Hypothesis
for the ZT improvement of layered cobalt oxide. Ions with greater atomic mass
(right) would increase ZT as they suppress thermal conductivity in the cobalt
oxide layers. (Image: Yugo Takashima et al, Journal of Materials Chemistry A,
October 13, 2020).
Waste heat
is a highly promising source of renewable energy; however, the efficiency of
using heat to generate energy has historically been much lower than
hydroelectric, wind or solar power. While there are a number of materials that
can be used for the generation of energy from waste heat, they all suffer from
various issues ranging from low stability to low efficiency. Nevertheless, the
fact that a large number of industries generate copious amounts of waste heat
have driven research into this field.
A team of
scientists led by Professor Hiromichi Ohta at the Research Institute for
Electronic Science (RIES), Hokkaido University, has recently developed a
layered cobalt oxide with a record-setting thermoelectric figure of merit for
metal oxides at room temperature.
Their
findings were published in the journal Journal of Materials Chemistry A
("Layered cobalt oxide epitaxial films exhibiting thermoelectric ZT = 0.11
at room temperature").
Thermoelectric
conversion is driven by the Seebeck effect: when there is a temperature
difference across a conducting material, an electric current is generated.
Historically, the efficiency of heat-to-electricity conversion of metal oxides
was very low; however, metal oxide-based thermoelectric devices are highly
desired due to their environmental compatibility.
The
thermoelectric conversion efficiency of a device depends on a key factor called
the thermoelectric figure of merit (ZT).
Hiromichi Ohta’s group has developed a layered cobalt oxide that exhibits a high ZT and is stable across a range of operating temperatures. Well-known sodium-cobalt oxide, where sodium and cobalt oxide layers alternate, shows a very low ZT of around 0.03, but the material developed by Ohta's group achieved a ZT of 0.11. The group replaced the sodium by other alkali or alkaline earth metals: calcium, strontium, and barium.
Correlation
between the atomic mass and thermoelectric figure of merit (ZT). As the atomic
mass of the alkali or alkaline earth metal increases, the ZT also increases.
Orange, calcium; yellow, sodium; purple, strontium; green, barium. (Image: Yugo
Takashima et al,
Journal of Materials Chemistry A, October 13, 2020)
The
layered barium-cobalt oxide material exhibited a record-setting ZT of 0.11 at
room temperature. The increase in ZT is directly caused by the decreased
thermal conductivity of barium. As the scientists hypothesized, the greater the
atomic mass, the lower the thermal conductivity, resulting in higher ZT. This
is due to the fact that heavier atoms suppress the vibrations in the cobalt
oxide layers caused by heating.
Further
research is required to optimize the material’s composition for higher efficacy
and stability, as well as determining the most useful practical applications.
Source:
Hokkaido University