全文获取类型
收费全文 | 39085篇 |
免费 | 114篇 |
国内免费 | 143篇 |
专业分类
系统科学 | 180篇 |
丛书文集 | 691篇 |
教育与普及 | 63篇 |
理论与方法论 | 155篇 |
现状及发展 | 18017篇 |
研究方法 | 1549篇 |
综合类 | 18087篇 |
自然研究 | 600篇 |
出版年
2013年 | 260篇 |
2012年 | 529篇 |
2011年 | 1100篇 |
2010年 | 221篇 |
2008年 | 620篇 |
2007年 | 737篇 |
2006年 | 697篇 |
2005年 | 737篇 |
2004年 | 810篇 |
2003年 | 658篇 |
2002年 | 689篇 |
2001年 | 1116篇 |
2000年 | 1054篇 |
1999年 | 751篇 |
1992年 | 711篇 |
1991年 | 511篇 |
1990年 | 571篇 |
1989年 | 562篇 |
1988年 | 515篇 |
1987年 | 547篇 |
1986年 | 631篇 |
1985年 | 786篇 |
1984年 | 557篇 |
1983年 | 493篇 |
1982年 | 440篇 |
1981年 | 462篇 |
1980年 | 527篇 |
1979年 | 1240篇 |
1978年 | 1015篇 |
1977年 | 972篇 |
1976年 | 752篇 |
1975年 | 821篇 |
1974年 | 1181篇 |
1973年 | 986篇 |
1972年 | 1047篇 |
1971年 | 1171篇 |
1970年 | 1584篇 |
1969年 | 1240篇 |
1968年 | 1105篇 |
1967年 | 1171篇 |
1966年 | 1038篇 |
1965年 | 807篇 |
1964年 | 263篇 |
1959年 | 435篇 |
1958年 | 794篇 |
1957年 | 567篇 |
1956年 | 430篇 |
1955年 | 421篇 |
1954年 | 469篇 |
1948年 | 292篇 |
排序方式: 共有10000条查询结果,搜索用时 484 毫秒
411.
Atelopus laetissimus is an endemic and threatened harlequin frog from the high mountain forests of Sierra Nevada de Santa Marta, Colombia. Knowledge of its reproductive biology is essential for understanding the intraspecific interactions that can help the conservation of Atelopus species. We quantified the energy, measured in body weight, invested by males and females of A. laetissimus for reproduction, and how this energetic investment is related to the survival of individuals and rainfall conditions in habitats during two years (2014 and 2015). Our results show plasticity in terms of reproductive phenology linked to rainfall with short- and long-duration breeding strategies. The first year of this study, 2014, had a precipitation level in accordance with the annual averages at the area. During this time frogs exhibit a short breeding period. Contrary to 2014, 2015 was a year with little precipitation, below the annual averages, which probably facilitated the females’ quick spawning in the creeks and a consequent reduction in the duration of amplexus and low breeding efforts by males. This, in turn, was related to a long breeding period that favors the survival and reproduction of males during the entire year. In 2014 we found a decrease of 25% to 30% body weight of potentially reproductive males, which may be attributed to a prolonged duration of amplectant events. 相似文献
412.
413.
M. Penchal Reddy R.A. Shakoor Gururaj Parande Vyasaraj Manakari F. Ubai A.M.A. Mohame Manoj Gupta 《自然科学进展(英文版)》2017,27(5):606-614
In the present study, nano-sized SiC (0, 0.3, 0.5, 1.0 and 1.5 vol%) reinforced aluminum (Al) metal matrix
composites were fabricated by microwave sintering and hot extrusion techniques. The structural (XRD, SEM),
mechanical (nanoindentation, compression, tensile) and thermal properties (co-efficient of thermal expansion-
CTE) of the developed Al-SiC nanocomposites were studied. The SEM/EDS mapping images show a
homogeneous distribution of SiC nanoparticles into the Al matrix. A significant increase in the strength
(compressive and tensile) of the Al-SiC nanocomposites with the addition of SiC content is observed. However,
it is noticed that the ductility of Al-SiC nanocomposites decreases with increasing volume fraction of SiC. The
thermal analysis indicates that CTE of Al-SiC nanocomposites decreases with the progressive addition of hard
SiC nanoparticles. Overall, hot extruded Al 1.5 vol% SiC nanocomposites exhibited the best mechanical and
thermal performance as compared to the other developed Al-SiC nanocomposites. 相似文献
414.
415.
综述了域上或交换代数上的线性(-)代数的相应的簇(范畴)的 Groebner-Shirshov 基理论的新成果,如:结合代数(包括群(半群)代数),自由代数的张量积,李代数,Di-代数,pre-李代数,Rota-Baxter代数,metabelian李代数,L-代数,半环代数,范畴代数,等.以上结果包含了许多应用,尤其是给出了一些著名结论的新的证明. 相似文献
416.
417.
418.
419.
Water mites Limnochares aquatica (L., 1758) during maintenance in the laboratory for a long period of time in constant conditions periodically produced certain whitish flocculent material consisting of long rigid unbranched tube-like threads 1.3 ± 0.3 µm in diameter crossing freely. These threads were studied using light-optical as well as transmission electron microscopical and scanning electron microscopical methods. Microbiological staining was also applied to the threads to exclude their bacterial or fungal origin. The thread wall is built of fine fibrils arranged at different angles to the long axis of threads that is reflected in a certain stratification of the wall. Threads are mostly hollow or may contain electron-dense homogeneous material. No cell components are present in the thread composition. Numerous dermal glands with their small slit-like orifice scattered throughout the mite body surface are thought to produce these threads. Most probably the thread formation is a reaction of mites to stress under laboratory conditions, and this is expected to be a type of defensive reaction. 相似文献
420.
Julhash U. Kazi Rohit A. Chougule Tianfeng Li Xianwei Su Sausan A. Moharram Kaja Rupar Alissa Marhäll Mohiuddin Gazi Jianmin Sun Hui Zhao Lars Rönnstrand 《Cellular and molecular life sciences : CMLS》2017,74(14):2679-2688
The type III receptor tyrosine kinase FLT3 is frequently mutated in acute myeloid leukemia. Oncogenic FLT3 mutants display constitutive activity leading to aberrant cell proliferation and survival. Phosphorylation on several critical tyrosine residues is known to be essential for FLT3 signaling. Among these tyrosine residues, Y842 is located in the so-called activation loop. The position of this tyrosine residue is well conserved in all receptor tyrosine kinases. It has been reported that phosphorylation of the activation loop tyrosine is critical for catalytic activity for some but not all receptor tyrosine kinases. The role of Y842 residue in FLT3 signaling has not yet been studied. In this report, we show that Y842 is not important for FLT3 activation or ubiquitination but plays a critical role in regulating signaling downstream of the receptor as well as controlling receptor stability. We found that mutation of Y842 in the FLT3-ITD oncogenic mutant background reduced cell viability and increased apoptosis. Furthermore, the introduction of the Y842 mutation in the FLT3-ITD background led to a dramatic reduction in in vitro colony forming capacity. Additionally, mice injected with cells expressing FLT3-ITD/Y842F displayed a significant delay in tumor formation, compared to FLT3-ITD expressing cells. Microarray analysis comparing gene expression regulated by FLT3-ITD versus FLT3-ITD/Y842F demonstrated that mutation of Y842 causes suppression of anti-apoptotic genes. Furthermore, we showed that cells expressing FLT3-ITD/Y842F display impaired activity of the RAS/ERK pathway due to reduced interaction between FLT3 and SHP2 leading to reduced SHP2 activation. Thus, we suggest that Y842 is critical for FLT3-mediated RAS/ERK signaling and cellular transformation. 相似文献