全文获取类型
收费全文 | 395篇 |
免费 | 14篇 |
专业分类
化学 | 247篇 |
晶体学 | 4篇 |
力学 | 25篇 |
数学 | 57篇 |
物理学 | 76篇 |
出版年
2023年 | 4篇 |
2022年 | 11篇 |
2021年 | 10篇 |
2020年 | 14篇 |
2019年 | 11篇 |
2018年 | 13篇 |
2017年 | 10篇 |
2016年 | 16篇 |
2015年 | 12篇 |
2014年 | 10篇 |
2013年 | 36篇 |
2012年 | 30篇 |
2011年 | 30篇 |
2010年 | 20篇 |
2009年 | 18篇 |
2008年 | 25篇 |
2007年 | 28篇 |
2006年 | 26篇 |
2005年 | 16篇 |
2004年 | 10篇 |
2003年 | 13篇 |
2002年 | 9篇 |
2001年 | 9篇 |
2000年 | 10篇 |
1999年 | 4篇 |
1998年 | 2篇 |
1997年 | 1篇 |
1996年 | 1篇 |
1995年 | 1篇 |
1994年 | 2篇 |
1992年 | 1篇 |
1990年 | 1篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1976年 | 1篇 |
排序方式: 共有409条查询结果,搜索用时 15 毫秒
81.
We study the hub covering problem which, so far, has remained one of the unstudied hub location problems in the literature. We give a combinatorial and a new integer programming formulation of the hub covering problem that is different from earlier integer programming formulations. Both new and old formulations are nonlinear binary integer programs. We give three linearizations for the old model and one linearization for the new one and test their computational performances based on 80 instances of the CAB data set. Computational results indicate that the linear version of the new model performs significantly better than the most successful linearization of the old model both in terms of average and maximum CPU times as well as in core storage requirements. 相似文献
82.
Karlo Jurica Vesna Benković Sunčana Sikirić Irena Brčić Karačonji Nevenka Kopjar 《Natural product research》2020,34(16):2354-2357
Abstract Although strawberry tree (Arbutus unedo L.) leaves have long been used as a herbal remedy, insufficient information is available on their nephrotoxicity. We assessed the safety of strawberry tree water leaf extract and its key component arbutin, administered per os to Lewis rats of both genders at 200?mg/kg b.w./day for 14 and 28 days. The effects of the tested compounds on DNA integrity in renal cells was evaluated using alkaline comet assay, while kidney function was studied using serum creatinine and urea levels. Strawberry tree water leaf extract showed high biocompatibility with kidney tissue. It did not impair DNA integrity of renal cells and kidney function, either in male or female rats. However, exposure to single arbutin affected the levels of primary DNA damage in renal cells which could be related to metabolic conversion of arbutin into hydroquinone, whose nephrotoxicity has previously been proven. 相似文献
83.
Masgrau L Ranaghan KE Scrutton NS Mulholland AJ Sutcliffe MJ 《The journal of physical chemistry. B》2007,111(11):3032-3047
Proton tunneling dominates the oxidative deamination of tryptamine catalyzed by the enzyme aromatic amine dehydrogenase. For reaction with the fast substrate tryptamine, a H/D kinetic isotope effect (KIE) of 55 +/- 6 has been reported-one of the largest observed in an enzyme reaction. We present here a computational analysis of this proton-transfer reaction, applying combined quantum mechanics/molecular mechanics (QM/MM) methods (PM3-SRP//PM3/CHARMM22). In particular, we extend our previous computational study (Masgrau et al. Science 2006, 312, 237) by using improved energy corrections, high-level QM/MM methods, and an ensemble of paths to estimate the tunneling contributions. We have carried out QM/MM molecular dynamics simulations and variational transition state theory calculations with small-curvature tunneling corrections. The results provide detailed insight into the processes involved in the reaction. Transfer to the O2 oxygen of the catalytic base, Asp128beta, is found to be the favored reaction both thermodynamically and kinetically, even though O1 is closer in the reactant complex. Comparison of quantum and classical models of proton transfer allows estimation of the contribution of hydrogen tunneling in lowering the barrier to reaction in the enzyme. A reduction of the activation free energy due to tunneling of 3.1 kcal mol-1 is found, which represents a rate enhancement due to tunneling by 2 orders of magnitude. The calculated KIE of 30 is significantly elevated over the semiclassical limit, in agreement with the experimental observations; a semiclassical value of 6 is obtained when tunneling is omitted. A polarization of the C-H bond to be broken is observed due to the close proximity of the catalytic aspartate and the (formally) positively charged imine nitrogen. A comparison is also made with the related quinoprotein methylamine dehydrogenase (MADH)-the much lower KIE of 11 that we obtain for the MADH/methylamine system is found to arise from a more endothermic potential energy surface for the MADH reaction. 相似文献
84.
Kara E. Humanski Santanna D. Patterson Paul J. Pecorin Madeline M. Perfitt Janet Romero William R. Sassack 《高分子科学杂志,A辑:纯化学与应用化学》2013,50(10):709-717
AbstractNovel trisubstituted ethylenes, ring-substituted propyl 2-cyano-3-phenyl-2-propenoates, RPhCH=C(CN)CO2C3H7 (where R is 4-chloro-3-fluoro, 2-fluoro-5-iodo, 2-fluoro-6-iodo, 2-trifluoromethyl, 3-trifluoromethyl, 4-trifluoromethyl, 2-trifluoromethoxy, 3-trifluoromethoxy, and 4-trifluoromethoxy) were prepared and copolymerized with styrene. The ethylenes were synthesized by the piperidine catalyzed Knoevenagel condensation of ring-substituted benzaldehydes and propyl cyanoacetate and characterized by CHN analysis, FTIR, 1H and 13C NMR. All the ethylenes were copolymerized with styrene in solution with radical initiation (ABCN) at 70?°C. The composition of the copolymers was calculated from nitrogen analysis, and the structures were analyzed by FTIR, 1H and 13C NMR. Decomposition of the copolymers in nitrogen (TGA) occurred in two steps, first in the 250–500?°C range with residue (0.5–3.1% wt.), which then decomposed in the 500–700?°C range. 相似文献
85.
Mohammed Kara Amine Assouguem Omkulthom Mohamed Al kamaly Safa Benmessaoud Hamada Imtara Hamza Mechchate Christophe Hano Abdou Rachid Zerhouni Jamila Bahhou 《Molecules (Basel, Switzerland)》2021,26(18)
Apple vinegar is a natural product widely used in food and traditional medicine as it contains many bioactive compounds. The apple variety and production methods are two factors that play a major role in determining the quality of vinegar. Therefore, this study aims to determine the quality of apple vinegar samples from different varieties (Red Delicious, Gala, Golden Delicious, and Starking Delicious) prepared by three methods using small apple pieces, apple juice, and crushed apple, through determining the physicochemical properties and antibacterial activity of these samples. The antibacterial activity was studied against five pathogenic bacteria: Staphylococcus aureus, Klebsiella pneumonia, Escherichia coli (ATB: 57), Escherichia coli (ATB: 97), and Pseudomonas aeruginosa, using two methods, disk diffusion and microdilution, for determining the minimum inhibitory concentrations and the minimum bactericidal concentrations. The results of this study showed that the lowest pH value was 3.6 for Stark Delicious, obtained by liquid fermentation, and the highest acetic acid values were 4.7 and 4% for the vinegar of Red Delicious and Golden Delicious, prepared by solid fermentation, respectively. The results of the antibacterial activity showed considerable activity of apple vinegar on the tested strains. Generally, the Staphylococcus aureus strain appears less sensitive and Pseudomonas aeruginosa seems to be very sensitive against all samples, while the other strains have distinct sensitivities depending on the variety studied and the method used. A higher antibacterial activity was found in vinegar obtained by the apple pieces method and the Red Delicious variety, with a low MIC and MBC recorded, at 1.95 and 3.90 µL/mL, respectively. This study has shown that the choice of both apple variety and production method is therefore an essential step in determining and aiming for the desired quality of apple vinegar. 相似文献
86.
The TiO2 nanoparticles are electrospun with polyacrylonitrile (PAN) polymer solution onto the discharged battery coal (DBC) electrode and the results are evaluated as a supercapacitor. The morphology and chemical composition of the synthesized TiO2 nanoparticles and PAN+TiO2 nanocomposite fibers were characterized by Scanning electron microscopy, thermogravimetry and FTIR analysis. Supercapacitor measurements and electrochemical characterizations of the electrodes examined by cyclic voltammetry and electrochemical impedance spectroscopy. Electrochemical measurements showed that the best current value was obtained from PAN and TiO2 coated DBC. The performances of both PAN and PAN+TiO2 coated DBC electrodes were investigated as supercapacitors. PAN+TiO2/DBC showed the best specific capacitance value of 156.00 F g−1 and PAN/DBC showed 74.93 F g−1. In addition, PAN+TiO2/DBC exhibited reliable stability performance over 2000.00 cycles. 相似文献
87.
Claeyssens F Ranaghan KE Manby FR Harvey JN Mulholland AJ 《Chemical communications (Cambridge, England)》2005,(40):5068-5070
Multiple profiles for the reaction from chorismate to prephenate in the enzyme chorismate mutase calculated with hybrid density functional combined quantum mechanics/molecular mechanics methods (B3LYP/6-31G(d)-CHARMM27) agree well with experiment, and provide direct evidence of transition-state stabilization by this important enzyme, which is at the centre of current debates about the nature of enzyme catalysis. 相似文献
88.
Hunka DE Herman DC Lormand KD Jaramillo DM Land DP 《Langmuir : the ACS journal of surfaces and colloids》2005,21(18):8333-8337
The decomposition of 1,1-dichloroethene on Pd(111) is investigated using conventional thermal desorption, laser-induced thermal desorption (LITD), and FT reflection absorption infrared spectroscopy (FT-RAIRS). The decomposition mechanism produces at least three hydrocarbon surface intermediates, including ethylidyne. Thermal desorption results differ between high and low coverages because of relative surface concentrations of Cl and H in combination with kinetic effects. 相似文献
89.
The magnetic poly(ethylene glycol dimethacrylate-n-vinylimidazole) (Fe3O4@poly (EGDMA@VIM)) microspheres were prepared by suspension polymerization method using magnetite Fe3O4 nano-powder and the porosity, morphology, chemical composition and structure of the magnetic polymer microspheres were characterized. The specific surface area and swelling ratio of the Fe3O4@poly(EGDMA@VIM) microspheres were found to be 278.6?m2·g–1 and 48%, respectively. The Fe3O4@poly(EGDMA@VIM) microspheres were used as an adsorbent for phenol removal. The effects of the parameters such as adsorbent dosage, temperature, pH and initial concentration of phenol solutions on the adsorption were investigated. The experimental adsorption equilibrium data obtained were fitted with Langmuir, Freundlich and Dubinin-Radushkevich isotherms and the pseudo-first-order, pseudo-second-order and intra–particle diffusion kinetic models. The adsorption equilibrium data agreed well with the Freundlich isotherm and the pseudo-second-order kinetic model. The maximum capacity of the Fe3O4@poly(EGDMA@VIM) microspheres was calculated to be 33.83?mg·g–1 at 298?K and natural pH from Langmuir isotherm. The Fe3O4@poly(EGDMA@VIM) microspheres were found to be reusable for removal of phenol after desorption for several times. The result indicated that the Fe3O4@poly(EGDMA@VIM) microspheres are potential candidate for removal of phenol in wastewaters. 相似文献
90.
Asl Genolu Sarkaya Bilgen Osman Ali Kara Nurdan Pazarlioglu Necati Beirli 《Biomedical chromatography : BMC》2016,30(2):88-96
In this study, antimicrobial pigment cinnabarinic acid (CA) was produced from Pycnoporus cinnabarinus in laboratory‐scale batch cultures. Magnetic poly(ethylene glycol dimethacrylate‐N‐methacryloyl‐l‐tryptophan methyl ester) [m‐poly(EGDMA‐MATrp)] beads (average diameter = 53–103 µm) were synthesized by copolymerizing of N‐methacryloyl‐l‐tryptophan methyl ester (MATrp) with ethylene glycol dimethacrylate (EGDMA) in the presence of magnetite (Fe3O4) and used for the adsorption of CA. The m‐poly(EGDMA‐MATrp) beads were characterized by N2 adsorption/desorption isotherms (Brunauer Emmet Teller), X‐ray photoelecron spectroscopy, scanning electron microscopy, infrared spectroscopy, thermal gravimetric analysis, electron spin resonance and swelling studies. The efficiency of m‐poly(EGDMA‐MATrp) beads for separation of CA from culture fluid was evaluated. The effects of pH, initial concentration, contact time and temperature on adsorption were analyzed. The maximum CA adsorption capacity of the m‐poly(EGDMA‐MATrp) beads was 272.9 mg g−1 at pH 7.0, 25 °C. All the isotherm data can be fitted with the Langmuir, Freundlich and Dubinin–Radushkevich isotherm models. The adsorption process obeyed pseudo‐second‐order kinetic model. Thermodynamic parameters ΔH = 5.056 kJ mol−1, ΔS = 52.44 J K−1 mol−1 and ΔG = −9.424 kJ mol−1 to ‐11.27 kJ mol−1 with the rise in temperature from 4 to 40 °C indicated that the adsorption process was endothermic and spontaneous. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献