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

Optical studies of UV-excited, polycrystalline α-alumina were performed as a function of the sintering temperature. The luminscence intensity in the 600–750 nm region increased with alumina density and originated from trace 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 commercial applications, this luminescent methodology has promise as a simple, nondestructive tool for detecting packing-density-gradients and microcracks in alumina-based ceramics at different stages of shape-forming and sintering. In high-density alumina, at T=300 K, sites A emit sharp R1/R2-lines at 14407.5/14436.5 cm−1 with ΔE(R2−R1) splitting of ∼30 cm−1, indicating a ruby-like high-field crystalline environment. The lower symmetry sites (B) are characterized by higher-energy, presumably 2E→4A2 doublet transitions (at 14746.7 cm−1 and ∼14818 cm−1) and 2E splitting of ∼71 cm−1. The larger splitting is attributed to anisotropy in the electron repulsion energy for Cr3+ ions residing in slightly larger volumes of the CrO6 and/or due to tetragonal crystal symmetry distortion. Luminescence from B sites is revealed for the first time; excitation using an argon-ion laser at 514.5 nm only reveals luminescence from A sites. UV-excited, luminescence based density calibration curves and density-dependent, color images of alumina samples, along with methodologies for density gradient mapping and microcrack detection, are described. It is shown that density gradients and microcracks can be visualized either by imaging the inherent luminescence of α-alumina or by fluorescence from an appropriate staining dye.