Papers by Author: Matteo Leoni

Abstract: Patterns calculated by applying the Debye function to faulted spherical nanoparticles are used to test the accuracy of modern Line Profile Analysis theory of faulting for small crystallites. The relative deviation of the determined fault density is found to be dependent on the fault position, and on the particle size. The study of the average pattern from systems of 100 particles (D = 9.8nm) shows an overestimated deviation of the determined fault density by as much as 30%.

Abstract: Recent advances in Line Profile Analysis of powder diffraction patterns must be paralleled by increasing attention to the quality and quantity of experimental data. The analysis of simulated data with different noise levels demonstrates the importance of statistical quality to reveal fine details of interest in the analysis of nanocrystalline materials, like the crystallite shape. It is also shown how synchrotron radiation diffraction can improve data quality with respect to laboratory measurements, both in terms of statistical quality and in terms of accessible information.

Abstract: The basics of the Whole Powder Pattern Modeling and its implementation in the PM2K software are briefly reviewed. The main features and functionalities, and most common line broadening models are introduced with the aid of working examples related to the instrumental profile and to a plastically deformed metal. A summary of the main expressions is reported in the appendix, together with a list of useful references.

Abstract: Whole Powder Pattern Modeling (WPPM) is especially useful to study line defects in powder and polycrystalline materials. As a result of recent progresses in this field, dislocation studies can be carried out for any slip system and crystal symmetry of the studied phases. Basic theory and procedures are described with the help of some representative cases of study. The use of the dislocation-related broadening in the PM2K software implementing the WPPM approach is shown with practical examples.

Abstract: The implementation of a physically-sound size broadening model for peak profiles in the Rietveld method is presented. TOPAS macros are provided and the results compared with analogous modelling performed according to advanced analysis methods such as the Whole Powder Pattern Modelling.

Abstract: The recent evolution of powder diffraction line profile analysis toward full pattern methods is discussed. Specific reference is made to the Whole Powder Pattern Modelling (WPPM), as applied to metals and ceramics subjected to strong plastic deformation. Examples concerning three different materials science studies are shown to illustrate features and potentialities of the WPPM approach.

Abstract: The residual stress in thin coatings of yttria stabilised-zirconia produced by Low Pressure Plasma Spraying were measured by X-ray Diffraction, using laboratory as well as synchrotron radiation sources. The specific microstructure, with absence of texture and fine distribution of nearly equiaxed grains, point out that despite the markedly anisotropic nature of cubic zirconia, coatings can be considered as macroscopically isotropic. This picture is also confirmed by the results of a parallel study, where the X-ray elastic constants were measured in-situ along two crystallographic directions ([440] and [620]): measured values agree fairly well with those calculated from single-crystal data under the Neerfeld-Hill assumptions. SR XRD provided a detailed information on the stress field across the thickness of the zirconia coatings. In particular the presence of a stress gradient was observed and modelled. The average stress is compressive, and increases with the coating thickness. Compression tend to increase from the surface toward the inside of the coating reaching a maximum of ~-1.0 GPa in a 24 µm coating.

Abstract: The role of grain surfaces and interfaces in nanocrystalline materials is investigated by X-ray diffraction (XRD). Two apparently distinct aspects show the importance of the interaction of a single grain with the environment in which it is placed: the role of grain-grain interaction in determining the residual stress in a polycrystalline thin film and the surface effects in single powder grains on the macroscopic average cell parameter determined by powder diffraction. A brief theoretical introduction of the phenomena and their possible modelling is presented together with a discussion of practical examples.