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   33947篇
  免费   125篇
  国内免费   193篇
系统科学   428篇
丛书文集   694篇
教育与普及   71篇
理论与方法论   181篇
现状及发展   15801篇
研究方法   1298篇
综合类   15432篇
自然研究   360篇
  2012年   448篇
  2011年   862篇
  2010年   227篇
  2009年   194篇
  2008年   529篇
  2007年   615篇
  2006年   608篇
  2005年   614篇
  2004年   712篇
  2003年   714篇
  2002年   801篇
  2001年   1106篇
  2000年   1031篇
  1999年   645篇
  1992年   547篇
  1991年   436篇
  1990年   506篇
  1989年   471篇
  1988年   424篇
  1987年   467篇
  1986年   493篇
  1985年   615篇
  1984年   445篇
  1983年   395篇
  1982年   386篇
  1981年   421篇
  1980年   455篇
  1979年   1085篇
  1978年   881篇
  1977年   872篇
  1976年   656篇
  1975年   724篇
  1974年   1102篇
  1973年   883篇
  1972年   872篇
  1971年   1034篇
  1970年   1294篇
  1969年   1088篇
  1968年   1007篇
  1967年   1034篇
  1966年   871篇
  1965年   686篇
  1964年   209篇
  1959年   373篇
  1958年   582篇
  1957年   419篇
  1956年   350篇
  1955年   345篇
  1954年   339篇
  1948年   209篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
The first record from Sicily of the introduced facelinid nudibranch Godiva quadricolor allowed the detection of trophic relationships with the polycerid Polycera hedgpethi, another non-native nudibranch, and with two bryozoan species, namely the naturalized Cheilostomatida Bugula neritina and the cryptogenic Ctenostomatida Amathia verticillata. The settlement of both nudibranchs was presumably promoted by a trophic shift of P. hedgpethi from the natural prey B. neritina towards the largely available and not exploited A. verticillata. This short food web, without evident links with native fauna and having G. quadricolor as the top predator, is described. A DNA barcoding approach was used to confirm the identity of this facelinid species and to explore the possible genetic divergence occurring among the samples analysed.  相似文献   
2.

This article presents a community learning model formulated by Engineers Without Borders Colombia with the aim of providing communities with tools to create sustainable productive solutions which have relevancy for members and for potential customers. The goal of this formulation is to promote learning processes that are guided by decisions made by community members to propose sustainable and replicable initiatives. The model applicability is evidenced through a case study devoted to strengthening community-led green businesses in the Guavio Province, Colombia by collecting lessons and conclusions. Ultimately, this collection will prove useful in replicating the learning model in other similar rural communities.

  相似文献   
3.
4.
根据我国当前电力系统运行情况,对电网无功功率补偿的必要性及通用补偿方法及其装置进行了具体阐述。  相似文献   
5.
以具体工程为例,介绍了建筑物地基处理的设计、施工中遵循的原则,施工中遇到特殊情况时采取的处理方法和必要的检测方法,对几种桩基的设计要求、施工工艺及过程、质量控制等情况进行了简单阐述。  相似文献   
6.
The shrink proofing on wool with treatment of protease named Argaenzyme STL was studied. The various pretreating auxiliaries, different parameters for protease treating process and the effect of stabilizer were discussed in detail. The varieties of some properties before and after protease treatment were also investigated.  相似文献   
7.
通过1998年、2000年的试验研究,系统分析了顶凌覆盖技术在高寒区节水增温保温、增产增收的机理及其应用效果。提出该技术可以缓解低温干旱特别是春旱对农业生产的困扰,是一项可以抗旱保春播增产增收的实用技术,在高寒地区具有较高的推广价值。  相似文献   
8.
Photon emission of phagocytes in relation to stress and disease.   总被引:1,自引:0,他引:1  
E M Lilius  P Marnila 《Experientia》1992,48(11-12):1082-1091
Phagocytes, the first-line cells of the body's defence mechanisms against invading pathogens, kill microorganisms by means of lysosomal degradative enzymes and highly toxic reactive oxygen intermediates. The reactive oxygen compounds are produced, in a process called the 'respiratory burst', by the NADPH oxidase complex in plasma membranes, and by myeloperoxidase in phagolysosomes after degranulation. These processes generate electronically excited states which, on relaxation, emit photons, giving rise to phagocyte chemiluminescence (CL). This paper describes the conditions for the measurement of CL, and reviews the activity of phagocytes from individuals undergoing stress or disease. The capability of phagocytes to emit photons reflects remarkably well the pathophysiological state of the host. In many cases even the magnitude of the stress, the presence of a pathogen in the body, or the activity of the disease can be estimated. Physiological changes, e.g. in the reproductive cycle, can also be predicted.  相似文献   
9.
Summary The authors show the antibiotic activity of water-insoluble salts of hydrosoluble antibiotics in the case of penicillin G (salts of iron, copper, silver, gold, and uranium) and streptomycin (oleate).  相似文献   
10.
1 Results Oxy-apatite materials are thought as zirconia-substitutes in Solid Oxide Fuel Cells due to their fast ionic conduction. However, the well known difficulties related to their densification prevent them from being used as such. This study presents strategies to obtain oxy-apatite dense materials and the influence of elaboration route on transport properties. Particular emphasis is put on the microstructure effect on ion conduction. By the combined use of freeze-drying and conventional or spark p...  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