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   32811篇
  免费   1446篇
  国内免费   113篇
电工技术   433篇
综合类   229篇
化学工业   5640篇
金属工艺   765篇
机械仪表   714篇
建筑科学   1111篇
矿业工程   99篇
能源动力   1029篇
轻工业   4430篇
水利工程   288篇
石油天然气   163篇
武器工业   6篇
无线电   2837篇
一般工业技术   4726篇
冶金工业   7370篇
原子能技术   276篇
自动化技术   4254篇
  2023年   165篇
  2022年   294篇
  2021年   880篇
  2020年   610篇
  2019年   684篇
  2018年   793篇
  2017年   870篇
  2016年   844篇
  2015年   651篇
  2014年   977篇
  2013年   1688篇
  2012年   1451篇
  2011年   1800篇
  2010年   1320篇
  2009年   1374篇
  2008年   1272篇
  2007年   1109篇
  2006年   927篇
  2005年   971篇
  2004年   996篇
  2003年   922篇
  2002年   876篇
  2001年   776篇
  2000年   605篇
  1999年   625篇
  1998年   2336篇
  1997年   1533篇
  1996年   1065篇
  1995年   713篇
  1994年   571篇
  1993年   612篇
  1992年   278篇
  1991年   343篇
  1990年   279篇
  1989年   238篇
  1988年   253篇
  1987年   197篇
  1986年   209篇
  1985年   225篇
  1984年   164篇
  1983年   136篇
  1982年   152篇
  1981年   152篇
  1980年   148篇
  1979年   114篇
  1978年   91篇
  1977年   189篇
  1976年   289篇
  1975年   94篇
  1973年   61篇
排序方式: 共有10000条查询结果,搜索用时 18 毫秒
81.
From visual data exploration to visual data mining: a survey   总被引:8,自引:0,他引:8  
We survey work on the different uses of graphical mapping and interaction techniques for visual data mining of large data sets represented as table data. Basic terminology related to data mining, data sets, and visualization is introduced. Previous work on information visualization is reviewed in light of different categorizations of techniques and systems. The role of interaction techniques is discussed, in addition to work addressing the question of selecting and evaluating visualization techniques. We review some representative work on the use of information visualization techniques in the context of mining data. This includes both visual data exploration and visually expressing the outcome of specific mining algorithms. We also review recent innovative approaches that attempt to integrate visualization into the DM/KDD process, using it to enhance user interaction and comprehension.  相似文献   
82.
Prevalence rates of trauma and posttraumatic stress disorder (PTSD) were estimated from a probability sample of 2,509 adults from 4 cities in Mexico. PTSD was assessed according to Diagnostic and Statistical Manual of Mental Disorders (American Psychiatric Association, 1994) criteria using the Composite International Diagnostic Interview (CIDI; WHO, 1997). Lifetime prevalence of exposure and PTSD were 76% and 11.2%, respectively. Risk for PTSD was highest in Oaxaca (the poorest city), persons of lower socioeconomic status, and women. Conditional risk for PTSD was highest following sexual violence, but nonsexual violence and traumatic bereavement had greater overall impact because of their frequency. Of lifetime cases, 62% became chronic; only 42% received medical or professional care. The research demonstrates the importance of expanding the epidemiologic research base on trauma to include developing countries around the world. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
83.
The product from the hydrogenation of an iron-tin treated Morwell, Victorian coal was separated into a number of fractions by solvent separation. Each of these fractions (tetrahydrofuran-insoluble materials, asphaltols, asphaltenes and oils) was reacted separately with hydrogen in tetralin both with and without added catalysts. The effect of added catalyst and of temperature, pressure, time and solvent on the hydrocracking and repolymerization reactions is discussed. The interconvertibility of the brown coal-derived products is clearly established, reinforcing the importance of incorporating reversibility of reactions in mechanistic models developed to describe coal hydroliquefaction.  相似文献   
84.
A digital signal processor-based control system for the permanent magnet-assisted reluctance synchronous machine, with the emphasis on dynamic performance, is proposed. A classical design approach is used to design the current and speed controllers for the machine. The stator current of the machine is controlled in such a way that the current angle in the dq synchronous reference frame is constant. The load-torque is estimated using a state space observer and compensation current based on the estimated load is used to improve the dynamic performance of the drive. The control system design is machine specific as it relies on data from finite-element analysis. Simulated and measured results on a 110-kW power level show that the resulting control system is stable and robust with good dynamic performance  相似文献   
85.
We consider the dynamics of a dark soliton in an elongated harmonically trapped Bose-Einstein condensate. A central question concerns the behavior at finite temperatures, where dissipation arises due to the presence of a thermal cloud. We study this problem using coupled Gross-Pitaevskii and N-body simulations, which include the mean field coupling between the condensate and thermal cloud. We find that the soliton decays relatively quickly even at very low temperatures, with the decay rate increasing with rising temperature.  相似文献   
86.
The thermal decomposition properties of Mg–Fe hydrotalcites were studied through in situ 57Fe Mössbauer spectroscopy and in situ X-ray diffraction. Abrupt changes in the quadrupolar splitting measured in the Mössbauer spectra revealed a phase transition from the starting lamellar structure to a new crystalline arrangement. By analyzing the Mössbauer parameters we show that the material is highly disordered in the 300–400 °C temperature range. This hypothesis is confirmed by the X-ray results whose diffractograms indicated the collapse of the lamellar structure and the formation of a solid solution.  相似文献   
87.
88.
Microscale wear of vitrified abrasive materials   总被引:2,自引:0,他引:2  
The study of bonding hard materials such as aluminium oxide and cubic boron nitride (cBN) and the nature of interfacial cohesion between these materials and glass is very important from the perspective of high precision grinding. Vitrified grinding wheels are typically used to remove large volumes of metal and to produce components with very high tolerances. It is expected that the same grinding wheel be used for both rough and finish machining operations. Therefore, the grinding wheel, and in particular its bonding system, is expected to react differently to a variety of machining operations. In order to maintain the integrity of the grinding wheel, the bonding system that is used to hold abrasive grains in place will react differently to forces that are placed on individual bonding bridges. This paper examines the role of vitrification heat treatment on the development of strength between abrasive grains and bonding bridges, and the nature of fracture and wear in vitrified grinding wheels that are used for precision grinding applications.  相似文献   
89.
High speed propulsion: Performance advantage of advanced materials   总被引:3,自引:0,他引:3  
High-speed air breathing propulsion systems have many attractive military and civil applications. The high propulsive efficiency of these systems allows the exploitation of speed, distance, and bigger payloads, or any combination of the three. The severe operating conditions of these systems require particular attention to overall thermal management of the engine/air-frame. Fuel-cooling the engine structure is a viable way of maintaining thermal balance over a range of flight conditions. Air Force applications have focused on using endothermic hydrocarbon fuels to address this issue because of their compatibility with the military operations. Recent ground tests of scramjet engines have demonstrated adequate performance utilizing state-of-the-art technology in materials. This progress has paved the way for an expendable flight test vehicle in the near future. In order to take full advantage of the capabilities of this propulsion system, advances in fuel-cooled structures, high temperature un-cooled materials, and increased heat capacity of hydrocarbon fuels will be needed to enable expendable systems to reach higher Mach numbers. An additional benefit would be realized in future reusable systems.  相似文献   
90.
Atmospheric corrosion of reference metals in Antarctic sites   总被引:1,自引:0,他引:1  
This paper presents the results obtained at three Antarctic test sites participating in the “Ibero-American Map of Atmospheric Corrosiveness” (MICAT), a project on atmospheric corrosion carried out during the period 1988–1994 at some 70 sites distributed across 12 countries of the Latin-American region, Spain and Portugal. The three Antarctic sites are located near the coastline.The singular climatic characteristics of Antarctic regions are related with the purity of the air, the absence of rainfall and the formation of ice on the metallic surface during an important part of the exposure time. However, electrochemical activity is possible below ice layers. This situation affects the structure and morphology of corrosion product films and the resulting corrosion rates of metallic surfaces.  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