全文获取类型
收费全文 | 68554篇 |
免费 | 276篇 |
国内免费 | 554篇 |
专业分类
系统科学 | 1265篇 |
丛书文集 | 667篇 |
教育与普及 | 296篇 |
理论与方法论 | 543篇 |
现状及发展 | 34997篇 |
研究方法 | 1410篇 |
综合类 | 28011篇 |
自然研究 | 2195篇 |
出版年
2013年 | 812篇 |
2012年 | 781篇 |
2011年 | 2882篇 |
2009年 | 622篇 |
2008年 | 999篇 |
2007年 | 1183篇 |
2006年 | 1254篇 |
2005年 | 1398篇 |
2004年 | 2480篇 |
2003年 | 2147篇 |
2002年 | 1807篇 |
2001年 | 1920篇 |
2000年 | 1482篇 |
1999年 | 1230篇 |
1998年 | 652篇 |
1997年 | 779篇 |
1994年 | 692篇 |
1993年 | 701篇 |
1992年 | 1176篇 |
1991年 | 996篇 |
1990年 | 1120篇 |
1989年 | 890篇 |
1988年 | 867篇 |
1987年 | 898篇 |
1986年 | 922篇 |
1985年 | 1158篇 |
1984年 | 958篇 |
1983年 | 837篇 |
1982年 | 883篇 |
1981年 | 908篇 |
1980年 | 1030篇 |
1979年 | 1803篇 |
1978年 | 1558篇 |
1977年 | 1501篇 |
1976年 | 1331篇 |
1975年 | 1348篇 |
1974年 | 1374篇 |
1973年 | 1508篇 |
1972年 | 1556篇 |
1971年 | 1692篇 |
1970年 | 2014篇 |
1969年 | 1670篇 |
1968年 | 1592篇 |
1967年 | 1510篇 |
1966年 | 1326篇 |
1965年 | 1035篇 |
1959年 | 606篇 |
1958年 | 853篇 |
1957年 | 675篇 |
1956年 | 593篇 |
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
61.
1 Results Acid-doped polybenzimidazoles[1] are particularly appealing because of high proton conductivity with no or low humidification and promising fuel cells performances. PBI, in fact, contains basic functional groups which can easily interact with strong oxo-acids, such as H3PO4 and H2SO4. The acid partially protonates the polymer and partially is freely dispersed in the polymer backbone, so allowing proton migration via Grotthuss mechanism along the anionic chains[2]. Anyway, a technological limit... 相似文献
62.
The RecQ family of DNA helicases is highly conserved throughout evolution and plays an important role in the maintenance of
genomic stability in all organisms. Mutations in three of the five known family members in humans, BLM, WRN and RECQL4, give rise to disorders that are characterized by predisposition to cancer and premature aging, emphasizing the importance
of studying the RecQ proteins and their cellular activities. Interestingly, three autosomal recessive disorders have been
associated with mutations in the RECQL4 gene: Rothmund-Thomson, RAPADILINO, and Baller-Gerold syndromes, thus making RECQL4 unique within the RecQ family of DNA
helicases. To date, however, the molecular function of RECQL4 and the possible cellular pathways in which it is involved remain
poorly understood. Here, we present an overview of recent findings in connection with RECQL4 and try to highlight different
directions the field could head, helping to clarify the role of RECQL4 in preventing tumorigenesis and maintenance of genome
integrity in humans.
Received 31 October 2006; received after revision 4 January 2007; accepted 5 February 2007 相似文献
63.
M.Hasegawa 《复旦学报(自然科学版)》2007,(5)
1 Results In the coordination system by using complexation with organic ligand, the ff emission of lanthanide(Ⅲ) (Ln(Ⅲ)) is induced the excitation energy transfer form the organic chromophore under the light-irradiation. However, there are not so much number of reports to discuss the energy relaxation mechanism in such complexes with Ln(Ⅲ). Recently, we succeeded firstly to estimate the rate constant of the energy transfer between the ligand and Ln(Ⅲ) in Pr(Ⅲ)-phenanthroline analogs[1]. Here, we will di... 相似文献
64.
65.
66.
67.
V.A.帕瑟根 《国外科技新书评介》2007,(2):7-7
分子间的范德华力是十分微弱的,但可以说普遍的存在着,在很多体系中起着巨大的作用,在物理学、化学、生物学等学科中得到了广泛的研究。但怎样计算范德华力?如何考虑各种条件对范德华力的影响? 相似文献
68.
Strell C Lang K Niggemann B Zaenker KS Entschladen F 《Cellular and molecular life sciences : CMLS》2007,64(24):3306-3316
The extravasation of leukocytes and tumor cells is a multi-step process with the involvement of various adhesion molecules
depending on the three steps rolling, adhesion, and diapedesis. We have developed an in vitro model, by which we investigated the rolling and adhesion of neutrophil granulocytes and MDA-MB-468 human breast carcinoma
cells to lung endothelial cells under physiological flow-conditions. We found that norepinephrine had an inhibitory function
on the fMLP-promoted adhesion of neutrophil granulocytes due to a down-regulation of β2-integrin. Furthermore, neutrophil
granulocytes serve as linking cells for the interaction of the MDA-MB-468 cells with the endothelium, which are both β2-integrin
negative, but express the β2-integrin ligand ICAM-1. In addition, we show here that N-cadherin is up-regulated on the endothelial
cells and on neutrophil granulocytes in response to fMLP. This up-regulation resulted in a significant increase of adherent
MDA-MB-468 cells, which are also N-cadherin positive.
Received 3 September 2007; received after revision 17 October 2007; accepted 22 October 2007 相似文献
69.
Numerous microRNAs (miRNAs) have been discovered in the genomes of higher eukaryotes, and functional studies indicate that they are important during development. However, little is known concerning the function of individual miRNAs. We approached this problem in zebrafish by combining identification of miRNA expression, functional analyses and experimental validation of potential targets. We show that miR-214 is expressed during early segmentation stages in somites and that varying its expression alters the expression of genes regulated by Hedgehog signaling. Inhibition of miR-214 results in a reduction or loss of slow-muscle cell types. We show that su(fu) mRNA, encoding a negative regulator of Hedgehog signaling, is targeted by miR-214. Through regulation of su(fu), miR-214 enables precise specification of muscle cell types by sharpening cellular responses to Hedgehog. 相似文献
70.
Advances in our understanding of cardiac development have fuelled research into cellular approaches to myocardial repair of
the damaged heart. In this collection of reviews we present recent advances into the basic mechanisms of heart development
and the resident and non-resident progenitor cell populations that are currently being investigated as potential mediators
of cardiac repair. Together these reviews illustrate that despite our current knowledge about how the heart is constructed,
caution and much more research in this exciting field is essential. The current momentum to evaluate the potential for cardiac
repair will in turn accelerate research into fundamental aspects of myocardial biology. 相似文献