全文获取类型
收费全文 | 13653篇 |
免费 | 35篇 |
国内免费 | 39篇 |
专业分类
系统科学 | 87篇 |
丛书文集 | 205篇 |
教育与普及 | 39篇 |
理论与方法论 | 102篇 |
现状及发展 | 6717篇 |
研究方法 | 674篇 |
综合类 | 5715篇 |
自然研究 | 188篇 |
出版年
2013年 | 137篇 |
2012年 | 261篇 |
2011年 | 438篇 |
2010年 | 125篇 |
2008年 | 271篇 |
2007年 | 280篇 |
2006年 | 293篇 |
2005年 | 318篇 |
2004年 | 270篇 |
2003年 | 260篇 |
2002年 | 264篇 |
2001年 | 318篇 |
2000年 | 316篇 |
1999年 | 219篇 |
1992年 | 191篇 |
1991年 | 168篇 |
1990年 | 176篇 |
1989年 | 174篇 |
1988年 | 179篇 |
1987年 | 183篇 |
1986年 | 186篇 |
1985年 | 250篇 |
1984年 | 162篇 |
1983年 | 150篇 |
1982年 | 125篇 |
1981年 | 131篇 |
1980年 | 178篇 |
1979年 | 448篇 |
1978年 | 339篇 |
1977年 | 385篇 |
1976年 | 238篇 |
1975年 | 253篇 |
1974年 | 418篇 |
1973年 | 355篇 |
1972年 | 383篇 |
1971年 | 377篇 |
1970年 | 502篇 |
1969年 | 443篇 |
1968年 | 426篇 |
1967年 | 417篇 |
1966年 | 363篇 |
1965年 | 262篇 |
1964年 | 85篇 |
1959年 | 124篇 |
1958年 | 220篇 |
1957年 | 176篇 |
1956年 | 160篇 |
1955年 | 137篇 |
1954年 | 139篇 |
1948年 | 109篇 |
排序方式: 共有10000条查询结果,搜索用时 250 毫秒
131.
Fracture toughness is very important when applying Damage Tolerance Design and Assessment Techniques. The traditional testing approach for obtaining fracture toughness values is costly and time consuming. In order to estimate the fracture toughness of ductile metals, the fracture mechanics theory, materials plastic deformation theory and materials constructive relationships are employed here. A series of formulae and a theoretical approach are presented to calculate fracture toughness values of different materials in the plane stress and plane strain conditions. Compared with test results, evaluated values have a good agreement. 相似文献
132.
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
133.
134.
135.
136.
137.
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
138.
139.
Dominique Eladari Régine Chambrey Nicolas Picard Juliette Hadchouel 《Cellular and molecular life sciences : CMLS》2014,71(15):2879-2895
Sodium absorption by the distal part of the nephron, i.e., the distal convoluted tubule, the connecting tubule, and the collecting duct, plays a major role in the control of homeostasis by the kidney. In this part of the nephron, sodium transport can either be electroneutral or electrogenic. The study of electrogenic Na+ absorption, which is mediated by the epithelial sodium channel (ENaC), has been the focus of considerable interest because of its implication in sodium, potassium, and acid–base homeostasis. However, recent studies have highlighted the crucial role played by electroneutral NaCl absorption in the regulation of the body content of sodium chloride, which in turn controls extracellular fluid volume and blood pressure. Here, we review the identification and characterization of the NaCl cotransporter (NCC), the molecule accounting for the main part of electroneutral NaCl absorption in the distal nephron, and its regulators. We also discuss recent work describing the identification of a novel “NCC-like” transport system mediated by pendrin and the sodium-driven chloride/bicarbonate exchanger (NDCBE) in the β-intercalated cells of the collecting system. 相似文献
140.
Juan C. Mayo Rosa M. Sainz Pedro González-Menéndez David Hevia Rafael Cernuda-Cernuda 《Cellular and molecular life sciences : CMLS》2017,74(21):3927-3940
Melatonin is a well-known, nighttime-produced indole found in bacteria, eukaryotic unicellulars, animals or vascular plants. In vertebrates, melatonin is the major product of the pineal gland, which accounts for its increase in serum during the dark phase, but it is also produced by many other organs and cell types. Such a wide distribution is consistent with its multiple and well-described functions which include from the circadian regulation and adaptation to seasonal variations to immunomodulatory and oncostatic actions in different types of tumors. The discovery of its antioxidant properties in the early 1990s opened a new field of potential protective functions in multiple tissues. A special mention should be made regarding the nervous system, where the indole is considered a major neuroprotector. Furthermore, mitochondria appear as one of the most important targets for the indole’s protective actions. Melatonin’s mechanisms of action vary from the direct molecular interaction with free radicals (free radical scavenger) to the binding to membrane (MLT1A and MLT1B) or nuclear receptors (RZR/RORα). Receptor binding has been associated with some, but not all of the indole functions reported to date. Recently, two new mechanisms of cellular uptake involving the facilitative glucose transporters GLUT/SLC2A and the proton-driven oligopeptide transporter PEPT1/2 have been reported. Here we discuss the potential importance that these newly discovered transport systems could have in determining the actions of melatonin, particularly in the mitochondria. We also argue the relative importance of passive diffusion vs active transport in different parts of the cell. 相似文献