全文获取类型
收费全文 | 133219篇 |
免费 | 11213篇 |
国内免费 | 8851篇 |
专业分类
化学 | 45688篇 |
晶体学 | 1958篇 |
力学 | 6459篇 |
综合类 | 1530篇 |
数学 | 40495篇 |
物理学 | 57153篇 |
出版年
2023年 | 593篇 |
2022年 | 2221篇 |
2021年 | 2118篇 |
2020年 | 1855篇 |
2019年 | 2160篇 |
2018年 | 1965篇 |
2017年 | 1939篇 |
2016年 | 2035篇 |
2015年 | 1858篇 |
2014年 | 3433篇 |
2013年 | 5134篇 |
2012年 | 3687篇 |
2011年 | 4667篇 |
2010年 | 4933篇 |
2009年 | 9997篇 |
2008年 | 11056篇 |
2007年 | 9663篇 |
2006年 | 8469篇 |
2005年 | 6240篇 |
2004年 | 5798篇 |
2003年 | 5707篇 |
2002年 | 8631篇 |
2001年 | 5226篇 |
2000年 | 4763篇 |
1999年 | 4396篇 |
1998年 | 3760篇 |
1997年 | 2836篇 |
1996年 | 2583篇 |
1995年 | 2887篇 |
1994年 | 2770篇 |
1993年 | 2237篇 |
1992年 | 1732篇 |
1991年 | 1261篇 |
1990年 | 1082篇 |
1989年 | 941篇 |
1988年 | 920篇 |
1987年 | 667篇 |
1986年 | 445篇 |
1985年 | 1298篇 |
1984年 | 888篇 |
1983年 | 669篇 |
1982年 | 954篇 |
1981年 | 1111篇 |
1980年 | 978篇 |
1979年 | 866篇 |
1978年 | 786篇 |
1977年 | 665篇 |
1976年 | 646篇 |
1974年 | 419篇 |
1973年 | 572篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
自由电子激光的预群聚 总被引:1,自引:0,他引:1
研究了自由电子激光预群聚产生的辐射,表明预群聚不仅可以提高效率而且使低增益得到增强,从而在理论上解释了由射频直线加速器驱动的自由电子激光预群聚器(prebuncher)的作用。同时还指出,预群聚有在不增加电子束能量情况下缩短激光波长和降低对电子束质量要求等作用。 相似文献
52.
C. Fallnich B. Ruffing Th. Herrmann A. Nebel R. Beigang R. Wallenstein 《Applied physics. B, Lasers and optics》1995,60(5):427-436
We report on a cw mode-locked non-critically phase matched KTP optical parametric oscillator synchronously pumped by a picosecond Ti:Sapphire laser. High average signal output power of up to 950 mW over a large tuning range has been achieved. For this OPO the influence of resonator-length detuning on the output power, pulse duration and spectral bandwidth has been investigated. The measured data are in good agreement with the results of a numerical simulation using a split-step Fourier method which considers the group-velocity mismatch, the group-velocity dispersion and the self-phase modulation. The numerical simulation also describes the measured strong pump depletion and its influence on the OPO output and efficiency. 相似文献
53.
54.
55.
HIRONOBU Ishihara 《Geometriae Dedicata》1997,67(3):309-336
Several classification results for ample vector bundles of rank 2 on Hirzebruch surfaces, and on Del Pezzo surfaces, are obtained. In particular, we classify rank 2 ample vector bundles with c2 less than seven on Hirzebruch surfaces, and with c2 less than four on Del Pezzo surfaces. 相似文献
56.
We investigate congruence classes and direct congruence classes of m-tuples in the complex projective space ℂP
n
. For direct congruence one allows only isometries which are induced by linear (instead of semilinear) mappings. We establish
a canonical bijection between the set of direct congruence classes of m-tuples of points in ℂP
n
and the set of equivalence classes of positive semidefinite Hermitean m×m-matrices of rank at most n+1 with 1's on the diagonal. As a corollary we get that the direct congruence class of an m-tuple is uniquely determined by the direct congruence classes of all of its triangles, provided that no pair of points of
the m-tuple has distance π/2. Examples show that the situation changes drastically if one replaces direct congruence classes by
congruence classes or if distances π/2 are allowed. Finally we do the same
kind of investigation also for the complex hyperbolic space ℂH
n
. Most of the results are completely analogous, however, there are also some interesting differences.
Received: 15 January 1996 相似文献
57.
Summary. We generalise and apply a refinement indicator of the type originally designed by Mackenzie, Süli and Warnecke in [15] and
[16] for linear Friedrichs systems to the Euler equations of inviscid, compressible fluid flow. The Euler equations are symmetrized
by means of entropy variables and locally linearized about a constant state to obtain a symmetric hyperbolic system to which
an a posteriori error analysis of the type introduced in [15] can be applied. We discuss the details of the implementation of the refinement
indicator into the DLR--Code which is based on a finite volume method of box type on an unstructured grid and present numerical results.
Received May 15, 1995 / Revised version received April 17, 1996 相似文献
58.
矩阵方程A^TXB=C的正定和半正定解 总被引:5,自引:1,他引:4
何楚宁 《高校应用数学学报(A辑)》1997,(4):475-480
给出了矩阵方程A^TXB=C在正定和半正定矩阵类中有解的充要条件及解的一般表达式。 相似文献
59.
XU Jun YAN Ai-Zhen XU Qin-Hua Department of Chemistry Nanjing University Nanjing Jiangsu China 《中国化学》1995,(2)
The strength of basic sites has been measured by pyrrole-IR on alkali metal cation exchanged β and X zeolites, as well as NaOH loaded Naβ. The influence of cation type and the structure of zeolites on their basicity has been studied. The acidic and basic properties of the samples were investigated by NH3-TPD and isopropanol reaction. It was shown that the strength of basic sites on samples could be characterized by the shift of vNH band in the pyrrole-IR spectra. The framework oxygen charges were calculated from the Sanderson electronegativity. The changes in basic properties with various alkali metal cation are consistent with the changes of local oxygen charges of the zeolite framework. 相似文献
60.
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