全文获取类型
收费全文 | 4643篇 |
免费 | 136篇 |
国内免费 | 16篇 |
专业分类
化学 | 3357篇 |
晶体学 | 48篇 |
力学 | 121篇 |
数学 | 556篇 |
物理学 | 713篇 |
出版年
2022年 | 21篇 |
2021年 | 42篇 |
2020年 | 51篇 |
2019年 | 58篇 |
2018年 | 34篇 |
2017年 | 38篇 |
2016年 | 108篇 |
2015年 | 73篇 |
2014年 | 102篇 |
2013年 | 198篇 |
2012年 | 264篇 |
2011年 | 316篇 |
2010年 | 177篇 |
2009年 | 135篇 |
2008年 | 259篇 |
2007年 | 284篇 |
2006年 | 296篇 |
2005年 | 259篇 |
2004年 | 219篇 |
2003年 | 218篇 |
2002年 | 196篇 |
2001年 | 59篇 |
2000年 | 58篇 |
1999年 | 33篇 |
1998年 | 46篇 |
1997年 | 50篇 |
1996年 | 78篇 |
1995年 | 38篇 |
1994年 | 42篇 |
1993年 | 39篇 |
1992年 | 23篇 |
1991年 | 38篇 |
1990年 | 39篇 |
1989年 | 33篇 |
1988年 | 39篇 |
1987年 | 23篇 |
1986年 | 27篇 |
1985年 | 57篇 |
1984年 | 59篇 |
1983年 | 33篇 |
1982年 | 62篇 |
1981年 | 60篇 |
1980年 | 54篇 |
1979年 | 43篇 |
1978年 | 52篇 |
1977年 | 46篇 |
1976年 | 36篇 |
1975年 | 39篇 |
1974年 | 34篇 |
1973年 | 35篇 |
排序方式: 共有4795条查询结果,搜索用时 46 毫秒
921.
N. P. Boley Paul De Bièvre Philip D. P. Taylor Adam Uldall 《Accreditation and quality assurance》2001,6(6):244-251
Many laboratories take part in proficiency testing schemes, external quality assessment programmes and other interlaboratory
comparisons. These have many similarities but also important differences in their modus operandi and evaluation of performance of participating laboratories. This paper attempts to highlight both the similarities and differences. It also puts particular emphasis on requirements
called ”target values for uncertainty” and their meaning.
Received: 24 January 2001 Accepted: 25 January 2001 相似文献
922.
A model describing electrochemical reactivity at nanoelectrode ensembles consisting of redox-molecule-based active sites immobilized on otherwise passivated electrode surfaces is presented. A mathematical treatment in terms of hemispherical diffusion of redox-active solutes to a layer of independent molecule-based nanoelectrode sites is shown to be equivalent to one in terms of a bimolecular diffusion-limited reaction between a layer of immobilized redox molecules and a reservoir of redox-active solutes. This equivalence derives from the fact that in both cases the mass-transfer problem is essentially that of hemispherical diffusion. The model is further developed to consider rate limitation by both the bimolecular redox reaction between the active-site molecule and redox molecules in solution and the heterogeneous redox reaction between the electrode and the active-site molecule. Analytical expressions are derived for the current–voltage relation corresponding to catalyzed electron transfer at an ensemble of redox-molecule-based nanoelectrode sites, and the expressions are used to interpret preliminary data for ultrasensitive electrochemical detection in flow streams via an electrochemical amplification process that is thought to involve redox mediation by individual analyte molecules adsorbed onto monolayer-coated electrodes. 相似文献
923.
Annamma John Daizy Philip Keith R. Morgan S. Devanarayanan 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2000,56(14):386
The FT IR and FT Raman spectra of Co(en)3Al3P4O16 · 3H2O (compound I) and [NH4]3[Co(NH3)6]3[Al2(PO4)4]2 · 2H2O (compound II) are recorded and analysed based on the vibrations of Co(en)33+, Co(NH3)63+, NH4+, Al---O---P, PO3, PO2 and H2O. The observed splitting of bands indicate that the site symmetry and correlation field effects are appreciable in both the compounds. In compound I, the overtone of CH2 deformation Fermi resonates with its symmetric stretching vibration. The NH4 ion in compound II is not free to rotate in the crystalline lattice. Hydrogen bonding of different groups is also discussed. 相似文献
924.
Zulfiqar Majeed William R. McWhinnie Philip R. Lowe 《Acta Crystallographica. Section C, Structural Chemistry》2000,56(3):e105-e106
The title selenenyl halide complex, 3‐iodo‐2‐phenyl‐3H‐3‐selenaindazole, C12H9IN2Se, has an almost planar conformation and a nearly ideal T‐shape for the Se(INC) moiety [Se—I 2.8122 (12), Se—C 1.881 (7) and Se—N2 2.051 (6) Å; C—Se—N 79.6 (3), C—Se—I 96.8 (2) and N—Se—I 176.17 (17)°]. This arrangement, together with the two selenium lone pairs, leads to a distorted trigonal‐bipyrimidal geometry about the Se atom. Intermolecular interactions are largely limited to stacking forces. 相似文献
925.
Brendan Twamley Ned J. Hardman Philip P. Power 《Acta Crystallographica. Section C, Structural Chemistry》2000,56(11):e514-e515
The sterically encumbered terphenyl halides 2′‐chloro‐2,2′′,4,4′′,6,6′′‐hexaisopropyl‐1,1′:3′,1′′‐terphenyl, C36H49Cl, (I), 2′‐bromo‐2,2′′,4,4′′,6,6′′‐hexaisopropyl‐1,1′:3′,1′′‐terphenyl, C36H49Br, (II), and 2′‐iodo‐2,2′′,4,4′′,6,6′′‐hexaisopropyl‐1,1′:3′,1′′‐terphenyl, C36H49I, (III), crystallize in space group Pnma. They are isomorphous and isostructural with a plane of symmetry through the centre of the molecule. The C–halide bond distances are 1.745 (3), 1.910 (4) and 2.102 (6) Å for (I)–(III), respectively. 相似文献
926.
Paul J. Wilson Alexander J. Blake Philip Mountford Martin Schrder 《Journal of organometallic chemistry》2000,600(1-2):71-83
The syntheses of the 1,3,5-trimethyl- and tri-tert-butyl-1,3,5-triazacyclohexane-supported imido complexes [M(NR)(R′3tach)Cl2] (M = Ti or Zr (NMR only); R = But or 2,6-C6H3Pri2; R′ = Me or But) are reported, along with that of the thermally robust dibenzyl derivative [Ti(NBut)(Me3tach)(CH2Ph)2]. The tert-butylimido ligand in [Ti(NBut)(Me3tach)Cl2] undergoes exchange with ArNH2 (Ar = 4-C6H4Me or 2,6-C6H4Me or 2,6-C6H3Pri2) to form the corresponding arylimides [Ti(NAr)(Me3tach)Cl2]. The Me3tach ring in [Ti(NR)(Me3tach)Cl2] undergoes slow exchange with But3tach or Me3tacn (1,4,7-trimethyl-1,4,7-triazacyclononane) to give the ring-exchanged products [Ti(NR)(But3tach)Cl2] and [Ti(NR)(Me3tacn)Cl2], respectively. The complexes [Ti(NR)(Me3tach)X2] (R = But or 2,6-C6H3Pri2; X = Cl or CH2Ph) exhibit room-temperature dynamic NMR behaviour via an unusual trigonal twist of the facially coordinated Me3tach ligand, and the activation parameters for these processes have been measured and are discussed. The X-ray structures of [Ti(NR)(But3tach)Cl2] (R = But or 2,6-C6H3Pri2) and [Ti(NBut)(Me3tach)(X)2] [X= Cl or CH2Ph) are reported. Me3tach and But3tach = 1,3,5-trimethyl- and tri-tert-butyl-1,3,5-triazacyclohexane, respectively. 相似文献
927.
David Hartley Robin W. Tucker Philip A. Tuckey Tevian Dray 《General Relativity and Gravitation》2000,32(3):491-503
Irregularities in the metric tensor of a signature-changing space-time suggest that field equations on such space-times might be regarded as distributional. We review the formalism of tensor distributions on differentiable manifolds, and examine to what extent rigorous meaning can be given to field equations in the presence of signature-change, in particular those involving covariant derivatives. We find that, for both continuous and discontinuous signature-change, covariant differentiation can be defined on a class of tensor distributions wide enough to be physically interesting. 相似文献
928.
Hema Tresa Varghese C. Yohannan Panicker Daizy Philip Joydeep Chowdhury Manash Ghosh 《Journal of Raman spectroscopy : JRS》2007,38(3):323-331
IR, Raman and surface enhanced Raman scattering (SERS) spectra of 3,5‐dinitrosalicylic acid (DNSA) were recorded and analysed. The vibrational wavenumbers were computed by the ab initio method using RHF/6–21G* basis and they were found to be in good agreement with the experimental values. The effect of the concentration dependence on the SERS intensity of the molecule was studied. The molecular plane assumes a tilted orientation with respect to the silver surface. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
929.
Francesca Bennet Tara M. Lovestead Philip J. Barker Thomas P. Davis Martina H. Stenzel Christopher Barner‐Kowollik 《Macromolecular rapid communications》2007,28(16):1593-1600
The products of the thermal degradation at 95 °C over 10 months of ω‐saturated and non‐saturated poly(methyl methacrylate) (pMMA) model compounds were identified with high accuracy via quadrupole ion trap and quadrupole ion trap‐time of flight (Q‐ToF) mass spectrometry. Analysis of the samples taken via these techniques indicated that degradation of vinyl terminated pMMA proceeds via the incorporation of oxygen via the formation of ethylene oxide type end groups, which subsequently rearrange under the expulsion of formaldehyde and 2‐oxo‐propionic acid methyl ester. The corresponding saturated model compounds were demonstrated to be stable over the same time period. The present findings highlight for the first time that poly(methyl methacrylate) degradation does not necessarily and exclusively proceed via radical intermediates.
930.
Abderrazak Ben‐Haida Howard M. Colquhoun Philip Hodge John L. Stanford 《Macromolecular rapid communications》2005,26(17):1377-1382
Summary: Blends of the poly(ether sulfone) derived from 4,4′‐biphenol and 4,4′‐dichlorodiphenylsulfone (Radel‐R™) with its homologous macrocyclic oligomers show greatly lowered melt viscosities relative to that of the parent polymer, potentially enabling more facile production and fabrication of fiber‐reinforced composite materials. The macrocycles can then undergo entropically driven ring‐opening polymerization in situ. The required blends can be obtained easily in one step, by carrying out polycondensations at concentrations lower than those usually used for polymer synthesis.