Bis(phosphinimino)methanide Borohydride Complexes of the Rare‐Earth Elements as Initiators for the Ring‐Opening Polymerization of ε‐Caprolactone: Combined Experimental and Computational Investigations

Rare‐earth‐metal borohydrides are known to be efficient catalysts for the polymerization of apolar and polar monomers. The bis‐borohydrides [{CH(PPh₂NSiMe₃)₂}La(BH₄)₂(THF)] and [{CH(PPh₂NSiMe₃)₂}Ln(BH₄)₂] (Ln=Y, Lu) have been synthesized by two different synthetic routes. The lanthanum and the lutetium complexes were prepared from [Ln(BH₄)₃(THF)₃] and K{CH(PPh₂NSiMe₃)₂}, whereas the yttrium analogue was obtained from in situ prepared [{CH(PPh₂NSiMe₃)₂}YCl₂]₂ and NaBH₄. All new compounds were characterized by standard analytical/spectroscopic techniques, and the solid‐state structures were established by single‐crystal X‐ray diffraction. The ring‐opening polymerization (ROP) of ε‐caprolactone initiated by [{CH(PPh₂NSiMe₃)₂}La(BH₄)₂(THF)] and [{CH(PPh₂NSiMe₃)₂}Ln(BH₄)₂] (Ln=Y, Lu) was studied. At 0 °C the molar mass distributions determined were the narrowest values (M?_w/M?_n=1.06–1.11) ever obtained for the ROP of ε‐caprolactone initiated by rare‐earth‐metal borohydride species. DFT investigations of the reaction mechanism indicate that this type of complex reacts in an unprecedented manner with the first B? H activation being achieved within two steps. This particularity has been attributed to the metallic fragment based on the natural bond order analysis.     

Renormalized solution for Stefan type problems: existence and uniqueness

We consider a class of nonlinear degenerate problems of Stefan type: $u_t- \Delta w -\nabla F(u,w)= g(\cdot,u), \ w\in \beta(u)$ where β is a maximal monotone graph in ${\mathbb{R}^2,}We consider a class of nonlinear degenerate problems of Stefan type:

ut- Dw -?F(u,w) = g(·,u),  w ? b(u)u_t- \Delta w -\nabla F(u,w)= g(\cdot,u), \ w\in \beta(u)      154. Optimal partial mass transportation and obstacle Monge–Kantorovich equation      Noureddine Igbida  Van Thanh Nguyen 《Journal of Differential Equations》2018,264(10):6380-6417 Optimal partial mass transport, which is a variant of the optimal transport problem, consists in transporting effectively a prescribed amount of mass from a source to a target. The problem was first studied by Caffarelli and McCann (2010) [6] and Figalli (2010) [12] with a particular attention to the quadratic cost. Our aim here is to study the optimal partial mass transport problem with Finsler distance costs including the Monge cost given by the Euclidian distance. Our approach is different and our results do not follow from previous works. Among our results, we introduce a PDE of Monge–Kantorovich type with a double obstacle to characterize active submeasures, Kantorovich potential and optimal flow for the optimal partial transport problem. This new PDE enables us to study the uniqueness and monotonicity results for the active submeasures. Another interesting issue of our approach is its convenience for numerical analysis and computations that we develop in a separate paper [14] (Igbida and Nguyen, 2018).      155. Vibrational and thermal properties of ScN and YN: quasi-harmonic approximation calculations and anharmonic effects      Tahri S  Qteish A  Al-Qasir II  Meskini N 《J Phys Condens Matter》2012,24(3):035401 The structural, vibrational and thermal properties of rocksalt ScN and YN are investigated by using a first-principles plane-wave approach. The results are discussed in comparison with the similarly calculated results for rocksalt MgO and zincblende AlN. The thermal expansivity (α(V)) computed within the quasi-harmonic approximation shows that there are significant anharmonic effects in ScN and YN, which are comparable to those in MgO. Since no experimental results are available for α(V) of either ScN or YN, the anharmonic effects are accounted for by a variant of the very recently introduced effective semiempirical ansatz (Phys. Rev. B 2009 79 104304) for calculating anharmonic free energy, which does not require any input from experiment. The validity of this very simple approach is demonstrated first by applying it to MgO. For the considered phase of AlN, the quasi-harmonic approximation is valid up to very high temperatures, and the thus obtained α(V) is in good agreement with experiment. The values of α(V) for semiconductor transition metal nitrides that crystallize in the rocksalt phase are higher than those for the zincblende phase of group-IIIB nitrides, and a major part of these differences is due to the crystal structure.      156. Elliptic problem involving diffuse measure data      Noureddine Igbida  Stanislas Ouaro 《Journal of Differential Equations》2012,253(12):3159-3183 In this paper, we study a suitable notion of solution for which a nonlinear elliptic problem governed by a general Leray-Lions operator is well posed for any diffuse measure data. In terms of the paper (Brezis et al., 2007, [10]), we study the notion of solution for which any diffuse measure is “good measure”.      157. A new triterpene from olea europea stem with antidiabetic activity      Ines Khlif  Khaled Hamden  Mohamed Damak  Noureddine Allouche 《Chemistry of Natural Compounds》2012,48(5):799-802       158. A new Fourier-related double scale analysis for instability phenomena in sandwich structures      Yin Liu  Kun Yu  Heng Hu  Salim Belouettar  Michel Potier-Ferry  Noureddine Damil 《International Journal of Solids and Structures》2012,49(22):3077-3088 In this paper, we present a new Fourier-related double scale analysis to study instability phenomena of sandwich structures. By using the technique of slowly variable Fourier coefficients, a zig–zag theory based microscopical sandwich model is transformed into a macroscopical one that offers three numerical advantages. Firstly, only the envelopes of instability patterns are evaluated and this leads to a significant improvement on computational efficiency, especially when dealing with high wavenumber wrinkling phenomena. Secondly, the proposed macroscopical model allows one to select modal wavelength, which makes easy to control non-linear calculations. Thirdly, in contrast to Landau–Ginzburg envelope equations, it may also remain valid away from the bifurcation point and the coupling between global and local instabilities can be accounted for. The established non-linear system is solved by asymptotic numerical method (ANM), which is more reliable and less time consuming than other iterative classical methods. The proposed double scale analysis yields accurate results with a significant reduced computational cost.      159. Optimization of concentric array resonators for wide band noise reduction      Jean-Michel Coulon  Noureddine AtallaAlain Desrochers 《Applied Acoustics》2016 In air duct noise control, Helmholtz resonators (HR) are considered as narrow band attenuators. For some applications they can be combined in line to form a wide band silencer. This study investigates the role of distance between HR side branch openings on the whole array attenuation. In the case of two resonators with same performance, the optimal distance can be calculated and corresponds to the quarter wave of HR mean frequency. On three or more HR arrays, relationships between resonators parameters and optimal lengths are much more complex. Tuning of such a device requires taking many geometrically coupled parameters into account; hence, design has to be automated. To operate this process, a 2D FEM COMSOL model has been coupled to a global MATLAB optimization solver. Among different types of constructions, arrays made of concentric resonators with transversal openings offers the most efficient and flexible design to optimize distance between openings. This methodology was applied to an existing turbo compressor silencer. Modifying openings and chambers arrangement, using the proposed approach increased the attenuation band by 10%. Another application concerning an air box for a two stroke engine was also investigated. This resulted in a 16L two chambers concept, being replaced by a more compact and more efficient, 8.3L wide band silencer, made of 8 resonators. With this approach it therefore becomes possible to handle available space and required noise attenuation on a required frequency band, all in one process.      160. C2-Symmetric Bis-Thioglycosides as Useful Ligands in Palladium-Catalyzed Asymmetric Allylic Alkylation: Synthesis of Both Enantiomers Using Natural Sugars as Ligand Precursors      Noureddine Khiar  Inmaculada Fernández  Cristina S. Araújo  Belén Suárez  Eleuterio Álvarez 《Phosphorus, sulfur, and silicon and the related elements》2013,188(5-6):1507-1508       [首页] « 上一页 [11] [12] [13] [14] [15] 16 [17] [18] [19] [20] [21] 下一页 » 末  页»

Optimal partial mass transportation and obstacle Monge–Kantorovich equation

Optimal partial mass transport, which is a variant of the optimal transport problem, consists in transporting effectively a prescribed amount of mass from a source to a target. The problem was first studied by Caffarelli and McCann (2010) [6] and Figalli (2010) [12] with a particular attention to the quadratic cost. Our aim here is to study the optimal partial mass transport problem with Finsler distance costs including the Monge cost given by the Euclidian distance. Our approach is different and our results do not follow from previous works. Among our results, we introduce a PDE of Monge–Kantorovich type with a double obstacle to characterize active submeasures, Kantorovich potential and optimal flow for the optimal partial transport problem. This new PDE enables us to study the uniqueness and monotonicity results for the active submeasures. Another interesting issue of our approach is its convenience for numerical analysis and computations that we develop in a separate paper [14] (Igbida and Nguyen, 2018).     

Vibrational and thermal properties of ScN and YN: quasi-harmonic approximation calculations and anharmonic effects

The structural, vibrational and thermal properties of rocksalt ScN and YN are investigated by using a first-principles plane-wave approach. The results are discussed in comparison with the similarly calculated results for rocksalt MgO and zincblende AlN. The thermal expansivity (α(V)) computed within the quasi-harmonic approximation shows that there are significant anharmonic effects in ScN and YN, which are comparable to those in MgO. Since no experimental results are available for α(V) of either ScN or YN, the anharmonic effects are accounted for by a variant of the very recently introduced effective semiempirical ansatz (Phys. Rev. B 2009 79 104304) for calculating anharmonic free energy, which does not require any input from experiment. The validity of this very simple approach is demonstrated first by applying it to MgO. For the considered phase of AlN, the quasi-harmonic approximation is valid up to very high temperatures, and the thus obtained α(V) is in good agreement with experiment. The values of α(V) for semiconductor transition metal nitrides that crystallize in the rocksalt phase are higher than those for the zincblende phase of group-IIIB nitrides, and a major part of these differences is due to the crystal structure.     

Elliptic problem involving diffuse measure data

In this paper, we study a suitable notion of solution for which a nonlinear elliptic problem governed by a general Leray-Lions operator is well posed for any diffuse measure data. In terms of the paper (Brezis et al., 2007, [10]), we study the notion of solution for which any diffuse measure is “good measure”.     

A new Fourier-related double scale analysis for instability phenomena in sandwich structures

In this paper, we present a new Fourier-related double scale analysis to study instability phenomena of sandwich structures. By using the technique of slowly variable Fourier coefficients, a zig–zag theory based microscopical sandwich model is transformed into a macroscopical one that offers three numerical advantages. Firstly, only the envelopes of instability patterns are evaluated and this leads to a significant improvement on computational efficiency, especially when dealing with high wavenumber wrinkling phenomena. Secondly, the proposed macroscopical model allows one to select modal wavelength, which makes easy to control non-linear calculations. Thirdly, in contrast to Landau–Ginzburg envelope equations, it may also remain valid away from the bifurcation point and the coupling between global and local instabilities can be accounted for. The established non-linear system is solved by asymptotic numerical method (ANM), which is more reliable and less time consuming than other iterative classical methods. The proposed double scale analysis yields accurate results with a significant reduced computational cost.     

Optimization of concentric array resonators for wide band noise reduction

In air duct noise control, Helmholtz resonators (HR) are considered as narrow band attenuators. For some applications they can be combined in line to form a wide band silencer. This study investigates the role of distance between HR side branch openings on the whole array attenuation. In the case of two resonators with same performance, the optimal distance can be calculated and corresponds to the quarter wave of HR mean frequency. On three or more HR arrays, relationships between resonators parameters and optimal lengths are much more complex. Tuning of such a device requires taking many geometrically coupled parameters into account; hence, design has to be automated. To operate this process, a 2D FEM COMSOL model has been coupled to a global MATLAB optimization solver. Among different types of constructions, arrays made of concentric resonators with transversal openings offers the most efficient and flexible design to optimize distance between openings. This methodology was applied to an existing turbo compressor silencer. Modifying openings and chambers arrangement, using the proposed approach increased the attenuation band by 10%. Another application concerning an air box for a two stroke engine was also investigated. This resulted in a 16L two chambers concept, being replaced by a more compact and more efficient, 8.3L wide band silencer, made of 8 resonators. With this approach it therefore becomes possible to handle available space and required noise attenuation on a required frequency band, all in one process.