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   358764篇
  免费   3964篇
  国内免费   1703篇
电工技术   5532篇
综合类   2830篇
化学工业   51164篇
金属工艺   15732篇
机械仪表   11374篇
建筑科学   8492篇
矿业工程   2122篇
能源动力   8041篇
轻工业   25885篇
水利工程   4232篇
石油天然气   6119篇
武器工业   43篇
无线电   42432篇
一般工业技术   71841篇
冶金工业   53320篇
原子能技术   7728篇
自动化技术   47544篇
  2021年   2153篇
  2019年   2063篇
  2018年   17629篇
  2017年   16608篇
  2016年   13582篇
  2015年   3065篇
  2014年   4552篇
  2013年   12120篇
  2012年   10068篇
  2011年   18790篇
  2010年   15489篇
  2009年   14068篇
  2008年   15626篇
  2007年   16613篇
  2006年   7902篇
  2005年   8383篇
  2004年   8250篇
  2003年   8086篇
  2002年   7211篇
  2001年   6923篇
  2000年   6644篇
  1999年   6807篇
  1998年   16338篇
  1997年   11602篇
  1996年   8955篇
  1995年   6851篇
  1994年   6023篇
  1993年   6059篇
  1992年   4406篇
  1991年   4202篇
  1990年   4073篇
  1989年   4055篇
  1988年   3759篇
  1987年   3387篇
  1986年   3392篇
  1985年   3739篇
  1984年   3460篇
  1983年   3217篇
  1982年   3024篇
  1981年   3038篇
  1980年   3030篇
  1979年   2829篇
  1978年   2869篇
  1977年   3141篇
  1976年   4096篇
  1975年   2416篇
  1974年   2379篇
  1973年   2396篇
  1972年   2018篇
  1971年   1781篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
Co-word analysis applied to patents through WPIL normalized title words appears to give a useful picture of a given field: we obtain both qualitative (themes) and quantitative information (weight of themes). It also gives information about the strategic aspects of the themes. Furthermore, in some cases, it is an indication of the future of certain themes that may help forecasting and management studies. Finally, it provides information about what could be a real technology growth process, in relation to the so-called translation model used in co-word analysis.  相似文献   
122.
123.
The variation of the dynamic elastic modulus in the immediate vicinity of the tip of the running crack was studied through an iterative procedure, based on the theoretical expressions for the stress-field components and the experimental relation between strain rate and elastic modulus. It was found that the elastic modulus varied strongly around the tip of the crack, both in radial and polar sense. Also it was observed that the polar distribution of the elastic modulus presented clear off-axis extrema in directions that were in good agreement with experimentally measured branching angles, thus indicating a possible relation between these two phenomena.  相似文献   
124.
125.
Good access to health facilities providing good first-level health care remains problematic in many developing countries. It is a hindrance to effective and efficient functioning of the hospital, as outpatient departments become overcrowded with patients from areas without health centres. In many cases the quality of care delivered to these patients is poor because within the district health system the hospital is not the best place for the supply of comprehensive, integrated and continuous care. Eventually, high hospital involvement in first-level care can jeopardize the delivery of adequate referral care for those patients who desperately need the hospital's technology and expertise. This paper provides an account of the way this problem was investigated and managed by the district health management team in the Murewa district in north-east Zimbabwe. The design of a comprehensive 'master plan' or 'coverage plan' is presented as well as the problems and difficulties encountered. The Murewa experience highlights the relevance of a coverage plan for rational and coherent health infrastructure planning at district level. The approach followed by the Murewa team illustrates the use of action research as an integral part of the management of district health systems.  相似文献   
126.
127.
128.
The deletion of nine residues from the C-terminus of the bacterialchloramphenicol acetyltransferase (CAT) results in depositionof the mutant protein in cytoplasmic inclusion bodies and lossof chloramphenicol resistance in Escherichia coli. This foldingdefect is relieved by C-terminal fusion of the polypeptide withas few as two residues. Based on these observations, efficientpositive selection for the cloning of DNA fragments has beendemonstrated. The cloning vector encodes a C-terminally truncatedCAT protein. Restriction sites in front of the stop codon allowthe insertion of target DNA, resulting in the production ofproperly folded CAT fusion proteins and regained chloramphenicolresistance. The positive selection of recombinants is accomplishedby growth of transformants on chloramphenicol-containing agarplates. The method appears particularly convenient for the cloningof DNA fragments amplified by the PCR because minimal informationto restore CAT folding can be included in the primers. The cloningof random sequences shows that the folding defect can be relievedby fusion to a wide variety of peptides, providing great flexibilityto the positive selection system. This vector may also contributeto the determination of the role of the C-terminus in CAT folding.  相似文献   
129.
130.
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