Transparent compact ceramics: Inherent physical issues

The overview focuses on inherent optical properties, governed by composition and the state of the crystallattice, and on the interference of these properties with the microstructural optimization of transparentceramics (e.g. the dependence of the tolerable pore size or grain size on the refractive index). Startingwith the general difference between the dispersive performances n(k) of glasses and transparent ceram-ics, examples will demonstrate the use of compositional changes for tuning the relationship of index,Abbe number, and anomalous dispersion. Perowskites and cubic zirconia are known candidates for suchdevelopments, but their mechanical strength is low. Therefore, physical conditions are investigated forbringing translucent tetragonal ZrO 2 to transparency. Another field of increasing interest is transparencyat extreme wave lengths affected by point defects, associated absorption and scattering mechanisms. 2008 Elsevier B.V. All rights reserved. 1. Introduction1.1. Translucence and transparency of sintered polycrystalsMaterials selection for transparent components starts with theconsideration that a minimum of light absorption means leastinteraction of the electromagnetic wave with the ceramic. A freeelectron gas as in metals will, obviously, not contribute to a behav-ior like this, and isolators are clearly preferred. But even in suchceramics different mechanisms can give rise to absorption: At short wave lengths the high energy of ultraviolet (UV) radia-tion initiates the transition of valence electrons into orbitalswith an increased state of energy. This photo-excitation of elec-trons and the associated absorption start when UV light is avail-able with a wave length that corresponds to a defined energygap and when the electronic transition in the solid body isallowed by quantum mechanical rules. On the infrared (IR) side of the spectrum, radiation does not exhi-bit enough energy to induce electronic transitions as with UV.Here the decreasing transmission (increasing absorption) isassociated with small energy differences in the vibrational(and rotational) states of molecules, amorphous networks or lat-tices (phonon modes). Point defects of the crystal lattice (e.g. oxygen vacancies or disor-der caused by impurities or by radiation) give rise to absorptionin most different parts of the spectrum. Known examples are thered color of ruby (single crystalline Al