全文获取类型
收费全文 | 74757篇 |
免费 | 8553篇 |
国内免费 | 5346篇 |
专业分类
电工技术 | 6622篇 |
技术理论 | 2篇 |
综合类 | 6361篇 |
化学工业 | 10279篇 |
金属工艺 | 4179篇 |
机械仪表 | 5094篇 |
建筑科学 | 6026篇 |
矿业工程 | 2412篇 |
能源动力 | 2245篇 |
轻工业 | 6567篇 |
水利工程 | 1951篇 |
石油天然气 | 3455篇 |
武器工业 | 1013篇 |
无线电 | 9537篇 |
一般工业技术 | 7332篇 |
冶金工业 | 2910篇 |
原子能技术 | 1134篇 |
自动化技术 | 11537篇 |
出版年
2024年 | 399篇 |
2023年 | 1296篇 |
2022年 | 2608篇 |
2021年 | 3620篇 |
2020年 | 2642篇 |
2019年 | 2073篇 |
2018年 | 2234篇 |
2017年 | 2644篇 |
2016年 | 2377篇 |
2015年 | 3556篇 |
2014年 | 4520篇 |
2013年 | 5208篇 |
2012年 | 6151篇 |
2011年 | 6251篇 |
2010年 | 5820篇 |
2009年 | 5486篇 |
2008年 | 5290篇 |
2007年 | 5062篇 |
2006年 | 4541篇 |
2005年 | 3615篇 |
2004年 | 2637篇 |
2003年 | 2039篇 |
2002年 | 2045篇 |
2001年 | 1680篇 |
2000年 | 1228篇 |
1999年 | 866篇 |
1998年 | 565篇 |
1997年 | 424篇 |
1996年 | 387篇 |
1995年 | 340篇 |
1994年 | 254篇 |
1993年 | 143篇 |
1992年 | 150篇 |
1991年 | 90篇 |
1990年 | 88篇 |
1989年 | 68篇 |
1988年 | 38篇 |
1987年 | 20篇 |
1986年 | 24篇 |
1985年 | 12篇 |
1984年 | 17篇 |
1983年 | 10篇 |
1982年 | 13篇 |
1981年 | 12篇 |
1980年 | 20篇 |
1979年 | 13篇 |
1977年 | 10篇 |
1976年 | 18篇 |
1959年 | 9篇 |
1951年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 51 毫秒
101.
In recent years, constructing a virtual backbone by nodes in a connected dominating set (CDS) has been proposed to improve the performance of ad hoc wireless networks. In general, a dominating set satisfies that every vertex in the graph is either in the set or adjacent to a vertex in the set. A CDS is a dominating set that also induces a connected sub‐graph. However, finding the minimum connected dominating set (MCDS) is a well‐known NP‐hard problem in graph theory. Approximation algorithms for MCDS have been proposed in the literature. Most of these algorithms suffer from a poor approximation ratio, and from high time complexity and message complexity. In this paper, we present a new distributed approximation algorithm that constructs a MCDS for wireless ad hoc networks based on a maximal independent set (MIS). Our algorithm, which is fully localized, has a constant approximation ratio, and O(n) time and O(n) message complexity. In this algorithm, each node only requires the knowledge of its one‐hop neighbours and there is only one shortest path connecting two dominators that are at most three hops away. We not only give theoretical performance analysis for our algorithm, but also conduct extensive simulation to compare our algorithm with other algorithms in the literature. Simulation results and theoretical analysis show that our algorithm has better efficiency and performance than others. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
102.
Effects of surfactant/silica and silica/cerium ratios on the characteristics of mesoporous Ce-MCM-41
Se Ho Park Bo Yune Song Tai Gyu Lee 《Journal of Industrial and Engineering Chemistry》2008,14(2):261-264
Using the surfactant CTMABr (cetyltrimethyl ammonium bromide) and cerium(IV) sulfate, mesoporous Ce-MCM-41 molecular sieves were produced under a hydrothermal condition with various surfactant/silica (surfactant/Si) and silica/cerium (Si/Ce) ratios. Changes to the structural traits caused by changing the molar ratios of both surfactant/Si and Si/Ce were investigated. XRD (X-ray diffraction), FT-IR (fourier transform infrared spectroscopy), and SEM (scanning electro microscopy) were used for the characterization of prepared mesoporous samples. Among the tested molar ratios, surfactant/Si ratio of 0.5 and 0.2 showed highest values of d1 0 0 and intensity, respectively, for the Si-MCM-41. XRD analysis also identified a quintessential hexagonal structure of Ce-MCM-41 for the Si/Ce molar ratio higher than 40 (maintaining the surfactant/Si ratio at 0.2). When cerium content was increased to have the Si/Ce molar ratio of 20, the hexagonal structure of Ce-MCM-41 was collapsed due to the structural stress of substituted cerium. FT-IR results confirmed calcination of Ce-MCM-41 and the incorporation of Ce4+ ions of cerium sulfate into the silica surface with proper removal of the surfactant. Rod-like shape with rounded edges of the prepared Ce-MCM-41 samples was identified by SEM. These results suggest surfactant/Si ratio of 0.2 and Si/Ce ratio of 40 for the production of Ce-MCM-41 with the highest level of crystallinity. 相似文献
103.
冯博 《西安邮电学院学报》2008,13(3):33-36
讨论了几种微蜂窝路径损耗模式,并用我国某城市测量的微蜂窝传播数据对COST231WI,修正COST231WI和Xia模式进行了检验,结果表明修正COST231模式的误差最小,Xia模式次之,COST231WI模式最差。从而得到修正COST231模式比较符合我国实际的微蜂窝传播预测模式。 相似文献
104.
105.
GAO Qinglu~ WU Bo~ GUO Guang~ .School of Management Wuhan University of Technology Wuhan China .School of Mechanical Engineering Huazhong University of Science Technology Wuhan China 《武汉理工大学学报》2006,28(Z2)
This article puts forward a scheduling method for nonlinear process plan shop floor.Task allocation and load bal- ance are realized by bidding mechanism.Though the agent interaction process,the execution of tasks is determined and the coherence of manufacturing decision is verified.The employment of heuristic index can help to optimize the system performance. 相似文献
106.
浅谈GPS在矿山测量中的应用 总被引:1,自引:0,他引:1
根据GPS的使用现状,结合测绘工作的一些感悟和经验,简单介绍GPS RTK(Real Time Kine-matic)工作原理及其在矿山测量、道路测量等实际工作的作业方法和注意的问题,及GPS在应用过程中的一些体会。 相似文献
107.
The catalysts of CeO2 and the mixture of CeO2 and CuO were prepared, and the activities of these catalysts for completely oxidizing benzene were studied.The results show that the optimal proportion of CeO2/CuO is 6: 4.The highest temperature at which benzene was completely oxidized on these catalysts at different airspeed was measured.Compared these catalysts with the noble metal used, our catalysts had superiority in the resources and the industrial cost besides good activities. 相似文献
108.
Yue Rong Yingbo Hua Swami A. Swindlehurst A.L. 《Signal Processing, IEEE Transactions on》2008,56(2):686-701
A space-time optimal power schedule for multiple distributed multiple-input multiple-output (MIMO) links without the knowledge of the instantaneous channel state information (CSI) at the transmitting nodes is proposed. A readily computable expression for the ergodic sum capacity of the MIMO links is derived. Based on this expression, which is a non-convex function of power allocation vectors, a projected gradient algorithm is developed to optimize the power allocation. For a symmetric set of MIMO links with independent identically distributed channels, it is observed that the space-time optimal power schedule reduces to a uniform isotropic power schedule when nominal interference is low, or to an orthogonal isotropic power schedule when nominal interference is high. Furthermore, the transition region between the latter two schedules is seen to be very sharp in terms of nominal interference-to-noise ratio (INR). For MIMO links with correlated channels, the corresponding space-time optimal power schedule is developed based on the knowledge of the channel correlation matrices. It is shown that the channel correlation has a great impact on the ergodic capacity and the optimality of different power scheduling approaches. 相似文献
109.
110.