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   34333篇
  免费   90篇
  国内免费   90篇
系统科学   301篇
丛书文集   591篇
教育与普及   73篇
理论与方法论   113篇
现状及发展   13989篇
研究方法   1334篇
综合类   17454篇
自然研究   658篇
  2013年   268篇
  2012年   436篇
  2011年   1112篇
  2010年   188篇
  2008年   528篇
  2007年   631篇
  2006年   639篇
  2005年   625篇
  2004年   612篇
  2003年   601篇
  2002年   530篇
  2001年   1190篇
  2000年   1132篇
  1999年   635篇
  1992年   642篇
  1991年   550篇
  1990年   577篇
  1989年   530篇
  1988年   541篇
  1987年   527篇
  1986年   536篇
  1985年   681篇
  1984年   537篇
  1983年   473篇
  1982年   389篇
  1981年   392篇
  1980年   488篇
  1979年   1050篇
  1978年   860篇
  1977年   803篇
  1976年   687篇
  1975年   798篇
  1974年   1029篇
  1973年   865篇
  1972年   896篇
  1971年   1067篇
  1970年   1370篇
  1969年   1030篇
  1968年   993篇
  1967年   948篇
  1966年   898篇
  1965年   634篇
  1964年   153篇
  1959年   375篇
  1958年   588篇
  1957年   438篇
  1956年   365篇
  1955年   340篇
  1954年   354篇
  1948年   248篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
641.
Cellular and Molecular Life Sciences -  相似文献   
642.
Ohne Zusammenfassung
Reviews

Compte rendu des publications

Resoconti delle pubblicazioni
  相似文献   
643.
644.
The main purpose of this paper is to exposit two very different, but very general, motivational schemes in the art of parameterization and a concrete example connecting them. We introduce a dynamic version of the DOMINATING SET problem and prove that it is fixed-parameter tractable(FPT). The problem is motivated by settings where problem instances evolve. It also arises in the quest to improve a natural greedy heuristic for the DOMINATING SET problem.  相似文献   
645.
In the present study, nano-sized SiC (0, 0.3, 0.5, 1.0 and 1.5 vol%) reinforced aluminum (Al) metal matrix composites were fabricated by microwave sintering and hot extrusion techniques. The structural (XRD, SEM), mechanical (nanoindentation, compression, tensile) and thermal properties (co-efficient of thermal expansion- CTE) of the developed Al-SiC nanocomposites were studied. The SEM/EDS mapping images show a homogeneous distribution of SiC nanoparticles into the Al matrix. A significant increase in the strength (compressive and tensile) of the Al-SiC nanocomposites with the addition of SiC content is observed. However, it is noticed that the ductility of Al-SiC nanocomposites decreases with increasing volume fraction of SiC. The thermal analysis indicates that CTE of Al-SiC nanocomposites decreases with the progressive addition of hard SiC nanoparticles. Overall, hot extruded Al 1.5 vol% SiC nanocomposites exhibited the best mechanical and thermal performance as compared to the other developed Al-SiC nanocomposites.  相似文献   
646.
Stimuli-responsive polymers have the extraor- dinary ability to change their physical and/or chemical state after they "detect" a change in their environment; their response depends dramatically on their chemical compo- sition. This property has been used for a plethora of applications; this review highlights their utility for human health. Specifically, this review will highlight efforts in the areas of sensing and biosensing, antimicrobial/antifouling coatings, tissue engineering and regenerative medicine, and drug delivery. Specific examples are given in each of these areas, with some focus on our work engineering poly(N- isopropylacrylamide)-based microgels and other respon- sive systems.  相似文献   
647.
Tin oxide (SnO2) nanoparticles were cost-effectively synthesized using nontoxic chemicals and green tea (Camellia sinensis) extract via a green synthesis method.The structural properties of the obtained nanoparticles were studied using X-ray diffraction,which indicated that the crystallite size was less than 20 nm.The particle size and morphology of the nanoparticles were analyzed using scanning electron microscopy and transmission electron microscopy.The morphological analysis revealed agglomerated spherical nanoparticles with sizes varying from 5 to 30 nm.The optical properties of the nanoparticles' band gap were characterized using diffuse reflectance spectroscopy.The band gap was found to decrease with increasing annealing temperature.The O vacancy defects were analyzed using photoluminescence spectroscopy.The increase in the crystallite size,decreasing band gap,and the increasing intensities of the UV and visible emission peaks indicated that the green-synthesized SnO2 may play future important roles in catalysis and optoelectronic devices.  相似文献   
648.
ABSTRACT

Here we document three cases of mimicry in coral reef fishes not previously reported in the literature involving two groupers (Epinephelus leucogrammicus and Plectropomus marisrubri) and a soapfish (Diploprion drachi) as mimics, and two wrasses (Larabicus quadrilineatus and Cheilinus quinquecinctus) and a blenny (Meiacanthus nigrolineatus) as models. All three cases are of aggressive mimicry, with a predatory species mimicking a harmless one, and in one of the cases, the mimicry is also Müllerian, where both the predator and harmless species are unpalatable.  相似文献   
649.
A polycaprolactone-grafted cellulose nanocrystal(PCL-g-CN) was prepared in ionic liquid and characterized by Fourier transform infrared spectrum(FT-IR),X-ray diffraction(XRD),and transmission electron microscopy(TEM).A peak assigned to the carbonyl group appears at 1 730 cm~(-1) in the FT-IR of PCL-g-CN,which confirms that the grafting reaction is successfully completed.The morphologies of PCL-g-CNs still maintain rod-like structure according to the TEM images.XRD results show that the crystal type of the PCL-g-CNs changed from cellulose Ⅰ to cellulose Ⅱ.The reasons for crystal transition of CNs turn out to be combined effects of anion and cation in ionic liquid with CNs.  相似文献   
650.
In this study, a powder mixture with an Al/TiO2 molar ratio of 10/3 was used to form an r-Al2Ti intermetallic matrix composite (IMC) reinforced withα-Al2O3 ceramic by a novel milling technique, called ...  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