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   480715篇
  免费   5050篇
  国内免费   1351篇
电工技术   7597篇
综合类   2498篇
化学工业   74741篇
金属工艺   26201篇
机械仪表   17246篇
建筑科学   10360篇
矿业工程   4770篇
能源动力   9269篇
轻工业   28257篇
水利工程   6571篇
石油天然气   15098篇
武器工业   30篇
无线电   46835篇
一般工业技术   105713篇
冶金工业   63919篇
原子能技术   14078篇
自动化技术   53933篇
  2021年   3846篇
  2019年   3751篇
  2018年   20989篇
  2017年   20175篇
  2016年   17354篇
  2015年   4950篇
  2014年   7749篇
  2013年   17225篇
  2012年   14150篇
  2011年   23461篇
  2010年   19570篇
  2009年   18506篇
  2008年   19775篇
  2007年   20373篇
  2006年   10841篇
  2005年   10789篇
  2004年   10444篇
  2003年   10019篇
  2002年   9269篇
  2001年   8706篇
  2000年   8551篇
  1999年   7946篇
  1998年   16731篇
  1997年   12616篇
  1996年   9670篇
  1995年   7562篇
  1994年   6765篇
  1993年   6869篇
  1992年   5545篇
  1991年   5580篇
  1990年   5583篇
  1989年   5459篇
  1988年   5315篇
  1987年   5047篇
  1986年   4900篇
  1985年   5347篇
  1984年   5120篇
  1983年   4823篇
  1982年   4551篇
  1981年   4693篇
  1980年   4558篇
  1979年   4707篇
  1978年   4889篇
  1977年   5190篇
  1976年   6184篇
  1975年   4398篇
  1974年   4434篇
  1973年   4541篇
  1972年   3985篇
  1971年   3672篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
The objective of this study was to characterise the nutritional potential of leaves and identify a diversity centre with low cyanide and high nutrient content among 178 Latin American cassava genotypes. This field-based collection represents the seven diversity centres, held at The International Center for Tropical Agriculture (CIAT Palmira, Colombia) by the Cassava Program. The cyanide, all-trans-β-carotene and lutein concentrations in cassava leaves ranged from 346 to 7484 ppm dry basis (db), from 174–547 μg g−1 db and 15–181 μg g−1 db, respectively. Cassava leaves also showed significant levels of essential amino acids leucine, lysine, phenylalanine, valine and threonine, and average total protein content of 26.24 g 100 g−1 db. Among seven diversity centres, South American rainforest group showed low cyanide and high carotene content in leaves. In addition, VEN77 and PAN51 genotypes stood out for having low cyanide in leaves and roots and high carotene in leaves. This genetic diversity can be used to select high potential progenitors for breeding purposes.  相似文献   
152.
Power Technology and Engineering - The major results from the survey and geotechnical monitoring during the leveling of the model foundation of the Zagorsk PSP-2 in test site No. 3 are presented....  相似文献   
153.
Russian Engineering Research - The machining precision in grinding a flexible shaft by sharp and blunt wheels is compared for a CNC system with a specified cycle of radial supply velocities. To...  相似文献   
154.
International Journal of Computer Vision - Anticipating future situations from streaming sensor data is a key perception challenge for mobile robotics and automated vehicles. We address the problem...  相似文献   
155.
The demand for food production has been constantly increasing due to rising population. In developed countries, for example, the emergence of regional production of old grains that are rarely utilized, along with the production of commonly consumed grains, has gained importance in recent years. These grains, known collectively as ancient or heirloom grains, have offered both farmers and consumers novel ways of cultivation and products with interesting taste, characteristics and nutritional value. Among the 30 000 plant species known, only five cereals currently provide more than 50% of the world's energy intake – bread wheat (Triticum aestivum), rice (Oryza sativa), sorghum (Sorghum bicolor), millets (Panicum sp.) and maize (Zea mays). The excessive utilization of these selected species has a great potential to cause genetic losses and difficulty in bridging future agricultural demands. Teff (Eragrostis tef), an ancient grain extensively cultivated in countries like Eritrea and Ethiopia, provides promising alternatives for new food uses since its nutritional value is significantly higher than most others cereal grains. The absence of gluten allows flexibility in food utilization since it can be directly substituted to gluten-containing products. The grain also offers an excellent balance of essential amino acids and minerals, which can fulfil the recommended daily intake and eliminates the need for fortification and enrichment. This review provides a general overview of the physical properties and nutritional composition of teff grains related to processing and applications in the food and feed industries. The current status of teff utilization, as well as the challenges in production and commercialization, and future opportunities is presented and discussed.  相似文献   
156.
Measurement Techniques - A comparative analysis of the results of estimation of measurement uncertainty is conducted. The results are obtained by a method described in the Handbook on the...  相似文献   
157.
Mobile Networks and Applications - Public cloud system offers Infrastructure-as-a-Service (IaaS) to deliver the computational resources on demand. Resource requirements of a cloud environment are...  相似文献   
158.
Russian Microelectronics - The recently developed nanomaterials and their production technologies as intellectual property objects (IPOs) are considered. The role of the informational-analytical...  相似文献   
159.
Strength of Materials - Creep constitutive equations have been derived for the materials that exhibit the properties of orthotropy (transversal isotropy) and transient creep under cyclic loading. A...  相似文献   
160.
Chorismate and isochorismate constitute branch-point intermediates in the biosynthesis of many aromatic metabolites in microorganisms and plants. To obtain unnatural compounds, we modified the route to menaquinone in Escherichia coli. We propose a model for the binding of isochorismate to the active site of MenD ((1R,2S, 5S,6S)-2-succinyl-5-enolpyruvyl-6-hydroxycyclohex-3-ene-1-carboxylate (SEPHCHC) synthase) that explains the outcome of the native reaction with α-ketoglutarate. We have rationally designed variants of MenD for the conversion of several isochorismate analogues. The double-variant Asn117Arg–Leu478Thr preferentially converts (5S,6S)-5,6-dihydroxycyclohexa-1,3-diene-1-carboxylate (2,3-trans-CHD), the hydrolysis product of isochorismate, with a >70-fold higher ratio than that for the wild type. The single-variant Arg107Ile uses (5S,6S)-6-amino-5-hydroxycyclohexa-1,3-diene-1-carboxylate (2,3-trans-CHA) as substrate with >6-fold conversion compared to wild-type MenD. The novel compounds have been made accessible in vivo (up to 5.3 g L−1). Unexpectedly, as the identified residues such as Arg107 are highly conserved (>94 %), some of the designed variations can be found in wild-type SEPHCHC synthases from other bacteria (Arg107Lys, 0.3 %). This raises the question for the possible natural occurrence of as yet unexplored branches of the shikimate pathway.  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