全文获取类型
收费全文 | 61519篇 |
免费 | 10687篇 |
国内免费 | 9234篇 |
专业分类
化学 | 43925篇 |
晶体学 | 1064篇 |
力学 | 3822篇 |
综合类 | 795篇 |
数学 | 7182篇 |
物理学 | 24652篇 |
出版年
2024年 | 205篇 |
2023年 | 1224篇 |
2022年 | 2377篇 |
2021年 | 2430篇 |
2020年 | 2547篇 |
2019年 | 2414篇 |
2018年 | 2086篇 |
2017年 | 2183篇 |
2016年 | 2895篇 |
2015年 | 3060篇 |
2014年 | 3639篇 |
2013年 | 4617篇 |
2012年 | 5518篇 |
2011年 | 5565篇 |
2010年 | 4062篇 |
2009年 | 3947篇 |
2008年 | 4291篇 |
2007年 | 3789篇 |
2006年 | 3463篇 |
2005年 | 2979篇 |
2004年 | 2281篇 |
2003年 | 1784篇 |
2002年 | 1709篇 |
2001年 | 1442篇 |
2000年 | 1431篇 |
1999年 | 1317篇 |
1998年 | 992篇 |
1997年 | 901篇 |
1996年 | 896篇 |
1995年 | 829篇 |
1994年 | 714篇 |
1993年 | 635篇 |
1992年 | 544篇 |
1991年 | 451篇 |
1990年 | 421篇 |
1989年 | 314篇 |
1988年 | 253篇 |
1987年 | 219篇 |
1986年 | 192篇 |
1985年 | 162篇 |
1984年 | 119篇 |
1983年 | 126篇 |
1982年 | 65篇 |
1981年 | 60篇 |
1980年 | 45篇 |
1979年 | 33篇 |
1977年 | 23篇 |
1976年 | 21篇 |
1975年 | 23篇 |
1972年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
42.
ZHONG Hui WAN Mingxi JIANG Yifeng WANG Supin 《声学学报:英文版》2006,25(2):104-115
The present study was to investigate the differential imaging method for detecting HIFU (High Intensity Focused Ultrasound)-induced lesions and the estimation of variation of attenuation for lesion evaluation with log spectral difference algorithm. Experiment results of bovine muscle and liver in vitro were acquired. Several algorithms for lesion detection - Absolute Difference (AD), Sum Absolute Differences (SAD) and Sum Squared Differences (SSD) - were analyzed with several window sizes and threshold values. Then three attenuation parameters were compared to evaluate the degree of tissue damage. It was found that variation of the mean attenuation △α was an effective parameter to evaluate lesions. 相似文献
43.
Haifeng Yang Guangming XieTianguang Chu Long Wang 《Nonlinear Analysis: Theory, Methods & Applications》2006
In the paper, commuting and stable feedback design for switched linear systems is investigated. This problem is formulated as to build up suitable state feedback controller for each subsystem such that the closed-loop systems are not only asymptotically stable but also commuting each other. A new concept, common admissible eigenvector set (CAES), is introduced to establish necessary/sufficient conditions for commuting and stable feedback controllers. For second-order systems, a necessary and sufficient condition is established. Moreover, a parametrization of the CAES is also obtained. The motivation comes from stabilization of switched linear systems which consist of a family of LTI systems and a switching law specifying the switching between them, where if all the subsystems are stable and commuting each other, then the total system is stable under arbitrary switching. 相似文献
44.
本文采用椭圆偏振光谱法研究了剂量为1×1016—3×1012cm-2的As+注入硅,及其在700℃退火后的光学性质。得出:当As+注入剂量增大到某一程度后,便呈非晶特性。低于临界剂量的样品,其n-λ,ε2-λ关系曲线随剂量的增大而往下方移动,呈有规律变化;退火后,在大于4000?波段,n-λ与ε2-λ曲线基本恢复到单晶硅状态。但在小于4000?的紫外区却未完全恢复,注入剂量越大,偏离单晶硅就越大。并指出,紫外光区是离子注入硅的信息敏感区;用有效质量模型计算出注入剂量与损伤度的关系。计算结果与实验符合得较好。
关键词: 相似文献
45.
Liyue Liu Yingge Yang Yafei Zhang 《Physica E: Low-dimensional Systems and Nanostructures》2004,24(3-4):343-348
The electrical conductivity was investigated for multi-walled carbon nanotubes (MWNTs) dissolved in chloroform and toluene, respectively. The electrical conductivity remarkably increased with increase in the content of MWNTs, which is in accordance with Archie's equation . Furthermore, a hypothesis of the electronic transport process was proposed to explain the difference between the solution and the solid compound. In addition, the temperature dependence of the electrical conductivity shows that log σ vs. 1/T exist in a good linear relationship. The activation energy of the electrical conductivity decreased with increase in concentration and an inflexion was observed at 60 °C in MWNT/toluene solution. 相似文献
46.
激光等离子体探针及应用研究 总被引:6,自引:1,他引:5
本文介绍用阴影、纹影相干涉方法确定等离子体折射系数变化的原理。分析了等离子体中的非线性效应和对光探测系统的基本要求。并简要叙述了实验上采用的后向喇曼探测束,给出了等离子体干涉、纹影和阴影的测量结果。 相似文献
47.
The Iyengar Type Inequalities with Exact Estimations and the Chebyshev Central Algorithms of Integrals 总被引:3,自引:0,他引:3
Xing Hua WANG Shi Jun YANG 《数学学报(英文版)》2005,21(6):1361-1376
In this paper, both low order and high order extensions of the Iyengar type inequality are obtained. Such extensions are the best possible in the same sense as that of the Iyengar inequality. hzrthermore, the Chebyshev central algorithms of integrals for some function classes and some related problems are also considered and investigated. 相似文献
48.
49.
证明了当超越亚纯函数的级小于1时,其Norel例外值最多只有一个.由于存在任何级的整函数,因此一个例外值总可达到,故所得结果不能再改进。 相似文献
50.
Fei Chang Dongheng Zhang Haijian Yang Haibin Song 《Journal of organometallic chemistry》2004,689(5):936-946
The synthesized 1-aryliminomethylenylnaphthalen-2-ol derivatives reacted with nickel chloride to form bis(1-aryliminomethylenylnaphthalen-2-oxy)nickel complexes. All resultant compounds were structurally characterized by elemental analyses, IR and H NMR, and the structures of the formed complexes were elucidated by X-ray crystal structure analysis. The complexes show high catalytic activities for the vinyl polymerization of norbornene in the presence of methylaluminoxane. The catalytic activity variations have been followed by gas chromatography through monitoring the conversion of norbornene. 相似文献