Doping Modification And Microstructure Control On ZnO-based Thermoelectric Material

Thermoelectric ?TE? materials have attracted much attention due to their potential applications in a variety of practical waste heat recovery systems, air conditioning and refrigeration. Thermoelectric oxides bear great potentials for mediate-to high-temperature thermoelectrical applications, owing to their chemical stability at high temperature, heat resistant, free of pollution, nonpoisonous, and ease manufacture. This topic focuses on n-type zinc oxide ?ZnO? thermoelectric materials, whose powders were prepared by solid phase reaction ?SSR? and hydrothermal synthesis method ?HS?, followed by spark plasma sintering?SPS? to the bulks. The carrier concentration and carrier mobility were optimized by Al, Ni, Ga doping and achieving textured and micro/nano structure, along with reducing the thermal conductivity, finally obtained a high thermoelectric performance.Dense and fine-grained Zn0.96Al0.04O thermoelectric ceramics were fabricated by SSR and SPS at 1173-1323 K. Apart from the host ZnO phase with a wurtzite structure, a trace of secondaryZnAl2O4 phase was segregated at the grain boundary and showed an increased trend with raising sintering temperature.The largest power factor 4.01 ×10^-4 Wm-1K-2 at 823 K was achieved in the bulk sintered at 1223 K. The thermal conductivity reduced significantly owing to the phonon scattering via refined grains and dispersed ZnAl2O4 phase and reached a low value 2.42 Wm-1K-1for the Zn0.96Al0.04O bulk sintered at 1173 K, whose maximum ZT value was 0.11 at 773 K.Textured Zn_1-xAlxO ?x=0,0.01,0.02,0.04? ceramics were prepared by SPS using rod-shaped powders synthesized by HS. An orientation degree of up to?0.6 was obtained along with a low Al3+solution level, no trace of ZnAl2O4 was detected in both the powders and sintered bulks due to a high pH value of 8.5-9.0 during hydrothermal synthesis. The textured structure of the resultant Zn_1-xAlxO bulks possesses high carrier mobility, leading to enhanced electrical conductivity, which increased from 68.2 Scm-1 to 749.2 Scm-1 for the Zn0.96Al0.04O composition as the temperature increased from 373 K to 573 K. As a result, the textured Zn0.96Al0.04O ceramic exhibited an enhanced power factorof up to 4.7 Wm-1K-2 at 573 K.Textured Zn0.98-xAl0.02NixO ?x=0,0.01,0.02,0.03,0.04? ceramics were prepared by SPS using rod-shaped powders synthesized by HS.The co-doped Ni2+ takes up usually the interstitial of ZnO lattice cell and expands its latticeduring the HS process, which favors the interstitial solution for Al3+ in the powders and bulks, leading to a high carrier concentration of ca. 1×1020 cm-3 at 0.01?x?0.04. The textured microstructure along with a high solubility of Al3+ contributes to an enhanced power factor up to 6.16×10^-4Wm-1K-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.The micro/nano structured Zn_1-xGaxO ?x=0,0.02,0.03,0.04? ceramics were prepared by SPS using micro/nanopowders synthesized by HS. A high solubility of Ga3+ and carrier mobility were obtained. The x=0.01 micro/nanostructured bulk obtains the highest 7.52×10^-4 Wm-1K-2 of power factor at 773 K. Micro/nano structure of powder was well maintained in the followed bulk, leading to more phonon scattering and reduced thermal conductivity. The lowest 5.15 Wm-1K-1 of thermal conductivity was obtained for the x=0.04 sample at 773 K. An optimized ZT value of 0.067 was obtained at 773 K in the x=0.04 sample.The micro/nano structured Zn_1-xTAlxO ?x=0,0.02,0.03,0.04? ceramics were prepared by SPS using micro/nanopowders synthesized by HS, which obtained the higher carrier mobility due to existed grains with similar orientation. The x=0.02 micro/nanostructured composite shows the highest 50.7cm2V-1s-1 of carrier mobility along with a higher carrier concentration of ca.1.5×1020cm-3, which exhibits similarly single crystal-like carrier mobility despite the large quantity of nano-grainboundaries. The lower k was obtained in our micro/nano structured Al-doped ZnO composites, the lowest k value 1.60 Wm-1K-1for the x=0.04 bulk was obtained at 1073 K. The ZT value of x=0.02 bulk reaches ca.0.36 at 1073 K.The C-doped ZnO ceramic was prepared by SPS combined with HS. As C source of TEA in the process of SPS, the ionization of C^x+ and C^x- were introduced into the ZnO crystal cell. Compared C-undoped ZnO sample, the C-doped ZnO sample increases the visible light absorption and reduces the band gap, whose power factor improves about 1.5 times.