Experimental investigation of the thermoelectric transport properties of Bi2Si2Te6-based and ZnSb-based materials

Abstract

In the context of energy crisis and global climate change, thermoelectricity, which can enable direction conversion between thermal energy and electrical power in an environmentally friendly manner, has attracted much research interest in recent years. The energy conversion efficiency of thermoelectric materials is governed by the dimensionless figure-of-merit zT = S2T/ρ(κe + κL), where S, T, ρ, κe and κL are the Seebeck coefficient, the absolute temperature, the electrical resistivity, the electronic thermal conductivity and the lattice thermal conductivity, respectively. The research focus of this dissertation is placed on improving the thermoelectric performance of Bi2Si2Te6-based and ZnSb-based materials and uncovering physical mechanisms that underlie their transport properties.

A layer-structure compound Bi2Si2Te6 is a narrow band gap semiconductor with an optical band gap of ~0.25 eV. Bipolar diffusion, which is commonly observed in narrow band gap semiconductors, is regarded as an obstacle to improving thermoelectric performance because it dramatically deteriorates the thermopower and increases the thermal conductivity at high temperatures. Here, we propose two effective strategies, either substitution of Bi with Mn, or replacement of Si with Ge, to optimize the carrier concentration and suppress the bipolar effect in Bi2Si2Te6. The electrical resistivity is largely reduced and a compensation of the Seebeck coefficient by minority charge carriers is eliminated because of increased carrier concentration. Meanwhile, both Mn doping at the Bi site and Ge alloying at the Si site can achieve a larger density-of-states effective mass which is favorable to the Seebeck coefficient. Accordingly, the maximum power factor is considerably improved from 4.4 μW cm−1 K−2 for pristine Bi2Si2Te6 to 9.6 μW cm−1 K−2 for Bi1.97Mn0.03Si2Te6 and 9.2 μW cm−1 K−2 for Bi2Si1.8Ge0.2Te6. Furthermore, the lattice thermal conductivity is largely diminished by ~40% at elevated temperatures, which is ascribed to the suppressed bipolar thermal conductivity and strengthened phonon scattering by extra substitutional point defects. Consequently, a peak zT value of unity is achieved in both Bi1.98Mn0.02Si2Te6 and Bi2Si1.8Ge0.2Te6 at 773 K.

ZnSb has long been regarded as a promising thermoelectric material due to its good thermodynamic stability and earth-abundant constituent elements. However, the thermoelectric performance of ZnSb is restricted by its low power factor and high lattice thermal conductivity. Herein, we demonstrate that there is an incredible improvement in the thermoelectric figure-of-merit of ZnSb by combining Cd alloying at the Zn site with Ge doping at the Sb site. Substitution of Zn with Cd can bring about a strong point defect scattering to phonon propagation, leading to the reduced phonon relaxation time. Meanwhile, the significant softening of acoustic phonons is introduced and thus group velocities of acoustic phonon modes are suppressed. Accordingly, a ~44% reduction in the lattice thermal conductivity is achieved in Zn0.7Cd0.3Sb at room temperature. Moreover, Ge doping at the Sb site in Zn0.7Cd0.3Sb can effectively optimize the carrier concentration and thus successfully enhance the power factor. As a result of the suppressed lattice thermal conductivity and optimized carrier concentration, a peak zT value as high as ~1.08 at around 564 K is attained in Zn0.7Cd0.3Sb0.96Ge0.04.