Most nanocrystalline materials, in powder or bulk form, show unit cell param- eters sensibly different from the coarse-grain or single-crystal counterpart. Even if the phenomenon is well known, a sound explanation is not always available: stoichiometric effects (e.g. related to vacancies or oxygen in excess) are frequently invoked to interpret the experimental evidence. Grain-surface relaxation is a possible alternative for data interpretation: due to the presence of the surface itself, atoms next to a surface are permanently displaced from their equilibrium lattice positions. The consequences of this, unveiled when the number of atoms in the surface layers starts to supersede the number of atoms in the rest of the grain (as in the case of nanocrystalline materials), are usually not considered in powder diffraction and are a main issue in this chapter. Ageneral Fourier-based method is therefore proposed to account for grain- surface relaxation effects on powder diffraction profiles (line shift and broad- ening). The model can be easily incorporated into the Whole Powder Pattern Fitting (WPPF) and Whole Powder Pattern Modelling (WPPM) algorithms, recently introduced for a non-destructive microstructural analysis of materi- als based on X-ray diffraction. The specific case of fcc materials is considered in detail. The proposed method accurately describes the diffraction patterns of nanocrystalline ce- ria produced by sol-gel and calcinated at various temperature, and provides microstructural parameters (crystallite size and distribution, dislocation con- tent, etc.) consistent with those refined by WPPF and WPPM. The average unit cell parameter after calcination at four different temperatures is correctly modelled by using only two grain surface relaxation parameters, kept con- stant for the studied sample series. The results of the non-destructive X-ray diffraction measurement are in good agreement with the direct Transmission Electron Microscopy (TEM) observations, giving a further validation of the proposed model.
|Titolo:||Grain Surface Relaxation Effects in Nanocrystalline Powders|
|Autori:||Leoni, Matteo; Scardi, Paolo|
|Titolo del volume contenente il saggio:||Diffraction Analysis of the Microstructure of Materials|
|Luogo di edizione:||Berlin|
|Anno di pubblicazione:||2004|
|Appare nelle tipologie:||02.1 Saggio su volume miscellaneo o Capitolo di libro (Essay or Book Chapter)|