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   43327篇
  免费   933篇
  国内免费   180篇
电工技术   543篇
综合类   631篇
化学工业   4677篇
金属工艺   592篇
机械仪表   867篇
建筑科学   914篇
矿业工程   390篇
能源动力   415篇
轻工业   2083篇
水利工程   613篇
石油天然气   49篇
武器工业   4篇
无线电   1868篇
一般工业技术   3475篇
冶金工业   21402篇
原子能技术   198篇
自动化技术   5719篇
  2024年   57篇
  2023年   201篇
  2022年   133篇
  2021年   128篇
  2019年   57篇
  2018年   457篇
  2017年   681篇
  2016年   1076篇
  2015年   795篇
  2014年   436篇
  2013年   451篇
  2012年   2156篇
  2011年   2488篇
  2010年   700篇
  2009年   793篇
  2008年   640篇
  2007年   663篇
  2006年   596篇
  2005年   3375篇
  2004年   2579篇
  2003年   2059篇
  2002年   852篇
  2001年   740篇
  2000年   279篇
  1999年   623篇
  1998年   6146篇
  1997年   3812篇
  1996年   2506篇
  1995年   1456篇
  1994年   1068篇
  1993年   1099篇
  1992年   246篇
  1991年   301篇
  1990年   307篇
  1989年   275篇
  1988年   290篇
  1987年   220篇
  1986年   197篇
  1985年   167篇
  1984年   69篇
  1983年   82篇
  1982年   129篇
  1981年   178篇
  1980年   192篇
  1979年   60篇
  1978年   100篇
  1977年   609篇
  1976年   1323篇
  1975年   99篇
  1971年   49篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
32.
33.
34.
35.
A modified natural surfactant was administered to a patient with life-threatening adult respiratory distress syndrome caused by viral pneumonia. Subsequently, there was a marked improvement in gas exchange. In order to assess the mechanism for improved oxygenation, computed tomography of the lungs was done. Quantitative analysis of the scans taken before and after surfactant administration indicates that improvement in gas exchange was largely due to the expansion of underinflated and collapsed lung areas. Although this is a single case report, it provides insight into the possible beneficial effect of instilled surfactant in severe respiratory distress from viral pneumonia.  相似文献   
36.
37.
Organochlorines are persistent and highly lipophilic environmental contaminants which bioaccumulate in the food chain. Some of these chemicals, 2,2-bis(p-chlorophenyl)-1,1,1-trichloroethane (DDT) and polychlorinated biphenyls (PCBs), have been suggested to be of significance in the aetiology of breast cancer. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an anti-oestrogen in animal studies and should be thus lower the risk of breast cancer. The other isomers of polychlorinated dibenzo-p-dioxins (PCDDs) or the chemically related polychlorinated dibenzofurans (PCDFs) have not been tested regarding carcinogenesis of the breast. The purpose of this study was to investigate whether PCDDs or PCDFs influence the risk for breast cancer. Consecutive patients who underwent surgery for a breast disease between 1993 and 1995 were recruited for the study. Cases were 22 patients with infiltrative breast cancer and controls were 19 patients operated for a benign breast disease during the same time period. Approximately 10 g of breast tissue free from tumour was taken from the specimen and frozen until analysis. Fat was extracted, cleaned and analysed with a high-resolution gas chromatograph coupled to a high-resolution mass spectrometer. Median concentrations of octachlorinated dibenzo-p-dioxin (OCDD) were 598 (170-14,880) and 396 (103-1,847) pg/g lipid in the cases and in the controls, respectively. In a multivariate logistic regression analysis controlling for other risk factors for breast cancer increased odds ratio (OR) was obtained for OCDD: 401-1000 pg/g lipid yielded OR 3.8, 95% confidence interval (CI) 0.4-39, > 1000 pg/g lipid gave OR 5.2, CI 0.4-72. When the lipid OCDD variable was examined as a continuous risk factor there was a 1.09 (9%), CI 0.95-1.25, increase in the adjusted OR for breast cancer per 100 unit (pg/g lipid) increase in OCDD. No differences were found between cases and controls for the other six tested PCDDs. Mean concentration of TCDD was in the cases 3.6 (1.0-7.9) and in the controls 3.3 (1.1-6.3) pg/g lipid. For PCDFs no significant differences were found between cases and controls. The results were not changed if oestrogen or progesterone receptor status, S-phase fraction and DNA ploidy were considered. Breast tissue concentration of OCDD was increased in cancer patients, whereas the concentrations of other PCDDs and PCDFs were equal in cases and controls.  相似文献   
38.
39.
40.
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