Structural engineering of Ca3Co4O9 thermoelectric thin films

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

Abstract:Oxide materials o�er 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, di�erent materials, di�erent structures, di�erent compositions. Big expectations lie in the use of thermoelectric thin �lms since they can be used on a small as well as on a large scale, so the range of applications is large. After the �rst �ndings 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 �lms have been grown on two di�erent 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 �lm 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 �lm and substrate. The structural properties of the di�erent samples show a lot of di�erences, so on the LSAT substrate the di�raction peaks of the thin �lm are much lower in intensity as compared to those on the Al2O3 substrate. The surface roughness of the thin �lms on the LSAT substrate is higher and the grains are smaller comparing them with the �lms on Al2O3. On top of the substrates there is a bu�er layer formed before the actual Ca3Co4O9 forms, which is di�erent in thickness for both substrate materials. There has been done a temperature variation for the deposition process and a thickness variation of the thin �lms. For grown �lms of 60nm thickness at deposition temperatures from 430 to 850 �C on both substrates there are maxima in the resistivity and the Seebeck coe�cient 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 coe�cients look the same on both substrates, but on LSAT both values are quite a bit higher than on Al2O3 (92.5�V/K and 5m cm as the best at 750 �C on Al2O3 and 13.3�V/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 �lm thickness of only 10nm no good thermoelectric performance was achieved, which is probably due to the bu�er layer between substrate and �lm. For the other thicknesses there is only slight variation, but on both substrates the �lm 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 di�raction 2�/! analysis. The Seebeck coe�cient and resistivity have also been measured at increasing temperature and here it has revealed that the �lms 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 �lm-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 �lms 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)
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