Negative Thermal Expansion Materials under Pressure: a New Route for High Performance Thermoelectrics
Synthesis and Properties of Functional Materials
Final Report Abstract
An ideal thermoelectric material should be an electrical conductor and thermal insulator simultaneously. This conflicting requirement poses a challenge, due to the strong coupling effect between the electronic and phononic transport. The overall goal of this project was to advance the fundamental science underlying the thermoelectric properties of negative thermal expansion materials under mechanical strain. The majority of research in this project was dedicated towards chalcopyrite compounds. The effect of compression on the thermal conductivity of CuGaS2, CuInS2, CuInTe2, and AgInTe2 chalcopyrites was studied at 300 K using phonon calculations. The thermal conductivity of CuGaS2 increases with pressure, which is a common behavior. Striking differences occur for the other three compounds. CuInTe2 and AgInTe2 exhibit a drop in the thermal conductivity upon increasing pressure, which is anomalous. AgInTe2 thus reaches a very low thermal conductivity of 0.2 W m^-1 K^-1 at 2.6 GPa, being beneficial for thermoelectrics. CuInS2 is an intermediate case. Based on the phonon dispersion data, the phonon frequencies of the acoustic modes for CuInTe2 and AgInTe2 decrease with increasing pressure, thereby driving the anomaly, while there is no significant pressure effect for CuGaS2. This leads to the negative Grüneisen parameter for CuInTe2 and AgInTe2, a decreased phonon relaxation time, and a decreased thermal conductivity. This softening of the acoustic modes upon compression was suggested to be due to a rotational motion of the chalcopyrite building blocks rather than a compressive oscillation. Since CuInTe2 is one of the most promising materials explored herein for thermoelectric applications, it was further studied in terms of alloying with Se on the Te site. For p-type CuIn(Se0.5Te0.5)2 at room temperature, the thermoelectric efficiency can be increased by a factor of two as the pressure in increased to 1.9 GPa. Since CuInSe2 is commonly grown on GaAs, this interface and interaction with intrinsic and extrinsic point defects (including dopants) were also investigated in this project. Cu vacancies and antisites, In vacancies, and extrinsic (O, H, C) point defects in CuInSe2 segregate at the interface with GaAs, being relevant for the transport properties. Finally, novel applications of thermoelectric devices were probed. By adjusting the population of the S-H bonds by deprotonating PSS in PEDOT:PSS, a conjugated polymeric system equivalent to inorganic superlattices, the linear coefficient of thermal expansion can be enhanced by 57%. Furthermore, thermoelectric MgAgSb and TiO2 alloyed with V were proposed to be biocompatible. Conveying scientific results in peer-reviewed journals is extremely important, but it is also crucial to explain our endeavor in a simple manner to a broad audience.
Publications
- Aspartic acid adsorption on thermoelectric surfaces, Appl. Surf. Sci. 496 (2019) 143716
D. Music, D. M. Holzapfel, F. Kaiser, and E. Wehr
(See online at https://doi.org/10.1016/j.apsusc.2019.143716) - First principles investigation of anomalous pressuredependent thermal conductivity of chalcopyrites, Materials 12 (2019) 3491
L. Elalfy, D. Music, and M. Hu
(See online at https://doi.org/10.3390/ma12213491) - Segregation of point defects at the CuInSe2(001)/GaAs(001) interface, Solid State Commun. 299 (2019) 113652
D. Music and P. Keuter
(See online at https://doi.org/10.1016/j.ssc.2019.113652) - Tuneable thermal expansion of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate, J. Phys.: Condens. Matter 31 (2019) 125101
D. Music and L. Elalfy
(See online at https://doi.org/10.1088/1361-648X/aafdda)
