Enhanced thermoelectric performance of yttrium-doped ZnO ceramics via secondary phase formation and conventional sintering

Abstract

Zinc oxide (ZnO)-based ceramics have been widely studied for thermoelectric applications due to their abundance, non-toxicity, cost-effectiveness, thermal stability, and high Seebeck coefficient. In this work, a series of yttrium (Y)-doped ZnO samples was synthesized using the sol-gel method followed by conventional sintering. The thermoelectric property measurements coupled with detailed structural characterization were systematically performed to establish a structure-property relationship. The X-ray diffraction (XRD) analysis confirms limited substitution of Y at the Zn-site in the lattice of ZnO. The surplus Y-doping results in the formation of the secondary phase Y₂O₃. The sample with composition Zn_0.98Y_0.02O exhibited the highest power factor and figure of merit (ZT) values of 0.47 μW cm^-1 K^-2 and 5.6 × 10^-5, respectively, at 575 K. The outlined study elucidates the effects of Y-doping in ZnO to understand the underlying transport phenomenon in ZnO ceramics.

Fig. 1. Schematic diagram demonstrating the synthesis route for pure and yttrium-doped ZnO ceramics through the sol–gel process.

Fig. 2. (a) XRD spectra of ZnO, Zn_0.99Y_0.01O, Zn_0.98Y_0.02O, and Zn_0.97Y_0.03O showing phase evolution and crystallinity. Standard JCPDS patterns for ZnO (01-079-2205) and Y₂O₃ (05-0574) are included for reference. (b) Magnified image of peaks shifting.

Fig. 3. (a) Lattice parameters a = b and c of ZnO and Y-doped ZnO as a function of yttrium (Y) concentration, showing a gradual increase with doping, where, crystallite size vs. doping concentration is given in the inset. (b) Unit cell volume and theoretical density of ZnO and Y-doped ZnO, both showing a rising trend with increasing Y content.

Fig. 4. Secondary electron (a) and BSE (b) images of the Zn_0.99Y_0.01O (Y-1%), and the corresponding images for the Zn_0.97Y_0.03O (Y-3%) sample are given in (c) and (d), respectively.

Fig. 5. Backscattered electron (BSE) image (a) along with EDS elemental mapping (b–d) of Zn_0.98Y_0.02O, and BSE image (e) along with EDS elemental mapping (f–h) of Zn_0.97Y_0.03O.

Fig. 6. Energy-dispersive spectroscopy data showing atomic and weight percentages of (a) ZnO and yttrium-doped zinc oxide (b) Zn_0.99Y_0.01O, (c) Zn_0.98Y_0.02O, (d) Zn_0.97Y_0.03O, respectively.

Fig. 7. Electrical conductivity (a) and Seebeck coefficient (b) of ZnO and Y-doped ZnO (Zn_0.99Y_0.01O, Zn_0.98Y_0.02O, Zn_0.97Y_0.03O) as functions of temperature.

Fig. 8. Temperature-dependent power factor (PF) of ZnO and yttrium-doped ZnO.

Fig. 9. Electronic thermal conductivity (K_e) and total thermal conductivity (K) (a) and (b), estimated (ZT) (c).