Thermoelectricity revisits silicon - Dr. Duckyoung Kim

JULY 19, 2018

New report from a team of scientists led by Dr. Duck Young Kim from HPSTAR, found that their recently synthesized silicon allotrope, namely Si₂₄, with nanoscale porous structure showed extremely favorable thermoelectric properties with slightly doping, comparing with normal silicon. This work is published in recent Nano Letters.

Thermoelectric(TE) materials are promising next generation energy materials owing to its capability to transform waste heat into useful electric energy.

How to evaluate a thermoelectric material? Physicists proposed a parameter called thermoelectric figure of merit-ZT, which indicates how efficiently a material converts heat to electricity. So materials with a high thermoelectric figure of merit are thus will be promising targets.

To increase the ZT value, a material needs to be a good electrical conductor as well as a good heat insulator. However, it is still challenging to achieve a high ZT in bulk materials.

“Silicon is commonly used in TE materials synthesis, while our newly synthesized silicon allotrope, Si24 shows even favorable features for a higher ZT”, said Dr. Duck Young Kim, the lead author.

The first advantage of Si24 is its intrinsic nanostructure as in the case of a good thermoelectric material, SnSe. This is in contrast to the previous studies for Si with postprocessings of extrinsic nanoscale structures.

The second is that the Si24 features regular array of nanosized pores as in skutterudites and clathrates, which makes it even more promising as a thermoelectric material.

“These indicate that an enhanced thermoelectric property can be achieved in a bulk Si material—cheap, nontoxic, Earth-abundant, and free of phase separation”, Dr. Duck young Kim added.

The team applied various calculations to slightly sodium doped Si24. The doped Si24 display superior TE property with an order-of magnitude higher ZT value compared with common silicon.

“The doped sodium atoms act as rattler in silicon cages which is similar to clathrates”, explained Dr. Duck Young Kim. “The doped guest atoms will interact with the host atoms, which will then suppress the lattice thermal conductivity”.

What’s more, the guest atoms will spontaneously donate many electrons to the host cage without affecting the electronic structures.

“In our study, we used fully first-principle calculations to estimate ZT values. We also provides a practical way to enhance the TE property by introducing rattling guest atoms to the pored host structures”, remarked Dr. Duck Young Kim.

Caption: (left) Ball-and-stick models of the Si24 and diamond-cubic phase of silicon crystals with orthogonal projection along the x-axis. (right) The maximum values of ZT (ZTmax) for a varying temperature are shown as squares. The enhancement (ratio of the ZTmax for Si24 to dSi) is also plotted as orange circles.