全文获取类型
收费全文 | 67829篇 |
免费 | 349篇 |
国内免费 | 617篇 |
专业分类
系统科学 | 1270篇 |
丛书文集 | 637篇 |
教育与普及 | 288篇 |
理论与方法论 | 543篇 |
现状及发展 | 35975篇 |
研究方法 | 1434篇 |
综合类 | 26456篇 |
自然研究 | 2192篇 |
出版年
2013年 | 830篇 |
2012年 | 773篇 |
2011年 | 2884篇 |
2009年 | 639篇 |
2008年 | 1028篇 |
2007年 | 1180篇 |
2006年 | 1217篇 |
2005年 | 1369篇 |
2004年 | 2598篇 |
2003年 | 2104篇 |
2002年 | 1807篇 |
2001年 | 1888篇 |
2000年 | 1381篇 |
1999年 | 1266篇 |
1998年 | 653篇 |
1997年 | 778篇 |
1996年 | 544篇 |
1994年 | 686篇 |
1993年 | 691篇 |
1992年 | 1167篇 |
1991年 | 955篇 |
1990年 | 1070篇 |
1989年 | 904篇 |
1988年 | 848篇 |
1987年 | 924篇 |
1986年 | 920篇 |
1985年 | 1122篇 |
1984年 | 947篇 |
1983年 | 768篇 |
1982年 | 880篇 |
1981年 | 922篇 |
1980年 | 1043篇 |
1979年 | 1772篇 |
1978年 | 1568篇 |
1977年 | 1548篇 |
1976年 | 1317篇 |
1975年 | 1343篇 |
1974年 | 1410篇 |
1973年 | 1530篇 |
1972年 | 1628篇 |
1971年 | 1679篇 |
1970年 | 1933篇 |
1969年 | 1629篇 |
1968年 | 1585篇 |
1967年 | 1472篇 |
1966年 | 1247篇 |
1965年 | 943篇 |
1959年 | 541篇 |
1958年 | 780篇 |
1957年 | 617篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
62.
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
63.
通过对“洋务运动博物馆”东立面形象的反思 ,讨论了建筑形态设计中隐喻手法的运用 ;就此引申 ,探讨传统问题 ,指出正确理解传统与创新的四个方面。 相似文献
64.
R. Poggioli A. V. Vergoni A. Bertolini 《Cellular and molecular life sciences : CMLS》1985,41(2):265-266
Summary Hydrochlorothiazide, acutely injected in rats, has a weak analgesic activity per se and potentiates and prolongs the antinociceptive effect of morphine.This work was supported in part by grants from Consiglio Nazionale delle Ricerche, and by Ministero della Pubblica Istruzione, Roma. 相似文献
65.
David H. Hubel 《自然科学进展(英文版)》2007,17(13):1-3
The visual field is topographically mapped onto the primary visual cortex (V1), forming a retinotopic map which is far more detailed for the foveal regions than for the periphery. We found that receptive field (RF) size in monkey V1 increases with eccentricity, and that a 1—2 m2 V1 region contains roughly a complete set of machinery necessary to analyze a visual-field area whose size is about that of the RFs of the cells. This allows V1 to be anatomically uniform, and is in sharp contrast with the retina. 相似文献
66.
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
67.
68.
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
69.
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
70.
Kang J.Y. Xu F.C. 《中国科学基金(英文版)》2004,(2):57-58,64
1 Introduction
Nanoscale science and technology have attracted greatattention since the novel properties become dominant for well-known materials as their sizes reduced to some critical dimension. These properties frequently originate in lattice distortions, structure transformations etc[1]. The variations of size and structure ofnanocrystals lead to the change of their electronic structures. Very recently, nanoscale materials with different electronic structures were utilized in fabricating nanoscale junctions[2].
…… 相似文献