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   1617426篇
  免费   25180篇
  国内免费   7137篇
电工技术   35016篇
综合类   6661篇
化学工业   276804篇
金属工艺   65832篇
机械仪表   46205篇
建筑科学   48511篇
矿业工程   11738篇
能源动力   50543篇
轻工业   126432篇
水利工程   16315篇
石油天然气   38038篇
武器工业   207篇
无线电   199950篇
一般工业技术   301369篇
冶金工业   217115篇
原子能技术   34396篇
自动化技术   174611篇
  2021年   15729篇
  2020年   12095篇
  2019年   14828篇
  2018年   18286篇
  2017年   17718篇
  2016年   22721篇
  2015年   17756篇
  2014年   28989篇
  2013年   88410篇
  2012年   38502篇
  2011年   53096篇
  2010年   45174篇
  2009年   52767篇
  2008年   48872篇
  2007年   46534篇
  2006年   47189篇
  2005年   42388篇
  2004年   43938篇
  2003年   43661篇
  2002年   42282篇
  2001年   39630篇
  2000年   37695篇
  1999年   37782篇
  1998年   62145篇
  1997年   49566篇
  1996年   42013篇
  1995年   34675篇
  1994年   31910篇
  1993年   31904篇
  1992年   27216篇
  1991年   24444篇
  1990年   24622篇
  1989年   23691篇
  1988年   22246篇
  1987年   20451篇
  1986年   19864篇
  1985年   23121篇
  1984年   22841篇
  1983年   20804篇
  1982年   19663篇
  1981年   19850篇
  1980年   18445篇
  1979年   18848篇
  1978年   18112篇
  1977年   18619篇
  1976年   20864篇
  1975年   16243篇
  1974年   15725篇
  1973年   15835篇
  1972年   13274篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
Chronic hepatitis C has been demonstrated to be associated with hepatic iron overload, and the hypothesis that the disease activity of hepatitis C is associated with iron cytotoxicity was tested in male volunteer blood donors. Sera with either antibody to hepatitis C virus or hepatitis B surface antigen were selected for determination of ferritin concentration and alanine aminotransferase activity. A correlation between serum ferritin concentration (Y; microgram/l) and alanine aminotransferase activity (X; IU/l) was found in donors with antibody to hepatitis C (log Y = 0.65 x log X + 0.98, r = 0.53, and P < 0.01). The correlation was lower in donors with hepatitis B surface antigen (r = 0.37; P < 0.01). Hepatitis C virus infection probably induces time-dependent iron accumulation associated with the progression of disease activity, while hepatitis B virus infection results in a variety of iron loads with different clinical features. The high disease activity related to hyperferritinemia suggests the presence of iron-induced liver damage in donors with hepatitis C.  相似文献   
962.
963.
Gilbenclamide, a widely used potent hypoglycaemic agent was solubllized using β -Cyclodextrin and β -Cyclodextrin derivatives. Complexes were prepared by kneading method in a molar ratio of 1:1 of the drug and the cyclodextrlns respectively. The Glibenclamide β -Cyelocextrin complex was characterized and evaluated by I.R. studies, Differential Scanning Calorimotry 6 X-ray diffractometry. The in-vitro dissolution rates of drug from inclusion complexes of β Cyclodextrins and its derivatives were compared. A significant Improvement In dissolution lor, rates of Gllbenclamide was observed with Inclusion complexes of all the Cyclodextrins. However, the solubilizing effect was more in case of β-Cyclodextrin derivatives.  相似文献   
964.
965.
Adam  J.A. 《Spectrum, IEEE》1994,31(10):24-26
The author focusses on advances in equipment that offset physically functional limitations. He discusses how disabled and elderly people are using aids that enhance mobility (walking a certain distance, lifting a weight, or climbing stairs) as well as sensory faculties (seeing print, hearing conversation, and speaking intelligibly)  相似文献   
966.
The time-scattering of an electromagnetic wave by a wedge-shaped region in a lossy medium, both with frequency dependent electric characteristics, is analyzed for plane wave and line source illuminations. New exact analytical expressions, satisfying explicitely the causality and allowing useful physical decompositions of the field, are obtained in these cases, using the particularities of the harmonic response in Sommerfeld-Maliuzhinets integral, especially original properties of the spectral function attached to it.  相似文献   
967.
Fractal error for detecting man-made features in aerial images   总被引:3,自引:0,他引:3  
A technique is proposed for aiding photointerpreters in detecting man-made features in aerial reconnaissance images. The technique, which uses a metric called fractal error, is based on the observed propensity of natural image features to fit a fractional Brownian motion model. Man-made features usually do not fit this model well, and consequently the fractal error metric may be used as a discriminant function for detecting man-made scene features  相似文献   
968.
The authors demonstrate 10 Gbit/s optical soliton transmission in a recirculating loop through the use of a monolithically integrated MQW-DFB-LD/MQW-EA modulator light source. The timing jitter due to the Gordon-Haus effect is successfully reduced by the optical bandpass filters conventionally used in the cascaded EDFAs to avoid the accumulation of amplified spontaneous emission. The transmission distance of 7200 km is achieved at a bit error rate of 10-9  相似文献   
969.
a-Si alloy three-stacked solar cells have been studied to improve the stabilized efficiency of a-Si: H based solar cells. Based on the analysis by the individual characterization method of the component cells in stacked type cells, the a-Si :H middle cell was replaced with an a-SiGe :H cell. Furthermore, the optical confinement technology was improved to obtain a high-output current with thin i-layer thickness in the a-SiGe :H bottom cell. By this device design, the initial conversion efficiency was improved up to 12.4% and more than a 10% stabilized efficiency was obtained in a-SiC :H/a-SiGe :H/a-SiGe :H three-stacked cells. These cell characteristics were confirmed by measurements at the JQA Organization (the former JMI Institute).  相似文献   
970.
Area- and time-specific marginal capacity costs of electricity distribution   总被引:2,自引:0,他引:2  
Marginal costs of electricity vary by time and location. In the past, researchers attributed the variations to factors related to electricity generation and transmission. These authors, however, have not analyzed possible variations in marginal distribution capacity costs (MDCC). The objectives of this paper are:
1. (i) to show that large MDCC variations are due to the dispersion in distribution capital expenditures by time and space,
2. (ii) to propose a method for quantifying the area- and time-specific MDCC in the presence of lumpy investments, and
3. (iii) to compare our MDCC estimates to those commonly used in the electric utility industry.
Our proposed method and its results were adopted by the California Public Utilities Commission (CPUC) in 1992 for Pacific Gas and Electric Company (PG&E), the largest privately owned electric utility in the U.S.  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