全文获取类型
收费全文 | 23360篇 |
免费 | 2802篇 |
国内免费 | 1564篇 |
专业分类
电工技术 | 1631篇 |
技术理论 | 2篇 |
综合类 | 2887篇 |
化学工业 | 2276篇 |
金属工艺 | 310篇 |
机械仪表 | 794篇 |
建筑科学 | 2551篇 |
矿业工程 | 1197篇 |
能源动力 | 752篇 |
轻工业 | 2584篇 |
水利工程 | 1669篇 |
石油天然气 | 1266篇 |
武器工业 | 216篇 |
无线电 | 2797篇 |
一般工业技术 | 1790篇 |
冶金工业 | 917篇 |
原子能技术 | 78篇 |
自动化技术 | 4009篇 |
出版年
2024年 | 122篇 |
2023年 | 395篇 |
2022年 | 744篇 |
2021年 | 863篇 |
2020年 | 898篇 |
2019年 | 813篇 |
2018年 | 774篇 |
2017年 | 893篇 |
2016年 | 1028篇 |
2015年 | 1017篇 |
2014年 | 1545篇 |
2013年 | 1602篇 |
2012年 | 1761篇 |
2011年 | 1938篇 |
2010年 | 1457篇 |
2009年 | 1403篇 |
2008年 | 1348篇 |
2007年 | 1577篇 |
2006年 | 1333篇 |
2005年 | 1113篇 |
2004年 | 908篇 |
2003年 | 778篇 |
2002年 | 609篇 |
2001年 | 481篇 |
2000年 | 379篇 |
1999年 | 374篇 |
1998年 | 278篇 |
1997年 | 240篇 |
1996年 | 198篇 |
1995年 | 187篇 |
1994年 | 152篇 |
1993年 | 111篇 |
1992年 | 67篇 |
1991年 | 74篇 |
1990年 | 60篇 |
1989年 | 42篇 |
1988年 | 43篇 |
1987年 | 17篇 |
1986年 | 23篇 |
1985年 | 10篇 |
1984年 | 15篇 |
1983年 | 14篇 |
1982年 | 10篇 |
1981年 | 2篇 |
1980年 | 9篇 |
1979年 | 4篇 |
1977年 | 2篇 |
1975年 | 2篇 |
1961年 | 2篇 |
1958年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
对未知组成的样品进行紫外光谱扫描 ,根据扫描结果以及各种溴指数测定方法的适用范围 ,选择测定未知物溴指数的适宜方法 ,以提高测定准确度。 相似文献
22.
目前采用的视频流调度算法,没有充分利用补丁流满足服务率高而占用系统资源少的优势,系统效率较低.为了提高效率,提出了一种基于MFQ的补丁优先算法,并给出了该算法的系统流程。通过对实验结果的比较,该算法在用户请求撤销率、节目调度不公平性和用户等待时间方面有明显的优越性. 相似文献
23.
本文提出一种多输入多输出(MIMO)天线系统中简单的扩展空时块编码(SSTBC)分集技术,采用沃尔什码来区分各天线发送数据子流。采用这种方法,在系统带宽一定时,不降低发送信息速率,同时接收机简单。不同天线的发送信息经过了所有收一发天线对之间的空间子信道,获得了所有路径的部分空间分集增益,仿真结果表明,这种增益的获得不受限于接收分集阶数,并且随发射天线的增加以一定的线性关系增加。 相似文献
24.
Carlos Fernandes Agostinho C. Rosa 《Soft Computing - A Fusion of Foundations, Methodologies and Applications》2008,12(10):955-979
Mate selection plays a crucial role in both natural and artificial systems. While traditional Evolutionary Algorithms (EA)
usually engage in random mating strategies, that is, mating chance is independent of genotypic or phenotypic distance between
individuals, in natural systems non-random mating is common, which means that somehow this mechanism has been favored during
the evolutionary process. In non-random mating, the individuals mate according to their parenthood or likeness. Previous studies
indicate that negative assortative mating (AM)—also known as dissortative mating—, which is a specific type of non-random mating, may improve EAs performance by maintaining the genetic diversity of the
population at a higher level during the search process. In this paper we present the Variable Dissortative Mating Genetic Algorithm (VDMGA). The algorithm holds a mechanism that varies the GA’s mating restrictions during the run by means of simple rule
based on the number of chromosomes created in each generation and indirectly influenced by the genetic diversity of the population.
We compare VDMGA not only with traditional Genetic Algorithms (GA) but also with two preceding non-random mating EAs: the
CHC algorithm and the negative Assortative Mating Genetic Algorithm (nAMGA). We intend to study the effects of the different methods in the performance of GAs and verify the reliability of
the proposed algorithm when facing an heterogeneous set of landscapes. In addition, we include the positive Assortative Mating Genetic Algorithm (pAMGA) in the experiments in order test both negative and positive AM mechanisms, and try to understand if and when negative
AM (or DM) speeds up the search process or enables the GAs to escape local optima traps. For these purposes, an extensive
set of optimization test problems was chosen to cover a variety of search landscapes with different characteristics. Our results
confirm that negative AM is effective in leading EAs out of local optima traps, and show that the proposed VDMGA is at least
as efficient as nAMGA when applied to the range of our problems, being more efficient in very hard functions were traditional
GAs usually fail to escape local optima. Also, scalability tests have been made that show VDMGA ability to decrease optimal
population size, thus reducing the amount of evaluations needed to attain global optima. We like to stress that only two parameters
need to be hand-tuned in VDMGA, thus reducing the tuning effort present in traditional GAs and nAMGA. 相似文献
26.
空时分集技术的最大优点在于在不增加带宽的情况下可以提高系统的可靠性,是目前移动通信的研究热点.常模算法是一种性能优良的码分多址(CDMA)盲多用户检测技术,能确保判决信号与实际传送信号之间的差错较小,误码率性能良好.文中提出将标准线性受限常模算法(LCCMA)与空时分组码(STBC)相结合,设计出一种收敛快、能够改善系统性能的基于2-空时分组码的多用户接收机. 相似文献
27.
28.
29.
Attempts have been made to extend SQL to work with multimedia databases. We are reserved on the representation ability of extended SQL to cope with the richness in content of multimedia data. In this paper we present an example of a multimedia database system, Computer Aided Facial Image Inference and Retrieval system (CAFIIR). The system stores and manages facial images and criminal records, providing necessary functions for crime identification. We would like to demonstrate some core techniques for multimedia database with CAFIIR system. Firstly, CAFIIR is a integrated system. Besides database management, there are image analysis, image composition, image aging, and report generation subsystems, providing means for problem solving. Secondly, the richness of multimedia data urges feature-based database for their management. CAFIIR is feature-based. A indexing mechanism,iconic index, has been proposed for indexing facial images using hierarchical self-organization neural network. The indexing method operates on complex feature measures and provides means for visual navigation. Thirdly, special retrieval methods for facial images have been developed, including visual browsing, similarity retrieval, free text retrieval and fuzzy retrieval. 相似文献
30.
通过引入归一化脉宽P和相对脉冲展宽因子q,提出了一个适用于IM/DD系统中各种眼图恶化量x和归一化脉宽P的色散限制改进公式。通过与其它理论的比较、计算机仿真和对实验数据的分析,验证了该公式的正确性,并给出了其它结果的限制条件。研究表明:当眼图恶化量x为1dB左右时,对2.5Gb/;外调制信号(α=0)而言,非色散位移光纤IM/DD系统的色散因子γ约等于0.79,所对应的系统再生中继距离约为900余公里。 相似文献