Thermoelectric Properties of ZnNiO/Polyparaphenylene Hybrids Prepared by Spark Plasma Sintering
ZnO is a promising high temperature thermoelectric (TE) material for power generation from waste heat due to its high melting point, high Seebeck coefficient and high carrier mobility. However, its practical application was limited by the high lattice thermal conductivity ????L and low electrical conductivity ????. In order to solve this problem, scientists have incorporated nanostructure into bulk materials to reduce ????L by effectively scattering phonons, but the ????L cannot be infinitely reduced because the phonon mean free path is larger than the interatomic distance. Therefore, we should find an efficient method to enhance the power factor (????S2) in order to further improve the ZT. Recently, a great number of theoretical researches have predicted that ZT, Seebeck coefficient and electrical conductivity can be concurrently optimized in molecular junctions where the organic orbits are well aligned with the chemistry potential of the contact. Furthermore, ????L can be also decreased through two possible mechanisms: (1) acoustic mismatch due to different phonon densities and velocities and (2) phonon scattering due to interface imperfections and boundaries. Based on these considerations, Zn0.975Ni0.025O/x wt% polyparaphenylene (x=0, 3, 5, 9) nanocomposites were fabricated by compressing the nanopowder through spark plasma sintering. The effect of different weight fractions of PPP nanoparticles on the electrical conductivity, Seebeck coefficient, thermal conductivity and ZT value was reported. The measured results showed that the power factor was significantly improved for Zn0.975Ni0.025O/9 wt% PPP due to the molecular junction effect. At the same time, the incorporation of PPP nanoparticles also results in a reduction of the thermal conductivity. The thermal conductivity decreases form 7.92 Wm-1K-1 for pure ZnO sample to 5.84 Wm-1K-1 for Zn0.975Ni0.025O sample, and further decreases to 3.1 Wm-1K-1 for Zn0.975Ni0.025O/9wt% PPP sample. Finally, the resultant maximum ZT =0.15 at 1000 K is 10-fold higher than pure ZnO (ZT = 0.015@1000 K). This value corresponds to a 50% enhancement compared to the Zn0.975Ni0.025O sample (ZT = 0.09@1000 K) fabricated by sol-gel method.