Thermoelectric (TE) materials could play a major role in future technologies. Even though the applications of these compounds have been explored for a long time, they are mostly limited to high-temperature devices. Recently, researchers from Osaka University have developed a new TE material with an improved power factor at room temperature. This study could help bring these materials out of the high-temperature niche into mainstream technologies.
TE materials display the thermoelectric effect by applying heat on one side and an electric current starts to flow. On the other side runs an external current through the device and a temperature gradient forms and one side becomes hotter than the other. By interconverting heat and electricity, TE materials can be used as power generators or refrigerators.
The ideal TE material combines high electrical conductivity, which allows the current to flow with low thermal conductivity, which prevents the temperature gradient from evening out. The power generation performance mainly depends on the “power factor,” which is proportional to both electrical conductivity and the Seebeck coefficient.
"Unfortunately, most TE materials are often based on rare or toxic elements," according to study co-author Sora-at Tanusilp. "To address this, we combined silicon — which is common in TE materials — with ytterbium, to create ytterbium silicide [YbSi2]. We chose ytterbium over other metals for several reasons. First, its compounds are good electrical conductors. Second, YbSi2 is non-toxic. Moreover, this compound has a specific property called valence fluctuation that makes it a good TE material at low temperatures."
One of the advantages of YbSi₂ is the Yb atoms occupy a mixture of valiance states, both +2 and +3. This function, called the Kondo resonance, increases the Seebeck coefficient by keeping metal-like high electrical conductivity at low temperature and therefore is the power factor.
Another advantage of YbSi₂ has an unusual layered structure. While Yb atoms occupy crystal planes similar to pure Yb metal, the Si atoms form hexagonal sheets between those planes, resembling the carbon sheets in graphite. This blocks the conduction of heat through the material and therefore keeps the thermal conductivity down, which preserves the temperature gradient. The researchers believe that that heat conduction is further suppressed by controlling the structure in nanoscale and traces of impurities and other defects.
The results of this study are encouragingly high power factor of 2.2 mWm-1K-2 at room temperature. This competitive with conventional TE materials based on bismuth telluride.
"The use of Yb shows we can reconcile the conflicting needs of TE materials through carefully selecting the right metals," explained Ken Kurosaki. "Room-temperature TEs, with moderate power, can be seen as complementary to the conventional high-temperature, high-power devices. This could help unlock the benefits of TE in everyday technology."
The paper on this research was published in Rapid Research Letter.