全文获取类型
收费全文 | 33508篇 |
免费 | 5558篇 |
国内免费 | 3635篇 |
专业分类
化学 | 22530篇 |
晶体学 | 367篇 |
力学 | 2291篇 |
综合类 | 260篇 |
数学 | 4086篇 |
物理学 | 13167篇 |
出版年
2024年 | 128篇 |
2023年 | 718篇 |
2022年 | 1292篇 |
2021年 | 1328篇 |
2020年 | 1422篇 |
2019年 | 1312篇 |
2018年 | 1230篇 |
2017年 | 1188篇 |
2016年 | 1666篇 |
2015年 | 1673篇 |
2014年 | 1883篇 |
2013年 | 2533篇 |
2012年 | 2954篇 |
2011年 | 2955篇 |
2010年 | 2007篇 |
2009年 | 1957篇 |
2008年 | 2122篇 |
2007年 | 1809篇 |
2006年 | 1673篇 |
2005年 | 1417篇 |
2004年 | 1072篇 |
2003年 | 840篇 |
2002年 | 775篇 |
2001年 | 628篇 |
2000年 | 617篇 |
1999年 | 727篇 |
1998年 | 566篇 |
1997年 | 564篇 |
1996年 | 525篇 |
1995年 | 498篇 |
1994年 | 397篇 |
1993年 | 369篇 |
1992年 | 308篇 |
1991年 | 267篇 |
1990年 | 250篇 |
1989年 | 196篇 |
1988年 | 163篇 |
1987年 | 129篇 |
1986年 | 108篇 |
1985年 | 110篇 |
1984年 | 77篇 |
1983年 | 56篇 |
1982年 | 65篇 |
1981年 | 27篇 |
1980年 | 18篇 |
1979年 | 15篇 |
1978年 | 8篇 |
1976年 | 8篇 |
1975年 | 7篇 |
1957年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
22.
应用七级目测法对浙江中部地区27个夏熟作物田样点的杂草进行了优势等级调查,将所得数据转换成重要值,以杂草在27个样点中的重要值为运算指标,应用主成分分析法和图论聚类中的最小生成树法,对24种杂草的生态学相似性进行了比较,指出了长期使用单一的除草剂导致农田杂草种群迅速更迭的原因:杂草对除草剂的敏感性差异及不同杂草间的生态学特性的差异。 相似文献
23.
Differential inequality method, bounding function method and topological degree are applied to obtain the existence criterions of at least one solution for the general fourth-order differential equations under nonlinear boundary conditions, and many existing results are complemented. 相似文献
24.
Jie Xue Liang Chen Li Zhou Zhifeng Jia Yanping Wang Xinyuan Zhu Deyue Yan 《Journal of Polymer Science.Polymer Physics》2006,44(15):2050-2057
α‐Cyclodextrin (α‐CD) has been complexed with various poly(ethylene glycol) (PEG) derivatives in aqueous solution. It has been found that the end groups of PEG derivatives affect the complexation kinetics greatly, but have only a little influence on the thermodynamic behavior. By increasing the hydrophobicity of end groups, the complexation speeds up rapidly. On the other hand, the bulky end groups slow down the threading of polymeric guests into the cavity of CD. By changing the hydrophobicity and the size of end groups, the complexation rate can be adjusted in the range of several orders of magnitudes, which should be quite useful in the design of new supramolecular systems. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2050–2057, 2006 相似文献
25.
Laser driven shocks can lead to a dynamic failure, called film spallation. Here, we use a modified laser spallation set-up
to measure the dynamic adhesion of thin films and we propose a novel diagnostic technology. Based on correlation theory, new
spallation criteria for characterizing the progressive damage at the interface between the film and the substrate are established,
such as interface delamination, film spallation and film expulsion. With the help of the theory, the degree of damage and
the dimension of damage (i.e. fracture), such as the minimum width of delamination radius, the thickness of the film etc.,
are estimated. Experiments are carried out on epoxy/stainless steel and epoxy/Al, and the experimental results show that their
dynamic bonding strengths are about 25 MPa and 20 MPa, respectively. The detailed results, analyses and discussions are presented
in this paper.
Received: 6 February 2001 / Accepted: 3 December 2001 / Published online: 11 February 2002 相似文献
26.
In situ flying height testing technology is becoming more and more important in slider–disk interaction analysis and manufacturing quality control of disk drives and head-related components. Triple harmonic method is a quite promising choice for in situ flying height analysis, compared with other in situ methods reported up to now. This paper reports results of investigations on engineering issues of applying triple harmonic method for in situ flying height analysis. The paper reports results of analysis on the effects of various testing conditions on flying height testing repeatability and accuracy. Results suggest that working at reasonable high channel density and working on the ratio between third and first harmonics will be an advantage in terms of both flying height testing sensitivity and testing repeatability. Comparing with media thickness effect, the gap-length variation among different heads will be important if it is to study flying height difference among different heads and the testing is at high channel density. Also, it is suggested to work at AC erased track, in order to reduce the non-linearity caused by hard transition. 相似文献
27.
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
28.
On interpolatory divergence-free wavelets 总被引:1,自引:0,他引:1
We construct interpolating divergence-free multiwavelets based on cubic Hermite splines. We give characterizations of the relevant function spaces and indicate their use for analyzing experimental data of incompressible flow fields. We also show that the standard interpolatory wavelets, based on the Deslauriers-Dubuc interpolatory scheme or on interpolatory splines, cannot be used to construct compactly supported divergence-free interpolatory wavelets.
29.
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
30.