全文获取类型
收费全文 | 8082篇 |
免费 | 323篇 |
国内免费 | 1295篇 |
出版年
2021年 | 68篇 |
2020年 | 43篇 |
2019年 | 142篇 |
2018年 | 120篇 |
2017年 | 334篇 |
2016年 | 234篇 |
2015年 | 311篇 |
2014年 | 466篇 |
2013年 | 330篇 |
2012年 | 283篇 |
2011年 | 304篇 |
2010年 | 302篇 |
2009年 | 112篇 |
2008年 | 123篇 |
2007年 | 165篇 |
2006年 | 119篇 |
2005年 | 181篇 |
2004年 | 151篇 |
2003年 | 342篇 |
2002年 | 1151篇 |
2001年 | 1383篇 |
2000年 | 147篇 |
1999年 | 204篇 |
1998年 | 96篇 |
1997年 | 74篇 |
1996年 | 97篇 |
1995年 | 68篇 |
1994年 | 65篇 |
1993年 | 136篇 |
1992年 | 124篇 |
1991年 | 111篇 |
1990年 | 146篇 |
1989年 | 168篇 |
1988年 | 255篇 |
1987年 | 111篇 |
1986年 | 96篇 |
1985年 | 107篇 |
1984年 | 69篇 |
1983年 | 111篇 |
1982年 | 52篇 |
1981年 | 64篇 |
1963年 | 30篇 |
1960年 | 41篇 |
1959年 | 70篇 |
1958年 | 68篇 |
1957年 | 49篇 |
1956年 | 90篇 |
1955年 | 120篇 |
1954年 | 57篇 |
1953年 | 49篇 |
排序方式: 共有9700条查询结果,搜索用时 12 毫秒
1.
2.
3.
S. Schultheiβ 《仿生工程学报(英文版)》2009,6(3):203-213
The spines of pencil and lance urchins Heterocentrotus mammillatus and Phyllacanthus imperialis were studied as a modelof light-weight material with high impact resistance.The complex and variable skeleton construction ("stereom") of body andspines of sea urchins consists of highly porous Mg-bearing calcium carbonate.This basically brittle material with pronouncedsingle-crystal cleavage does not fracture by spontaneous catastrophic device failure but by graceful failure over the range of tensof millimeter of bulk compression instead.This was observed in bulk compression tests and blunt indentation experiments onregular,infiltrated and latex coated sea urchin spine segments.Microstructural characterization was carried out using X-raycomputer tomography,optical and scanning electron microscopy.The behavior is interpreted to result from the hierarchicstructure of sea urchin spines from the rnacroscale down to the nanoscale.Guidelines derived from this study see ceramics withlayered porosity as a possible biomimetic construction for appropriate applications. 相似文献
4.
5.
追溯山茱萸科植物的性状进化和生物地理学历史——方法选择的影响? 总被引:4,自引:0,他引:4
This study compares results on reconstructing the ancestral state of characters and ancestral areas of distribution in Cornaceae to gain insights into the impact of using different analytical methods. Ancestral character state reconstructions were compared among three methods (parsimony, maximum likelihood, and stochastic character mapping) using MESQUITE and a full Bayesian method in BAYESTRAITS and inferences of ancestral area distribution were compared between the parsimony-based dispersal-vicariance analysis (DIVA) and a newly developed maximum likelihood (ML) method. Results indicated that among the six inflorescence and fruit characters examined, "perfect" binary characters (no homoplasy, no polymorphism within terminals, and no missing data) are little affected by choice of method, while homoplasious characters and missing data are sensitive to methods used. Ancestral areas at deep nodes of the phylogeny are substantially different between DIVA and ML and strikingly different between analyses including and excluding fossils at three deepest nodes. These results, while raising caution in making conclusions on trait evolution and historical biogeography using conventional methods, demonstrate a limitation in our current understanding of character evolution and biogeography. The biogeographic history favored by the ML analyses including fossils suggested the origin and early radiation of Cornus likely occurred in the late Cretaceous and earliest Tertiary in Europe and intercontinental disjunctions in three lineages involved movements across the North Atlantic Land Bridge (BLB) in the early and mid Tertiary. This result is congruent with the role of NALB for post-Eocene migration and in connecting tropical floras in North America and Africa, and in eastern Asia and South America. However, alternative hypotheses with an origin in eastern Asia and early Trans-Beringia migrations of the genus cannot be ruled out. 相似文献
7.
目的
介绍国外大型医用设备管理的经验,为完善大型医用设备管理办法提供借鉴。方法 通过浏览各国卫生部或相关部门的官方网站,并通过Google Scholar搜索引擎检索国外大型医用设备管理的相关信息。结果 荷兰和日本等国的市场准入机制、欧盟的上市后监控和警戒系统、加拿大的缺陷设备召回机制、美国的需要许可证制度(CON)都是行之有效的大型医用设备管理经验。结论 国外大型医用设备管理的市场准入、上市后监管和缺陷设备召回、政府强制审核和规范配置程序等管理经验,可以为新一轮医改中进一步完善大型医用设备管理提供参考。 相似文献
8.