Sintering aids, their role and behaviour in the production of transparent ceramics

Transparent ceramics have been in the spotlight as advanced optical ceramic materials for the last two decades, both as optically transparent materials with excellent mechanical properties and with rare earth doping for applications in photonics, and even more so with the wider use of compact diode-pumped lasers. Sintering aids have been widely used in the production of a variety of transparent ceramics, removing porosity, enhancing the optical quality and providing a more uniform microstructure. This review aims to provide an overview of the state of the art and to point out both the most promising approaches and additives, as well as the important issues in the production of transparent ceramics and their properties related to the use of sintering aids, e.g. their incorporation in the ceramic crystal structure, the presence of secondary phase derived from sintering aids, or their double use as charge compensators.