Crystalline microstructure of sepiolite influenced by grinding

Abstract

The crystalline microstructure of ground sepiolite has been investigated. A reference sample of sepiolite and products of its comminution by dry grinding were studied through X-ray diffraction pattern analysis, specific surface measurements by nitrogen adsorption and complementary analysis of field emission scanning electron microscope images. A statistical model of polycrystals was applied to describe and determine the crystalline microstructure of the studied specimens. The model parameters characterizing the microstructure were prevalent crystallite shape, volume-weighted crystallite size distribution and second-order crystalline lattice strain distribution, and they were determined for each sample by modelling a selected part of the X-ray diffraction pattern and fitting the simulated pattern to a measured one. A strict correlation of microstructure parameters with grinding time and with specific surface magnitudes was observed. A parallelepiped with edge-length ratios almost independent of grinding time (for longer times) was found to be the predominant crystallite shape. The crystallite size distributions were found to be close to logarithmic normal ones, with the mean values decreasing with increasing grinding time and the standard-deviation-to-mean-value ratios approximately constant. The second-order crystalline lattice strain distributions were found to be close to some simple function with the mean value equal to zero, the mean deviation increasing with increasing grinding time and the standard-to-mean-deviation ratios approximately constant. It was demonstrated that the specific surface can be calculated on the basis of the microstructure characteristics. Some details of the relation between crystallites and crystalline grains were explained by comparing the results of analyses via X-ray diffraction and scanning electron microscopy.

Crystalline Microstructure of Corundum Fillers Determined from Powder X-Ray Diffraction Patterns

“…Figure 3 includes the obtained XRD patterns, and shows minor phases (sodium aluminates) identified in addition to corundum. The crystalline microstructure of corundum specimens, characterised by a prevalent crystallite shape, a volume-weighted crystallite size distribution and a second-order crystalline lattice strain distribution was determined by method described previously [16]. The crystallite shapes were well modelled as hexagonal prisms, which might be similar to possible forms as in Fig.…”

“…[15]). The method of microstructural analysis of powder XRD patterns applied in [16] has been used in the present work for microstructural evaluation of industrial alpha-alumina fillers. This method exploits a model of polycrystalline material combined with a description of XRD line profiles including contributions from crystallite shapes and sizes and from strains, enabling the determination of the crystallite size distribution and strain distribution together with the estimation of prevalent crystallite shape.…”

Crystal structure and microstructure of synthetic hexagonal magnesium–cobalt cordierite solid solutions (Mg2−2xCo2xAl4Si5O18)

“…A quantitative crystalline microstructure analysis of samples was performed by means of a mathematical model of polycrystalline material which has been discussed in detail earlier (Kojdecki, 2004;Kojdecki & Mielcarek, 2000;Kojdecki et al, 2005Kojdecki et al, , 2007Kojdecki, Ruiz de Sola et al, 2009). In this model a polycrystalline sample is interpreted as a statistical population of crystallites, which are assumed to be domains with perfect crystalline order inside, each of them scattering X-rays in a coherent way independently of one another, being separated by small-angle or large-angle boundaries and randomly oriented in space.…”

Análisis microestructural comparativo (difracción de rayos X y FESEM) en sepiolita

“…Para la molienda más avanzada se ha reconocido una disminución de superficie específica previamente observada en la bibliografía e interpretada como debida al recubrimiento de partículas con material amorfo (7), que ocuparía asimismo límites entre cristalitos (8).…”

“…La bibliografía existente sobre el procesamiento de este mineral incluye mucha información sobre la evolución de sus características microestructurales (micromorfología, tamaño y distribución de partícula, porosidad, superficie específica) pero ningún trabajo, salvo el de Cornejo y Hermosín (7), incluye datos del estudio de la microestructura por difracción de rayos X (tamaño de cristalito y distorsiones de red). Más recientemente, se han aplicado procedimientos avanzados de análisis microestructural por difracción de rayos X (AMDRX) a sepiolita (8). Es, por ello, útil mostrar resultados obtenidos con métodos de uso habitual en AMDRX, en la molienda de la sepiolita de referencia considerada en (8) y analizar también la evolución micromorfológica en microscopía electrónica de emisión campo (FESEM).…”