J.C. Niepce

Some recent developments in mechanical activation and mechanosynthesis

After outlining the general characteristics of high-energy ball-milling, mechanochemical synthesis is argued to be an attractive method for the synthesis and transformation of materials. Phase transformations induced by milling, annealing of mechanically activated iron silicides, mechanically activated self-heat sustaining reactions (MASHS), for instance of FeAl, are first discussed. The route, which starts from mechanosynthesized powders to reach consolidated alumina-(Fe,Ti) composites, yields materials whose original morphologies and some mechanical properties are then described.

Neutron Diffraction Study of the in Situ Oxidation of UO2

This paper discusses uranium oxide crystal structure modifications that are observed during the low-temperature oxidation which transforms UO(2) into U(3)O(8). The symmetries and the structural parameters of UO(2), beta-U(4)O(9), beta-U(3)O(7), and U(3)O(8) were determined by refining neutron diffraction patterns on pure single-phase samples. Neutron diffraction patterns were also collected during the in situ oxidation of powder samples at 483 K. The lattice parameters and relative ratios of the four pure phases were measured during the progression of the isothermal oxidation. The transformation of UO(2) into U(3)O(8) involves a complex modification of the oxygen sublattice and the onset of complex superstructures for U(4)O(9) and U(3)O(7), associated with regular stacks of complex defects known as cuboctahedra, which consist of 13 oxygen atoms. The kinetics of the oxidation process are discussed on the basis of the results of the structural analysis.

Enhancement of self-sustaining reaction by mechanical activation: case of an FeSi system

Mechanical high energy ball milling of an Fe2Si elemental powders mixture was used to activate a self sustaining combustion reaction or so-called self-propagating high-temperature synthesis (SHS) to form iron disilicide, a reaction for which the thermodynamic criterion is not favorable. A complete characterization of the milled powders before reaction was performed with energy dispersive X-ray spectrometry, specific surface measurements and X-ray diffraction profile analysis. Thermal and structural information describing the combustion front initiated by heating up a sample to 400°C in a Fe‐Si system is communicated. In order to isolate the phases involved in the gasless reaction, a time-resolved X-ray diffraction experiment was designed to study in situ the formation of silicide phases (FeSi and b-FeSi2) produced by the new process called MASHS (mechanically activated self-propagating high-temperature synthesis).

High magnification SEM observations for two types of granularity in a high burnup PWR fuel rim

Abstract Rim microstructure of an UO2 pellet irradiated up to 61 GWd/tU, observed with high magnification scanning electron microscopy, presented two types of subgrains: polyhedral and round subgrains. Round subgrains were also observed on the surface of fabrication pores along the pellet radius. These round-shaped subgrains seem to be associated to free surface rearrangement. This new type of round subgrains was proved to be independent of the rim effect. Observations of the fuel structure in the vicinity of rim area evidenced two types of defects specific to rim effect: planar defects and small intergranular pores. Hypothesis is made that the accumulation of planar defects could generate polyhedral subgrains, a new description of the rim effect is proposed including this planar defect. These observations prove that a careful examination of subgrains morphology is necessary to identify rim effect.

Dielectric and E.P.R. studies of Mn-doped barium titanate

Manganese introduced in BaTiO3 as a substituant of titanium is able to trap the electrons produced during a sintering under a very low oxygen pressure. However it strongly modifies the vibrational ...

Thermodynamic considerations of the grain size dependency of material properties

Phase transitions which depend on grain size induce very interesting properties in materials such as zirconia or barium titanate. A new and rigorous thermodynamic treatment of this type of phase transition is proposed with consideration of the surface phenomena. An interpretation is given of the observed differences when the material—particularly BaTiO3—under consideration is a fine grain powder or is a fine grain ceramic.ZusammenfassungKorngrößenabhängige Phasenumwandlungen verursachen bei Materialien wie Zirkonerde oder Bariumtitanat sehr interessante Eigenschaften. Unter Berücksichtigung der Oberflächenerscheinungen wurde ein neue und gründliche thermodynamische Behandlung dieses Typs von Phasenumwandlungen vorgeschlagen. Es wird eine Erörterung der beobachteten Differenzen in Abhängigkeit davon angestellt, ab das untersuchte Material—hier BaTiO3—ein feingekörntes Pulver oder eine feingekörnte Keramik ist.

Mechanically Activated SHS Reaction in the Fe-Al System: In Situ Time Resolved Diffraction Using Synchrotron Radiation

The Mechanical Activation Self propagating High temperature Synthesis (M.A.S.H.S.) processing is a new way to produce nanocrystalline iron aluminide intermetallic compounds. This process is maily the combination of two steps ; in the one hand, a mechanical activation where the Fe - Al powder mixture was milled during a short time at given energy and frequency of shocks and in the other hand, a Self propagating High temperature Synthesis (S.H.S.) reaction, for which the exothermicity of the Fe + Al reaction is used. This fast propagated MASHS reaction has been in -situ investigated using the Time Resolved X - Ray Diffraction (TRXRD) using a X - ray synchrotron beam and an infrared thermography camera, allowing the coupling of the materials structure and the temperature field. The effects of the initial mean compositions, of the milling conditions as well as of the compaction parameters on the MASHS reaction are reported.

Dielectric Properties of Barium Strontium Titanate / Non Ferroelectric Oxide Ceramic Composites

Barium strontium titanate ceramics present high dielectric permittivity and tunability. In order to reduce their permettivity and loss tangent while keeping tunability, various composites of barium strontium titanate oxide...

Texture, structure and domain microstructure of ferroelectric PZT thin films

Abstract Ferroelectric thin films with different compositions from PZT 50 50 to PbTiO 3 were prepared by a modified sol-gel process and deposited by spin-coating on Pt/Ti/SiO 2 /Si(100)wafer. The ferroelectric and dielectric properties of the thin films dependent on thin film composition are presented and compared with bulk ceramic properties. The results correlated with the crystal structure and ferroelectric domain microstucture of PZT crystallites determined by X-ray diffraction. The influence of the sol-gel film process on the ferroelectric properties, the crystallite texture, structure and domain microstructure is discussed.

Main recent contributions to SHS from France

Both in situ TRXRD and IR thermography, on the one hand, and different levels of modeling, on the other, have generated a strong progress in the knowledge and control of numerous SHS reactions. The SHS of simple binary materials, such as intermetallics (FeAl, MoSi2, NbAl3, etc.), oxides (e.g. ZrO2), carbides (e.g. SiC) or nitrides, more complex materials, such as mullite, SiAlONs, MAX phases, composites (SiO2-Al2O3, NiAl-ZrO2), powders in their more complicated states, such as well controlled microstructures, fully densified intermetallics, smart composites, and hard coatings carried out by GFA researchers greatly contributed to the worldwide competition to harness the potential of SHS. On the occasion of the 40th anniversary of SHS, this paper is giving an overview of the main results obtained by GFA over the last ten years.