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   252650篇
  免费   3170篇
  国内免费   857篇
电工技术   4809篇
综合类   193篇
化学工业   36087篇
金属工艺   9504篇
机械仪表   7545篇
建筑科学   6075篇
矿业工程   885篇
能源动力   7228篇
轻工业   21121篇
水利工程   2202篇
石油天然气   3230篇
武器工业   15篇
无线电   34284篇
一般工业技术   49981篇
冶金工业   48652篇
原子能技术   5076篇
自动化技术   19790篇
  2021年   2158篇
  2020年   1560篇
  2019年   1914篇
  2018年   3273篇
  2017年   3311篇
  2016年   3399篇
  2015年   2221篇
  2014年   3956篇
  2013年   11486篇
  2012年   6237篇
  2011年   8580篇
  2010年   6910篇
  2009年   7892篇
  2008年   8407篇
  2007年   8300篇
  2006年   7374篇
  2005年   6508篇
  2004年   6288篇
  2003年   6558篇
  2002年   6044篇
  2001年   6517篇
  2000年   5984篇
  1999年   6398篇
  1998年   16500篇
  1997年   11418篇
  1996年   8816篇
  1995年   6616篇
  1994年   5913篇
  1993年   5845篇
  1992年   4134篇
  1991年   4023篇
  1990年   3862篇
  1989年   3652篇
  1988年   3561篇
  1987年   3017篇
  1986年   2946篇
  1985年   3208篇
  1984年   2927篇
  1983年   2829篇
  1982年   2636篇
  1981年   2547篇
  1980年   2463篇
  1979年   2305篇
  1978年   2110篇
  1977年   2607篇
  1976年   3569篇
  1975年   1764篇
  1974年   1733篇
  1973年   1665篇
  1972年   1469篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
952.
953.
954.
Over 2000 electrocution deaths were identified among U.S. construction workers from 1980 to 1991, with the highest mean annual crude mortality rate (2.5 per 100,000 people), and second highest mean age-adjusted rate (2.7 per 100,000 people) of all industries. Although the crude fatality rates showed a downward trend, construction workers are still about four times more likely to be electrocuted at work than are workers in all industries combined. Nearly 40% of the 5083 fatal electrocutions in all industries combined occurred in construction, and 80% were associated with industrial wiring, appliances, and transmission lines. Electrocutions ranked as the second leading cause of death among construction workers, accounting for an average of 15% of traumatic deaths in the industry from 1980 to 1991. The study indicates that the workers most at risk of electrical injury are male, young, nonwhite, and electricians, structural metal workers, and laborers. The most likely time of injury is 11 a.m. to 3 p.m. from June to August. Focusing prevention on these populations and characteristics through better methods of worker and supervisor electrical safety training, use of adequate protective clothing, and compliance with established procedures could minimize the average annual loss of 168 U.S. construction workers.  相似文献   
955.
The phrase “not much mathematics required” can imply a variety of skill levels. When this phrase is applied to computer scientists, software engineers, and clients in the area of formal specification, the word “much” can be widely misinterpreted with disastrous consequences. A small experiment in reading specifications revealed that students already trained in discrete mathematics and the specification notation performed very poorly; much worse than could reasonably be expected if formal methods proponents are to be believed  相似文献   
956.
957.
The functional ability of a muscle is closely related to the activities of the mitochondria, which are energy-producing organelles in muscle cells. The development of the mammalian masticatory muscle progresses dramatically when feeding behavior changes from suckling to mastication, but it is unclear how the energy-producing systems of the mitochondria change. In this paper, the development of rat masticatory muscle mitochondria was investigated in terms of enzyme activities of the mitochondrial respiratory chain and the structural and numerical development of mitochondria, especially regarding the change in feeding behavior from suckling to mastication. Using isolated mitochondria from the masticatory muscle, we measured succinate dehydrogenase, NADH dehydrogenase, succinate-O2 oxidoreductase, and NADH-O2 oxidoreductase. These were found to be increased in the 15-day postnatal rat compared with the 0- to 10-day postnatal rat. The structural development of mitochondria was gradual in the 0- to 15-day postnatal rat. However, a notable increase was found in the cross-sectional area of mitochondria between 10 and 15 days postnatally. The number of mitochondria per muscle fiber was apparently constant during the same period. We demonstrated that the change in feeding behavior was well-correlated with an increase in mitochondrial enzyme activity, also supported by the early structural development of mitochondria.  相似文献   
958.
959.
960.
We performed a cross-sectional survey of stroke and acute myocardial infarction (AMI) in Okinawa, Japan with a census population of about 1.2 million. A total of 3,644 cases of first-ever stroke and 898 cases of initial acute myocardial infarction (AMI) were detected. The age-adjusted annual incidence rate for stroke was 105 per 100,000 standard population of Japan, and that of AMI was 26. The case-fatality rate of stroke within 28 days of onset was 12.8%, and that of AMI was 22.2%. Of the stroke cases, 51.4% were diagnosed as brain infarction, 38.7% as brain hemorrhage, and 9.3% as subarachnoid hemorrhage. The diagnosis of stroke subtypes were confirmed by computed tomography or magnetic resonance imaging in 98.4% of all stroke cases. In Okinawa, the incidence rate of AMI was still considerably lower than that in the Western population, and the rate of stroke was similar to that in the Western population.  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