全文获取类型
收费全文 | 16942篇 |
免费 | 38篇 |
国内免费 | 76篇 |
专业分类
系统科学 | 53篇 |
丛书文集 | 174篇 |
教育与普及 | 67篇 |
理论与方法论 | 49篇 |
现状及发展 | 7676篇 |
研究方法 | 676篇 |
综合类 | 8193篇 |
自然研究 | 168篇 |
出版年
2012年 | 200篇 |
2011年 | 349篇 |
2009年 | 101篇 |
2008年 | 267篇 |
2007年 | 288篇 |
2006年 | 269篇 |
2005年 | 287篇 |
2004年 | 327篇 |
2003年 | 292篇 |
2002年 | 283篇 |
2001年 | 469篇 |
2000年 | 495篇 |
1999年 | 343篇 |
1994年 | 304篇 |
1992年 | 271篇 |
1991年 | 225篇 |
1990年 | 282篇 |
1989年 | 252篇 |
1988年 | 261篇 |
1987年 | 284篇 |
1986年 | 290篇 |
1985年 | 339篇 |
1984年 | 234篇 |
1983年 | 259篇 |
1982年 | 214篇 |
1981年 | 206篇 |
1980年 | 245篇 |
1979年 | 510篇 |
1978年 | 436篇 |
1977年 | 393篇 |
1976年 | 317篇 |
1975年 | 389篇 |
1974年 | 475篇 |
1973年 | 422篇 |
1972年 | 431篇 |
1971年 | 563篇 |
1970年 | 612篇 |
1969年 | 533篇 |
1968年 | 532篇 |
1967年 | 459篇 |
1966年 | 408篇 |
1965年 | 295篇 |
1964年 | 130篇 |
1959年 | 168篇 |
1958年 | 322篇 |
1957年 | 253篇 |
1956年 | 202篇 |
1955年 | 201篇 |
1954年 | 193篇 |
1948年 | 167篇 |
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
191.
As a newly developed precision technology, casting-forging complex near net forming process is utilized to produce complex components with a short lead time, low cost and high precision, thus to accelerate the response speed of the market and enhance the competitive power of products. In this paper, the casting-forging complex near net forming process of alternator claw pole was developed and investigated with a combination of experimental and numerical simulation method. Qualified near net workpiece was manufactured, mechanical parameter and relative field information during the forming process was also obtained. While the alternator claw-pole is processed with this technology, the forming force is small, the process is short and the quality of forgings is perfect. Therefore, the complex casting-forging near net forming process of claw-pole is an energy and material saving technology, which will have a vast developing and application prospect in the future. 相似文献
192.
John Davenport Julia Davenport Cheong-Hoong Diong K.H. Low 《Journal of Natural History》2016,50(33-34):2097-2105
The pig-nosed freshwater turtle Carettochelys insculpta Ramsay, 1886 has paddle-shaped foreflippers that resemble those of sea turtles. These turtles exhibit a wide range of swimming capabilities. As well as swimming by the action of synchronized foreflippers alone, they sometimes used alternate hindlimb action at the same time. They could swim by ipsilaterally synchronized action of all four limbs, or by hindlimb action alone (combined with stabilizer/lift function of the foreflippers). The turtles also showed flexibility in bottom-walking. Besides the ipsilaterally synchronized quadrupedal action characteristic of other freshwater turtles, they exhibited a bipedal walking mechanism never previously described. Propelled by alternate action of the hindlimbs, the animals held the head and plastron above the substratum, with the large foreflippers acting to provide anterior lift and stability against roll and yaw. Because both hindlimbs were sometimes off the substratum simultaneously during bipedal locomotion, their duty factors were < 0.5, implying a bipedal run. 相似文献
193.
194.
195.
Luiz A. Rocha Joseph D. DiBattista Tane H. Sinclair-Taylor Michael L. Berumen 《Journal of Natural History》2020,54(15-16):1019-1023
ABSTRACT Here we document three cases of mimicry in coral reef fishes not previously reported in the literature involving two groupers (Epinephelus leucogrammicus and Plectropomus marisrubri) and a soapfish (Diploprion drachi) as mimics, and two wrasses (Larabicus quadrilineatus and Cheilinus quinquecinctus) and a blenny (Meiacanthus nigrolineatus) as models. All three cases are of aggressive mimicry, with a predatory species mimicking a harmless one, and in one of the cases, the mimicry is also Müllerian, where both the predator and harmless species are unpalatable. 相似文献
196.
正Published online:14 March 2014óScience China Press and Springer-Verlag Berlin Heidelberg 2014Erratum to:Chin.Sci.Bull.(2014)59(5–6):528–532DOI 10.1007/s11434-013-0060-1In the original publication of this paper,the first name and the last name of the first author has been documented 相似文献
197.
Rike Wallbrecher Wouter P. R. Verdurmen Samuel Schmidt Petra H. Bovee-Geurts Felix Broecker Anika Reinhardt Toin H. van Kuppevelt Peter H. Seeberger Roland Brock 《Cellular and molecular life sciences : CMLS》2014,71(14):2717-2729
Binding to negatively charged heparan sulfates (HS) at the cell surface is considered the first step in the internalization of cationic cell-penetrating peptides (CPPs). However, little is known about the relation of the characteristics of the HS-CPP interaction such as affinity, stoichiometry, and clustering with uptake. In this study, we investigated a collection of mutants of a cyclic CPP derived from human lactoferrin with respect to HS binding and uptake. The thermodynamic parameters of HS binding were determined by isothermal titration calorimetry, clustering of HS was investigated by dynamic light scattering, and cellular uptake by flow cytometry and confocal microscopy. Whereas mutations of non-arginine amino acids that are conserved across lactoferrins of different mammalia only had a minor effect on uptake efficiency, changes in the number of arginine residues influenced the uptake significantly. In general, introduction of arginine residues and cyclization improved the HS affinity and the ability to cluster HS. In particular, there was a strong negative correlation between stoichiometry and uptake, indicating that crosslinking of HS is the driving force for the uptake of arginine-rich CPPs. Using glycan microarrays presenting a collection of synthetic HS, we show that a minimal chain length of HS is required for peptide binding. 相似文献
198.
鉴于构建流行病动力学模型、探索流行病传播规律对疫情防控具有十分重要的理论意义和实际应用价值,在已有的均匀混合模型基础上,针对个体接触关系异质化越发明显,且每个个体都处在不同的接触关系中,建立了兼顾个体状态与接触追踪的动态小世界网络模型。模拟了新冠病毒在社会中的传播过程。通过对比仿真结果,说明了所建模型的合理性。在此基础上,仿真计算了网络拓扑结构与接种免疫人数占比共同作用下对新冠病毒传播的影响,分析得到群体免疫临界值。说明所建传播模型合理,接种疫苗实现群体免疫可行。 相似文献
199.
200.
We have taken a new approach to test the commonly accepted, but recently questioned, principle of clonal inheritance of vertebrate mitochondrial DNA (mtDNA) by relating its inheritance to a female-specific marker of nuclear DNA. Whereas this is impossible in organisms with male heterogamy (such as mammals), we show here that genealogies of mtDNA and the female-specific W chromosome of a bird species are completely concordant. Our results indicate that inheritance of mtDNA is free of detectable recombination effects over an evolutionary timescale. 相似文献