全文获取类型
收费全文 | 920篇 |
免费 | 32篇 |
国内免费 | 6篇 |
专业分类
化学 | 562篇 |
晶体学 | 5篇 |
力学 | 41篇 |
数学 | 131篇 |
物理学 | 219篇 |
出版年
2023年 | 8篇 |
2022年 | 11篇 |
2021年 | 35篇 |
2020年 | 27篇 |
2019年 | 22篇 |
2018年 | 18篇 |
2017年 | 13篇 |
2016年 | 45篇 |
2015年 | 34篇 |
2014年 | 30篇 |
2013年 | 72篇 |
2012年 | 62篇 |
2011年 | 50篇 |
2010年 | 48篇 |
2009年 | 51篇 |
2008年 | 94篇 |
2007年 | 79篇 |
2006年 | 54篇 |
2005年 | 28篇 |
2004年 | 32篇 |
2003年 | 16篇 |
2002年 | 25篇 |
2001年 | 16篇 |
2000年 | 10篇 |
1999年 | 8篇 |
1998年 | 4篇 |
1997年 | 9篇 |
1996年 | 8篇 |
1995年 | 5篇 |
1994年 | 6篇 |
1993年 | 4篇 |
1992年 | 1篇 |
1991年 | 4篇 |
1990年 | 3篇 |
1989年 | 1篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 4篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1970年 | 1篇 |
排序方式: 共有958条查询结果,搜索用时 375 毫秒
81.
82.
83.
Luminescent Di‐ and Trinuclear Boron Complexes Based on Aromatic Iminopyrrolyl Spacer Ligands: Synthesis,Characterization, and Application in OLEDs 下载免费PDF全文
Dr. D. Suresh Dr. Clara S. B. Gomes Patrícia S. Lopes Dr. Cláudia A. Figueira Bruno Ferreira Prof. Pedro T. Gomes Dr. Roberto E. Di Paolo Prof. António L. Maçanita Prof. M. Teresa Duarte Dr. Ana Charas Prof. Jorge Morgado Diogo Vila‐Viçosa Prof. Maria José Calhorda 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(25):9133-9149
New bis‐ and tris(iminopyrrole)‐functionalized linear (1,2‐(HNC4H3‐C(H)?N)2‐C6H4 ( 2 ), 1,3‐(HNC4H3‐C(H)?N)2‐C6H4 ( 3 ), 1,4‐(HNC4H3‐C(H)?N)2‐C6H4 ( 4 ), 4,4′‐(HNC4H3‐C(H)?N)2‐(C6H4‐C6H4) ( 5 ), 1,5‐(HNC4H3C‐(H)?N)2‐C10H6 ( 6 ), 2,6‐(HNC4H3C‐(H)?N)2‐C10H6 ( 7 ), 2,6‐(HNC4H3C‐(H)?N)2‐C14H8 ( 8 )) and star‐shaped (1,3,5‐(HNC4H3‐C(H)?N‐1,4‐C6H4)3‐C6H3 ( 9 )) π‐conjugated molecules were synthesized by the condensation reactions of 2‐formylpyrrole ( 1 ) with several aromatic di‐ and triamines. The corresponding linear diboron chelate complexes (Ph2B[1,3‐bis(iminopyrrolyl)‐phenyl]BPh2 ( 10 ), Ph2B[1,4‐bis(iminopyrrolyl)‐phenyl]BPh2 ( 11 ), Ph2B[4,4′‐bis(iminopyrrolyl)‐biphenyl]BPh2 ( 12 ), Ph2B[1,5‐bis(iminopyrrolyl)‐naphthyl]BPh2 ( 13 ), Ph2B[2,6‐bis(iminopyrrolyl)‐naphthyl]BPh2 ( 14 ), Ph2B[2,6‐bis(iminopyrrolyl)‐anthracenyl]BPh2 ( 15 )) and the star‐shaped triboron complex ([4′,4′′,4′′′‐tris(iminopyrrolyl)‐1,3,5‐triphenylbenzene](BPh2)3 ( 16 )) were obtained in moderate to good yields, by the treatment of 3 – 9 with B(C6H5)3. The ligand precursors are non‐emissive, whereas most of their boron complexes are highly fluorescent; their emission color depends on the π‐conjugation length. The photophysical properties of the luminescent polyboron compounds were measured, showing good solution fluorescence quantum yields ranging from 0.15 to 0.69. DFT and time‐dependent DFT calculations confirmed that molecules 10 and 16 are blue emitters, because only one of the iminopyrrolyl groups becomes planar in the singlet excited state, whereas the second (and third) keeps the same geometry. Compound 13 , in which planarity is not achieved in any of the groups, is poorly emissive. In the other examples ( 11 , 12 , 14 , and 15 ), the LUMO is stabilized, narrowing the gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO–LUMO), and the two iminopyrrolyl groups become planar, extending the size of the π‐system, to afford green to yellow emissions. Organic light‐emitting diodes (OLEDs) were fabricated by using the new polyboron complexes and their luminance was found to be in the order of 2400 cd m?2, for single layer devices, increasing to 4400 cd m?2 when a hole‐transporting layer is used. 相似文献
84.
This paper describes the automation of a fully electrochemical system for preconcentration, cleanup, separation and detection, comprising the hyphenation of a thin layer electrochemical flow cell with CE coupled with contactless conductivity detection (CE-C?D). Traces of heavy metal ions were extracted from the pulsed-flowing sample and accumulated on a glassy carbon working electrode by electroreduction for some minutes. Anodic stripping of the accumulated metals was synchronized with hydrodynamic injection into the capillary. The effect of the angle of the slant polished tip of the CE capillary and its orientation against the working electrode in the electrochemical preconcentration (EPC) flow cell and of the accumulation time were studied, aiming at maximum CE-C?D signal enhancement. After 6 min of EPC, enhancement factors close to 50 times were obtained for thallium, lead, cadmium and copper ions, and about 16 for zinc ions. Limits of detection below 25 nmol/L were estimated for all target analytes but zinc. A second separation dimension was added to the CE separation capabilities by staircase scanning of the potentiostatic deposition and/or stripping potentials of metal ions, as implemented with the EPC-CE-C?D flow system. A matrix exchange between the deposition and stripping steps, highly valuable for sample cleanup, can be straightforwardly programmed with the multi-pumping flow management system. The automated simultaneous determination of the traces of five accumulable heavy metals together with four non-accumulated alkaline and alkaline earth metals in a single run was demonstrated, to highlight the potentiality of the system. 相似文献
85.
Ana C. Gomes Rita P. Lopes Isabel S. Gonçalves 《Journal of organometallic chemistry》2011,696(22):3543-3550
The ligand N-(n-propyl)-2-pyridylmethanimine (pyim) and an immobilised analogue of this ligand (MCM-41-pyim) were prepared by the condensation reaction of 2-pyridinecarboxaldehyde with either propylamine or aminopropyl groups covalently attached to the ordered mesoporous silica MCM-41. Free and immobilised tetracarbonyl complexes of the type cis-[Mo(CO)4(L)] (L = pyim (1), MCM-41-pyim) were then prepared by microwave-assisted heating of a mixture of Mo(CO)6 and the organic ligand or ligand-silica in toluene at 110 °C for 30-45 min. The products were characterised by NMR spectroscopy (1H, 13C and 29Si, in solution and in the solid state), elemental analysis, N2 adsorption, and FT-IR spectroscopy. When used as catalyst precursors for the epoxidation of cis-cyclooctene by tert-butylhydroperoxide at 55 °C (1 mol% catalyst (Mo), no additional co-solvent), 1,2-epoxy-cyclooctane was obtained as the only reaction product in quantitative yield after 5 h for 1 and 36% yield after 24 h for the supported complex. The use of the ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate as co-solvent led to lower catalytic activities (epoxide selectivity was always 100%) but allowed the catalyst/IL mixtures (homogeneous mixture for IL+1 and a biphasic solid + IL system for IL+MCM-41-pyim/Mo) to be easily recovered and reused in subsequent runs without loss of catalytic performance. 相似文献
86.
Marcelo Kobelnik Douglas Lopes Cassimiro Diógenesdos Dos Santos Clóvis Augusto Ribeiro Marisa Spirandeli Crespi 《中国化学》2011,29(11):2271-2277
Solid state compounds of general formula Sr(Diclof)2·5.7H2O and Ba(Diclof)2·4.8H2O were obtained. Thermogravimetry (TG), differential scanning calorimetry (DSC), X‐ray diffraction powder patterns and microscopy analysis were used to characterize these compounds. Details concerning the dehydration and thermal decomposition as well as data of kinetic parameters have been described here. The kinetic behaviors of these stages were evaluated from several heating rates with samples masses of 2 and 5 mg in open crucibles under a nitrogen atmosphere. The results of the present study improve the knowledge on these compounds including their dehydration and thermal stability. The obtained data depend on the mass of the sample which results in two kinetic behavior patterns. 相似文献
87.
Paulina V. F. Santos Ilanna C. Lopes Victor C. Diculescu Mário César U. de Araújo Ana Maria Oliveira‐Brett 《Electroanalysis》2011,23(10):2310-2319
The electrochemical behaviour of Nodularin (NOD), a hepatotoxic cyclic pentapeptide, was studied at a glassy carbon electrode. NOD electrochemical oxidation is an irreversible, pH‐independent process, involving the transfer of one electron. Upon incubation in different pH electrolytes, chemical degradation of NOD was electrochemically detected by the appearance of a new oxidation peak. The chemically degraded NOD (cdNOD), undergoes an irreversible, pH‐dependent oxidation, and its redox products are reversibly oxidised. The charge transfer properties of cdNOD as well as of its redox metabolites were investigated. Mechanisms for NOD oxidation, NOD chemical degradation and oxidation of cdNOD and its metabolites were proposed. 相似文献
88.
Araújo MJ Mattar NS Reis AS Serra IC Fialho EM Assunção AK Dutra RP Nogueira AM Libério SA Guerra RN Lopes AS Ribeiro MN Nascimento FR 《Natural product research》2011,25(11):1037-1046
The propolis of Scaptotrigona aff. postica is popularly used in Maranh?o State, Brazil, for treating wounds and respiratory illnesses. Nevertheless, little is known about the chemical composition of this propolis and the adverse effects of its use. Hence, this study is a pharmacognostic characterisation of the propolis hydroalcoholic extract (PHE) from S. aff. postica. The methodology consisted of an evaluation of the sensory and chemical parameters. Chemical analysis of PHE indicated high concentrations of phenolic and triterpens substances, and the absence of steroids. Additionally, we evaluated the acute toxicity of propolis using 48 Swiss male and female mice. The animals received single doses of PHE (1000, 2000 or 4000 mg kg?1) orally and were observed for 14 days. After this period, the mice were sacrificed and the blood was used for biochemical and haematological evaluation. PHE did not induce any death, and the acute treatment significantly reduced serum concentrations of alanine aminotransferase and alkaline phosphatase. The resultant data indicate that PHE from S. aff. postica has low toxicity when used orally, even in high doses. 相似文献
89.
Serrano A Sousa MM Hallett J Lopes JA Oliveira MC 《Analytical and bioanalytical chemistry》2011,401(2):735-743
A new analytical approach based on high-performance liquid chromatography with diode array detector (HPLC-DAD) and multivariate
data analysis was applied and assessed for analyzing the red dye extracted from cochineal insects, used in precious historical
textiles. The most widely used method of analysis involves quantification of specific minor compounds (markers), using HPLC-DAD.
However, variation in the cochineal markers concentration, use of aggressive dye extraction methods and poor resolution of
HPLC chromatograms can compromise the identification of the precise insect species used in the textiles. In this study, a
soft extraction method combined with a new dye recovery treatment was developed, capable of yielding HPLC chromatograms with
good resolution, for the first time, for historical cochineal-dyed textiles. After principal components analysis (PCA) and
mass spectrometry (MS), it was possible to identify the cochineal species used in these textiles, in contrast to the accepted
method of analysis. In order to compare both methodologies, 7 cochineal species and 63 historical cochineal insect specimens
were analyzed using the two approaches, and then compared with the results for 15 historical textiles in order to assess their
applicability to real complex samples. The methodology developed here was shown to provide more accurate and consistent information
than the traditional method. Almost all of the historical textiles were dyed with Porphyrophora sp. insects. These results emphasize the importance of adopting the proposed methodology for future research on cochineal
(and related red dyes). Mild extraction methods and HPLC-DAD/MSn analysis yield distinctive profiles, which, in combination with a PCA reference database, are a powerful tool for identifying
red insect dyes. 相似文献
90.
Milton C. Lopes Filho Evelyne Miot Helena J. Nussenzveig Lopes 《Journal of Nonlinear Science》2011,21(5):685-703
The vortex-wave system is a coupling of the two-dimensional vorticity equation with the point-vortex system. It is a model
for the motion of a finite number of concentrated vortices moving in a distributed vorticity background. In this article,
we prove existence of a weak solution to this system with an initial background vorticity in L
p
, p>2, up to the time of first collision of point vortices. 相似文献