全文获取类型
收费全文 | 47011篇 |
免费 | 363篇 |
国内免费 | 650篇 |
专业分类
系统科学 | 470篇 |
丛书文集 | 768篇 |
教育与普及 | 155篇 |
理论与方法论 | 186篇 |
现状及发展 | 18823篇 |
研究方法 | 1612篇 |
综合类 | 25466篇 |
自然研究 | 544篇 |
出版年
2014年 | 452篇 |
2013年 | 670篇 |
2012年 | 928篇 |
2011年 | 1482篇 |
2010年 | 647篇 |
2009年 | 665篇 |
2008年 | 1255篇 |
2007年 | 1291篇 |
2006年 | 1274篇 |
2005年 | 1149篇 |
2004年 | 1033篇 |
2003年 | 957篇 |
2002年 | 982篇 |
2001年 | 1528篇 |
2000年 | 1408篇 |
1999年 | 957篇 |
1994年 | 445篇 |
1992年 | 824篇 |
1991年 | 612篇 |
1990年 | 732篇 |
1989年 | 684篇 |
1988年 | 638篇 |
1987年 | 723篇 |
1986年 | 659篇 |
1985年 | 835篇 |
1984年 | 651篇 |
1983年 | 542篇 |
1982年 | 510篇 |
1981年 | 519篇 |
1980年 | 601篇 |
1979年 | 1308篇 |
1978年 | 1057篇 |
1977年 | 1031篇 |
1976年 | 852篇 |
1975年 | 887篇 |
1974年 | 1171篇 |
1973年 | 1045篇 |
1972年 | 1054篇 |
1971年 | 1146篇 |
1970年 | 1505篇 |
1969年 | 1142篇 |
1968年 | 1143篇 |
1967年 | 1105篇 |
1966年 | 972篇 |
1965年 | 692篇 |
1959年 | 375篇 |
1958年 | 645篇 |
1957年 | 417篇 |
1956年 | 385篇 |
1955年 | 365篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
32.
Vitamin B12 (methylcobalamin) was administered orally (3 mg/day) to 9 healthy subjects for 4 weeks. Nocturnal melatonin levels after exposure to bright light (ca. 2500 lx) were determined, as well as the levels of plasma melatonin over 24 h. The timing of sleep was also recorded. Vitamin B12 was given blind to the subjects and crossed over with placebo. We found that the 24-h melatonin rhythm was significantly phase-advanced (1.1 h) in the vitamin B12 trial as compared with that in the placebo trial. In addition, the 24-h mean of plasma melatonin level was much lower in the vitamin B12 trial than with the placebo. Furthermore, the nocturnal melatonin levels during bright light exposure were significantly lower in the vitamin B12 trial than with the placebo. On the other hand, vitamin B12 did not affect the timing of sleep. These findings raise the possibility that vitamin B12 phase-advances the human circadian rhythm by increasing the light sensitivity of the circadian clock. 相似文献
33.
34.
35.
1 Results This paper presents the development of platinum nanocomposites structures based on organically modified c.a.2 nm core platinum nanoparticles.The chemical modification of the 4-mercaptoaniline functionalized particles by various in coming molecules is evidenced and precisely quantified.The particles can be dissolved like molecules in various solvents depending on the features of the new crown and X-rays shows that the interparticle distance is affected by the crown modification.These platinum n... 相似文献
36.
1 Results The development of the concepts of nanotechnology has given an important impact on the design of new polymer based materials which are in most cases characterized by a multiphase morphology. When at least one phase has nanometric dimension(s) the system can be considered as a nanocomposite where the interface is not only determining for the adhesion but also may play a role in some bulk properties. Indeed in nanostructured multiphase solids the interface is significant as a bulk component. The... 相似文献
37.
1 Results Pyrimidinones (PMs) are a class of important heterocycles which have been well documented throughout the literature due to their biological importance. They exhibit a wide range of pharmaceutical and therapeutic properties[1].A rapid and efficient one-pot method for the synthesis of 4,6-diarylpyrimidin-2(1H)- ones and related heterocycles is described.The condensation of acetophenone derivatives,aldehydes and urea in the presence of sulfamic acid was employed to synthesize a variety of pyrimid... 相似文献
38.
介绍了语音数字化存储也回放系统设计方法,给出了电路图,讨论了系统应用情况,并给出其在银行排队系统中的应用方案。 相似文献
39.
论档案信息资源的开发与利用 总被引:1,自引:0,他引:1
高玲娜 《科技情报开发与经济》2002,12(4):51-51,56
简要介绍了开发利用档案信息资源的重要意义,提出了提高档案信息资源利用率的措施和方法,包括:做好档案的收集工作,实现档案的现代化管理,为用户提供多层次的服务等。 相似文献
40.