全文获取类型
收费全文 | 55419篇 |
免费 | 7246篇 |
国内免费 | 3874篇 |
专业分类
电工技术 | 5515篇 |
技术理论 | 1篇 |
综合类 | 4555篇 |
化学工业 | 7916篇 |
金属工艺 | 3194篇 |
机械仪表 | 3621篇 |
建筑科学 | 4343篇 |
矿业工程 | 1974篇 |
能源动力 | 1590篇 |
轻工业 | 4695篇 |
水利工程 | 1644篇 |
石油天然气 | 2653篇 |
武器工业 | 774篇 |
无线电 | 6889篇 |
一般工业技术 | 5861篇 |
冶金工业 | 2197篇 |
原子能技术 | 576篇 |
自动化技术 | 8541篇 |
出版年
2024年 | 444篇 |
2023年 | 1451篇 |
2022年 | 2834篇 |
2021年 | 3861篇 |
2020年 | 2700篇 |
2019年 | 2020篇 |
2018年 | 2032篇 |
2017年 | 2260篇 |
2016年 | 1993篇 |
2015年 | 2763篇 |
2014年 | 3326篇 |
2013年 | 3783篇 |
2012年 | 4373篇 |
2011年 | 4447篇 |
2010年 | 3949篇 |
2009年 | 3726篇 |
2008年 | 3605篇 |
2007年 | 3260篇 |
2006年 | 2879篇 |
2005年 | 2502篇 |
2004年 | 1587篇 |
2003年 | 1157篇 |
2002年 | 1053篇 |
2001年 | 927篇 |
2000年 | 809篇 |
1999年 | 636篇 |
1998年 | 423篇 |
1997年 | 358篇 |
1996年 | 321篇 |
1995年 | 249篇 |
1994年 | 210篇 |
1993年 | 129篇 |
1992年 | 104篇 |
1991年 | 79篇 |
1990年 | 58篇 |
1989年 | 42篇 |
1988年 | 34篇 |
1987年 | 25篇 |
1986年 | 33篇 |
1985年 | 11篇 |
1984年 | 10篇 |
1983年 | 3篇 |
1982年 | 11篇 |
1981年 | 9篇 |
1980年 | 13篇 |
1979年 | 12篇 |
1977年 | 4篇 |
1976年 | 6篇 |
1959年 | 7篇 |
1951年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
吐哈盆地巴喀油田特低渗砂岩油层裂缝分布特征 总被引:10,自引:3,他引:7
巴喀油田中侏罗统砂岩属特低渗透裂缝 孔隙型储集层 ,平均渗透率为 0 .3× 10 - 3μm2 ;地下基质岩块孔隙度为7.4%,渗透率为 0 .16× 10 - 3μm2 。砂岩储集层发育的主要裂缝类型有构造层理缝、正向正交缝、斜向正交缝和羽裂等 ,裂缝宽度一般为 0 .3~ 3 .2mm ,纵向切深为 0 .1~ 0 .8m ,水平延伸长度为 0 .2~ 2 .1m。裂缝以半充填为主 ,其分布受岩性、层厚和构造位置的影响 ,纵向、横向均呈带状分布 ,被裂缝切割而成的基质岩块尺寸取决于裂缝频率 ,岩块长轴方向以北西西向为主 ,与构造轴向基本一致。图 3表 4参 8 相似文献
52.
Jinman Kim Weidong Cai Dagan Feng Hao Wu 《IEEE transactions on information technology in biomedicine》2006,10(3):598-607
The advances in digital medical imaging and storage in integrated databases are resulting in growing demands for efficient image retrieval and management. Content-based image retrieval (CBIR) refers to the retrieval of images from a database, using the visual features derived from the information in the image, and has become an attractive approach to managing large medical image archives. In conventional CBIR systems for medical images, images are often segmented into regions which are used to derive two-dimensional visual features for region-based queries. Although such approach has the advantage of including only relevant regions in the formulation of a query, medical images that are inherently multidimensional can potentially benefit from the multidimensional feature extraction which could open up new opportunities in visual feature extraction and retrieval. In this study, we present a volume of interest (VOI) based content-based retrieval of four-dimensional (three spatial and one temporal) dynamic PET images. By segmenting the images into VOIs consisting of functionally similar voxels (e.g., a tumor structure), multidimensional visual and functional features were extracted and used as region-based query features. A prototype VOI-based functional image retrieval system (VOI-FIRS) has been designed to demonstrate the proposed multidimensional feature extraction and retrieval. Experimental results show that the proposed system allows for the retrieval of related images that constitute similar visual and functional VOI features, and can find potential applications in medical data management, such as to aid in education, diagnosis, and statistical analysis. 相似文献
53.
54.
Biao Chen Hao Wang 《Signal Processing, IEEE Transactions on》2004,52(7):2047-2057
Blind deterministic estimation of the orthogonal frequency division multiplexing (OFDM) frequency offset via oversampling is proposed in this paper. This method utilizes the intrinsic phase shift of neighboring sample points incurred by the frequency offset that is common among all subcarriers. The proposed method is data efficient - it requires only a single OFDM symbol to achieve reliable estimation, hence making it more suitable to systems with stringent delay requirement and mobility-induced channel variation. The proposed scheme is devised to perfectly retrieve frequency offset in the absence of noise. Quite remarkably, we show that in the presence of channel noise, this intuitive scheme is indeed the maximum likelihood estimate of the carrier frequency offset. The possible presence of virtual carriers are also accommodated in the system model, and some interesting observations are obtained. The Cramer-Rao lower bound is derived for the oversampling-based signal model, and we show through numerical simulation that the proposed algorithm is efficient. Practical issues such as identifiability, the front-end filter bandwidth, and the possible presence of correlated noises are also carefully addressed. 相似文献
55.
用CMOS工艺实现非接触IC卡天线的集成化设计 总被引:2,自引:0,他引:2
论述了用CMOS工艺实现非接触式IC卡天线的集成化需要考虑的各个方面,建立了集成天线的模型,给出了合理的设计方案,并通过实验验证了模型和设计方案.实验结果表明,采用片上天线完全可以提供非接触式IC卡工作所需要的能量.在频率为2 2 .5 MHz、感应强度为6×10 - 4 T的磁场中,面积为2 m m×2 mm的集成天线可以为10 kΩ的负载提供1.2 2 5 m W的能量. 相似文献
56.
57.
Data mining extracts implicit, previously unknown, and potentially useful information from databases. Many approaches have been proposed to extract information, and one of the most important ones is finding association rules. Although a large amount of research has been devoted to this subject, none of it finds association rules from directed acyclic graph (DAG) data. Without such a mining method, the hidden knowledge, if any, cannot be discovered from the databases storing DAG data such as family genealogy profiles, product structures, XML documents, task precedence relations, and course structures. In this article, we define a new kind of association rule in DAG databases called the predecessor–successor rule, where a node x is a predecessor of another node y if we can find a path in DAG where x appears before y. The predecessor–successor rules enable us to observe how the characteristics of the predecessors influence the successors. An approach containing four stages is proposed to discover the predecessor–successor rules. © 2006 Wiley Periodicals, Inc. Int J Int Syst 21: 621–637, 2006. 相似文献
58.
大型往复式压缩机功率大、占地面积大,管道振动问题也更为突出。笔者结合实践经验,对大型往复式压缩机的配管设计进行探讨,说明了平面布置与管道防振设计的密切关系,并介绍了管道防振设计的方法和步骤,以及相对简单的复杂管系气柱固有频率的转移矩阵计算方法。 相似文献
59.
60.
Hao Wang Ying Lin Biao Chen 《Signal Processing, IEEE Transactions on》2003,51(10):2613-2623
We investigate non data-aided channel estimation for cyclically prefixed orthogonal frequency division multiplexing (OFDM) systems. By exploiting channel diversity using only two receive antennas, a blind deterministic algorithm is proposed. Identifiability conditions are derived that guarantee the perfect channel retrieval in the absence of noise. In the presence of noise, the proposed method has the desired property of being data efficient-only a single OFDM block is needed to achieve good estimation performance for a wide range of SNR values. The algorithm is also robust to input symbols as it does not have any restriction on the input symbols with regard to their constellation or their statistical properties. In addition, this diversity-based algorithm is computationally efficient, and its performance compares favorably to most existing blind algorithms. 相似文献