Increasing interest in an effciency enhancement of existing energy sources led to an extended research in the field of thermoelectrics. Especially skutterudites with their high power factor (electric conductivity times Seebeck coefficient squared) are suitable thermoelectric materials. However, a further improvement of their thermoelectric properties is necessary. The relatively high thermal conductivity can be decreased by introducing loosely bound guest ions, whereas atom substitution or nanostructuring (as thin films) could yield an increased power factor.
The present work proves the feasibility to deposit single phase skutterudite thin films by MBE technique. In this regard CoSby and FeSby film series were deposited with three different methods: i) codeposition at elevated temperatures, ii) codeposition at room temperature followed by post-annealing, and iii) modulated elemental reactant method. The structural and thermoelectric properties of these films were investigated by taking the thermal stability of the film and the substrate properties into account. Compared to the stoichiometric Sb content of skutterudites of 75 at.%, a small excess of Sb is necessary for achieving single phase skutterudite films. It was found, that the deposited single phase CoSb3 films reveal bipolar conduction (and therefore a low Seebeck coefficient), whereas FeSb3 films show p-type conduction and very promising power factors at room temperature.
The need of substrates with a low thermal conductivity and a suitable thermal expansion coefficient is also demonstrated. A high thermal conductivity influences the measurements of the Seebeck coefficient and the obtained values will be underestimated by thermal shortening of the film by the substrate. If the thermal expansion coefficient of film and substrate differ strongly from each other, crack formation at the film surface was observed.
Furthermore, the realization of controlled doping by substitution as well as the incorporation of guest ions was successfully shown. Hence, this work is a good starting point for designing skutterudite based thin film structures. Two successful examples for such structures are given: i) a thickness series, where a strong decrease of the resistivity was observed for films with a thickness lower than 10nm, and ii) a FexCo1-xSb3 gradient film, for which the gradient was maintained even at an annealing temperature of 400°C.:Contents
1 Introduction
2 Nanostructured thermoelectric materials
2.1 Thermoelectric materials and ZT
2.2 Recent developments in improving ZT in thin films
3 Thermoelectric transport theory
3.1 Electronic transport coefficients
3.2 Lattice thermal conductivity
4 Skutterudites as promising thermoelectric material
4.1 CoSb3
4.1.1 Structural properties of skutterudites
4.1.2 Band structure of CoSb3 and density of states
4.1.3 Thermoelectric properties of CoSb3
4.1.4 Synthesis of CoSb3 thin films
4.2 FeSb3
4.2.1 Structural and thermoelectric properties of FeSb3 thin films
4.2.2 Synthesis of FeSb3 thin films
5 Experimental methods
5.1 Basic methods for structural characterization
5.2 Electric characterization: Resistivity and Hall measurements using van der Pauw geometry
5.3 Thermoelectric characterization (Seebeck coefficient)
5.4 Thermal characterization methods
6 Deposition of skutterudite thin films
6.1 Deposition chamber and deposition parameters
6.2 Deposition methods
6.3 Composition control of skutterudite films
7 Control of structural properties by the used deposition method
7.1 Structural properties of CoSb3 thin films
7.1.1 Crystallization characteristics of CoSb3 films
7.1.2 Comparison of films deposited with different deposition methods
7.1.3 Influence of different deposition parameters on the film properties
7.2 Structural properties of FeSb3 thin films
7.2.1 Crystallization behaviour
7.2.2 Structural properties of post-annealed Fe-Sb films prepared by
codeposition
7.2.3 Influence of the heating rate on the film properties
8 CoSb3 and FeSb3 composition series
8.1 CoSby composition series
8.1.1 Films deposited at elevated temperatures
8.1.2 Annealed films
8.2 FeSby composition series
9 Influence of various substrates on the film properties
9.1 Substrate influence on the film morphology
9.2 Substrate influence on thermoelectric properties and measurements
10 FexCo1-xSb3 - controlled doping by substitution of Co with Fe
10.1 Properties of codeposited FexCo1-xSb3 films
10.2 Properties of FexCo1-xSb3 films deposited via MERM
11 Filled CoSb3 thin films
12 Examples for nanostructured thin film approaches
12.1 CoSb3 thickness series
12.2 FexCo1-xSb3 gradient films
13 Summary and Outlook
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:20228 |
| Date | 02 April 2015 |
| Creators | Daniel, Marcus |
| Contributors | Albrecht, Manfred, Seyller, Thomas, Technische Universität Chemnitz |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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