全文获取类型
收费全文 | 413473篇 |
免费 | 5236篇 |
国内免费 | 1996篇 |
专业分类
化学 | 229255篇 |
晶体学 | 5922篇 |
力学 | 16621篇 |
综合类 | 67篇 |
数学 | 48495篇 |
物理学 | 120345篇 |
出版年
2020年 | 3733篇 |
2019年 | 4108篇 |
2018年 | 5282篇 |
2017年 | 5257篇 |
2016年 | 7713篇 |
2015年 | 4915篇 |
2014年 | 7331篇 |
2013年 | 17640篇 |
2012年 | 14248篇 |
2011年 | 17306篇 |
2010年 | 12249篇 |
2009年 | 11909篇 |
2008年 | 16231篇 |
2007年 | 16402篇 |
2006年 | 15369篇 |
2005年 | 13996篇 |
2004年 | 12481篇 |
2003年 | 11118篇 |
2002年 | 10907篇 |
2001年 | 11550篇 |
2000年 | 9074篇 |
1999年 | 6725篇 |
1998年 | 5685篇 |
1997年 | 5711篇 |
1996年 | 5575篇 |
1995年 | 4771篇 |
1994年 | 4895篇 |
1993年 | 4778篇 |
1992年 | 5151篇 |
1991年 | 5202篇 |
1990年 | 4954篇 |
1989年 | 4863篇 |
1988年 | 4712篇 |
1987年 | 4624篇 |
1986年 | 4571篇 |
1985年 | 5964篇 |
1984年 | 6157篇 |
1983年 | 5030篇 |
1982年 | 5498篇 |
1981年 | 5222篇 |
1980年 | 4821篇 |
1979年 | 5192篇 |
1978年 | 5477篇 |
1977年 | 5481篇 |
1976年 | 5488篇 |
1975年 | 5105篇 |
1974年 | 5227篇 |
1973年 | 5354篇 |
1972年 | 3938篇 |
1971年 | 3257篇 |
排序方式: 共有10000条查询结果,搜索用时 397 毫秒
211.
Ya. V. Fattakhov M. F. Galyautdinov T. N. L’vova I. B. Khaibullin 《Technical Physics》1997,42(12):1457-1459
The first results regarding the formation of a two-dimensional periodic structure of local melting regions on a silicon surface
upon pulsed light irradiation are presented. The conditions are established, and the mechanism of the formation of such structures
is discussed.
Zh. Tekh. Fiz. 67, 97–99 (December 1997) 相似文献
212.
A converging perturbation series that can be summed analytically has been obtained for intersubband transitions of electrons
coherently tunneling through the middle of a dimensionally quantized level in an asymmetric double-barrier structure in a
high-frequency terahertz electric field. The possibility of a substantial increase in tunneling current accompanied by either
absorption or emission of a photon has been demonstrated. The quantum efficiency of radiative transitions between dimensionally
quantized levels can be up to 66%.
Zh. éksp. Teor. Fiz. 112, 237–245 (July 1997) 相似文献
213.
L. K. Aminov I. N. Kurkin D. A. Lukoyanov K. P. Chernov 《Physics of the Solid State》1997,39(8):1184-1186
Results are reported for measurements of the spin-lattice relaxation times of E′1 centers in quartz glass, produced by neutron irradiation, with the measurements made at two frequencies 9.25 and 24.0 GHz
over a wide temperature interval 1.5–300 K. The experimental data are interpreted on the basis of interaction mechanisms of
the spins with two-level systems with excitation energies ∼6, ∼26, and ∼420 cm−1. A small modification of the existing theory allows us to explain a number of features of the observed temperature and frequency
dependence of the relaxation rate. The results are compared with the data available in the literature on spin-lattice relaxation
of irradiation centers in crystalline quartz and quartz glass.
Fiz. Tverd. Tela (St. Petersburg) 39, 1335–1337 (August 1997) 相似文献
214.
It is shown that equations describing the dynamics of Darwinian systems (DS) far from the bifurcation points may be expressed
in Hamiltonian form. The cases of DS with constant organization and DS with a constant flux through the system are considered.
The configurational part of phase space is formed by variables containing information on the structure of the system. Momentum
variables may be regarded as specific rates of multiplication. The evolution of DS with constant organization in this phase
space is expressed as uniform rectilinear motion. In the case of DS with a constant flux, the motion occurs in some effective
constant and uniform field. The meaning of the elements of the Hamiltonian structure is described in terms of theoretical
biology.
Tomsk State University. Scientific-Research Institute of Biological Systems, Tomsk State University. Translated from Izvestiya
Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 23–28, July, 1997. 相似文献
215.
Rafael López 《Annals of Global Analysis and Geometry》1997,15(3):201-210
In this paper we study constant mean curvature compact surfaces with two Jordan curves in parallel planes as boundary and we investigate the point at which the surface inherits the symmetries of its boundary. 相似文献
216.
William J. Letts Bambi L. Bailey Kathryn Scantlebury 《School science and mathematics》1997,97(4):192-199
With recent national calls for the reform of science education have come standards that delineate not only science content but also assessment, pedagogy, and teachers’ professional development. If teachers must teach science differently, then teacher preparation must change. This study asked 31 inservice secondary science teachers to complete a survey about topics for inclusion in a secondary science methods course. Respondents ranked a list of prespecified topics and had an opportunity to suggest other topics for inclusion in the course. Results showed that the majority of prespecified potential topics were judged important enough by these teachers to warrant inclusion in a methods course, though no individual added topic appeared on more than two surveys. Results demonstrate that these teachers believe teaching many of the traditional topics in science methods courses is still needed. In addition, they advocated the inclusion of several topics that either represent recent technological and theoretical advances, or longstanding ideas that have recently received considerable attention. 相似文献
217.
Reynaldo C. Castells Eleuterio L. Arancibia Angel M. Nardillo 《Journal of solution chemistry》1993,22(1):85-94
The retention behavior of eight halomethanes and four saturated hydrocarbons was measured in gas chromatographic stationary phases consisting in tri-n-octylamine (TOA), squalane (SQ) and six TOA+SQ mixtures, at 55.0, 58.5, 62.5 and 65.0°C. Equlibrium constants for complex formation were extracted from experimental data by using a lattice model developed by Martire. The results may be interpreted in terms of the formation of weak hydrogen-bonded complexes, with sociation constants of about 0.13 L-mol–1 for haloforms and 0.07 L-mol–1 for dihalomethanes at 60°C. 相似文献
218.
Daniel L. Feldheim Del R. Lawson Charles R. Martin 《Journal of Polymer Science.Polymer Physics》1993,31(8):953-957
We report a strong dependence of the thermal stability of Nafion® perfluorosulfonate ionomer on the nature of the counterion associated with the fixed sulfonate site. These results were obtained using thermal gravimetric analysis on a series of alkali metal and alkyl ammonium cation-exchanged Nafion films. We have found that the temperature of decomposition of Nafion is inversely dependent on the size of the exchanged cation; i.e., Nafion films show improved thermal stability as the size of the counter cation decreases. We attribute this inverse relationship of thermal stability with counterion size to an initial decomposition reaction which is strongly influenced by the strength of the sulfonate-coun-terion interaction. © 1993 John Wiley & Sons, Inc. 相似文献
219.
J. E. Fischer E. Werwa P. A. Heiney 《Applied Physics A: Materials Science & Processing》1993,56(3):193-196
We have used both reflection-geometry and grazing-incidence-geometry X-ray scattering to study thin films of C60 evaporated onto mica substrates via a hot-wall technique. The growth mode yields close-packed C60 planes, which are parallel to the substrate surface and which exhibit out-of-plane correlation lengths of 850 Å. In the film plane the C60 is at best pseudo-epitaxial, with a 0.9° distribution of crystallite orientations, a 450 Å in-plane correlation length, and a 3.7% lattice mismatch, better than obtained by other thin film techniques but far from the accepted definition of single crystal thin film epitaxy. 相似文献
220.
Summary Consider the solution of one-dimensional linear initial-boundary value problems by a finite element method of lines using a piecewiseP
th
-degree polynomial basis. A posteriori estimates of the discretization error are obtained as the solutions of either local parabolic or local elliptic finite element problems using piecewise polynomial corrections of degreep+1 that vanish at element ends. Error estimates computed in this manner are shown to converge in energy under mesh refinement to the exact finite element discretization error. Computational results indicate that the error estimates are robust over a wide range of mesh spacings and polynomial degrees and are, furthermore, applicable in situations that are not supported by the analysis.This research was partially supported by the U.S. Air Force Office of Scientific Research, Air Force Systems Command, USAF, under Grant Number AFOSR 90-0194; by the U.S. Army Research Office under Contract Number DAAL03-91-G-0215; and by the National Science Foundation under Institutional Infrastructure Grant Number CDA-8805910 相似文献