Effect of electron doping on thermoelectric properties for narrow-bandgap intermetallic compound RuGa2

Abstract

The maximum dimensionless figure of merit, ZTmax, as a function of the chemical potential of the narrow-bandgap intermetallic compound RuGa2 was calculated by using the Boltzmann transport equation with a simple rigid band approach under the constant relaxation time assumption. The calculation, including the effect of the group velocity, indicates that ZTmax over unity would be achieved by electron doping rather than hole doping. Based on this calculation, the effects of Ir substitution for Ru on the thermoelectric properties for RuGa2 have been investigated in the temperature range from 373 K to 973 K. Indeed, a relatively large ZT value of 0.31 for n-type material was obtained in the nominal composition of Ir3.0Ru30.4Ga66.6. The discussion includes the validity of the rigid band approximation and further enhancement of ZT from theoretical and experimental aspects.

Effects of Transition Metal (TM: Co, Rh, Ni and Pd) Substitution for Ru on Thermoelectric Properties for Intermetallic Compound RuGa₂

“…S of undoped RuGa 2 exhibits a large positive value of 300 µVK ¹1 at 373 K. In the TM substituted samples, the sign of S changes from positive to negative and S exhibits a large value of 150 < «S« < 350 µVK ¹1 at about 373 K, indicating that the chemical potential, µ, shifts to the conduction band. Figure 4 shows the magnitude of S at 373 K, «S 373K «, as a function of carrier concentration at room temperature n 300K for undoped RuGa 2 , 7) and samples doped with Re, 8) Ir 10) and TM substituted RuGa 2 . Undoped and Re substituted RuGa 2 exhibit a strong correlation between «S« and carrier concentration shown with dotted line.…”

“…In this case, electrons with low carrier mobility would contribute to the transport, as expected from the E-k dispersion relation. 10,18,19) With increasing temperature, @f F.D. /@E broadens and µ shifts to lower energy, and thus thermally excited holes contribute to the conduction.…”

“…10) Mother ingots were synthesized by an arc-melting technique in a purified argon (6N) atmosphere. They were crushed to an average particle size of less than 20 µm with an agata mortar and pestle then synthesized by a sintering method (SPS-515S, Fuji Electronic Industrial Co.).…”

“…15) The maximum ZT found among these intermetallic compounds is 0.50 at 773 K for RuGa 2 compound. 7) Although the effects of hole 8) and electron doping 10) on the thermoelectric properties above room temperature have been investigated for RuGa 2 , we have investigated various kinds of electron dopants effects on the electrical conductivity and Seebeck coefficient for RuGa 2 compound: we selected single electron doping elements (Co and Rh) and double electron doping elements (Ni and Pd). Furthermore, to elucidate the effects of TM substitution on the electronic density of states, N(E), the Korringa-Kohn-Rostoker Green's function method under a coherent potential approximation (KKR-CPA) 16,17) has been employed.…”

Thermoelectric Properties of Al&ndash;Ga&ndash;Pd&ndash;Re Icosahedral Quasicrystals

“…Most of high S 2 · and high ZT materials are pseudogap or narrow-gap compounds such as doped-PbTe, 3,4) Zn 4 Sb 3 , 5) skutterudites, 6) clathrates, 7) silicides, 8) and gallides compounds. 9) AlPdMn and AlPdRe icosahedral quasicrystals (QCs) are potential thermoelectric materials 10) because their quasiperiodic complex crystal structure brings glass-like low thermal conductivities of about 1 Wm ¹1 K ¹1 . These quasicrystals also have large Seebeck coefficients of over 90 µV K ¹1 because of the pseudogap formed near E F .…”