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   28157篇
  免费   2192篇
  国内免费   31篇
电工技术   184篇
综合类   14篇
化学工业   5939篇
金属工艺   479篇
机械仪表   597篇
建筑科学   867篇
矿业工程   54篇
能源动力   662篇
轻工业   5652篇
水利工程   322篇
石油天然气   125篇
武器工业   1篇
无线电   1230篇
一般工业技术   4739篇
冶金工业   5561篇
原子能技术   85篇
自动化技术   3869篇
  2024年   55篇
  2023年   305篇
  2022年   303篇
  2021年   776篇
  2020年   698篇
  2019年   731篇
  2018年   1313篇
  2017年   1288篇
  2016年   1339篇
  2015年   1014篇
  2014年   1182篇
  2013年   2456篇
  2012年   1855篇
  2011年   1688篇
  2010年   1360篇
  2009年   1175篇
  2008年   1221篇
  2007年   1117篇
  2006年   739篇
  2005年   600篇
  2004年   578篇
  2003年   545篇
  2002年   447篇
  2001年   349篇
  2000年   280篇
  1999年   346篇
  1998年   1931篇
  1997年   1230篇
  1996年   762篇
  1995年   431篇
  1994年   329篇
  1993年   340篇
  1992年   110篇
  1991年   95篇
  1990年   82篇
  1989年   66篇
  1988年   96篇
  1987年   90篇
  1986年   69篇
  1985年   87篇
  1984年   74篇
  1983年   51篇
  1982年   45篇
  1981年   74篇
  1980年   69篇
  1978年   39篇
  1977年   113篇
  1976年   212篇
  1975年   36篇
  1973年   46篇
排序方式: 共有10000条查询结果,搜索用时 481 毫秒
71.
72.
73.
Comparative genomic hybridization (CGH) was used to detect copy number changes of DNA sequences in the Ewing family of tumours (ET). We analysed 20 samples from 17 patients. Fifteen tumours (75%) showed copy number changes. Gains of DNA sequences were much more frequent than losses, the majority of the gains affecting whole chromosomes or whole chromosome arms. Recurrent findings included copy number increases for chromosomes 8 (seven out of 20 samples; 35%), 1q (five samples; 25%) and 12 (five samples; 25%). The minimal common regions of these gains were the whole chromosomes 8 and 12, and 1q21-22. High-level amplifications affected 8q13-24, 1q and 1q21-22, each once. Southern blot analysis of the specimen with high-level amplification at 1q21-22 showed an amplification of FLG and SPRR3, both mapped to this region. All cases with a gain of chromosome 12 simultaneously showed a gain of chromosome 8. Comparison of CGH findings with cytogenetic analysis of the same tumours and previous cytogenetic reports of ET showed, in general, concordant results. In conclusion, our findings confirm that secondary changes, which may have prognostic significance in ET, are trisomy 8, trisomy 12 and a gain of DNA sequences in 1q.  相似文献   
74.
Motor vehicle accidents are the main cause of death and disability between 1 and 4 years of age. Since 1980 the Academy of Pediatrics has been promoting the correct use of car seats. The major reason that car seats are not fulfilling their full potential is their incorrect or lack of use. In order to evaluate parental knowledge about car seat use for children and their actual use in the population visiting HURRA, a survey was performed which demonstrated that only 57.6% of the parents interviewed had infant car seats and that only 83.3% of owners actually use it. As many as 94.4% had correct knowledge about car seat use, but the majority of correct information was not provided by the medical staff. The majority of parents use seat belts for their own protection but a significantly smaller percentage use car seats for their own children.  相似文献   
75.
76.
The complete nucleotide sequence (4,826 bp) of the cryptic plasmid pGA1 from Corynebacterium glutamicum was determined. DNA sequence analysis revealed four putative coding regions (open reading frame A [ORFA], ORFA2, ORFB, and ORFC). ORFC was identified as a rep gene coding for an initiator of plasmid replication (Rep) according to the high level of homology of its deduced amino acid sequence with the Rep proteins of plasmids pSR1 (from C. glutamicum) and pNG2 (from Corynebacterium diphtheriae). This function was confirmed by deletion mapping of the minimal replicon of pGA1 (1.7 kb) which contains only ORFC. Deletion derivatives of pGA1 devoid of ORFA exhibited significant decreases in the copy number in C. glutamicum cells and displayed segregational instability. Introduction of ORFA in trans into the cells harboring these deletion plasmids dramatically increased their copy number and segregational stability. The ORFA gene product thus positively influences plasmid copy number. This is the first report on such activity associated with a nonintegrating bacterial plasmid. The related plasmids pGA1, pSR1, and pNG2 lacking significant homology with any other plasmid seem to be representatives of a new group of plasmids replicating in the rolling-circle mode.  相似文献   
77.
78.
79.
80.
A cross-sectional study was performed to obtain risk factors for hepatitis B disease, HBsAg carriers and immunised personnel, among 2470 workers in a general hospital in Madrid, Spain. The data obtained were analyzed with multiple logistic regression to obtain beta coefficients for variables. The results of the analysis show that being a nurse or being regularly exposed to blood are the most important risk factors for hepatitis B acquisition. The length of time working at the same job activity was also a risk factor. The resulting beta coefficients allow the construction for a hepatitis non-immunised, HBsAg carrier and immunised HBV status, which can select subjects for a hepatitis B vaccination program.  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