全文获取类型
收费全文 | 2388篇 |
免费 | 73篇 |
国内免费 | 15篇 |
专业分类
化学 | 1947篇 |
晶体学 | 10篇 |
力学 | 31篇 |
数学 | 242篇 |
物理学 | 246篇 |
出版年
2023年 | 9篇 |
2022年 | 34篇 |
2021年 | 32篇 |
2020年 | 39篇 |
2019年 | 35篇 |
2018年 | 27篇 |
2017年 | 27篇 |
2016年 | 57篇 |
2015年 | 43篇 |
2014年 | 54篇 |
2013年 | 126篇 |
2012年 | 151篇 |
2011年 | 176篇 |
2010年 | 112篇 |
2009年 | 101篇 |
2008年 | 177篇 |
2007年 | 163篇 |
2006年 | 130篇 |
2005年 | 161篇 |
2004年 | 153篇 |
2003年 | 113篇 |
2002年 | 127篇 |
2001年 | 27篇 |
2000年 | 24篇 |
1999年 | 22篇 |
1998年 | 27篇 |
1997年 | 21篇 |
1996年 | 26篇 |
1995年 | 13篇 |
1994年 | 15篇 |
1993年 | 14篇 |
1992年 | 20篇 |
1991年 | 17篇 |
1990年 | 11篇 |
1989年 | 16篇 |
1988年 | 7篇 |
1986年 | 9篇 |
1985年 | 12篇 |
1984年 | 12篇 |
1983年 | 9篇 |
1982年 | 17篇 |
1981年 | 8篇 |
1980年 | 11篇 |
1979年 | 10篇 |
1978年 | 8篇 |
1977年 | 10篇 |
1976年 | 16篇 |
1975年 | 11篇 |
1974年 | 5篇 |
1973年 | 7篇 |
排序方式: 共有2476条查询结果,搜索用时 15 毫秒
981.
Ayhan MM Singh A Hirel C Gürek AG Ahsen V Jeanneau E Ledoux-Rak I Zyss J Andraud C Bretonnière Y 《Journal of the American Chemical Society》2012,134(8):3655-3658
The concept of octupolar molecules has considerably enlarged the engineering of second-order nonlinear optical materials by giving access to 2D and 3D architectures. However, if the archetype of octupolar symmetry is a cube with alternating donor and acceptor groups at the corners, no translation of this ideal structure into a real molecule has been realized to date. This may be achieved by designing a bis(phthalocyaninato)lutetium(III) double-decker complex with a crosswise ABAB phthalocyanine bearing alternating electron-donor and electron-acceptor groups. In this communication, we present the first step toward this goal with the synthesis, crystal structure determination, and measurement of the molecular first-order hyperpolarizability β by harmonic light diffusion, of an original lutetium(III) sandwich complex displaying the required ABAB-type alternation for one face of the cube. This structure is characterized by an intense absorption in the near-IR due to an intervalence transition and exhibits the highest quadratic hyperpolarizability ever reported for an octupolar molecule, √<β(2)(HLS)> = 5750 × 10(-30) esu. 相似文献
982.
Catherine A. Bliss Isabel M. Kloumann Kameron Decker Harris Christopher M. Danforth Peter Sheridan Dodds 《Journal of computational science》2012,3(5):388-397
The advent of social media has provided an extraordinary, if imperfect, ‘big data’ window into the form and evolution of social networks. Based on nearly 40 million message pairs posted to Twitter between September 2008 and February 2009, we construct and examine the revealed social network structure and dynamics over the time scales of days, weeks, and months. At the level of user behavior, we employ our recently developed hedonometric analysis methods to investigate patterns of sentiment expression. We find users’ average happiness scores to be positively and significantly correlated with those of users one, two, and three links away. We strengthen our analysis by proposing and using a null model to test the effect of network topology on the assortativity of happiness. We also find evidence that more well connected users write happier status updates, with a transition occurring around Dunbar's number. More generally, our work provides evidence of a social sub-network structure within Twitter and raises several methodological points of interest with regard to social network reconstructions. 相似文献
983.
Jonathan M. Breiner Shelly Sheats Harkness Carla C. Johnson Catherine M. Koehler 《School science and mathematics》2012,112(1):3-11
Educational reformation has proceeded slowly despite the many calls to improve science and mathematics for our students. The acronym STEM (science, technology, engineering, and mathematics) has been adopted by numerous programs as an important focus for renewed global competitiveness for the United States, but conceptions of what STEM entails often vary among stakeholders. This paper examines the conceptions of STEM held by faculty members from a public Research I institution in the middle of a regional “STEM movement.” Faculty members responded to two open‐ended questions: (1) What is STEM? and (2) How does STEM influence and/or impact your life? Although 72% of these faculty members possessed a relevant conception of STEM, the results suggest that they do not share a common conceptualization of STEM. Their conception is most likely based on their academic discipline or how STEM impacts their daily lives. STEM faculty members were likely to have a neutral or positive conception where non‐STEM faculty members often had negative feelings about STEM. 相似文献
984.
We construct stationary flat three-dimensional Lorentzian manifolds with singularities that are obtained from Euclidean surfaces with cone singularities and closed one-forms on these surfaces. In the application to (2?+?1)-gravity, these spacetimes correspond to models containing massive particles with spin. We analyse their geometrical properties, introduce a generalised notion of global hyperbolicity and classify all stationary flat spacetimes with singularities that are globally hyperbolic in that sense. We then apply our results to (2?+?1)-gravity and analyse the causality structure of these spacetimes in terms of measurements by observers. In particular, we derive a condition on observers that excludes causality violating light signals despite the presence of closed timelike curves in these spacetimes. 相似文献
985.
Chartoire A Lesieur M Falivene L Slawin AM Cavallo L Cazin CS Nolan SP 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(15):4517-4521
The bigger the better: The new well-defined [Pd(IPr*)(cin)Cl] pre-catalyst is described. This complex proves to be highly active in the Suzuki-Miyaura cross-coupling for the synthesis of tetra-ortho-substituted biaryls under mild conditions. IPr* is reported as the largest N-heterocyclic carbene (NHC) to date for [Pd(NHC)(cin)Cl] complexes, explaining the high reactivity observed for this complex in this challenging transformation. 相似文献
986.
Mélanie Lavergne Mohammed Derkaoui Catherine Delmau Didier Letourneur Catherine Le Visage 《Macromolecular bioscience》2012,12(7):901-910
Human ECFCs contribute to vascular repair. For this reason, they are considered as valuable cell therapy products in ischemic diseases. Porous scaffolds are prepared that are composed of natural polysaccharides, pullulan and dextran, by chemical crosslinking without use of organic solvents. These porous scaffolds, which have pores with an average size of 42 µm and a porosity of 21%, preserve the viability and the proliferation of cord‐blood ECFCs. After 7 d of culture in porous scaffolds, ECFCs express endothelial markers (CD31 and vWf) and maintain endothelial functions. The cultured cells can be easily retrieved by enzymatic degradation of the porous scaffolds. In vitro results suggest that the porous scaffold could allow cell delivery of ECFCs for treatment of vascular diseases.
987.
The potential of electrospun pullulan/dextran (P/D) nanofibers (average diameter = 323 nm) for vascular tissue engineering applications is explored. The mechanical properties of the nanofibers are of the same order of magnitude as that of human arteries (Young's modulus ≈0.88 MPa; tensile strength ≈0.35 MPa). It is demonstrated that the nanofiber topography enables cell adhesion and that the endothelial phenotype is maintained on the nanofibers. Moreover, P/D nanofibers support a stable confluent monolayer of endothelial cells over 14 d. SMCs seeded on nanofibers display similar levels of alpha smooth muscle actin and a lower proliferation rate than cells on 2D cultures. The observations suggest that nanofibers promote a shift to a quiescent contractile phenotype in SMCs.
988.
989.
990.