全文获取类型
收费全文 | 917篇 |
免费 | 38篇 |
国内免费 | 11篇 |
专业分类
化学 | 590篇 |
晶体学 | 6篇 |
力学 | 27篇 |
数学 | 171篇 |
物理学 | 172篇 |
出版年
2023年 | 5篇 |
2022年 | 12篇 |
2021年 | 33篇 |
2020年 | 28篇 |
2019年 | 13篇 |
2018年 | 20篇 |
2017年 | 18篇 |
2016年 | 54篇 |
2015年 | 30篇 |
2014年 | 36篇 |
2013年 | 78篇 |
2012年 | 78篇 |
2011年 | 64篇 |
2010年 | 44篇 |
2009年 | 30篇 |
2008年 | 68篇 |
2007年 | 45篇 |
2006年 | 53篇 |
2005年 | 33篇 |
2004年 | 29篇 |
2003年 | 23篇 |
2002年 | 24篇 |
2001年 | 7篇 |
2000年 | 13篇 |
1999年 | 8篇 |
1998年 | 7篇 |
1997年 | 5篇 |
1996年 | 5篇 |
1995年 | 7篇 |
1994年 | 6篇 |
1993年 | 14篇 |
1992年 | 8篇 |
1991年 | 4篇 |
1990年 | 7篇 |
1989年 | 3篇 |
1988年 | 4篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 4篇 |
1978年 | 3篇 |
1976年 | 3篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1973年 | 5篇 |
1972年 | 2篇 |
1969年 | 3篇 |
1968年 | 2篇 |
1937年 | 2篇 |
排序方式: 共有966条查询结果,搜索用时 46 毫秒
101.
Barkleit A Tsushima S Savchuk O Philipp J Heim K Acker M Taut S Fahmy K 《Inorganic chemistry》2011,50(12):5451-5459
Thermodynamic parameters for the complexation of Eu(3+) with pyromellitic acid (1,2,4,5-benzenetetracarboxylic acid, BTC) as a model system for polymerizable metal-complexing humic acids were determined using temperature-dependent time-resolved laser-induced fluorescence spectroscopy (TRLFS) and isothermal titration calorimetry (ITC). At low metal and ligand concentrations (<50 μM Eu(3+), <1 mM BTC), a 1:1 monomeric Eu-BTC complex was identified in the range of 25-60 °C. At elevated concentrations (>500 μM Eu(3+) and BTC) a temperature-dependent polymerization was observed, where BTC monomers are linked via coordinating shared Eu(3+) ions. The two methods lead to comparable thermodynamic data (ΔH = 18.5 ± 1.5/16.5 ± 0.1 kJ mol(-1); ΔS = 152 ± 5/130 ± 5 J mol(-1) K(-1); TRLFS/ITC) in the absence of polymerization. With the onset of polymerization, TRLFS reveals the water coordination number of the lanthanide, whereas calorimetry is superior in determining the thermodynamic data in this regime. Evaluating the heat uptake kinetics, the monomer and polymer formation steps could be separated by "time-resolved" ITC, revealing almost identical binding enthalpies for the sequential reactions. Structural features of the complexes were studied by Fourier-transform infrared (FTIR) spectroscopy in combination with density functional theory (DFT) calculations showing predominantly chelating coordination with two carboxylate groups in the monomeric complex and monodentate binding of a single carboxylate group in the polymeric complex of the polycarboxylate with Eu(3+). The data show that pyromellitic acid is a suitable model for the study of metal-mediated polymerization as a crucial factor in determining the effect of humic acids on the mobility of heavy metals in the environment. 相似文献
102.
Hossini I Harrad MA Ait Ali M El Firdoussi L Karim A Valerga P Puerta MC 《Molecules (Basel, Switzerland)》2011,16(7):5886-5895
Friedel-Craft acylation at 100 °C of 2,5,9,9-tetramethyl-6,7,8,9-tetrahydro-5H-benzocycloheptene [ar-himachalene], a sesquiterpenic hydrocarbon obtained by catalytic dehydrogenation of α-, β- and γ-himachalenes, produces a mixture of two compounds: (3,5,5,9-tetramethyl-6,7,8,9-tetrahydro-5H-benzocyclohepten-2-yl)-ethanone (2, in 69% yield), with a conserved reactant backbone, and 3, with a different skeleton, in 21% yield. The crystal structure of 3 reveals it to be 1-(8-ethyl-8-hydroperoxy-3,5,5-trimethyl-5,6,7,8-tetrahydronaphthalen-2-yl)-ethanone. In this compound O-H…O bonds form dimers. These hydrogen-bonds, in conjunction with weaker C-H…O interactions, form a more extended supramolecular arrangement in the crystal. 相似文献
103.
Harrad MA Valerga P Puerta MC Houssini I Ali MA El Firdoussi L Karim A 《Molecules (Basel, Switzerland)》2011,16(1):367-372
A recyclable catalyst, Ni(0)-CMC-Na, composed of nickel colloids dispersed in a water soluble bioorganic polymer, sodium carboxymethylcellulose (CMC-Na), was synthesized by a simple procedure from readily available reagents. The catalyst thus obtained is stable and highly active in alkene hydrogenations. 相似文献
104.
Hosni K Msaâda K Ben Taârit M Chahed T Marzouk B 《Natural product communications》2011,6(11):1731-1734
The chemical composition of the essential oils obtained by hydrodistillation from the aerial parts of the Tunisian Hypericum perforatum and H. ericoides ssp. roberti was elucidated by a combination of GC and GC-MS analyses. The main constituents of the oil of H. perforatum were alpha-pinene (11.8%), alpha-ylangene (10.4%), germacrene-D (9.5%), n-octane (6.5%) and alpha-selinene (5.9%). The oil of H. ericoides ssp. roberti exhibited a higher amount of aliphatic and branched hydrocarbons and the main constituents were n-octane (29.1%), alpha-pinene (10.9%), pulegone (7.7%) and acetophenone (7%). Both qualitative and quantitative differences were observed between the studied oils. This chemical variability seems likely to result from the genetic variability, since samples of both species were collected at the same location and processed under the same conditions. 相似文献
105.
Zahra Kamali Bahram Khani Robati Karim Hedayatian 《Czechoslovak Mathematical Journal》2011,61(2):551-563
In this paper, we discuss the hypercyclicity, supercyclicity and cyclicity of the adjoint of a weighted composition operator
on a Hilbert space of analytic functions. 相似文献
106.
The ruled residue theorem characterises residue field extensions for valuations on a rational function field. Under the assumption that the characteristic of the residue field is different from 2 this theorem is extended here to function fields of conics. The main result is that there is at most one extension of a valuation on the base field to the function field of a conic for which the residue field extension is transcendental but not ruled. Furthermore the situation when this valuation is present is characterised. 相似文献
107.
F. A. Nour El-Dien 《光谱学快报》2013,46(3):347-357
Abstract Novel reaction between trioxalatoferrate (III) complex (A) and some di- and trihydric phenols were studied and found to form interesting mixed ligand chelates of iron (III) in the ratio 1:2:1 (Fe : oxalate : phenol) forming blue to violet colors at pH 4.0 to 6.5 and λmax = 580 to 590 nm. These reactions were used for indirect volumetric and spectrophotometric microdetermination of catechol (PC), pyrogallol (PG), dopamine hydrochloride (DHCl), adrenaline hydrogen tartrate (AHT) and sulbutamol sulfate (SS) via EDTA titration using complex (A) as an indicator. PC, DHC1 and PG were determined by EDTA titration within the concentration ranges of 0.55-2.2, 0.95-3.79 and 0.65-2.52 μg ml?1, respectively. AHT was determined in the concentration range of 96 to 204 μg ml?1 and SS was also determined in the range 5.75 to 57.7 μg ml?1. Adrenaline in ampoules coming from two Egyptian companies: sulbutamol in sulbovent liquids, and dopamine in five urine samples of Egyptian tumor patients, was also determined using the suggested procedure with high accuracy. 相似文献
108.
109.
An asymptotic formula for the voltage potential in the case of a near-surface conductivity inclusion
Habib Ammari Hyeonbae Kang Karim Touibi 《Zeitschrift für Angewandte Mathematik und Physik (ZAMP)》2006,57(2):234-243
We rigorously derive an asymptotic expansion of the steady-state voltage potentials in the presence of a conductivity inclusion
of small volume that is close to a planar surface. This new formula is motivated by the practically important inverse problem
of imaging a conductivity inclusion near a planar interface.
Partly supported by ACI Jeunes Chercheurs (0693) from the Ministry of Education and Scientific Research, France.
Partially supported by grant R02-2003-000-10012-0 from the Korea Science and Engineering Foundation.
Received: December 2, 2003; revised: January 18, 2004 相似文献
110.
Karim Kadir Tetsuo Sakai Itsuki Uehara Lars Eriksson 《Acta Crystallographica. Section C, Structural Chemistry》2001,57(9):999-1000
Yttrium tricopper dialuminium, YCu3Al2, is isostructural with hexagonal CaCu5, in which each Cu atom at the 3g(½,0,½) position in space group P6/mmm (No. 191) is partially replaced by an Al atom. The hydrogen‐uptake properties are usually enhanced in other AB5 structures by aluminium substitution. YCu5 does not show any hydrogen absorption, and the goal of the present work is to investigate whether aluminium substitution could expand the metal‐atom lattice enough to provide better interstitial positions for hydrogen storage. However, no enthalpy change was observed up to 773 K under 3 MPa static H2 pressure by differential thermal analysis (DTA) for the title compound. The compound does not show any significant hydrogen absorption/desorption in the pressure‐composition isotherms (P–C–T diagrams) in the temperature range 298–673 K under 3.3 MPa H2 pressure. 相似文献