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   4295篇
  免费   193篇
  国内免费   141篇
电工技术   277篇
综合类   740篇
化学工业   447篇
金属工艺   143篇
机械仪表   144篇
建筑科学   328篇
矿业工程   97篇
能源动力   69篇
轻工业   232篇
水利工程   30篇
石油天然气   141篇
武器工业   7篇
无线电   289篇
一般工业技术   392篇
冶金工业   519篇
原子能技术   118篇
自动化技术   656篇
  2024年   5篇
  2023年   33篇
  2022年   53篇
  2021年   92篇
  2020年   95篇
  2019年   66篇
  2018年   78篇
  2017年   81篇
  2016年   113篇
  2015年   129篇
  2014年   180篇
  2013年   237篇
  2012年   234篇
  2011年   256篇
  2010年   233篇
  2009年   224篇
  2008年   239篇
  2007年   363篇
  2006年   350篇
  2005年   308篇
  2004年   249篇
  2003年   230篇
  2002年   175篇
  2001年   137篇
  2000年   84篇
  1999年   93篇
  1998年   66篇
  1997年   54篇
  1996年   29篇
  1995年   29篇
  1994年   26篇
  1993年   18篇
  1992年   8篇
  1991年   4篇
  1990年   3篇
  1989年   7篇
  1987年   2篇
  1986年   2篇
  1983年   2篇
  1981年   2篇
  1979年   2篇
  1966年   2篇
  1964年   5篇
  1963年   3篇
  1961年   8篇
  1960年   2篇
  1959年   2篇
  1958年   2篇
  1956年   2篇
  1955年   2篇
排序方式: 共有4629条查询结果,搜索用时 15 毫秒
1.
Garzan oil field is located at the south east of Turkey. It is a mature oil field and the reservoir is fractured carbonate reservoir. After producing about 1% original oil in place (OOIP) reservoir pressure started to decline. Waterflooding was started in order to support reservoir pressure and also to enhance oil production in 1960. Waterflooding improved the oil recovery but after years of flooding water breakthrough at the production wells was observed. This increased the water/oil ratio at the production wells. In order to enhance oil recovery again different techniques were investigated. Chemical enhanced oil recovery (EOR) methods are gaining attention all over the world for oil recovery. Surfactant injection is an effective way for interfacial tension (IFT) reduction and wettability reversal. In this study, 31 different types of chemicals were studied to specify the effects on oil production. This paper presents solubility of surfactants in brine, IFT and contact angle measurements, imbibition tests, and lastly core flooding experiments. Most of the chemicals were incompatible with Garzan formation water, which has high divalent ion concentration. In this case, the usage of 2-propanol as co-surfactant yielded successful results for stability of the selected chemical solutions. The results of the wettability test indicated that both tested cationic and anionic surfactants altered the wettability of the carbonate rock from oil-wet to intermediate-wet. The maximum oil recovery by imbibition test was reached when core was exposed 1-ethly ionic liquid after imbibition in formation water. Also, after core flooding test, it is concluded that considerable amount of oil can be recovered from Garzan reservoir by waterflooding alone if adverse effects of natural fractures could be eliminated.  相似文献   
2.
The ‐centroid problem or leader–follower problem is generalized considering different customer choice rules where a customer may use facilities belonging to different firms, if the difference in travel distance (or time) is small enough. Assuming essential goods, some particular customer choice rules are analyzed. Linear programming formulations for the generalized ‐medianoid and ‐centroid problems are presented and an exact solution approach is applied. Some computational examples are included.  相似文献   
3.
A series of 2-phenyloxazoles bearing an amide group at position 4 were designed and synthesized for evaluation as potential inhibitors of human recombinant monoamine oxidases (hrMAOs). Results of kinetics experiments demonstrated that all compounds behave as competitive MAO inhibitors, with good selectivity toward the MAO-B isoform. The most potent and selective derivatives are characterized by inhibition constant (Ki) values in the sub-micromolar range and a good selectivity index (Ki MAO-A/Ki MAO-B>50). Some derivatives were also found to be able to inhibit MAO activity in nerve growth factor (NGF)-differentiated PC12 cells, taken as a model of neuronal cells. In particular, 2-(2-hydroxyphenyl)-N-phenyloxazole-4-carboxamide (compound 4 a ) may be a promising new scaffold, exerting the highest selectivity and inhibitory effect toward MAOs in NGF-differentiated PC12 cell lysates, without compromising cell viability. Molecular docking analysis allowed a rationalization of the experimentally observed binding affinity and selectivity.  相似文献   
4.
Although previous research has investigated widespread use of social media, especially Facebook, by youth attending college, the conditions under which these media foster adjustment to college remain unclear. This study tested a model illuminating pathways linking social competence to college adjustment via students’ perceptions about the usefulness of Facebook and ways in which they used the medium. Self-report survey data from 321 college students (M age = 20.09; 58% female; 84% Caucasian) attending a major Midwestern university supported the proposed model, indicating that higher social competence could foster or impede college adjustment, depending upon how it was related to beliefs about the usefulness of different Facebook functions and how these perceptions, in turn, were associated with patterns of Facebook use. Findings underscore the importance of considering connections among personal attributes, perception of media effectiveness, and media behaviors in assessing the implications of social media for users’ psychosocial well-being.  相似文献   
5.
6.
We report a general template strategy for rational fabrication of a new class of nanostructured materials consisting of multicore shell particles. Our approach is demonstrated by encapsulating Au or Pt nanoparticles in silica shells. Other superstructures of these hollow shells, like dimers, trimers, and tetramers can also be formed by nanoparticle‐mediated self‐assembly. We have also used the as‐prepared multicore Au–silica hollow particles to perform the first studies of Ostwald ripening in confined microspace, in which chloride was found to be an efficient mediating ligand. After treatment with aqua regia, Au–Cl complex is formed inside the shell, and is found to be very active under in situ transmission electron microscopy observations while confined in a microcell. This aspect of the work is expected to motivate further in situ studies of confined crystal growth.  相似文献   
7.
制造业企业竞争情报系统建设浅析   总被引:1,自引:0,他引:1  
余立 《信息技术》2004,28(5):91-93
讨论了制造业企业信息化进程中包含竞争情报系统的必要性。论述了该类企业竞争情报系统的内涵及功能特征。较详细地介绍了相应系统常用外部数据资源内容及各自应用特点;此外本文还简要地说明了该类系统现有开发支持工具软件、开发关键步骤等内容。  相似文献   
8.
A cohort of 2nd-grade students provided comparisons of academic and social competence based on school retention/promotion decisions. Sample groups were (a) retained, (b) at risk for retention, (c) special education, and (d) promoted. Findings suggested most children with academic deficiencies are identified by schools early and are sorted into educational treatments differing in intensity that represent a continuum of competence. The authors provide empirical evidence counter to the assumptions that retained students have the requisite ability to catch up and have more problem behaviors than other low-achieving students. The relevance of high-stakes test scores for promotion/retention decisions and the parallels between schools' implementation of retention policy and implementation of regulations for identifying children with disabilities are included in the discussion. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
9.
放置过滤器浇注系统的设计   总被引:3,自引:1,他引:2  
综述了常用的网型、芯型、泡沫陶瓷过滤器的研究和应用情况。指出应根据过滤器类型,确定其在浇注系统中的安放位置并相应改变该处的结构和截面积,以便在使用过滤器净化金属的同时,保证充型速度不变。  相似文献   
10.
接触低浓度一氧化碳工人行为功能改变   总被引:1,自引:0,他引:1  
文章选择接触一氧化碳(CO)工人61人为观察组(另选56人作为对照组)进行了WHO推荐的神经行为核心测试组合(WHO-NCTB)研究。结果表明:接触组在情感状态、简单反应时、数字跨度、手工敏捷度、数字译码和视觉保留等项目的标准得分显著低于对照组(P<0.05),并且在简单反应时和手工敏捷度两项指标上存在剂量-反应关系。提示长期反复接触CO可对中枢视觉系统功能状态产生不良改变。  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