In-Situ Synthesis of Sub-Stoichiometric Titanium Dioxide for Spray-Formed Thermoelectric Generators
GILDERSLEEVE V E. 1, VAßEN R. 1
1 Forschungszentrum Jülich GmbH, Institute of Energy and Climate Research, Materials Synthesis and Processing (IEK-1), Jülich, Germany
The rapid acceleration of climate change in conjunction with increase global energy demand has necessitated the world to look for alternative sources of energy with reduced greenhouse gas emissions. Significant technological delays have prevented renewable energy sources from acting as the primary suppliers to global main power grids. Therefore, alternative ‘clean’ energy sources must be considered – both to augment these lagging technologies as well as potentially retrofit to existing applications. Recently, existing energy conversion processes have been given significant attention to determine if there are pathways for efficiency improvements or lost energy recovery.
A large portion of underutilized energy falls under the form of wasted heat. For instance, the exhaust gases in combustion processes (automotive, turbine, etc.). The reclamation of waste heat into the form of usable energy is a topic of constant interest in the materials and engineering communities. However, much research in this field of thermoelectric power generators (TEGs) is built based on the analysis of sintered bulk materials. For an optimal thermoelectric generator, the material must have a high Seebeck coefficient [uV/K], a high electrical conductivity [S/m], and a low thermal conductivity [W/m*K]. In sintered materials, it is possible to optimize the material chemistry and crystallographic structure toward achieving optimal Seebeck coefficient and electrical conductivity. However, these sintered thermoelectric materials typically will have >95% densities, yielding a significantly high thermal conductivity (close to the bulk values). Therefore, there is an opportunity to seek alternative materials processes which might yield similar chemical and crystallographic properties while also having more microstructural control.
In this presentation, the opportunity to use thermal spray processing to form thermoelectric devices is proposed. Thermal spray processing offers enhanced material flexibility, such that almost any of the thermoelectric materials that have been studied using sintering processes can be revisited. Here, plasma spray processing by standard DC and cascaded arc processes is used to deposit titanium dioxide (TiO2) powdered material. TiO2 when reduced to TiO2-x acts as an n-type semiconductive material due to the formation of oxygen vacancies. When using argon-hydrogen plasmas to spray oxides, it is common to expect some degree of reduction in the coating – typically dependent on the flow rate of hydrogen gas used. In this work, the effect of hydrogen content in the plasma flame, its influence on the inflight reduction of TiO2 particles, and the thermoelectric performance of reduced TiO2-x coatings will be shown. Through control and fine tuning of the process parameters, relationships between spraying parameters and thermoelectric performance will be demonstrated.
