首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   664篇
  免费   17篇
  国内免费   18篇
化学   239篇
晶体学   6篇
力学   8篇
综合类   11篇
数学   102篇
物理学   333篇
  2024年   4篇
  2023年   1篇
  2022年   5篇
  2021年   11篇
  2020年   23篇
  2019年   17篇
  2018年   18篇
  2017年   6篇
  2016年   19篇
  2015年   31篇
  2014年   41篇
  2013年   49篇
  2012年   43篇
  2011年   44篇
  2010年   38篇
  2009年   61篇
  2008年   73篇
  2007年   36篇
  2006年   32篇
  2005年   18篇
  2004年   22篇
  2003年   22篇
  2002年   14篇
  2001年   7篇
  2000年   15篇
  1999年   7篇
  1998年   8篇
  1997年   6篇
  1996年   4篇
  1995年   3篇
  1994年   5篇
  1993年   2篇
  1992年   2篇
  1991年   1篇
  1989年   2篇
  1988年   1篇
  1987年   2篇
  1985年   1篇
  1981年   1篇
  1978年   1篇
  1977年   1篇
  1976年   1篇
  1970年   1篇
排序方式: 共有699条查询结果,搜索用时 15 毫秒
71.
黎占亭 《有机化学》2000,20(5):655-662
轮烷是一类由两端带有大的基团的线性分子和有机环化合物组成的互相锁连的分子化合物。主要综述了近年来这类超分子化合物的合成方法进展、在合成中的应用及其作为分子器件方面的研究进展。  相似文献   
72.
开挖过程的非线性理论分析   总被引:4,自引:2,他引:2       下载免费PDF全文
本文从岩土介质力学非线性和变形破坏过程自组织非线性的角度对开挖过程岩土体非线性行为进行了探讨。以此为依据,分析了岩土从微观破坏、滑移面形成、直至整体变形破坏全过程的自组织发生机理,在此基础上,建立了伴随开挖过程岩土体变形破坏的非线性动力学描述方程。  相似文献   
73.
针对现有视频数据库系统存在的不足,根据视频数据的特点,提出并实现了一种基于MPEG-7标准的视频数据组织与管理方法.该方法将视频内容的描述数据按照不同层次的视频内容单元进行组织,并针对不同的描述数据,分别生成结构索引、文本索引和特征索引,以满足基于内容的视频浏览和检索的需求.实验证明,通过视频描述数据库对视频数据进行管理,不仅降低了服务器的存储要求,而且为基于内容的视频检索奠定了基础,具有很好的通用性和可扩展性.  相似文献   
74.
75.
Syndiotactic polypropylene (sPP) was quenched from the melt in an ice‐water bath, and changes in the structural organization, during the aging time, were followed by X‐rays, differential scanning calorimetry, and transport properties of dichloromethane at low activities. After 1 month, an increase of crystallinity from 19 to 26% was observed. In addition, the results of sorption and diffusion indicated a consistent increase of an intermediate phase not crystalline, yet impermeable to the vapors. The study of the mechanical properties showed that there is a remarkable increase of all the mechanical parameters with the aging time, and this effect was associated to the increase of the intermediate phase. The elastic modulus increased three times in the first hours of aging. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 173–180, 1999  相似文献   
76.
    
Previous works suggest that musical networks often present the scale-free and the small-world properties. From a musician's perspective,the most important aspect missing in those studies was harmony. In addition to that,the previous works made use of outdated statistical methods. Traditionally,least-squares linear regression is utilised to fit a power law to a given data set. However,according to Clauset et al. such a traditional method can produce inaccurate estimates for the power law exponent. In this paper,we present an analysis of musical networks which considers the existence of chords (an essential element of harmony). Here we show that only 52.5% of music in our database presents the scale-free property,while 62.5% of those pieces present the small-world property. Previous works argue that music is highly scale-free; consequently,it sounds appealing and coherent. In contrast,our results show that not all pieces of music present the scale-free and the small-world properties. In summary,this research is focused on the relationship between musical notes (Do,Re,Mi,Fa,Sol,La,Si,and their sharps) and accompaniment in classical music compositions. More information about this research project is available at https://eden.dei.uc.pt/~vitorgr/MS.html.https://doi.org/10.1209/0295-5075/121/38005  相似文献   
77.
    
Subways are crucial in modern transportation systems of metropolises. To quantitatively evaluate the potential risks of subway networks suffered from natural disasters or deliberate attacks, real data from seven Chinese subway systems are collected and their population distributions and anti-risk capabilities are analyzed. Counterintuitively, it is found that transfer stations with large numbers of connections are not the most crucial, but the stations and lines with large betweenness centrality are essential, if subway networks are being attacked. It is also found that cycles reduce such correlations due to the existence of alternative paths. To simulate the data-based observations, a network model is proposed to characterize the dynamics of subway systems under various intensities of attacks on stations and lines. This study sheds some light onto risk assessment of subway networks in metropolitan cities.https://doi.org/10.1209/0295-5075/121/48004  相似文献   
78.
    
Networks are designed to satisfy given objectives under specific requirements. While the static connectivity of networks is normally analyzed and corresponding design principles for static robustness are proposed, the challenge still remains of how to design survivable networks that maintain the required level of connectivity during their whole lifespan, against component aging. We introduce network survivability as a new concept to evaluate the networks overall performance during their whole lifespan, considering both network connectivity and network duration. We develop a framework for designing a survivable network by allocating the expected lifetimes of its components, given a limited budget. Based on percolation theory and simulation, we find that the maximal network survivability can be achieved with a quantitative balance between network duration and connectivity. For different survivability requirements, we find that the optimal design can be separated into two categories: strong dependence of lifetime on node's degree leads to larger network lifetime, while weak dependence generates stronger network connectivity. Our findings could help network design, by providing a quantitative prediction of network survivability based on network topology.https://doi.org/10.1209/0295-5075/122/36003  相似文献   
79.
    
A new and simple method for fabrication of nanofiber scaffolds with gradations in fiber organization is reported. The nanofiber organization, achieved by deposition of random fibers on the uniaxially aligned nanofiber mat in a gradient manner, directed morphological changes of applied adipose‐derived stem cells. These morphological changes and resultant biochemical changes can help mimic the structural orientation of complex biomechanical structures like the collagen fiber structure at the tendon‐to‐bone insertion site. In addition, chemical gradients can be established through nanoencapsulation in this novel scaffold allowing for enhanced biomedical applications.

  相似文献   

80.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号