Thermoelectric (TE) conversion offers a carbon-free power generation from geothermal, waste, body
or solar heat, and shows promise to be the next-generation energy conversion technology.
At the core of such TE conversion, there lies an all solid-state thermoelectric device which
enables energy conversion without the emission of noise, vibrations, or pollutants.
To this, a research team from POSTECH, Korea, proposed a way to design the next-generation
thermoelectric device that exhibits remarkably simple manufacturing process and structure
compared to the conventional ones, while displaying improved energy conversion efficiency using
the spin Seebeck effect (SSE).
A POSTECH joint research team has succeeded in designing a highly efficient thermoelectric
device by optimizing the properties of both the interior and surface of the magnetic material
that makes up the SSE thermoelectric device.
This is a pioneering study to show the possibility of fabricating a next-generation thermoelectric
device by utilizing the SSE, which has remained in fundamental research.
These research findings were recently published in the online edition of Energy and Environmental
Science, an international academic journal in the field of energy.
Conventional TE devices rely on the charge Seebeck effect, a thermoelectric effect wherein
a charge current is generated in the direction parallel to an applied temperature gradient
in a solid material.
This longitudinal geometry complicates the device structure and limits manufacturing
such TE devices.
The research team fabricated a nickel ferrite (NFO) - platinum (Pt) bilayer SSE device by
thinking outside the box - they realized that the device structure can be made much simpler
by utilizing the transverse TE effect where a charge current is generated in the direction
perpendicular to the applied temperature gradient . When a temperature gradient is applied in
the height direction of the device, the spin current generated in the NFO magnetic material
is transferred to the interface between NFO and Pt, injected into the Pt, then converted
into an electric current inside the Pt.
The current generated at this time flows in a direction perpendicular to the applied temperature
gradient.
By using this SSE, it is possible to construct a device structure that is simpler and easier
to scale up compared to conventional thermoelectric devices.
In order to utilize the SSE device, it requires dramatic improvement in structure and efficiency.
To this, the research team devised a simple heat treatment method to improve the efficiency
of the NFO-Pt thermoelectric device.
It was discovered through an observation using a scanning transmission electron microscopy,
that a unique microstructure can be formed inside the NFO material by heating it at a
high temperature of 1200°C or higher for a certain duration, then heating at a higher
temperature for a given time then cooling.
In addition, the team also confirmed that the same heat treatment technique can also
significantly improve the quality of the interface between NFO and Pt.
Finally, it was shown that these two effects can greatly improve the thermoelectric efficiency
of the device.
In this study, the researchers have explained and presented the principles behind fabricating
the next-generation TE device with a much simpler structure than conventional ones using
the SSE and a method that can dramatically improve its efficiency through a simple heat
treatment technique.