全文获取类型
收费全文 | 26024篇 |
免费 | 58篇 |
国内免费 | 87篇 |
专业分类
系统科学 | 288篇 |
丛书文集 | 611篇 |
教育与普及 | 70篇 |
理论与方法论 | 97篇 |
现状及发展 | 12232篇 |
研究方法 | 1013篇 |
综合类 | 11588篇 |
自然研究 | 270篇 |
出版年
2013年 | 137篇 |
2012年 | 320篇 |
2011年 | 650篇 |
2010年 | 151篇 |
2008年 | 411篇 |
2007年 | 492篇 |
2006年 | 434篇 |
2005年 | 466篇 |
2004年 | 412篇 |
2003年 | 464篇 |
2002年 | 417篇 |
2001年 | 795篇 |
2000年 | 794篇 |
1999年 | 462篇 |
1992年 | 452篇 |
1991年 | 363篇 |
1990年 | 405篇 |
1989年 | 387篇 |
1988年 | 385篇 |
1987年 | 388篇 |
1986年 | 409篇 |
1985年 | 517篇 |
1984年 | 351篇 |
1983年 | 331篇 |
1982年 | 276篇 |
1981年 | 320篇 |
1980年 | 375篇 |
1979年 | 870篇 |
1978年 | 684篇 |
1977年 | 711篇 |
1976年 | 530篇 |
1975年 | 544篇 |
1974年 | 853篇 |
1973年 | 693篇 |
1972年 | 669篇 |
1971年 | 786篇 |
1970年 | 1018篇 |
1969年 | 880篇 |
1968年 | 804篇 |
1967年 | 813篇 |
1966年 | 683篇 |
1965年 | 508篇 |
1964年 | 147篇 |
1959年 | 300篇 |
1958年 | 480篇 |
1957年 | 347篇 |
1956年 | 318篇 |
1955年 | 279篇 |
1954年 | 276篇 |
1948年 | 183篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
241.
XIA Chun XU Guangxian LIN Changyou HU Tuanjun YAN Ruoqian George F GAO 《自然科学进展(英文版)》2004,14(5):403-410
In order to clarify the molecular sequences,allelic polymorphism and the tertiary structure of grass carp (Ctenophayngodon idellus) MHC class I,and to further study their relationship with disease resistances,grass carp MHC class I gene (Ctid-MHC I) was cloned from a cDNA library and the allelic polymorphism in the population was investigated.The results showed that most of the variations exist in the peptide-binding domain (PBD) and high polymorphism was identified in the Ctid-MHC I allelic genes from 12 individuals.Based on the genetic distance,Ctid-MHC class I can be classified into 6 types (from Ctid-MHC I-UA to Ctid-MHC I-UF) which were subdivided into 9 lineages (from A to I).Comparison of the Ctid-MHC I among animals and humans showed that the key amino acids of the peptide binding sites are conserved.Analysis of the tertiary structure of the PBD between Grass carp and human crystallographic data of HLA-A2,the variation with insertion or deletion was found in eight regions (A~H).The phylogenetic tree of MHC class I indicates the evolution of MHC class I among grass carp,fish,amphibian,birds,higher vertebrates and humans. 相似文献
242.
243.
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
244.
245.
Sandeep Chauhan Vikas Verma Ujjwal Prakash P. C. Tewari Dinesh Khanduja 《矿物冶金与材料学报》2017,24(8):918-925
Induction hardening of dense Fe-Cr/Mo alloys processed via the powder-metallurgy route was studied. The Fe-3Cr-0.5Mo, Fe-1.5Cr-0.2Mo, and Fe-0.85Mo pre-alloyed powders were mixed with 0.4wt%, 0.6wt%, and 0.8wt% C and compacted at 500, 600, and 700 MPa, respectively. The compacts were sintered at 1473 K for 1 h and then cooled at 6 K/min. Ferrite with pearlite was mostly observed in the sintered alloys with 0.4wt% C, whereas a carbide network was also present in the alloys with 0.8wt% C. Graphite at prior particle boundaries led to deterioration of the mechanical properties of alloys with 0.8wt% C, whereas no significant induction hardening was achieved in alloys with 0.4wt% C. Among the investigated samples, alloys with 0.6wt% C exhibited the highest strength and ductility and were found to be suitable for induction hardening. The hardening was carried out at a frequency of 2.0 kHz for 2-3 s. A case depth of 2.5 mm was achieved while maintaining the bulk (interior) hardness of approximately HV 230. A martensitic structure was observed on the outer periphery of the samples. The hardness varied from HV 600 to HV 375 from the sample surface to the interior of the case hardened region. The best combination of properties and hardening depth was achieved in case of the Fe-1.5Cr-0.2Mo alloy with 0.6wt% C. 相似文献
246.
Shachi P. Patel Suzanne J. Randle Sarah Gibbs Anne Cooke Heike Laman 《Cellular and molecular life sciences : CMLS》2017,74(8):1553-1566
G1 phase cell cycle proteins, such as cyclin-dependent kinase 6 (Cdk6) and its activating partners, the D-type cyclins, are important regulators of T-cell development and function. An F-box protein, called F-box only protein 7 (Fbxo7), acts as a cell cycle regulator by enhancing cyclin D-Cdk6 complex formation and stabilising levels of p27, a cyclin-dependent kinase inhibitor. We generated a murine model of reduced Fbxo7 expression to test its physiological role in multiple tissues and found that these mice displayed a pronounced thymic hypoplasia. Further analysis revealed that Fbxo7 differentially affected proliferation and apoptosis of thymocytes at various stages of differentiation in the thymus and also mature T-cell function and proliferation in the periphery. Paradoxically, Fbxo7-deficient immature thymocytes failed to undergo expansion in the thymus due to a lack of Cdk6 activity, while mature T cells showed enhanced proliferative capacity upon T-cell receptor engagement due to reduced p27 levels. Our studies reveal differential cell cycle regulation by Fbxo7 at different stages in T-cell development. 相似文献
247.
Wilson J. E. M. Costa Axel M. Katz José Leonardo O. Mattos Pedro F. Amorim Beatrizz O. Mesquita Paulo J. Vilardo 《Journal of Natural History》2019,53(47-48):2905-2928
ABSTRACT This study is primarily directed to the most poorly known species of the genus Trichomycterus, comprising five nominal species (T. florensis, T. immaculatus, T. nigricans, T. paquequerensis and T. santaeritae) endemic to south-eastern Brazil. One of them, T. nigricans, is the type species of the genus, involved in taxonomic problems for over 150 years. A detailed historical review, accompanied by examination of type specimens and recent collections, revealed that the correct type locality of T. nigricans is in the vicinity of Rio de Janeiro, not Santa Catarina as commonly appears in the literature; specimens previously misidentified as T. nigricans from Santa Catarina belong to a possibly undescribed species of the genus Cambeva; T. paquequerensis is a synonym of T. immaculatus, and T. florensis is a synonym of T. santaeritae; and the hypothesis that T. santaeritae is closely related to the Amazon Sarcoglanidinae is refuted. The three valid species are redescribed. These species are members of a clade also including T. caipora that is highly supported by molecular data, diagnosed by a pronounced posterior maxillary process and caudal fin emarginate at least in larger specimens. A subclade comprising T. caipora, T. nigricans and T. santaeritae is diagnosed by a long maxilla and a bifid anterior extremity of hypobranchial 3. 相似文献
248.
249.
250.
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