Enhanced Al/Ni co-doping and power factor in textured ZnO thermoelectric ceramics prepared by hydrothermal synthesis and spark plasma sintering

In this work, we report a significant enhancement of thermoelectric power factor in textured Al/Ni-doped ZnO bulks, which were prepared by spark plasma sintering (SPS) Zn0.98-xAl0.02NixO (x = 0, 0.01, 0.02, 0.03, 0.04) microrod-shaped powders synthesized by hydrothermal synthesis (HS). The co-doped Ni2+ takes up usually the interstitial ZnO lattice cell and expands its lattice during the HS process, which favors the interstitial solution for Al3+ in the powders. The improved solubility of Al3+ in the powders was retained in the bulks after SPS due to its short sintering time, leading to a high carrier concentration of ca. 1 x 10(20) cm(-3) at 0.01 <= x <= 0.04. The co-doping mechanism of Al3+ and/or Ni2+ during the HS and SPS process was also discussed in detail concerning their solution types. The textured microstructure along with a high solubility of Al3+ contributes to an enhanced power factor up to 6.16 x 10(-4) Wm(-1)K(-2) at 673 K in the Zn0.97Al0.02Ni0.01O bulk and an optimized ZT value of 0.057 at 773 K in the Zn0.96Al0.02Ni0.02O bulk. (C) 2015 Elsevier B.V. All rights reserved.