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   551756篇
  免费   7063篇
  国内免费   1974篇
电工技术   10234篇
综合类   389篇
化学工业   80913篇
金属工艺   20435篇
机械仪表   15460篇
建筑科学   13438篇
矿业工程   2028篇
能源动力   14561篇
轻工业   51027篇
水利工程   4859篇
石油天然气   7246篇
武器工业   13篇
无线电   69543篇
一般工业技术   101921篇
冶金工业   112439篇
原子能技术   9680篇
自动化技术   46607篇
  2021年   3847篇
  2019年   3680篇
  2018年   5917篇
  2017年   5950篇
  2016年   6411篇
  2015年   4628篇
  2014年   7788篇
  2013年   24378篇
  2012年   13622篇
  2011年   19086篇
  2010年   15039篇
  2009年   17412篇
  2008年   18004篇
  2007年   18101篇
  2006年   16302篇
  2005年   15175篇
  2004年   14581篇
  2003年   14236篇
  2002年   13524篇
  2001年   14256篇
  2000年   13131篇
  1999年   14204篇
  1998年   37256篇
  1997年   25620篇
  1996年   19553篇
  1995年   14676篇
  1994年   12611篇
  1993年   12529篇
  1992年   8775篇
  1991年   8435篇
  1990年   8132篇
  1989年   7865篇
  1988年   7536篇
  1987年   6279篇
  1986年   6204篇
  1985年   7277篇
  1984年   6564篇
  1983年   5985篇
  1982年   5534篇
  1981年   5736篇
  1980年   5219篇
  1979年   4997篇
  1978年   4792篇
  1977年   5735篇
  1976年   7468篇
  1975年   4140篇
  1974年   3995篇
  1973年   4050篇
  1972年   3280篇
  1971年   2973篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
871.
Image processing allows the automation of the business transaction, turning “paper factories” into “image factories.” This burgeoning new area of computerization needs new paradigms, theories, and methods to maximize its effectiveness at cutting time and costs. This paper investigates what this new area of computerization can gain from CIM technology. It examines the similarities between the processing of computer images in a paperless business environment and the processing of materials into products in the manufacturing plant. These similarities will allow us to apply mature, well-tested CIM techniques to emerging image factories.  相似文献   
872.
Foreword     
  相似文献   
873.
Erosion and corrosion of metallic materials in particle containing aqueous solutions The combined effect and mutual influence, respectively, of erosion and corrosion of metallic materials in particle containing flows was investigated. Materials under investigation are carbon steel of different hardness levels, ferritic chromium steel, austenitic stainless steel and brass. Corrosive medium was deionized water, acid (pH 4.5), alkaline (pH 10) and chloride (3% NaCl) solution. Quartz particles were used as abrasive at various particle concentrations, grain sizes and flow velocities. For comparison, tests were performed with pure water and under conditions of idle corrosion. Mechanical or corrosive influences dominate depending on material and test conditions, in some cases, synergistic effects of both mechanisms cause an increased mass loss.  相似文献   
874.
Surveyed the prevalence of weight reducing and weight gaining in 1,373 high school students. 63% of the girls and 16.2% of the boys reported being on weight-reducing regimens; 9.1% of the girls and 28.4% of the boys were trying to gain weight. Most female reducers and male gainers were already normal weight. Whites and Hispanics were more likely to be reducing, whereas Blacks were more likely to be gaining. Exercise and moderate caloric reduction were most popular for weight reducing, and a small but significant number were regularly using fasting, vomiting, laxatives, and appetite suppressants. The rate of weight reducing in female high school adolescents has increased significantly since similar surveys of American youths 20 yrs ago. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
875.
876.
877.
The exfiltration of wastewater from sewer systems in urban areas causes a deterioration of soil and possibly groundwater quality. Beside the simulation of transport and degradation processes in the unsaturated zone and in the aquifer the analysis of the potential impact requires the estimation of quantity and temporal variation of wastewater exfiltration. Exfiltration can be assessed by the application of a leakage model. The hydrological approach was originally developed to simulate the interactions between the groundwater and surface water, it was adapted to allow for modelling of interactions between groundwater and sewer system. In order to approximate the exfiltration specific model parameters infiltration specific parameters were used as a basis. Scenario analysis of the exfiltration in the City of Dresden from 1997 to 1999 and during the flood event in August 2002 shows the variation and the extent of exfiltration rates.  相似文献   
878.
Segregation of reactive metals at the bonding interface has been observed in various ceramic and/or metal joints bonded with reactive metal-bearing braze alloys. When a d.c. of 20 mAcm−2 is applied to the ceramic/braze/ceramic system at a brazing temperature of, say, 1373 K, the electric field assists the segregation at the braze-ceramic interface on the cathode side and suppresses the segregation at the interface on the anode side. This may imply that reactive metal atoms in the braze can migrate as a cation. E.m.f. measurement on the ceramic (AIN or ZrB2)-metal foil systems with increasing temperature shows that a negative e.m.f. to the ceramic pole appears from about 900 K for AIN and from 500 K for ZrB2, as does the thermally stimulated current in polymers. These temperatures coincide well with those where the electrical conductivity of AIN and ZrB2, respectively, begins to increase with increasing temperature. Therefore, it is considered that the polarization of the ceramics may take place and assist the migration, and consequently segregation, of reactive metals in braze alloys to the braze-ceramic interface during brazing.  相似文献   
879.
This paper attempts to measure the effect of a court-ordered desegregation plan on three aspects of the housing market in Dallas County, Texas. The indicators include sale price, number of sales, and proportion of home sales financed by conventional mortgages. Monthly data from 1969 to 1979 are analyzed using Box-Jenkins time series models. After best fit models are determined, various intervention terms are introduced into the models to assess the impact of both a 1975 Circuit Court Order and a 1976 District Court Order. The analysis suggests neither order had a measurable effect on the housing market in Dallas.  相似文献   
880.
An investigation of the effect of reaction conditions on product distribution in the skeletal isomerisation reaction of linear butenes has been carried out. The main reaction routes over ferrierite have been identified. Beside the main product isobutene, major by-product formation occurs. The unwanted reactions include dimerisation of butene to form octenes, hydrogen transfer yielding small amounts of saturated C3 and C4 hydrocarbons and disproportionation producing propene and pentenes. The most abundant by-products were pentene and propene, though these were not formed in equimolar amounts as could be expected. Oligomerisation experiments of propene over ferrierite produced large amounts of butene and pentene, revealing the presence of adsorbed nonene. The cracking of this surface species to hexene and propene is the most likely reaction route for the excess propene formation. This additional path to propene formation operates mainly at temperatures above 623 K.  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