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The peak dimensionless thermoelectric figure-of-merit (ZT) of Bi2Te3-based n-type single crystals is about 0.85 in the ab plane at room temperature, which has not been improved over the last 50 years due to the high thermal conductivity of 1.65 W m−1 K−1 even though the power factor is 47 × 10−4 W m−1 K−2. Credit Nano Letters

Abstract:
Thermoelectric materials promise everything from clean power for cars to clean power from the sun, but making these materials widely useful has been a challenge.

Grain orientation boosts thermoelectric performance

Chestnut Hill, MA | Posted on January 23rd, 2011

Now researchers from Boston College, MIT and GMZ Energy have developed an inexpensive, simple mechanical process for achieving a major increase in the efficiency of a common thermoelectric material: bismuth telluride selenide (BiTeSe), which has been used in commercial devices since the 1950s. "Power generation applications for thermoelectrics are not big now because the materials aren't good enough," said MIT professor Gang Chen. He believes their findings could pave the way for a new generation of products - from semiconductors and air conditioners to car exhaust systems and solar power technology - that run cleaner.

Xiao Yan and his colleagues from BC, MIT and GMZ Energy achieved a 22% improvement in peak thermoelectric figure of merit (ZT - see 1 below) from 0.85 to 1.04 at 125 degrees C in Bi₂Te_2.7Se_0.3 by repressing the as-pressed samples. The main improvement is the large increase of electrical conductivity with only small increase of thermal conductivity and similar Seebeck coefficient. "We want to attain the single-crystal-like high power factor (see 2 below) by preparing preferential grain orientation while maintaining low thermal conductivity by nanocomposite approach," said Boston College professor Zhifeng Ren.

An innovative mechanical process technique was employed by Xiao Yan and his co-workers from BC, MIT and GMZ Energy. As-pressed samples were initially obtained by ball milling the mixture of individual element materials into alloyed BiTeSe nanopowders and then hot pressing the powder into bulk forms with nano constituents. Then as-pressed bulks were pressed again at elevated temperatures in a bigger diameter die to obtain re-pressed bulk samples. "During repressing process, lateral flow takes place, which helps to orient the grains and thus improve the power factor," explained Ren and Chen.

This work was published in Nano Letters, pubs.acs.org/doi/abs/10.1021/nl101156v

(1) ZT is a measure of the thermoelectric performance of a material

(2) Power factor is defined as a product of squared Seebeck coefficient and electrical conductivity.

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