In-doped multifilled n-type skutterudites with ZT = 1.8

In this paper we maximize the thermoelectric (TE) figure of merit, ZT, of n-type skutterudites, (In, Sr, Ba, Yb)_yCo₄Sb₁₂, via three different routes: (i) find the optimum fraction of In as fourth filler (ii) check the influence of powder particle, grain, and crystallite size on the TE properties and (iii) check thermal stability. Filled n-type (Sr, Ba, Yb)_yCo₄Sb₁₂ was mixed in three different proportions with In_0.4Co₄Sb₁₂, ball milled (regular or high-energy (HB) ball milling) and hot-pressed. Particle size analyses and SEM pictures of the broken surfaces of the hot pressed samples document that only HB produces uniform particles/grains with average crystallite sizes ∼100 nm, proven by transmission electron microscopy. X-ray Rietveld refinements combined with EDX indicate that in all cases indium entered the icosahedral voids of the skutterudite. Temperature dependent physical properties of all three regularly ball-milled samples show that increasing In-content infers an increasing electrical resistivity, increasing Seebeck coefficient but a decreasing total thermal conductivity. Although ZT (823 K) is in the same range as for the sample without In, the ZT values in the whole temperature range are higher and consequently the TE-conversion efficiency, η is at least 10% higher. Annealing the samples at 600 °C for three days shows minor changes in structure and thermoelectric properties, indicating TE stability. The HB sample, due to uniformly small particles, equally sized grains and crystallites, exhibits a high power factor (4.4 mW/m K^-2 at 730 K) and a very low thermal conductivity leading to an outstanding high ZT = 1.8 at 823 K (η_max = 17.5%).