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   8438篇
  免费   476篇
  国内免费   11篇
电工技术   115篇
综合类   38篇
化学工业   2001篇
金属工艺   226篇
机械仪表   179篇
建筑科学   605篇
矿业工程   21篇
能源动力   279篇
轻工业   820篇
水利工程   86篇
石油天然气   43篇
武器工业   1篇
无线电   653篇
一般工业技术   1441篇
冶金工业   551篇
原子能技术   59篇
自动化技术   1807篇
  2023年   75篇
  2022年   72篇
  2021年   292篇
  2020年   184篇
  2019年   202篇
  2018年   270篇
  2017年   233篇
  2016年   313篇
  2015年   234篇
  2014年   339篇
  2013年   628篇
  2012年   485篇
  2011年   616篇
  2010年   427篇
  2009年   492篇
  2008年   455篇
  2007年   412篇
  2006年   341篇
  2005年   291篇
  2004年   270篇
  2003年   254篇
  2002年   184篇
  2001年   124篇
  2000年   114篇
  1999年   88篇
  1998年   112篇
  1997年   113篇
  1996年   82篇
  1995年   80篇
  1994年   80篇
  1993年   80篇
  1992年   56篇
  1991年   58篇
  1990年   60篇
  1989年   67篇
  1988年   49篇
  1987年   47篇
  1986年   48篇
  1985年   53篇
  1984年   61篇
  1983年   45篇
  1982年   38篇
  1981年   37篇
  1980年   39篇
  1979年   32篇
  1978年   34篇
  1977年   33篇
  1976年   39篇
  1975年   40篇
  1973年   36篇
排序方式: 共有8925条查询结果,搜索用时 234 毫秒
101.
To further our understanding on policymaking and policy change we need to recognize the significance of individual key actors in policy and planning processes. This article theorizes on the characteristics and policy influence of inside activism in which individual public officials act strategically from inside public administration to change government policy and action in line with a civic engagement and value commitment. Based on initial empirical findings from Swedish local government, we argue that inside activism is empirically relevant but not satisfactorily covered by other key actor concepts. We theorize that inside activism is 1) dualistic: open, deliberative, consensus-seeking and tacit, tactical, power-driven; 2) influential through informal networking inside and outside of government; and 3) dynamic as it varies over time and between critical situations. Due to current trends in society and public administration (e.g. governance), we expect inside activism to be increasingly relevant and we encourage further theoretical, empirical as well as normative research and discussion on this phenomenon.  相似文献   
102.
This article contributes to an understanding of policy change. Exploring a local land planning case, it investigates how an environmental advocacy coalition effectively challenged road and housing plans, with the result that an area was instead developed into a nature reserve. In the course of the article, the value and practical effectiveness of the Advocacy Coalition Framework (ACF) is examined and developed with the help of two central concepts—the “value network” and “inside activist”. The outcome of the case is explained by the powerful influence of a value network of ornithologists, with particular inside activists of that network playing important roles in presenting a challenge to development of the area. Instead of trying to build an abstract theory of the ACF, the article argues the need to develop the ACF as a framework, opening it up to insights from policy network and social constructionist research as well as from practical processes.  相似文献   
103.
104.
105.
The temporal evolution of atmospheric lead deposition and its possible sources were assessed in eastern Canada and in western Scotland, using blanket peat bogs as geochemical archives. Short cores were taken from two remote sites located close to the sea. Significant lead enrichments in the upper layers at both sites reflect the increasing emission of lead into the atmosphere due to anthropogenic activities during the last century. At the Scottish site, a region under aeolian influence from Europe, anthropogenic derived lead could be recognized by the distinctive unradiogenic composition (206Pb/207Pb ratios down to approximately 1.115), being clearly different from the pre-industrial values (206Pb/207Pb approximately 1.166). In contrast, the lead pollution in eastern Canada (influenced by North American sources) is identified by a more radiogenic lead isotope composition (206Pb/207Pb ratios up to approximately 1.199) compared to preindustrial values (206Pb/207Pb approximately 1.161). Emission inventories and isotope characteristics suggest that industrial (coal burning, mining) and traffic (leaded gasoline) outputs are the most likely sources during the first and the second half of the 20th century, respectively, in both, western Scotland and eastern Canada alike. The Scottish record is in line with previous studies of past atmospheric lead deposition. However, the Canadian deposit suggests that the wind derived, pre-industrial lead, is less radiogenic as previously implied using sediment archives. These results are thus the first to report pre-industrial lead isotope ratios and concentrations of atmospheric derived aerosols in North America.  相似文献   
106.
Journal of Applied Electrochemistry - The consumption of critical raw materials, especially those in permanent magnets of Nd–Fe–B and Sm–Co-type, has significantly grown in the...  相似文献   
107.
The rate of hydrolytic degradation of tissue‐engineered scaffolds made from bioresorbable polyesters is dependent on several factors. Some are related to the properties of the degrading polymeric material, but others are related to the geometry of the porous structure and the operating environment. It is well known that the rate of hydrolytic degradation of a given object, porous or nonporous, is lower when it is exposed to dynamic conditions, a flowing medium, than when it operates in static conditions. The most likely reason is the more efficient removal of the acidic degradation products from the vicinity of the polymeric material when it is operating in a flowing medium. In this article, we present a new phenomenological reaction–diffusion model of aliphatic polymer degradation. The model can be used to predict the significance of various factors in in vitro degradation tests, with particular reference to the flow of the degradation medium, and the frequency of medium replacement in the case of static conditions. The developed model was used to simulate the degradation of poly(dl ‐lactide‐co‐glycolide) scaffolds with different porosities subjected to static and dynamic testing conditions. The results confirm that the porosity of the scaffold had a significant influence on the degradation rate. It was shown that the combination of dynamic conditions and high porosity effectively reduced the mass loss and molecular weight loss of the degrading polymer. However, the effect of changes in the velocity of the flowing medium had a negligible effect on the rate of degradation. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40280.  相似文献   
108.
109.
Waste expanded polystyrene (EPS) represents a source of valuable chemical products like styrene and other aromatics. The catalytic degradation was carried out in a batch reactor with a mixture of polystyrene (PS) and catalyst at 450 °C for 30 min in case of Mg and at 400 °C for 2 h both for MgO and MgCO3 catalysts. At optimum degradation conditions, EPS was degraded into 82.20±3.80 wt%, 91.60±0.20 wt% and 81.80±0.53 wt% liquid with Mg, MgO and MgCO3 catalysts, respectively. The liquid products obtained were separated into different fractions by fractional distillation. The liquid fractions obtained with three catalysts were compared, and characterized using GC-MS. Maximum conversion of EPS into styrene monomer (66.6 wt%) was achieved with Mg catalyst, and an increase in selectivity of compounds was also observed. The major fraction at 145 °C showed the properties of styrene monomer. The results showed that among the catalysts used, Mg was found to be the most effective catalyst for selective conversion into styrene monomer as value added product.  相似文献   
110.
The development of motion capturing devices poses new challenges in the exploitation of human‐motion data for various application fields, such as computer animation, visual surveillance, sports, or physical medicine. Recently, a number of approaches dealing with motion data have been proposed, suggesting characteristic motion features to be extracted and compared on the basis of similarity. Unfortunately, almost each approach defines its own set of motion features and comparison methods; thus, it is hard to fairly decide which similarity model is the most suitable for a given kind of human‐motion retrieval application. To cope with this problem, we propose the human motion model evaluator, which is a generic framework for assessing candidate similarity models with respect to the purpose of the target application. The application purpose is specified by a user in form of a representative sample of categorized motion data. Respecting such categorization, the similarity models are assessed from the effectiveness and efficiency points of view using a set of space‐complexity, information‐retrieval, and performance measures. The usability of the framework is demonstrated by case studies of three practical examples of retrieval applications focusing on recognition of actions, detection of similar events, and identification of subjects. Copyright © 2015 John Wiley & Sons, Ltd.  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