全文获取类型
收费全文 | 102篇 |
免费 | 5篇 |
专业分类
化学 | 70篇 |
力学 | 8篇 |
数学 | 11篇 |
物理学 | 18篇 |
出版年
2022年 | 2篇 |
2021年 | 3篇 |
2020年 | 1篇 |
2019年 | 1篇 |
2018年 | 1篇 |
2016年 | 4篇 |
2015年 | 4篇 |
2014年 | 2篇 |
2013年 | 14篇 |
2012年 | 9篇 |
2011年 | 2篇 |
2010年 | 3篇 |
2009年 | 5篇 |
2008年 | 3篇 |
2007年 | 7篇 |
2006年 | 2篇 |
2005年 | 5篇 |
2004年 | 6篇 |
2003年 | 2篇 |
2002年 | 3篇 |
2000年 | 6篇 |
1999年 | 2篇 |
1998年 | 3篇 |
1996年 | 3篇 |
1995年 | 4篇 |
1994年 | 1篇 |
1993年 | 3篇 |
1990年 | 1篇 |
1986年 | 2篇 |
1984年 | 2篇 |
1976年 | 1篇 |
排序方式: 共有107条查询结果,搜索用时 15 毫秒
41.
Estrella Saniger Beatriz Delgado Josefa M. González-Pérez Miguel Á. Gallo Joaquín M. Campos 《Tetrahedron》2004,60(50):11453-11464
The synthesis of (RS)-2- or (RS)-3-methoxy-2,3-dihydro-5H-1,4-benzodioxepins and (RS)-5- or (RS)-3-methoxy-2,3,5,6-tetrahydro-8H-benzo-[1,4,7]-trioxecins has been developed. The mechanism of such a reaction via the boron trifluoride etherate-promoted transformation of 2-(methoxyethoxymethoxy)benzyloxyacetaldehyde dimethyl acetals or 2-(methoxyethoxymethoxymethyl)phenyloxyacetaldehyde dimethyl acetals has been proposed. Transannular versions of the reaction results in the facile ring contraction of 12-membered intermediates to the 10- and to 7-membered benzene-fused O,O-acetals. The characterization of the by-products strongly supports the mechanisms proposed. 相似文献
42.
Estrella Johnson Jessica Ellis Chris Rasmussen 《International Journal of Mathematical Education in Science & Technology》2016,47(4):491-504
This paper is based on a large-scale empirical study designed to investigate Calculus I programmes across the United States to better understand the relationship between instructors' concerns about coverage, instructional practices, and the nature of the material covered. We found that there was no association between instructors feeling pressured to go through material quickly to cover all the required topics and intended pacing. Furthermore, our results suggest that both intended pacing and feelings of pressure are poor indicators of instructional practices. 相似文献
43.
J.?ValverdeEmail author J.?L.?Escalona E.?Freire J.?Domínguez 《Nonlinear dynamics》2005,42(2):137-163
In this paper, a modified Jeffcott model is proposed and studied in order to shed light into the dynamics of a complex system,
the Short Electrodynamic Tether (SET), which is similar to an unbalanced rotor. Due to the internal damping, a geometrically
linear SET model appears to be unstable as predicted by the linear rotordynamics theory. Some studies in the field of rotordynamics
suggest that this instability caused by internal damping do not appear if geometric nonlinearities are taken into account
in the system equations of motion. Stability and bifurcation analysis have been carried out on the modified Jeffcott model,
which accounts for geometric nonlinearities, orthotropy in the shaft's cross section, and a viscous damping-based internal
damping model. The stability results analytically obtained have been compared with a nonlinear multibody model by means of
time simulations and good agreement has been found. 相似文献
44.
Ana E. Platero‐Prats Dr. Víctor A. de la Peña‐O'Shea Dr. Natalia Snejko Prof. Ángeles Monge Prof. Enrique Gutiérrez‐Puebla 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(38):11632-11640
Herein, we present a Ca‐based metal–organic framework named AEPF‐1, which is an active and selective catalyst in olefin hydrogenation reactions. AEPF‐1 exhibits a phase transition upon desorption of guest molecules. This structural transformation takes place by a crystal to crystal transformation accompanied by the loss of single‐crystal integrity. Powder diffraction methods and computational studies were applied to determine the structure of the guest‐free phase. This work also presents data on the exceptional adsorption behavior of this material, which is shown to be capable of separating polar from nonpolar organic solvents, and is a good candidate for selective solvent adsorption under mild conditions. 相似文献
45.
46.
Carolina Zuriaga-Monroy Jos-Manuel Martínez-Magadn Estrella Ramos Rodolfo Gmez-Balderas 《Journal of molecular catalysis. A, Chemical》2009,313(1-2):49-54
Molybdenum disulfide nanoparticles are of interest for their extensive use in heterogeneous catalysis. Here, we report a systematic density functional theory study carried out to investigate the electronic effects of Co(Ni) mono-substitutions on triangular molybdenum sulfide models of nanometric scale. On the basis of the electronic structure, the charge distribution and the Δ(ELUMO − EHOMO) gap analysis, the triangle molecular model with nickel substitution is identified as more favorable for inducing the best catalytic performance. Nickel consistently induces stronger electronic rearrangements than cobalt, on the molybdenum first-neighbor atoms, which are connected with its higher promoting effect. Charge distribution analysis points out a chemical reduction on the molybdenum sites when the cluster is doped. Moreover, nickel substitution produces smaller Δ(ELUMO − EHOMO) gaps than cobalt substitutions, revealing that Ni-doped clusters are more reactive. 相似文献
47.
Miriam Sanz Brígida Fernández de Simón Estrella Cadahía Enrique Esteruelas Angel M. Muñoz Teresa Hernández Isabel Estrella Ernani Pinto 《Journal of mass spectrometry : JMS》2012,47(7):905-918
The phenolic composition of heartwood extracts from Fraxinus excelsior L. and F. americana L., both before and after toasting in cooperage, was studied using LC‐DAD/ESI‐MS/MS. Low‐molecular weight (LMW) phenolic compounds, secoiridoids, phenylethanoid glycosides, dilignols and oligolignols compounds were detected, and 48 were identified, or tentatively characterized, on the basis of their retention time, UV/Vis and MS spectra, and MS fragmentation patterns. Some LMW phenolic compounds like protocatechuic acid and aldehyde, hydroxytyrosol and tyrosol, were unlike to those for oak wood, while ellagic and gallic acid were not found. The toasting of wood resulted in a progressive increase in lignin degradation products with regard to toasting intensity. The levels of some of these compounds in medium‐toasted ash woods were much higher than those normally detected in toasted oak, highlighting vanillin levels, thus a more pronounced vanilla character can be expected when using toasted ash wood in the aging wines. Moreover, in seasoned wood, we found a great variety of phenolic compounds which had not been found in oak wood, especially oleuropein, ligstroside and olivil, along with verbascoside and isoverbascoside in F. excelsior, and oleoside in F. americana. Toasting mainly provoked their degradation, thus in medium‐toasted wood, only four of them were detected. This resulted in a minor differentiation between toasted ash and oak woods. The absence of tannins in ash wood, which are very important in oak wood, is another peculiar characteristic that should be taken into account when considering its use in cooperage. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
48.
Barrio L Zhou G González ID Estrella M Hanson J Rodriguez JA Navarro RM Fierro JL 《Physical chemistry chemical physics : PCCP》2012,14(7):2192-2202
This work analyzes the influence of cerium content (6-15 wt%) on a TiO(2) support over the structure and water gas shift (WGS) activity of Pt catalysts. The structural properties of these Pt/Ce-TiO(2) catalysts were characterized by XRD, TEM and XANES. Physicochemical characterization of the catalysts showed differences in the structure and dispersion of Ce entities on the support with Ce loading. For the samples with low ceria content (6 wt%), cerium is deposited on the support in the form of CeO(x) clusters in a highly dispersed state in close interaction with the Ti atoms. The formation of CeO(x) clusters at low Ce-loading on the support facilitates the dispersion of small particles of Pt and improves the reducibility of ceria component at low temperatures. The changes in platinum dispersion and support reducibility with Ce-loading on the TiO(2) support lead to significant differences in the WGS activity. Pt supported on the sample with lower Ce content (6 wt%) shows better activity than those corresponding to catalysts with higher Ce content (15 wt%). Activity measurements coupled with catalysts characterization suggest that the improvement in the reducibility of the support with lower Ce content was associated with the presence of CeO(x) clusters of high reducibility that improve the chemical activity of the oxide-metal interfaces at which the WGS reaction takes place. 相似文献
49.
Coherent electronic transport through a molecular device is studied using non-equilibrium Green's function (NEGF) formalism.
Such device is made of atomic nanowire which is connected to ferromagnetic electrodes. The molecule itself is described with
the help of Hubbard model (Coulomb interactions are treated by means of the Hartree-Fock approximation), while the coupling
to the electrodes is modeled through the use of a broad-band theory. It was shown that magnetoresistance varies periodically
with increasing length of the atomic wire (in the linear response regime) and oscillates with increasing bias voltage (in
the nonlinear response regime). Since the TMR effect for analyzed structures is predicted to be large (tens of percent), these
junctions seem to be suitable for application as magnetoresistive elements in future electronic circuits. 相似文献
50.
M. Aguilar S. Martín J. F. Vega A. Muoz‐Escalona J. Martínez‐Salazar 《Journal of Polymer Science.Polymer Physics》2005,43(21):2963-2971
The processability of a metallocene‐catalyzed polyethylene blended in the melt with low amounts of ultra‐high molecular weight polyethylene was evaluated. Our morphological and thermal studies revealed that the blends were structurally heterogeneous, formed by a matrix of metallocene polyethylene and homogeneously distributed particles of ultra‐high molecular weight material. The visible particles were smaller than those of the reactor powder. Also observed was some degree of interaction between both phases. Rheological data indicated an intense effect of morphology on viscoelastic functions, beyond that expected for a system composed of noninteractive phases. Collectively, our findings suggest the existence, to some extent, of a homogeneous phase at the molecular level composed of a fraction of ultra‐high molecular weight species and metallocene polyethylene. This gives rise to a striking behavior during processing in that the blends show improved extrusion performance the higher their ultra‐high molecular weight polyethylene content. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2963–2971, 2005 相似文献