Connection between alumina doping methodology and performance: a review

Alumina (Al 2 O 3 ) is a versatile ceramic whose properties can be precisely tailored through selective doping and processing. Introduction of dopants into the alumina structure has been shown to modify these properties, and as a result, there is a large body of work that is concerned with the preparation and testing of doped alumina materials. To date, this library of work has not been summarised or reviewed; this paper aims to bring together published studies that focus on doped alumina, draw connections between synthesis methods, dopants, and performance, and provide recommendations for future directions. Across alkali/alkaline, transition-metal, and lanthanide families, dopants modulate densification kinetics, grain-boundary mobility, phase stability, and optical properties. Many dopants have low bulk solubility in alumina, which can be controlled and influenced by oxidation state, grain size or shape, or temperature. The segregation of these dopants to grain boundaries in doped alumina is a key mechanism for the property changes observed in these materials, including abnormal grain growth, stabilisation of surface area at high temperatures, and increased or hindered densification. Luminescence can be induced in alumina through new luminescent centres at dopant ions, from alumina defects caused by the presence of dopant ions or processing conditions, or from the presence of secondary phases. In general, elements such as Mg, Ca, Ti, Y, and Zr are associated with changes in sintering behaviour; Cr, Eu, and Mn are associated with new phosphor materials, and lanthanide dopants such as La and Ce are associated with the stabilisation of alumina surface area at high temperatures. Although the literature surrounding doped alumina is comprehensive, this review has identified several areas for future work which would strengthen the field. These recommendations include: moving towards complete reporting of reagent purity composition; prioritisation of dopant state characterisation, such as through synchrotron XANES experiments; greater attention to dopant distribution throughout samples, including bridging the characterisation gap between macro (XRD) and micro (electron microscopy) methods of determining secondary phases; performing method comparisons for given dopants to determine effects from chemistry differences as opposed to physical differences such as particle size and shape which result from different methods; and further exploration of lanthanide and rare earth dopants in alumina.