collaboration between osaka university (japan) and hitachi, ltd. has led to the development of a new thermoelectric material, ytterbium silicide (ybsi­₂), which has demonstrated a high power factor even at room temperature.

three-dimensional crystal structure of ybsi₂, (b) view along the a-axis, and (c) along the c-axis. (kurosaki et al./osaka university)

typically, as explained by an article on the university website, thermoelectric materials have been limited to high-temperature devices but ybsi₂ was selected because it is non-toxic and because of its valance fluctuation makes it useful at lower temperatures.

“the first advantage of ybsi₂ is that the yb atoms occupy a mixture of valence states, both +2 and +3,’ the article continued. “this fluctuation, also known as kondo resonance, increases the seebeck coefficient with keeping metal-like high electrical conductivity at low temperature, and therefore the power factor.”

the second advantage for ybsi₂ is its layered structure that includes yb atoms acting as crystal planes with si atoms forming hexagonal sheets in between the planes to block heat conduction through the material, lowers the thermal conductivity, and preserving the temperature gradient necessary for thermoelectricity.

“the result is an encouragingly high power factor of 2.2 mwm^-1k^-2 at room temperature,” the article said. “this is competitive with conventional te materials based on bismuth telluride.”

the research was recently published in physica status solidi rapid research letters. the abstract read:

“metal silicide-based thermoelectric (te) materials have attracted attention in the past two decades, because they are less toxic, with low production cost and high chemical stability. here, we study the te properties of ytterbium silicide ybsi₂ with a specific layered structure and the mixed valence state of yb²⁺ and yb³⁺.

“ybsi₂ exhibits large seebeck coefficient, s, accompanied by high electrical conductivity, σ, leading to high power factor, s²σ, of 2.2 mw m⁻¹ k⁻² at room temperature, which is comparable to those of state-of-the-art te materials such as bi₂te₃ and pbte.

“moreover, ybsi₂ exhibits high grüneisen parameter of 1.57, which leads to relatively low lattice thermal conductivity, κ_lat, of 3.0 w m⁻¹ k⁻¹ at room temperature. the present study reveals that ybsi₂ can be a good candidate of te materials working near room temperature.”