A SAXS/WAXS XAFS study of crystallisation in cordierite glass

Abstract

New Cr X-ray absorption fine structure (XAFS) data have been combined with the results of small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) experiments to probe in detail the crystallisation mechanism in cordierite (Mg2Al4Si5O18) glass doped with 0.34 mol% Cr2O3. By direct comparison with chromo-aluminate spinels (MgCr2xAl2(1 - x)O4) Cr XAFS is used to determine the composition of the devitrified Cr species. This is identified as MgCr(0.18)Al(1.82)O4, which can be directly related to the Cr content in the starting glass and as a result the total crystalline volume in the fully developed ceramic is predicted to be 4%. In situ WAXS not only reveals the presence of the spinel phase but also a silica-rich stuffed quartz phase. This grows independently of the spinel and is probably nucleated from the glass surface. From our knowledge of the compositions of both crystalline phases we are able to deduce that the SAXS contrast between the surrounding glass and the spinel crystallites is 30 times greater than that between the quartz crystallites and the glass matrix, and therefore dominates the measured scattered intensity and the SAXS invariant that is derived from it. As a consequence we are able to show that the spinel crystalline volume fraction inherent in the SAXS is in close agreement with the 4% value obtained from the Cr XAFS. Furthermore in situ SAXS reveals the gradual development of the spinel particle size and shape during heat treatment. This is conducted in the super-cooled region just above the glass transition temperature, Tg. By employing a two-step annealing process nucleation can be separated from growth and from time-resolved SAXS measurements the alumino-chromate nanocrystals are found to be closely monodispersed. Over a total time course of 600 min they grow from rough crystallites to smooth spherical particles of radius 21 +/- 2 nm, with a final density of (1.2 +/- 0.4) x 10(21) m(-3). As the process of ceramic formation takes place in the viscous melt, growth is indeed found to be limited by diffusion and is complete when all the Cr is exhausted. We use this comprehensive in situ study of crystallisation in cordierite glass to demonstrate the advantages of combining SAXS, WAXS and XAFS for probing the time-resolved chemistry, the microstructure and its development from nucleation sites, that underpins the processing of nanoparticle ceramics.

Characterization of Ceramics by X‐Ray and Neutron Small‐Angle Scattering

“…SAS methods have characterized and quantified many of the defect morphologies that control the desired properties of structural and electronic ceramics and glasses 4,26–29,43–50,75,94,96–125 . These include perovskite materials of current interest for SOFC applications and for high‐ T C superconductors.…”

“…(1) Structural/Electronic Ceramics and Glasses, Perovskite Applications SAS methods have characterized and quantified many of the defect morphologies that control the desired properties of structural and electronic ceramics and glasses. 4,[26][27][28][29][43][44][45][46][47][48][49][50]75,94, These include perovskite materials of current interest for SOFC applications and for high-T C superconductors. In order to distinguish between the frequently present multiple component microstructures SANS studies can exploit the neutron isotope effect for one or more of the components, or exploit the neutron magnetic interaction with any atoms within a component that possess a finite magnetic moment.…”

“…The quantitative parameters obtained complement the visual information gained from scanning and transmission electron microscopy (SEM and TEM), computed X-ray microtomography (XMT), the phase composition information obtained from X-ray and neutron diffraction (XRD and ND), or information from other methods ranging from nuclear magnetic resonance (NMR) to light scattering. [37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52] SAS has played a role in materials research since the development of laboratory SAXS methods by Guinier and others in the 1950s. [53][54][55][56] SANS became available at neutron research reactors from the mid-1960s onwards, and more especially with the development of large-scale cold-neutron facilities from the early 1970s, 57 and again in the 1990s.…”

“…This kind of SAS measurement has a similar geometry to, but is quite distinct from, X-ray or neutron reflectivity studies of thin films and surface features. 67,95 Taking advantage of these developments, SAS studies of ceramic systems now represent a growing field with major research activities in areas that include: structural and electronic ceramics and glasses; [26][27][28][29][43][44][45][46][47][48][49][50]94, perovskites, high T C superconductors, solid oxide fuel cells (SOFCs) and H storage materials; 4,75,[117][118][119][120][121][122][123][124][125] sintering, cavitation and damage effects; [5][6][7][8][9][10][11][40][41][42]86,[126][127][128][129][130][131][132][133][134][135]…”

Spectroscopic and structural properties of Cr3+ in silicate glasses: Cr3+ does not probe the average glass structure

“…Cr 3+ is widely used in silicate glasses due to its favorable absorption and emission properties [5,6]. It is also a nucleating agent for particular glass compositions such as alumino-silicate glasses [7]. However, the local structure around Cr 3+ in silicate glasses is not well known as only scarce and contradictory structural data are available [8,9].…”