Thermoelectric oxide materials for efficient energy harvesting
In view of global energy and environmental issues, the necessity to utilize our global energy sources more efficiently becomes relevant. Since most energy is still being discharged into the environment as waste heat, significant amount of renewable energy remains unused. A promising method to convert (waste) heat directly into electrical energy is thermoelectric power generation. Thermoelectric power generation exploits the properties of p- and n-type thermoelectric materials by combining these into a thermoelectric module as shown in figure 1.
Figure 1 Thermoelectric module. Sales, Science (2002)
The basic building blocks of a thermoelectric module are the p- and n-type thermoelectric materials, which develop an opposite electrical potential under the presence of a temperature gradient. The efficiency of energy conversion of a thermoelectric module is determined by the properties of the thermoelectric materials and the dimensionless figure of merit ZT is a good measure for this efficiency. The ZT value can be improved by maximizing the thermopower and electrical conductivity or by minimizing the thermal conductivity.
To date the best thermoelectric materials are found in superlattice structures based on semiconductors, such as Bi2Te3, which can have a ZT value up to 3. These superlattice structures have an improved ZT value, because the thermal conductivity is significantly reduced by phonon scattering at the interfaces (fig. 2). Implementation of these materials into practical thermoelectric applications has been hampered due to the presence of toxic and/or scarce elements and poor chemical stability at high temperatures. The use of oxide compounds as thermoelectric materials has enormous potential to overcome these problems, however their thermal conductivity has to be reduced to obtain comparable properties.
Figure 2 Development of highest ZT value. A strong increase was realized by using superlattice structures. Here, a similar approach is investigated for oxide thermoelectric materials.
This project aims at developing and improving thermoelectric oxide materials. One of the approaches is to study the thermoelectric properties of thin films of oxide materials. By controlling the crystal structure and chemical composition at the atomic scale, an increase of the ZT value is expected for thin films.
The aim of this project is to design and fabricate high-quality oxide superlattice structures with improved thermoelectric properties. The main challenge will be to control atomic interdiffusion and interface defects, which will have a significant influence on phonon scattering in these structures.
PhD student: Peter Brinks
Supervisor: Mark Huijben