Yucheng Lan

NbFeSb-based p-type half-Heuslers for power generation applications

We report a peak dimensionless figure-of-merit (ZT) of ∼1 at 700 °C in a nanostructured p-type Nb0.6Ti0.4FeSb0.95Sn0.05 composition. Even though the power factor of the Nb0.6Ti0.4FeSb0.95Sn0.05 composition is improved by 25%, in comparison to the previously reported p-type Hf0.44Zr0.44Ti0.12CoSb0.8Sn0.2, the ZT value is not increased due to a higher thermal conductivity. However, the higher power factor of the Nb0.6Ti0.4FeSb0.95Sn0.05 composition led to a 15% increase in the power output of a thermoelectric device in comparison to a device made from the previous best material Hf0.44Zr0.44Ti0.12CoSb0.8Sn0.2. The n-type material used to make the unicouple device is the best reported nanostructured Hf0.25Zr0.75NiSn0.99Sb0.01 composition with the lowest hafnium (Hf) content. Both the p- and n-type nanostructured samples are prepared by ball milling the arc melted ingot and hot pressing the finely ground powders. Moreover, the raw material cost of the Nb0.6Ti0.4FeSb0.95Sn0.05 composition is more than six times lower compared to the cost of the previous best p-type Hf0.44Zr0.44Ti0.12CoSb0.8Sn0.2. This cost reduction is crucial for these materials to be used in large-scale quantities for vehicle and industrial waste heat recovery applications.

Investigating the thermoelectric properties of p-type half-Heusler Hfx(ZrTi)1−xCoSb0.8Sn0.2 by reducing Hf concentration for power generation

Based on the fact that Hf is much more expensive than other commonly used elements in HfCoSb-based half-Heusler materials, we studied the thermoelectric properties of the p-type half-Heusler Hfx(ZrTi)1−xCoSb0.8Sn0.2 by reducing Hf concentration. A peak ZT of ∼1.0 was achieved at 700 °C with the composition of Hf0.19Zr0.76Ti0.05CoSb0.8Sn0.2 by keeping the Hf/Zr ratio at 1/4 and Hf/Ti at 4/1. This composition has much reduced cost and similar thermoelectric performance compared with our previously reported best p-type half-Heusler: Hf0.44Zr0.44Ti0.12CoSb0.8Sn0.2. Due to the decreased usage of Hf, it is more favorable for consideration in applications. In addition, a higher output power is expected because of the higher power factor even though the conversion efficiency is the same due to the same ZT.

Nanoporous gallium nitride square microtubes

Porous gallium nitride microtubes were self-fabricated from gallium nitride submicron irregular structures. The microtubes were of square cross-section. Electron diffractions indicated that the microtubes were composed of zincblende gallium nitride. Electron energy-loss spectrum and photoluminescence spectrum of the microtubes were collected and compared with that of single crystals.