全文获取类型
收费全文 | 2202篇 |
免费 | 44篇 |
国内免费 | 19篇 |
专业分类
化学 | 1232篇 |
晶体学 | 7篇 |
力学 | 67篇 |
数学 | 491篇 |
物理学 | 468篇 |
出版年
2020年 | 16篇 |
2019年 | 37篇 |
2017年 | 18篇 |
2016年 | 40篇 |
2015年 | 38篇 |
2014年 | 26篇 |
2013年 | 71篇 |
2012年 | 92篇 |
2011年 | 80篇 |
2010年 | 67篇 |
2009年 | 55篇 |
2008年 | 87篇 |
2007年 | 94篇 |
2006年 | 83篇 |
2005年 | 63篇 |
2004年 | 42篇 |
2003年 | 44篇 |
2002年 | 41篇 |
2001年 | 50篇 |
2000年 | 43篇 |
1999年 | 29篇 |
1998年 | 24篇 |
1997年 | 18篇 |
1996年 | 43篇 |
1995年 | 33篇 |
1994年 | 31篇 |
1993年 | 39篇 |
1992年 | 40篇 |
1991年 | 26篇 |
1990年 | 28篇 |
1989年 | 21篇 |
1988年 | 23篇 |
1987年 | 32篇 |
1986年 | 21篇 |
1985年 | 27篇 |
1984年 | 20篇 |
1983年 | 29篇 |
1982年 | 37篇 |
1981年 | 26篇 |
1980年 | 20篇 |
1979年 | 35篇 |
1978年 | 38篇 |
1977年 | 36篇 |
1976年 | 24篇 |
1975年 | 27篇 |
1974年 | 30篇 |
1973年 | 33篇 |
1971年 | 25篇 |
1970年 | 23篇 |
1969年 | 19篇 |
排序方式: 共有2265条查询结果,搜索用时 15 毫秒
201.
Flow rate analysis of a surface tension driven passive micropump 总被引:2,自引:0,他引:2
A microfluidic passive pumping method relying on surface tension properties is investigated and a physical model is developed. When a small inlet drop is placed on the entrance of a microfluidic channel it creates more pressure than a large output drop at the channel exit, causing fluid flow. The behavior of the input drop occurs in two characteristic phases. An analytical solution is proposed and verified by experimental results. We find that during the first phase the flow rate is stable and that this phase can be prolonged by refilling the inlet drop to produce continuous flow in the microchannel. 相似文献
202.
Technical cellulose-2.5-acetates (CA 2.5) were characterized regarding their carbohydrate composition in comparison to the raw material. The association of the CA 2.5 samples in acetone was studied by size exclusion chromatography (SEC) using various acetone grades and styrene divinylbenzene copolymer columns. In HPLC grade acetone with and without addition of 1% water up to three different pre-humps eluted in front of the main fraction of the polymer. The evaluation of the main peak by light scattering measurements resulted in high molar masses indicating that for these technical CA 2.5 samples even the main fraction is not dissolved without association. No pre-humps or association phenomena were observed after addition of 1 ppm LiBr to HPLC grade acetone or with p.a. grade acetone. In addition pre-hump enriched and pre-hump free fractions were isolated by fractionated precipitation. The carbohydrate composition of these fractions was determined and correlated with their association pattern in SEC investigations. 相似文献
203.
Portuondo-Campa E Tortschanoff A van Mourik F Moser JE Kornherr A Chergui M 《The journal of physical chemistry. B》2006,110(15):7835-7844
Broadband transient absorption (TA) spectroscopy, three-pulse photon echo peak shift (3PEPS), and anisotropy decay measurements were used to study the solvation dynamics in bulk water and interfacial water at ZrO(2) surfaces, using Eosin Y as a probe. The 3PEPS results show a multiexponential behavior with two subpicosecond components that are similar in bulk and interfacial water, while a third component of several picoseconds is significantly lengthened at the interface. The bandwidth correlation function from TA spectra exhibits the same behavior, and the TA spectra are well reproduced using the doorway-window picture with the time constants from PEPS. Our results suggest that interfacial water is restricted to a thickness of less than 5 A. Also the high-frequency collective dynamics of water does not seem to be affected by the interface. On the other hand, the increase of the third component may point to a slowing down of diffusional motion at the interface, although other effects, may play a role, which are discussed. 相似文献
204.
Letzel M Kirchhoff D Grützmacher HF Stein D Grützmacher H 《Dalton transactions (Cambridge, England : 2003)》2006,(16):2008-2016
The unimolecular reactions of radical cations and cations derived from phenylarsane, C6H5AsH2 (1) and dideutero phenylarsane, C6H5AsD2 (1-d2), were investigated by methods of tandem mass spectrometry and theoretical calculations. The mass spectrometric experiments reveal that the molecular ion of phenylarsane, 1*+, exhibits different reactivity at low and high internal excess energy. Only at low internal energy the observed fragmentations are as expected, that is the molecular ion 1*+ decomposes almost exclusively by loss of an H atom. The deuterated derivative 1-d2 with an AsD2 group eliminates selectively a D atom under these conditions. The resulting phenylarsenium ion [C6H5AsH]+, 2+, decomposes rather easily by loss of the As atom to give the benzene radical cation [C6H6]*+ and is therefore of low abundance in the 70 eV EI mass spectrum. At high internal excess energy, the ion 1*+ decomposes very differently either by elimination of an H2 molecule, or by release of the As atom, or by loss of an AsH fragment. Final products of these reactions are either the benzoarsenium ion 4*+, or the benzonium ion [C6H7]+, or the benzene radical cation, [C6H6]*+. As key-steps, these fragmentations contain reductive eliminations from the central As atom under H-H or C-H bond formation. Labeling experiments show that H/D exchange reactions precede these fragmentations and, specifically, that complete positional exchange of the H atoms in 1*+ occurs. Computations at the UMP2/6-311+G(d)//UHF/6-311+G(d) level agree best with the experimental results and suggest: (i) 1*+ rearranges (activation enthalpy of 93 kJ mol(-1)) to a distinctly more stable (DeltaH(r)(298) = -64 kJ mol(-1)) isomer 1 sigma*+ with a structure best represented as a distonic radical cation sigma complex between AsH and benzene. (ii) The six H atoms of the benzene moiety of 1 sigma*+ become equivalent by a fast ring walk of the AsH group. (iii) A reversible isomerization 1+<==>1 sigma*+ scrambles eventually all H atoms over all positions in 1*+. The distonic radical cation 1*+ is predisposed for the elimination of an As atom or an AsH fragment. The calculations are in accordance with the experimentally preferred reactions when the As atom and the AsH fragment are generated in the quartet and triplet state, respectively. Alternatively, 1*(+) undergoes a reductive elimination of H2 from the AsH2 group via a remarkably stable complex of the phenylarsandiyl radical cation, [C6H5As]*+ and an H2 molecule. 相似文献
205.
We present a detailed study of the photophysical properties of a tetraphenoxy-substituted perylene bisimide derivative. The probe molecules were immobilized in a Shpol'skii matrix of hexadecane and investigated by single-molecule spectroscopy at cryogenic temperatures. By using single-molecule spectroscopic techniques we reveal the triplet substate kinetics and the fluorescence quantum yield, and we provide an estimate for the S1-S0 transition dipole moment. 相似文献
206.
Core-shell nanostructures from single poly(N-vinylcaprolactam) macromolecules: stabilization and visualization 总被引:1,自引:0,他引:1
Bronstein LM Kostylev M Tsvetkova I Tomaszewski J Stein B Makhaeva EE Okhapkin I Khokhlov AR 《Langmuir : the ACS journal of surfaces and colloids》2005,21(7):2652-2655
Formation of core-shell poly(N-vinylcaprolactam) (PVCL) single-molecule nanostructures due to interaction of PVCL with metal ions was studied using transmission electron microscopy, 13C NMR, and light scattering. This study demonstrates that addition of CoCl2 to PVCL in its globular conformation yields unimolecular core-shell polymer particles with the core decorated with Co(II) ions. The crucial condition for formation of well-defined unimolecular nanostructures is the presence of stable globular aggregates in aqueous solution. Moreover, the metal ions should have a sufficiently high coordination number (higher than 2) to provide a cross-linking and stabilization of the core. 相似文献
207.
Reisner E Arion VB Rufińska A Chiorescu I Schmid WF Keppler BK 《Dalton transactions (Cambridge, England : 2003)》2005,(14):2355-2364
Reaction of the antitumor complex trans-[Ru(III)Cl4(Hind)2]- (Hind = indazole) with an excess of dimethyl sulfoxide (dmso) in acetone afforded the complex trans,trans,trans-[Ru(II)Cl2(dmso)2(Hind)2] (1). Two other isomeric compounds trans,cis,cis-[Ru(II)Cl2(dmso)2(Hind)2] (2) and cis,cis,cis-[Ru(II)Cl2(dmso)2(Hind)2] (3) have been obtained on refluxing cis-[Ru(II)Cl(2)(dmso)(4)] with 2 equiv. of indazole in ethanol and methanol, respectively. Isomers 1 and 2 react with acetonitrile yielding the complexes trans-[Ru(II)Cl2(dmso)(Hind){HN=C(Me)ind}].CH3CN (4.CH3CN) and trans,cis-[Ru(II)Cl2(dmso)2{HN=C(Me)ind}].H2O (5.H2O), respectively, containing a cyclic amidine ligand resulting from insertion of the acetonitrile C triple bond N group in the N1-H bond of the N2-coordinated indazole ligand in the nomenclature used for 1H-indazole. These are the first examples of the metal-assisted iminoacylation of indazole. The products isolated have been characterized by elemental analysis, IR spectroscopy, UV-vis spectroscopy, electrospray mass-spectrometry, thermogravimetry, differential scanning calorimetry, 1H NMR spectroscopy, and solid-state 13C CP MAS NMR spectroscopy. The isomeric structures of 1-3 and the presence of a chelating amidine ligand in 4 and 5 have been confirmed by X-ray crystallography. The electrochemical behavior of 1-5 and the formation of 5 have been studied by cyclic voltammetry. 相似文献
208.
Rogatsky E Jayatillake H Goswami G Tomuta V Stein D 《Journal of the American Society for Mass Spectrometry》2005,16(11):1805-1811
The development of a sensitive assay for the quantitative analysis of carbohydrates from human plasma using LC/MS/MS is described in this paper. After sample preparation, carbohydrates were cationized by Cs(+) after their separation by normal phase liquid chromatography on an amino based column. Cesium is capable of forming a quasi-molecular ion [M + Cs](+) with neutral carbohydrate molecules in the positive ion mode of electrospray ionization mass spectrometry. The mass spectrometer was operated in multiple reaction monitoring mode, and transitions [M + 133] --> 133 were monitored (M, carbohydrate molecular weight). The new method is robust, highly sensitive, rapid, and does not require postcolumn addition or derivatization. It is useful in clinical research for measurement of carbohydrate molecules by isotope dilution assay. 相似文献
209.
210.
Karl Hensen Ralf Mayr‐Stein Bjrn Spangenberg Michael Bolte 《Acta Crystallographica. Section C, Structural Chemistry》2000,56(5):610-613
The reaction products of an addition reaction of five different silicon tetrahalides with the aromatic nitrogen base 4‐methylpyridine are presented. The following five structures are isomorphous: (I) tetrachlorobis(4‐methylpyridine)silicon, C12H14Cl4N2Si, (II) bromotrichlorobis(4‐methylpyridine)silicon, C12H14BrCl3N2Si, (III) dibromodichlorobis(4‐methylpyridine)silicon, C12H14Br2Cl2N2Si, (IV) tribromochlorobis(4‐methylpyridine)silicon, C12H14Br3ClN2Si, and (V) tetrabromobis(4‐methylpyridine)silicon, C12H14Br4N2Si. The molecules of (I) and (V), with D2h symmetry, have crystallographic C2h symmetry, while the molecules of (II), (III) and (IV) have a lower molecular symmetry, but as a result of the disorder of the halogen ligands, they appear to be of the same crystallographic symmetry. The environment around the Si atom can be described as a slightly distorted octahedron with the methylpyridine ligands occupying axial positions and the four halogen ligands in the equatorial plane. In spite of the different substitution pattern of the silicon centre, there are only insignificant differences between these five structures. 相似文献