Papers by Author: B. Ouladdiaf

Abstract: Pyrrhotite [1] is a common ferrimagnetic mineral in terrestrial rocks and has been identified recently as the major remanence carrier in Martian rocks, as in SNC (Shergotty-Nakhla-Chassigny) type meteorites. This compound undergoes a low temperature magnetic transition around 32 K with a change in the natural magnetic remanence [2]. This transition is going to be of growing importance in paleomagnetism and rock magnetism. To determine the structure change at 32K neutron diffraction was performed on two crystal samples at 50K and 20K (D10-ILL). Based on the magnetic torque measurements, performed on the same samples, a model of low-temperature triclinic structure was build. This model, explains previous Mössbauer experiments [4], the torque measurements, the magneto-crystalline anisotropy decreasing.

Abstract: We give the necessary formulas to extract pure magnetic diffraction signals from the difference spectra between two neutron texture measurements, one operated with a sample at zero-magnetic field, and the other under a magnetic field. This enables us to calculate the total-magnetic-scattering ODF, and the polarisation-magnetic-scattering ODF, of an iron sample. Using the developed approach it is shown that under some hypotheses one can describe the three dimensional orientation of the magnetic moments in the sample under a given magnetic field.

Abstract: The orientation distributions of α-Al2O3 textured ceramics are determined from neutron diffraction and SEM-EBSD. A curved position-sensitive detector coupled to a tilt angle (χ) scan allowed the whole neutron diffraction pattern treatment in the combined Rietveld-WIMV-Popa algorithm. Analyses from neutron and electron diffraction data gave similar results if EBSD data are smoothed to account for grain statistics. Four textured alumina ceramics were prepared by slipcasting under a high magnetic field and sintered at 800°C, 1300°C, 1400°C and 1600°C. The inverse pole figures and EBSD-mapping highlights the influence of the magnetic field and sintering temperature on the texture development. The inverse pole figures calculated for the fiber direction show a major (001) component for all the samples. With the increasing sintering temperature, the texture strength is enhanced and the c-axis distribution is sharper. The effectiveness of the combined approach for determining the crystallite size is also evident. As a global trend, the calculated crystallite size and observed grain size are similar and increase with the increasing sintering temperature. The mechanism of the texture development in the sintered specimens is certainly initiated from the preferred orientation of the green body after slip-casting under a high magnetic field. The basal texture is enhanced during sintering by selective anisotropic grain growth. We evidenced here the powerfulness of the Rietveld texture analysis correlated to SEM-EBSD calculation to provide a basis for the correlation of texture, microstructural parameters and anisotropic properties.

Abstract: Orientation distributions of polyphased (Bi,Pb)2Sr2Ca2Cu3O10+d superconducting textured materials are determined from neutron diffraction analysis. The quantitative texture analysis of neutron data was accomplished by using the combined Rietveld-WIMV-Popa algorithms, implemented in the program package Materials Analysis Using Diffraction (MAUD). Curved position-sensitive detector and 4-circle diffractometry allow the whole diffraction pattern treatment. Transport critical current densities, measured on different samples, are strongly dependent of the calculated texture strengths, crystallite sizes and phase ratios. The results prove the interest of the combined approach for a quantitative texture analysis of complex materials. Texture to anisotropic physical properties relationship is discussed.

Abstract: A high speed hot forging process was applied to Nd-Fe-B and Nd-Fe-V as cast alloys in order to develop both the microstructure and the crystallographic texture appropriate for permanent anisotropic magnet properties. Neutron diffraction texture analyses are used to demonstrate the effect of the hot forging process on both kind of alloys. Microstructural changes are an important feature on forging in both cases. Coercivity is developed in the Nd-Fe-B alloy mainly from grain size reduction and disappearance of free iron. Stabilisation of the Nd(Fe,V)12 hard magnetic phase is achieved from the iron and Nd-rich microstructure of the starting Nd-Fe-V material. A comparison of the crystallographic textures of Nd2Fe14B and Nd(Fe,V)12 phases is done to account for the development of extrinsic magnetic anisotropy. In both cases, a nearly fibre texture is obtained in correlation with the symmetry of the deformation. However, the orientations are quite different in both alloys and the consequences on the magnetic properties are evidenced.