全文获取类型
收费全文 | 1344篇 |
免费 | 61篇 |
国内免费 | 19篇 |
专业分类
化学 | 1057篇 |
晶体学 | 8篇 |
力学 | 26篇 |
数学 | 148篇 |
物理学 | 185篇 |
出版年
2024年 | 1篇 |
2023年 | 19篇 |
2022年 | 20篇 |
2021年 | 22篇 |
2020年 | 28篇 |
2019年 | 40篇 |
2018年 | 18篇 |
2017年 | 27篇 |
2016年 | 54篇 |
2015年 | 49篇 |
2014年 | 39篇 |
2013年 | 76篇 |
2012年 | 113篇 |
2011年 | 120篇 |
2010年 | 49篇 |
2009年 | 52篇 |
2008年 | 108篇 |
2007年 | 89篇 |
2006年 | 124篇 |
2005年 | 88篇 |
2004年 | 72篇 |
2003年 | 49篇 |
2002年 | 76篇 |
2001年 | 12篇 |
2000年 | 18篇 |
1999年 | 8篇 |
1998年 | 5篇 |
1997年 | 9篇 |
1996年 | 5篇 |
1994年 | 4篇 |
1993年 | 4篇 |
1992年 | 3篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1986年 | 1篇 |
1984年 | 2篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1976年 | 2篇 |
1975年 | 3篇 |
1973年 | 1篇 |
1965年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有1424条查询结果,搜索用时 15 毫秒
81.
Van Parijs SM Corkeron PJ Harvey J Hayes SA Mellinger DK Rouget PA Thompson PM Wahlberg M Kovacs KM 《The Journal of the Acoustical Society of America》2003,113(6):3403-3410
Comparative analyses of the roar vocalization of male harbor seals from ten sites throughout their distribution showed that vocal variation occurs at the oceanic, regional, population, and subpopulation level. Genetic barriers based on the physical distance between harbor seal populations present a likely explanation for some of the observed vocal variation. However, site-specific vocal variations were present between genetically mixed subpopulations in California. A tree-based classification analysis grouped Scottish populations together with eastern Pacific sites, rather than amongst Atlantic sites as would be expected if variation was based purely on genetics. Lastly, within the classification tree no individual vocal parameter was consistently responsible for consecutive splits between geographic sites. Combined, these factors suggest that site-specific variation influences the development of vocal structure in harbor seals and these factors may provide evidence for the occurrence of vocal dialects. 相似文献
82.
Costa DP Crocker DE Gedamke J Webb PM Houser DS Blackwell SB Waples D Hayes SA Le Boeuf BJ 《The Journal of the Acoustical Society of America》2003,113(2):1155-1165
Changes in the diving behavior of individual free-ranging juvenile northern elephant seals, Mirounga angustirostris, exposed to the acoustic thermometry of the ocean climate (ATOC) sound source were examined using data loggers. Data loggers were attached to the animals and measured swim speed, maximum depth of dive, dive duration, surface interval, descent and ascent rate, and descent and ascent angle along with sound pressure level (SPL). The ATOC sound source was at a depth of 939 m and transmitted at 195 dB re: 1 microPa at 1 m centered at 75 Hz with a 37.5-Hz bandwidth. Sound pressure levels (SPL) measured at the seal during transmissions averaged 128 dB and ranged from 118 to 137 dB re: 1 microPa for the 60-90 Hz band, in comparison to ambient levels of 87-107 dB within this band. In no case did an animal end its dive or show any other obvious change in behavior upon exposure to the ATOC sound. Subtle changes in diving behavior were detected, however. During exposure, deviations in descent rate were greater than 1 s.d. of the control mean in 9 of 14 seals. Dive depth increased and descent velocity increased in three animals, ascent velocity decreased in two animals, ascent rate increased in one animal and decreased in another, and dive duration decreased in only one animal. There was a highly significant positive correlation between SPL and descent rate. The biological significance of these subtle changes is likely to be minimal. This is the first study to quantify behavioral responses of an animal underwater with simultaneous measurements of SPL of anthropogenic sounds recorded at the animal. 相似文献
83.
84.
85.
86.
87.
Sean McGuinness 《Graphs and Combinatorics》2001,17(1):135-148
Let ? be the family of finite collections ? where ? is a collection of bounded, arcwise connected sets in ℝ2 where for any S,T∈? such that S∩T≠?, it holds that S∩T is arcwise connected. Given ? is triangle-free, and provided the chromatic number χ(G) of the intersection graph G=G(?) of ? is sufficiently large, there exists α>1 independent of ? such that there is a subcollection ?′⊂? of at most 5 sets with the property that the sets of ? surrounded by ?′ induce an intersection graph H where .
Received: November 13, 1995 Final version received: December 3, 1998 相似文献
88.
The first total synthesis of polemannones B and C, higly oxygenated benzoxanthenones derivatives from Polemannia montana, is reported employing our new catalytic Cu(II)/sparteine system for β,β-phenolic couplings and tandem inverse-electron demand Diels-Alder reaction cascade in 75-90% yield. 相似文献
89.
Houxiang Sean Tang Theresa J. Hermel‐Davidock Stephen F. Hahn Dan J. Murray Robert C. Cieslinski Nikhil E. Verghese Ha Q. Pham 《Journal of Polymer Science.Polymer Physics》2009,47(4):393-406
Microdeformation behavior in nanostructured block copolymer‐toughened epoxy resins, or templated epoxy thermosets, was studied using an in situ tensile deformation technique performed directly in a transmission electron microscope. The observed microdeformation modes were found to correlate well with the macroscopic mechanical properties of the materials. In the order of decreasing macroscopic fracture toughness, the microdeformation modes were observed to change from large uniform plastic deformation over an extensive area, to localized plastic deformation bands, to little plastic deformation observed in the most brittle material. A similar trend was also observed when samples of the same material were tested at different temperatures, reflecting changes in the deformation mechanism as a function of temperature. Structural defects were observed in nanotoughening phases when plastic deformation was observed. The implication of the observed microdeformation modes to the macroscopic toughening mechanisms is discussed in the context of the micromorphology of the nanometer sized toughening phases and parameters of the epoxy matrix chemistry such as bromination, molecular weight, and interfacial miscibility. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 393–406, 2009 相似文献
90.
Sean McGuinness 《Combinatorica》2005,25(4):439-450
Let G be a k-connected graph G having circumference c ≥ 2k. It is shown that for k ≥ 2, then there is a bond B which intersects every cycle of length c-k + 2 or greater. 相似文献