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   176878篇
  免费   1641篇
  国内免费   199篇
电工技术   2662篇
综合类   148篇
化学工业   28625篇
金属工艺   9658篇
机械仪表   5883篇
建筑科学   3334篇
矿业工程   1913篇
能源动力   2927篇
轻工业   9861篇
水利工程   2810篇
石油天然气   8700篇
武器工业   12篇
无线电   14997篇
一般工业技术   39763篇
冶金工业   29143篇
原子能技术   6972篇
自动化技术   11310篇
  2021年   1981篇
  2019年   1783篇
  2018年   3255篇
  2017年   3342篇
  2016年   3683篇
  2015年   1936篇
  2014年   3297篇
  2013年   6807篇
  2012年   4787篇
  2011年   5995篇
  2010年   4800篇
  2009年   5222篇
  2008年   5349篇
  2007年   5189篇
  2006年   4392篇
  2005年   4015篇
  2004年   3810篇
  2003年   3686篇
  2002年   3591篇
  2001年   3565篇
  2000年   3434篇
  1999年   3310篇
  1998年   7380篇
  1997年   5353篇
  1996年   3999篇
  1995年   3012篇
  1994年   2698篇
  1993年   2810篇
  1992年   2293篇
  1991年   2348篇
  1990年   2433篇
  1989年   2363篇
  1988年   2335篇
  1987年   2223篇
  1986年   2270篇
  1985年   2328篇
  1984年   2235篇
  1983年   2170篇
  1982年   2017篇
  1981年   2218篇
  1980年   2076篇
  1979年   2299篇
  1978年   2443篇
  1977年   2460篇
  1976年   3163篇
  1975年   2259篇
  1974年   2315篇
  1973年   2353篇
  1972年   2157篇
  1971年   1920篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
An in vitro culture system is described which allows an analysis of the signals responsible for the survival, growth and functional maturation of afferent lymph dendritic cells (ALDC), a subpopulation of migrating dermal dendritic cells involved in antigen carriage and presentation to T-cells. Purified ALDC survived and grew for up to 30 days in lymph node conditioned medium and survived 14 days in recombinant ovine (rov) TNF-alpha whereas none were detected after 24 h in rov GM-CSF, rov IFN-gamma or rh M-CSF. However, when rov GM-CSF was added to cultures along with rov TNF-alpha, increased numbers of ALDC compared with input numbers (growth) were recorded on Days 14 and 21. In contrast, when 50-200 units ml-1 of rov IFN-gamma were added to cultures of ALDC along with TNF-alpha or rov TNF-alpha plus rov GM-CSF, cell survival and growth was inhibited. Antibody blocking studies confirmed the cytokine specificity of these effects. ALDC cultured in rov TNF-alpha or rov TNF-alpha plus rov GM-CSF retained MHC Class-II and ov CD-1 antigen expression and accessory function for autologous ov CD-4 T-cell proliferation, although at reduced levels compared with freshly isolated cells. Neither fresh nor cultured ALDC expressed coagulation factor XIIIa.  相似文献   
992.
993.
994.
995.
996.
The basic ideas underlying an Estelle-C compiler, which accepts an Estelle protocol specification and produces a protocol implementation in C, are presented. The implementation of the ISO (International Organization for Standardization) class-2 transparent protocol, using the semiautomatic approach, is discussed. A manual implementation of the protocol is performed and compared to the semiautomatic implementation. The semiautomatic approach to protocol implementation offers several advantages over the conventional manual one, including correctness and modularity in protocol implementation code, conformance to the specification, and reduction in implementation time. Finally, ongoing development of a new Estelle-C compiler is presented  相似文献   
997.
The chromium content in foods is relatively low and the most foods have a content below 0.1 ppm. Only spices, cacao, cacao products, poppy and whole meal products contain higher contents. The often published high chromium content of corn germ oil could not be verified. We did not find any chromium enrichment in the entrails of a cow. In addition the chromium contents of each food are strong deviated. The analysed values were sometimes lower than in literature. We expect that the elder values are higher because of different contaminations in some cases.  相似文献   
998.
As a result of measuring the temperature (concentration) field behind the source, the transfer coefficients are determined for flow in a tube behind a system of parallel channelsZ/d = 50) and in an annual channel of complex cross section.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 55, No. 6, pp. 897-902, December, 1988.  相似文献   
999.
1000.
BACKGROUND: This study is part of the European Community Respiratory Health Survey (ECRHS), which uses a common methodology in different areas throughout the world. This paper describes the prevalences of reported asthma, asthma-like symptoms and nasal allergies, their relationships to age group and sex, and the relationships of asthma-like symptoms to current asthma, in the general population aged 20-44 years of three French urban areas. METHODS: The study population of 2804 subjects in Grenoble, 3774 in Montpellier and 3152 in Paris (18th district), randomly selected from electoral rolls, answered a postal questionnaire (stage I of ECRHS). The response rates were 77.8%, 68.6% and 74.4%, respectively. RESULTS: The prevalences were approximately 14% for wheezing, 16% for chest tightness and 4.5% for nocturnal shortness of breath in the three areas. Asthma attacks in the last 12 months were reported by 2.7% of subjects in Grenoble, 3.5% of subjects in Montpellier and 4.0% of subjects in Paris (P = 0.02). For nasal allergies, the prevalences were 28.0%, 34.3% and 30.8%, respectively (P < 0.001). Asthma was inversely correlated to age (higher prevalence in the youngest) but was not related to sex. Neither age distribution nor sex ratio explained the differences between areas. Among the asthma-like symptoms, wheezing and nocturnal shortness of breath correlated strongly with asthma, chest tightness correlated moderately and nocturnal coughing correlated poorly. CONCLUSION: The prevalences observed were higher than expected from previous comparable French studies in young adults. These results are consistent with the hypothesis of a recent increase of asthma and allergies.  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