Magnetically textured y-Fe2O3 nanoparticles in a silica gel matrix: structural and magnetic properties

International audienceThis paper is devoted to magnetic and structural properties of anisotropic g -Fe2O3 superparamagnetic particles dispersed in a transparent xerogel matrix. The effect of frozen anisotropy axes and magnetic texture, induced by a magnetic field applied during the solidification of the matrix on the in-field magnetization process, is studied by alternating gradient force magnetometry and first and second order magneto-optical effects. The changes of magnetization curves with respect to the ferrofluid solution at the same particle concentration are interpreted on the basis of an existing statistical approach extended to systems with particle size distribution, which has to be taken into account for real samples. A very good agreement between the experiment and theory was achieved for a log-normal distribution of diameters which well resembles that deduced from electron microscopy observations in different imaging modes. This structural analysis states the parameter values used in calculations and confirms the relevance of basic assumptions of the model for the specimens studied. The experimental results and the related theoretical discussion should be of use to understand magnetic properties of other magnetically textured superparamagnetic system

Positron annihilation in latex templated macroporous silica films: pore size and ortho-positronium escape

International audienceDepth profling of positron annihilation characteristics has been used to investigate the pore size distribution in macroporous PMMA latex templated SiO2 films deposited on glass or Si and prepared with 11-70% porosity. The correlation between the annihilation characteristics shows that o-Ps escape (re-emission) into vacuum occurs in all films with a porosity threshold that is pore size dependent. For 60 ± 2% porosity, the o-Ps reemission yield decreases from ~ 0:25 to ~ 0:11 as the pore size increases from 32 to 75 nm. The o-Ps reemission yield is shown to vary linearly with the specific surface area per mass unit and the slope is independent of pore size, 9:1±0:4 g cm-1. For 32 nm pores, the o-Ps annihilation lifetimes in the films, 17(2)ns and 106(5) ns, show that o-Ps annihilates from micropores with small effective size (1:4 ± 4 nm) and from macropores with large effective size (~ 32 nm). Above the porosity threshold, the o-Ps-escape model predicts the annihilation lifetime in the films to be 19±2 ns. Our results imply that o-Ps effciently detects the microporosity present in the silica walls. At low porosity, its capture into the micropores competes with its capture into the macropores. At higher porosity (when the distance between micropores and macropores become small), this capture into the micropores assists the capture into the macropores

Effect of chromophore-chromophore electrostatic interactions in the NLO response of functionalized organic-inorganic sol-gel materials

In the last years, important non-linear optical results on sol-gel and polymeric materials have been reported, with values comparable to those found in crystals. These new materials contain push-pull chromophores either incorporated as guest in a high Tg polymeric matrix (doped polymers) or grafted onto the polymeric matrix. These systems present several advantages; however they require significant improvement at the molecular level - by designing optimized chromophores with very large molecular figure of merit, specific to each application targeted. Besides, it was recently stated in polymers that the chromophore-chromophore electrostatic interactions, which are dependent of chromophore concentration, have a strong effect into their non-linear optical properties. This has not been explored at all in sol-gel systems. In this work, the sol-gel route was used to prepare hybrid organic-inorganic thin films with different NLO chromophores grafted into the skeleton matrix. Combining a molecular engineering strategy for getting a larger molecular figure of merit and by controlling the intermolecular dipole-dipole interactions through both: the tuning of the push-pull chromophore concentration and the control of TEOS (Tetraethoxysilane) concentration, we have obtained a r33 coefficient around 15 pm/V at 633 nm for the classical DR1 azo-chromophore and a r33 around 50 pm/V at 831 nm for a new optimized chromophore structure.Comment: 10 pages, 11 figures, 1 tabl

Pb Mg1/3 Nb2/3 O3 (PMN) MULTILAYER CAPACITORS

Des condensateurs milticouches ont été réalisés à partir d'une chamotte 3 PbO - MgO - Nb2O5 dopée en oxyde de plomb, en utilisant comme électrode interne un alliage 70 Ag - 30 Pd. La nature et la répartition des phases dans les chips varient suivant l'importance du dopage en oxyde de plomb (6 - 8 % en poids) et les conditions de frittage (TMax : 850°C - 1000°C, à l'air ou sous atmosphère contrôlée en plomb). On trouve, à travers une section d'un condensateur, une phase pyrochlore particulièrement développée en surface alors que la zone active est riche en phase perovskite associée éventuellement à une phase vitreuse à base d'oxyde de plomb, celle-ci abaissant la constante diélectrique du condensateur.Multilayer capacitors were made from a 3PbO - MgO - Nb2O5 mixing doped with lead oxide, and a 70 Ag - 30 Pd alloy as internal electrode. The phase nature and distribution in chips depend on the additions of lead oxide (6-8 wt %) and the sintering step (TMax : 850 - 1000°C, in air or in a PbO controlled atmosphere). Through a chip cross section, one can note a pyrochlore type phase especially developped in the surface layer while the chip active area is essentially constituted of a perovskite type phase (PMN) and of an eventual glassy phase based on lead oxide, which strongly lowers the chip dielectric constant

Nuclear magnetic relaxation of 23Na in beta-alumina electrolytes

Nuclear spin-lattice relaxation times have been measured in 23Na beta-alumina between 120 and 800 K. It is shown that the relaxation is governed by the modulation of the quadrupolar hamiltonian due to translational diffusion of the ions. NMR provides a direct access to the hopping frequency of the ions. The data suggest the possible existence of two kinds of local motions characterized by the activation energies 0.2 and 0.1 eV.Les temps de relaxation nucléaire spin-réseau ont été mesurés dans l'alumine beta au sodium entre 120 et 800 K. On a montré que la relaxation était due à la modulation de l'hamiltonien quadrupolaire par la diffusion des ions par translation. La RMN permet d'atteindre directement la fréquence de saut des ions. Les résultats suggèrent l'existence de 2 sortes de mouvements locaux caractérisés par les énergies d'activation 0,2 et 0,1 eV

S.A.X.S. STUDY OF NASIGEL AND NASIGLAS (SODIUM SUPERIONIC GELS AND GLASSES)

Des gels monolithiques ont été préparés dans le système Na2O-ZrO2-SiO2-P2O5, par la méthode sol/gel, (hydrolyse - polycondensation d'alkoxydes). L'évolution structurale pendant la gélification et la transition gel/verre a été étudiée par diffusion centrale de rayons X pour deux compositions. Des inhomogénéités peuvent exister dans le gel, elles correspondent probablement à la présence de clusters de zircone cubique qui se dissolvent progressivement dans la matrice silicatée lors de la transition gel/verre.Monolithic gels have been prepared in the Na2O-ZrO2-SiO2-P2O5 system by a low temperature chemical polymerization from metal or non metal alkoxide hydrolysis. We present here, for two typical compositions, structural results from a small angle X-ray scattering study (S.A.X.S.), concerning sol/gel, gel/xerogel and xerogel/glass transitions. Gels can exhibit inhomogeneities, probably due to cubic zirconia clusters, which progressively disappear during the gel/glass conversion

ALUMINO-SILICATE AEROGELS

Des aérogels d'alumino-silicates, de densité comprise entre 50 et 250 kg/m3, ont été élaborés à partir du précurseur mixte (BuO)2Al-O-Si (OEt)3. La diffusion centrale des rayons X montre que les structures des aérogels et des alcogels d'alumino-silicates sont formées d'unités élémentaires dont l'assemblage par un mécanisme d'agrégation conduit à des particules fractales en masse. L'auto-similarité est mise en évidence dans les aérogels pour un large domaine de densité.Alumino-silicate aerogels have been prepared from the (BuO)2Al-O-Si (OEt)3 double precursor with different densities (from 50 to 250 kg/m3). SAXS study indicates that both alumino-silicate alcogel and aerogel structures can be represented by primary units which stick together into mass-fractal clusters (D = 1.8-1.9) by a mechanism of cluster-cluster aggregation. Self similarity is displayed in the alumino-silicate aerogels in a large range of densities

Growth process and structure of aluminosilicate gels

Small-angle X-ray scattering and (29Si—27Al) NMR were used to study the growth process and structure of sols and gels in the silicon-aluminium ester /H2O/i PrOH system. The structure of elementary particles is governed by a chemical growth process with the formation of Al-O-Al linkages, and the self-similar structure of the gel (fractal dimension of 1.8) results from a cluster-cluster growth process.La croissance et la structure des sols et gels dans le système ester silicique /H2O/i PrOH ont été étudiés par RMN et diffusion centrale des rayons X. La structure des particules élémentaires est gouvernée par un processus chimique avec formation de liaisons Al-O-Al et la structure fractale du gel (dimension 1,8) résulte d'une agrégation de clusters