全文获取类型
收费全文 | 77979篇 |
免费 | 11949篇 |
国内免费 | 8765篇 |
专业分类
化学 | 41525篇 |
晶体学 | 735篇 |
力学 | 3383篇 |
综合类 | 458篇 |
数学 | 6628篇 |
物理学 | 21591篇 |
无线电 | 24373篇 |
出版年
2024年 | 335篇 |
2023年 | 2022篇 |
2022年 | 2717篇 |
2021年 | 3297篇 |
2020年 | 3238篇 |
2019年 | 2948篇 |
2018年 | 2506篇 |
2017年 | 2636篇 |
2016年 | 3455篇 |
2015年 | 3839篇 |
2014年 | 4513篇 |
2013年 | 5618篇 |
2012年 | 6635篇 |
2011年 | 6601篇 |
2010年 | 4886篇 |
2009年 | 4836篇 |
2008年 | 5078篇 |
2007年 | 4439篇 |
2006年 | 4097篇 |
2005年 | 3568篇 |
2004年 | 2677篇 |
2003年 | 2235篇 |
2002年 | 2131篇 |
2001年 | 1749篇 |
2000年 | 1489篇 |
1999年 | 1686篇 |
1998年 | 1302篇 |
1997年 | 1165篇 |
1996年 | 1145篇 |
1995年 | 1010篇 |
1994年 | 872篇 |
1993年 | 746篇 |
1992年 | 603篇 |
1991年 | 524篇 |
1990年 | 416篇 |
1989年 | 310篇 |
1988年 | 225篇 |
1987年 | 198篇 |
1986年 | 198篇 |
1985年 | 161篇 |
1984年 | 109篇 |
1983年 | 106篇 |
1982年 | 66篇 |
1981年 | 48篇 |
1980年 | 30篇 |
1979年 | 21篇 |
1977年 | 21篇 |
1976年 | 22篇 |
1975年 | 24篇 |
1957年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
181.
分子生物学中基因无方向的反向基因组重排问题在数学上已被证明是一个NP困难问题.基于断点图的概念,给出一个时间复杂性为O(max{b^(π),nb(π)}),空间复杂性为0(n)的求其近似最优解的算法.其中n为基因组中基因个数,π=(π1,π2,…,πn)表示n个基因的一种排列,b(π)表示排列π中的断点数.数据实验的结果表明,该近似算法可以求得较好的结果. 相似文献
182.
183.
Recent technological advances have made it possible to support long lifetime and large volume streaming data transmissions
in sensor networks. A major challenge is to maximize the lifetime of battery-powered sensors to support such transmissions.
Battery, as the power provider of the sensors, therefore emerges as the key factor for achieving high performance in such
applications. Recent study in battery technology reveals that the behavior of battery discharging is more complex than we
used to think. Battery powered sensors might waste a huge amount of energy if we do not carefully schedule and budget their
discharging. In this paper we study the effect of battery behavior on routing for streaming data transmissions in wireless
sensor networks. We first give an on-line computable energy model to mathematically model battery discharge behavior. We show
that the model can capture and describe battery behavior accurately at low computational complexity and thus is suitable for
on-line battery capacity computation. Based on this battery model we then present a battery-aware routing (BAR) protocol to
schedule the routing in wireless sensor networks. The routing protocol is sensitive to the battery status of routing nodes
and avoids energy loss. We use the battery data from actual sensors to evaluate the performance of our protocol. The results
show that the battery-aware protocol proposed in this paper performs well and can save a significant amount of energy compared
to existing routing protocols for streaming data transmissions. Network lifetime is also prolonged with maximum data throughput.
As far as we know, this is the first work considering battery-awareness with an accurate analytical on-line computable battery
model in sensor network routing. We believe the battery model can be used to explore other energy efficient schemes for wireless
networks as well. 相似文献
184.
Seyeong Choi Alouini M.-S. Qaraqe K.A. Hong-Chuan Yang 《Wireless Communications, IEEE Transactions on》2008,7(2):495-499
We propose and analyze new finger assignment techniques that are applicable for RAKE receivers in the soft handover (SHO) region. Specifically, extending the results for the case of two-base station (BS), we consider the multi-BS situation, attack the statistics of several correlated generalized selection combining (GSC) stages, and provide closed-form expressions for the statistics of the output signal-to-noise ratio (SNR). By investigating the tradeoff among the error performance, the average number of required path estimations/comparisons, and the SHO overhead, we show through numerical examples that the new schemes offer commensurate performance in comparison with more complicated GSC-based diversity systems while requiring a smaller estimation load and SHO overhead. 相似文献
185.
学生学习知识,一般来说,是从生动的具体的直观开始的,生动的直观能使学生获得关于事物生动而具体的感性认识。但是,感性认识并没有揭露事物的本质,人们对事物本质的认识要通过一系列的抽象来完成。 相似文献
186.
Yang Dai Faquan Li Xuewu Cheng Zhiling Jiang Shunsheng Gong 《Optics & Laser Technology》2007,39(7):1374-1379
In Shack–Hartmann wavefront sensor (SHWS), the behavior of the irradiance pattern produced by the micro-lens array is important for an accurate centroid estimation. In this paper, the behavior of a micro-lens array in SHWS is analyzed using Fourier optics, and reveals that in addition to the main, expected spots, secondary spots with smaller intensities also appeared as a result of diffraction by the small dimensions of the micro-lens and interference from the different micro-lenses. This result is confirmed by comparing with a irradiance pattern taken from an actual SHWS. The additional error in centroid estimation caused by these secondary spots is discussed and relationship to the parameters of the micro-lens of SHWS is analyzed. 相似文献
187.
188.
Rhythmic growth of ring‐banded spherulites in blends of liquid crystalline methoxy‐poly(aryl ether ketone) (M‐PAEK) and poly(aryl ether ether ketone) (PEEK) has been investigated by means of differential scanning calorimetry (DSC), polarized light microscopy (PLM), and scanning electron microscopy (SEM) techniques. The measurements reveal that the formation of the rhythmically grown ring‐banded spherulites in the M‐PAEK/PEEK blends is strongly dependent on the blend composition. In the M‐PAEK‐rich blends, upon cooling, an unusual ring‐banded spherulite is formed, which is ascribed to structural discontinuity caused by a rhythmic radial growth. For the 50:50 M‐PAEK/PEEK blend, ring‐banded spherulites and individual PEEK spherulites coexist in the system. In the blends with PEEK as the predominant component, M‐PAEK is rejected into the boundary of PEEK spherulites. The cooling rate and crystallization temperature have great effect on the phase behavior, especially the ring‐banded spherulite formation in the blends. In addition, the effects of M‐PAEK phase transition rate and phase separation rate on banded spherulite formation is discussed. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3011–3024, 2007 相似文献
189.
Pengbo Jiang Tiexin Cheng Yan Liu Xianghao Cui Yingli Bi Kaiji Zhen 《Reaction Kinetics and Catalysis Letters》2004,82(1):49-56
The novel phase transfer catalysts S-8 [4-(dimethyloctylammonium) propansultan] and DB-X [1,4-bis(triethylmethylammonium)benzene dibromide] were synthesized and employed for high conversion synthesis of dichlorocyclopropane from various olefins.This revised version was published online in December 2005 with corrections to the Cover Date. 相似文献
190.
随着系统芯片(SoC)集成更多的功能并采用更先进的工艺,它所面临的高性能与低功耗的矛盾越来越突出.动态电压调整(DVS)技术可以在不影响处理器性能的前提下,通过性能预测软件根据处理器的繁忙程度调整处理器的工作电压和工作频率,达到降低芯片功耗的目的.文中讨论了DVS技术降低功耗的可能性,介绍了如何利用两种不同的DVS技术让处理器根据当前的工作负荷运行在不同的性能水平上,以节省不必要的功耗. 相似文献