Thermal and thermoelectric properties of zinc oxide ceramics alloyed with metals

Abstract

The paper studies the thermal, electrical and thermoelectric properties of ZnO–MexOy ceramics with 1 ≤ x, y ≤ 3, where Me = Al, Co, Fe, Ni, Ti. The samples were made on the basis of ceramic sintering technology of powder mixtures of two or more oxides in an open atmosphere with variations in temperature and duration of annealing. Structural and phase studies of ceramics indicate that the addition of powders of MexOy alloying agents to ZnO powder with a wurtzite structure after the synthesis process leads to the release of secondary phases such as Znx(Me)yO4 spinels and a 4-fold increase in the porosity of the resulting ceramics. Studies of thermal conductivity at room temperature indicate the predominance of the lattice contribution. The decrease in thermal conductivity during doping is due to an increase in phonon scattering due to the influence of the following factors: (1) the size factor when replacing zinc ions in the ZnO (wurtzite) crystal lattice with metal ions from the added MexOy oxides; (2) the formation of defects – point, grain boundaries (microstructure grinding); (3) increase in porosity (decrease in density); and (4) formation of additional phase particles (such as spinels Znx(Mе)yO4). The effect of these factors in the substitution of zinc ions with metals (Co, Al, Ti, Ni, Fe) leads to an increase in the thermoelectric Q-factor of ZT by 4 orders of magnitude (due to a decrease in electrical resistivity and thermal conductivity with a relatively small decrease in the coefficient of thermal EMF). The reason for the decrease in electrical resistance is the more uniform redistribution of alloying metal ions in the wurtzite lattice, resulting in an increase in the number of donor centers, formed with an increase in the duration of annealing.