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   3796篇
  免费   203篇
  国内免费   223篇
电工技术   93篇
技术理论   8篇
综合类   978篇
化学工业   267篇
金属工艺   39篇
机械仪表   105篇
建筑科学   406篇
矿业工程   348篇
能源动力   43篇
轻工业   141篇
水利工程   158篇
石油天然气   88篇
武器工业   9篇
无线电   211篇
一般工业技术   198篇
冶金工业   531篇
原子能技术   13篇
自动化技术   586篇
  2023年   42篇
  2022年   43篇
  2021年   78篇
  2020年   72篇
  2019年   62篇
  2018年   50篇
  2017年   62篇
  2016年   73篇
  2015年   97篇
  2014年   223篇
  2013年   174篇
  2012年   244篇
  2011年   308篇
  2010年   260篇
  2009年   297篇
  2008年   233篇
  2007年   261篇
  2006年   279篇
  2005年   244篇
  2004年   164篇
  2003年   152篇
  2002年   122篇
  2001年   101篇
  2000年   81篇
  1999年   77篇
  1998年   36篇
  1997年   44篇
  1996年   71篇
  1995年   25篇
  1994年   25篇
  1993年   22篇
  1992年   6篇
  1991年   12篇
  1990年   7篇
  1989年   7篇
  1988年   12篇
  1987年   6篇
  1986年   6篇
  1982年   9篇
  1979年   9篇
  1974年   9篇
  1971年   8篇
  1966年   8篇
  1965年   9篇
  1964年   11篇
  1958年   5篇
  1957年   6篇
  1956年   6篇
  1955年   12篇
  1954年   7篇
排序方式: 共有4222条查询结果,搜索用时 15 毫秒
1.
信息化时代推动了我国科研水平飞速增长,为各行业、各领域提供先进的技术与设备支持,以此提升我国整体的GDP水平。文章认为在提升科研水平的过程中,科研项目管理系统起到举足轻重的作用,为科研活动的规范性、合理性与可行性提供了参考标准。  相似文献   
2.
Plant specialized metabolites (SMs) play an important role in the interaction with the environment and are part of the plant defense response. These natural products are volatile, semi-volatile and non-volatile compounds produced from common building blocks deriving from primary metabolic pathways and rapidly evolved to allow a better adaptation of plants to environmental cues. Specialized metabolites include terpenes, flavonoids, alkaloids, glucosinolates, tannins, resins, etc. that can be used as phytochemicals, food additives, flavoring agents and pharmaceutical compounds. This review will be focused on Mediterranean crop plants as a source of SMs, with a special attention on the strategies that can be used to modulate their production, including abiotic stresses, interaction with beneficial soil microorganisms and novel genetic approaches.  相似文献   
3.
贾承造 《石油学报》2021,41(12):1445-1464
石油和天然气是全球和中国最重要的一次能源。石油工业的生存发展是由油气资源、市场、技术和社会政治经济环境等要素决定的,其中,技术进步是最活跃和最关键的因素之一。中国已成为全球油气生产消费大国,中国石油工业上游的发展也高度依赖石油科技的进步。中国石油工业已形成了先进完整的理论技术研发和装备制造体系,支撑了油气产量持续稳产和增产。未来是中国社会经济发展的关键期,石油工业必将面对重大挑战与新的技术需求,大力实施国家创新战略,发展具有国际领先水平的新一代勘探开发理论技术,支撑油气产业发展,保障国家油气能源安全。中国石油工业上游在未来面临的重大挑战与技术需求包括:①满足中国未来现代化建设的巨大油气需求和保障油气供应安全,这当中必须加大中国油气勘探开发,同时进一步扩大全球及"一带一路"油气投资与生产;②实现中国石油长期稳产2×108t/a以上;③实现中国天然气产量上升至3 000×108m3/a并长期稳产;④发展海洋及深水油气勘探开发先进技术与装备;⑤形成新一代石油工程服务技术装备和数字化转型。中国石油工业上游未来的科技攻关方向和研发重点包括:①先进的石油大幅度提高采收率技术;②大气田勘探与复杂气田提高采收率技术;③非常规油气勘探开发技术;④海洋及深水油气勘探开发技术及装备;⑤"一带一路"油气勘探开发技术;⑥新一代石油工程服务技术装备和数字化转型。  相似文献   
4.
1,4-Butanediol (1,4-BDO) is a four-carbon diol used for industrial applications such as organic solvents, and the production of adhesives, fibers and polyurethanes. 1,4-BDO currently is produced through several petrochemical routes: hydrogenation of maleic anhydride, isomerization of propylene oxide, acetoxylation of butadiene, and the reaction between formaldehyde and acetylene. The current trends in 1,4-BDO production involve the utilization of renewable sources such as biomass. In this context, the present study aimed to identify promising technologies of 1,4-BDO production through prospecting methodology based on the analyses of patents and scientific article, describing the most relevant aspects of those emerging technologies. An increasing amount of 1,4-BDO production focused on biotechnological routes has been reported, with the US heavily involved in the development of new technologies. This study tracked three promising technologies which have potential for application in a biorefinery context because those processes involve (i) production of 1,4-BDO from sugars, classified herein as the biotechnological route; (ii) production of intermediates from sugar fermentation followed by catalytic conversion into 1,4-BDO, classified herein as the hybrid route, and (iii) furan/furfural conversion into 1,4-BDO. © 2020 Society of Chemical Industry  相似文献   
5.
This study investigates how the preparation and re-pelletization of BSCCO ceramic superconductors affects their structural, magnetic, electrical, and mechanical properties. Samples were prepared using conventional methods including the dry solid state (SS) reaction and wet ammonium nitrate (AN) precipitation, with three variations prepared for each technique. The fabricated samples were then characterized with X-ray powder diffraction (XRD) and scanning electron microscopy. The density, resistance vs. temperature characteristics, AC susceptibility vs. temperature characteristics, and magnetic hysteresis properties were measured. In addition, Vicker's microhardness was measured and revealed that all six samples exhibit the reverse indentation size effect (RISE). Microhardness modeling was also conducted. Calculations with Meyer's law, the Hays and Kendall model, and the proportional sample resistance model indicate that the samples are far from the plateau region, whereas the indentation-induced cracking model was consistent with the experimental results. The elastic modulus, Young's modulus, yield strength, and brittleness index were also calculated for each sample.  相似文献   
6.
Due to its outstanding ability in processing large quantity and high-dimensional data, machine learning models have been used in many cases, such as pattern recognition, classification, spam filtering, data mining and forecasting. As an outstanding machine learning algorithm, K-Nearest Neighbor (KNN) has been widely used in different situations, yet in selecting qualified applicants for winning a funding is almost new. The major problem lies in how to accurately determine the importance of attributes. In this paper, we propose a Feature-weighted Gradient Decent K-Nearest Neighbor (FGDKNN) method to classify funding applicants in to two types: approved ones or not approved ones. The FGDKNN is based on a gradient decent learning algorithm to update weight. It updatesthe weight of labels by minimizing error ratio iteratively, so that the importance of attributes can be described better. We investigate the performance of FGDKNN with Beijing Innofund. The results show that FGDKNN performs about 23%, 20%, 18%, 15% better than KNN, SVM, DT and ANN, respectively. Moreover, the FGDKNN has fast convergence time under different training scales, and has good performance under different settings.  相似文献   
7.
This paper reviews recent studies, that not only includes both experiments and modeling components, but celebrates a close coupling between these techniques, in order to provide insights into the plasticity and failure of polycrystalline metals. Examples are provided of studies across multiple-scales, including, but not limited to, density functional theory combined with atom probe tomography, molecular dynamics combined with in situ transmission electron miscopy, discrete dislocation dynamics combined with nanopillars experiments, crystal plasticity combined with digital image correlation, and crystal plasticity combined with in situ high energy X-ray diffraction. The close synergy between in situ experiments and modeling provides new opportunities for model calibration, verification, and validation, by providing direct means of comparison, thus removing aspects of epistemic uncertainty in the approach. Further, data fusion between in situ experimental and model-based data, along with data driven approaches, provides a paradigm shift for determining the emergent behavior of deformation and failure, which is the foundation that underpins the mechanical behavior of polycrystalline materials.  相似文献   
8.
An evaluation of XML queries such as XQuery or XPath expressions represents a challenging task due to its complexity. Many algorithms have been introduced to cope with this problem. Some of them, called binary joins, evaluate separated parts of a query and subsequently merge intermediate results, while the others, called holistic twig joins, evaluate a query as a whole. Moreover, these algorithms also differ in what index data structure they use to handle XML data. There exist cost-based approaches utilizing binary joins and various index data structures; however, they share a limitation. The limitation is that they cannot perform a join between query nodes not having a direct XPath relationship. Such a join can be advantageous especially if their joint selectivity is high. Since holistic joins work with all query nodes they overcome this limitation. In this article, we introduce such a holistic twig join called CostTwigJoin. To the best of our knowledge, CostTwigJoin is the first holistic join capable of combining various index data structures during an evaluation of an XML query. Usage of the holistic join has yet another advantage for cost-based approaches: an optimizer does not have to resolve the order of binary joins; therefore, the search space is reduced. In this article, we perform thorough experiments on hundreds of queries to evaluate our approach and demonstrate its advantages.  相似文献   
9.
Mainly known for its links to the periodical market and radical politics, this article recontextualizes the editorship of William Nicholson (1753–1815) in terms of its roots in the metropolitan natural philosophical circles of the second half of the 18th century as well as its impact on experimenters and men of science after 1797. The article argues that Nicholson's editorship of the Journal of Natural Philosophy, Chemistry, and the Arts was a means to expand his philosophical significance among natural philosophers at home and abroad—and was, in fact, a form of epistemological subversion that challenged the “Banksian Learned Empire.”  相似文献   
10.
文章将对中国皮革工业的发展历程与现在基本的环境状况作出详尽的介绍与分析,提出必须以科学发展观为指导,加大力度深入探讨皮革工业的环境建设,并且积极的树立健康良好的皮革工业环境建设观念,以此来促进皮革工业健康可持续的发展。  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