Probing the crystalline environment of alpha-alumina via luminescence of metal ion impurities: an optical method of ceramic flaw detection

Optical studies of W-excited, polycrystalline alpha-alumina were performed as a function of the sintering temperature. The luminscence intensity in th e 600-750 nm region increased with alumina density and originated from trac e amounts of Cr3+ impurities residing in two different sites, labeled A and B. Since trace amounts of Cr3+ exist in most aluminas that are used in com mercial applications, this luminescent methodology has promise as a simple, nondestructive tool for detecting packing-density-gradients and microcrack s in alumina-based ceramics at different stages of shape-forming and sinter ing. In high-density alumina, at T = 300 K, sites A emit sharp R-1/R-2-line s at 14407.5/14436.5 cm(-1) with Delta E(R-2 - R-1) splitting of similar to 30 cm(-1), indicating a ruby-like high-field crystalline environment. The lower symmetry sites (B) are characterized by higher-energy, presumably E-2 --> (4)A(2) doublet transitions (at 14746.7 cm(-1) and similar to 14818 cm (-1)) and E-2 splitting of similar to 71 cm(-1). The larger splitting is at tributed to anisotropy in the electron repulsion energy for Cr3+ ions resid ing in slightly larger volumes of the CrO6 and/or due to tetragonal crystal symmetry distortion. Luminescence from B sites is revealed for the first t ime; excitation using an argon-ion laser at 514.5 nm only reveals luminesce nce from A sites. UV-excited, luminescence based density calibration curves and density-dependent, color images of alumina samples, along with methodo logies for density gradient mapping and microcrack detection, are described . It is shown that density gradients and microcracks can be visualized eith er by imaging the inherent luminescence of cc-alumina or by fluorescence fr om an appropriate staining dye. Published by Elsevier Science S.A.