Reinvestigation of the Thermoelectric Properties of Fe‐Substituted Icosahedral Al–Pd–Re Quasicrystals

Abstract

Icosahedral quasicrystals (QCs) have a glass‐like, low thermal conductivity because of their complex crystal structures, and a moderate power factor because of deep pseudogap formation in their electronic density of states (DOS) at the Fermi level originating from icosahedral symmetry and covalent bonding nature. These features are beneficial for high‐performance thermoelectric materials. Densified bulk samples of icosahedral Al–Pd–Re–Fe QCs are synthesized by arc‐melting, annealing, and spark plasma sintering methods. It is found that a 2/1‐approximant crystal (AC) can be produced by higher Fe substitution [Al71Pd20(Re1–xFex)9 (x≥0.75)]. The thermoelectric properties of Al71Pd20(Re1–xFex)9 (where x = 0, 0.2, 0.4, 0.65, 0.7, and 0.75) are evaluated between 373 and 973 K. Fe substitution enhances the power factor and reduces the phonon thermal conductivity by a concept of “weakly bonded rigid heavy clusters” (WBRHCs) and an increase in the carrier concentration, enhancing the dimensionless figure of merit (zT). The 2/1‐AC phase has a much higher electrical conductivity and a moderate Seebeck coefficient in the mid‐temperature region due to a considerable increase in carrier concentration; this results in the highest power factor of 900 μW m−1 K−2, and zT value for the 2/1‐Al−Pd−Re−Fe AC is 0.26 at 573 K.

Thermoelectric Properties of Co-Substituted Al–Pd–Re Icosahedral Quasicrystals

“…We confirmed that all synthesized bulk samples were identified as an F -type quasi-crystalline phase [ 26 ], as shown in Figure 1 A. In the Al–Pd–Re–Co system, the formation of 2/1- or 1/1-approximant crystal was not realized, unlike in Al–Pd–Re–Ru [ 27 ] and Al–Pd–Re–Fe [ 16 ] quaternary systems. We observed peak shifting to a higher degree with increasing x , indicating that the quasi-lattice constant ( a R ) decreased by Co substitution for Re ( Figure 1 B).…”

“…The relative density of the sample with x = 0 was 95.7% [ 16 ], indicating that a dense bulk sample was fabricated by the SPS process. The ideal densities of the Co-substituted samples were calculated from d bulk (relative density: ~100%, 6.30 g cm –3 ) of the sample with x = 0 [ 22 ] and the rate of change in average atomic weight ( Table 1 ).…”

“…Figure 3 a–c show the temperature dependence of σ , S , and S 2 σ from 300 to 873 K for the Al 71 Pd 20 (Re 1− x Co x ) 9 ( x = 0 [ 16 ], 0.1, 0.2, 0.3, 0.4, 0.5) samples. All samples had a similar positive temperature coefficient of σ (semiconducting behavior), with a varying magnitude of 100–280 Ω −1 cm −1 at 300 K. The calculated activation energy from the Arrhenius plot was 0.11 eV for the sample with x = 0 and increased to 0.15 eV with Co substitution.…”

“…Table 1 lists T S and bulk densities ( d bulk ) obtained from the Archimedes method for Al 71 Pd 20 (Re 1− x Co x ) 9 ( x = 0, 0.1, 0.2, 0.3, 0.4, 0.5). Here, the data for the sample with x = 0 are quoted from a reported paper [ 16 ].…”

“…Regarding thermoelectric applications, i -Al–Pd–Re QC has been widely investigated from experimental [ 14 ] and theoretical [ 15 ] points of view. The i -Al–Pd–Re QC exhibits promising thermoelectric properties because of its moderate σ (~280 Ω −1 cm −1 ), relatively high S (~100 μV K −1 ), and low κ total (~1.3 W m −1 K −1 ) at room temperature [ 16 ], which are brought about by pseudo-gap formation at the Fermi energy ( E F ) [ 17 ] and their complex crystal structures with icosahedral symmetry [ 18 ]. Recently, we obtained a relatively large zT max value of 0.26 at 573 K for the Fe-substituted Al–Pd–Re system [ 16 ]: 2/1-Al–Pd–Re–Fe approximant crystal showed the highest S 2 σ of ~900 μW m −1 K −2 at 573 K. To date, we have succeeded in improving the thermoelectric performance of only p-type materials based on i -Al–Pd–Re QC [ 14 , 16 ] and isostructural i -Al–Pd–Mn QC [ 19 , 20 ].…”