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   644931篇
  免费   8555篇
  国内免费   1972篇
电工技术   11762篇
综合类   600篇
化学工业   97736篇
金属工艺   25259篇
机械仪表   18579篇
建筑科学   15911篇
矿业工程   2958篇
能源动力   17621篇
轻工业   57474篇
水利工程   6304篇
石油天然气   11315篇
武器工业   39篇
无线电   72841篇
一般工业技术   123514篇
冶金工业   126852篇
原子能技术   13210篇
自动化技术   53483篇
  2021年   5727篇
  2020年   4096篇
  2019年   5277篇
  2018年   9055篇
  2017年   8874篇
  2016年   9316篇
  2015年   6565篇
  2014年   10859篇
  2013年   29348篇
  2012年   17409篇
  2011年   24049篇
  2010年   19009篇
  2009年   21381篇
  2008年   21760篇
  2007年   21460篇
  2006年   18752篇
  2005年   17217篇
  2004年   16492篇
  2003年   16090篇
  2002年   15358篇
  2001年   15463篇
  2000年   14428篇
  1999年   15278篇
  1998年   39562篇
  1997年   27744篇
  1996年   21173篇
  1995年   15952篇
  1994年   13764篇
  1993年   13471篇
  1992年   9732篇
  1991年   9174篇
  1990年   8960篇
  1989年   8744篇
  1988年   8192篇
  1987年   7314篇
  1986年   7152篇
  1985年   8016篇
  1984年   7486篇
  1983年   6704篇
  1982年   6315篇
  1981年   6426篇
  1980年   6058篇
  1979年   5976篇
  1978年   5725篇
  1977年   6930篇
  1976年   9069篇
  1975年   4978篇
  1974年   4735篇
  1973年   4866篇
  1972年   4002篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Foreword     
  相似文献   
992.
993.
A survey of the state of SQA in Japan finds that the situation is much like, but slightly different than, that in the US.  相似文献   
994.
995.
996.
The quantities of olefins, polyaromatic hydrocarbons and char formed during thermal decomposition in closed systems of alkylbenzenes are substantially reduced if the decomposition takes place in the presence of a good acceptor molecule such as pyrene or adamantane. The chemistry seems to be applicable to the upgrading of whole crude oils by a thermal treatment at 425-450°C in a closed system for 20-25 minutes.  相似文献   
997.
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   
998.
New functionalized styrene–maleimide copolymers were prepared by free radical copolymerization of styrene (St) and N‐4‐carboxybutylmaleimide (NBMI) in chloroform, using 2,2′‐azobisisobutyronitrile (AIBN) as initiator. Monomer and copolymer characterization was carried out by 1H‐ and 13C‐NMR. Copolymer composition was determined by elemental analysis and Fourier‐transform infrared (FTIR) spectroscopy. The glass transition temperature (from DSC) and the thermogravimetric analysis (TGA) of the copolymers were consistent with the thermal behavior and stability observed for alternating St–maleimide copolymers. St–NBMI copolymers crosslinked with divinylbenzene (DVB) were also synthesized and their cation exchange properties evaluated in order to assess the capacity of the new copolymers to bind metallic ions. Copyright © 2005 Society of Chemical Industry  相似文献   
999.
New electroactive and photoactive conjugated copolymers consisting of alternating 2,7‐carbazole and oligothiophene moieties linked by vinylene groups have been developed. Different oligothiophene units have been introduced to study the relationship between the polymer structure and the electronic properties. The resulting copolymers are characterized by UV‐vis spectroscopy, size‐exclusion chromatography, and thermal and electrochemical analyses. Bulk heterojunction photovoltaic cells from different copolymers and a soluble fullerene derivative, [6,6]‐phenyl‐C61 butyric acid methyl ester, have been fabricated, and promising preliminary results are obtained. For instance, non‐optimized devices using poly(N‐(4‐octyloxyphenyl)‐2,7‐carbazolenevinylene‐alt‐3″,4″‐dihexyl‐2,2′;5′,2″;5″,2″′;5″′,2″″‐quinquethiophenevinylene 1″,1″‐dioxide) as an absorbing and hole‐carrier semiconductor exhibit power conversion efficiency up to 0.8 % under air mass (AM) 1.5 illumination. These features make 2,7‐carbazolenevinylene‐based and related polymers attractive candidates for solar‐cell applications.  相似文献   
1000.
Genotype by environment interaction (GEI) is a relevant topic in many fields of applied biology. Normal parametric approaches following analysis of variance are not suitable when large differences between error variances within environments are present. We illustrate an APL program following Yamada's approach to estimate GEIs and ancillary statistics when two environments are considered. The choice of APL environment is based on its value as a useful tool in algorithm implementation and problem-solving. Its use is suggested in biological applications where matrix algebra is involved and an understanding of the computing complexity of problems is generally not required.  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