全文获取类型
收费全文 | 148480篇 |
免费 | 12441篇 |
国内免费 | 13617篇 |
专业分类
化学 | 74993篇 |
晶体学 | 1201篇 |
力学 | 3025篇 |
综合类 | 1123篇 |
数学 | 14809篇 |
物理学 | 24004篇 |
无线电 | 55383篇 |
出版年
2024年 | 315篇 |
2023年 | 1480篇 |
2022年 | 2914篇 |
2021年 | 3533篇 |
2020年 | 3992篇 |
2019年 | 3402篇 |
2018年 | 3110篇 |
2017年 | 4737篇 |
2016年 | 5008篇 |
2015年 | 5160篇 |
2014年 | 7198篇 |
2013年 | 10262篇 |
2012年 | 10506篇 |
2011年 | 9410篇 |
2010年 | 7461篇 |
2009年 | 9014篇 |
2008年 | 9586篇 |
2007年 | 10160篇 |
2006年 | 9267篇 |
2005年 | 7848篇 |
2004年 | 7177篇 |
2003年 | 6147篇 |
2002年 | 6722篇 |
2001年 | 4315篇 |
2000年 | 3829篇 |
1999年 | 3390篇 |
1998年 | 2969篇 |
1997年 | 2416篇 |
1996年 | 2079篇 |
1995年 | 1913篇 |
1994年 | 1613篇 |
1993年 | 1300篇 |
1992年 | 1152篇 |
1991年 | 803篇 |
1990年 | 653篇 |
1989年 | 574篇 |
1988年 | 421篇 |
1987年 | 317篇 |
1986年 | 302篇 |
1985年 | 245篇 |
1984年 | 255篇 |
1983年 | 145篇 |
1982年 | 218篇 |
1981年 | 186篇 |
1980年 | 197篇 |
1979年 | 187篇 |
1978年 | 169篇 |
1977年 | 128篇 |
1976年 | 109篇 |
1973年 | 67篇 |
排序方式: 共有10000条查询结果,搜索用时 906 毫秒
881.
Yozo Miura Hiroyuki Dote Hiroyuki Kubonishi Kenichi Fukuda Tomoko Saka 《Journal of polymer science. Part A, Polymer chemistry》2007,45(6):1159-1169
A dendritic macroinitiator having 16 TEMPO‐based alkoxyamines, Star‐16 , was prepared by the reaction of a dendritic macroinitiator having eight TEMPO‐based alkoxyamines, [G‐3]‐OH , with 4,4′‐bis(chlorocarbonyl)biphenyl. The nitroxide‐mediated radical polymerization (NMRP) of styrene (St) from Star‐16 gave 16‐arm star polymers with PDI of 1.19–1.47, and NMPR of 4‐vinylpyridine from the 16‐arm star polymer gave 16‐arm star diblock copolymers with PDI of 1.30–1.43. The ring‐opening polymerization of ε‐caprolactone from [G‐3]‐OH and the subsequent NMRP of St gave AB8 9‐miktoarm star copolymers with PDI of 1.30–1.38. The benzyl ether linkages of the 16‐arm star polymers and the AB8 9‐miktoarm star copolymers were cleaved by treating with Me3SiI, and the resultant poly(St) arms were investigated by size exclusion chromatography (SEC). The SEC results showed PDIs of 1.23–1.28 and 1.18–1.22 for the star polymers and miktoarm stars copolymers, respectively, showing that they have well‐controlled poly(St) arms. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1159–1169, 2007. 相似文献
882.
N-Acylalkylation of neutral and anionic N-nucleophiles with α-halocarbonyl compounds was investigated by quantum chemical methods
in terms of the density functional theory and by experimental methods for 2,3-dihydroimidazo[2,1-b]quinazolin-1(10)H-5-one, its N-anion, and simpler model structures. High reactivity of these reagents is determined primarily by stabilization
of transition states (TS) by bridge bonds involving halogen or nitrogen atoms rather than by conjugation, as has been commonly
accepted. Bridged TS are formed by both the substitution mechanism S
N
2 and the addition-elimination mechanism. α-Haloalkyl-substituted zwitterions, which are potential intermediates of stepwise
N-acylalkylation of neutral N-nucleophiles, do not exist in the isolated state, but they are rather efficiently stabilized
upon solvation. These zwitterions, as well as analogous O-anions generated from anionic N-nucleophiles, can serve as intermediates
of N-acylalkylation, as was demonstrated by localization of the corresponding TS.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1150–1164, June, 2007. 相似文献
883.
Ignacio Villanueva 《Mathematische Nachrichten》2003,250(1):98-103
We clarify and prove in a simpler way a result of Taskinen about symmetric operators on C(Kn), K an uncountable metrizable compact space. To do this we prove that, for any compact space K and any n ∈ ?, the symmetric injective n–tensor product of C(K), , is complemented in C(BC(K)*), a result of independent interest. The techniques we develop allow us to extend the result in several directions. We also show that the hypothesis of metrizability and uncountability cannot be removed. 相似文献
884.
C. Bréchignac Ph. Cahuzac F. Carlier C. Colliex M. de Frutos N. Kébaïli J. Le Roux A. Masson B. Yoon 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2003,24(1-3):265-268
We studied shape relaxation of nano-fractal islands,
during annealing, after their growth from antimony cluster
deposition on graphite surface. Annealing at
180°C shows evidence of an increase
of the fractal branch width with time followed by branch
fragmentation, without changing the fractal dimension. The time
evolution of the width of the arm suggests the surface
self-diffusion mechanism as the main relaxation process. With
Monte Carlo simulations, we confirmed the observed behavior.
Comparison is done with our previous results on fragmentation of
nano-fractal silver islands when impurity added to the incident
cluster promotes rapid fragmentation by surface self-diffusion
enhancement [1]. 相似文献
885.
Algebraic properties of cryptosystem PGM 总被引:2,自引:0,他引:2
In the late 1970s Magliveras invented a private-key cryptographic system calledPermutation Group Mappings (PGM). PGM is based on the prolific existence of certain kinds of factorization sets, calledlogarithmic signatures, for finite permutation groups. PGM is an endomorphic system with message space ℤ|G| for a given finite permutation groupG. In this paper we prove several algebraic properties of PGM. We show that the set of PGM transformations ℐ
G
is not closed under functional composition and hence not a group. This set is 2-transitive on ℤ|G| if the underlying groupG is not hamiltonian and not abelian. Moreover, if the order ofG is not a power of 2, then the set of transformations contains an odd permutation. An important consequence of these results
is that the group generated by the set of transformations is nearly always the symmetric group ℒ|G|. Thus, allowing multiple encryption, any permutation of the message space is attainable. This property is one of the strongest
security conditions that can be offered by a private-key encryption system.
S. S. Magliveras was supported in part by NSF/NSA Grant Number MDA904-82-H0001, by U.S. West Communications, and by the Center
for Communication and Information Science of the University of Nebraska. 相似文献
886.
887.
本文从智能网的基本概念入手,论述了利用 ZXJ10程控交换系统为发展智能网业务提供支持功能的技术策略以及分阶段实现的过程,并对各个阶段能够实现的业务作了简要介绍。 相似文献
888.
M. Dubé C. Daneault V. Vuorinen M. Alava M. Rost 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,56(1):15-26
We investigate the structure and dynamics of the interface between
two immiscible liquids in a three-dimensional disordered porous
medium. We apply a phase-field model that includes explicitly
disorder and discuss both spontaneous and forced imbibition.
The structure of the interface is dominated by a length scale
ξ× which arises from liquid conservation. We further show
that disorder in the capillary and permeability act on different
length scales and give rise to different scalings and
structures of the interface properties. We conclude with a
range of applications. 相似文献
889.
首先将无线传感器网络的路由问题转化成求解最小Steiner树问题,然后给出了求解无线传感器网络路由的蚁群优化算法,并对算法的收敛性进行了证明.最后对找到最优解后信息素值的变化进行了分析.即在限制信息素取值的条件下,当迭代次数充分大时,该算法能以任意接近于1的概率找到最优解,并且当最优解找到后,最优树边上的信息素单调增加,而最优解以外边上的信息素在有限步达到最小值. 相似文献
890.
A theoretical study of the electron dynamics in image potential states on Cu(1 0 0) surfaces with different types of adsorbates is presented. Scattering of the image state electron by an adsorbate induces inter-band and intra-band transitions leading respectively to the population decay and to the dephasing of the image state. We compare results obtained with low coverage (typically 1 adsorbate atom per 1000 surface atoms) Cs, Ar, and a model electronegative adsorbates. As follows from our results, Cs adsorbates lead to both appreciable dephasing and decay, while electronegative adsorbates mostly affect the dephasing rate. The effect of low coverage Ar adsorbates is small, consistent with their neutrality. 相似文献