Improving Thermoelectric Properties of Inorganic Clathrates by Atomic and Microscale Structure Engineering
Doctoral thesis, 2021

As more countries are aiming for a carbon-neutral economy, technologies that utilize renewable energies are increasingly being considered. Thermoelectric materials enable the direct conversion between a thermal gradient and an electrical potential gradient, and are thus exploited for applications such as waste heat recovery. One of the prominent thermoelectric materials is the inorganic clathrate. Extensive research has been conducted over the past few decades to utilize its properties. However, several problems still remain ambiguous.

In this thesis, we have studied the atomic and microscale structure of the clathrates and investigated its impact on the thermoelectric properties. Especially, with a combination of experiment and theoretical calculations, the existence of an order-disorder phase transition is confirmed. It further influences the electrical transport properties, since the band structure changes after the phase transition. The degree of chemical ordering can be controlled by the synthesis method, because materials reach different equilibrium states in either route. In addition, it is found that atomic vacancies can induce a peculiar transition effect in the electrical resistivity.

In order to investigate its influence on the thermoelectric performance, a novel method is employed, where Ba8(AlxGa1–x)16Ge30 clathrates are synthesized by alloying Ba8Al16Ge30 with Ba8Ga16Ge30. This way a heterostructure is created, which contains the quaternary clathrate main phase and aggregates of Al particles. Consequently, the charge carrier mobility is largely improved to a value higher than that of the single crystal, while the lattice thermal conductivity is reduced due to the enhanced phonon scattering at different length scales.

A greatly improved understanding of the process-structure-property relationship of clathrates is achieved in this thesis. The methodologies used, as well as the key findings, can be applicable for other material systems, and hence facilitate the future research in the thermoelectrics field.

Microstructure

Thermal properties

Crystal structure

Electrical transport properties

Chemical ordering

Clathrates

Thermoelectrics

10:an, Kemigården 4, Chalmers. Zoom password: 548153
Opponent: Prof. Nini H. Pryds, Technical University of Denmark

Author

Yifei Zhang

Chalmers, Chemistry and Chemical Engineering, Applied Chemistry, Applied Surface Chemistry

Order-Disorder Transition in Inorganic Clathrates Controls Electrical Transport Properties

Chemistry of Materials,; Vol. 33(2021)p. 4500-4509

Journal article

Zhang Y., Brorsson J., Kamiyama T., Saito T., Erhart P., Palmqvist A. E. C. Investigating the Chemical Ordering in Quaternary Clathrate Ba8AlxGa16–xGe30.

Zhang Y., Brorsson J., Qiu R., Erhart P., Palmqvist A. E. C. Effect of Al/Ga Ratio on Atomic Vacancies Content and Thermoelectric Properties in Clathrates Ba8AlxGa16–xGe30.

Zhang Y., Palmqvist A. E. C. Effect of Ce Doping on the Thermoelectric Properties of Clathrates Ba8AlxGa16–xGe30.

Carbon neutral has become the most popular word nowadays, as the world largest economies US, EU and China are all aiming for a long-term economy with net-zero greenhouse gas emissions. To fulfill the economic, environmental, and political demands, technology development has attracted enormous attention over the past years, especially the energy-conversion and energy-storage applications. Waste heat exists everywhere: when your phone is being charged the battery gets hot because some electrical energy transfers to waste heat, or when you are driving, the engine is also hot because some mechanical energy converts to waste heat. According to International Energy Agency, only 32 % energy is used while most of the energy is, unfortunately, wasted as heat. Therefore, it is a great interest if we can utilize the waste heat and convert it to electricity. Thermoelectric materials provide such solution.

Thermoelectric materials have been extensively studied: nanotechnology has boosted the thermoelectric performance of the state-of-art materials, the newly discovered materials have leveled the whole field to a more compelling state. In this thesis, we have studied inorganic clathrates, one of the prominent thermoelectric materials. Especially, we have investigated the atomic structure with a combination of experiment and theoretical calculation, and are able to tackle some problems that remained ambiguous previously. Moreover, by engineering the microstructure we have improved the thermoelectric performance of the clathrates. We believe that, the methodologies in this thesis are applicable to other materials systems and can facilitate the future research of thermoelectric materials.

SwedNESS

Swedish Foundation for Strategic Research (SSF) (GSn15-0008), 2017-01-01 -- 2020-12-31.

Swedish Foundation for Strategic Research (SSF) (GSn15-0008), 2016-07-01 -- 2021-06-30.

Subject Categories

Inorganic Chemistry

Areas of Advance

Materials Science

ISBN

978-91-7905-575-2

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5042

Publisher

Chalmers University of Technology

10:an, Kemigården 4, Chalmers. Zoom password: 548153

Online

Opponent: Prof. Nini H. Pryds, Technical University of Denmark

More information

Latest update

10/29/2021