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Structural engineering of Ca3Co4O9 thermoelectric thin films

Ihns, Melanie (2013) Structural engineering of Ca3Co4O9 thermoelectric thin films.

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Abstract:Oxide materials offer new possibilities for thermoelectric devices because of their natural abun- dance, non-toxicity and good performance and therefore they are studied in all their variety, different materials, different structures, different compositions. Big expectations lie in the use of thermoelectric thin films since they can be used on a small as well as on a large scale, so the range of applications is large. After the first findings with NaxCoO2 now the more interesting material is Ca3Co4O9 be- cause due to the non-volatility of the Na NaxCoO2 materials are not stable in air environment without an extra capping layer. In this research epitaxial Ca3Co4O9 thin films have been grown on two different substrates using pulsed laser deposition. As substrate materials Al2O3 and (La0:3Sr0:7)(Al0:65Ta0:35)O3 (LSAT) are chosen. According to the crystal lattice structure the mismatch between the film and the Al2O3 substrate should be small, since they both have hexagonal unit cells with relatively similar lattice parameters. But LSAT has a cubic unit cell, so here there should be a large mismatch between film and substrate. The structural properties of the different samples show a lot of differences, so on the LSAT substrate the diffraction peaks of the thin film are much lower in intensity as compared to those on the Al2O3 substrate. The surface roughness of the thin films on the LSAT substrate is higher and the grains are smaller comparing them with the films on Al2O3. On top of the substrates there is a buffer layer formed before the actual Ca3Co4O9 forms, which is different in thickness for both substrate materials. There has been done a temperature variation for the deposition process and a thickness variation of the thin films. For grown films of 60nm thickness at deposition temperatures from 430 to 850 °C on both substrates there are maxima in the resistivity and the Seebeck coefficient found for 430, 750, and 850 °C, while for 650 °C there is on both substrates the lowest thermo- electric performance. The curves of the resistivity and Seebeck coefficients look the same on both substrates, but on LSAT both values are quite a bit higher than on Al2O3 (92.5µV/K and 5m cm as the best at 750 °C on Al2O3 and 13.3µV/K and 21.39m cm on LSAT). For thickness variation a range of 10 to 120nm has been used at the best performing tem- perature of 750 °C. With a film thickness of only 10nm no good thermoelectric performance was achieved, which is probably due to the buffer layer between substrate and film. For the other thicknesses there is only slight variation, but on both substrates the film of 90nm thickness has a somewhat worse performance. Interestingly all samples that performed worse than the others in their measurement series showed a shift to the left in the diffraction n 2θ/ω analysis. The Seebeck coefficient and resistivity have also been measured at increasing temperature and here it has revealed that the films on LSAT show a stable performance up to 700 °C, while with the Al2O3 substrate it is stable only up to 600 °C. At these temperatures the resistivity increases abruptly when cooling the sample back down to room temperature. The thermal conductivity of both film-substrate combinations has been measured in the US, resulting in 1.2 and 2.1W/mK respectively for Al2O3 and LSAT. Based on the results obtained in this thesis it is concluded that Ca3Co4O9 thin films can play an important role in the application of thermoelectric materials.
Item Type:Essay (Master)
Faculty:TNW: Science and Technology
Subject:33 physics
Programme:Applied Physics MSc (60436)
Link to this item:https://purl.utwente.nl/essays/63574
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