1971年 | 2627篇 |
1968年 | 2577篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Principles and recent developments in ultrasound contrast agents 总被引:6,自引:1,他引:5
The behaviour of gas bubbles and gas encapsulated spheres as echographic contrast agents is reviewed. Compared with rigid spheres, gas bubbles are superior scattering agents and they offer a number of useful properties which can be exploited in a variety of ways. The analysis of their velocity of sound, back-scatter intensity, second harmonic emission and resonant frequency opens up new perspectives in the development of contrast agents for echocardiographic research with potential clinical applications. 相似文献
992.
993.
New concepts for the study of incompressible plane or axisymmetric flows are analysed by the stream tube method. Flows without eddies and pure vortex flows are considered in a transformed domain where the mapped streamlines are rectilinear or circular. The transformation between the physical domain and the computational domain is an unknown of the problem. In order to solve the non-linear set of relevant equations, we present a new algorithm based on a trust region technique which is effective for non-convex optimization problems. Experimental results show that the new algorithm is more robust compared to the Newton-Raphson method. 相似文献
994.
Summary Oblique propagating magnetohydrodynamic waves with various wave forms and amplitudes are observed both at the Earth's foreshock
and at comets. The possibility of interpreting some observational results in terms of nonlinear evolution of one- and two-dimensional
hydromagnetic waves is investigated. For this purpose both analytical and numerical techniques are employed. It is found that
an initial monochromatic wave changes its polarization giving origin to magnetosonic shocks and rotational discontinuities;
the time evolution of density-magnetic-field correlation is studied, as a function of the plasma parameters and of the propagation
angle. In the two-dimensional case both a transverse instability and a self-focusing effect may take place. Moreover, a two-dimensional
magnetosonic solution is found, in which the density fluctuations are driven by the total pressure fluctuation as in a one-dimensional
simple wave. These theoretical predictions compare well with the features observed in the solar-wind waves.
Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990. 相似文献
995.
996.
997.
P. J. Skevington M. A. G. Halliwell M. H. Lyons S. J. Amin M. A. Z. Rejman-Greene G. J. Davies 《Journal of Crystal Growth》1992,120(1-4):328-332
InP/InGaAs multiple quantum well structures with up to 200 periods have been grown by CBE. These structures exhibit exceptional lateral uniformity, measured as ±1 Å in period, ±13 ppm in lattice mismatch and ±0.5 nm in wavelength across a 2 inch wafer. Good surface morphology, sharp interfaces and excellent growth control have all been demonstrated. 相似文献
998.
Sea nodules were extensively studied over a wide range of temperature ranging from 77K to 1175K using Mossbauer effect and
ESR. The Mossbauer studies of the nodules at 77K and room temperature show a quadrupole doublet whereas at higher temperatures
magnetic spectra were obtained starting at around 875K which ultimately gives a hyperfine field of around 390 KOe at 1175
K. The Mossbauer spectra recorded at 30K did not show any significant change in the room temperature spectra. The analysis
of the spectra upto 775K showed two positions of FeR+, vie., octahedral and tetrahedral which were varified by ESR of the diluted samples. 相似文献
999.
T Christensen E B Jensen F Junker H Dalb?ge F Abildgaard J J Led 《Acta chemica Scandinavica (Copenhagen, Denmark : 1989)》1992,46(1):97-99
Biosynthetic human growth hormone specifically 13C-labelled in the carbonyl positions of all 26 leucine residues has been obtained by recombinant DNA techniques using 13C-labelled leucine and an E. coli strain that requires leucine. It is shown that, on the whole, the labelling is specific with no significant mislabelling as would have been the case had the 13C-labelled leucine been metabolized. 相似文献
1000.
The authors acknowledge valuable discussions with Jonathan Hamilton and Mamoru Kaneko as well as very detailed comments of two referees. 相似文献