Here, we report on the time dependence of a synthesis procedure for generation of both n- and p-type bismuth telluride-based materials. To initiate the reaction, the starting materials were first ...mechanical pre-reacted. The Rietveld refinements of X-ray diffraction (XRD) data collected after different milling times demonstrate that Bi
Te
was formed after only 10 min, and longer milling times do not alter the composition. To complete the phase formation, the powders were treated by field-assisted sintering and heat treatment afterwards. The effect of this fast procedure on the structural and thermoelectric properties was investigated. Samples were obtained with relative densities above 99%. A clear preferred orientation of the crystallites in the samples is evidenced by Rietveld refinements of XRD data. The thermoelectric characteristics demonstrate a good performance despite the short milling time. Further, it was demonstrated for this fast synthesis that the physical transport properties can be varied with well-known n- and p-type dopants like CHI
or Pb. For these non-optimized materials, a
value of 0.7 (n-type) and 0.9 (p-type) between 400 and 450 K was achieved. The long-term stability is demonstrated by repeated measurements up to 523 K showing no significant alteration of the thermoelectric performance.
The influence of low‐level metal cation substitution in the thermoelectric material NiCr2S4, treated via field‐assisted sintering, is investigated in X‐ray diffraction (XRD) and transmission electron ...microscopy (TEM) studies. NiCr2S4 and Mn0.1Ni0.9Cr2S4 can be synthesized and compacted as phase‐pure pellets, while In0.1Ni0.9Cr2S4 appears as a mixture of different phases. XRD investigations reveal that Mn can be incorporated into the host material's Ni lattice sites, while In is mainly incorporated into additional phases. Both NiCr2S4 and Mn0.1Ni0.9Cr2S4 form a structure of chemically segregated, nanoscale domains, which appear significantly more pronounced for Mn0.1Ni0.9Cr2S4. All materials exhibit similar, promising thermal conductivities around 2.0 W m−1 K−1, with Seebeck coefficients ranging from −55 to −65 μV K−1. Only the electrical conductivity is noticeably influenced by the substitutions, with the highest value of 504 S cm−1 obtained for the pristine material, and subsequently declining for both substituted phases.
Pristine NiCr2S4 and cation substitutions M0.1Ni0.9Cr2S4 (M = Mn, In) are synthesized and compacted via field‐assisted sintering technique (FAST). NiCr2S4 and Mn0.1Ni0.9Cr2S4 are obtained as phase‐pure pellets and exhibit the formation of nanoscale domains. In0.1Ni0.9Cr2S4 remains homogeneous and forms additional phases. Cation substitution mainly affects the electrical conductivity, which is lowered by the addition of Mn or In.
The influence of low‐level metal cation substitution in the thermoelectric material NiCr
2
S
4
, treated via field‐assisted sintering, is investigated in X‐ray diffraction (XRD) and transmission ...electron microscopy (TEM) studies. NiCr
2
S
4
and Mn
0.1
Ni
0.9
Cr
2
S
4
can be synthesized and compacted as phase‐pure pellets, while In
0.1
Ni
0.9
Cr
2
S
4
appears as a mixture of different phases. XRD investigations reveal that Mn can be incorporated into the host material's Ni lattice sites, while In is mainly incorporated into additional phases. Both NiCr
2
S
4
and Mn
0.1
Ni
0.9
Cr
2
S
4
form a structure of chemically segregated, nanoscale domains, which appear significantly more pronounced for Mn
0.1
Ni
0.9
Cr
2
S
4
. All materials exhibit similar, promising thermal conductivities around 2.0 W m
−1
K
−1
, with Seebeck coefficients ranging from −55 to −65 μV K
−1
. Only the electrical conductivity is noticeably influenced by the substitutions, with the highest value of 504 S cm
−1
obtained for the pristine material, and subsequently declining for both substituted phases.
Phase relations in Cr3S4 and the substituted system Cr3S4–x
Se
x
are studied to determine the influence of chemical substitutions on the thermoelectric properties. In addition to the expected ...equilibrium phase crystallizing in the monoclinic space group I2/m, some samples exhibit a defect phase with Cr2S3‐like structure. The defect phase can be observed in a few samples prior to sintering, with the majority being phase‐pure Cr3S4. The defect phase can, however, be introduced in phase‐pure samples through in situ heating. It can be proven that the defect phase has an influence on the thermoelectric properties, by lowering the electrical and thermal conductivity, while increasing the Seebeck coefficient. Substitution in the anion lattice of Cr3S4 with Se lowers the thermal conductivity. The improvement is mainly achieved through a reduction of the electronic contribution to the thermal conductivity, leading to total values as low as 1.6 Wm−1 K−1 for the substituted system in comparison to the pristine material 2.3 Wm−1 K−1.
Herein, phase relations in Cr3S4 and the substituted system Cr3S4–x
Se
x
are studied. A defect phase occurs that exhibits a Cr2S3‐like crystal structure but retains the stoichiometry of Cr3S4. Electrical and thermal conductivities are affected and both are lowered by the inclusion of the defect phase. Anion substitution with Se suppresses the defect phase and lowers the thermal conductivity.
Phase relations in Cr
3
S
4
and the substituted system Cr
3
S
4–
x
Se
x
are studied to determine the influence of chemical substitutions on the thermoelectric properties. In addition to the expected ...equilibrium phase crystallizing in the monoclinic space group
I
2/
m
, some samples exhibit a defect phase with Cr
2
S
3
‐like structure. The defect phase can be observed in a few samples prior to sintering, with the majority being phase‐pure Cr
3
S
4
. The defect phase can, however, be introduced in phase‐pure samples through in situ heating. It can be proven that the defect phase has an influence on the thermoelectric properties, by lowering the electrical and thermal conductivity, while increasing the Seebeck coefficient. Substitution in the anion lattice of Cr
3
S
4
with Se lowers the thermal conductivity. The improvement is mainly achieved through a reduction of the electronic contribution to the thermal conductivity, leading to total values as low as 1.6 Wm
−1
K
−1
for the substituted system in comparison to the pristine material 2.3 Wm
−1
K
−1
.
The effects of low-level partial cation substitution in Cr
2−
x
M
x
S
3
with M = Ti, V or Sn and
x
= 0.05 and 0.1 have been investigated regarding the long- and short-range crystal structures and ...thermoelectric properties. All substituted compounds crystallized in the equilibrium phase of Cr
2
S
3
, adopting the space group
R
3
¯
. Electron beam irradiation led to a phase transformation from space group
R
3
¯
to
P
3
¯
1
c
with a subsequent appearance of diffuse scattering, indicating short-range ordering of cations in the partially occupied cation layers. Substitution of Cr by V led to a reduction in electrical conductivity and subsequently to a lower thermoelectric performance in comparison to the pristine material. In contrast, substitution with Ti yielded an improvement of the performance due to a higher electrical conductivity and a reasonably high Seebeck coefficient. Both Sn-substituted compounds contained only traces of Sn. Surprisingly, a significant improvement of the electrical conductivities could be observed in comparison to the pristine material as well as the other Cr
2−
x
M
x
S
3
materials.
A thermoelectric triple-phase p-type Ca3Co4O9-NaxCoO2-Bi2Ca2Co2O9 (CCO–NCO–BCCO) 2D nanocomposite was obtained from pressureless sintering in air. The anisotropic thermoelectric properties of the ...nanocomposite exhibit a high electrical conductivity of 116 S cm−1 and a power factor of 6.5 μW cm−1 K−2 perpendicular to the pressing direction at 1073 K in air. A corresponding zT value of 0.35 was obtained. Three co-doped, thermoelectrically active misfit-layered materials were stacked to form a triple-phase nanocomposite, which combines the advantages of all three materials. The resulting nanocomposite enables simultaneous increases of the isothermal electrical conductivity σ and the Seebeck coefficient α by charge carrier concentration engineering and synergistic effects. The Bi2Ca2Co2O9 and NaxCoO2 phases were stabilized in a Ca3Co4O9 matrix at high temperatures. To evaluate the application of the nanocomposite in high-temperature thermoelectric generators, the representation of the electrical conductivity and power factor in a Ioffe plot was more appropriate than the zT value.
The influence of sintering parameters on the physical properties and the chemical structure of rhombohedral Cr2S3 (rh‐Cr2S3) is investigated using high pressures and high temperatures. The ...densification of the powder is performed by applying the high‐pressure field‐assisted sintering technique/spark plasma sintering. Using a titanium–zirconium–molybdenum (TZM) alloy as sintering tool, it is possible to increase the magnitude of the applied pressure to several hundred MPa at temperatures as high as 1223 K. A relative density of up to 99.9% is achieved at a sintering temperature of 1223 K and a pressure of 395 MPa. The presence of phase‐pure rh‐Cr2S3 is proven by X‐ray diffraction analysis and transmission electron microscopy. The Seebeck coefficients of the self‐doped samples change drastically with the sintering temperatures ranging between −650 and −350 μV K−1. The densities and the thermal conductivities of the sintered samples increase with increasing sintering temperatures. The electrical conductivity is largely increased compared with the thermal conductivity potentially due to the current‐assisted high‐pressure sintering.
Highly densified rhombohedral Cr2S3 is successfully prepared by high‐pressure field‐assisted sintering technique using titanium–zirconium–molybdenum alloy sintering tools. A theoretical density of 99.9% is achieved by sintering at 1223 K and 395 MPa. The investigation of the microstructure proves that the initial structure is preserved. The electrical and the thermal conductivities indicate a strong dependency of the microstructure.
The influence of sintering parameters on the physical properties and the chemical structure of rhombohedral Cr
2
S
3
(rh‐Cr
2
S
3
) is investigated using high pressures and high temperatures. The ...densification of the powder is performed by applying the high‐pressure field‐assisted sintering technique/spark plasma sintering. Using a titanium–zirconium–molybdenum (TZM) alloy as sintering tool, it is possible to increase the magnitude of the applied pressure to several hundred MPa at temperatures as high as 1223 K. A relative density of up to 99.9% is achieved at a sintering temperature of 1223 K and a pressure of 395 MPa. The presence of phase‐pure rh‐Cr
2
S
3
is proven by X‐ray diffraction analysis and transmission electron microscopy. The Seebeck coefficients of the self‐doped samples change drastically with the sintering temperatures ranging between −650 and −350 μV K
−1
. The densities and the thermal conductivities of the sintered samples increase with increasing sintering temperatures. The electrical conductivity is largely increased compared with the thermal conductivity potentially due to the current‐assisted high‐pressure sintering.
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