全文获取类型
收费全文 | 54660篇 |
免费 | 258篇 |
国内免费 | 556篇 |
专业分类
系统科学 | 1240篇 |
丛书文集 | 249篇 |
教育与普及 | 286篇 |
理论与方法论 | 525篇 |
现状及发展 | 31468篇 |
研究方法 | 788篇 |
综合类 | 18731篇 |
自然研究 | 2187篇 |
出版年
2013年 | 816篇 |
2012年 | 619篇 |
2011年 | 2647篇 |
2009年 | 618篇 |
2008年 | 834篇 |
2007年 | 904篇 |
2006年 | 1002篇 |
2005年 | 1173篇 |
2004年 | 2296篇 |
2003年 | 1888篇 |
2002年 | 1527篇 |
2001年 | 1308篇 |
2000年 | 830篇 |
1999年 | 935篇 |
1998年 | 649篇 |
1997年 | 778篇 |
1996年 | 537篇 |
1994年 | 684篇 |
1993年 | 701篇 |
1992年 | 871篇 |
1991年 | 741篇 |
1990年 | 819篇 |
1989年 | 644篇 |
1988年 | 623篇 |
1987年 | 611篇 |
1986年 | 678篇 |
1985年 | 847篇 |
1984年 | 752篇 |
1983年 | 632篇 |
1982年 | 780篇 |
1981年 | 813篇 |
1980年 | 893篇 |
1979年 | 1325篇 |
1978年 | 1200篇 |
1977年 | 1200篇 |
1976年 | 1083篇 |
1975年 | 1071篇 |
1974年 | 913篇 |
1973年 | 1173篇 |
1972年 | 1249篇 |
1971年 | 1257篇 |
1970年 | 1346篇 |
1969年 | 1247篇 |
1968年 | 1202篇 |
1967年 | 1061篇 |
1966年 | 888篇 |
1965年 | 720篇 |
1964年 | 481篇 |
1958年 | 562篇 |
1957年 | 472篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
基于治理理论和委托—代理模型,本研究以35 个国际单项体育联合会善治
评价为对象,设计了以透明度、程序民主、制衡、团结等维度作为关键维度的指标体系,
并使用文献资料法、问卷调查法和访谈法等方法,对数据进行搜集整理。研究发现,国
际单项体育联合会平均得分为45.5(满分100),善治状况评价为“一般”,决策和工作
机制不透明、选举和工作程序失范、利益相关者参与不足等,导致组织内部投机主义盛
行,国际单项体育联合会合法性面临危机。研究建议:1)严格执行选举程序,健全主席
和高管资格审查机制;2)完善信息公开制度,提升组织透明度;3)规范工作流程,充
实组织行政程序正当性;4)多元主体参与,健全组织内外部审查制度;5)明确最长任
期,保证组织官员积极性。 相似文献
992.
Selective functionalization of mesoporous silica nanospheres (MSNs) is crucial for nanoengineering of MSNs.
Herein, we have combined “surface-protected etching strategy” and “cationic surfactant assisted etching
strategy” to prepare functionalized MSNs with externally attached amino groups. The externally attached NH2
groups endow the catalysts with excellent catalytic performance for nitroaldol reaction between nitromethane
and benzaldehyde. In addition, those NH2-MSNs can also be used to support gold nanoparticles, which display
very good catalytic performance for reduction of 4-nitrophenol. It can be envisioned that the synthesis protocol
developed in this work could also be extended to nanoengineered MSNs, which provides opportunities for nanoreactors
design. 相似文献
993.
John Davenport Julia Davenport Cheong-Hoong Diong K.H. Low 《Journal of Natural History》2016,50(33-34):2097-2105
The pig-nosed freshwater turtle Carettochelys insculpta Ramsay, 1886 has paddle-shaped foreflippers that resemble those of sea turtles. These turtles exhibit a wide range of swimming capabilities. As well as swimming by the action of synchronized foreflippers alone, they sometimes used alternate hindlimb action at the same time. They could swim by ipsilaterally synchronized action of all four limbs, or by hindlimb action alone (combined with stabilizer/lift function of the foreflippers). The turtles also showed flexibility in bottom-walking. Besides the ipsilaterally synchronized quadrupedal action characteristic of other freshwater turtles, they exhibited a bipedal walking mechanism never previously described. Propelled by alternate action of the hindlimbs, the animals held the head and plastron above the substratum, with the large foreflippers acting to provide anterior lift and stability against roll and yaw. Because both hindlimbs were sometimes off the substratum simultaneously during bipedal locomotion, their duty factors were < 0.5, implying a bipedal run. 相似文献
994.
995.
Aloysius Domingo David Amar Karen Grütz Lillian V. Lee Raymond Rosales Norbert Brüggemann Roland Dominic Jamora Eva Cutiongco-dela Paz Arndt Rolfs Dirk Dressler Uwe Walter Dimitri Krainc Katja Lohmann Ron Shamir Christine Klein Ana Westenberger 《Cellular and molecular life sciences : CMLS》2016,73(16):3205-3215
996.
997.
998.
Rodney G.Downey ;Judith Egan ;Michael R.Fellows ;Frances A.Rosamond ;Peter Shaw 《清华大学学报》2014,(4):329-337
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
999.
Patricia A. Burrowes María Celeste Martes Mónica Torres-Ríos Ana V. Longo 《Journal of Natural History》2017,51(11-12):643-656
Pathogen-mediated changes in host behaviour can result from hosts altering their habitat preferences. Although infection risk with pathogenic fungus Batrachochytrium dendrobatidis in amphibians is associated with environments favouring its growth, the relationship with microhabitat use has not been examined. Here, we aim to determine if microhabitats used by frogs during their nocturnal activity predict B. dendrobatidis prevalence and infection intensity. Our focal host, Eleutherodactylus coqui, is a habitat generalist that uses multiple habitats from the forest floor to the canopy. We analysed data on B. dendrobatidis occurrence in 157 adults and 122 juveniles at El Yunque National forest in Puerto Rico. We categorized each individual’s nocturnal microhabitat as forest floor, curled palm fronds in the floor, arboreal bromeliads and foliage or tree trunks 50 cm to 2.5 m above ground. We found that frogs on the forest floor had the greatest B. dendrobatidis prevalence (73%), compared with those active in vegetation above ground (55%). Overall, the probability of B. dendrobatidis infection in frogs using microhabitats on the forest floor was twice as great as for those on arboreal substrates. Differences in B. dendrobatidis prevalence and intensity in E. coqui may be explained by specific abiotic conditions of microenvironments (temperature and humidity) affecting both pathogen and host, and by the age-specific ecological requirements of hosts. Adults were found to be most active in microhabitats where individuals had lower infection burdens, suggesting pathogen-modulated habitat choice. This work has important implications for the evolutionary dynamics of enzootic diseases and provides data that may inform potential mitigation strategies against a generalist amphibian pathogen. 相似文献
1000.