全文获取类型
收费全文 | 20497篇 |
免费 | 984篇 |
国内免费 | 101篇 |
专业分类
化学 | 13132篇 |
晶体学 | 121篇 |
力学 | 420篇 |
数学 | 2898篇 |
物理学 | 3427篇 |
无线电 | 1584篇 |
出版年
2023年 | 138篇 |
2022年 | 180篇 |
2021年 | 315篇 |
2020年 | 425篇 |
2019年 | 433篇 |
2018年 | 283篇 |
2017年 | 270篇 |
2016年 | 648篇 |
2015年 | 597篇 |
2014年 | 705篇 |
2013年 | 1087篇 |
2012年 | 1201篇 |
2011年 | 1321篇 |
2010年 | 792篇 |
2009年 | 738篇 |
2008年 | 1100篇 |
2007年 | 1087篇 |
2006年 | 1010篇 |
2005年 | 928篇 |
2004年 | 833篇 |
2003年 | 697篇 |
2002年 | 653篇 |
2001年 | 350篇 |
2000年 | 324篇 |
1999年 | 309篇 |
1998年 | 270篇 |
1997年 | 259篇 |
1996年 | 291篇 |
1995年 | 276篇 |
1994年 | 245篇 |
1993年 | 233篇 |
1992年 | 216篇 |
1991年 | 186篇 |
1990年 | 189篇 |
1989年 | 184篇 |
1988年 | 163篇 |
1987年 | 136篇 |
1986年 | 120篇 |
1985年 | 176篇 |
1984年 | 173篇 |
1983年 | 134篇 |
1982年 | 159篇 |
1981年 | 165篇 |
1980年 | 141篇 |
1979年 | 145篇 |
1978年 | 166篇 |
1977年 | 128篇 |
1976年 | 110篇 |
1975年 | 122篇 |
1974年 | 97篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Quantitative H–Al distances in acid sites of two zeolites with MFI and IFR framework topology were obtained by numerical simulation of 1H{27Al} rotational echo adiabatic passage double resonance (REAPDOR) experiments. A 27Al offset-dependent data set yields for each resolved 1H NMR line a corresponding nuclear electric quadrupole coupling constant of the neighboring 27Al site. This information is used for analyzing a second data set for on-resonance irradiation, where the dipolar evolution time (number of rotor cycles) was varied, to yield the 1H–27Al dipolar coupling constant. Numerical simulations indicate that the REAPDOR method does not depend significantly on the polar angles, defining the orientation of the electric field gradient tensor of 27Al with respect to the Al–H dipolar vector. In contrast, the transfer of populations in double resonance sequence is sensitive to these angles, and it can be thus used to measure them. 相似文献
42.
43.
Bubble pressure points of ethanol–1,1,1,2,3,3,3-heptafluoropropane (HFC-227ea refrigerant) mixtures from the third Industrial Fluid Properties Simulation Challenge are computed using publicly available molecular simulation software. Several published force fields are compared against the known answers provided in the contest guidelines and the best force fields are used to make predictions for the unknown results. 相似文献
44.
Lee Stephen; Harrison Martin C.; Robinson Carol L. 《Teaching Mathematics and its Applications》2007,26(3):111-118
In the past 6 years changes have occurred in GCE A-levels. Inparticular, there have been several major changes in A-levelMathematics courses. As engineering students are usually requiredto have studied A-level Mathematics, or its equivalent, thesechanges have had an effect on their prior mathematical knowledge.Moreover, engineering students traditionally obtained a goodgrounding in mechanics as part of their A-level Mathematicsqualification. However, mechanics, which was once included inthe core syllabus, is now optional. This article investigatesthe current availability and uptake of mechanics modules withinA-level Mathematics courses in schools. Comparisons are drawnbetween these results and results of a survey of schools in2004 and surveys of first year engineering students conductedin 2004 and 2005. It is found that there is a decline in theavailability of mechanics modules and the uptake of more thanone mechanics module has also decreased. The implications ofthese findings for engineering educators are discussed. 相似文献
45.
Xiaoming Zhou Martin Jacobsson Henk Uijterwaal Piet Van Mieghem 《International Journal of Communication Systems》2008,21(6):643-663
Although packet delay and loss are two important parameters of the Internet performance, to the best of our knowledge, the evolution of large‐scale IPv6 delay and loss performance has previously not been studied. In this paper, we analyze more than 600 end‐to‐end IPv6 paths between about 26 testboxes of RIPE Network Coordination Centre over two years, and compare the delay and loss performance over time with their IPv4 counterparts. We present and discuss the measurement methodologies and show that IPv6 paths have a higher delay and loss than their IPv4 counterparts. The main reason for the worse performance stems from IPv6‐in‐IPv4 tunnels rather than from native IPv6 paths and such tunnels are still widely used today. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
46.
It can be conjectured that the colored Jones function of a knot can be computed in terms of counting paths on the graph of
a planar projection of a knot. On the combinatorial level, the colored Jones function can be replaced by its weight system.
We give two curious formulas for the weight system of a colored Jones function: one in terms of the permanent of a matrix
associated to a chord diagram, and another in terms of counting paths of intersecting chords.
Electronic supplementary material to this article is available at and is accessible to authorized users.
* S. G. was partially supported by an NSF and by an Israel-US BSF grant.
† M. L. was partly supported by GAUK 158 grant and by the Project LN00A056 of the Czech Ministry of Education. 相似文献
47.
48.
We close a gap in the proof of Theorem 4.1 in our paper “The complexity of counting graph homomorphisms” [Random Structures Algorithms 17 (2000), 260–289]. © 2004 Wiley Periodicals, Inc. Random Struct. Alg., 2004 相似文献
49.
Martin Vohralík 《Comptes Rendus Mathematique》2004,339(7):525-528
We consider the lowest-order Raviart–Thomas mixed finite element method for elliptic problems on simplicial meshes in two or three space dimensions. This method produces saddle-point type problems for scalar and flux unknowns. We show how to easily eliminate the flux unknowns, which implies an equivalence between this method and a particular multi-point finite volume scheme, without any approximate numerical integration. We describe the stencil of the final matrix and give sufficient conditions for its symmetry and positive definiteness. We present a numerical example illustrating the performance of the proposed method. To cite this article: M. Vohralík, C. R. Acad. Sci. Paris, Ser. I 339 (2004). 相似文献
50.
在空间电荷主导电子束研究中,纵向聚焦非常重要,在输运过程中由于强空间电荷力的影响,束团将很快变长并崩溃,马里兰大学的电子环设计用于方形束团和抛物线形束团的研究,环中需要用三个感应腔对束团进行纵向聚焦.文中首先介绍了方形束团和抛物线形束团的纵向束流动力学,推导了两种束团纵向聚焦所需要的5MHz重复率高压脉冲波形,并给出了实现纵向聚焦的感应腔设计及其特殊要求. 相似文献