全文获取类型
收费全文 | 4048篇 |
免费 | 93篇 |
国内免费 | 19篇 |
专业分类
化学 | 2865篇 |
晶体学 | 33篇 |
力学 | 79篇 |
数学 | 549篇 |
物理学 | 634篇 |
出版年
2023年 | 15篇 |
2022年 | 65篇 |
2021年 | 74篇 |
2020年 | 59篇 |
2019年 | 46篇 |
2018年 | 50篇 |
2017年 | 33篇 |
2016年 | 104篇 |
2015年 | 91篇 |
2014年 | 99篇 |
2013年 | 236篇 |
2012年 | 260篇 |
2011年 | 302篇 |
2010年 | 175篇 |
2009年 | 145篇 |
2008年 | 267篇 |
2007年 | 274篇 |
2006年 | 246篇 |
2005年 | 224篇 |
2004年 | 163篇 |
2003年 | 136篇 |
2002年 | 135篇 |
2001年 | 74篇 |
2000年 | 57篇 |
1999年 | 56篇 |
1998年 | 33篇 |
1997年 | 48篇 |
1996年 | 53篇 |
1995年 | 44篇 |
1994年 | 41篇 |
1993年 | 40篇 |
1992年 | 49篇 |
1991年 | 25篇 |
1990年 | 21篇 |
1989年 | 20篇 |
1988年 | 15篇 |
1987年 | 22篇 |
1986年 | 18篇 |
1985年 | 40篇 |
1984年 | 30篇 |
1983年 | 25篇 |
1982年 | 29篇 |
1981年 | 31篇 |
1980年 | 18篇 |
1979年 | 19篇 |
1978年 | 14篇 |
1977年 | 18篇 |
1976年 | 22篇 |
1975年 | 15篇 |
1973年 | 18篇 |
排序方式: 共有4160条查询结果,搜索用时 0 毫秒
21.
Bogumil Brzezinski Zbigniew Rozwadowski Teresa Dziembowska Georg Zundel 《Journal of Molecular Structure》1998,440(1-3):73-79
Three di-Schiff bases of 2-hydroxy-5-methyl-isophthalaldehyde with 4-R-anilines (R=H, CH3, OCH3) and their 1:1 complexes with HClO4 were studied by FT-IR, 1H, and 13C NMR spectroscopy in acetonitrile and [2H3]acetonitrile solutions, respectively. In di-Schiff bases intramolecular OH…N hydrogen bonds have been detected; however, they show no proton polarizability. Hydrogen-bonded systems with fast proton fluctuation and large proton polarizability have been found in the 1:1 complexes of di-Schiff bases with HClO4. 相似文献
22.
Serrano MC Portolés MT Vallet-Regí M Izquierdo I Galletti L Comas JV Pagani R 《Macromolecular bioscience》2005,5(5):415-423
Tissue engineering offers the potential of providing vessels that can be used to replace diseased and damaged native blood vessels. The endothelization of a synthetic vascular graft minimizes the failures associated with blood clotting and platelet activation. The aim of this study was to culture vascular-derived endothelial and smooth muscle cells on both untreated and NaOH-treated poly(epsilon-caprolactone) (PCL) films, a biocompatible and bio-resorbable polymer, and to evaluate the behavior of both cell types as a preliminary study for vascular graft development. PCL films were prepared by hot pressing; characterized by DSC, IR, SEM, and scanning force microscopy; and treated with NaOH to increase the surface hydrophilicity before cell culture. Endothelial and smooth muscle cells, isolated from pig cava vein, were characterized by immunofluorescence and confocal microscopy studies of endothelial nitric oxide synthase and alpha-smooth muscle actin. Good adhesion, growth, viability and morphology of both the endothelial and smooth muscle cells on PCL films were obtained, but a light stimulation of mitochondrial activity was observed during short culture times. NaOH treatment improved the adhesion and enhanced the proliferation in both cell types. This verified the possible use of this modified polymer as a support in the preparation of a synthetic vascular graft. [Diagram: see text] SEM micrograph of smooth muscle cells cultured on NaOH-treated PCL film. (Original magnification: 1000x). 相似文献
23.
Stability constant for mercury binding by commercial and natural humic acids (HA) were determined using a new potentiometric mercury(II) sensor based on dithiosalicylic acid modified carbon paste electrode. The sensor present a high selective and sensitive response to mercury(II) ions, and a low detection limit of 1.8×10?8 M. The potentiometric titrations curves of humic acids against mercury(II) ions were modeled. For 1.00×10?7 to 3.00×10?4 M mercury(II) ion concentration levels the results are consistent with the presence of two different binding sites in the humic acid macromolecule. The strongest binding sites (log K1 ranging from 10.1 to 6.8) are probably due to interaction with carboxylic acid and amine groups in the molecule, whereas weakest binding sites (log K2 ranging from 8.8 to 4.5) can be associated to phenolic groups. 相似文献
24.
Barberá J Puig L Romero P Serrano JL Sierra T 《Journal of the American Chemical Society》2005,127(1):458-464
The work described here concerns a challenge of general interest in supramolecular chemistry: the achievement of chiral helical organizations with controlled structures. This work provides a strategy to obtain supramolecular polymers in which a chiral helical conformation has been induced by a noncovalent association, that is, through hydrogen bonding. Polycatenar 2,4,6-triarylamino-1,3,5-triazines, which organize into columnar mesophases and are susceptible to H-bonding interactions, were chosen as a starting point to build up the chiral supramolecular structure. The stacking of these mesogens has been forced to wind in a helical way by means of H-bond association with (R)-3-methyladipic acid, within the mesophase. The optically active columnar organization has been studied in depth by optical microscopy, differential scanning calorimetry (DSC), X-ray diffraction, and circular dichroism. Formation of stable complexes between the triazine units and (R)-3-methyladipic acid has also been investigated by means of NMR diffusion-ordered spectroscopy (DOSY) experiments in chloroform. 相似文献
25.
Determination of four nitroimidazole residues in poultry meat by liquid chromatography-mass spectrometry 总被引:8,自引:0,他引:8
A multi-residue liquid chromatography-mass spectrometry (LC-MS) assay method is described for the determination of four nitroimidazoles in poultry muscle. The extraction procedure is based on liquid-liquid extraction with ethyl acetate followed by an evaporation step. A deuterated internal standard is used. The LC separation was made on a C18 bonded silica column with an aqueous formic acid (0.2%) solution-methanol-acetonitrile (81:13:6) mobile phase. Following electrospray ionization, the protonated molecular ion [M+H]+ is obtained for each compound. Monitoring several ions for each nitroimidazole provides the specificity required for confirmatory assay. Validation of the method was performed to estimate linearity, intra-day and inter-day repeatability, accuracy and detection limit. The present method is capable of identifying nitroimidazole residues in muscle at levels below 5 microg/kg. 相似文献
26.
Orellana M Arriola P Del Río R Schrebler R Cordova R Scholz F Kahlert H 《The journal of physical chemistry. B》2005,109(32):15483-15488
During the electrochemical oxidation of Prussian blue (PB) to Prussian yellow (PY), an electrocatalytic oxygen production proceeds at the electrode when aqueous electrolyte solutions are used. The formed oxygen is scavenged by the PY, probably by absorption, and it is consumed during the electrochemical reduction of PY to PB by a heterogeneous chemical reaction of PB with oxygen to PY and hydrogen peroxide. Because of this catalytic regeneration of PY, it is impossible to determine the amount of low-spin iron by chronocoulometry using a potential program in which PB is first oxidized to PY and then the charge is measured to reduce PY to PB. The latter charge is biased by the electrocatalytic PY regeneration. 相似文献
27.
Optimisation of Supercritical Carbon Dioxide Systems for Complexation of Naproxen : Beta-Cyclodextrin 总被引:1,自引:0,他引:1
Susana Junco Teresa Casimiro Nuno Ribeiro Manuel Nunes Da Ponte Helena M. Cabral Marques 《Journal of inclusion phenomena and macrocyclic chemistry》2002,44(1-4):69-73
Supercritical carbon dioxide (scCO2)offers several attractive scenarios for thepharmaceutical processing as an alternativeto aqueous and organic solvents. In thiswork naproxen, a widely used non steroidalanti-inflammatory drug with analgesic andanti-inflammatory properties, was chosenas a model drug. Its complexation with cyclodextrinsimproves the rate and extent of dissolutionof the drug, increase its rate of absorption and mayreduce the unpleasant side-effects of the drug.The interest in using this supercritical technologyled us to develop an experimental unit for the useof supercritical CO2 as a processing medium forthe complexation of naproxen with beta cyclodextrin (CD). 相似文献
28.
Antonio Doménech María Teresa Doménech‐Carbó Howell G. M. Edwards 《Electroanalysis》2007,19(18):1890-1900
Multivariate chemometric methods are applied for identifying earth pigments from square‐wave voltammetric measurements performed at pigment‐modified paraffin‐impregnated graphite electrodes allowing for a separation between hematite‐based earths, French ochres, Spanish ochres, siennas, umbers and green earths. This methodology is applied to the identification of the pigments in samples from the ceiling frescoes of Antonio Palomino (dated 1707) in the vaulted nave of the Sant Joan del Mercat church in Valencia (Spain). These frescoes suffered considerable damage by fire during the Spanish Civil War in 1936, resulting in severe chemical and chromatic alterations. Electrochemical data, supported by scanning electron microscopy/energy dispersive X‐ray microanalysis and Raman spectroscopy, indicate that thermal stress induced the transformation of goethite‐based yellow ochres into hematite and magnetite. 相似文献
29.
Angel Alberola Luis Calvo Teresa Rodríguez Rodríguez Carmen Saudo 《Journal of heterocyclic chemistry》1992,29(2):445-450
The reactivity of 3-methyl-5-phenylisoxazole against electrophilic compounds in the presence of different bases is studied. With n-BuLi, alkylated products at C-4 position and C-3 methyl group, and, in a few cases, dialkylated isoxazoles are obtained. When the reactions are carried out with LICA, the nature of the alkylated products depends on the alkyl halide used. By using LICA-TMEDA, as deprotonating system, regio-selective reaction at the C-3 methyl group is found. 相似文献
30.
13-Hydroxyacenaphtho[1,2-b]quinolizinium bromide (13-HQBr)was selected as a fluorescence indicator to determine basic compounds in non-aqueous media. This compound possesses an acidic phenolic hydroxyl group. It presents varying absorption (ROH, 408, 430 nm; RO(-) 456, 478 nm) and excitation spectra (ROH, 425 nm; RO, 471 nm) depending on the pH of the media, but the same emission fluorescence spectrum (ROH = RO(-), 526 nm) at different pH in buffered aqueous solutions. However, in acidic non-aqueous media (acetic, formic and trifluoroacetic acids), it can be observed that the fluorescence emission spectra differ for the ionized (lambda(em) = 530 nm) and non-ionized (lambda(em) = 440, 470 nm) forms. The fluorescence intensity at the characteristic peaks depends on the acid-base equilibria in the ground and excited states. Therefore, this property could be used to evaluate the concentration of basic compounds, showing a good linearity range between fluorescence intensity and basic sample concentration. 相似文献