全文获取类型
收费全文 | 283篇 |
免费 | 12篇 |
专业分类
化学 | 219篇 |
晶体学 | 11篇 |
力学 | 3篇 |
数学 | 29篇 |
物理学 | 33篇 |
出版年
2023年 | 1篇 |
2022年 | 2篇 |
2021年 | 19篇 |
2020年 | 8篇 |
2019年 | 9篇 |
2018年 | 11篇 |
2017年 | 8篇 |
2016年 | 14篇 |
2015年 | 7篇 |
2014年 | 13篇 |
2013年 | 23篇 |
2012年 | 26篇 |
2011年 | 25篇 |
2010年 | 22篇 |
2009年 | 7篇 |
2008年 | 17篇 |
2007年 | 14篇 |
2006年 | 9篇 |
2005年 | 19篇 |
2004年 | 12篇 |
2003年 | 9篇 |
2002年 | 4篇 |
2001年 | 4篇 |
2000年 | 4篇 |
1999年 | 4篇 |
1998年 | 1篇 |
1996年 | 1篇 |
1990年 | 2篇 |
排序方式: 共有295条查询结果,搜索用时 187 毫秒
61.
The work deals with the problem of determination of automobile petrol of various octane numbers and petroleum contamination of some objects on the basis of analysis of photoluminescence spectra of automobile petrol samples. For this purpose steady state luminescence properties of samples of automobile petrol of different types being in sale were measured. Samples of automobile petrol diluted in hexane were prepared and their luminescence spectra were measured at room and liquid nitrogen temperatures of samples. We constructed concentration dependences of luminescence intensity of both wide band luminescence of liquid solutions obtained at room temperature and peak intensities of luminescence lines of quasi linear spectra of solutions frozen at 77 K. Possibility to use luminescence method for analysis of petrol pollutions on some objects is illustrated by results of investigation of pine-wood pieces contaminated by automobile petrol. 相似文献
62.
Natalija van Well Sitaram Ramakrishnan Ketty Beauvois Navid Qureshi Eric Ressouche Markos Skoulatos Robert Georgii Oksana Zaharko Sander van Smaalen 《Annalen der Physik》2020,532(7):2000147
The triangular antiferromagnetic Cs2CuCl4-xBrx mixed system is studied by neutron single-crystal diffraction in magnetic field. It shows a rich magnetic phase diagram consisting of four regimes depending on the Br concentration and is characterized by different exchange coupling mechanisms. For the investigated compositions from regime I (0 < x ≤ 1.5), a critical magnetic field Bc is found for a Br concentration x = 0.8 at Bc = 8.10(1) T and for x = 1.1 at Bc = 7.73(1) T and from regime IV (3.2 < x < 4) for x = 3.3 at Bc = 0.99(3) T. For magnetic fields larger than the respective Bc, magnetic superlattice reflections of these compounds are not found. The incommensurate magnetic wave vector q = (0, 0.470, 0) appears below the ordering temperature TN = 0.51(1) K for Cs2CuCl3.2Br0.8, and q = (0, 0.418, 0) below TN = 1.00(6) K for Cs2CuCl0.3Br3.7. Neutron diffraction experiments at around 60 mK for x = 3.7 in a magnetic field show the critical magnetic field at Bc = 7.94(16) T and the formation of the second magnetic phase at around 8.5 T depending on the temperature. Inelastic neutron scattering experiments for the compound from regime III (2 < x ≤ 3.2) with x = 2.2 show dynamical correlations at a temperature around 50 mK giving evidence for a spin liquid phase. 相似文献
63.
64.
65.
Oksana Yakimova 《Bulletin des Sciences Mathématiques》2010,134(6):579-587
Let g be a classical Lie algebra, e∈g a nilpotent element and ge⊂g the centraliser of e. We prove that ge=[ge,ge] if and only if e is rigid. It is also shown that if e∈[ge,ge], then the nilpotent radical of ge coincides with [ge(1),ge], where ge(1)⊂ge is an eigenspace of a characteristic of e corresponding to the eigenvalue 1. 相似文献
66.
Libor Mat
jka Oksana Dukh Ji
í Brus William J. Simonsick Jr Bohumil Meissner 《Journal of Non》2000,270(1-3):34-47
The sol–gel polymerization of 3-glycidyloxypropyltrimethoxysilane (GTMS) was followed by size exclusion chromatography and 29Si NMR. Extensive non-random cyclization under formation of polyhedral cycles – cubic cages – predominates at the beginning of the reaction. Structure growth of polysilsesquioxanes proceeds by combining the incompletely condensed cage frameworks. The extent of the cage formation increases with dilution and the amount of water and depends appreciably on a catalyst. The cage fraction was isolated from a reaction mixture using preparative size exclusion chromatography and identified by 29Si NMR spectroscopy and electrospray ionization mass spectrometry (ESI-MS). High content of polyhedral cages prevents gelation of the trifunctional GTMS monomer. Reaction of pendant epoxy groups is much slower; however, at a late reaction stage the epoxy hydrolysis can be significant. Under some conditions, like base catalysis, polysilsesquioxane clusters are crosslinked by intermolecular condensation of SiOH with hydrolyzed epoxy groups and the system gels. The cage with epoxide functionalities may serve as a rigid precursor of crosslinking. 相似文献
67.
Valentyna V. Semenaka Oksana V. Nesterova Volodymyr N. Kokozay Roman I. Zybatyuk Oleg V. Shishkin Roman Boca Carlos J. Gómez-García Juan M. Clemente-Juan Julia Jezierska 《Polyhedron》2010
Four novel heterometallic complexes [Co2Cr2(NCS)4(HDea)2(Dea)2]·4dmf (1), [Co2Cr2(NCS)4(HDea)2(Dea)2]·4dmso (2), [Mn2Cr2(NCS)4(HDea)2(Dea)2(dmf)2]·2dmf (3) and [Mn2Cr2(NCS)4(HDea)2(Dea)2(dmso)2]·4dmso (4) have been prepared using zerovalent cobalt (1, 2) or manganese (3, 4), Reineckes salt, ammonium thiocyanate and a non-aqueous solution of diethanolamine (H2Dea) in air. The single X-ray analysis reveals that all compounds have similar centrosymmetric crystal structures based on a tetranuclear {M2Cr2(μ3-O)2(μ-O)4} (M = Co, Mn) core. Variable-temperature magnetic susceptibility measurements of 1, 2 and 4 show antiferromagnetic coupling between the magnetic centers, while 3 exhibits a ferromagnetic behavior. 相似文献
68.
Anton Savitsky Oksana Gopta Mahir Mamedov John H. Golbeck Alexander Tikhonov Klaus M?bius Alexey Semenov 《Applied magnetic resonance》2010,37(1-4):85-102
Photosystem I (PS I) mutants from the cyanobacterium Synechocystis sp. PCC 6803 bearing point mutations to the axial ligands of A0A (M688NPsaA) and A0B (M668NPsaB) were studied by high-field W-band electron paramagnetic resonance (EPR) spectroscopy. It was found that the EPR observables of PS I from the M668NPsaB mutant were virtual identical to that of the wild type (WT), and are clearly distinct from the M688NPsaA mutant. In particular, the P 700 ·+ decay kinetics in the M688NPsaA mutant is significantly slower than in the WT or the M668NPsaB mutant. The analysis of the out-of-phase electron–electron dipolar electron spin echo envelope modulation shows that in the M668NPsaB mutant, the estimated distance of 26.0 ± 0.3 Å agrees well with the 25.8 Å distance for the P 700 ·+ A 1A ·? radical pair measured in the X-ray crystal structure. In the M688NPsaA mutant, two populations are found with estimated distances of 26.0 ± 0.3 and 25.0 ± 0.3 Å in a ratio of 0.7–0.3, which agree well with the 25.8 Å distance for the P 700 ·+ A 1A ·? radical pair and the 24.6 Å distance for the P 700 ·+ A 1B ·? radical pair measured in the X-ray crystal structure. The data confirm that under the experimental conditions employed in this work, which involve dark-adapted samples without the pre-reduction of the iron–sulfur clusters, electron transport in cyanobacterial PS I is asymmetrical at 100 K, with the majority of electron transfer taking place through the A-branch of cofactors. 相似文献
69.
Multiconfiguration molecular mechanics (MCMM) is a general algorithm for constructing potential energy surfaces for reactive systems (Kim, Y.; Corchado, J. C.; Villà, J.; Xing, J.; Truhlar, D. G. J. Chem. Phys. 2000, 112, 2718). This paper illustrates how the performance of the MCMM method can be improved by adopting improved molecular mechanics parameters. We carry out calculations of reaction rate constants using variational transition state theory with optimized multidimensional tunneling on the MCMM PESs for three hydrogen transfer reactions, and we compare the results to direct dynamics. We find that the MCMM method with as little as one electronic structure Hessian can describe the dynamically important regions of the ground-electronic state PES, including the corner-cutting-tunneling region of the reaction swath, with practical accuracy. 相似文献
70.
In this paper we consider monic polynomials such that their coefficients coincide with their zeros. These polynomials were first introduced by S. Ulam. We combine methods of algebraic geometry and dynamical systems to prove several results. We obtain estimates on the number of Ulam polynomials of degree N. We provide additional methods to obtain algebraic identities satisfied by the zeros of Ulam polynomials, beyond the straightforward comparison of their zeros and coefficients. To address the question about the existence of orthogonal Ulam polynomial sequences, we show that the only Ulam polynomial eigenfunctions of hypergeometric type differential operators are the trivial Ulam polynomials \(\{x^N\}_{N=0}^\infty \). We propose a family of solvable N-body problems such that their stable equilibria are the zeros of certain Ulam polynomials. 相似文献