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   519篇
  免费   141篇
  国内免费   194篇
系统科学   45篇
教育与普及   4篇
现状及发展   8篇
研究方法   52篇
综合类   745篇
  2024年   2篇
  2023年   12篇
  2022年   16篇
  2021年   19篇
  2020年   3篇
  2019年   6篇
  2018年   25篇
  2017年   19篇
  2016年   26篇
  2015年   16篇
  2014年   67篇
  2013年   30篇
  2012年   55篇
  2011年   47篇
  2010年   18篇
  2009年   22篇
  2008年   54篇
  2007年   85篇
  2006年   38篇
  2005年   44篇
  2004年   71篇
  2003年   57篇
  2002年   34篇
  2001年   27篇
  2000年   29篇
  1999年   10篇
  1998年   3篇
  1997年   4篇
  1996年   1篇
  1995年   2篇
  1994年   4篇
  1993年   2篇
  1992年   1篇
  1991年   1篇
  1990年   1篇
  1986年   1篇
  1980年   1篇
  1979年   1篇
排序方式: 共有854条查询结果,搜索用时 16 毫秒
161.
The 252 Cf neutron brachytherapy devices were designed and manufactured after solving those problems: the shield difficulties for 252Cf neutron; the method and facilities for 252Cf neutron source deliver; the dose calculation and measurement for 252Cf neutron. From 1999 to 2009, more than twenty 252Cf neutron brachytherapy devices had been installed in China. The Chinese radiation oncologists implemented the 252 Cf neutron braehytherapy on more than 5 000 different kinds of cancer patients, including cervix cancer; endometrium cancer and achieved ideal results. For special presentation, they had breakthrough on radical treatment for low lying rectal adenocarcinoma with sphincter preservation and melanoma patients after the modification of 252Cf neutron intracavitary applicator and dose calibration.  相似文献   
162.
The genome sequence of the rice blast fungus Magnaporthe grisea   总被引:8,自引:0,他引:8  
Magnaporthe grisea is the most destructive pathogen of rice worldwide and the principal model organism for elucidating the molecular basis of fungal disease of plants. Here, we report the draft sequence of the M. grisea genome. Analysis of the gene set provides an insight into the adaptations required by a fungus to cause disease. The genome encodes a large and diverse set of secreted proteins, including those defined by unusual carbohydrate-binding domains. This fungus also possesses an expanded family of G-protein-coupled receptors, several new virulence-associated genes and large suites of enzymes involved in secondary metabolism. Consistent with a role in fungal pathogenesis, the expression of several of these genes is upregulated during the early stages of infection-related development. The M. grisea genome has been subject to invasion and proliferation of active transposable elements, reflecting the clonal nature of this fungus imposed by widespread rice cultivation.  相似文献   
163.
This paper introduced the design of the hybrid powertrain of the Fuel Cell City Bus demonstrated in 2008 Beijing Olympic Games. The configuration of the hybrid fuel cell powertrain was introduced. The safety of hydrogen storage and delivery system, the hydrogen leakage alarm system were developed. The real-time distributed control and diagnosis system based on the Time Trigger Controller Area Network (TTCAN) with 10 ms basic control period was developed. The concept and implementation of processor (or controller) monitor and process (or task) monitor technique based on the TTCAN were applied in this paper. The fault tolerant control algorithm of the fuel cell engine and the battery management system were considered. The demonstration experience verified that the fault tolerant control was very important for the fuel cell city bus.  相似文献   
164.
Calcination and acid leaching of coal kaolin were studied to determine an effective and economical preparation method of calcined kaolin. Thermogravimetric-differential thermal analysis (TG-DTA) and X-ray diffraction (XRD) demonstrated that 900℃ was the suitable temperature for the calcination. Leaching tests showed that hydrochloric acid was more effective for iron dissolution from raw coal kaolin (RCK), whereas oxalic acid was more effective on iron dissolution from calcined coal kaolin (CCK). The iron dissolution from CCK was 28.78wt%, which is far less effective than the 54.86wt% of RCK under their respective optimal conditions. Through analysis by using M?ssbauer spectroscopy, it is detected that nearly all of the structural ferrous ions in RCK were removed by hydrochloric acid. However, iron sites in CCK changed slightly by oxalic acid leaching because nearly all ferrous ions were transformed into ferric species after firing at 900℃. It can be concluded that it is difficult to remove the structural ferric ions and ferric oxides evolved from the structural ferrous ions. Thus, iron removal by acids should be conducted prior to calcination.  相似文献   
165.
INTRODUCTIONIt was reported that organic-solvent pulping process emerged in 1893,and then kleirert, proved that the lignin delignfied in the cooking was soluble in the ethanol water (EW) more than in the ethanol only, obtained a correlative patent in 1932.Diebold who worked in Paper association of Canada prompted the process, and pronounced a patent in the 1978[1-2]. In addition, non-wood fiber material occupies a much higher proportion to the material used in papermaking of china, so lo…  相似文献   
166.
INTRODUCTIONIn mechanical pulp bleaching, a higher initial pulp brightness makes it much easier to achieve a higher bleached pulp brightness and greater bleaching efficiency. Generally, there is a substantial increase in light absorbance (i.e. chromophore formation) that may lead to a brightness loss in thermomechanical pulping (TMP) processes. It is reported that easily oxidized structures, predominantly in the lignin portion of the wood material, are converted into colored structures b…  相似文献   
167.
Magnesium alloys containing long period stacking ordered (LPSO) phase have been received a great deal of attention in the last decade owing to their excellent comprehensive properties of mechanical strength and corrosion resistance. In this paper, some fundamental aspects of LPSO containing Mg alloys have been reviewed, including: (1) microstructural characterization, formation conditions and the associated phase transformation of LPSO phases in Mg alloys; (2) deformation mechanism of LPSO phases and their influence on the deformation mechanism of the Mg matrix; (3) effect of LPSO structure on the mechanical performance such as tensile strength, creep resistance, fracture toughness and fatigue strength; (4) corrosion behavior of LPSO containing Mg alloys and their possible applications as the biomaterials. Moreover, some remaining unsolved issues of the LPSO containing Mg alloys and the future target about how to further improve their service properties have been also described.  相似文献   
168.
IP over WDM is being envisioned as one of the most attractive architectures for the next generation Internet. This paper introduces two novel backbone node structures for IP over WDM networks. These node structures can be used as improvement approaches to DPDP (default path and dedicated path) method. The flow classifiers, which are the main components of these node structures, and the performance metrics of these structures, are also discussed.  相似文献   
169.
In complicated application environment such as CIMS (Computer Integrate d Manufacture System) enterprise, it will bring great benefits to integrate dist ributed knowledge sources. But the difficulties of knowledge sharing and reuse s eriously encumbrance the implementation of knowledge integration. In this paper, we describe how a framework of knowledge-integrated system based on ontology ( KISO) can be used to support integrating distributed knowledge sources.  相似文献   
170.
研究调查中美老年人三大非正式支持--家庭、朋友和邻里,以及心理健康--自尊、抑郁和孤独的来源.研究结果发现:家庭的支持是美国老年人的健康最期望来源,而在中国则为邻里与朋友的支持.  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