Determination of alpha and beta alumina in ceramic alumina by X-ray diffraction

Determination of alpha alumina (α-Al2O3) content has been one of the oldest X-ray diffraction (XRD) methods of analysis. However, rhombohedral aluminas may have a variety of morphological forms for which there are marked differences in relative intensities of their major XRD reflections (the preferred orientation effect). In addition, a specimen X-ray diffracting power may vary making it difficult or impossible to employ a reference material. Consequently, comparing intensities of selected reflections from a sample with those of a reference material may cause a serious analytical error. A conventional XRD and a Rietveld approach were used to analyze nine commercial ceramic aluminas, three NIST standards and two special grades of alumina for alpha and beta alumina (β-Al2O3=Na2O·11Al2O3) content. The samples were also analyzed by scanning electron microscopy (SEM) to assess morphology. Increasing the number of XRD reflections analyzed from two to eight did not improve the reliability of the quantification process. The morphological differences among samples could not have been minimized that way. Conventional XRD (two to eight reflections) indicated alpha alumina values varying from 88 to 106% while the Rietveld analysis (full pattern) attributed 100% for all but two. These two samples were found to contain beta alumina. Initial Rietveld refinements indicate that the beta phase contains only a fraction of the total Na2O content. The preferred orientation was computed for 14 specimens and assessed using a relative scale. The Rietveld least-square refinements would not only provide relatively quick, but also reliable information on alpha and beta content, as well as unit cell parameters of alpha and beta phases.