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排序方式: 共有109条查询结果,搜索用时 15 毫秒
1.
On the role of acidity in catalytic oxidation 总被引:2,自引:0,他引:2
The role of the catalyst surface acid-base properties on the heterogeneously-catalyzed oxidation reaction mechanisms is discussed. Acid-base properties depend on the covalent/ionic character of the metal-oxygen bonds and are involved in some steps of the oxidation reactions, such as the activation of the C---H hydrocarbon bonds, the step associated with the evolution of alkoxide species and the desorption/overoxidation of the partial oxidation products. Thus they participate with the cation redox properties in determining the selective/unselective catalyst behavior. 相似文献
2.
Roser Cervellera Xavier Ramis Josep Maria Salla Ana Mantecn Angels Serra 《应用聚合物科学杂志》2006,102(3):2086-2093
Diglycidyl ether of bisphenol A or 3,4‐epoxycyclohexylmethyl 3,4‐epoxycyclohexane carboxylate were mixed with different proportions of 4‐methyl‐1,3‐dioxolan‐2‐one and cured using lanthanide triflates as initiators. In order to compare the materials obtained, conventional initiators such as boron trifluoride complexes and N,N‐dimethylaminopyridine were also tested. The curing process was followed by differential scanning calorimetry (DSC) and Fourier transform IR in attenuated total reflectance mode. This technique proved that the carbonate accelerates the curing process because it helps to form the active initiating species, although it was not chemically incorporated into the network and remained entrapped in the material. The DSC kinetic study was also reported. © 2006 Wiley Periodicals Inc. J Appl Polym Sci 102: 2086–2093, 2006 相似文献
3.
Servando González Xavier Fernández‐Francos Josep Maria Salla Angels Serra Ana Mantecón Xavier Ramis 《应用聚合物科学杂志》2007,104(5):3406-3416
Mixtures of diglycidylether of bisphenol A (DGEBA) with different proportions of γ‐caprolactone (γ‐CL) were cured with ytterbium triflate as initiator. The curing was studied with differential scanning calorimetry (DSC) and thermo mechanical analysis (TMA). The results are presented in the form of a time–temperature–transformation diagram. The kinetic analysis was performed by means of the isoconversional integral procedure and the kinetic model was also determined using the Coats–Redfern method. Gelation was determined by means of combined experiences of DSC and TMA. The relationship between the glass transition temperature (Tg) and the degree of conversion α was determined by DSC. Using the isoconversional lines and the Tg‐α relationship, the vitrificacion curve was obtained. The methodology developed makes it possible to obtain the TTT diagram using only no‐isothermal experiments with equivalent results to those using classical isothermal procedures. The addition of γ‐CL accelerates the curing and reduces the shrinkage after gelation and consequently the internal stresses in the material. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007 相似文献
4.
Baldassarri Sandra Passerino Liliana Ramis Silvia Riquelme Inma Perales Francisco J. 《Universal Access in the Information Society》2021,20(2):239-254
Universal Access in the Information Society - Technology and videogames have been proven as motivating tools for working attention and complex communication skills, especially in children with... 相似文献
5.
Photocuring of cycloaliphatic epoxy formulations using polyesters with multiarm star topology as additives 下载免费PDF全文
Marjorie Flores Xavier Fernández‐Francos Xavier Ramis Marco Sangermano Francesc Ferrando Àngels Serra 《应用聚合物科学杂志》2014,131(6)
Multiam star polyesters were synthesized by growing poly(ε‐caprolactone) (PCL) arms from hyperbranched polyesters cores of different molecular weight and used as polymeric modifiers in UV‐curable cationic formulations based on a biscycloaliphatic epoxy resin. The effect of the multiarm stars on the curing kinetics has been investigated by real‐time FTIR. The thermal‐mechanical properties of the photocured thermosets have been studied with calorimetry and dynamomechanical and thermogravimetric analysis. Impact strength tests have been performed to assess their effect on the toughness of the cured materials. An accelerative effect of these modifiers has been observed as a consequence of the participation of the hydroxyl groups of the modifiers in the cationic curing of the epoxy resin. A modest increase in toughness accompanied by a decrease in the glass transition are observed, as a consequence of the incorporation of the modifiers into the network structure, leading to homogeneous, in situ reinforced materials. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 40005. 相似文献
6.
The anionic curing initiated by 1‐methyl imidazole of diglycidyl ether of bisphenol A with a hyperbranched polymer (HBP) containing long aliphatic chains in the structure were studied. The hydroxyl groups present as chain ends in the HBP structure played an important role in the curing kinetics, as demonstrated by differential scanning calorimetry, Fourier transform infrared spectroscopy and rheological studies. Properties such as shrinkage on curing and thermomechanical characteristics were also investigated. The structure of the HBP, which contains long aliphatic chains and reactive hydroxyl groups as chain ends, flexibilizes the network significantly, improving the impact resistance without notably affecting either the glass transition temperature or the microhardness of the modified thermosets. Copyright © 2012 Society of Chemical Industry 相似文献
7.
Xavier Fernández‐Francos Wayne D Cook Josep M Salla Àngels Serra Xavier Ramis 《Polymer International》2009,58(12):1401-1410
BACKGROUND: Blends of epoxy resin oligomers, diglycidylether of bisphenol‐A (DGEBA), and a bislactone, 1,6‐dioxaspiro[4.4]nonan‐2,7‐dione (s(γBL)), were anionically copolymerized using two tertiary amines as anionic initiators. Their curing rheology and gelation behaviour were studied to provide a more comprehensive knowledge of the curing of these previously studied systems. RESULTS: The activation energy for gelation was found to be similar to that previously measured using differential scanning calorimetry and appeared to increase in the presence of the bislactone. The reaction rate during copolymerization of DGEBA with s(γBL) was slower than DGEBA homopolymerization alone because the alkoxide attack on the epoxide is faster than the reaction of the carboxylate ion and the epoxy group. The effect of the initiator type on the gel conversion was small and was presumably due to differences in the kinetic chain length caused by amine regeneration from the quaternary amine. For the same initiator and at a constant ratio of DGEBA/s(γBL), an increase in the hydroxyl concentration of the DGEBA oligomer raised the gel conversion. For a DGEBA oligomer with low hydroxyl levels, an increase in the concentration of s(γBL) increased the gel conversion; however, for a DGEBA oligomer with high hydroxyl levels, increasing s(γBL) concentration decreased the gel conversion. CONCLUSION: These results are interpreted in terms of the effect of initiation rate and chain transfer rate on the kinetic chain length. The glass transition temperature of the gel was found to be controlled by the fraction of the aliphatic s(γBL) and the amount of plasticizing sol in the matrix. Copyright © 2009 Society of Chemical Industry 相似文献
8.
Performance Analysis of Multiuser Detectors for Uplink Quasi-synchronous MC-CDMA Systems 总被引:1,自引:1,他引:1
Felip Riera-Palou Guillem Femenias Jaume Ramis 《Wireless Personal Communications》2007,43(4):1511-1521
This paper analyses and compares the average bit error rate (BER) of different multiuser detectors (MUD) in the uplink of
a multicarrier code- division multiple access (MC-CDMA) system. In particular, maximum likelihood, zero-forcing, minimum mean-square
error and interference cancellation-based multiuser detectors have been analysed for the special case of uncorrelated subcarriers.
The derived BER expressions are based upon previous results on diversity combining and also on recent findings on multiple
input multiple output (MIMO) architectures. The subcarrier correlation is considered in the context of physical parameters
currently under discussion for future wireless systems to give an indication up to what extent the assumption of uncorrelated
subcarrier fading is plausible. 相似文献
9.
Group-orthogonal multi-carrier code division multiple access (GO-MC-CDMA) has recently been proposed as a promising technique for the uplink segment of wireless systems. In this paper we propose and analyze a related scheme, group-orthogonal multi-carrier code division multiplexing (GO-MC-CDM), suitable for the downlink segment of the future generation of wireless systems. The proposed system is shown to offer a similar bit error rate (BER) performance as the downlink version of GO-MC-CDMA at a fraction of its computational complexity. An analytical expression for the BER when using maximum likelihood (ML) detection is derived providing valuable insight into the parameters affecting the system performance and providing a basis for its optimization. Simulation results using parameters and (correlated) channel models aiming at the next generation of wireless systems are provided confirming the analytically derived results. 相似文献
10.
B. VauzourJ.J. Santos D. BataniS.D. Baton M. KoenigPh. Nicolaï F. PerezF.N. Beg C. BenedettiR. Benocci E. BrambrinkS. Chawla M. CouryF. Dorchies C. FourmentM. Galimberti L.A. GizziR. Heathcote D.P. HigginsonJ.J. Honrubia S. HulinR. Jafer L.C. JarrotL. Labate K. LancasterP. Köster A.J. MacKinnonP. McKenna A.G. McPheeW. Nazarov J. PasleyR. Ramis Y. RheeC. Regan X. RibeyreM. Richetta F. SerresH.-P. Schlenvoigt G. SchurtzA. Sgattoni C. SpindloeX. Vaisseau L. VolpeV. Yahia 《Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment》2011,653(1):176-180