全文获取类型
收费全文 | 1390篇 |
免费 | 39篇 |
国内免费 | 17篇 |
专业分类
化学 | 825篇 |
晶体学 | 20篇 |
力学 | 49篇 |
数学 | 270篇 |
物理学 | 282篇 |
出版年
2023年 | 6篇 |
2022年 | 50篇 |
2021年 | 51篇 |
2020年 | 39篇 |
2019年 | 25篇 |
2018年 | 35篇 |
2017年 | 26篇 |
2016年 | 60篇 |
2015年 | 37篇 |
2014年 | 46篇 |
2013年 | 90篇 |
2012年 | 88篇 |
2011年 | 80篇 |
2010年 | 63篇 |
2009年 | 66篇 |
2008年 | 69篇 |
2007年 | 93篇 |
2006年 | 73篇 |
2005年 | 74篇 |
2004年 | 68篇 |
2003年 | 38篇 |
2002年 | 45篇 |
2001年 | 14篇 |
2000年 | 17篇 |
1999年 | 18篇 |
1998年 | 12篇 |
1997年 | 16篇 |
1996年 | 23篇 |
1995年 | 6篇 |
1994年 | 12篇 |
1993年 | 8篇 |
1992年 | 12篇 |
1991年 | 8篇 |
1990年 | 5篇 |
1989年 | 7篇 |
1988年 | 8篇 |
1985年 | 4篇 |
1984年 | 6篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1977年 | 5篇 |
1974年 | 2篇 |
1960年 | 2篇 |
1954年 | 2篇 |
1923年 | 4篇 |
1922年 | 5篇 |
排序方式: 共有1446条查询结果,搜索用时 15 毫秒
91.
Tomasz Smolinski Danuta Wawszczak Andrzej Deptula Wieslawa Lada Tadeusz Olczak Marcin Rogowski Marta Pyszynska Andrzej Grzegorz Chmielewski 《Journal of Radioanalytical and Nuclear Chemistry》2017,313(1):69-77
A micro-cloud point extraction method was discussed for preconcentration and spectrophotometric quantification of U(VI). The method depends on complex formation between U(VI) and 2-(4-sulphophenyloazo)-1,8-dihydroxy-3,6-naphtalenedisulphonic acid (SPADNS) at pH 7.0 and subsequent extraction of the complex in a mixed surfactant medium (cethyltrimethyl ammonium bromide and Triton X-114). The separation was carried out in the presence of 1% Na2SO4 at room temperature. The calibration curve was linear up to 3000 µg L?1. The enrichment factor, detection limit and precision were 16.0, 1.05 µg L?1, and 2.3%, respectively. The method was employed for the determination of U(VI) in real samples with different matrices. 相似文献
92.
A series of vanadium(V) complexes with unsymmetrical tridentate Schiff base ligands, obtained by the single condensation of R(−)-1,2-diaminopropane with salicylaldehyde and its derivatives, 2-hydroxy-1-naphthaldehyde, 2-hydroxyacetophenone, 1-hydroxy-2-acetonaphthone and 2-hydroxybenzophenone, were prepared. The complexes were characterized by elemental analysis and by their IR, CD, UV–Vis, 1D (1H, 51V) and 2D (COSY, NOESY, gHSQC) NMR spectra. Crystal structures of the mononuclear complex {R(−)-2-amino-1-N-[(2′-oxido-κO-4′,6′-dimethoxyphenyl)methylene]aminopropane-κ2N}dioxidovanadium(V), VO2(C12H17N2O3), 4, and of the dinuclear complex, di-μ-oxido-bis({R(−)-2-[1-(2-aminopropylimino)ethyl]-4-methylphenolato-κ3N,N′,O}oxidovanadium(V)), V2O4(C11H15N2O)2, 5, have been obtained by X-ray diffraction studies. The structure of 4 was revealed to be a distorted trigonal–bipyramidal coordination geometry, rarely encountered in VO2(tridentate Schiff base) complexes. Complexes 2 and 3 have the ability to catalyze the oxidation of prochiral sulfide substrates PhSR (R = Me, Bz) utilizing hydrogen peroxide or cumene hydroperoxide (CHPO) as the oxidant. 相似文献
93.
Jaworska M Hrynczyszyn PB Wełniak M Wojtczak A Nowicka K Krasiński G Kassassir H Ciesielski W Potrzebowski MJ 《The journal of physical chemistry. A》2010,114(47):12522-12530
Two analogous Schiff bases, (S,E)-2-((1-hydroxy-3-methyl-1,1-diphenylbutan-2-ylimino)methyl)phenol (1) and (S,Z)-2-hydroxy-6-((1-hydroxy-3-methyl-1,1-diphenylbutan-2-ylamino)methylene)cyclohexa-2,4-dienone (2), exist in the solid state as phenol-imine and keto-amine tautomers, respectively. Their crystal structures were solved using the X-ray diffraction method. Sample 1 forms orthorhombic crystals of space group P2(1)2(1)2(1), while 2 forms monoclinic crystals of space group P2(1). In each sample, one molecule is in the asymmetric unit of the crystal structure. One-dimensional and two-dimensional solid state NMR techniques were used for structure assignment and for inspection of the (13)C and (15)N δ(ii) of the chemical shift tensor (CST) values. NMR study indicates that the span (Ω = δ(11)-δ(33)) and the skew (κ = 3(δ(22)-δ(iso)/Ω) are extremely sensitive to change in the tautomeric form of the Schiff bases. Theoretical calculations of NMR shielding parameters for 1 and 2 and a model compound with reduced aliphatic residue were performed using the GIAO method with B3LYP functional and 6-311++g(d,p) basis sets. From comparative analysis of the experimental and theoretical parameters, it was concluded that the position of hydrogen in the intramolecular bridge has tremendous influence on (13)C and (15)N CST parameters. Inspection of Ω and κ parameters allowed for the establishment of the nature of the hydrogen bonding and the assignment of the equilibrium proton position in the intramolecular bridges in the solid state. 相似文献
94.
Magdalena Śliwka-Kaszyńska Grzegorz Gorczyca Marek Ślebioda 《Journal of chromatography. A》2010,1217(3):329-336
Twelve calix[4]arene stationary phases in 1,3-alternate conformation, synthesized in the authors’ laboratory, were characterized in terms of their surface coverage, hydrophobic selectivity, aromatic selectivity, shape selectivity, hydrogen bonding capacity and ion-exchange capacity. The set of tests commonly used for evaluation of commercially available stationary phases was applied to assess fundamental chromatographic properties of the calixarene phases. The new calixarene phases were compared to each other, to Caltrex and LiChrosorb C-18 columns. Principal component analysis has been used to provide comparison between 1,3-alternate calix[4]arene phases and commercially available phenyl, fluorophenyl and fluoroalkyl columns. 相似文献
95.
Grzegorz Dutkiewicz B. P. Siddaraju H. S. Yathirajan B. Narayana Maciej Kubicki 《Journal of chemical crystallography》2010,40(11):970-974
Abstract
The title compound, C22H28F3N3OS2+ × 2(C6H2N3O7 −) × (CH3)2SO—the picrate salt of the potent antipsychotic drug, fluphenazine—crystallizes in the triclinic P-1 space group with unit cell parameters a = 10.6333(12) ?, b = 11.9696(12) ?, c = 17.7036(15) ?, α = 103.265(9)°, β = 98.414(9)°, γ = 102.702(10)°. The ionic fragments: the fluphenazinium dictation, and two picrate anions, are joined by means of strong N–H···O and weak C–H···O hydrogen bonds into the chains of alternating cations and anions, expanding along [010] direction. Within the chain, the picrates interact by means of short π···π interactions: the mean distance between the planes is 3.366 ?; additional interaction of the same type between one of the picrates and the phenyl ring of the phenothiazine ring system additionally strengthen the pattern. The phenothiazine ring exists in a typical, “butterfly-like” conformation, with two terminal rings planar and the central ring in a slightly flattened boat form. This conformation results in the dihedral angle between the terminal rings of 41.76(5)°. The aliphatic chain which substitutes phenothiazine at N-position is not in an extended conformation, the torsion angles along this chain are 75.95(14)° and −163.96(10)°. The structure contains also the solvent—dimethylsulphoxide molecules, which are connected with the cation–anion structure by means of strong O–H···O hydrogen bonds. 相似文献96.
Okunev VD Samoilenko ZA Szewczyk A Szymczak R Szymczak H Lewandowski SJ Aleshkevych P Wi?ckowski J Khmelevskaya VS Antoshina IA 《J Phys Condens Matter》2011,23(41):415702
We have reinforced local superconductivity in ferromagnetic Fe(67)Cr(18)B(15) metallic glasses by ion irradiation. Superconductivity in this medium appears due to the presence of large-scale layered clusters of metallic Fe-Cr phase, 150-230 ? in size, with a ferromagnetic (or superparamagnetic) Fe-rich core and nonmagnetic Cr-rich superconducting shell. Here we show that due to the intensification of concentration phase separation in the Fe-Cr clusters under ion (Ar(+)) irradiation, the volume of the superconducting phase increases from the initial 0.4-0.5% up to 7-8%. After irradiation, the resistivity jump Δρ/ρ in the temperature range T=3.1-3.6 K increases ~14 times, reaching 19%, as compared to 1.36% for the initial sample. In the interval of T=3.1-3.6 K, the rate of resistance change reaches 79% K(-1) for the irradiated sample instead of 3.6% K(-1) for the initial sample. In the same temperature interval, the rate of magnetoresistance change increases from 3% K(-1) for the initial sample up to 70% K(-1) after irradiation. 相似文献
97.
98.
Aleksandra Swiercz Edmund K. Burke Mateusz Cichenski Grzegorz Pawlak Sanja Petrovic Tomasz Zurkowski Jacek Blazewicz 《Central European Journal of Operations Research》2014,22(3):567-589
This paper introduces a new approach to applying hyper-heuristic algorithms to solve combinatorial problems with less effort, taking into account the modelling and algorithm construction process. We propose a unified encoding of a solution and a set of low level heuristics which are domain-independent and which change the solution itself. This approach enables us to address NP-hard problems and generate good approximate solutions in a reasonable time without a large amount of additional work required to tailor search methodologies for the problem in hand. In particular, we focused on solving DNA sequencing by hybrydization with errors, which is known to be strongly NP-hard. The approach was extensively tested by solving multiple instances of well-known combinatorial problems and compared with results generated by meta heuristics that have been tailored for specific problem domains. 相似文献
99.
Luigi C. Berselli Roger Lewandowski 《Annales de l'Institut Henri Poincaré (C) Analyse Non Linéaire》2012
We consider a 3D Approximate Deconvolution Model ADM which belongs to the class of Large Eddy Simulation (LES) models. We aim at proving that the solution of the ADM converges towards a dissipative solution of the mean Navier–Stokes equations. The study holds for periodic boundary conditions. The convolution filter we first consider is the Helmholtz filter. We next consider generalized convolution filters for which the convergence property still holds. 相似文献
100.