全文获取类型
收费全文 | 308篇 |
免费 | 19篇 |
国内免费 | 1篇 |
专业分类
化学 | 238篇 |
晶体学 | 3篇 |
力学 | 5篇 |
数学 | 21篇 |
物理学 | 61篇 |
出版年
2024年 | 1篇 |
2023年 | 6篇 |
2022年 | 14篇 |
2021年 | 24篇 |
2020年 | 9篇 |
2019年 | 15篇 |
2018年 | 12篇 |
2017年 | 11篇 |
2016年 | 19篇 |
2015年 | 18篇 |
2014年 | 14篇 |
2013年 | 18篇 |
2012年 | 30篇 |
2011年 | 18篇 |
2010年 | 17篇 |
2009年 | 18篇 |
2008年 | 15篇 |
2007年 | 14篇 |
2006年 | 17篇 |
2005年 | 8篇 |
2004年 | 6篇 |
2003年 | 4篇 |
2002年 | 4篇 |
2001年 | 1篇 |
2000年 | 2篇 |
1999年 | 3篇 |
1998年 | 1篇 |
1996年 | 1篇 |
1995年 | 2篇 |
1994年 | 1篇 |
1993年 | 1篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1984年 | 1篇 |
1970年 | 1篇 |
排序方式: 共有328条查询结果,搜索用时 406 毫秒
141.
142.
Wiebke Bedenknecht Guilherme Oliveira Mota Christian Reiher Mathias Schacht 《Journal of Graph Theory》2019,90(2):137-149
Erdős conjectured that every n-vertex triangle-free graph contains a subset of vertices that spans at most edges. Extending a recent result of Norin and Yepremyan, we confirm this conjecture for graphs homomorphic to so-called Andrásfai graphs. As a consequence, Erdős' conjecture holds for every triangle-free graph G with minimum degree and if the degree condition can be relaxed to . In fact, we obtain a more general result for graphs of higher odd-girth. 相似文献
143.
Adriano V. Reis Marcos R. Guilherme Thais A. Moia Luiz H. C. Mattoso Edvani C. Muniz Elias B. Tambourgi 《Journal of polymer science. Part A, Polymer chemistry》2008,46(7):2567-2574
Synthesis and characterization of a new hydrogel were carried out using a chemically modified starch (starch‐M) consisting of coupling C?C bounds coming from glycidil methacrylate (GMA) onto the polysaccharide structure. 13C NMR, 1H NMR, and FT‐IR spectroscopies were used to confirm the incorporation of such groups onto the starch‐M. The hydrogel was prepared by a crosslinking polymerization of starch‐M using sodium persulfate as an initiating agent. The starch‐M hydrogel shows morphology clearly different from that of the raw starch film due to the presence of voids on its surface. The swelling process of the starch‐M hydrogel was not significantly affected by changes on the temperature or on pH of the surrounding liquid, indicating the such behavior can be then understood by a diffusional process, resulting from its physical–chemical interactions with the solvent. The values of the diffusional exponent n were on the order of 0.45–0.49 for the range of pHs investigated, demonstrating that the water transport mechanism of starch‐M hydrogel is more dependent on Fickian diffusion, that is, controlled by water diffusion. Such starch‐M hydrogel is a promising candidate to be used in transporting and in preserving acid‐responsive drugs, such as corticoids, for the treatment of colon‐specific diseases, for example, Crohn's disease. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2567–2574, 2008 相似文献
144.
Smith SJ Peralta RA Jovito R Horn A Bortoluzzi AJ Noble CJ Hanson GR Stranger R Jayaratne V Cavigliasso G Gahan LR Schenk G Nascimento OR Cavalett A Bortolotto T Razzera G Terenzi H Neves A Riley MJ 《Inorganic chemistry》2012,51(4):2065-2078
A mixed-valence complex, [Fe(III)Fe(II)L1(μ-OAc)(2)]BF(4)·H(2)O, where the ligand H(2)L1 = 2-{[[3-[((bis(pyridin-2-ylmethyl)amino)methyl)-2-hydroxy-5-methylbenzyl](pyridin-2-ylmethyl)amino]methyl]phenol}, has been studied with a range of techniques, and, where possible, its properties have been compared to those of the corresponding enzyme system purple acid phosphatase. The Fe(III)Fe(II) and Fe(III)(2) oxidized species were studied spectroelectrochemically. The temperature-dependent population of the S = 3/2 spin states of the heterovalent system, observed using magnetic circular dichroism, confirmed that the dinuclear center is weakly antiferromagnetically coupled (H = -2JS(1)·S(2), where J = -5.6 cm(-1)) in a frozen solution. The ligand-to-metal charge-transfer transitions are correlated with density functional theory calculations. The Fe(III)Fe(II) complex is electron paramagnetic resonance (EPR)-silent, except at very low temperatures (<2 K), because of the broadening caused by the exchange coupling and zero-field-splitting parameters being of comparable magnitude and rapid spin-lattice relaxation. However, a phosphate-bound Fe(III)(2) complex showed an EPR spectrum due to population of the S(tot) = 3 state (J= -3.5 cm(-1)). The phosphatase activity of the Fe(III)Fe(II) complex in hydrolysis of bis(2,4-dinitrophenyl)phosphate (k(cat.) = 1.88 × 10(-3) s(-1); K(m) = 4.63 × 10(-3) mol L(-1)) is similar to that of other bimetallic heterovalent complexes with the same ligand. Analysis of the kinetic data supports a mechanism where the initiating nucleophile in the phosphatase reaction is a hydroxide, terminally bound to Fe(III). It is interesting to note that aqueous solutions of [Fe(III)Fe(II)L1(μ-OAc)(2)](+) are also capable of protein cleavage, at mild temperature and pH conditions, thus further expanding the scope of this complex's catalytic promiscuity. 相似文献
145.
Roberto Hazenfratz Guilherme Mongeló Casimiro S. Munita Eduardo G. Neves 《Journal of Radioanalytical and Nuclear Chemistry》2012,291(1):43-48
In this work, 50 ceramic fragments from the Lago Grande and 30 from the Osvaldo archaeological site were compared to assess
elemental similarities. The aim is to perform a preliminary comparison between the sites, which are located in the central
Amazon, Brazil. The analytical technique employed to obtain the ceramics elemental composition was instrumental neutron activation
analysis (INAA). The data set obtained was explored by the multivariate statistical techniques of cluster, principal component
and discriminant analysis. The analyzed elements were: Na, Lu, U, Yb, La, Th, Cr, Cs, Sc, Fe, Eu, Ce and Hf. The results showed
the existence of at least two compositional groups for Lago Grande and Osvaldo. Each compositional group of Osvaldo archaeological
site matches with one group of Lago Grande. Correlated with the archaeological background, the results suggest commercial
or cultural exchange in the region, which is an indicative of socio-cultural interactions between those sites. 相似文献
146.
Helio G. Bonacorso Guilherme P. Bortolotto Jussara Navarini Liliane M.F. Porte Carson W. Wiethan Nilo Zanatta Marcos A.P. Martins Alex F.C. Flores 《Journal of fluorine chemistry》2010,131(12):1297-1301
This study describes a simple and efficient procedure to synthesize a novel series of fourteen 4-substituted N-(5-pyridinyl-1H-1,2,4-triazol-3-yl)-6-(trifluoromethyl)pyrimidin-2-amines, where the 4-substituents are H, CH3, C6H5, 4-FC6H4, 4-CH3C6H4, 4-CH3OC6H4 and 2-Furyl; from the cyclocondensation reaction of N-[5-(pyridinyl)-1H-1,2,4-triazol-3-yl]guanidines with 4-alkoxy-4-alkyl(aryl/heteroaryl)-1,1,1-trifluoroalk-3-en-2-ones. The reactions were carried out in ethanol under reflux for 18 h and led to 40-68% yields. N-(Pyridyl-triazolyl)guanidine precursors were further obtained from reactions of cyanoguanidine with nicotinic or isonicotinic acid hydrazides and the halogenated enones from trifluoroacetylation of enolethers or acetals. 相似文献
147.
Ieda Rottava Priscila F. Cortina Camila E. Grando André R. S. Colla Eduarda Martello Rogério L. Cansian Geciane Toniazzo Helen Treichel Octávio A. C. Antunes Enrique G. Oestreicher Débora de Oliveira 《Applied biochemistry and biotechnology》2010,162(3):719-732
This work is focused on the biotransformation of R-(+)-limonene and (?)-β-pinene to bioflavor production. To carry out the present study, 405 microorganisms were tested for their ability to bioconvert the substrates. From the isolated microorganisms, 193 were selected in the prescreening using mineral medium for limonene degradation. At the screening step, eight strains were able to convert R-(+)-limonene and 15 to transform (?)-β-pinene, both in α-terpineol. The highest concentration in α-terpineol from R-(+)-limonene was about 3,450 mg/L for Penicillium sp. isolated from eucalyptus steam. From (?)-β-pinene, the highest product concentration of 675.5 mg/L was achieved using an Aspergillus sp. strain isolated from orange tree stem. 相似文献
148.
Guilherme Dias Rodrigues Luis Henrique Mendes da Silva Luis Antonio Minim 《Talanta》2010,80(3):1139-8
A greener and more sensitive spectrophotometric procedure has been developed for the determination of phenol and o-cresol that exploits an aqueous two-phase system (ATPS) using a liquid-liquid extraction technique. An ATPS is formed mostly by water and does not require organic solvent. Other ATPS components used in this study were the polymer, polyethylene oxide, and some salts (i.e., Li2SO4, Na2SO4 or K2HPO4 + KOH). The method is based on the reaction between phenol, sodium nitroprusside (NPS) and hydroxylamine hydrochloride (HL) in an alkaline medium (pH 12.0), producing the complex anion [Fe2(CN)10]10− that spontaneously concentrates in the top phase of the system. The linear range was 1.50-500 μg kg−1 (R ≥ 0.9997; n = 8) with coefficients of variation equal to 0.38% for phenol and 0.30% for o-cresol (n = 5). The method yielded limits of detection (LODs) of 1.27 and 1.88 μg kg−1 and limits of quantification (LOQs) of 4.22 and 6.28 μg kg−1 for phenol and o-cresol, respectively. Recoveries between 95.7% and 107% were obtained for the determination of phenol in natural water and wastewater samples. In addition, excellent agreement was observed between this new ATPS method and the standard 4-aminoantipyrine (4-AAP) method. 相似文献
149.
Pescim GF Marrach G Vannuci-Silva M Souza LA Menegário AA 《Analytical and bioanalytical chemistry》2012,404(5):1581-1588
Saccharomyces cerevisiae immobilized in agarose gel is proposed as a binding agent for the diffusive gradients in thin films (DGT) technique for determination of Pb in river water and seawater. DGT samplers were assembled with the proposed binding agent (25-mm disk containing 20%, m/v, S. cerevisiae and 3.0%, m/v, agarose) and a diffusive layer of cellulose (3MM Chr chromatography paper of 25-mm diameter). The effects of some DGT parameters (e.g., immersion time, ionic strength, and pH) were evaluated. Elution of Pb from the binding agent was effectively done with 1.75 mol L(-1) HNO(3). The deployment curve (between 2 and 24 h) was characterized by a significant uptake of Pb (346 ng Pb h(-1)) and good linear regression (R(2) = 0.9757). The experimental results are in excellent agreement with the predicted theoretical curve for mass uptake. Consistent results were found for solutions with ionic strengths of 0.005 mol L(-1) or greater and within a pH range of 4.5-8.5. Interferences from Cu (20:1), Mn (20:1), Fe (20:1), Zn (20:1), Ca (250:1), and Mg (250:1) in Pb retention were negligible. Determination of Pb in spiked river water samples (from the Corumbataí and Piracicaba rivers) performed using the proposed device was in agreement with total dissolved Pb, whereas measurements in seawater suggest that of the various species of Pb present in the samples, only cationic Pb species are adsorbed by the agarose-yeast gel disks. The in situ concentration of Pb obtained at two different sites of the Rio Claro stream (Corumbataí basin) were 1.13 ± 0.01 and 1.34 ± 0.04 μg L(-1). For 72-h deployments, a detection limit of 0.75 μg L(-1) was calculated. The combination of inductively coupled plasma optical emission spectroscopy and in situ deployments of DGT samplers during the 72-h period makes possible the determination of labile Pb in river water. 相似文献
150.
Jorge Casalderrey-Solana José Guilherme Milhano Paloma Quiroga-Arias 《Physics letters. [Part B]》2012
We study the time structure of vacuum jet evolution via a simple uncertainty principle estimate in the kinematic range explored by current heavy ion collisions at the LHC. We observe that a large fraction of the partonic splittings occur at large times, of the order of several fm. We compare the time distribution of vacuum splittings with the distribution of path lengths traversed by jets in a heavy ion collision. We find that if no medium induced modification of the jet dynamics were present, a very large fraction (larger than 80% for inclusive jets) of the jet splittings would occur outside of the medium. We confront this observation with current available data on jet properties in heavy ion collisions and discuss its implications for the dynamics of jet–medium interactions. 相似文献