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Title: Current assisted sintering of thermoelectric materials
Author: Gucci, F.
ISNI:       0000 0004 7971 8444
Awarding Body: Queen Mary, University of London
Current Institution: Queen Mary, University of London
Date of Award: 2019
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The performance of thermoelectric materials (ZT), their capability of converting a temperature gradient into electricity, is dependent not only on their composition but also how they were processed (pressure-less, hot pressed or Spark Plasma Sintered (SPS) etc). SPS is a state of art process where current passes mostly through a graphite die, small or none through the sample, causing rapid Joule heating (typically 100 °C/min). A newly developed processing technique, called flash sintering, passes current directly through the sample achieving higher heating rates (100 °C/s or more). Thermoelectric materials could benefit from rapid heating, but they are too electrically conductive for traditional flash and too mechanically weak for Flash-SPS. Multi-physic software was used to develop and optimise a new process hybrid Flash-SPS (hFSPS), which uses a thin walled stainless-steel tube to constrain the powders and redirect the current to reduce Peltier cooling (a source of uneven sintering). HFSPS was used to sinter a skutterudite, a chalcopyrite and a half-Heusler which were compared to a reference SPSed sample. The rapid heating of hFSPS resulted in better phase purity (93 vs 90 %) when reactively sintering a skutterudite and an increase in ZT (0.81 vs 0.46 at 500 °C). HFSPS produced a Half-Heusler with higher power factor and lower thermal conductivity leading to an improved ZT (0.44 vs 0.35 at 350 °C) with the same density (92.5%). hFSPS reduced the amount of sulphur loss of chalcopyrite during sintering resulting in lower electrical resistivity (100 μohm*m vs 300 μohm*m). Flashed samples also showed improved ZT (0.21 vs 0.07 at 350 °C) and an improved thermal stability. A brief study was also performed showing a modest improvement on the oxidation resistance of Mg2.1Si0.48Sn0.5Sb0.013 protected by a hybrid coating when aged for 120 h at 500 °C, while no success was obtained for Higher Manganese Silicide.
Supervisor: Not available Sponsor: CoACH (Marie Skłodowska-Curie (CoACH-ETN))
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Sintering ; Thermoelectric Materials ; Spark Plasma Sintering