全文获取类型
收费全文 | 383980篇 |
免费 | 30179篇 |
国内免费 | 16106篇 |
专业分类
电工技术 | 22276篇 |
技术理论 | 59篇 |
综合类 | 24496篇 |
化学工业 | 63703篇 |
金属工艺 | 21491篇 |
机械仪表 | 24570篇 |
建筑科学 | 29952篇 |
矿业工程 | 12658篇 |
能源动力 | 11238篇 |
轻工业 | 24215篇 |
水利工程 | 6592篇 |
石油天然气 | 25031篇 |
武器工业 | 3259篇 |
无线电 | 42707篇 |
一般工业技术 | 44350篇 |
冶金工业 | 20148篇 |
原子能技术 | 3701篇 |
自动化技术 | 49819篇 |
出版年
2024年 | 1684篇 |
2023年 | 6396篇 |
2022年 | 11343篇 |
2021年 | 15635篇 |
2020年 | 11980篇 |
2019年 | 9651篇 |
2018年 | 10947篇 |
2017年 | 12228篇 |
2016年 | 10890篇 |
2015年 | 14804篇 |
2014年 | 18921篇 |
2013年 | 22458篇 |
2012年 | 24225篇 |
2011年 | 26603篇 |
2010年 | 22817篇 |
2009年 | 21787篇 |
2008年 | 21320篇 |
2007年 | 20426篇 |
2006年 | 20915篇 |
2005年 | 18275篇 |
2004年 | 11979篇 |
2003年 | 10609篇 |
2002年 | 9831篇 |
2001年 | 8868篇 |
2000年 | 8863篇 |
1999年 | 10217篇 |
1998年 | 8458篇 |
1997年 | 7025篇 |
1996年 | 6557篇 |
1995年 | 5502篇 |
1994年 | 4508篇 |
1993年 | 3205篇 |
1992年 | 2576篇 |
1991年 | 2021篇 |
1990年 | 1553篇 |
1989年 | 1264篇 |
1988年 | 1050篇 |
1987年 | 704篇 |
1986年 | 554篇 |
1985年 | 351篇 |
1984年 | 249篇 |
1983年 | 208篇 |
1982年 | 209篇 |
1981年 | 135篇 |
1980年 | 148篇 |
1979年 | 76篇 |
1978年 | 34篇 |
1977年 | 38篇 |
1976年 | 55篇 |
1975年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
42.
WC/Cu复合材料制备及其高温性能 总被引:11,自引:0,他引:11
用机械合金化法结合冷变形,制备了WC/Cu复合材料,研究了冷变形后复合材料的组织特征和高温退火时韵性能变化。结果表明:烧结后的材料经冷变形,组织呈显著纤维状,WC颗粒弥散分布,密度明显提高,达到理论密度的99.2%;复合材料经600~900℃高温退火,强度和硬度略有下降,塑性则有大幅提高;900℃退火时未发生明显的再结晶,界面结合良好;所制备的WC/Cu复合材料有优良的综合性能。 相似文献
43.
By exploiting a general cyclostationary (CS) statistics-based framework, this letter develops a rigorous and unified asymptotic (large sample) performance analysis setup for a class of blind feedforward timing epoch estimators for linear modulations transmitted through time nonselective flat-fading channels. Within the proposed CS framework, it is shown that several estimators proposed in the literature can be asymptotically interpreted as maximum likelihood (ML) estimators applied on a (sub)set of the second- (and/or higher) order statistics of the received signal. The asymptotic variance of these ML estimators is established in closed-form expression and compared with the modified Crame/spl acute/r-Rao bound. It is shown that the timing estimator proposed by Oerder and Meyr achieves asymptotically the best performance in the class of estimators which exploit all the second-order statistics of the received signal, and its performance is insensitive to oversampling rates P as long as P/spl ges/3. Further, an asymptotically best consistent estimator, which achieves the lowest asymptotic variance among all the possible estimators that can be derived by exploiting jointly the second- and fourth-order statistics of the received signal, is also proposed. 相似文献
44.
45.
论文提出了一种混沌通信系统的噪音衰减算法,该算法利用混沌同步现象在接收端获得正确的噪音估计值,从而从接收信号中滤除噪音恢复出正确的信号。通过数值仿真试验表明该方法是可行的。 相似文献
46.
基于模式理论光栅椭偏参数反演的数值模拟 总被引:5,自引:2,他引:3
将一种广泛用于求解系统优化问题的方法——正单纯形法,求解光栅的椭偏方程。首先,利用求解光栅的傅立叶模式理论对TE和TM波的复反射系数进行求解。然后计算出其相应的椭偏参数(△,Ψ),并在该值的基础上加入不同偏差的随机高斯噪声,将加入噪声后的值(△m,Ψm)作为模拟测量值。最后使用优化算法进行反演。通过对几种常用面形光栅椭偏参数的数值模拟,一方面表明傅立叶模式理论计算光栅的椭偏参数不仅精度高。而且速度快;另一方面表明利用正单纯形法得到的光栅参数值很接近于正演时假设的参数值,从而从理论上证明了利用椭偏法测量光栅各种光学参数的可行性。 相似文献
47.
48.
49.
We consider the problem of designing a network of optical cross-connects (OXCs) to provide end-to-end lightpath services to large numbers of label switched routers (LSRs). We present a set of heuristic algorithms to address the combined problem of physical topology design (i.e., determine the number of OXCs required and the fiber links among them) and logical topology design (i.e., determine the routing and wavelength assignment for the lightpaths among the LSRs). Unlike previous studies which were limited to small topologies with a handful of nodes and a few tens of lightpaths, we have applied our algorithms to networks with hundreds or thousands of LSRs and with a number of lightpaths that is an order of magnitude larger than the number of LSRs. In order to characterize the performance of our algorithms, we have developed lower bounds which can be computed efficiently. We present numerical results for up to 1000 LSRs and for a wide range of system parameters such as the number of wavelengths per fiber, the number of transceivers per LSR, and the number of ports per OXC. The results indicate that it is possible to build large-scale optical networks with rich connectivity in a cost-effective manner, using relatively few but properly dimensioned OXCs. 相似文献
50.
This paper extends an analysis developed for tubular permanent-magnet machines to account for the effect of fringing associated with the finite length of the ferromagnetic armature core. The magnetic field distribution, established by using an analytical technique formulated in the cylindrical coordinate system, provides an accurate means of evaluating the effect of the fringing flux on the thrust force and the back-electromotive force. Finite-element calculations confirm the analytically derived results. In Part II of the paper, the analysis is used to predict the cogging force that results from end effects and to facilitate minimization of the force. 相似文献