全文获取类型
收费全文 | 7809篇 |
免费 | 992篇 |
国内免费 | 95篇 |
专业分类
化学 | 69篇 |
晶体学 | 4篇 |
力学 | 55篇 |
综合类 | 47篇 |
数学 | 727篇 |
物理学 | 478篇 |
无线电 | 7516篇 |
出版年
2024年 | 26篇 |
2023年 | 104篇 |
2022年 | 126篇 |
2021年 | 156篇 |
2020年 | 248篇 |
2019年 | 151篇 |
2018年 | 150篇 |
2017年 | 243篇 |
2016年 | 321篇 |
2015年 | 454篇 |
2014年 | 724篇 |
2013年 | 609篇 |
2012年 | 737篇 |
2011年 | 689篇 |
2010年 | 472篇 |
2009年 | 443篇 |
2008年 | 495篇 |
2007年 | 548篇 |
2006年 | 447篇 |
2005年 | 367篇 |
2004年 | 295篇 |
2003年 | 278篇 |
2002年 | 246篇 |
2001年 | 170篇 |
2000年 | 92篇 |
1999年 | 63篇 |
1998年 | 41篇 |
1997年 | 36篇 |
1996年 | 38篇 |
1995年 | 20篇 |
1994年 | 13篇 |
1993年 | 8篇 |
1992年 | 9篇 |
1991年 | 5篇 |
1990年 | 7篇 |
1989年 | 5篇 |
1988年 | 5篇 |
1987年 | 4篇 |
1986年 | 9篇 |
1985年 | 13篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 8篇 |
1979年 | 5篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有8896条查询结果,搜索用时 15 毫秒
161.
162.
163.
164.
本文简要介绍了势能面搜索的两个主要问题,即过渡态搜索和全局势能面搜索,并针对这两个问题介绍了本研究组最近发展的3种方法,即限制最小化双子算法、偏置势函数辅助限制性最小化双子算法以及势能面随机行走方法.这3种方法均只需计算一阶梯度,能够用于快速自动化的搜索过渡态以及势能面.通过几个典型例子分别说明了3种方法的特点及优势. 相似文献
165.
166.
Francesco Chiti Romano Fantacci Andrea Tassi 《Wireless Communications and Mobile Computing》2015,15(7):1131-1141
This paper deals with the performance evaluation and optimization of an efficient hybrid automatic repeat request (HARQ) scheme suitable for applications delivered over lossy multicast communication channels. In particular, different from previously investigated strategies, this paper proposes a modified HARQ scheme based on the symbol combining principle (MHARQ‐SC) where multiple copies of a same packet are consecutively transmitted at each transmission opportunity. By considering as the performance metrics, the mean packet delivery delay, and the energy consumption per information packet, this paper presents suitable performance evaluation and optimization strategies tailored for multicast communications. For the sake of comparisons, it has been also analyzed, under the same operational conditions, the performance of different HARQ schemes optimized for multicast communications. Numerical results have been provided to validate the proposed performance evaluation and optimization approaches in the case of the MHARQ‐SC scheme. An important result devised here is that the reported analytical results clearly highlight the performance gain of the proposed strategy in comparison with all the other considered alternatives. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
167.
Wireless sensor and actuator networks combine a large number of sensors and a lower number of actuators that are connected with wireless medium, providing distributed sensing and executing appropriate tasks in a special region of interest. To accomplish effective sensing and acting tasks, efficient coordination mechanism among the nodes is required. As an attempt in this direction, this paper develops a collaborative control and estimation mechanism, which addresses the nodes coordination in a distributed manner. First, we propose a regional controllability‐based virtual force algorithm as an actuator deployment strategy to enhance area coverage after an initial random placement of actuators. During this process, a dynamic coordination mechanism is adopted to control nodes. This mechanism incorporates two components, namely, proportional‐integral‐derivative neural network and recursive least squares‐based Kalman filter algorithms. Taking advantage of feedback control and online learning technology, the proposed coordination mechanism schedules the corresponding nodes on the basis of the characteristics of current events, utilizes proportional‐integral‐derivative neural network controller inside each actuator to improve system transient and steady‐state responses, and deals with system state/parameter estimation problems according to the recursive least squares‐based Kalman filter algorithm, so as to achieve better control accuracy. Simulations demonstrate the effectiveness of our proposed methods. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
168.
Xin Guan Min Chen Tomoaki Ohtsuki 《Wireless Communications and Mobile Computing》2015,15(12):1633-1648
In sparse mobile wireless networks, normally, the mobile nodes are carried by people, and the moving activity of nodes always happens in a specific area, which corresponds to some specific community. Between the isolated communities, there is no stable communication link. Therefore, it is difficult to ensure the effective packet transmission among communities, which leads to the higher packet delivery delay and lower successful delivery ratio. Recently, an additional ferry node was introduced to forward packets between the isolated communities. However, most of the existing algorithms are working on how to control the trajectory of only one ferry work in the network. In this paper, we consider multiple ferries working in the network scenario and put our main focus on the optimal packet selection strategy, under the condition of mutual influence between the ferries and the buffer limitation. We introduce a non‐cooperative Bayesian game to achieve the optimal packet selection strategy. By maximizing the individual income of a ferry, we optimize the network performance on packet delivery delay and successful delivery ratio. Simulation results show that our proposed packet selection strategy improves the network performance on packet delivery delay and successful delivery ratio. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
169.
170.
Wei J Liu Z Chai Z Yuan J Lian J Shen GX 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2007,186(2):358-363
In this paper, a digital wireless transmission system based on 802.11b standard for magnetic resonance imaging (MRI) application is designed and built for the first time to eliminate the interference aroused by coil array cables. The analysis shows that the wireless receiver has a very high sensitivity to detect MRI signals. The modulation technique of differential quadrature phase shift keyed (DQPSK) can be applied to MRI data transmission with rate of 2 Mbps and bandwidth of 2 MHz. The bench test verifies that this wireless link has a dynamic range over 86 dB supporting up to 3 T MRI system data transmission. The 2D spin echo imaging of phantom is performed and the SNR of the image obtained by the wireless transmission can be comparable with that got by the coaxial cables. 相似文献