Spinel Ferrite Nanoparticles: Correlation of Structure and Magnetism

This chapter focuses on the relationship between structural and magnetic properties of cubic spinel ferrite MFe2O4 (M = Mg, Mn, Fe, Co, Ni, Cu and Zn) nanoparticles (NPs). First, a brief overview of the preparation methods yielding well‐developed NPs is given. Then, key parameters of magnetic NPs representing their structural and magnetic properties are summarized with link to the relevant methods of characterization. Peculiar features of magnetism in real systems of the NPs at atomic, single‐particle, and mesoscopic level, respectively, are also discussed. Finally, the significant part of the chapter is devoted to the discussion of the structural and magnetic properties of the NPs in the context of the relevant preparation routes. Future outlooks in the field profiting from tailoring of the NP properties by doping or design of core‐shell spinel‐only particles are given

Studies on the hydrothermal synthesis of CdxZn1-x S compounds

In this study series of CdxZn1-xS solid solutions with different amounts of Cd and Zn were synthesized by the hydrothermal treatment of aqueous solutions containing CdCl2, Na2S · 9 H2O and ZnSO4 · 7 H2O. The aim was to examine the influence of Zn concentration and processing conditions (hydrothermal temperature and duration) on the structure of the obtained powders and their photocatalytic activity (in water splitting process). The obtained photocatalysts (with and without Pd co-catalyst) were analysed by X-ray diffraction (XRD), atomic absorption spectroscopy (AAS), ultraviolet-visible spectroscopy (UV-VIS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET) method and gas chromatography (GC). The XRD results confirmed the crystallinity of the compounds and transition from hexagonal to cubic phase with increasing Zn content. Complete transformation from hexagonal to cubic phase did not take place, and both phases were present in almost all samples. BET analysis showed the importance of the pore distribution and pore size, especially in the case of photocatalysts with different duration treatment. GC measurements of the photocatalysts without and with Pd co-catalyst confirmed the production of hydrogen for all tested compounds. The best photocatalytic performance was achieved by the sample Zn50230/72-Pd prepared at 230 °C, for 72 hours, with 50% Zn and in the presence of Pd co-catalyst. The synthesis implied neither stabilizer nor organic compound

Material effect in the fuel coolant interaction (structural characterization of the steam explosion debris and solidification mechanism)

Ce travail a été réalisé en cotutelle entre l Université Charles à Prague (République Tchèque) et l'Université de Strasbourg (France). Il a également profité d une coopération entre l'Institut de Chimie Inorganique de l'Académie des Sciences de République Tchèque et le Commissariat à l'Énergie Atomique et aux Énergies Alternatives (CEA, Cadarache, France). Les résultats des travaux ont contribué au projet OCDE / AEN Serena 2 (Programme portant sur l étude des effets d'une explosion de vapeur dans un réacteur nucléaire à eau). La thèse présentée se situe dans le domaine de la sûreté nucléaire et de la science des matériaux. Elle traite de l Interaction Combustible-Réfrigérant (ICR, ou FCI en anglais pour Fuel-Coolant Interaction) susceptible d intervenir lors d un accident grave de réacteur nucléaire et actuellement à l étude dans les programme de R&D. Au cours d un accident de fusion d un coeur de réacteur, les matériaux fondus peuvent interagir avec le liquide de refroidissement (eau légère), aussi appelé réfrigérant. Cette interaction peut se produire à l'intérieur de la cuve ou, en cas de rupture de celle-ci, à l'extérieur. Ces deux scénarios sont couramment appelés Interaction Combustible-Réfrigérant en- et hors- cuve et se distinguent de par les conditions du réacteur lors de l accident : pression du système, degré de sous refroidissement de l eau, etc. L'interaction entre le combustible fondu et le liquide de refroidissement peut évoluer vers une détonation thermique appelée explosion de vapeur qui peut endommager le réacteur, voire compromettre l'intégrité du confinement. Des expériences récentes ont montré que la composition du combustible a un effet majeur sur l apparition et le rendement d une telle explosion. En particulier, des comportements différents ont été observés entre un matériau simulant, l'alumine, qui explose très facilement, et diverses compositions de corium prototypique (80 m. % UO2, 20% m.% ZrO2). Cet effet matériau a suscité un intérêt nouveau pour les analyses post-expériences des débris issus de l ICR afin de déterminer les mécanismes qui interviennent au cours de ces phénomènes extrêmement rapides. La thèse est organisée en neuf chapitres. Le chapitre 1 constitue une introduction générale et présente le contexte d un accident grave d un réacteur nucléaire. Quelques exemples d accidents graves (Three Miles Island 1979, Tchernobyl 1986 et Fukushima 2011) sont brièvement abordés. Le chapitre 2 résume les aspects théoriques de l'interaction combustible-réfrigérant. Il est divisé en quatre parties correspondant aux quatre étapes généralement rencontrées lors du mécanisme d ICR i) Prémélange - le combustible fondu, versé dans l'eau, se fragmente en gouttelettes grossières qui s isolent d un film de vapeur. ii) Déclenchement le film de vapeur entourant les gouttes de combustible est déstabilisé, permettant ainsi la fragmentation fine du combustible. iii) Propagation - la fragmentation du combustible se propage à l ensemble du prémélange, augmentant ainsi la surface de contact entre le combustible fondu et l eau. Ceci conduit à une production intense de vapeur à grande échelle. iv) Expansion (explosion) - l'énergie thermique transférée du combustible à l'eau est transformée en travail mécanique de la vapeur.[...]This work has been performed under co-tutelle supervision between Charles University in Prague (Czech Republic) and Strasbourg University (France). It also profited from the background and cooperation of Institute of Inorganic Chemistry Academy of Science of the Czech Republic and French Commission for Atomic and Alternative energies (CEA Cadarache). Results of the work contribute to the OECD/NEA project Serena 2 (Program on Steam Explosion Resolution for Nuclear Applications).Presented thesis can be classed in the scientific field of nuclear safety and material science. It is aimed on the socalled molten nuclear Fuel Coolant Interaction (FCI) that belongs among the recent issues of the nuclear reactorsevere accident R&D. During the nuclear reactor melt down accident the melted reactor load can interact with the coolant (light water). This interaction can be located inside the vessel or outside in the case of vessel break-up. These two scenarios are commonly called in- and ex-vessel FCI and they differ in the conditions such as initial pressure of the system, water sub-cooling etc. The Molten fuel coolant interaction can progress into thermal detonation called steam explosion that can challenge the reactor or containment integrity.Recent experiments have shown that the melt composition has a major effect on the occurrence and yield of such explosion. In particular, different behaviors have been observed between simulant material (alumina), which has important explosion efficiency, and some prototypic corium compositions (80 w. % UO2, 20% w. % ZrO2). This material effect has launched a new interest in the post-test analyses of FCI debris in order to estimate the processes occurring during these extremely rapid phenomena. The thesis is organized in nine chapters. The chapter 1 gives the general introduction and context of the nuclear reactor accident. Major nuclear accidents (Three Miles Island 1979, Chernobyl 1986 and Fukushima 2011) are briefly described. The chapter 2 summarizes the theoretical aspects of the fuel coolant interaction. It is divided in four thematic fields according to the FCI progression. In general, FCI has four stages: i) Premixing hot melt is poured in water and fragmented in coarse droplets surrounded by steam filmii) Triggering steam film around melt droplets is destabilized allowing fine fragmentation iii) Propagation the fine fragmentation propagate through the premixture increasing the melt water interface area, which leads to large steam production iv) Expansion (explosion) Thermal energy transferred from the melt to water is changed into mechanical workof the steam.The chapter 3 summarizes the research conducted in different experimental facilities using nonradioactive simulant or radioactive prototypic materials. The chapter 4 shows the results of thermodynamic calculations, by which thepossible chemici reactions between melts and water/steam at high temperatures were modeled. Second part presentsthe results of 1D calculations of radiation heat transfer from FCI materials to water/steam. The chapter 5 describes the material analyses of non-radioactive simulant debris coming from MISTEE experimental research program (KTH, Sweden) and PREMIX, ECO facilities (FZK, Germany). The chapters 6 to 8 describe the material analyses of radioactive prototypic debris coming from KROTOS research program (CEA, France). The KROTOS KS2 test used melt composition 70 w. % UO2 and 30 w. % ZrO2, the KS4 test 80 w. % UO2 and 20 w. % ZrO2, the last KS5 test used suboxidized melt 80.1 w. % UO2 and 11.4 w. % ZrO2 and 8.5 w. % metallic Zr. The chapter 9 concludes the work and presents future perspectives.STRASBOURG-Bib.electronique 063 (674829902) / SudocSudocFranceF

Préparation de nanocomposites magnétiques de grenat/SiO2 et sphinelle/SiO2 par la méthode sol-gel et caractérisation

STRASBOURG-Sc. et Techniques (674822102) / SudocSudocFranceF

Hydrothermally grown porous FeVO4 nanorods and their integration as active material in gas-sensing devices

Controllable fabrication of highly porous iron vanadate (FeVO4) thick film consisting of disordered nanorods suitable for gas penetration and permeation was achieved by hydrothermal synthesis of fervanite-like FeVO4 center dot 1.1H(2)O. The subsequent dehydration to FeVO4 was investigated by Fe-57 Mossbauer spectroscopy (DQS), DTA, magnetic susceptibility (c) and electron microscopy (REM/TEM). Their integration in gas sensing devices as porous layer via polymer-blended (PVDF) doctor-blading approach was successfully demonstrated followed by investigations of their electric properties and oxygen sensing capability. The probed I-U behaviour and UV-Vis measurements confirmed the semiconducting nature of triclinic FeVO4 (E-g = 2.72 eV) and exhibited an activation energy for electric conduction of 0.46 eV. The best sensitivity of 0.29 +/- 0.01 (m = -3.4 +/- 0.1) could be obtained at an optimal working temperature of 250 degrees C

Multiscale magnetization in cobalt-doped ferrite nanocubes

The magnetization of cobalt ferrite nanocubes of similar size, but with varying Co/Fe ratio, is extensively characterized on atomistic and nanoscopic length scales. Combination of X-ray diffraction, Mossbauer spectroscopy, magnetization measurements and polarized small-angle neutron scattering (SANS) reveals that a lower amount of cobalt leads to an enhanced magnetization. At the same time, magnetic SANS confirms no or negligible near-surface spin disorder in these highly crystalline, homogeneously magnetized nanoparticles, resulting in an exceptionally hard magnetic material with high coercivity

Nanocomposite of CeO2 and High-Coercivity Magnetic Carrier with Large Specific Surface Area

We succeeded in the preparation of CoFe2O4/CeO2 nanocomposites with very high specific surface area (up to 264 g/m2). First, highly crystalline nanoparticles (NPs) of CoFe2O4 (4.7 nm) were prepared by hydrothermal method in water-alcohol-oleic acid system. The oleate surface coating was subsequently modified by ligand exchange to citrate. Then the NPs were embedded in CeO2 using heterogeneous precipitation from diluted Ce3+ sulphate solution. Dried samples were characterized by Powder X-Ray Diffraction, Energy Dispersive X-Ray Analysis, Scanning and Transmission Electron Microscopy, Mössbauer Spectroscopy, and Brunauer-Emmett-Teller method. Moreover, detailed investigation of magnetic properties of the bare NPs and final composite was carried out. We observed homogeneous embedding of the magnetic NPs into the CeO2 without significant change of their size and magnetic properties. We have thus demonstrated that the proposed synthesis method is suitable for preparation of extremely fine CeO2 nanopowders and their nanocomposites with NPs. The morphology and magnetic nature of the obtained nanocomposites make them a promising candidate for magnetoresponsive catalysis

Formation of Nanoparticles of E-Fe2O3 from Yttrium Iron Garnet in a Silica Matrix: An Unusually Hard Magnet with a Morin-Like Transition below 150 K

The elusive ε-Fe2O3 has been obtained as nanoparticles by vacuum heat treatment of yttrium iron garnet in a silica matrix at 300 °C followed by annealing at 1000 °C for up to 10 h in air and employing formamide as a gel modifier. Its nuclear structure is temperature independent as observed from the neutron powder diffraction patterns and has been modeled by the published structures on analogous MM‘O3 compounds. It displays complex magnetic properties that are characterized by two transitions:  one at 480 K from a paramagnet (P) to canted antiferromagnet (CAF1) and the second at ca. 110 K from the canted antiferromagnet (CAF1) to another canted antiferromagnet (CAF2) that has a smaller resultant magnetic moment (i.e., smaller canting angle). The latter transition resembles that of Morin for α-Fe2O3 at 260 K. The magnetization shows unusual history dependence:  it has a bifurcation below 100 K if the field is applied at low temperatures after zero-field-cooled, whereas the bifurcation is above 150 K if the field is applied at high temperatures. The magnetic hardness first increases slightly from 300 to 200 K, then it drastically decreases to zero at 100 K and follows a further increase down to 2 K. The coercive field reaches an unexpected and quite exceptional 22 kOe at 200 K. There appears to be a further ill-defined metamagnetic transition below 50 K, characterized by a doubling of the measured magnetization in 50 kOe. The AF1−AF2 transition is accompanied by sharp peaks in both the real and imaginary components of the ac-susceptibility due to the hard−soft effect, and their peak maxima shift to lower temperatures on increasing the frequency. Mössbauer spectra are characterized by a change in hyperfine field of the tetrahedral Fe by ca. 40% around the transition, suggesting a change of geometry

Field Dependence of Magnetic Disorder in Nanoparticles

The performance characteristics of magnetic nanoparticles toward application, e.g., in medicine and imaging or as sensors, are directly determined by their magnetization relaxation and total magnetic moment. In the commonly assumed picture, nanoparticles have a constant overall magnetic moment originating from the magnetization of the single-domain particle core surrounded by a surface region hosting spin disorder. In contrast, this work demonstrates the significant increase of the magnetic moment of ferrite nanoparticles with an applied magnetic field. At low magnetic field, the homogeneously magnetized particle core initially coincides in size with the structurally coherent grain of 12.8(2) nm diameter, indicating a strong coupling between magnetic and structural disorder. Applied magnetic fields gradually polarize the uncorrelated, disordered surface spins, resulting in a magnetic volume more than 20% larger than the structurally coherent core. The intraparticle magnetic disorder energy increases sharply toward the defect-rich surface as established by the field dependence of the magnetization distribution. In consequence, these findings illustrate how the nanoparticle magnetization overcomes structural surface disorder. This new concept of intraparticle magnetization is deployable to other magnetic nanoparticle systems, where the in-depth knowledge of spin disorder and associated magnetic anisotropies are decisive for a rational nanomaterials design

Low temperature superparamagnetic nanocomposites obtained by Fe(acac)3-SiO2-PVA hybrid xerogel thermolysis

Fe(acac)3/silica/PVA hybrid xerogel nanocomposite was obtained by one pot acid catalysed sol-gel synthesis using the homogeneous mixture of iron(III) acetylacetonate (Fe(acac)3), tetraethylorthosilicate (TEOS), and polyvinyl alcohol (PVA). Nominal composition ratio of iron oxide/silica was 15/85 (weight percent). Nitric acid was used as catalyst. Another sample of Fe(acac)3/silica xerogel without PVA addition was prepared in the similar processing conditions. Based on thermal analysis studies, the thermal behaviour of both xerogel samples was unveiled and it allowed choosing the optimal calcination temperatures in order to obtain iron oxide silica magnetic nanocomposite samples. The two xerogel (with and without PVA) samples were thermally treated, in air, at 220, 260 and 300 °C and characterized by different techniques. XRD investigations revealed phase composition evolution with calcination temperature, from cubic spinel phase (maghemite) to hexagonal stable hematite containing nanocomposite of 10–20 nm average crystallite size. These findings were confirmed by Mössbauer spectroscopy. Up to 300 °C, the surface area and total pores volume increased with temperature for all samples. By calcination at the same temperature, the hybrid xerogel containing PVA resulted in significantly higher magnetization and free volume values in comparison with the sample without PVA