Doping Effects on Internally Coupled Seebeck Coefficient, Electrical, and Thermal Conductivities in Aluminum-Doped TiO2

TiO2 is widely available as a chemically stable and harmless material. Recently, it has received a great deal of attention as a practical thermoelectric material. In this work, we studied the effects of aluminum (Al) doping concentration and lattice parameter on the thermoelectric properties of TiO2 at both room and high temperatures (475 degrees C). We found that the Al doping leads to a compression in the lattice constant and an increase in carrier concentration and consequently increases the electrical conductivity in the TiO2. We observed that the Al-doped TiO2 thin film shows a negative Seebeck coefficient and its value linearly decreases with increasing electrical conductivity. This result indicates that the Seebeck effect is developed by entropy-driven thermally assisted charge diffusion between low- and high-temperature surfaces. Further studies found that doping-induced reduction on entropy difference through electrical conductivity is accountable for the decrease of the Seebeck effect in the Al-doped TiO2. However, the Al doping can lead to phonon scattering at grain boundary interfaces and consequently decreases thermal conductivity. Therefore, the Al doping can essentially function as a mechanism to separately adjust electrical and thermal conductivities in the Al-doped TiO2. With Al doping, we obtained a maximal figure of merit (Z value) to be 1.30 at 475 degrees C and 0.48 At 23 degrees C from the Al-doped TiO2 at the doping concentration of 3%. The comparison of Z values between room and high temperatures confirms that thermally assisted charge diffusion and phonon scattering are two critical parameters for the development of efficient thermoelectric function in the Al-doped TiO2.