全文获取类型
收费全文 | 373篇 |
免费 | 15篇 |
专业分类
化学 | 268篇 |
晶体学 | 2篇 |
力学 | 13篇 |
数学 | 32篇 |
物理学 | 73篇 |
出版年
2023年 | 5篇 |
2022年 | 12篇 |
2021年 | 24篇 |
2020年 | 13篇 |
2019年 | 10篇 |
2018年 | 9篇 |
2017年 | 7篇 |
2016年 | 18篇 |
2015年 | 16篇 |
2014年 | 21篇 |
2013年 | 30篇 |
2012年 | 20篇 |
2011年 | 27篇 |
2010年 | 14篇 |
2009年 | 18篇 |
2008年 | 21篇 |
2007年 | 19篇 |
2006年 | 20篇 |
2005年 | 11篇 |
2004年 | 12篇 |
2003年 | 10篇 |
2002年 | 7篇 |
2001年 | 4篇 |
2000年 | 5篇 |
1999年 | 2篇 |
1998年 | 3篇 |
1997年 | 3篇 |
1996年 | 4篇 |
1995年 | 6篇 |
1994年 | 2篇 |
1993年 | 1篇 |
1992年 | 1篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1988年 | 3篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 2篇 |
排序方式: 共有388条查询结果,搜索用时 15 毫秒
361.
In this work, methodologies were developed to determine aluminum (Al), cadmium chromium and lead in drinking water by electrothermal atomic absorption spectrometry using permanent modifiers. No use of modifier, iridium, ruthenium, rhodium and zirconium (independently, 500 μg) were tested to each one analyte through the pyrolysis and atomization temperatures curves. As the matrix is very simple, did not had occurred problems with the background for all metals. The best results obtained for cadmium and chromium was with the use of rhodium permanent modifier. For lead and aluminum, the best choice was the use of zirconium. The selection for the modifier took into account the sensitivity, form of the absorption pulse and low atomization temperature (what contributes to elevate the useful life of the graphite tube). For aluminum using zirconium permanent, the best pyrolysis and atomization temperatures were respectively, of 1000 and 2500 °C with a characteristic mass (1% of absorbance, mo) of 19 pg (recommended of 20 pg). For cadmium, with use of rhodium the best temperatures for the pyrolysis and atomization were respectively of 400 and 1100 °C, with a symmetrical peak and with a mo of 1.0 pg (recommended of 1.0 pg). For chromium with rhodium permanent, the best temperatures for pyrolysis and atomization were respectively of 1000 and 2200 °C, with symmetrical peak and mo of 5.3 pg (recommended of 5.5 pg). For lead with zirconium permanent, the best temperatures for pyrolysis and atomization were of 700 and 2400 °C, with symmetrical peak and with mo of 30 pg (recommended of 20 pg). Water samples spiked with each one of the metals in four different levels inside of the acceptable values presented recoveries always close to 100%. The detection limits were of 0.1 μg l−1 for cadmium; 0.2 μg l−1 for chromium; 0.5 μg l−1 for lead and 1.4 μg l−1 for aluminum. 相似文献
362.
John Burgess John Fawcett Vitor Palma Syeda R. Gilani 《Acta Crystallographica. Section C, Structural Chemistry》2001,57(3):277-280
The structures of five complexes of fluorine-containing bidentate salicylideneamine Schiff base ligands are reported. These are the bis-ligand copper(II) complexes of the Schiff bases derived from salicylaldehyde and 4-fluoro-, [Cu(C13H9FNO)2], 3-fluoro-4-methyl-, [Cu(C14H11FNO)2], 3,5-bis(trifluoromethyl)-, [Cu(C15H8F6NO)2], and 4-trifluoromethoxyanilines, [Cu(C14H9F3NO2)2], and the bis-ligand oxovanadium(IV) complex of the Schiff base derived from salicylaldehyde and 4-trifluoromethoxyaniline, [VO(C14H9F3NO2)2]. Three of the copper complexes have square-planar coordination at the metal, imposed by the virtue of symmetry, but the immediate coordination environment of the copper in the 3,5-bis(trifluoromethyl) complex is intermediate between square planar and tetrahedral. The coordination environment at the metal of the vanadium complex can be described as distorted square pyramidal. 相似文献
363.
Walysson Ferreira de Paiva Ingredy Bastos Braga João Vitor de Assis Sandra Milena Bonilla Castañeda Ángel Gabriel Sathicq Valeria Palermo Gustavo Pablo Romanelli Ricardo Natalino Marcio José da Silva Felipe Terra Martins Gustavo Senra Gonçalves de Carvalho Giovanni Wilson Amarante Sergio Antonio Fernandes 《Tetrahedron》2019,75(27):3740-3750
The application of immobilizing calixarene onto the surface of silica using a sol-gel method, as an efficient heterogeneous catalyst (CX4SO3HSi(n)) for multicomponent Povarov reaction has been described. Catalytic activity of the CX4SO3HSi(n) for the synthesis of julolidines under greener and environmentally benign conditions in simple and efficient method was explored. Notably, only 0.5 mol% of the catalyst is sufficient to catalyze the Povarov solvent-free reaction under microwave-assistance. Besides, this protocol allows the construction of four new C–C bonds and two C–N bonds in a single step. To the best of our knowledge, this consists the first silica support calix[4]arene as a heterogeneous catalyst for multicomponent synthesis of julolidines. 相似文献
364.
365.
Haroldo T. Hattori Vitor M. Schneider Carmem L. Barbosa Osni Lisba 《Optics & Laser Technology》2000,32(1):59
Several gradient-index (GRIN) tapers have been analyzed in the literature, whether with a transverse parabolic index distribution or with a hyperbolic secant index profile. In this work, a new class of GRIN tapers, formed by grouping together several sections of transverse parabolic index devices is analyzed, focusing the attention on some of its limitations and advantages over ordinary tapers. Numerical simulations based upon geometric optics are conducted to analyze the behavior of these tapers. 相似文献
366.
David L.N. de F. Amorim Sergio P.B. Proença Julio Flórez-López 《International Journal of Solids and Structures》2013
A simplified model of cracking and damage in RC circular elements is proposed. The model can be used for the structural assessment of arches and rings. The constitutive equations are based on lumped damage mechanics which is an adaptation of fracture and continuum damage mechanics to the theory of frames with plastic hinges. An arch element is assumed to be the assemblage of an elastic circular component and two inelastic hinges where the main inelastic effects, plastic yielding of the longitudinal reinforcement and concrete cracking, are concentrated. Deformations in the elastic part are assumed to be small but the model may include some geometrically nonlinear effects due to large displacements or rotations of the hinges. The numerical examples presented in the paper show that the model describes correctly the global behavior of two structures including the softening phase. 相似文献
367.
Vaz PD Nolasco M Fonseca N Amado AM Costa AM Félix V Drew MG Goodfellow BJ Ribeiro-Claro PJ 《Physical chemistry chemical physics : PCCP》2005,7(16):3027-3034
The crystal structure of 4-phenyl-benzaldehyde reveals the presence of a dimer linked by the C=O and C9-H groups of adjacent molecules. In the liquid phase, the presence of C-H...O bonded forms is revealed by both vibrational and NMR spectroscopy. A DeltaH value of -8.2 +/- 0.5 kJ mol(-1) for the dimerisation equilibrium is established from the temperature-dependent intensities of the bands assigned to the carbonyl-stretching modes. The NMR data suggest the preferential engagement of the C(2,6)-H and C(10/12)/C(11)-H groups as hydrogen bond donors, instead of the C(9)-H group. While ab initio calculations for the isolated dimers are unable to corroborate these NMR results, the radial distribution functions obtained from molecular dynamics simulations show a preference for C(2,6)-H and C(10/12)/C(11)-H...O contacts relative to the C(9)-H...O ones. 相似文献
368.
Matthews SE Schmitt P Felix V Drew MG Beer PD 《Journal of the American Chemical Society》2002,124(7):1341-1353
A new class of cryptand-like ionophore based on a bis calix[4]arene structure has been developed. These molecules proved highly selective for complexation of potassium over all group I metal cations and barium. A range of symmetric and asymmetric calix[4]tubes featuring either alkyl or phenyl substituents at the upper rim have been synthesized in exceptional yield. Alteration of the calix[4]arene upper rim environment enables fine-tuning of the rate of potassium uptake, which can be evaluated through the conformational change on binding by NMR studies. Selectivity of potassium complexation has been demonstrated using NMR and electrospray mass spectral techniques. Molecular modeling studies, derived from crystallographic data, confirm that the potassium metal cation is complexed via the axial route, passing through the calix[4]arene annulus, and provide evidence for complexation rate and selectivity. 相似文献
369.
Pessoa JC Calhorda MJ Cavaco I Costa PJ Correia I Costa D Vilas-Boas LF Félix V Gillard RD Henriques RT Wiggins R 《Dalton transactions (Cambridge, England : 2003)》2004,(18):2855-2866
Oxovanadium(IV) complexes with ligands derived from the reaction of salicylaldehyde with L-cysteine and with D- and D,L-penicillamine are prepared. The compounds are characterised by elemental analysis, spectroscopy (UV-VIS, CD, EPR), TG, DSC and magnetic susceptibility measurements (9-295 K). We discuss several aspects related to the structure of these complexes in the solid state and in solution; in particular, the possibility of forming thiazolidine complexes, and their comparison with the characterised complexes is studied by molecular mechanics and density functional theory calculations. The solution structures depend on pH and solvent, and while with L-Cys the spectroscopic results show trends similar to those of the L-Ala and L-Ser systems up to ca. pH 8-9, where thiolate coordination starts being detected, the penicillamine system is quite distinct, namely thiolate coordination occurs for pH > 6.5. In the presence of salicylaldehyde and V(IV)O the desulfydration of cysteine proceeds rapidly, but no similar reaction occurs with penicillamine, although its decomposition is also activated. The DFT calculations do not indicate any energetic basis for this distinct reactivity, which possibly results from different complexes present in the Cys and Pen systems. In the cysteine system, the N-salicylidenedehydroalanine-V(IV)O complex V is believed to form in an intermediate stage of the desulfydration. Further, addition of several nucleophiles to the cysteine reaction mixtures produce amino acid derivatives by a Michael-type base-catalysed addition, a result compatible with the formation of V. The products of these reactions were analysed by TLC and HPLC, and in some cases isolated. 相似文献
370.