Magnetic anisotropy in epitaxial Mn5Ge3 films

High crystalline quality Mn 5 Ge 3 films with thicknesses ranging 4–200 nm have been grown on Ge(111) substrates by solid phase epitaxy. The basal hexagonal plane of Mn 5 Ge 3 is in epitaxy with the Ge(111) plane. Magnetic properties of the films have been investigated as a function of the film thickness and the magnetization curves have been analyzed using a theory that includes a description of magnetic domains in uniaxial thin films. The results clearly indicate the existence of a critical thickness below which the magnetic stripe phase disappears. We have determined the value of this thickness to lie between 10 and 25 nm from the analysis of experimental magnetization curves and the theoretical fit of the in-plane remanent magnetization. Although analogies can be drawn between the behavior observed in our system and that of hcp Co, we have shown that the critical thickness is considerably smaller in Mn 5 Ge 3 ; this has the potential to open new fields of applications for Mn 5 Ge 3 thin films in magnetic recording and spintronics

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Soxhlet assisté par micro-ondes (conception et application à l'extraction des matières grasses)

De manière générale, la préparation d'échantillon peut être décomposée en deux étapes distinctes : extraction et analyse. Alors que l'étape analytique requiert quelques minutes, l'étape d'extraction nécessite plusieurs heures (ou jours). C'est le cas de la méthode de Soxhlet, inventée en 1879 qui est aujourd'hui la procédure de référence pour l'extraction solide-liquide. Depuis la découverte des effets thermiques des micro-ondes par Spencer en 1946, l'impact des micro-ondes sur la matière a été largement caractérisé et validé par la communauté scientifique. Rapidement, les micro-ondes ont été utilisées pour l'extraction dans le but de contrecarrer les limites des techniques d'extraction conventionnelles. Notre travail a consisté en la conception, l'optimisation et la valorisation d'une technique d'extraction destinée au dosage des matières grasses dans les matrices alimentaires solides : le Soxhlet assisté par micro-ondes. Une première partie du document est consacrée à la compréhension des fondements et considérations théoriques de l'énergie micro-ondes. Une seconde partie du manuscrit rappelle les techniques employées pour la conception et valorisation du Soxhlet assisté par micro-ondes. Enfin, la dernière partie du manuscrit traite des résultats obtenus et s'articule selon trois sections distinctes : dans un premier chapitre, la mise au point, l'optimisation et l'application du nouveau dispositif sont décrites. L'appareillage permet des extractions rapides, non destructrices et généralisables à diverses matrices alimentaires discutées dans un second chapitre. L'approche écologique de l'invention, combinée à l'emploi du d-limomène, comme alternative au n-hexane conventionnel, fait l'objet d'un troisième et dernier chapitreIn most cases, sample preparation can be divided in two distinctive steps : extraction and analysis. While analytical step requires few minutes, the extraction step requires several hours (or days). This applies to the Soxhlet method, invented in 1879, which today is the reference for solid-liquid extraction. Since the discovery of thermal effects of microwaves by Spencer in 1946, the impact of microwaves on the matter has been widely characterized and validated by the scientific community. Quickly, microwaves were used for extraction to counteract limitations of conventional extraction procedures. Our work consisted of design, optimization and development of a new extraction technique for the determination of fat in solid food samples : the microwave-integrated Soxhelt. The first part of the document is devoted to understanding the theoretical foundations and considerations of microwave energy. A second part of the manuscript refers to the techniques used for the design and optimization of microwave-integrated Soxhlet. Finally, the last part of the manuscript deals with the results obtained and is divided into three distint sections : in a first chapter, the development, optimization and implementation of the new device are described. The apparatus allows rapid and non-destructive extractions that can be generalized for various food matrices. This topic is discussed in a second chapter.The ecological approach of the invention, combined with the use d-limonene, as an alternative to conventional n-hexane, is the subjet of a third and final chapterAVIGNON-BU Centrale (840072102) / SudocSudocFranceF

Sonochemistry of actinides: from ions to nanoparticles and beyond

International audienceSonochemistry studies chemical and physical effects in liquids submitted to power ultrasound. These effects arise not from a direct interaction of molecules with sound waves, but rather from the acoustic cavitation: the nucleation, growth, and implosive collapse of microbubbles in liquids submitted to power ultrasound. The violent implosion of bubbles leads to the formation of chemically reactive species. In principle, each cavitation bubble can be considered as a microreactor initiating chemical reactions at mild conditions. In addition, microjets and shock waves accompanied bubble collapse produce fragmentation, dispersion and erosion of solid surfaces or particles. Microbubbles oscillating in liquids also enable nucleation and precipitation of nanosized actinide compounds with specific morphology. This review focuses on the versatile sonochemical processes with actinide ions and particles in homogeneous solutions and heterogeneous systems. The redox reactions in aqueous solutions, dissolution or precipitation of refractory solids, synthesis of actinide nanoparticles, and ultrasonically driving decontamination are considered. The guideline for further research is also discussed

Comportement Sonochimique d'Oxydes d'Actinides Nanostructurés

International audienceLes matériaux nanostructurés peuvent être définis comme des matériaux ayant une structurationde l’ordre de quelques nanomètres (typiquement de 1 à 10 nanomètres). La nanostructurationdes matériaux est un sujet d’étude important en raison de leurs propriétés physiques et chimiquesintéressantes qui peuvent être optimisées en fonction de leur taille, morphologie et structure. Lesparticules nanométriques qui composent ces matériaux possèdent un grand rapport surface survolume et permettent d’offrir une plus grande quantité de sites actifs par rapport à d’autresmatériaux de taille plus importante. Par conséquent, les matériaux nanostructurés peuvent trouverdes applications dans la catalyse, le biomédical, la synthèse de matériaux luminescents.Cependant, la nanostructuration des oxydes d’actinides et leur réactivité qui en résulte sont peurapportées dans la littérature. Des études récentes ont montré que la nanostructuration d’oxydesde plutonium semble jouer un rôle dans la formation sonochimique de colloïdes de Pu en solutionsaqueuses

Total Lipid Extraction of Food Using d-Limonene as an Alternative to n-Hexane

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La Sonochimie, une Chimie sans Réactifs

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Simultaneous H/D and 13^{13}C/12^{12}C anomalous kinetic isotope effects during the sonolysis of water in the presence of carbon monoxide

International audienceSplitting of water molecules driven by ultrasound plays a central role in sonochemistry. While studies of sonoluminescence revealed the formation of a plasma inside the cavitation bubble, much less is known about the contribution of plasma chemical processes to the sonochemical mechanisms. Herein, we report for the first time sonochemical processes in water saturated with pure CO. The presence of CO causes a large increase in the H/D kinetic isotope effect (KIE) to $\alpha_H$ = 14.6 ± 1.8 in a 10% H$_2$O/D$_2$O mixture under 20 kHz ultrasound.The anomalous H/D KIE is attributed to electron quantum tunneling in the plasma produced by cavitation. In addition, CO$_2$ formed simultaneously with hydrogen during the sonochemical process is enriched with the $^{13C}$C isotope, which indicates a VV pumping mechanism typical for non-equilibrium plasma. Both observed KIEs unambiguously point to the contribution of quantum effects in sonochemical mechanisms

Sonochemical redox reactions of Pu( iii ) and Pu( iv ) in aqueous nitric solutions

International audienceThe behavior of Pu(IV) and Pu(III) was investigated in aqueous nitric solutions under ultrasound irradiation (Ar, 20 kHz). In the absence of anti-nitrous reagents, ultrasound has no effect on Pu(IV), while Pu(III) can be rapidly oxidized to Pu(IV) due to the autocatalytic formation of HNO$_2$ induced by HNO$_3$ sonolysis. In the presence of anti-nitrous reagents (sulfamic acid or hydrazinium nitrate), Pu(IV) can be sonochemically reduced to Pu(III). The reduction follows a first order reaction law and leads to a steady state where Pu(IV) and Pu(III) coexist in solution. The reduction process is attributed to the sonochemical generation of H$_2$O$_2$ in solution. The kinetics attributed to the reduction of Pu(IV) are however higher than those related to the formation of H$_2$O$_2$ which, after several hypotheses, is explained by the sonochemical erosion of the titanium-based sonotrode. Titanium particles thereby generated can be solubilized under ultrasound and generate Ti(III) as an intermediate species, a strong reducing agent able to react with Pu(IV)