Ido Koresh, MSc candidate
Department of Materials Science and Engineering
Technion – Israel Institute of Technology
In the thermoelectric (TE) effect thermal energy is converted into electrical energy and vice-versa, which is applicable for waste heat recovery as well as heat-exchange or refrigerating. TE performance is highly sensitive to the finest features of microstructure, and can be tailored by manipulating material’s microstructure. Among the most promising TE materials that offer a desirable combination between reasonable conversion efficiency, chemical and structural stability at high service temperatures, as well as low cost, is ZnO.
In the present study we investigate the effects of precipitation on thermal conductivity of nickel-doped ZnO for TE waste heat recovery at high temperatures. The presence of precipitates with high number density values is essential for phonon scattering, thereby reducing lattice thermal conductivity and enhancing the energy conversion efficiency. A 3 at. %super-saturated Ni-doped ZnO solid solution was prepared by sintering at 1400 oC followed by air-quenching at room temperature. Formation of nanometer to sub-micrometer size NiO-precipitates is, then, initiated by controlled nucleation and growth aging heat treatments at 750, 800, and 900 oC for different durations.
We investigated the microstructure evolution in terms of grain size, precipitate number density, average radius, and volume fraction applying electron microscopy. Thermal conductivity was determined applying the laser flash analysis (LFA) technique at temperatures between 50 and 700 oC. We observed a reduction of thermal conductivity at 50 oC from 18.1 W·m-1K-1 for the as-quenched samples down to 17.4 and 17.6 W·m-1K-1 upon aging at 750 oC for 8 and 16 h, respectively. We associate this behavior to the relatively high precipitate number density, e.g. 1.3·1020 m-3 upon aging at 750 oC for 16 h. We discuss the combined effects of precipitates, impurities, and grain boundaries, on phonon scattering, and draw the correlation between microstructure and thermal conductivity in Ni-doped ZnO.
Supervisor: Asst. Prof. Yaron Amouyal