全文获取类型
收费全文 | 13560篇 |
免费 | 3765篇 |
国内免费 | 1291篇 |
专业分类
化学 | 1158篇 |
晶体学 | 31篇 |
力学 | 2470篇 |
综合类 | 672篇 |
数学 | 5064篇 |
物理学 | 9221篇 |
出版年
2024年 | 69篇 |
2023年 | 283篇 |
2022年 | 277篇 |
2021年 | 368篇 |
2020年 | 258篇 |
2019年 | 397篇 |
2018年 | 202篇 |
2017年 | 459篇 |
2016年 | 454篇 |
2015年 | 620篇 |
2014年 | 1110篇 |
2013年 | 746篇 |
2012年 | 755篇 |
2011年 | 942篇 |
2010年 | 938篇 |
2009年 | 996篇 |
2008年 | 1168篇 |
2007年 | 863篇 |
2006年 | 824篇 |
2005年 | 765篇 |
2004年 | 796篇 |
2003年 | 771篇 |
2002年 | 667篇 |
2001年 | 578篇 |
2000年 | 468篇 |
1999年 | 400篇 |
1998年 | 327篇 |
1997年 | 318篇 |
1996年 | 294篇 |
1995年 | 309篇 |
1994年 | 231篇 |
1993年 | 199篇 |
1992年 | 179篇 |
1991年 | 162篇 |
1990年 | 169篇 |
1989年 | 128篇 |
1988年 | 38篇 |
1987年 | 32篇 |
1986年 | 10篇 |
1985年 | 14篇 |
1984年 | 4篇 |
1983年 | 12篇 |
1982年 | 9篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1959年 | 2篇 |
1951年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
951.
952.
基于人眼视觉系统可感知的阈值信噪比,提出预测红外热像仪最小可分辨温差(MRTD)方法。该方法根据频谱理论和红外热成像系统的二维调制传递函数,并结合匹配滤波器的概念,描述4条带标准靶的空间频谱函数。利用此空间频谱函数研究了4条带标准靶经过红外热成像系统的调制传递之后人眼视觉系统的感知信噪比。基于人眼视觉系统可感知的阈值信噪比,建立一个新的MRTD模型。依据此MRTD模型和视距模型,实现红外热像仪视距的预测。仿真结果表明,该方法预测的视距与实验数据有很好的一致性。 相似文献
953.
954.
955.
对于2阶Duffing系统与3阶单模激光Lorenz系统,用主动控制方法设计了一种同步控制器,实现二者之间的缩阶反同步。依据罗斯 霍维兹判据,给出误差系统在原点处渐进稳定的条件。数值试验证实:在控制器作用下,2阶Duffing系统的状态变量与3阶单模激光Lorenz投影子系统的状态变量振动的幅值相同但方向相反,且在随机噪声干扰下反同步仍能很好地实现。这表明所设计的控制器对于不同阶的Duffing系统与Lorenz系统的缩阶反同步的实现是可行的、有效的,而且具有很好的鲁棒性。 相似文献
956.
In this paper, the generalized synchronization of two unidirectionally coupled Ginzburg-Landau equations is studied theoretically. It is demonstrated that the drive-response system has bounded attraction domain and compact attractors. It is derived that the correction equation has asymptotically stable zero solutions under certain conditions and that the sufficient conditions for smooth generalized synchronization and Hölder continuous generalized synchronization exist in the coupling system. Numerical result analysis shows the correctness of theory. 相似文献
957.
A new kind of nonlinear fractional-order chaotic phenomenon in coupled systems: coexistence of anti-phase and complete synchronization 下载免费PDF全文
In this paper, we have found a kind of interesting nonlinear phenomenon hybrid synchronization in linearly coupled fractional-order chaotic systems. This new synchronization mechanism, i.e., part of state variables are anti- phase synchronized and part completely synchronized, can be achieved using a single linear controller with only one drive variable. Based on the stability theory of the fractional-order system, we investigated the possible existence of this new synchronization mechanism. Moreover, a helpful theorem, serving as a determinant for the gain of the controller, is also presented. Solutions of coupled systems are obtained numerically by an improved Adams Bashforth-Moulton algorithm. To support our theoretical analysis, simulation results are given. 相似文献
958.
A method of interference correction for improving the sensitivity of non-dispersive infrared (NDIR) gas analysis system is demonstrated. Based on the proposed method, the interference due to water vapor and carbon dioxide in the NDIR NO analyzer is corrected. After interference correction, the absorbance signal at the NO filter channel is only controlled by the absorption of NO, and the sensitivity of the analyzer is improved greatly. In the field experiment for pollution source emission monitoring, the concentration trend of NO monitored by NDIR analyzer is in good agreement with the differential optical absorption spectroscopy NO analyzer. Small variations of NO concentration can also be resolved, and the measuring correlation coefficient of the two analyzers is 94.28%. 相似文献
959.
We experimentally investigate the transmission performance of 60-GHz signals over standard single-mode fiber (SSMF) and wireless links at different bit rates. Experimental results show that in a transmission of over 10-km SSMF and 1.3-m wireless link, bit rate reaches up to 5 Gb/s and bit error rate (BER) is less than 10 4 . The main limiting factor in such radio-over-fiber (ROF) systems is intersymbol interferences caused by the so-called walk-off effect when BER is below 10 8 . In addition, a transmission of over 20-km SSMF without chromatic dispersion compensation is briefly investigated. For a BER of 10 8 , the optical penalty is 2 dB. 相似文献
960.
We propose employing multi-beam propagating technology to mitigate the influence of atmospheric scintillation to the wireless optical code division multiple access (WOCDMA) system and then deduce the bit error rate (BER) formulas of systems in weak and strong scintillations,respectively.According to simulation experiment results,multi-beam propagation can improve the system performance very well compared with single-beam propagating technique.Moreover,the more beams we use,the better the performance we get.When the received optical power is-30 dBm,the BER of the system employing four beams is 5 and 1 dB lower than that of using single-beam propagating technique in weak and strong scintillations,respectively. 相似文献