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   2653篇
  免费   69篇
  国内免费   4篇
电工技术   35篇
综合类   6篇
化学工业   372篇
金属工艺   87篇
机械仪表   30篇
建筑科学   76篇
矿业工程   10篇
能源动力   62篇
轻工业   171篇
水利工程   17篇
石油天然气   7篇
无线电   341篇
一般工业技术   343篇
冶金工业   861篇
原子能技术   8篇
自动化技术   300篇
  2022年   30篇
  2021年   25篇
  2020年   24篇
  2019年   27篇
  2018年   29篇
  2017年   31篇
  2016年   34篇
  2015年   31篇
  2014年   59篇
  2013年   130篇
  2012年   87篇
  2011年   105篇
  2010年   60篇
  2009年   69篇
  2008年   102篇
  2007年   99篇
  2006年   88篇
  2005年   86篇
  2004年   68篇
  2003年   68篇
  2002年   61篇
  2001年   40篇
  2000年   52篇
  1999年   43篇
  1998年   203篇
  1997年   121篇
  1996年   106篇
  1995年   64篇
  1994年   59篇
  1993年   53篇
  1992年   29篇
  1991年   24篇
  1990年   44篇
  1989年   29篇
  1988年   31篇
  1987年   21篇
  1986年   29篇
  1985年   35篇
  1984年   20篇
  1983年   23篇
  1982年   25篇
  1981年   17篇
  1980年   24篇
  1978年   20篇
  1977年   27篇
  1976年   50篇
  1975年   17篇
  1973年   29篇
  1972年   23篇
  1971年   15篇
排序方式: 共有2726条查询结果,搜索用时 968 毫秒
121.
Seven independent groups estimated the location of North American cities using both spatial and numeric response modes and a variety of perceptual and memory supports. These supports included having location markers for each city color coded by nation and identified by name, giving participants the opportunity to see and update all their estimates throughout the task, and allowing them to respond directly on a map. No manipulation mitigated the influence of categories on the judgments, but some manipulations improved within-region ordinal accuracy. The data provide evidence that the city and regional levels are independent, spatial and numeric response modalities affect accuracy differently at the different levels, biases at the regional level have multiple sources, and accurate spatial cues improve estimates primarily by limiting the use of global landmarks to partition the response space. Results support J. Huttenlocher, L. V. Hedges, and S. Duncan's (1991) theory of spatial location estimates and extend it to the domain of real-world geography. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
122.
Within the last years there has been increasing interest in direct liquid fuel cells as power sources for portable devices and, in the future, power plants for electric vehicles and other transport media as ships will join those applications. Methanol is considerably more convenient and easy to use than gaseous hydrogen and a considerable work is devoted to the development of direct methanol fuel cells. But ethanol has much lower toxicity and from an ecological viewpoint ethanol is exceptional among all other types of fuel as is the only chemical fuel in renewable supply. The aim of this study is to investigate the possibility of using direct alcohol fuel cells fed with alcohol mixtures. For this purpose, a comparative exergy analysis of a direct alcohol fuel cell fed with alcohol mixtures against the same fuel cell fed with single alcohols is performed. The exergetic efficiency and the exergy loss and destruction are calculated and compared in each case. When alcohol mixtures are fed to the fuel cell, the contribution of each fuel to the fuel cell performance is weighted attending to their relative proportion in the aqueous solution. The optimum alcohol composition for methanol/ethanol mixtures has been determined.  相似文献   
123.
Amylomaltase-treated starch (ATS) is an excellent creaminess enhancer in yoghurt. Small amounts of ATS raised the creaminess perception of low-fat yoghurt (1.5%) to that of full-fat yoghurt (5%). In this way, a reduction in fat-related energy value could be achieved from 45 to 21.5 kcal/100 g product. The functionality of ATS in set yoghurt resulted from discrete domains of ATS that resemble the microstructural behaviour of fat particles. The microstructure of the yoghurt is dominated by the protein and the ATS domains are enclosed in or bound to this protein network. The perceived creaminess resulted from in-mouth melting of these ATS domains due to a combined effect of their physical melting and hydrolysis by amylase present in the saliva.  相似文献   
124.
A chloride-based inorganic sol–gel route was used for preparing pure and metal (osmium, nickel, palladium, platinum)-doped SnO2 sol. SnCl4 was first reacted with propanol, then the resulting compound was hydrolyzed and subsequently mixed with solutions of the metal dopants. The obtained sols were used for depositing thin films by spin coating or for preparing powders by solvent evaporation at 110°C. FTIR spectroscopy and thermal analysis of the powders revealed that chlorine still bound to tin stabilized the sol against gelation by hindering the condensation reactions. Film characterizations showed that platinum and palladium, unlike nickel and osmium, were likely to form nanoparticles in the SnO2 lattice. This result was discussed with regard to the different ways that platinum and palladium, on one hand, and nickel and osmium, on the other, modified the growth of SnO2 grains and the film roughness and morphology. Dopants that formed nanoparticles (platinum, palladium) resulted in the roughest film, while dopants that did not form particles (nickel, osmium) resulted in SnO2 grain size very close to that of pure SnO2.  相似文献   
125.
126.
Field observations suggest that while pressure depletion is a local process that occurs dominantly in reservoirs being exploited, it triggers a redistribution of effective stress in a more extensive domain that involves not just the adjacent strata, but the entire overburden and surrounding rocks out to considerable distances. In an infinite or semi-infinite domain, a petroleum reservoir can be simulated as a displacement discontinuity, permitting use of an efficient displacement discontinuity boundary element method to calculate surrounding stress and displacement changes. A 3D poroelastic finite element method is used to account for the local reservoir model. By coupling the displacement discontinuity and finite element models, a 3D poroelastic reservoir in an infinite or semi-infinite domain is simulated. The numerical model has been verified and numerical examples are given. Results show that the relative deformation properties of surrounding strata have significant effects on pressure drawdown and reservoir deformation during production, and thus should be taken into account.  相似文献   
127.
Governments are increasingly reliant on the reacquisition of water rights as a mechanism for recovering overexploited basins. Yet, serious concerns have recently been raised about the efficacy and operational dimensions of existing programs. Water buyback is typically implemented as the purchase of a fixed quantity of water rights from the agricultural sector at the price set by the Water Authority. This paper seeks to analyze whether the use of water buyback in its current form represents a sensible means of recovering overexploited basins. The results??which are particularly relevant to contexts characterised by poor enforcement regimes and widespread illegal water use??highlight the need for greater scrutiny of current programs and call for additional work to improve the design of reacquisition policies in the context of water resource management.  相似文献   
128.
Leo Ainola 《热应力杂志》2013,36(7):614-623
By investigating axisymmetric stress fields with integrated photoelasticity, the stress components σz and σrz can be determined directly from the experimental data. Stress components σr and σθ are usually determined using the equilibrium and compatibility equations. In this article it is shown that the stress function for an axisymmetric thermoelastic stress field can be determined on the basis of experimental data, obtained with integrated photoelasticity. Knowledge of the stress function permits one to calculate all the stress components as well as the temperature field in the test object.  相似文献   
129.
130.
The purpose of this study was to investigate the impact of low fat diets in children aged 2 to 5. Eighty two children (40 females and 42 males) attending a school cafeteria (Province of Buenos Aires, Argentina), in a cross sectional study, were evaluated. Body weight (W), height (H) and body composition (BC) by bioimpedance were recorded. The anthropometric raw data were processed as Z-score of the weight-for-age (WEZ) and of the height-for-age (HAZ). Serum insulin-like growth factor 1 (IGF-1) and Zinc/haemoglobin ratio (Zn/Hb) were also measured. Results showed that 73.2% of the children were adequate (A) according WEZ, 13.4% were lean (L) and 13.4% overweight (O). 8.5% presented simultaneously impairment in WEZ and HAZ. Body fat percentage and energy metabolism were higher in O than in L and A (p < 0.05). Serum IGF-1's children--aged 4 to 5 years--with HAZ deficit were low than adequate HAZ ones. No statistical differences in Zn/Hb ratio between A, L and O were found. This cross sectional study suggests metabolic disorders in young children attending school cafeterias. These conclusions will allow to design balanced diets in order to optimize the resources, promote optimal growth and development and prevent adult diseases through dietary practices in childhood.  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