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   23867篇
  免费   1821篇
  国内免费   783篇
电工技术   1515篇
技术理论   2篇
综合类   2093篇
化学工业   2880篇
金属工艺   528篇
机械仪表   873篇
建筑科学   2380篇
矿业工程   868篇
能源动力   767篇
轻工业   989篇
水利工程   895篇
石油天然气   841篇
武器工业   257篇
无线电   2268篇
一般工业技术   1308篇
冶金工业   1607篇
原子能技术   196篇
自动化技术   6204篇
  2025年   147篇
  2024年   347篇
  2023年   345篇
  2022年   445篇
  2021年   616篇
  2020年   648篇
  2019年   608篇
  2018年   543篇
  2017年   715篇
  2016年   905篇
  2015年   895篇
  2014年   1299篇
  2013年   1210篇
  2012年   1361篇
  2011年   1630篇
  2010年   1224篇
  2009年   1336篇
  2008年   1255篇
  2007年   1401篇
  2006年   1322篇
  2005年   1062篇
  2004年   885篇
  2003年   875篇
  2002年   760篇
  2001年   661篇
  2000年   497篇
  1999年   394篇
  1998年   302篇
  1997年   232篇
  1996年   196篇
  1995年   183篇
  1994年   145篇
  1993年   113篇
  1992年   100篇
  1991年   65篇
  1990年   88篇
  1989年   44篇
  1988年   44篇
  1987年   48篇
  1986年   33篇
  1985年   141篇
  1984年   241篇
  1983年   285篇
  1982年   205篇
  1981年   311篇
  1980年   119篇
  1979年   38篇
  1978年   42篇
  1975年   22篇
  1974年   22篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
A 1% Pd catalyst (38% dispersion) was prepared by impregnating a γ-alumina with palladium acetylacetonate dissolved in acetone. The behaviour of this catalyst in oxidation and steam reforming (SR) of propane was investigated. Temperature-programmed reactions of C3H8 with O2 or with O2 + H2O were carried out with different stoichiometric ratios S(S =[O2]/5[C3H8]). The conversion profiles of C3H8 for the reaction carried out in substoichiometry of O2 (S < 1) showed two discrete domains of conversion: oxidation at temperatures below 350°C and SR at temperatures above 350°C. The presence of steam in the inlet gases is not necessary for SR to occur: there is sufficient water produced in the oxidation to form H2 and carbon oxides by this reaction. Contrary to what was observed with Pt, an apparent deactivation between 310 and 385°C could be observed with Pd in oxidation. This is due to a reduction of PdOx into Pd0, which is much less active than the oxide in propane oxidation. Steam added to the reactants inhibits oxidation while it prevents the reduction of PdOx into Pd0. Compared to Pt and to Rh, Pd has a higher thermal resistance: no deactivation occurred after treatment up to 700°C and limited deactivation after treatment up to 900°C, provided that the catalyst is maintained in an oxygen-rich atmosphere during the cooling.  相似文献   
2.
Abstract This paper describes an approach to the design of interactive multimedia materials being developed in a European Community project. The developmental process is seen as a dialogue between technologists and teachers. This dialogue is often problematic because of the differences in training, experience and culture between them. Conditions needed for fruitful dialogue are described and the generic model for learning design used in the project is explained.  相似文献   
3.
The 226Ra and 228Ra content of the lichens Umbilicaria cylindrica, U. deusta, U. murina and U. hirsuta has been determined as a function of the growth altitude above sea level, based on data derived from material collected from localities in southwest Poland. These data are interpreted as graphs and formulae to show the influence of environmental factors, particularly altitude, on the concentration of 226Ra and 228Ra, and the coefficients of accumulation calculated.  相似文献   
4.
龙潭拱坝坝高90m,砼上坝运输是砼施工过程中的中间环节,控制进度的关键,必须因时因地制宜地选择适当的运输机械和运输方案,使砼运输能做到保质、保量、及时和经济,通过论证比较,结合本工程现实情况,砼上坝运输不采用缆机而采用门机方案.  相似文献   
5.
Abstract The key research question for this study was to ask whether or not innovative teaching practices would lead to the development of learning outcomes essential for preparing the younger generation for the challenges of life in the knowledge society of the 21st century, and if so, how are the pedagogical features related to the different learning outcomes. Preliminary analyses of the case study data collected from the SITES M2 Study in Hong Kong reveal that where the development of more significant learning gains were observed, the cases possess characteristics additional to the criteria defined in the Study for selection of innovation. More importantly, it was found that the impact of the pedagogical practices was not determined simply by the aggregation of characteristics of the practices per se , nor on the technologies used, but on whether 'empowerment' permeates the curriculum goal and process. Further, this paper claims that these affective and socio-cognitive learning outcomes are more important as preparation for lifelong learning in the 21st century than 'knowledge management competencies'.  相似文献   
6.
A stepwise degradation of a kerogen concentrate of Green River oil shale was effected by oxidation with perchloric acid of varying boiling point and concentration. Dissolution of the kerogen concentrate occurred between 160 and 170°C. Several model compounds were also oxidized with perchloric acid. The model compounds were polyethylene, graphite, nylon, and polyethylene oxide. The results indicated that the kerogen was similar to polyethylene oxide. The results of this study and previous work were used to present a model for the structure of the kerogen of Green River oil shale.  相似文献   
7.
Abstract  In the past, authoring systems have been used to generate computer-assisted learning materials that have often followed the rather restrictive programmed learning format. A number of innovative ways of using recently developed systems to develop CAL packages that include a wider range of learning activities (access to data bases, simulations, interactive video, etc.) are described. Analogous to the concept of a media selection model, a computer delivery mode selection model is presented as an aid to decision making for designers. This increase in flexibility should broaden the applicability of computerassisted learning (CAL) to a wider range of educational objectives and raise the level of the cognitive emphasis in learning packages, as well as facilitate the implementation of visually appealing materials. The paper also implies a standard of acceptability for authoring systems.  相似文献   
8.
Krystal M. Lee 《Energy》1985,10(7):785-791
Low temperature corrosion from flue gases is a common problem in energy recovery systems. We present a simple computation procedure to estimate the dewpoint of combustion gases, which is the limiting design temperature to prevent corrosion. If the composition of fuel oil and the percentage of CO2 or O2 in the flue gases are known, the air ratio, percentage of H2O, p.p.m. of SO3 in the flue gas, and the dewpoint can be estimated with an error of 1.9%. The results indicate that the dewpoint is high because the combustion conditions are not appropriately adjusted, even if low sulfur fuel oil is used. We conclude that both the fuel quality and the combustion conditions are controlling variables in the design and operation of energy recovery systems.  相似文献   
9.
Despite the apparent maturity of the learning design field, and the variety of tooling available to support it, adoption among the teacher community (one of its alleged main targets) is still low. There is a lack of research on teachers' perception and use of different technological learning design tools, as existing evaluations are often restricted to a single tool. In order to explore whether there are common factors hampering teacher adoption, and which tool features might appeal to different teachers, more studies involving multiple authoring tools are needed. This paper provides a first step in this direction, describing a mixed methods study performed around a professional development workshop with 18 university teachers from multiple disciplines. This workshop exposed teachers to two different authoring tools (WebCollage and EDIT2), as they learned to create computer-supported collaborative learning (CSCL) designs and implement them. The findings of our interpretive study (which included questionnaires, observations, or group discussion recordings) support the idea that there is no single tool or set of features that are globally perceived as better, although our evidence also highlights certain factors as important for participant teachers – amongst others, the integration of learning designs with the ICT platforms for enactment, as well as with other tools that they already use in their everyday practice.  相似文献   
10.
A number of new azo disperse dyes with polyfluoroalkyl groups attached to the nitrogen atom of the coupling component have been prepared. The influence of polyfluoroalkyl groups on the colour and properties of azo dyes has been studied. The replacement of hydrogen atoms of the methyl group of the coupling component by polyfluoroalkyl radicals causes a hypsochromic shift of absorption maximum in a neutral solution and deepening of the dye colour in an acid solution. All tested dyes showed a high lightfastness on acetate and polyamide fibres.  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