全文获取类型
收费全文 | 6642篇 |
免费 | 142篇 |
国内免费 | 180篇 |
专业分类
系统科学 | 140篇 |
丛书文集 | 141篇 |
教育与普及 | 175篇 |
理论与方法论 | 10篇 |
现状及发展 | 1991篇 |
研究方法 | 211篇 |
综合类 | 4284篇 |
自然研究 | 12篇 |
出版年
2022年 | 78篇 |
2021年 | 85篇 |
2015年 | 102篇 |
2014年 | 95篇 |
2013年 | 95篇 |
2012年 | 147篇 |
2011年 | 203篇 |
2010年 | 188篇 |
2009年 | 212篇 |
2008年 | 211篇 |
2007年 | 244篇 |
2006年 | 159篇 |
2005年 | 223篇 |
2004年 | 202篇 |
2003年 | 150篇 |
2002年 | 157篇 |
2001年 | 267篇 |
2000年 | 264篇 |
1999年 | 221篇 |
1998年 | 111篇 |
1997年 | 115篇 |
1996年 | 106篇 |
1995年 | 79篇 |
1994年 | 68篇 |
1993年 | 64篇 |
1992年 | 144篇 |
1991年 | 127篇 |
1990年 | 114篇 |
1989年 | 97篇 |
1988年 | 102篇 |
1987年 | 95篇 |
1985年 | 101篇 |
1984年 | 82篇 |
1983年 | 63篇 |
1982年 | 67篇 |
1980年 | 75篇 |
1979年 | 146篇 |
1978年 | 134篇 |
1977年 | 107篇 |
1976年 | 79篇 |
1975年 | 73篇 |
1974年 | 98篇 |
1973年 | 103篇 |
1972年 | 90篇 |
1971年 | 111篇 |
1970年 | 101篇 |
1969年 | 97篇 |
1968年 | 95篇 |
1967年 | 87篇 |
1966年 | 86篇 |
排序方式: 共有6964条查询结果,搜索用时 15 毫秒
941.
Falcke H Apel WD Badea AF Bähren L Bekk K Bercuci A Bertaina M Biermann PL Blümer J Bozdog H Brancus IM Buitink S Brüggemann M Buchholz P Butcher H Chiavassa A Daumiller K de Bruyn AG de Vos CM Di Pierro F Doll P Engel R Gemmeke H Ghia PL Glasstetter R Grupen C Haungs A Heck D Hörandel JR Horneffer A Huege T Kampert KH Kant GW Klein U Kolotaev Y Koopman Y Krömer O Kuijpers J Lafebre S Maier G Mathes HJ Mayer HJ Milke J Mitrica B Morello C Navarra G Nehls S Nigl A Obenland R Oehlschläger J 《Nature》2005,435(7040):313-316
The nature of ultrahigh-energy cosmic rays (UHECRs) at energies >10(20) eV remains a mystery. They are likely to be of extragalactic origin, but should be absorbed within approximately 50 Mpc through interactions with the cosmic microwave background. As there are no sufficiently powerful accelerators within this distance from the Galaxy, explanations for UHECRs range from unusual astrophysical sources to exotic string physics. Also unclear is whether UHECRs consist of protons, heavy nuclei, neutrinos or gamma-rays. To resolve these questions, larger detectors with higher duty cycles and which combine multiple detection techniques are needed. Radio emission from UHECRs, on the other hand, is unaffected by attenuation, has a high duty cycle, gives calorimetric measurements and provides high directional accuracy. Here we report the detection of radio flashes from cosmic-ray air showers using low-cost digital radio receivers. We show that the radiation can be understood in terms of the geosynchrotron effect. Our results show that it should be possible to determine the nature and composition of UHECRs with combined radio and particle detectors, and to detect the ultrahigh-energy neutrinos expected from flavour mixing. 相似文献
942.
Cellular responses to mild heat stress 总被引:12,自引:0,他引:12
Since its discovery in 1962 by Ritossa, the heat shock response has been extensively studied by a number of investigators to understand the molecular mechanism underlying the cellular response to heat stress. The most well characterized heat shock response is induction of the heat shock proteins that function as molecular chaperones and exert cell cycle regulatory and anti-apoptotic activities. While most investigators have focused their studies on the toxic effects of heat stress in organisms such as severe heat stress-induced cell cycle arrest and apoptosis, the cellular response to fever-ranged mild heat stress has been rather underestimated. However, the cellular response to mild heat stress is likely to be more important in a physiological sense than that to severe heat stress because the body temperature of homeothermic animals increases by only 1–2°C during febrile diseases. Here we provide information that mild heat stress does have some beneficial role in organisms via positively regulating cell proliferation and differentiation, and immune response in mammalian cells.Received 14 May 2004; received after revision 2 August 2004; accepted 16 August 2004 相似文献
943.
Mammalian lactoferrin (Lf) receptors are suggested to have pivotal roles for mediating multiple functions of Lf. In this review, we focus on current knowledge of the structure and function of mammalian Lf receptors, mainly the first cloned Lf receptor that has been shown to be expressed in the infant small intestine at high levels but also in virtually all other tissues. The small intestinal Lf receptor takes up iron from Lf into cells and presumably exerts other physiological functions. Other Lf receptors in various tissues have also been reported to mediate some functions of Lf, such as modulating immune function, inhibiting platelet aggregation and enhancing collagen gel contractile strength. The detailed mechanisms behind the receptor-Lf interactions still need to be elucidated. 相似文献
944.
Tulapurkar AA Suzuki Y Fukushima A Kubota H Maehara H Tsunekawa K Djayaprawira DD Watanabe N Yuasa S 《Nature》2005,438(7066):339-342
There is currently much interest in the development of 'spintronic' devices, in which harnessing the spins of electrons (rather than just their charges) is anticipated to provide new functionalities that go beyond those possible with conventional electronic devices. One widely studied example of an effect that has its roots in the electron's spin degree of freedom is the torque exerted by a spin-polarized electric current on the spin moment of a nanometre-scale magnet. This torque causes the magnetic moment to rotate at potentially useful frequencies. Here we report a very different phenomenon that is also based on the interplay between spin dynamics and spin-dependent transport, and which arises from unusual diode behaviour. We show that the application of a small radio-frequency alternating current to a nanometre-scale magnetic tunnel junction can generate a measurable direct-current (d.c.) voltage across the device when the frequency is resonant with the spin oscillations that arise from the spin-torque effect: at resonance (which can be tuned by an external magnetic field), the structure exhibits different resistance states depending on the direction of the current. This behaviour is markedly different from that of a conventional semiconductor diode, and could form the basis of a nanometre-scale radio-frequency detector in telecommunication circuits. 相似文献
945.
946.
947.
Poulet F Bibring JP Mustard JF Gendrin A Mangold N Langevin Y Arvidson RE Gondet B Gomez C Berthé M Erard S Forni O Manaud N Poulleau G Soufflot A Combes M Drossart P Encrenaz T Fouchet T Melchiorri R Bellucci G Altieri F Formisano V Fonti S Capaccioni F Cerroni P Coradini A Korablev O Kottsov V Ignatiev N Titov D Zasova L Pinet P Schmitt B Sotin C Hauber E Hoffmann H Jaumann R Keller U Forget F;Omega Team 《Nature》2005,438(7068):623-627
The recent identification of large deposits of sulphates by remote sensing and in situ observations has been considered evidence of the past presence of liquid water on Mars. Here we report the unambiguous detection of diverse phyllosilicates, a family of aqueous alteration products, on the basis of observations by the OMEGA imaging spectrometer on board the Mars Express spacecraft. These minerals are mainly associated with Noachian outcrops, which is consistent with an early active hydrological system, sustaining the long-term contact of igneous minerals with liquid water. We infer that the two main families of hydrated alteration products detected-phyllosilicates and sulphates--result from different formation processes. These occurred during two distinct climatic episodes: an early Noachian Mars, resulting in the formation of hydrated silicates, followed by a more acidic environment, in which sulphates formed. 相似文献
948.
Interest in mapping the global distribution of malaria is motivated by a need to define populations at risk for appropriate resource allocation and to provide a robust framework for evaluating its global economic impact. Comparison of older and more recent malaria maps shows how the disease has been geographically restricted, but it remains entrenched in poor areas of the world with climates suitable for transmission. Here we provide an empirical approach to estimating the number of clinical events caused by Plasmodium falciparum worldwide, by using a combination of epidemiological, geographical and demographic data. We estimate that there were 515 (range 300-660) million episodes of clinical P. falciparum malaria in 2002. These global estimates are up to 50% higher than those reported by the World Health Organization (WHO) and 200% higher for areas outside Africa, reflecting the WHO's reliance upon passive national reporting for these countries. Without an informed understanding of the cartography of malaria risk, the global extent of clinical disease caused by P. falciparum will continue to be underestimated. 相似文献
949.
Antigenic activity of lobster (Homarus vulgaris) arginine kinase and its cyanogen bromide fragments]
Y Benyamin Y Robin F Regnouf 《Comptes rendus des séances de l'Académie des sciences. Série D, Sciences naturelles》1977,284(19):1955-1958
The antigenic saturation of lobster arginine kinase (38 000 daltons) by its specific antibodies has been studied. It was found that seven antigenic binding sites are simultaneously reactive on the surface of the enzyme in the presence of a large excess of antibodies or of their Fab fragments. After cyanogen bromide cleavage, the antigenic reactivity is distributed on several fragments of various sizes. 相似文献
950.
Ultrastructural correlation of water reabsorption in isolated rat cauda epididymidis 总被引:1,自引:0,他引:1
Y. C. Wong P. Y. D. Wong C. H. Yeung 《Cellular and molecular life sciences : CMLS》1978,34(4):485-487
Summary Electron microscopic study was made on the water reabsorption of the epithelial cells of the rat cauda epididymidis. It was shown that when the epididymal duct was reabsorbing water at a maximal rate, widely dilated intercellular spaces were seen. It is suggested that the standing gradient model of water reabsorption first proposed for the gall bladder may also operate in the cauda epididymidis.This work was supported in part by the World Health Organization and grant number 158/254 of Hong Kong University. 相似文献