The carrier concentration in the p-type half-Heusler compound Ti0.3Zr0.35Hf0.35CoSb1−xSnx was optimized, which is a fundamental approach to enhance the performance of thermoelectric materials. The ...optimum carrier concentration is reached with a substitution level x = 0.15 of Sn, which yields the maximum power factor, 2.69 × 10−3 W m−1 K−2, and the maximum ZT = 0.8. This is an enhancement of about 40% in the power factor and the figure of merit compared to samples with x = 0.2. To achieve low thermal conductivities in half-Heusler compounds, intrinsic phase separation is an important key point. The present work addresses the influence of different preparation procedures on the quality and reproducibility of the samples, leading to the development of a reliable fabrication method.
Semiconducting half‐Heusler compounds based on NiSn and CoSb have attracted attention because of their good performance as thermoelectric materials. Nanostructuring of the materials was ...experimentally established through phase separation in (T1−x′Tx″)T(M1−yMy′) alloys when mixing different transition metals (T, T′, T″) or main group elements (M, M′). The electric transport properties of such alloys depend not only on their micro‐ or nanostructure but also on the atomic‐scale electronic structure. In the present work, the influence of the band structure and density of states on the electronic transport and thermoelectric properties is investigated in detail for the constituents of phase‐separated half‐Heusler alloys. The electronic structure is calculated using different theoretical schemes for ordered and disordered materials. It is found that chemical disorder scattering influences the electronic transport properties in all substituted materials. Substitution in NiSn‐based compounds leads to high performance n‐type materials but only moderate p‐type thermoelectric properties. The latter is caused by the influence of the valence band on the conductivity. For CoSb‐based compounds, it is found that Sb substitution with Sn keeps the bands close to the Fermi energy intact. The resulting substituted alloys are excellent p‐type materials because of the characteristic valence band structure in the Λ direction.
The figure shows the fcc crystal structure (C1b) of the half‐Heusler compounds (prototype: MgAgAs, cF12, F:4‾3m, 216).
The structural properties of (Ti,Zr,Hf)CoSb thermoelectric Half-Heusler compounds were investigated by synchrotron radiation based techniques. The short-range order, in particular the environment of ...the Co atoms, was studied by extended X-Ray absorption fine structure spectroscopy and the long range order by powder X-Ray diffraction. Structural models were obtained for the single phase materials TiCoSb0.85Sn0.15, ZrCoSb0.85Sn0.15, and HfCoSb0.85Sn0.15. These models were transferred for the phase-separated material Ti0.5Hf0.5CoSb0.85Sn0.15. As a result, proving that each Half-Heusler phase was well ordered, apart from the intermixing of Ti and Hf on its designated crystallographic lattice site.
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The effect of thermal cycling upon the thermoelectric performance of state‐of‐the‐art p‐type half‐Heusler materials was investigated and correlated with the impact on the structural properties. We ...simulated a heat treatment of the material similar to actual applications in the mid‐temperature range, such as occurs during the energy conversion from an automotive exhaust pipe. We compared three different compositions based on the (Ti/Zr/Hf)CoSb1−xSnx system. The best and most reliable performance was achieved using Ti0.5Hf0.5CoSb0.85Sn0.15, which reached a maximum figure of merit ZT of 1.1 at 700 °C. The intrinsic phase separation and resulting microstructuring, which are responsible for the outstanding thermoelectric performance, were stable even after 500 heating and cooling cycles. The standard deviation of the obtained ZT values lies within 2–3 %, which is significantly smaller than the measurement errors.
Towards application: We have simulated the heat treatment similar to actual applications in the mid‐temperature range such as in an automotive exhaust pipe. Even after 500 heating and cooling cycles the intrinsic microstructure is stable. The material Ti0.5Hf0.5CoSb0.85Sn0.15 shows the best and most reliable thermoelectric performance reaching a maximum figure of merit ZT of 1.1 at 700 °C.
The half-Heusler system CoTi
1
-
x
Y
x
Sb (
Y
= Sc, V, Mn, Fe) has been investigated by means of an
ab initio
-based mean-field model which provides phase diagrams of alloys. Co(Ti,
Y
)Sb materials ...show a miscibility gap, which leads to spontaneous demixing within a spinodal region. The results are compared with experimental investigations of microstructure and transport properties of the alloys. The thermoelectric properties of the solid solution were investigated comprehensively by measuring the temperature dependence of the Seebeck coefficient as well as electrical and thermal conductivity. Compared with pure CoTiSb, the thermal conductivity of substituted CoTi
0.9
Y
0.1
Sb was significantly reduced by approximately 53% for
Y
= V. Here, we report on the effect of phase separation in the Co(Ti,
Y
)Sb system and its consequences for the thermoelectric figure or merit.
(ProQuest: ... denotes formulae and/or non-USASCII text omitted; see image) The half-Heusler system CoTi...Sb (Y = Sc, V, Mn, Fe) has been investigated by means of an ab initio-based mean-field model ...which provides phase diagrams of alloys. Co(Ti,Y)Sb materials show a miscibility gap, which leads to spontaneous demixing within a spinodal region. The results are compared with experimental investigations of microstructure and transport properties of the alloys. The thermoelectric properties of the solid solution were investigated comprehensively by measuring the temperature dependence of the Seebeck coefficient as well as electrical and thermal conductivity. Compared with pure CoTiSb, the thermal conductivity of substituted CoTi...Sb was significantly reduced by approximately 53% for Y = V. Here, we report on the effect of phase separation in the Co(Ti,Y)Sb system and its consequences for the thermoelectric figure or merit.
Abstract
The effect of thermal cycling upon the thermoelectric performance of state‐of‐the‐art p‐type half‐Heusler materials was investigated and correlated with the impact on the structural ...properties. We simulated a heat treatment of the material similar to actual applications in the mid‐temperature range, such as occurs during the energy conversion from an automotive exhaust pipe. We compared three different compositions based on the (Ti/Zr/Hf)CoSb
1−
x
Sn
x
system. The best and most reliable performance was achieved using Ti
0.5
Hf
0.5
CoSb
0.85
Sn
0.15
, which reached a maximum figure of merit
ZT
of 1.1 at 700 °C. The intrinsic phase separation and resulting microstructuring, which are responsible for the outstanding thermoelectric performance, were stable even after 500 heating and cooling cycles. The standard deviation of the obtained
ZT
values lies within 2–3 %, which is significantly smaller than the measurement errors.
The effect of thermal cycling upon the thermoelectric performance of state-of-the-art p-type half-Heusler materials was investigated and correlated with the impact on the structural properties. We ...simulated a heat treatment of the material similar to actual applications in the mid-temperature range, such as occurs during the energy conversion from an automotive exhaust pipe. We compared three different compositions based on the (Ti/Zr/Hf)CoSb sub(1-x)Sn sub(x) system. The best and most reliable performance was achieved using Ti sub(0.5)Hf sub(0.5)CoSb sub(0.85)Sn sub(0.15), which reached a maximum figure of merit ZT of 1.1 at 700 degree C. The intrinsic phase separation and resulting microstructuring, which are responsible for the outstanding thermoelectric performance, were stable even after 500heating and cooling cycles. The standard deviation of the obtained ZT values lies within 2-3%, which is significantly smaller than the measurement errors. Towards application: We have simulated the heat treatment similar to actual applications in the mid-temperature range such as in an automotive exhaust pipe. Even after 500heating and cooling cycles the intrinsic microstructure is stable. The material Ti sub(0.5)Hf sub(0.5)CoSb sub(0.85)Sn sub(0.15) shows the best and most reliable thermoelectric performance reaching a maximum figure of merit ZT of 1.1 at 700 degree C.
Improvements in the thermoelectric properties of Half-Heusler materials have been achieved by means of a micrometer-scale phase separation that increases the phonon scattering and reduces the lattice ...thermal conductivity. A detailed study of the p-type Half-Heusler compounds Ti(1-x)Hf(x)CoSb0.85Sn0.15 using high-resolution synchrotron powder X-ray diffraction and element mapping electron microscopy evidences the outstanding thermoelectric properties of this system. A combination of intrinsic phase separation and adjustment of the carrier concentration via Sn substitution is used to realize a record thermoelectric figure of merit for p-type Half-Heusler compounds of ZT around 1.15 at 710C in Ti0.25Hf0.75CoSb0.85Sn0.15. The phase separation approach can form a significant alternative to nanostructuring processing time, energy consumption and increasing the thermoelectric efficiency.
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