Transparent polycrystalline YAG with controlled dopant distribution and functional gradient microstructure

Active laser materials with layered or gradient structure are very interesting especially for high power laser applications, because they offer better thermal management possibilities compared to materials with homogeneous doping. In the case of YAG, thermal conductivity decreases with increasing dopant content, and thus the introduction of parts with lower or no dopant content is favourable for more efficient cooling and can help to avoid thermal lensing. This contribution presents transparent YAG ceramic materials with controlled dopant (Yb3+ or Nd3+) distribution prepared via solid state reaction sintering of oxide powders. The oxides were mixed in stoichiometric ratio, the mixture homogenized in ethanol and spray dried. Layered samples with layers of different dopant concentration were prepared by pressing of the mixed powders followed by sintering in high vacuum. The sintered materials displayed good optical transmittance and high laser efficiency. XRD analysis was used to identify the lattice parameters of the YAG crystal cell with different doping levels. The microstructure of each layer was examined by means of optical microscopy and electron microscopy coupled with EDS, followed by quantitative image analysis, so that the effect of dopant on the microstructure (e.g. the mean intercept length of the grains) could be observed. Attention was paid to the boundaries between the layers and the diffusion profiles. The thermal diffusivity of the samples was measured by the laser flash method at room temperature and elevated temperatures. The resulting thermal conductivities of the functional gradient microstructures are compared to those of the uniform materials and to the Wiener bounds for conductivity.