全文获取类型
收费全文 | 1360篇 |
免费 | 29篇 |
国内免费 | 9篇 |
专业分类
化学 | 916篇 |
晶体学 | 16篇 |
力学 | 17篇 |
数学 | 175篇 |
物理学 | 274篇 |
出版年
2023年 | 6篇 |
2022年 | 12篇 |
2021年 | 19篇 |
2020年 | 23篇 |
2019年 | 15篇 |
2018年 | 20篇 |
2017年 | 21篇 |
2016年 | 24篇 |
2015年 | 30篇 |
2014年 | 38篇 |
2013年 | 80篇 |
2012年 | 73篇 |
2011年 | 103篇 |
2010年 | 75篇 |
2009年 | 78篇 |
2008年 | 103篇 |
2007年 | 102篇 |
2006年 | 74篇 |
2005年 | 81篇 |
2004年 | 74篇 |
2003年 | 50篇 |
2002年 | 35篇 |
2001年 | 31篇 |
2000年 | 26篇 |
1999年 | 20篇 |
1998年 | 10篇 |
1997年 | 14篇 |
1996年 | 11篇 |
1995年 | 8篇 |
1994年 | 7篇 |
1993年 | 15篇 |
1992年 | 12篇 |
1991年 | 9篇 |
1990年 | 7篇 |
1989年 | 7篇 |
1988年 | 10篇 |
1987年 | 4篇 |
1985年 | 14篇 |
1984年 | 6篇 |
1983年 | 6篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1980年 | 9篇 |
1979年 | 5篇 |
1978年 | 2篇 |
1977年 | 6篇 |
1976年 | 3篇 |
1974年 | 2篇 |
1966年 | 3篇 |
1894年 | 1篇 |
排序方式: 共有1398条查询结果,搜索用时 0 毫秒
81.
A new mode of magnetophoresis is described that is capable of separating micron-sized superparamagnetic beads from complex mixtures with high sensitivity to their size and magnetic moment. This separation technique employs a translating periodic potential energy landscape to transport magnetic beads horizontally across a substrate. The potential energy landscape is created by superimposing an external, rotating magnetic field on top of the local fixed magnetic field distribution near a periodic arrangement of micro-magnets. At low driving frequencies of the external field rotation, the beads become locked into the potential energy landscape and move at the same velocity as the traveling magnetic field wave. At frequencies above a critical threshold, defined by the bead's hydrodynamic drag and magnetic moment, the motion of a specific population of magnetic beads becomes uncoupled from the potential energy landscape and its magnetophoretic mobility is dramatically reduced. By exploiting this frequency dependence, highly efficient separation of magnetic beads has been achieved, based on fractional differences in bead diameter and/or their specific attachment to two microorganisms, i.e., B. globigii and S. cerevisiae. 相似文献
82.
Leandro J. dos Santos Mara R. C. Couri Inácio Luduvico Rosemeire B. Alves Maria A. F. Prado Rossimiriam P. Freitas Gil 《合成通讯》2013,43(18):3059-3066
Five new nitrogen heterocycles, mono‐and disubstituted tetrazoles with potential synthetic and pharmacological interest, were synthesized from α, α‐trehalose via the alkylation of commercial tetrazoles. This method appears to have broad scope with respect to the variations at positions 1 and 2 of tetrazole. 相似文献
83.
Basis sets ranging in size from (16, 10, 7) to (20, 14, 11) have been derived for the atoms Y–Cd. Separate sets represent the energy optimized wave functions for each of the s2dn, s1dn+1, and s0dn+2 configurations. The energies from the largest sets are within 3 mhartrees of the values obtained in numerical Hartree–Fock calculations. Reasonable Hartree–Fock s2dn– s1dn+1 and s2dn– s0dn+2 excitation energies may be obtained either using the largest basis sets, or using d-orbitals optimized for the s0dn+2 configurations. The basis sets are slightly unbalanced in favor of the s-functions and in disfavor of the d-functions, but various alternative basis sets may be derived by combining parts of the five parent sets. The convergence of radial expectation values is discussed. 相似文献
84.
E. Pinilla Gil L. Calvo Blazquez R. M. Garcia-Monco Carra A. Sanchez Misiego 《Fresenius' Journal of Analytical Chemistry》1989,335(8):1002-1004
Summary A differential pulse polarographic method for the determination of oxytetracycline in urine and human serum in acid media (HClO4 of pH 2) is proposed. The effects of the amount of sample taken and the concentration of HClO4 present were investigated. The detection limit was 5.5×10–6 mol/l. The standard deviation of the determination of 5.5×10–5 mol/l of oxytetracycline in 2 ml of urine was 1.7×10–6 mol/l and that of the determination of 5.5×10–5 mol/l of oxytetracycline in 2 ml of human serum was 1.9×10–6 mol/l.
Bestimmung von Oxytetracyclin in Urin und Humanserum durch Differential-Pulspolarography相似文献
85.
Katrin Meier Raul Cardoso‐Gil Ulrich Burkhardt Caroline Curfs Michael Hanfland Yuri Grin PD Dr. Ulrich Schwarz 《无机化学与普通化学杂志》2012,638(10):1446-1451
The rare‐earth metal germanides RE2Ge9 (RE = Nd, Sm) have been prepared by thermal decomposition of the metastable high‐pressure phases REGe5 at ambient pressure. The compounds adopt an orthorhombic unit cell with a = 396.34(4) pm; b = 954.05(8) pm and c = 1238.4(1) pm for Nd2Ge9 and a = 395.46(7) pm; b = 946.4(2) pm and c = 1232.1(3) pm for Sm2Ge9. Crystal structure refinements reveal space group Pmmn (No. 59) for Nd2Ge9. The atomic pattern resembles an ordered defect variety of the pentagermanide motif REGe5 (RE = La; Nd, Sm, Gd, Tb) comprising corrugated germanium layers. These condense into a three‐dimensional network interconnected by eight‐coordinated germanium atoms. The resulting framework channels along [100] enclose the neodymium atoms. With respect to the atomic arrangement of the pentagermanides, half of the interlayer germanium atoms are eliminated in an ordered way so that occupied and empty germanium columns alternate along [001]. The rare‐earth metal atoms of both types of compounds, REGe5 and RE2Ge9, exhibit the electronic states 4f 3 and 4f 5 (oxidation state +3) for neodymium and samarium, respectively, evidencing that the modification of the germanium network leaves the electron configuration of the metal atoms unaffected. 相似文献
86.
Virginia Bravo Salvador Gil Ana M. Costero María N. Kneeteman Ursula Llaosa Pedro M.E. Mancini Luís E. Ochando Margarita Parra 《Tetrahedron》2012,68(24):4882-4887
The synthesis of two new 2,7-disubstituted phenanthrene-based bis oximes is described. The ability of these two compound for complexing heavy metal cations have been studied and complexation constants and complex stoichiometry for Cr3+ and Fe3+ complex have been determined. The fluorescent properties of ligand 2 make this compound able to act as a sensor able to discriminate between Cr3+ and Fe3+. Detection limits for these two cations have been evaluated. 相似文献
87.
Hong J Kim YH Gil JH Cho K Jung JH Han SY 《Rapid communications in mass spectrometry : RCM》2002,16(22):2089-2093
The structural determination of sn-1 and sn-2 hexadecanoic lysophosphatidylcholine (LPC) regioisomers was carried out using fast atom bombardment tandem mass spectrometry (FAB-MS/MS). The collision-induced dissociation (CID) of protonated and sodiated molecules produced diverse product ions due mainly to charge remote fragmentations. Based on the information obtained from the CID spectra of protonated and sodiated molecules, sn-1 and sn-2 hexadecanoic LPC isomers could be discriminated. Especially, the abundance ratio of the diagnostic ion pair [m/z 224/226] in the CID spectra of [M + H](+) ions was shown to be greatly different. Moreover, the CID-MS/MS spectra of sodium-adducted molecules for hexadecanoic LPC isomers showed characteristic product ions such as [M + Na - 103](+), [M + Na - 85](+), and [M + Na - 59](+), by which their regio-specificity can be differentiated. 相似文献
88.
Mauricio Alcolea Palafox Nerea Iza Manuel Gil Jos Luís Núez 《International journal of quantum chemistry》2002,89(1):25-47
A quantum chemical study of several complex monocyclic 4‐benzoyl‐4‐phenyl‐β‐lactam derivatives was carried out using cyclobutane, azetidine, 2‐azetidinone, 1‐methyl‐2‐azetidinone, and 3‐methyl‐2‐azetidinone as model compounds. The optimum geometry was obtained for the different conformations. The planarity of the ring was discussed in terms of the influence of the substituents on the amide resonance. To better analyze the amide resonance and the activity of the β‐lactam ring, a vibrational study was also carried out. To examine the influence of solvent polarity on the carbonyl bands, the Fourier transform–infrared (FT‐IR) spectra of the β‐lactam monocyclic derivatives were recorded in CCl4, C6H6, and CHCl3 solutions. The normal vibrations of the β‐lactam ring in the model compounds were characterized and used in the analysis of the β‐ring of more complex derivatives. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002 相似文献
89.
Kh. Kh. Gil’manov R. G. Romanova A. A. Lamberov R. R. Gil’mullin 《Russian Journal of Applied Chemistry》2010,83(1):76-83
Calorimetry, thermogravimetry, X-ray diffraction analysis, and low-temperature adsorption of nitrogen were used to study the
structure of bimetallic Pt—Sn catalysts on zinc–alumina spinel supports. The effect of the porous structure of the catalyst
samples synthesized on their catalytic activity in dehydrogenation of 2–methyl butane was analyzed. 相似文献
90.
Maria J. Alves Vera C.M. Duarte Hélio Faustino António Gil Fortes 《Tetrahedron: Asymmetry》2010,21(15):1817-1820
Erythrose benzylidene-acetal 1,3-butadienes are studied as partners in Diels–Alder cycloadditions. A high diastereofacial improvement is found in cases where both the alcohol function is protected and a π–π interaction between the diene and dienophile is possible. Several competing factors have been studied independently in order to explain its influence on the selectivity of the cycloadditions. 相似文献