UC San Diego

Transparent ceramics have many features that recommend them over single crystals for use as laser amplifiers. Some features, such as greater mechanical toughness and an absence of extended crystalline defects, are intrinsic to polycrystalline materials. Other advantages accrue from ceramic processing: ceramics sinter more rapidly than crystals grow from a melt, at lower temperatures. Ceramic processes are more readily scaled than Czochralski growth, facilitating larger apertures. Unlike a uniform melt, a ceramic green structure can have controlled concentration gradients, resulting in a multifunctional device upon sintering. Identifying diffusion mechanisms in a suitable host material and quantifying diffusion for a dopant with appropriate energy levels are key steps toward tailoring laser ceramics to the specifications of device designers. Toward that end, this study was the first to identify the mechanism and rate of Nd diffusion in YAG. Grain boundary diffusion was shown to dominate Nd transport under conditions relevant to laser ceramics fabrication. Based on a definition of grain boundary width as 1Å, this process occurs at a rate of DGB = 6.4 x 10⁵ ± 2.0 x 10⁵ exp(-491 ± 64kJ/(mol K))m²/s. Mechanism identification and the first published kinetics measurement were made possible by the introduction of a heat treatment method that isolates microstructural change from dopant diffusion : the concentration of grain boundaries was kept great enough to allow rapid diffusion, but low enough to limit the driving force for coarsening. Sintering of fine- grained and phase-pure precursor powder for 4 min at 1700°C produced 0.8[mu]m grains; subsequent diffusion heat treatments at up to 1650°C for up to 64 h caused negligible coarsening, while achieving diffusion distances of up to 23 [mu]m