University of Twente Student Theses


Possible enhancement of thermoelectric p-type oxides by creating superlattice structures with pulsed laser deposition

Nawrath, Chris (2013) Possible enhancement of thermoelectric p-type oxides by creating superlattice structures with pulsed laser deposition.

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Abstract:Advanced thermoelectric materials are part of considerations about the future energy handling. They promise to be a direct and easy method to convert waste heat into easy usable electrical energy. General problem in optimizing thermoelectric properties seems to be the coupled behavior of electrical and thermal properties. Superlattice structures are a promising approach to lower thermal transport and at the same time to preserve the electrical conductivity. In this work 3 superlattices of the thermoelectric oxides NaxCoO2 and Ca3Co4O9 are prepared by pulsed laser deposition on top of [La0.3Sr0.7] [Al0.65Ta0.35]O3 (LSAT) substrates. X-ray diraction (XRD) scans and resistivity measurements are done of the samples and the Seebeck coefficients are determined. Furthermore it is theoretically considered how thermal properties can be affected by superlattice structures. XRD measurements showed, that Ca3Co4O9 tends not to grow crystalline under the used deposition temperatures (430°C). Therefore a reference sample is used of Ca3Co4O9 on top of a thin layer of NaxCoO2. All samples showed similar crystallographic ordering and the peak around 16° was used to look at the differences in detail. All superlattices showed interplanar distances in between the two reference samples and in sequence of the total thickness ratio of both components. Shape and height of the peaks showed, that NaxCoO2 still seemed to grow more crystalline than Ca3Co4O9. The measured Seebeck coefficient of the NaxCoO2 thin layer was as expected 79.3 µV/K and the one of the Ca3Co4O9 reference sample with 136.7 µV/K lower than other reported values. The superlattices were all in between those two values, showing a mixed Seebeck coefficient. That indicates, that the Seebeck coefficient is not influenced by the superlattice structure in particular. All samples containing both materials, NaxCoO2 as well as Ca3Co4O9, showed rising resistivities, suggesting a reaction occurring. They did not get stable in the course of the entire bachelor project. Furthermore a change in color was observed spreading from the edges of the samples. Either there is a new undiscovered reaction occurring between the two materials or the earlier found reactions between NaxCoO2 and air (possibly achieving the sample from the edges or through an insufficient capping layer) appear again. Theoretical calculations showed that it would be favorable to compose superlattices with smaller thicknesses of the single layers than done in this work. Worse properties were reported for thin films with layers thinner than approx. 10 nm. However the superlattices seem not to suffer from this problem.
Item Type:Essay (Bachelor)
Faculty:TNW: Science and Technology
Subject:51 materials science
Programme:Chemical Engineering BSc (56960)
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