全文获取类型
收费全文 | 645篇 |
免费 | 34篇 |
国内免费 | 8篇 |
专业分类
化学 | 395篇 |
晶体学 | 3篇 |
力学 | 28篇 |
数学 | 77篇 |
物理学 | 184篇 |
出版年
2023年 | 2篇 |
2022年 | 3篇 |
2021年 | 11篇 |
2020年 | 11篇 |
2019年 | 11篇 |
2018年 | 14篇 |
2017年 | 11篇 |
2016年 | 20篇 |
2015年 | 17篇 |
2014年 | 25篇 |
2013年 | 35篇 |
2012年 | 26篇 |
2011年 | 55篇 |
2010年 | 35篇 |
2009年 | 29篇 |
2008年 | 37篇 |
2007年 | 57篇 |
2006年 | 40篇 |
2005年 | 29篇 |
2004年 | 18篇 |
2003年 | 19篇 |
2002年 | 17篇 |
2001年 | 10篇 |
2000年 | 11篇 |
1999年 | 14篇 |
1998年 | 11篇 |
1997年 | 7篇 |
1996年 | 5篇 |
1995年 | 7篇 |
1994年 | 3篇 |
1993年 | 12篇 |
1992年 | 11篇 |
1991年 | 4篇 |
1990年 | 7篇 |
1989年 | 5篇 |
1988年 | 5篇 |
1986年 | 2篇 |
1985年 | 7篇 |
1984年 | 6篇 |
1983年 | 2篇 |
1982年 | 6篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 4篇 |
1977年 | 2篇 |
1975年 | 2篇 |
1974年 | 3篇 |
1973年 | 6篇 |
1971年 | 2篇 |
1958年 | 1篇 |
排序方式: 共有687条查询结果,搜索用时 258 毫秒
81.
In this paper, the role played by the excitation of surface phonons in the far infrared optical absorption of small metallic spheres is explored. We find that for frequencies ranging between 100 cm-3 and 200 cm-1 this absorption mechanism is dominating over the ones coming from considering electronic excitations or the Joule heating. 相似文献
82.
83.
Marcos A. P. Martins Alex F. C. Flores Rogrio A. Freitag Nilo Zanatta 《Journal of heterocyclic chemistry》1996,33(4):1223-1226
The investigation of the effect of the cyclopentane ring on the regiochemistry of the reactions of 2-acetylcyclopentanones 1a-d and β-methoxyvinyl trifluoromethyl ketone derivative 2a with hydroxylamine hydrochloride is reported. The reactions give regiospecifically the 4,5-trimethylene-4,5-dihydroisoxazoles 3a-d in good yields. 相似文献
84.
It is proposed that the X-ray intensity I at the forbidden 222 reflection is useful as an order parameter in the phase transition from a diamond-lattice semiconductor to a metal. Available X-ray data as a function of pressure on Si is shown to lend some support to the proposal. 相似文献
85.
In the Heck reaction between aryl halides and n-butyl acrylate, the palladacycle {Pd[kappa(1)-C, kappa(1)-N-C=(C(6)H(5))C(Cl)CH(2)NMe(2)](mu-Cl)}(2), 1, is merely a reservoir of the catalytically active Pd(0) species [1](Pd colloids or highly active forms of low ligated Pd(0) species) that undergoes oxidative addition of the aryl halide on the surface with subsequent detachment, generating homogeneous Pd(II) species. The main catalytic cycle is initiated by oxidative addition of iodobenzene to [1], followed by the reversible coordination of the olefin to the oxidative addition product. All the unimolecular subsequent steps are indistinguishable kinetically and can be combined in a single step. This kinetic model predicts that a slight excess of alkene relative to iodobenzene leads to a rapid rise in the Pd(0) concentration while when using a slight excess of iodobenzene, relative to alkene, the oxidative addition product is the resting state of the catalytic cycle. Competitive experiments of various bromoarenes and iodoarenes with n-butyl acrylate catalyzed by 1 and CS, CP, and NCN palladacycles gave the same rho value (2.4-2.5 for Ar-Br and 1.7-1.8 for Ar-I) for all palladacycles employed, indicating that they generate the same species in the oxidative addition step. The excellent fit of the slope with the sigma(0) Hammett parameter and the entropy of activation of -43 +/- 8 J mol(-1) K(-1) are consistent with an associative process involving the development of only a partial charge in the transition state for the oxidative step of iodobenzene. 相似文献
86.
We introduce the concept of a strict l∞-metric projector, based in the definition of strict approximation, to prove that for each matrix A of order m×n with coefficients in the field R of real numbers there exists a set of operators G: Rm → Rn homogeneous and continuous, but not necessarily linear (strict generalized inverse) such that AGA = A and 6AGy?y6 is minimized for all y, when the norm is the l∞ norm. We investigate the properties of these operators and prove that there are two distinguished operators A-1∞, β and A-1∞ which are extensions of the generalized inverse introduced by Newman and Odell in the case of a strictly convex norm. 相似文献
87.
The effects of the temperature on the highly correlated two-dimensional electron gas of a Si-111(1 × 1) surface have been analysed within a Hubbard Hamiltonian, going beyond the Hartree-Fock approximation. Our results show that a Kondo-like peak appearing at low T, disappears around 700 K; moreover, we find that the surface chemical potential shifts towards the valence band. All these results offer an explanation to recent experimental data of Yokotsuka et al. 相似文献
88.
Sánchez-Méndez A Benito JM de Jesús E de la Mata FJ Flores JC Gómez R Gómez-Sal P 《Dalton transactions (Cambridge, England : 2003)》2006,(45):5379-5389
The substituted bis(pyrazolyl)methane ligands RCH(3,5-Me2pz)2(R=SiMe3, CH2Ph, G1, G2, and G3; Gn=Fréchet-type dendritic wedges of generation n) have been prepared starting from H2C(3,5-Me2pz)2. Reaction of these didentate ligands with [NiBr2(DME)] is a straightforward procedure that allows the synthesis of the nickel(II) complexes [NiBr2{RCH(3,5-Me2pz)2}]. The molecular structure of compound (R=CH2Ph) has been determined by X-ray diffraction studies. The nickel centre coordinates two bromine and two nitrogen atoms in a tetrahedral environment, and the metallacycle Ni(NN)2C adopts a boat conformation with the benzyl group in an axial position. 1H NMR studies have been carried out to characterize these paramagnetic nickel compounds in solution. Valuable information about the disposition of the ligands and dendritic wedges in solution has been obtained thanks to the influence of the paramagnetic centre on the proton resonances. 相似文献
89.
Sum frequency generation surface vibrational spectroscopy and kinetic measurements using gas chromatography have been used to identify a reactive surface intermediate in situ during hydrogenation of benzene on a Pt(111) single crystal surface at Torr pressures. Upon adsorption at 310 K, both chemisorbed and physisorbed benzene coexist on the surface, a result which has not previously been observed. Kinetic measurements show a linear compensation effect for the production of both cyclohexane and cyclohexene. From these data the isokinetic temperature was identified and correlated to the chemisorbed benzene species, which were probed by means of vibrational spectroscopy. Additionally, chemisorbed benzene was determined to be a reactive intermediate, which is critical for hydrogenation. 相似文献
90.
We consider the existence of solutions of a nonlinear Riemann-Hilbert problem for a quasilinear -equation on a bordered Riemann surface.
The first author was supported in part by a grant ``Analiza in geometrija' P1-0291 from the Ministry of Higher Education,
Science and Technology of the Republic of Slovenia.
The second author was supported in part by grants from FEDER y Ministerio de Ciencia y Tecnología numbers BFM2001-3894 MTM
2004-05878 and Consejería de Educacion Cultura y Deportes del Gobierno de Canarias, PI 2001/091. 相似文献