Experimental Studies on Anisotropic Thermoelectric Properties and Structures of n-Type Bi 2 Te 2.7 Se 0.3 Xiao Yan, † Bed Poudel, ‡ Yi Ma, ‡ W. S. Liu, † G. Joshi, † Hui Wang, † Yucheng Lan, † Dezhi Wang, † Gang Chen,* ,§ and Z. F. Ren* ,† † Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467, ‡ GMZ Energy, Inc., 11 Wall Street, Waltham, Massachusetts 02453, and § Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 ABSTRACT The peak dimensionless thermoelectric figure-of-merit (ZT) of Bi 2 Te 3 -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 Wm -1 K -2 . In samples with random grain orientations, we found that the thermal conductivity can be decreased by making grain size smaller through ball milling and hot pressing, but the power factor decreased with a similar percentage, resulting in no gain in ZT. Reorienting the ab planes of the small crystals by repressing the as-pressed samples enhanced the peak ZT from 0.85 to 1.04 at about 125 °C, a 22% improvement, mainly due to the more increase on power factor than on thermal conductivity. Further improvement is expected when the ab plane of most of the small crystals is reoriented to the direction perpendicular to the press direction and grains are made even smaller. KEYWORDS Bi 2 Te 2.7 Se 0.3 , thermoelectric material, anisotropy, grain orientation S olid-state thermoelectric (TE) converters are recently receiving increasing attention due to their potential to make important contributions to the effort on reducing CO 2 and greenhouse gas emission and providing cleaner forms of energy. 1 Bismuth telluride based single crystal like bulk solid solutions, including p-type Bi x Sb 2-x Te 3 and n-type Bi 2 Te 3-y Se y , still remain the best TE materials used at near room temperature. 2,3 One notable attribute about the Bi 2 Te 3 -based single crystal bulks is the lamellar structure and the weak van der Waals bonding between Te (1) -Te (1) , which is responsible for the easy cleavage along the planes perpendicular to the c-axis. 4,5 Originating from this unique structural anisotropy, thermoelectric properties of n-type Bi 2 Te 3-y Se y single crystal solid solutions prepared by traveling heater method shows strong anisotropy. 6 The electrical and thermal conductivities along the cleavage planes (perpendicular to the c-axis) are about four and two times larger than those along the c-axis, respectively. Even though the Seebeck coefficient is nearly isotropic, the ther- moelectric figure-of-merit Z along the cleavage planes is approximately two times as large as that along the c-axis. At room temperature a maximum dimensionless thermo- electric figure-of-merit ZT was achieved at 0.85 (Z ) 2.9 × 10 -3 K -1 ) for solid solutions with a 2.5 atom % Se replacing Te:Bi 2 Te 2.925 Se 0.075 that has a power factor (defined as S 2 σ, where S is the Seebeck coefficient and σ the electrical conductivity) of 47 × 10 -4 Wm -1 K -2 and thermal conduc- tivity of 1.65 W m -1 K -1 in which the lattice contribution is 1.27 W m -1 K -1 (ref 6). In principle, ZT could be greatly improved if we can decrease the thermal conductivity by breaking the single crystal into individual nanosize grains and thus increase phonon scattering due to the significantly increased grain boundaries of nanograins 7,8 while maintaining the high power factor by keeping the preferential orientation of grains. 9,10 Ball milling and direct current hot pressing proves to be a successful approach to make nanostructured com- posite (nanocomposite) 11-13 with a 40% peak ZT improve- ment from 1 to 1.4 in p-type nanostructured bulk bismuth antimony telluride by decreasing the thermal conductivity. 11 Following this approach, we have successfully synthesized n-type Bi 2 Te 2.7 Se 0.3 bulk samples by ball milling and dc hot pressing and achieved a significantly lower thermal conduc- tivity of 1.06 W m -1 K -1 (with a lattice contribution of 0.7 Wm -1 K -1 , much lower than the 1.27 W m -1 K -1 in single crystals) due to the increase of grain boundaries, in com- parison to the 1.65 W m -1 K -1 in the case of single crystal bulk samples. However, ZT was not enhanced at all because of a much lower power factor of 25 × 10 -4 Wm -1 K -2 , in comparison to the 47 × 10 -4 Wm -1 K -2 in single crystal bulk samples. We suspect that the reason for the lower power factor is due to the randomness of the small grains. Therefore, the challenge is to improve the power factor to the level close to that of the single crystal like bulks while keeping the low thermal conductivity owing to the fine grains. In literature, there have been a couple of methods *To whom correspondence should be addressed, gchen2@mit.edu and renzh@bc.edu. Received for review: 4/01/2010 Published on Web: 07/30/2010 pubs.acs.org/NanoLett © 2010 American Chemical Society 3373 DOI: 10.1021/nl101156v | Nano Lett. 2010, 10, 3373–3378