全文获取类型
收费全文 | 509篇 |
免费 | 29篇 |
国内免费 | 2篇 |
专业分类
化学 | 357篇 |
晶体学 | 1篇 |
力学 | 17篇 |
数学 | 75篇 |
物理学 | 90篇 |
出版年
2023年 | 8篇 |
2022年 | 9篇 |
2021年 | 24篇 |
2020年 | 19篇 |
2019年 | 17篇 |
2018年 | 15篇 |
2017年 | 7篇 |
2016年 | 30篇 |
2015年 | 22篇 |
2014年 | 16篇 |
2013年 | 28篇 |
2012年 | 37篇 |
2011年 | 35篇 |
2010年 | 23篇 |
2009年 | 22篇 |
2008年 | 38篇 |
2007年 | 38篇 |
2006年 | 27篇 |
2005年 | 33篇 |
2004年 | 19篇 |
2003年 | 20篇 |
2002年 | 14篇 |
2001年 | 3篇 |
2000年 | 4篇 |
1999年 | 3篇 |
1997年 | 2篇 |
1995年 | 4篇 |
1994年 | 1篇 |
1993年 | 4篇 |
1992年 | 2篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1989年 | 3篇 |
1987年 | 1篇 |
1985年 | 2篇 |
1983年 | 1篇 |
1978年 | 1篇 |
1957年 | 1篇 |
1940年 | 1篇 |
1935年 | 3篇 |
1906年 | 1篇 |
排序方式: 共有540条查询结果,搜索用时 328 毫秒
91.
Bueno C Mikelsons L Maretti L Scaiano JC Aspée A 《Photochemistry and photobiology》2008,84(6):1535-1542
The photophysical properties of the nitroxide prefluorescent probes 4-(3-hydroxy-2-methyl-4-quinolinoyloxy)-2,2,6,6-tetramethyl-piperidin-4-yl) ester free radical (QT) and 2,3,4,6,7,8-hexahydro-quinolizino [1,10,9-gh] coumarin-3-carboxylic acid (1-oxyl-2,2,6,6-tetramethyl-piperidin-4-yl) ester free radical (C343T) were evaluated as a function of pH and solvent properties. The absorbance of QT showed high pH sensitivity. The pKa values for the different ionization forms involved in the acid-base equilibrium of the quinoline chromophore were determined in the ground and excited states. The fluorescence lifetimes of QT, and N-hydroxylamine (QTH) and quinoline methyl ester (QMe) derivatives, showed that the intramolecular quenching efficiency by the nitroxide moiety is independent of the quinoline ionization form. The fluorescence and absorbance of C343T were highly sensitive to solvent polarity in agreement with a charged transfer excited state of the chromophore. However, we noted a decrease in the intramolecular fluorescence quenching efficiency by the nitroxide moiety when changing the polarity of the solvent from hexane to water. This behavior has been attributed to a suppression of an energy transfer mechanism in the nitroxide quenching process in very polar solvents. The results obtained in micelles allow us to propose QT and C343T as sensors for pH and micropolarity, respectively, in addition to their role as monitors for free radicals or hydrogen transfer from phenols. 相似文献
92.
Dr. Alexis Bordet Prof. Dr. Walter Leitner 《Angewandte Chemie (International ed. in English)》2023,62(33):e202301956
The rapidly growing importance of green hydrogen and renewable carbon resources as essential feedstocks for sustainable chemical value chains opens room for disruptive innovations regarding chemical production processes. The fluctuation and variability associated with non-fossil energy and raw material supply holds many challenges for catalysts to cope with the resulting dynamics. However, many new opportunities also arise once catalyst design starts to aim at performance that is “adaptive” rather than “task-specific”. In this Scientific Perspective, we propose to define adaptivity in catalysis on the basis of three essential properties that are reversibility, rapidity, and robustness (R3 rule). Promising design strategies and selected examples are described to substantiate the scientific concept and to highlight its potential for chemical energy conversion. 相似文献
93.
We investigated the impact of calcium and magnesium ions on the deposition kinetics of flagellated and nonflagellated Pseudomonas aeruginosa onto an alginate conditioning film in a radial stagnation point flow system. The bacterial deposition/adhesion behavior was related to structural changes of the alginate film in the presence of the divalent cations. Our results showed that adhesion of nonmotile bacteria was governed by cation bridging interactions between high-affinity sites at the bacterial surface and either clean or alginate-conditioned substrate surfaces. For motile bacteria, the adhesion onto clean quartz was governed by electrostatic interactions while adhesion onto alginate-conditioned quartz was dependent on the structure and viscoelastic properties of the alginate film in the presence of calcium or magnesium. We demonstrate that bacterial adhesion behavior is governed both by the effects of divalent cations on the surface properties of the bacteria and the substrate and by the type of specific interactions occurring between these two surfaces. 相似文献
94.
Irene Zacharaki Christos Kontoyannis Alexis Lycourghiotis Christos Kordulis 《Colloids and surfaces. A, Physicochemical and engineering aspects》2008,324(1-3):208-216
Mesoporous anatase was prepared following sol–gel and using urea as template. The influence of calcination temperature on the phase stability, nanocrystal/aggregate size, pore size distribution and specific surface area as well as on the acid–base behavior in aqueous solutions was studied using X-ray diffraction, laser-Raman and diffuse reflectance spectroscopies, scanning electron microscopy and laser scattering as well as N2 adsorption–desorption isotherms and potentiometric mass titrations.The crystal structure was kept constant upon calcination over the whole temperature range, 200–500 °C. In this range anatase is constituted from primary nanocrystals. These are assembled into larger, rather spherical, clusters of about 30–40 nm and then into aggregates of various sizes (0.2–0.3 μm and 2–100 μm) with a distribution centered at about 12 μm. Increase of the calcination temperature caused an increase in the size of the primary nanocrystals from 8.1 nm at 200 °C to 17.1 nm at 500 °C, whereas calcination does not influence the morphology at micro-scale. Moreover, increase of the calcination temperature from 200 °C to 500 °C brings about a shift in the mean pore diameter from 47 nm to 91 nm accompanied by a decrease in the specific surface area and pore volume. The above effects were related with the aforementioned increase in the size of the primary nanocrystals. The value of pzc and the values of surface charge determined at various pH do not practically depend on the calcination temperature. The absence of pore space confinement effects was explained in terms of the structure and size of the interface development between the anatase surface and the electrolytic solution. 相似文献
95.
Alexis Morales Jochem Struppe Enrique Meléndez 《Journal of inclusion phenomena and macrocyclic chemistry》2008,60(3-4):263-270
The possible inclusion complexes of Cp2NbCl2 into α-, β-, and γ-CD hosts have been investigated. The existence of a true inclusion complex in the solid state was confirmed
by a combination of thermogravimetric analysis, FTIR, PXRD, and 13C CP-MAS NMR spectroscopies. The solid-state results demonstrated that α-cyclodextrin does not form inclusion complexes with
Cp2NbCl2 whereas β- and γ-cyclodextrins do form such complexes. PXRD, NMR, and thermal analysis showed that the organometallic molecules
of Cp2NbCl2OH are included in the cavities of β- and γ-cyclodextrins, possibly adopting a symmetrical conformation in the complex, with
each glucose unit in a similar environment. In solution, 1H NMR experiments suggest that niobocene has a shallow penetration on the β-CD leading to upfield shift on H-3 signal with
a minor perturbation on the H-5 proton while for γ-CD, both H-3 and H-5 are shifted upfield substantially. This suggests that
niobocene penetrates deeper into the γ-CD cavity than in the β-CD cavity, as a result of the cavity size.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
96.
We present DFT calculations for adsorption of the first row of transition metal atoms on a MgO(1 0 0) surface and on a surface exhibiting defects. Some atoms exhibit a high adsorption energy on the defect (e.g. Co, Ni and Cu), but others (Ca, Sc) rather adsorb on a clean surface and another set is indifferent to the presence of defect. The adsorption becomes energetically unfavorable when the σ anti-bonding orbitals become populated; this is worse on a defective surface than on a terrace. The π back-donation to the surface contributes to favor the adsorption on the center. 相似文献
97.
Magnetically Induced Continuous CO2 Hydrogenation Using Composite Iron Carbide Nanoparticles of Exceptionally High Heating Power
下载免费PDF全文
![点击此处可从《Angewandte Chemie (International ed. in English)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Alexis Bordet Dr. Lise‐Marie Lacroix Dr. Pier‐Francesco Fazzini Dr. Julian Carrey Dr. Katerina Soulantica Dr. Bruno Chaudret 《Angewandte Chemie (International ed. in English)》2016,55(51):15894-15898
The use of magnetic nanoparticles to convert electromagnetic energy into heat is known to be a key strategy for numerous biomedical applications but is also an approach of growing interest in the field of catalysis. The heating efficiency of magnetic nanoparticles is limited by the poor magnetic properties of most of them. Here we show that the new generation of iron carbide nanoparticles of controlled size and with over 80 % crystalline Fe2.2C leads to exceptional heating properties, which are much better than the heating properties of currently available nanoparticles. Associated to catalytic metals (Ni, Ru), iron carbide nanoparticles submitted to magnetic excitation very efficiently catalyze CO2 hydrogenation in a dedicated continuous‐flow reactor. Hence, we demonstrate that the concept of magnetically induced heterogeneous catalysis can be successfully applied to methanation of CO2 and represents an approach of strategic interest in the context of intermittent energy storage and CO2 recovery. 相似文献
98.
From evidence-based policy making to policy analytics 总被引:1,自引:0,他引:1
This paper aims at addressing the problem of what characterises decision-aiding for public policy making problem situations. Under such a perspective it analyses concepts like “public policy”, “deliberation”, “legitimation”, “accountability” and shows the need to expand the concept of rationality which is expected to support the acceptability of a public policy. We then analyse the more recent attempt to construct a rational support for policy making, the “evidence-based policy making” approach. Despite the innovation introduced with this approach, we show that it basically fails to address the deep reasons why supporting the design, implementation and assessment of public policies is such a hard problem. We finally show that we need to move one step ahead, specialising decision-aiding to meet the policy cycle requirements: a need for policy analytics. 相似文献
99.
In this paper, we study the existence of positive solutions for the quasilinear elliptic singular problemwhere \({c,\lambda >0}\), \({\gamma \in (0,1)}\), f is strictly increasing and derivable in \({[0,\infty)}\) with \({f(0)>0}\). We show that there exists \({\lambda^*>0}\) such that \({(0,\lambda^*]}\) is the maximal set of values such there exists solution. In addition, we prove that for \({\lambda<\lambda^*}\) there exists minimal and bounded solutions. Moreover, we give sufficient conditions for existence and regularity of solutions for \({\lambda=\lambda^*}\).
相似文献
$$\left\{\begin{array}{ll}-\Delta u + c\,\frac{|\nabla u|^2}{u^\gamma} = \lambda\,f(u), \quad \quad \mbox{in $\Omega$},\\ u=0, \quad \qquad \qquad \qquad \quad \, \, \, \, \, \mbox{on $\partial$$\Omega$},\end{array}\right.$$
100.
Combination of sorption properties of polydimethylsiloxane and solid‐phase extraction sorbents in a single composite material for the passive sampling of polar and apolar pesticides in water
下载免费PDF全文
![点击此处可从《Journal of separation science》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Alexis Martin Christelle Margoum Marina Coquery Jérôme Randon 《Journal of separation science》2016,39(20):3990-3997
Passive sampling techniques have been developed as an alternative method for in situ integrative monitoring of trace levels of neutral pesticides in environmental waters. The objective of this work was to develop a new receiving phase for pesticides with a wide range of polarities in a single step. We describe the development of three new composite silicone rubbers, combining polydimethylsiloxane mechanical and sorption properties with solid‐phase extraction sorbents, prepared as a receiving phase for passive sampling. A composite silicone rubber composed of polydimethylsiloxane/poly(divinylbenzene‐co‐N‐vinylpyrrolidone) was selected by batch experiments for its high sorption properties for pesticides with octanol‐water partition coefficients ranging from 2.3 to 5.5. We named this composite material “Polar/Apolar Composite Silicone Rubber”. A structural study by scanning electron microscopy confirmed the homogeneous dispersion of the sorbent particles and the encapsulation of particles within the polydimethylsiloxane matrix. We also demonstrate that this composite material is resistant to common solvents used for the back‐extraction of analytes and has a maximal resistance temperature of 350°C. Therefore, the characteristics of the “Polar/Apolar Composite Silicone Rubber” meet most of the criteria for use as a receiving phase for the passive sampling of pesticides. 相似文献