Taylor Sparks, Materials 265, Nov 19 2008

SuperlatticesBulk Thermoelectrics

http://www.thermoelectrics.caltech.edu/science_page.htm

Slack’s concept of “Phonon-Glass, Electron Crystal”

• Hicks & Hicks & Hicks & Hicks & DresselhausDresselhausDresselhausDresselhaus: : : : Quantum Confinement • Increase the density of states by decreasing the dimensions• Within 17-55 Å wells had 5 times the ZT of bulk PbTe!• Overall improvement was 2 times better

Quantum well PbTe(smaller Eg)

Barrier Pb0.927Eu0.073Te (larger Eg)

a is the width of the quantum well

L. D. Hicks and M. S. Dresselhaus, Phys. Rev. B 47474747, 16631 (1993)

•OhtaOhtaOhtaOhta et al:et al:et al:et al: 2DEG based on SrTiO3 and SrTi0.8Nb0.2O3

Ohta, H., Kim, S., Mune, Y., Mizoguchi, T., Nomura, K., Ohta, S., Nomura, T., Nakanishi Y., Ikuhara, Y., Hirano, M., Hosono, H., Koumoto, K. Nature Materials, 6666, 129-134 (2007).

•OhtaOhtaOhtaOhta et al:et al:et al:et al: 2DEG has ZTRT=2.4, >800 HV/K• Overall structure has effective ZTRT=0.24, did not measure κ directly

Ohta, H., Kim, S., Mune, Y., Mizoguchi, T., Nomura, K., Ohta, S., Nomura, T., Nakanishi Y., Ikuhara, Y., Hirano, M., Hosono, H., Koumoto, K. Nature Materials, 6666, 129-134 (2007).

Dragoman et al: Dragoman et al: Dragoman et al: Dragoman et al: Graphene based thermoelectrics

• The best Seebeck coefficient was 850 HV/K in SrTiO3 superlattice structures• The Seebeck coefficient in these graphene structures is 30 mV/K which is 35 times greater!

Dragoman, M., Dragoman, D., Physical Review Letters, 91919191, 203116 (2007).

Quantum devices• molecular devices, superlattices, carbon nanotubes, nanowires,• Seebeck coefficient is given by

For maximum S value, the shape of the transmission TQ(E) should consist of a series of sharp peaks, indicative of a narrow energy spectrum for electrons.

• VenkatasubramanianVenkatasubramanianVenkatasubramanianVenkatasubramanian: : : : Quantum Confinement • Superlattice with 10 Å wells, 50 Å barriers has ZT300K=2.4 ZT instead of ZT300K=1 of bulk BiTe

Venkatasubramanian, R., Siivola, E., Colpitts, T., O'Quinn, B., Nature, 413413413413, (2001)

Venkatasubramanian, R., Siivola, E., Colpitts, T., O'Quinn, B., Nature, 413413413413, (2001)

4x’s better cooling

Harman Technique

VenkatasubramanianVenkatasubramanianVenkatasubramanianVenkatasubramanian: cont’d: cont’d: cont’d: cont’d

Nano-Engineered Superlattices• Rayleigh scattering occurs based on the scattering cross section b6/λ4 where b is the size of the nanodot and λ is the acoustic phonon wavelength.• Heat carrying phonons from <1 nm to 10 Hm. • Random distribution best

Kim, W., Zide, J., Gossard, A., Klenov, D., Stemmer, S., Shakouri, A., Majunmdar, A. Physical Review Letters, 96969696, 045901 (2006).

Variable nanodotconcentration

Variable seperationdistance

Random nanodot size and distribution