Preparation and Thermoelectric Properties of Co-Doped ZnO Synthesized by Sol-Gel.

Zinc oxide (ZnO) has attracted increasing attention as one of the most promising n-type thermo-electric materials, but its practice use was limited by high thermal conductivity and low electrical conductivity. Therefore, we herein prepared Co-doped ZnO nanoparticles by sol-gel method and then compressed nanoparticles into bulk materials through spark plasma sintering. The thermo-electric properties, including electrical conductivity, Seebeck coefficient, thermal conductivity, and ZT value, have been investigated. We found that the substitution of Co2+ causes the decrease of bandgap and the increase of carrier concentration, thus the improvement of electrical conductivity. At the same time, the Co-induced lattice distortion and nanoparticles reduce the thermal conductivity by shortening the mean free path (MFP) of the phonons. The resultant ZT is 0.037 for Zn0.9Co0.1O, which is more than 23-fold higher than that of the pure ZnO samples.

Nanofluids Containing γ-Fe2O3 Nanoparticles and Their Heat Transfer Enhancements.

Homogeneous and stable magnetic nanofluids containing γ-Fe2O3 nanoparticles were prepared using a two-step method, and their thermal transport properties were investigated. Thermal conductivities of the nanofluids were measured to be higher than that of base fluid, and the enhanced values increase with the volume fraction of the nanoparticles. Viscosity measurements showed that the nanofluids demonstrated Newtonian behavior and the viscosity of the nanofluids depended strongly on the tested temperatures and the nanoparticles loadings. Convective heat transfer coefficients tested in a laminar flow showed that the coefficients increased with the augment of Reynolds number and the volume fraction.