Testing and Modeling of Functionally Graded Aluminum‐Doped Zinc Oxide Using Spark Plasma Sintering and Discrete Powder Layers of Varying Composition

Functionally graded material (FGM) ZnO is made via spark plasma sintering/field‐assisted sintering technique (SPS/FAST) by varying the Al dopant content along the z ‐axis or pressing direction. A wide range of Al content (0–5 wt%) is used by adding Al powder to nanosized ZnO powder. Thermoelectric (TE) measurements are done on the FGM and individual layers made separately. X‐ray diffraction (XRD) shows two phases, ZnO and spinel phase (ZnAl 2 O 4 ). High‐resolution Raman spectroscopy reveals doped ZnO and spinel phase (ZnAl 2 O 4 ) spatially and shows segregation in the layer with highest Al content. Electron backscatter diffraction (EBSD) reveals noticeable grain growth with decreasing Al content, and there is a common, random preferred orientation in all layers. The tested properties are used to simulate efficiency curves for a discretely graded, five‐layer FGM as well as a homogenous material, where both graded structures provide an opportunity to widen the current density ranges and thus the temperature range of useful energy conversion.