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   51718篇
  免费   1217篇
  国内免费   216篇
电工技术   648篇
综合类   650篇
化学工业   6636篇
金属工艺   745篇
机械仪表   1084篇
建筑科学   1424篇
矿业工程   421篇
能源动力   567篇
轻工业   2822篇
水利工程   665篇
石油天然气   59篇
武器工业   5篇
无线电   2408篇
一般工业技术   4797篇
冶金工业   22940篇
原子能技术   254篇
自动化技术   7026篇
  2023年   244篇
  2022年   234篇
  2021年   246篇
  2020年   117篇
  2019年   165篇
  2018年   619篇
  2017年   778篇
  2016年   1234篇
  2015年   964篇
  2014年   660篇
  2013年   963篇
  2012年   2523篇
  2011年   2961篇
  2010年   1070篇
  2009年   1141篇
  2008年   1035篇
  2007年   1053篇
  2006年   924篇
  2005年   3645篇
  2004年   2844篇
  2003年   2263篇
  2002年   1081篇
  2001年   909篇
  2000年   445篇
  1999年   771篇
  1998年   6394篇
  1997年   3991篇
  1996年   2653篇
  1995年   1588篇
  1994年   1184篇
  1993年   1230篇
  1992年   375篇
  1991年   378篇
  1990年   395篇
  1989年   357篇
  1988年   386篇
  1987年   286篇
  1986年   252篇
  1985年   246篇
  1984年   165篇
  1983年   144篇
  1982年   199篇
  1981年   259篇
  1980年   260篇
  1979年   145篇
  1978年   167篇
  1977年   675篇
  1976年   1375篇
  1975年   161篇
  1973年   94篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
102.
OBJECTIVE: Experiences obtained with nonoperative treatment (NOT), i.e. total prohibition of per oral food intake for a minimum of 7 days, administration of combinations of broad-spectrum antibiotics, and parenteral hyperalimentation, are described in the management of esophageal perforations. SUMMARY BACKGROUND DATA: The place, value, and indication of NOT in the management of esophageal perforation has not yet been unequivocally defined. As a result, contradictory data have been published regarding the outcome of NOT. METHODS: During the past 15 years (1979 to 1994), 20 of 86 patients (23.3%) with esophageal perforation have been treated nonoperatively from the outset. In this group, perforations were located to the upper, middle, and lower third of the esophagus in 50%, 30%, and 20%, respectively. In the operative management group (OT)--in which conservative (drainage, endeprothesis), reconstructive (suture, reinforced suture), and radical (resection) surgical methods were applied--lesions were preponderantly located in the lower one third of the esophagus (56.1%--37/66). As to the interval between the perforation and the onset of treatment, 14 patients had been diagnosed within 24 hours, whereas in 6 cases treatment had been begun beyond 24 hours. RESULTS: NOT could be successfully carried out in 16 patients; the decision to use NOT had to be revised in 4 other cases (Table 1). Two patients were lost; the mortality rate was 10% (2 of 20). The rate of complications was lower in the NOT group (20%, or 4 of 20) than in the OT group (50%, or 33 of 66). CONCLUSIONS: NOT can be suggested for the treatment of intramural perforations. In the case of transmural perforation, this approach should be taken into consideration if the esophageal lesion is circumscribed, is not in neoplastic tissue, is not in the abdominal cavity, and is not accompanied by simultaneous obstructive esophageal disease; in addition, symptoms and signs of septicemia should be absent.  相似文献   
103.
A focus on the importance of leisure in the development of student nurses has been neglected in the 1990s. This study considers 444 prospective nurses on conventional, diploma and degree courses in two areas of England. It examines their activities associated with socializing, their social support networks and their use of clubs, sports and hobbies before entering nursing. It is evident that students enter nurse training with a wide variety of leisure experiences. Changing recruitment patterns and the structure of nurse training has an effect on the leisure needs of prospective nurses which must be addressed.  相似文献   
104.
105.
BACKGROUND: Disturbed fibrinolytic function may influence the progression of coronary atherosclerosis and contribute to thrombotic cardiovascular (CV) events. METHODS AND RESULTS: In the Angina Prognosis Study in Stockholm (APSIS), patients with stable angina pectoris were studied prospectively during double-blind treatment with metoprolol or verapamil. Various measures of fibrinolytic function were studied in 631 (of 809) patients. During a median follow-up time of 3.2 years (2132 patient-years), 32 patients suffered a CV death, 21 had a nonfatal myocardial infarction (MI), and 77 underwent revascularization. Plasma levels of tissue plasminogen activator (TPA) activity and antigen (ag), plasminogen activator inhibitor (PAI-1) activity at test, and TPA responses to exercise were determined at baseline and after 1 month's treatment and were related to subsequent fatal and nonfatal CV events. Univariate Cox regression analysis revealed that elevated levels of TPA-ag at rest (P < .05), high PAI-1 activity (P < .05), and low TPA-ag responses to exercise (P < .05) were associated with increased risk of subsequent CV death. After adjustment for baseline risk factors, TPA-ag independently predicted CV death or MI. In addition, PAI-1 activity independently predicted CV death or MI in male patients. Verapamil treatment was associated with a 10% decrease of TPA-ag levels and metoprolol treatment with a 2% increase (P < .001 for treatment difference). CONCLUSIONS: Plasma TPA-ag levels at rest, and among male patients PAI-1 activity as well, independently predict subsequent CV death or MI in patients with stable angina pectoris.  相似文献   
106.
We study experimentally and theoretically two polarization effects in a vertical cavity surface emitting laser submitted to optical feedback. In a first experiment, we obtain flips between two linearly polarized laser modes up to a frequency of 50 MHz using an external cavity with a polarizer. In a second experiment, polarization self modulation is demonstrated up to a frequency of 2.6 GHz, using a quarter wave plate instead. Numerical calculations, based on a four levels model for the active medium, show a good agreement with the experiments.  相似文献   
107.
A theoretical model is presented by which a true expression of pregnancy rate resulting from stimulated cycles can be calculated. This includes the transfer of both fresh and cryopreserved embryos. It is concluded that the total reproductive potential of a single cycle of stimulation can only be evaluated by including pregnancies arising from all fresh and frozen embryo transfers resulting from that cycle.  相似文献   
108.
109.
The tibial diaphysis osteotomy with limited separation of periosteum on the ends of the fragments with subsequent local infection by Staphylococcus aureus culture was conducted. In 90% of observations chronic inflammation was revealed in affected bones while morphological investigation conduction. An acute traumatic disorders of intraosseous blood circulation (the bone infarction) and local staphylococcal infection play the main role in the traumatic osteomyelitis pathogenesis.  相似文献   
110.
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