全文获取类型
收费全文 | 7445篇 |
免费 | 201篇 |
国内免费 | 15篇 |
专业分类
化学 | 4188篇 |
晶体学 | 35篇 |
力学 | 210篇 |
综合类 | 1篇 |
数学 | 1212篇 |
物理学 | 2015篇 |
出版年
2021年 | 61篇 |
2020年 | 89篇 |
2019年 | 78篇 |
2018年 | 107篇 |
2017年 | 88篇 |
2016年 | 180篇 |
2015年 | 147篇 |
2014年 | 186篇 |
2013年 | 351篇 |
2012年 | 387篇 |
2011年 | 443篇 |
2010年 | 231篇 |
2009年 | 199篇 |
2008年 | 406篇 |
2007年 | 390篇 |
2006年 | 386篇 |
2005年 | 399篇 |
2004年 | 313篇 |
2003年 | 257篇 |
2002年 | 219篇 |
2001年 | 179篇 |
2000年 | 120篇 |
1999年 | 90篇 |
1998年 | 65篇 |
1997年 | 87篇 |
1996年 | 89篇 |
1995年 | 73篇 |
1994年 | 56篇 |
1993年 | 60篇 |
1992年 | 62篇 |
1991年 | 53篇 |
1990年 | 47篇 |
1989年 | 54篇 |
1988年 | 54篇 |
1987年 | 48篇 |
1986年 | 69篇 |
1985年 | 93篇 |
1984年 | 102篇 |
1983年 | 76篇 |
1982年 | 106篇 |
1981年 | 88篇 |
1980年 | 93篇 |
1979年 | 85篇 |
1978年 | 97篇 |
1977年 | 80篇 |
1976年 | 72篇 |
1975年 | 67篇 |
1974年 | 66篇 |
1973年 | 68篇 |
1971年 | 39篇 |
排序方式: 共有7661条查询结果,搜索用时 15 毫秒
11.
In this work, the melting-point depression and molecular dynamics of hexamethyldisilane confined within five controlled pore
glasses, with mean diameters ranging from 7.9 to 23.9 nm, are studied by high-field (9.4 T) nuclear magnetic resonance (NMR),
and the results are discussed with reference to the bulk substance. The melting-point depression in pores with radiusR follows the simplified Gibbs-Thompson equation ΔT=k
p/(R−s) with ak
p value of 74 K · nm and ans value of 1 nm. To our knowledge, this is the first time thek
p value of hexamethyldisilane is reported. Proton spin-lattice relaxation times (T
1), spin-spin relaxation times (T
2), and diffusivities (D) are reported as a function of temperature. The confinement in the pores gives rise to substantial changes in the molecular
dynamics and the phase behavior. The line-shape measurements reveal a two-phase system assigned to a relatively mobile component
at the pore walls and a crystalline solid at the center of the pores. However, theT
2 measurements show that the mobile phase also embraces a minor component attributed to nonfrozen liquid in pockets or micropores.
The diffusivity of the major narrow-line component is approximately three orders of magnitude larger than that in the plastic
bulk phase, reflecting fast diffusion of mobile molecules. Below the melting region,T
1 of the narrow line is significantly shorter thanT
1 of the broad line, suggesting that the molecular reorientation is more hindered close to the surface than at the center of
the pore. 相似文献
12.
Trond Stølen Gustavsen 《Compositio Mathematica》2003,138(2):199-231
We relate the equisingular deformation theory of plane curve singularities and sandwiched surface singularities. We show the existence of a smooth map between the two corresponding deformation functors and study the kernel of this map. In particular we show that the map is an isomorphism when a certain invariant is large enough. 相似文献
13.
Jacob Schach Møller 《Annales Henri Poincare》2005,6(6):1091-1135
In this paper we analyze the bottom of the energy-momentum spectrum of the translation invariant Nelson model, describing
one electron linearly coupled to a second quantized massive scalar field. Our results are valid for all values of the coupling
constant and include an HVZ theorem, non-degeneracy of ground states, existence of isolated groundstates in dimensions 1 and
2, non-existence of ground states embedded in the bottom of the essential spectrum in dimensions 3 and 4, (i.e., at total
momenta where no isolated groundstate eigenvalue exists), and we study regularity and monotonicity properties of the bottom
of the essential spectrum, as a function of total momentum.
Communicated by Joel Feldman
submitted 04/11/04, accepted 17/02/05 相似文献
14.
Xiaojuan Hao Camilla Nilsson Martin Jesberger Martina H. Stenzel Eva Malmstrm Thomas P. Davis Emma
stmark Christopher Barner‐Kowollik 《Journal of polymer science. Part A, Polymer chemistry》2004,42(23):5877-5890
The synthesis and characterization of novel first‐ and second‐generation true dendritic reversible addition–fragmentation chain transfer (RAFT) agents carrying 6 or 12 pendant 3‐benzylsulfanylthiocarbonylsulfanylpropionic acid RAFT end groups with Z‐group architecture based on 1,1,1‐hydroxyphenyl ethane and trimethylolpropane cores are described in detail. The multifunctional dendritic RAFT agents have been used to prepare star polymers of poly(butyl acrylate) (PBA) and polystyrene (PS) of narrow polydispersities (1.4 < polydispersity index < 1.1 for PBA and 1.5 < polydispersity index < 1.3 for PS) via bulk free‐radical polymerization at 60 °C. The novel dendrimer‐based multifunctional RAFT agents effect an efficient living polymerization process, as evidenced by the linear evolution of the number‐average molecular weight (Mn) with the monomer–polymer conversion, yielding star polymers with molecular weights of up to Mn = 160,000 g mol?1 for PBA (based on a linear PBA calibration) and up to Mn = 70,000 g mol?1 for PS (based on a linear PS calibration). A structural change in the chemical nature of the dendritic core (i.e., 1,1,1‐hydroxyphenyl ethane vs trimethylolpropane) has no influence on the observed molecular weight distributions. The star‐shaped structure of the generated polymers has been confirmed through the cleavage of the pendant arms off the core of the star‐shaped polymeric materials. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5877–5890, 2004 相似文献
15.
Yuri M. Boiko Anders Bach Jørgen Lyngaae-Jørgensen 《Journal of Polymer Science.Polymer Physics》2004,42(10):1861-1867
Films of amorphous polystyrene (PS) with a weight-average molecular weight (Mw) of 225 × 103 g/mol were bonded in a T-peel test geometry, and the fracture energy (G) of a PS/PS interface was measured at the ambient temperature as a function of the healing time (th) and healing temperature (Th). G was found to develop with (th)1/2 at Th = Tg-bulk − 33 °C (where Tg-bulk is the glass-transition temperature of the bulk sample), and log G was found to develop with 1/Th at Tg-bulk − 43 °C ≤ Th ≤ Tg-bulk − 23 °C. The smallest measured value of G = 1.4 J/m2 was at least one order of magnitude larger than the work of adhesion required to reversibly separate the PS surfaces. These three observations indicated that the development of G at the PS/PS interface in the temperature range investigated (<Tg-bulk) was controlled by the diffusion of chain segments feasible above the glass-transition temperature of the interfacial layer, in agreement with our previous findings for fracture stress development at several polymer/polymer interfaces well below Tg-bulk. Close values of G = 8–9 J/m2 were measured for the symmetric interfaces of polydisperse PS [Mw = 225 × 103, weight-average molecular weight/number-average molecular weight (Mw/Mn) = 3] and monodisperse PS (Mw = 200 × 103, Mw/Mn = 1.04) after healing at Th = Tg-bulk − 33 °C for 24 h. This implies that the self-bonding of high-molecular-weight PS at such relatively low temperatures is not governed by polydispersity. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1861–1867, 2004 相似文献
16.
In the data envelopment analysis (DEA) efficiency literature, qualitative characterizations of returns to scale (increasing, constant, or decreasing) are most common. In economics it is standard to use the scale elasticity as a quantification of scale properties for a production function representing efficient operations. Our contributions are to review DEA practices, apply the concept of scale elasticity from economic multi-output production theory to DEA piecewise linear frontier production functions, and develop formulas for scale elasticity for radial projections of inefficient observations in the relative interior of fully dimensional facets. The formulas are applied to both constructed and real data and show the differences between scale elasticities for the two valid projections (input and output orientations). Instead of getting qualitative measures of returns to scale only as was done earlier in the DEA literature, we now get a quantitative range of scale elasticity values providing more information to policy-makers. 相似文献
17.
18.
L. Landström D. Brodoceanu K. Piglmayer D. Bäuerle 《Applied Physics A: Materials Science & Processing》2006,84(4):373-377
Extraordinary optical transmission through metal-coated close-packed monolayers has been observed. The monolayers consist of silica (a-SiO2) or polystyrene microspheres that form two-dimensional close-packed lattices by self-assembly. Metal layers of Ag, Au and Ni with different thicknesses (larger than the skin depth) were evaporated onto such lattices by means of standard techniques. The optical transmission spectra investigated between 300 and 2500 nm show pronounced peaks that scale with the diameter and the optical properties of the composite slabs. The enhanced transmission observed is most likely mediated via plasmons. PACS 78.66.-w; 81.16.Dn; 82.70.Dd 相似文献
19.
Bahattin Gümgüm Nermin Biricik Feyyaz Durap Ismail Özdemir Nevin Gürbüz Wee Han Ang Paul J. Dyson 《应用有机金属化学》2007,21(8):711-715
Palladium(II) complexes with N,N‐bis(diphenylphosphino)aniline ligands catalyse the Heck reaction between styrene and aryl bromides, affording stilbenes in good yield. The structures of two of the complexes used as pre‐catalysts have been determined by single‐crystal X‐ray diffraction. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
20.