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   2740篇
  免费   12篇
  国内免费   2篇
电工技术   5篇
综合类   5篇
化学工业   86篇
金属工艺   37篇
机械仪表   28篇
建筑科学   28篇
矿业工程   2篇
能源动力   13篇
轻工业   61篇
水利工程   7篇
石油天然气   117篇
无线电   39篇
一般工业技术   107篇
冶金工业   2169篇
原子能技术   3篇
自动化技术   47篇
  2021年   10篇
  2020年   12篇
  2018年   5篇
  2017年   6篇
  2016年   9篇
  2015年   5篇
  2014年   11篇
  2013年   26篇
  2012年   23篇
  2011年   32篇
  2010年   18篇
  2009年   19篇
  2008年   23篇
  2007年   22篇
  2006年   18篇
  2005年   24篇
  2004年   68篇
  2003年   36篇
  2002年   37篇
  2001年   23篇
  2000年   20篇
  1999年   81篇
  1998年   619篇
  1997年   396篇
  1996年   266篇
  1995年   139篇
  1994年   116篇
  1993年   138篇
  1992年   20篇
  1991年   33篇
  1990年   27篇
  1989年   20篇
  1988年   28篇
  1987年   23篇
  1986年   26篇
  1985年   24篇
  1984年   10篇
  1983年   12篇
  1982年   18篇
  1981年   13篇
  1980年   19篇
  1979年   7篇
  1978年   8篇
  1977年   74篇
  1976年   142篇
  1975年   9篇
  1973年   4篇
  1972年   5篇
  1968年   4篇
  1951年   3篇
排序方式: 共有2754条查询结果,搜索用时 15 毫秒
101.
OBJECTIVES: To determine the effect of adding salmeterol 50 micrograms twice daily for six months to current treatment in subjects with asthma who control their inhaled corticosteroid dose according to a management plan. DESIGN: A double blind, randomised crossover study. SETTING: Nottingham. SUBJECTS: 101 subjects with mild or moderate asthma taking at least 200 micrograms twice daily of beclomethasone dipropionate or budesonide. INTERVENTIONS: Salmeterol 50 micrograms twice daily and placebo for six months each, with a one month washout. Subjects adjusted inhaled steroid dose according to guidelines. MAIN OUTCOME MEASURE: Reduction in inhaled steroid use, exacerbations of asthma, and use of oral steroids. RESULTS: Data were available for 87 subjects. When compared with placebo salmeterol treatment was associated with a 17% reduction in inhaled steroid use (95% confidence interval 12% to 22%) with no significant difference in the number of subjects who had an exacerbation (placebo 25%, salmeterol 16%) or use of oral steroids. For secondary end points salmeterol treatment was associated with higher morning and evening peak expiratory flow and forced expiratory volume in one second; a reduction in symptoms, bronchodilator use and airway responsiveness to methacholine; and no effect on serum potassium concentration, 24 hour heart rate, or the final forced expiratory volume in one second achieved during a salbutamol dose-response study. CONCLUSIONS: In subjects who adjusted their inhaled steroid treatment according to guidelines the addition of salmeterol 50 micrograms twice daily was associated with a reduction in inhaled steroid use and improved lung function and symptom control.  相似文献   
102.
We have previously shown that prolonged interferon-beta (IFN-beta) treatment of RS485 cells (NIH3T3 cells transformed with multiple copies of an LTR-cHa-ras oncogene) resulted in the phenotypic reversion of 1%-5% of the culture, depending on the conditions used. This reversion persisted after IFN-beta was discontinued, although the revertants retained the LTR-cHa-ras and continued to express ras mRNA and p21. Clones were prepared of such persistent revertant cell lines (PRs). Expression of lysyl oxidase (LOX), which appears to act as a suppressor of ras transformation, was downregulated in RS485 and upregulated in the PRs. When retinoic acid (RA) was combined with IFN-beta treatment of the RS485 cultures, a different mechanism of reversion predominated. Following 60 days of treatment with 20 IU/ml of IFN-beta and 10 microM RA, all of the multiple (3-5) copies of the transforming LTR-c-Ha-ras originally present in RS485 cells were deleted from the genome in 72% of 54 revertant cell lines isolated. As in the case of revertants observed after treatment with IFN-beta alone, LOX mRNA expression was upregulated in all of the revertants that resulted from the treatment with IFN plus RA. The level of LOX mRNA expression acts, therefore, as an indicator of transformation in this system.  相似文献   
103.
104.
105.
Previous research has documented differences in the pattern of EEG activation between 3-month-old infants of depressed mothers and infants of nondepressed mothers. In the present study, EEG was recorded in even younger 1-month-old infants of depressed and nondepressed mothers. The infants of depressed mothers exhibited greater relative right frontal EEG asymmetry (due to reduced left frontal activation), and this pattern at 1 month was significantly related to 3-month EEG asymmetry. Right frontal EEG asymmetry was also related to more frequent negative facial expressions (sad and pre-cry faces) during the Brazelton exam. Finally, the infants of depressed mothers showed more indeterminate sleep, were less active, and cried less than infants of nondepressed mothers.  相似文献   
106.
107.
108.
African lions, Panthera leo, engage in many cooperative activities including hunting, care of young, and group territoriality, but the contribution of juvenile lions to these activities has never been documented. Here we present experimental evidence that juvenile lionesses make a gradual transition to group-territorial defence between weaning (8 months) and sexual maturity (42 months). When challenged by simulated intruders played from a loud-speaker, juvenile females (but not males) become progressively more likely to join the adult females in territorial defence with age, and their behaviour is affected by both the number of defending adults and the number of intruders. We interpret the ability of juveniles to assess relative numbers as an adaptation for assessing the risk of territorial conflict according to their own fighting ability, and the ability of their pride of successfully defend the territory. The difference between the sexes reflects the greater value of the natal territory to philopatric females. Adult females display a variety of strategies when defending the territory, including unconditional and conditional forms of cooperation. We show here that individuals display the rudiments of these strategies as juveniles.  相似文献   
109.
Metallurgical and Materials Transactions A - Uranium alloyed with 10 wt pct Mo (U-10Mo) undergoes discontinuous precipitation (DP) when subjected to sub-eutectoid heat treatment. It is hypothesized...  相似文献   
110.
介绍了以钻开油层完井液屏蔽暂堵技术为主的综合配套钻井技术。吐哈地区鄯善、丘陵、温米油田435口开发井的应用结果表明,该技术能有效地减少钻井过程中的储层损害,较好地保护油井的原始产能,使储层流动效率平均提高了30%~40%。  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