KEYWORDS Energy crisis; Renewable energy; Energy harvesting; Semiconductor;
Thermoelectric effect; Thermoelectric generators; skutterudites;
Figure of merit; Seebeck coefficient; Electrical resistivity; Thermal
conductivity; Electron scattering; Phonon scattering; Defect
engineering; Band engineering; Contact resistance; Diffusion barrier;
Thermoelectric device.

Thermoelectric (TE) devices are useful for solid-state energy conversion technology and
offer a direct conversion of heat to electricity. These devices have great potential to
generate useful energy by recycling waste heat. Many alloy systems like half-Heuslers,
skutterudites, silicides, tellurides, selenides, clathrates, and Zintl phases are widely
explored as potential materials for TE devices. Among the potential TE materials,
Co4Sb12-based skutterudites are highly solicited for intermediate temperature range
(450-750 K) applications. Co4Sb12-skutterudites contain a cage compound with two
voids in its structure, which can be filled with rattling atoms acting as phonon scatters. The
filler atoms alter the electronic structure, reducing thermal conductivity and improving
TE properties.
The single, double, and multiple atom filling approaches are being adopted in
skutterudites to reduce the lattice thermal conductivity (κl). But there are practical issues
in using the multiple fillers in terms of stability, reproducibility, ease of handling, and
cost. The highly reactive nature of the alkali and alkaline-earth metals are not suitable to
be filled in the skutterudite cages. Also, the low melting point of chalcogen metals are
not suitable fillers for skutterudites as they show sublimation at elevated temperatures
in a long thermal run.The highly reactive nature of the alkali and alkaline-earth metals
is unsuitable for filling the skutterudite cages. The chalcogen metals are unsuitable
fillers for skutterudites as they have low melting points and show sublimation at high

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temperatures during long thermal runs.
Therefore, a suitable filler for skutterudites is still challenging. Considering
all the above aspects of filling strategies and importance, we have chosen two novel
candidates for single filling study in the skutterudite cages: the refractory metal tantalum
(Ta) and rare-earth metal Dysprosium (Dy). The essential criterion for selecting Ta and
Dy as a filler atom is that it has a covalent radius of 1.70 A and 1.33, smaller than the void
radius (1.892 A); hence, it can be filled in the void space quickly. A series of refractory
Ta-filled Co4Sb12 (TaxCo4Sb12 (x = 0, 0.4, 0.6, and 0.8)) and Dy-filled Co4Sb12 samples
(DyxCo4Sb12 (x = 0, 0.4, and 0.6)) are synthesized using a solid-state synthesis route
as ball milling and the spark plasma sintering route. The structural analysis of the
samples using X-ray diffraction reveals the existence of a single skutterudite phase.The
characterization of the structure and thermoelectric properties will be shown in detail.
In the case of TaxCo4Sb12, SEM analysis confirmed nanometer-sized equiaxed
grains are present in the Ta0.2Co4Sb12 and Ta0.4Co4Sb12 samples, and bimodal
distributions of equiaxed grains and elongated grains are observed in Ta0.6Co4Sb12 and
Ta0.8Co4Sb12 samples.The dominant carrier type changes from electrons (n-type) to
holes (p-type) with an increase in Ta concentration in the samples. The power factor of
the Ta0.6Co4Sb12 sample is increased to 2.12 mW/mK2

at 623 K due to the 10-fold
reduction in electrical resistivity. The lowest lattice thermal conductivity observed for
Ta0.6Co4Sb12 indicates the rattling action of Ta atoms and grain boundary scattering.
The figure of merit (ZT) of ∼ 0.4 is obtained in the Ta0.6Co4Sb12 sample, which is
comparable to single metal-filled p-type skutterudites reported to date.
In the case of DyxCo4Sb12, the SEM analysis confirmed bimodal distribution
of grains (85-210 nm) in all DyxCo4Sb12 samples. Dy0.2Co4Sb12 and Dy0.4Co4Sb12
samples shown n-type conduction , while Dy0.6Co4Sb12 was p-type behaviour. This
hole dominant conduction in Dy0.6Co4Sb12 sample shows that electrons donated by Dy

do not neutralize the effects of point defects. The higher n-type Seebeck coefficient
values obtained in Dy0.2Co4Sb12 and Dy0.4Co4Sb12 samples. The highest value of
PF achieved in n-type(Dy0.6Co4Sb12) is ∼0.45 mW/mK2

and p-type (Dy0.6Co4Sb12 is

∼1.85 mW/mK2

at 623 K. The lattice thermal conductivity of DyxCo4Sb12 samples
were 50% lower than TaxCo4Sb12 samples.The lattice thermal conductivity falls with the
increase in temperature because of the phonon-phonon Umklapp scattering. The total
thermal conductivity of DyxCo4Sb12 samples were lower than TaxCo4Sb12 samples. As
it can understand that the Dy has a larger size than Ta, and it can give more resonance
scattering.The highest dimensionless figure of merit (ZT = 0.43 ± 0.05 at 673 K) was
obtained in Dy0.6Co4Sb12. The value of ZT ∼ 0.43 is comparable to many rare earth-filled
skutterudites.

The single-filled skutterudites showed a bipolar transition. Another work issues
the Ni-doping in Co-site for the suppression of bipolar transition. The thermoelectric
properties of nanostructured Ni-doped Dy-filled CoSb3 skutterudites (Dy0.4Co4xNixSb12
(x =0, 0.4, and 0.8)) have been studied. The structural analysis of the samples using
X-ray diffraction reveals the existence of a single skutterudite phase in Ni-doped samples,
irrespective of the Ni concentration. Microstructure studies using transmission electron
microscopy and scanning electron microscopy show the existence of nanometer (∼60
nm) size equiaxed grains in the investigated samples. A few recrystallized elongated
grains (∼200 nm) are observed in the Dy0.4Co3.2Ni0.8Sb12 sample. The power factor of
theDy0.4Co3.2Ni0.8Sb12 sample is enhanced to 5.2 mW/mK2

, the highest power factor
for the doped ternary skutterudites reported so far. The power factor enhancement is due
to the substantially reduced electrical resistivity with increased Ni concentration at higher
temperatures. The lattice thermal conductivity is drastically reduced to ∼ 0.3W/mK at
773 K in the Dy0.4Co3.2Ni0.8Sb12 sample due to the enhanced phonon scattering from
Ni-induced point defects and grain boundaries. As a result, a huge increase in the figure
of merit (ZT ∼ 1.4 ) at 773 K is observed in the Dy0.4Co3.2Ni0.8Sb12 sample, the highest
among those of the single element filled Co4Sb12 skutterudites reported so far at this

temperature. Hence, Ni doping could enhance the thermoelectric efficiency of Dy-filled
Co4Sb12 skutterudites.

The advantage of secondary-phase-induced carrier filtering on the thermoelectric
properties has paved the way for developing cost-effective, highly efficient thermoelectric
materials. We have chosen the skutterudite nanocomposites approach to achieve
interface carrier filtering for enhancement of figure of merit of Dy0.4Co3.2Ni0.8Sb12
sample. The skutterudite nanocomposites are prepared by dispersing rare-earth oxides
nanoparticles (Y b2O3, Sm2O3, La2O3) in the skutterudite (Dy0.4Co3.2Ni0.8Sb12) matrix.
The nanoparticles/skutterudite interfaces act as efficient carrier filters, thereby significantly
enhancing the Seebeck coefficient without compromising the electrical conductivity.
As a result, the highest power factor of ∼6.5 W/mK2

is achieved in the skutterudite
nanocomposites. The non-uniform strain distribution near the nanoparticles due to the
local lattice misfit and concentration fluctuations affect the heat carriers, reducing the
lattice’s thermal conductivity. Moreover, the three-dimensional atom probe analysis
reveals the formation of Ni-rich grain boundaries in the skutterudite matrix, which
also facilitates the reduction of lattice thermal conductivity. Both the factors, i.e., the
reduction in lattice thermal conductivity and the enhancement of the power factor, lead to
an enormous increase in ZT up to ∼1.84 at 723 K and an average ZT of about 1.56 in the
temperature range from 523 to 723 K, the highest among the single-filled skutterudites
reported so far.
Solid-state TE devices typically suffer from lower conversion efficiency, lack
of reliable high-temperature device fabrication process and long-term stability. To
achieve high-performance thermoelectric (TE) devices, constructing a good interfacial
contact between TE materials and electrodes is as crucial as a high figure of merit. We
designed novel Ni-Cr-Cu/Co4Sb12 skutterudite thermoelectric joints to deal with the
interfacial issues. Ni-Cr-Cu electrode powder is prepared by ball milling, and a one-step
sintering route is adopted to fabricate the Ni-Cr-Cu/Co4Sb12 joints. The processed

Ni-Cr-Cu/skutterudite joint interface is continuous without having any crack and has
low contact resistance (