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   2803篇
  免费   35篇
  国内免费   21篇
电工技术   17篇
综合类   1篇
化学工业   195篇
金属工艺   10篇
机械仪表   24篇
建筑科学   54篇
矿业工程   5篇
能源动力   27篇
轻工业   68篇
水利工程   9篇
石油天然气   5篇
无线电   130篇
一般工业技术   174篇
冶金工业   1960篇
原子能技术   18篇
自动化技术   162篇
  2022年   10篇
  2021年   13篇
  2020年   10篇
  2019年   12篇
  2018年   16篇
  2017年   18篇
  2016年   19篇
  2015年   19篇
  2014年   17篇
  2013年   61篇
  2012年   29篇
  2011年   52篇
  2010年   33篇
  2009年   42篇
  2008年   28篇
  2007年   31篇
  2006年   32篇
  2005年   33篇
  2004年   24篇
  2003年   34篇
  2002年   18篇
  2001年   25篇
  2000年   23篇
  1999年   74篇
  1998年   598篇
  1997年   367篇
  1996年   237篇
  1995年   134篇
  1994年   110篇
  1993年   119篇
  1992年   35篇
  1991年   49篇
  1990年   39篇
  1989年   38篇
  1988年   22篇
  1987年   40篇
  1986年   34篇
  1985年   30篇
  1984年   15篇
  1983年   10篇
  1982年   16篇
  1981年   22篇
  1980年   24篇
  1979年   9篇
  1978年   11篇
  1977年   47篇
  1976年   92篇
  1975年   12篇
  1973年   9篇
  1971年   7篇
排序方式: 共有2859条查询结果,搜索用时 15 毫秒
1.
2.
3.
4.
The virtual agency concept is now used within the United States Government as an alliance approach to manage large research and development (R&D) processes across departments. This paper examines the history of the virtual agency concept and its important characteristics. The paper identifies the potential benefits and associated risks involved in managing R&D within a virtual agency. Three cases are examined where the virtual agency concept has been applied to R&D programs: the High Performance Computing and Communications initiative, the Next Generation Internet, and the Partnership for a New Generation of Vehicles. The case studies indicate that the R&D process is attempting to balance formal process controls with the agility to adapt rapidly to new research opportunities. Virtual agencies can be used to improve organizational efficiency, improve knowledge transfer, increase interoperability through standards, provide better alignment of agency missions with national policy, and introduce increased flexibility into the R&D process. At the same time, the virtual agency concept has major risks including inefficiencies due to organizational complexity, the danger of collective myopia, the problem of adopting standards too early, the difficulty of reaching objectives in a loose organizational structure, and the problem of properly balancing the tension between agency mission objectives and national policy agendas.  相似文献   
5.
Two groups of Brangus steers produced by nuclear transplantation cloning were used in parallel studies investigating the impact of calf- and yearling-feeding. The first group (n = 8) were fed as calves (CF; n = 4) or yearlings (YF; n = 4) to a constant age end point of 16 mo. The second group (n = 10) were fed as calves (CF; n = 5) or yearlings (YF; n = 5) to a constant live weight end point (530 kg). When slaughtered at the same age, CF and YF steers did not differ (P > .05) in feedlot ADG, but the CF steers were heavier and had higher dressing percentages, numeric yield grades, and quality grades (P < .05). Top loin steaks from the groups of steers did not differ (P > .05) in palatability traits. When fed to a constant live weight, the YF steers gained more rapidly (P < .05) and had lower (P < .05) numeric yield grades than did CF steers. Again CF steers had higher (P < .05) dressing percentages. There was no difference (P > .05) between the treatments in carcass quality grade or meat palatability characteristics. Thus, when finished to a constant weight end point, YF steers gained more rapidly, with no adverse effects on carcass quality grade or palatability traits; however, CF steers consistently produced higher dressing percentages, largely due to greater external fatness.  相似文献   
6.
7.
The ratio of the transverse relaxation rate to the longitudinal relaxation rate of water protons (T1/T2) at 90 MHz can be used to determine the relative amounts of free Mn2+ ions (the hexaquo ion) and chelated manganese ions when both species are present in the same aqueous solution. This technique is demonstrated for simple aqueous solutions containing known concentrations of both the free Mn2+ ion and a manganese(III) porphyrin. The accuracy of the method is found to be +/- 7.3% for the solutions considered in this report. The determinations of the relative amounts of the free Mn2+ ion and the manganese(III) porphyrin by this T1/T2 method are shown to agree well with those determined by nuclear magnetic resonance dispersion profiles. The theoretical basis of this ratio method as well as the scope and limitations in its application to aqueous solutions containing both free Mn2+ ions and chelated manganese species other than manganese(III) porphyrins are also discussed. This work demonstrates a unique use of NMR as a means for the quantitative analysis of manganese in which the effects of the metal ion on the solvent, and not the metal itself, are observed.  相似文献   
8.
The short contact time dissolution of vitrinite and inertinite concentrates of an Australian bituminous coal was studied under isothermal conditions with tetralin as solvent. The reactivity of the two maceral types and the yields and nature of the products were determined for a range of reaction temperatures (400, 420 and 440 °C) and reaction times (2–20 min). The dissolution of vitrinite proceeded at a significantly greater rate than for inertinite and was associated with greater selectivity to soluble non-gaseous products. At very short reaction times (2 min), the vitrinite underwent solubilizing reactions to yield products of higher molecular weight and greater oxygen functionality than was the case for inertinite, for which changes in the chemical nature of the products were negligible over the range of conditions studied. At longer reaction times (5 min), the composition of the vitrinite-derived products was similar to that of the inertinite-derived products.  相似文献   
9.
SB McNeil 《Canadian Metallurgical Quarterly》1997,23(3):287-8; discussion 289-90, 317
Jehovah's Witnesses refuse blood transfusions for themselves and for their children. This action can be difficult for health professionals to understand and can lead to tensions between the staff and family. For one family, their refusal of blood for their child lead to a greater understanding of their religion and its beliefs for those who cared for them. Interspersed with their story are the medical reasons their son required blood, the reasons Jehovah's Witnesses refuse blood transfusions, and what the acceptable alternatives are to Jehovah's Witnesses. This article will share the thoughts and feelings of the family and the nursing staff who cared for the family during this crisis.  相似文献   
10.
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