全文获取类型
收费全文 | 761篇 |
免费 | 26篇 |
国内免费 | 6篇 |
专业分类
化学 | 529篇 |
晶体学 | 4篇 |
力学 | 15篇 |
数学 | 102篇 |
物理学 | 143篇 |
出版年
2023年 | 6篇 |
2022年 | 37篇 |
2021年 | 31篇 |
2020年 | 31篇 |
2019年 | 27篇 |
2018年 | 30篇 |
2017年 | 23篇 |
2016年 | 39篇 |
2015年 | 18篇 |
2014年 | 29篇 |
2013年 | 37篇 |
2012年 | 56篇 |
2011年 | 45篇 |
2010年 | 29篇 |
2009年 | 21篇 |
2008年 | 27篇 |
2007年 | 22篇 |
2006年 | 39篇 |
2005年 | 30篇 |
2004年 | 21篇 |
2003年 | 21篇 |
2002年 | 21篇 |
2001年 | 10篇 |
2000年 | 7篇 |
1999年 | 5篇 |
1997年 | 5篇 |
1996年 | 8篇 |
1995年 | 9篇 |
1994年 | 8篇 |
1993年 | 3篇 |
1992年 | 5篇 |
1991年 | 4篇 |
1990年 | 3篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 7篇 |
1985年 | 4篇 |
1984年 | 4篇 |
1983年 | 7篇 |
1982年 | 6篇 |
1981年 | 14篇 |
1980年 | 8篇 |
1979年 | 6篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1976年 | 5篇 |
1975年 | 5篇 |
1974年 | 3篇 |
1972年 | 3篇 |
1970年 | 2篇 |
排序方式: 共有793条查询结果,搜索用时 15 毫秒
771.
In this paper, we are interested in the survivable network design problem (SNDP) for last mile communication networks called (L-SNDP). Given a connected, weighted, undirected graph \(\mathrm{{G}} = (\mathrm{V, E})\); a set of infrastructure nodes and a set of customers C including two customer types where customers in the subset C1 require a single connection (type-1) and customers in the subset C2 need to be redundantly connected (type-2). The aim is to seek a sub-graph of G with the smallest weight in which all customers are connected to infrastructure nodes and the connections are protected against failures. This is a NP-hard problem and it has been solved only with the objective of minimizing the network cost. In this paper, we introduce a new multi-objective approach to solve L-SNDP called ML-SNDP. These objectives are to minimize the network cost (total cost) and to minimize the maximal amount of sharing links between connections. Results of computational experiments reported show the efficiency of our proposal. 相似文献
772.
773.
774.
Gour N Kedracki D Safir I Ngo KX Vebert-Nardin C 《Chemical communications (Cambridge, England)》2012,48(44):5440-5442
For the very first time, highly efficient synthesis of DNA-peptide hybrids to scaffold self-assembled nanostructures is described. Oligonucleotide conjugation to the diphenylalanine dipeptide triggers a morphological transition from fibrillar to vesicular structures which may potentially be used as delivery vehicles, since they exhibit pH triggered release. 相似文献
775.
In this paper, the nonexistence of parabolic boundary points of infinite type of certain domains in is showed. 相似文献
776.
It is well known that the Voigt profile does not well describe the (measured) shapes of isolated lines. This is due to the neglect of the intermolecular collision-induced velocity changes and of the speed dependence of the collisional parameters. In this paper, we present a new line profile model for pure H(2)O which takes both of these effects into account. The speed dependence of the collisional parameters has been calculated by a semi-classical method. The velocity changes have been modeled by using the Keilson-Storer collision kernel with two characteristic parameters. The latter have been deduced from classical molecular dynamics simulations which also indicate that, for pure H(2)O, the correlation between velocity-changing and state-changing collisions is not negligible, a result confirmed by the analysis of measured spectra. A partially correlated speed-dependent Keilson-Storer model has thus been adopted to describe the line-shape. Comparisons between simulated spectra and measurements for four self-broadened lines in the near-infrared at various pressures show excellent agreements. 相似文献
777.
Anna V. Gulevskaya Huong T.L. Nguyen Alexander S. Tyaglivy Alexander F. Pozharskii 《Tetrahedron》2012,68(2):488-498
The reactivity of 3-alkynyl-2-chloroquinoxalines, 2,3-dialkynylquinoxalines, and some related pyrazine derivatives toward carbanions of the CH-acids (malononitrile, ethyl cyanoacetate, diethyl malonate, 1,3-dimethylbarbituric acid) has been studied. Most of the observed reactions proceeded as tandem or cascade cyclizations yielding polynuclear heterocyclic compounds. The process starts with the addition of a nucleophile to an alkyne triple bond. Depending on the nature of the used C-nucleophile, the thus formed anionic adduct underwent further cyclization via (i) intramolecular nucleophilic substitution of the chlorine atom, (ii) intramolecular acylation of the ring nitrogen atom by the ester side chain or (iii) intramolecular nucleophilic attack on the second CC bond. Reactions of 3-alkynyl-2-chloro- and 2,3-dialkynylpyrazines with 1,3-dimethylbarbituric acid in the presence of t-BuOK were accompanied by recyclization of the 1,3-dimethylbarbituric acid moiety. 相似文献
778.
The reaction of Al, Ga, or In metals and H5IO6 in aqueous media at 180 °C leads to the formation of Al(IO3)3, Ga(IO3)3, or In(IO3)3, respectively. Single-crystal X-ray diffraction experiments have shown In(IO3)3 contains the Te4O9-type structure, while both Al(IO3)3 and Ga(IO3)3 are known to exhibit the polar Fe(IO3)3-type structure. Crystallographic data for In(IO3)3, trigonal, space group , a=9.7482(4) Å, c=14.1374(6) Å, V=1163.45(8) Z=6, R(F)=1.38% for 41 parameters with 644 reflections with I>2σ(I). All three iodate structures contain group 13 metal cations in a distorted octahedral coordination environment. M(IO3)3 (M=Al, Ga) contain a three-dimensional network formed by the bridging of Al3+ or Ga3+ cations by iodate anions. With In(IO3)3, iodate anions bridge In3+ cations in two-dimensional layers. Both materials contain distorted octahedral holes in their structures formed by terminal oxygen atoms from the iodate anions. The Raman spectra have been collected for these metal iodates; In(IO3)3 was found to display a distinctively different vibrational profile than Al(IO3)3 or Ga(IO3)3. Hence, the Raman profile can be used as a rapid diagnostic tool to discern between the different structural motifs. 相似文献
779.
780.
Penner S Bera P Pedersen S Ngo LT Harris JJ Campbell CT 《The journal of physical chemistry. B》2006,110(48):24577-24584
The interaction of O(2) with small Pd particles (2-10 nm) supported on an alpha-Al(2)O(3)(0001) single crystal under both ultrahigh vacuum (UHV) and high-pressure conditions has been studied by temperature-programmed desorption (TPD), temperature-programmed low-energy ion scattering (TP-LEIS), and X-ray photoelectron spectroscopy (XPS). A low O(2) exposure (30 L) at 500 K leads to surface oxygen adatoms on the Pd nanoparticles, which desorb in TPD as O(2) in a peak at approximately 880 K. Surface O adatoms on the smallest Pd particles move to subsurface sites starting at 400 K, and they almost all move subsurface by approximately 750 K, desorbing mainly at considerably higher temperature. The dominant oxygen species above 700 K is subsurface, implying that it is more stable than oxygen adatoms on Pd. Exposures of the Pd nanoparticles to 25 Torr O(2) at 373-473 K readily convert the Pd to a species whose Pd XPS peak shifts by the same amount as the binding energy difference between bulk Pd and bulk PdO. We attribute this to PdO nanoparticles (or a thin film of PdO on or under the Pd for the larger particles). The decomposition of the PdO on these nanoparticles to Pd in an equilibrium O(2) pressure of 10-7 Torr does not occur until approximately 750 K, or approximately 200 K higher than the equilibrium decomposition of bulk PdO. This is attributed to the higher energy of Pd nanoparticles compared to bulk Pd and, for the larger particles, to the adhesion energy of the PdO film to the Pd, both of which stabilize the PdO on these Pd nanoparticles relative to bulk PdO. This PdO-like film on the larger particles may be similar to the ordered oxide thin film previously reported to form on Pd(111) but may also reside at the alpha-Al(2)O(3) interface and be partially stabilized by adhesion to this interface. 相似文献