全文获取类型
收费全文 | 54929篇 |
免费 | 4628篇 |
国内免费 | 3109篇 |
专业分类
化学 | 27987篇 |
晶体学 | 521篇 |
力学 | 3243篇 |
综合类 | 229篇 |
数学 | 12284篇 |
物理学 | 18402篇 |
出版年
2024年 | 67篇 |
2023年 | 574篇 |
2022年 | 744篇 |
2021年 | 945篇 |
2020年 | 1104篇 |
2019年 | 1018篇 |
2018年 | 2087篇 |
2017年 | 2304篇 |
2016年 | 1965篇 |
2015年 | 1750篇 |
2014年 | 1917篇 |
2013年 | 2591篇 |
2012年 | 5445篇 |
2011年 | 4701篇 |
2010年 | 3408篇 |
2009年 | 3103篇 |
2008年 | 2258篇 |
2007年 | 2063篇 |
2006年 | 1936篇 |
2005年 | 5543篇 |
2004年 | 4714篇 |
2003年 | 2955篇 |
2002年 | 1072篇 |
2001年 | 796篇 |
2000年 | 603篇 |
1999年 | 797篇 |
1998年 | 634篇 |
1997年 | 586篇 |
1996年 | 529篇 |
1995年 | 504篇 |
1994年 | 426篇 |
1993年 | 365篇 |
1992年 | 453篇 |
1991年 | 403篇 |
1990年 | 372篇 |
1989年 | 292篇 |
1988年 | 243篇 |
1987年 | 180篇 |
1986年 | 134篇 |
1985年 | 130篇 |
1984年 | 91篇 |
1983年 | 76篇 |
1982年 | 70篇 |
1981年 | 40篇 |
1976年 | 80篇 |
1974年 | 40篇 |
1973年 | 48篇 |
1972年 | 39篇 |
1969年 | 37篇 |
1966年 | 37篇 |
排序方式: 共有10000条查询结果,搜索用时 343 毫秒
121.
合成了一种新型金属杂冠醚配合物[Mn6(4-amashz)6(DMF)6]·12DMF,并用X射线衍射法对其结构进行了表征.研究了该化合物修饰金电极催化氧还原的性质,结果表明,该化合物具有平面碟状结构,对在碱性溶液中的氧有显著的催化还原作用. 相似文献
122.
Improved sample wettability was obtained by oxygen plasma functionalization of pyrolytic graphite. The samples were exposed to highly dissociated oxygen plasma with the density of 1 × 1016 m−3, the electron temperature of about 5.5 eV and the density of neutral oxygen atoms of 8 × 1021 m−3 for 20 s. The surface wettability was measured by a contact angle of water drop. The contact angle dropped from original 112° down to about 1°. The functional groups were detected by XPS analyses. The survey spectrum showed a substantial increase of oxygen concentration on the surface, while high-resolution analyses showed additional oxygen was bonded onto the graphite surface in the form of C-O polar functional group responsible for the increase of the surface energy. 相似文献
123.
Kj?rand Iversen Per Nilsson 《International Journal of Computers for Mathematical Learning》2007,12(2):113-133
This paper focuses on the different ways in which students in lower secondary school (14–16 year olds) experience compound
random events, presented to them in the form of combined junctions. A carefully designed ICT environment was developed enabling
the students to interact with different representations of such structures. Data for the analysis was gathered from two interview
sessions. The analysis of the interaction is based on constructivist principles on learning; i.e. we adopted a student-oriented
perspective, taking into consideration the different ways students try to make sense of chance encounters.
Our results show how some students give priority to geometrical and physical concerns, and we discuss how seeking causal explanations
of random phenomena may have encouraged this. With respect to numerically oriented models a division strategy appears to stand
out as the preferred one. 相似文献
124.
125.
Baoqiang Li Dechang Jia Yu Zhou Qiaoling Hu Wei Cai 《Journal of magnetism and magnetic materials》2006
Chitosan/magnetite nanocomposite was synthesized induced by magnetic field via in situ hybridization in ambient condition. Results of XRD patterns and TEM micrographs indicated that magnetite particles with 10–20 nm were dispersed in chitosan homogeneously. An interesting result is that magnetite nanoparticles were assembled to form chain-like structures under the influence of the external magnetic field, which mimics the magnetite chains inside of magnetotatic bacteria. The saturated magnetization (Ms) of nano-magnetite in chitosan was 50.54 emu/g, which is as high as 54% of bulk magnetite. The remanence (Mr) and coercivity (Hc) were 4 emu/g and14.8 Oe, respectively, which indicated that magnetite nanoparticles were superparamagnetic. The key of route is that a pre-precipitated chitosan hydrogel membrane, used as chemical reactor, which controlled the precipitation of chitosan precipitation and in situ transformation of magnetite from the precursor simultaneously in the magnetic field environment. 相似文献
126.
Piotr Ko?cielniak 《Topology and its Applications》2007,154(9):1951-1955
Let M be a compact manifold with dimM?2. We prove that some iteration of the generic homeomorphism on M is semiconjugated to the shift map and has infinite topological entropy (Theorem 1.1). 相似文献
127.
系统如图1所示,滑轮与绳的质量忽略,绳不可伸长.设系统所有的部位都没有摩擦,物体B借助导轨(图中未画出来)被限定在沿物体C的右侧面运动,试求物体C的运动加速度。 相似文献
128.
The regression problem in learning theory is investigated with least square Tikhonov regularization schemes in reproducing
kernel Hilbert spaces (RKHS). We follow our previous work and apply the sampling operator to the error analysis in both the
RKHS norm and the L2 norm. The tool for estimating the sample error is a Bennet inequality for random variables with values in Hilbert spaces.
By taking the Hilbert space to be the one consisting of Hilbert-Schmidt operators in the RKHS, we improve the error bounds
in the L2 metric, motivated by an idea of Caponnetto and de Vito. The error bounds we derive in the RKHS norm, together with a Tsybakov
function we discuss here, yield interesting applications to the error analysis of the (binary) classification problem, since
the RKHS metric controls the one for the uniform convergence. 相似文献
129.
Covering numbers of precompact symmetric convex subsets of Hilbert spaces are investigated. Lower bounds are derived for sets containing orthogonal subsets with norms of their elements converging to zero sufficiently slowly. When these sets are convex hulls of sets with power-type covering numbers, the bounds are tight. The arguments exploit properties of generalized Hadamard matrices. The results are illustrated by examples from machine learning, neurocomputing, and nonlinear approximation. 相似文献
130.
We give counterexamples to the following conjecture of Auslander: given a finitely generated module M over an Artin algebra Λ, there exists a positive integer nM such that for all finitely generated Λ-modules N, if ExtΛi(M,N)=0 for all i?0, then ExtΛi(M,N)=0 for all i?nM. Some of our examples moreover yield homologically defined classes of commutative local rings strictly between the class of local complete intersections and the class of local Gorenstein rings. 相似文献