Peter Brinks

Size effects in Thermoelectric Cobaltate Heterostructures

Promotion date: September 5.

Promotor: Guus Rijnders

Assistant Promotor: Mark Huijben

Thermoelectric energy conversion is a promising technology as it can be used to directly convert a temperature gradient into electric energy. Major improvements of the efficiency could be achieved by engineering new materials. Main challenge is to improve the performance from the interconnection of: the Seebeck coefficient, the electrical conductivity and the thermal conductivity. Further improvement is by suppression of phonon transport, for example to use phonon scattering at the interfaces in thin films and superlattices.

Layered cobaltates, such as NaxCoO2 and Ca3Co4O9 could result in promising performances. In this thesis heterostructures of layered cobaltates are studied. Epitaxial thin films are grown by pulsed laser deposition on various single crystal substrates. Stability issues are overcome by the in-situ deposition of an amorphous AIOx capping layer, resulting in chemically stable NaxCoO9 thin films.

Size effects are studied as well, by fabricating thin films with thicknesses between 5 nm and 250 nm. Because of the preservation of the Seebeck coefficient, these ultrathin layers are very promising as building blocks in superlattice structures. The thermal stability of thin films NaxCoO2 and Ca3Co4O9 is improved by heating the samples in an oxygen-rich environment, leading to significant improvements of the thermoelectric properties at elevated temperatures.

Finally, thermoelectric superlattices of cobaltates are investigated. The electronic properties are preserved and the thermal conductivity is suppressed by enhanced phonon scattering at the interfaces. A new approach is studied, in which two thermoelectric materials, NaxCoO2 and Ca3Co4O9 , are combined into a superlattice, without any electrically insulating materials. Here, a high degree of crystalline ordering was achieved because of the good crystallinity match. Because of that the electronic properties are comparable, leading to a promising and novel approach to improve the thermoelectric performances of cobaltate-based materials.

In what sense was your PhD work application oriented?

Thermoelectric generators are used in some applications already converting waste heat into electricity, for example in the automobile industry and in space applications such as the Mars Curiosity Rover.

Because of the absence of moving parts in this kind of generators, applications are thinkable in industry as well, provided that material properties can be further improved and controlled. Then the efficiency of these generators will increase, to make applications on a larger scale economically feasible.

The work on the capping layer, for example, fits in this pursuit. This was the first result during the PhD work, which led to a publication and to stable samples to be further studied. After that the properties of these layers could be improved by a factor of two, by further controlling the crystalline structure of the layers.

Combining materials in superlattices also is important for future applications. The interfaces between the materials could lead to unexpected results. By continuing the research on these effects, understanding of the physical phenomena taking place here will increase. This could lead to new design tools to further improve large scale materials properties.

Can you recall some special moments happening during your PhD working period?

The purchase of new equipment enabling us to study the performances of the thin layers at high temperatures, was certainly one of them. The oxides fabricated should be stable at 800 K, or even higher. These temperatures are plausible when applied in future practical situations.

The Ca3Co4O9 structures appeared to be more stable under these severe conditions, compared to NaxCoO2. Also we were able to vary the background gases in experiments allowing us to notice, for example, if the oxides used were resistant to different environments. Being able to perform these range of experiments made us gain ground internationally in this field of research, as we were one of the few groups to study these effects.

In what magazines were your findings published?

The first article was published in The Royal Society of Chemicals (RSC) Advances. Also publications appeared in Advanced Energy Materials and in Applied Physical Letters. Part of my thesis will also be used for a book chapter. Three articles are under preparation still, at this very moment. The next half year I hope to finish them as I will work on this topic a little further.

On conferences and symposia the work presented, received special attention from researchers working on these type of materials from another point of view: more chemically oriented towards enhancing the properties by mixing powder components suitable for large scale applications. By preparing the thin layers and experimenting directly with these, we gained special insights in specific properties that were helpful for these scientists and researchers.

In what way did you develop personally, as a researcher and scientist?

I learned to communicate my results better, starting right from the start when new experiments were designed. It appeared very important being able to communicate the results to my direct colleagues and other interested people in the field. By reporting all steps accurately, one is able to create impact and to build up a new knowledge base in a systematic way.

What, in your opinion, is important for Mesa+ to stay successful in future?

The meetings of the Strategic Research Organisation of Mesa+ I found very interesting and useful. I learned to position energy topics researchers at Mesa+ in a much broader perspective.

Also it is important, in my opinion, to transfer knowledge and skills to the next generation of PhD’s and post-docs in an earlier stage. This will allow some overlap between them and the researchers actually working in the nano lab already. In that way the potential of the special equipment present at Mesa+ can be used even more effectively.