|M.Sc Student||Koresh Ido|
|Subject||Effects of Precipitation on the Thermal Conductivity of|
Doped Thermoelectric Zine Oxide
|Department||Department of Materials Science and Engineering||Supervisor||Professor Yaron Amouyal|
|Full Thesis text|
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 the microstructure, and can be tailored by manipulating material’s microstructure. Zinc oxide (ZnO) is 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.
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 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 °C 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 °C for different durations.
We investigate the microstructure evolution in terms of grain size, precipitate number density, average radius, and volume fraction applying electron microscopy. Thermal conductivity was determined using the laser flash analysis (LFA) technique as well as the electrical conductivity and thermopower at temperatures between 50 and 700 oC. We observed reduction of thermal conductivity at 50 oC from 18.1 W•m-1K-1 in 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 relate this behavior to the relatively high precipitate number density, e.g. 1. 6 •1020 m-3 upon aging at 750 oC for 16 h. On the other hand, nucleation and growth of NiO-precipitates reduce electrical conductivity at 50 oC from 9.7 S•cm-1 in the as-quenched specimen down to 4 and 2.2 S•cm-1 upon aging at 750 oC for 8 and 24 h, respectively. The thermopower exhibits different trend, in which the as-quenched samples and those aged at 750 oC for 8 and 24 h show about the same value of 240 µV•K-1 at 50 oC. In conclusion, aging heat treatments yield an overall decrease of ZT and a maximum ZT-value of 0.08 at 700 oC obtain for the as-quenched samples. Finally, we discuss the combined effects of precipitates, impurities, and grain boundaries, on both thermal and electrical conductivities, and draw the correlation between microstructure and these transport properties in Ni-doped ZnO.