Screening of thermoelectric performance of half-Heusler materials and their alloys from first principles
Master thesis
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2021Metadata
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- Master's theses (RealTek) [1847]
Abstract
Thermoelectric (TE) generators have seen a lot of renewed interest over the last years.
Along with increasing computational power, the approach in the search for new TE
materials has seen a shift to first principle calculations. This study employs a highthroughput-like method for first principle calculations on half-Heusler (HH) materials.
Based on the high-throughput (HT) result a subgroup is then selected for volumetric
band alignment (VBA). VBA approximates alloying by varying the volume of the
crystal structure. Since this approximation can be performed on primitive cells it
is computationally inexpensive and can be applied to higher numbers of materials
than more expensive supercell calculations. By combining HT screening with VBA
this study develops a way to search a large group of materials, but also optimize the
thermoelectric figure of merit (ZT) by alloying. Finally, supercell calculations are
performed to give an indication of the accuracy of the VBA approximation.
From the first set of calculations for 67 materials, several showed good TE properties.
LaRhTe was found to have both a very low thermal conductivity and a reasonably
good power factor giving it a predicted ZT = 2.7, for its relaxed structure. Out of
the 67 materials, 11 were chosen for VBA, some based on high ZT and others based
on promising features in the band structure.
The volume of the 11 materials was adjusted by changing the lattice constant up
and down 5% and new first principle calculations were performed. The VBA showed
promising results for some compounds. As the volume changed, materials such as
LiZnSb and LaPtSb had an impressive increase in ZT, up to as high as 3.5 for the
latter. In general, the only materials that did not show an increase in ZT were
materials where two or more band valleys were already well aligned. The result from
VBA was attempted to be verified by supercell calculations. The unfolded band
structures showed a similar trend at the Γ-point compared to the VBA calculations,
but the results were inconclusive. Termoelektriske (TE) generatorer har sett økende interesse de siste˚arene. Med økende
datakraft har tilnærmingen i søket etter TE-materialer skiftet mot førsteprinsipp
beregninger. Denne studien benyttes en høy-gjennomstrømsmetode for førsteprinsipp
beregninger p˚a halv-Heusler-materialer. Basert p˚a resultatene ble en undergruppe
valgt til volumetrisk b˚and-tilpasning (VBA). VBA approksimerer legeringer ved ˚a
variere volumet til krystall-strukturen. Siden denne approksimasjonen kan utføres
p˚a primitive celler er den beregnings-messig billig og kan bli utført p˚a høyere antall
materialer enn dyrere supercelleberegninger. Ved ˚a kombinere disse to metodene
utvikler denne studien en m˚ate ˚a søke gjennom et stort antall materialer og samtidig
optimere det termoelektriske verdien (ZT) ved legering. Til slutt ble supercelleberegninger utført for ˚a gi en indikasjon p˚a nøyaktigheten til VBA-approksimasjonen.
Fra de første beregningene p˚a 67 materialer viste flere gode TE-egenskaper. LaRhTe
ble funnet til˚a ha b˚ade lav termisk ledningsevne og høy effektfaktor, noe som resulterer
i en beregenet ZT p˚a 2.7 for den relakserte krystallstrukturen. Av de 67 materialene
ble 11 valgt til VBA, noen basert p˚a høy ZT, andre p˚a grunn av lovende trekk i
b˚andstrukturen.
Volumet til de 11 utvalgte materialene ble endre ved˚a justere opp og ned gitterkonstanten
5%, s˚a ble nye førsteprinsippsberegninger utført. Den volumetriske b˚and-tilpasningen
viste lovende resultater. LaPtSb og LiZnSb viste begge stor økning i ZT, opp til s˚a
mye som 3.5 for LaPtSb. Generelt viste alle materialer en økning i ZT med mindre
to eller flere b˚and allerede var godt tilpasset i energiniv˚a. Resultatene fra VBA ble
forsøkt verifisert med supercelleberegninger. B˚andstrukturene viste likhetstrekk til
VBA-beregningene rund Γ-punktet, men dette resultatet ble ansett til˚a være usikkert.