全文获取类型
收费全文 | 319659篇 |
免费 | 4423篇 |
国内免费 | 1087篇 |
专业分类
化学 | 174104篇 |
晶体学 | 4553篇 |
力学 | 13154篇 |
综合类 | 7篇 |
数学 | 37296篇 |
物理学 | 96055篇 |
出版年
2020年 | 2080篇 |
2019年 | 2075篇 |
2018年 | 2085篇 |
2017年 | 1995篇 |
2016年 | 4067篇 |
2015年 | 3398篇 |
2014年 | 4645篇 |
2013年 | 14459篇 |
2012年 | 11033篇 |
2011年 | 13683篇 |
2010年 | 8526篇 |
2009年 | 8501篇 |
2008年 | 12581篇 |
2007年 | 12828篇 |
2006年 | 12435篇 |
2005年 | 11341篇 |
2004年 | 10206篇 |
2003年 | 9024篇 |
2002年 | 8836篇 |
2001年 | 10206篇 |
2000年 | 7793篇 |
1999年 | 6185篇 |
1998年 | 4966篇 |
1997年 | 4781篇 |
1996年 | 4823篇 |
1995年 | 4429篇 |
1994年 | 4156篇 |
1993年 | 3966篇 |
1992年 | 4473篇 |
1991年 | 4322篇 |
1990年 | 4064篇 |
1989年 | 3875篇 |
1988年 | 4165篇 |
1987年 | 3818篇 |
1986年 | 3718篇 |
1985年 | 5380篇 |
1984年 | 5473篇 |
1983年 | 4427篇 |
1982年 | 4797篇 |
1981年 | 4834篇 |
1980年 | 4598篇 |
1979年 | 4701篇 |
1978年 | 4713篇 |
1977年 | 4697篇 |
1976年 | 4637篇 |
1975年 | 4543篇 |
1974年 | 4388篇 |
1973年 | 4552篇 |
1972年 | 2583篇 |
1971年 | 1889篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
131.
132.
F. A. van Goor S. V. Mitko V. N. Ochkin W. J. Witteman 《Journal of Russian Laser Research》1997,18(2):107-120
Experimental and theoretical results are presented on an XeF(C?A) blue-green laser driven by 5-kJ energy. The laser was pumped by a ferrite-induced discharge of 90 cm in length. The output energy of 0.22 J was obtained with a plane-parallel resonator. A program to simulate laser operation has been developed. Numerical results for a wide range of conditions are compared with experiments performed by us and by other authors. It is found that intracavity refractive losses limit laser operation for XeF pressures above 3 torr. The laser efficiency strongly depends on the discharge-to-cavity length ratio. Possible ways to increase the laser power and efficiency are discussed. 相似文献
133.
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
134.
Whittum D.H. Ebihara K. Hiramatsu S. Kishiro J.-i. Monaka T. Ozaki T. Takayama K. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1993,21(1):136-141
Studies of microwave amplification with an in-focused electron beam drawn from an induction injector are reported. A free-electron laser (FEL) operating at 9.4 GHz and employing ion-focusing within the interaction region has achieved power in excess of 30 MW at 9.4 GHz, with a beam energy of 0.8 MeV and current of 0.7 kA. Peak gain is 20 dB/m, with no saturation after 15 wiggler periods. Also reported are the first evolution and detuning data for an ion-channel laser/maser (ICL). Two shortcomings of the prematurely halted ICL studies are poor frequency discrimination and a large axial plasma gradient. Prospects for operation with an upgraded 1.6 MeV accelerator are discussed 相似文献
135.
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
136.
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
137.
Chromatographia - A theoretical model is used to predict retention times of divalent metal cations in single-column ion chromatography using one or more ligands in the eluent. Stability constants... 相似文献
138.
139.
140.
Summary A system for capillary electrophoresis combined with enzymatic assay has been evaluated for the two enzymes glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase. Instrumentation included a post-column reactor coupled to the separation capillary by a liquid junction. A technique for generating a substrate solution flow into the reactor by utilizing two high voltage supplies is proposed. This method offers a high degree of freedom in optimizing the separation and enzymatic reaction conditions individually. Possibilities for improving the enzymatic assay sensitivity were also examined. 相似文献