Thermoelectric properties of icosahedral Al-Pd-(Mn or Re) quasicrystals: Improvement of the ZT value by Ga substitution for Al atoms

We report the thermoelectric properties of polygrain icosahedral Al ₇₁Pd ₂₀Mn ₉ and Al ₇₁Pd ₂₀Re ₉ quasicrystals (QCs). The microstructure of polygrain (arc-melted and annealed) Al-Pd-Re QCs is highly porous. We synthesized sintered pellets of the Al ₇₁Pd ₂₀Re ₉ QC by the spark plasma sintering (SPS) method, resulting in an improvement in their relative density from 65% to more than 99%. This enables direct comparison of Al ₇₁Pd ₂₀Mn ₉ QCs with Al ₇₁Pd ₂₀Re ₉ QCs. The maximum dimensionless figures of merit ZT were 0.18 and 0.12 for Al ₇₁Pd ₂₀Mn ₉ and Al ₇₁Pd ₂₀Re ₉ QCs, respectively. For further improvement of the ZT value, we substituted Ga for Al atoms in the Al ₇₁Pd ₂₀Mn ₉ QC in accordance with the guiding principle of weakly bonded rigid heavy clusters (WBRHCs). The sound velocity and Debye temperature beneficially decreased with increasing Ga concentration, indicating weakening of the intercluster bond strength, as predicted for WBRHCs. The maximum ZT value was increased by a factor of 1.4 from 0.18 for the Al ₇₁Pd ₂₀Mn ₉ QC to 0.26 for the Al ₆₈Ga ₃Pd ₂₀Mn ₉ QC by the addition of an alloying effect to that of the WBRHCs.