RESUMO
Doping in semiconductors is a widely implemented strategy for manipulation of carrier concentration, which is a critical parameter to regulate the thermoelectric performance. Stoichiometric BaCu2Te2 shows high hole concentration and unstable transport properties owing to the inherent Cu vacancy and dynamic precipitation behavior. In this work, Te has been partially substituted by Cl in BaCu2Te2 to suppress the overhigh hole concentration. Due to the high electronegativity of Cl, strong Cl-Cu bonds can significantly inhibit the Cu migration and the consequent dynamic precipitation. Meanwhile, nano-precipitate BaCl2 distributes in the grain boundary, acting as ionic blocking layers. Therefore, the thermal stability of the samples can be essentially improved via chemical bonding strengthening and grain boundary engineering. In terms of thermal transport, the introduced point defects and second phase strengthen the short-wavelength and medium-wavelength phonon scattering, leading to further reduced thermal conductivity. Eventually, the repeatable ZT value of BaCu2Te1.98Cl0.02 reached 1.22 at 823 K, which is higher by 19.6% compared with 1.02 of pristine BaCu2Te2. The average ZTs of BaCu2Te2-xClx (x = 0, 0.02, 0.04, and 0.06) in the temperature range of 323-823 K are 0.737 for x = 0.02, 0.689 for x = 0.04, and 0.667 for x = 0.06, which are 24.6, 17.2, and 13.4% higher than the average ZT of 0.588 corresponding to the undoped sample, respectively. The study shows that synergetic enhancements of thermal stability and thermoelectric properties can be achieved by strengthening chemical bonding and constructing ionic blocking layers in the grain boundary, which can be applied to other fast-ionic conductor thermoelectric materials.
RESUMO
Donor-doped strontium titanate (SrTiO3) is one of the most promising n-type oxide thermoelectric materials. Routine doping of La at Sr site can change the charge scattering mechanism, and meanwhile can significantly increase the power factor in the temperature range of 423-773 K. In addition, the introduction of Sc partially substitutes Sr, thus further increasing the electron concentration and optimizing the electrical transport properties. Moreover, the excess Sc in the form of Sc2O3 composite suppresses multifrequency phonon transport, leading to low thermal conductivity of κ = 3.78 W·m-1·K-1 at 773 K for sample Sr0.88La0.06Sc0.06TiO3 with the highest doping content. Thus, the thermoelectric performance of SrTiO3 can be significantly enhanced by synergistic optimization of electrical transport and thermal transport properties via cation doping and composite engineering.