Thermoelectric Properties of Nanograined ZnO

Bulk ZnO with a grain size of 20 nm was successfully obtained by pulsed electric current sintering. The crystalline size was almost identical to that of the raw particles, because the sintering temperature was as low as 200A degrees C. A pressure of 500 MPa effectively enhanced densification, leading to a relative density of > 90% at 200A degrees C. The small grain size led to a low thermal conductivity of 3 W/m K at room temperature, due to enhanced boundary scattering. The Seebeck coefficient was higher than that of micrograined ZnO with similar Ga doping (0.3 at.% Ga). However, the resistivity was increased by more than 1000 times. The temperature dependence of conductivity showed thermally activated conduction behavior, while that of micrograined ZnO exhibited metallic-like behavior. The thermoelectric properties suggest that a carrier trap in the nanograined ZnO hinders carrier transport. Surface modification of the ZnO nanoparticles by heat treatment in H(2) resulted in observable photoluminescence which was quenched in the starting nanoparticles, and led to a decrease in the resistivity of the sintered bulk, which indicates that control of surface defects on the nanoparticles is crucial for enhancement of the thermoelectric properties of nanograined ZnO.