首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   310458篇
  免费   5361篇
  国内免费   1042篇
电工技术   4641篇
综合类   2514篇
化学工业   46500篇
金属工艺   13339篇
机械仪表   9727篇
建筑科学   8565篇
矿业工程   1996篇
能源动力   6917篇
轻工业   24097篇
水利工程   3729篇
石油天然气   4451篇
武器工业   9篇
无线电   34362篇
一般工业技术   62208篇
冶金工业   42889篇
原子能技术   5605篇
自动化技术   45312篇
  2021年   2367篇
  2020年   1785篇
  2019年   2206篇
  2018年   17436篇
  2017年   16416篇
  2016年   13389篇
  2015年   3266篇
  2014年   4385篇
  2013年   11393篇
  2012年   9554篇
  2011年   17884篇
  2010年   14947篇
  2009年   13017篇
  2008年   14454篇
  2007年   15362篇
  2006年   6895篇
  2005年   7325篇
  2004年   6864篇
  2003年   6752篇
  2002年   5789篇
  2001年   5311篇
  2000年   5270篇
  1999年   5282篇
  1998年   12241篇
  1997年   8742篇
  1996年   6935篇
  1995年   5098篇
  1994年   4627篇
  1993年   4430篇
  1992年   3491篇
  1991年   3305篇
  1990年   3067篇
  1989年   3133篇
  1988年   3014篇
  1987年   2558篇
  1986年   2464篇
  1985年   2958篇
  1984年   2754篇
  1983年   2533篇
  1982年   2260篇
  1981年   2410篇
  1980年   2194篇
  1979年   2282篇
  1978年   2267篇
  1977年   2531篇
  1976年   3377篇
  1975年   1980篇
  1974年   1885篇
  1973年   1882篇
  1972年   1516篇
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
101.
102.
Variations of Incomplete Open Cubes is the major project by the twentieth-century conceptual artist Sol LeWitt. In this paper we interpret the enumerative component of the project as embeddings of graphs. This formulation permits use of an algorithm to check the completeness of the list of the structures produced by the artist. Our conclusion is that the artist found the correct number of structures (that is, 122), but that his list contains a mistake in the presentation of a pair of incomplete cubes, a discovery that appears not to have been noted before.  相似文献   
103.
An explicit extraction of the retinal vessel is a standout amongst the most significant errands in the field of medical imaging to analyze both the ophthalmological infections, for example, Glaucoma, Diabetic Retinopathy (DR), Retinopathy of Prematurity (ROP), Age-Related Macular Degeneration (AMD) as well as non retinal sickness such as stroke, hypertension and cardiovascular diseases. The state of the retinal vasculature is a significant indicative element in the field of ophthalmology. Retinal vessel extraction in fundus imaging is a difficult task because of varying size vessels, moderately low distinction, and presence of pathologies such as hemorrhages, microaneurysms etc. Manual vessel extraction is a challenging task due to the complicated nature of the retinal vessel structure, which also needs strong skill set and training. In this paper, a supervised technique for blood vessel extraction in retinal images using Modified Adaboost Extreme Learning Machine (MAD-ELM) is proposed. Firstly, the fundus image preprocessing is done for contrast enhancement and in-homogeneity correction. Then, a set of core features is extracted, and the best features are selected using “minimal Redundancy-maximum Relevance (mRmR).” Later, using MAD-ELM method vessels and non vessels are classified. DRIVE and DR-HAGIS datasets are used for the evaluation of the proposed method. The algorithm’s performance is assessed based on accuracy, sensitivity and specificity. The proposed technique attains accuracy of 0.9619 on the DRIVE database and 0.9519 on DR-HAGIS database, which contains pathological images. Our results show that, in addition to healthy retinal images, the proposed method performs well in extracting blood vessels from pathological images and is therefore comparable with state of the art methods.  相似文献   
104.
105.
106.
107.
The design of a microstrip‐fed annular‐ring slot antenna (ARSA) with circular polarization (CP) radiation is initially studied. To obtain CP radiation with broad 3‐dB axial ratio (AR) bandwidth that can cover the WiMAX 2.3 GHz (2305–2320 MHz, 2345–2360 MHz) and WLAN 2.4 GHz (2400–2480 MHz) bands, a novel technique of extending an inverted L‐shaped slot from the bottom section of the annular‐ring is proposed. To suppress the harmonic modes induced by the CP ARSA, the technique of integrating a defected ground structure into the annular‐ring slot is further introduced. From the measured results, 10‐dB impedance bandwidth and 3‐dB AR bandwidth of 44.86 and 9.68% were achieved by the proposed harmonic suppressed CP ARSA. Furthermore, average gain and radiation efficiency of ~4.7 dBic and 71%, respectively, were also exhibited across the bands of interest. © 2014 Wiley Periodicals, Inc. Int J RF and Microwave CAE 25:337–345, 2015.  相似文献   
108.
109.
Indoles are privileged structures in medicinal and bioorganic chemistry that are particularly well suited to serve as platforms for diversity. Among many other therapeutic areas, the indole scaffold has been used to design aromatic compounds useful to interfere with enzymes engaged in the regulation of substrate acylation status, such as sirtuins. However, the planarity of the indole ring is not necessarily optimal for all target enzymes, especially when functionalization with aromatic side chains is required. Replacement of flat scaffolds by nonplanar molecular cores dominated by sp3 hybridization is a common strategy to avoid the disadvantages associated with poor solubility and high promiscuity, while covering less-well-explored areas of chemical space. Thus, we synthesized fragment-like tetrahydroindoles suitable for fragment-based drug discovery as well as a well-characterized small library intended as multipurpose screening compounds. For proof of principle, these compounds were screened against sirtuins 1–3, enzymes known to be addressable by indoles. We found that 2,6,6-trimethyl-4-oxo-4,5,6,7-tetrahydro-1H-indole-3-carboxamides are potent and selective SIRT2 inhibitors. Compound 16 t displayed an IC50 value of 0.98 μm and could serve as exquisite starting point for hit-to-lead profiling.  相似文献   
110.
Polymer Bulletin - Jute and glass composites of epoxy resin of (2E, 6E)-bis(4-hydroxybenzylidene)cyclohexanone (EBHBC) were prepared by compressing molding technique using three different hardeners...  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号