Transparent ceramic composites: Macro and micro, the "hows" and "what fors"

Transparent ceramics stand as cutting-edge class of materials that benefit from the shaping possibilities of ceramic technology and from the crystalline structure that offers superior performance compared to glasses. Transparency may be obtained only when the material is free of defects that scatter light, viz. pores or secondary phases. Mostly, this means a requirement of a fully dense, single-phase, defect-free microstructure. However, the impact of small scatterers on transparency diminishes as their size decreases, allowing the development of multiphase materials, composites, that are transparent in the IR and even in the visible range for nanometric grain sizes. Conversely, another important topic in the field of transparent ceramics are macroscopic composites. The increasing optical quality of transparent ceramics in the past years has ignited a growing interest, especially in optics and photonics [1]. Transparent ceramics stand as counterparts to more traditionally used single crystals, which may have the same composition, but are obtained by different processes, mostly based on growth from melt. This process is time- and energy-consuming, and above all imposes significant limitations on the final shape of the components, which is obtained by machining. Transparent ceramics, in contrast, take advantage of the shaping flexibility of ceramic processing, in particular to produce composite or gradient structures without the need of post-processing and bonding [2]. Unlike the nanocomposites mentioned above, these composites are macroscopic, mostly with relatively small differences in chemical composition among the different parts. In the case of simple shapes and planar interfaces such structures may be obtained by diffusion bonding of polished single crystals, but the process is demanding and expensive. Ceramic processing allows us to shape such structures in the green state with a high degree of freedom, avoiding intermediate cutting, polishing and bonding steps. The aim of the presentation is to illustrate the possibilities and benefits of transparent ceramics with a particular emphasis on multimaterial components, spanning both nano- and macro-composites. The application potential will be illustrated on specific examples involving structures for lasers or protective domes and the shaping possibilities will be discussed. References [1] J. Hostasa, "Ceramics for laser technologies", pp. 110-124 in Encyclopedia of Materials: Technical Ceramics and Glasses - Vol. 3. Ed. M. Pomeroy, Elsevier, 2021. [2] F. Tian et al., J. Eur. Ceram. Soc., 42 (2022) 1833-1851.