Nanostructured YbAgCu 4 for Potentially Cryogenic Thermoelectric Cooling Machhindra Koirala, † Hui Wang, † Mani Pokharel, ‡ Yucheng Lan, † Chuanfei Guo, † Cyril Opeil, ‡ and Zhifeng Ren †, * † Department of Physics and TcSUH, University of Houston, Houston, Texas 77204, United States ‡ Department of Physics, Boston College, Chesnut Hill, Massachusetts 02467, United States ABSTRACT: We have studied the thermoelectric properties of nanostructured YbAgCu 4 materials. A high power factor of ∼131 μW cm -1 K -2 has been obtained at 22 K for nanostructured samples prepared by ball milling the arc melted ingot into nanopowder and hot pressing the nano- powder. The implementation of nanostructuring method decreased the thermal conductivity at 42 K by 30-50% through boundary scattering comparing with the previously reported value of polycrystalline YbAgCu 4 . A peak dimension- less thermoelectric figure-of-merit, ZT, of 0.11 has been achieved at 42 K, which may find potential applications for cryogenic cooling below 77 K. The nanostructuring approach can be extended to other heavy Fermion materials to achieve high power factor and low thermal conductivity and ultimately higher ZT. KEYWORDS: Thermoelectric, heavy Fermions, Kondo lattice, boundary scattering, YbAgCu 4 T he ability of thermoelectric (TE) materials for converting heat into electricity and vice versa is very important for power generation 1,2 as well as solid-state cooling. 3 For the aerospace program where the weight and size compatibility of the devices is important, thermoelectric devices are more useful for cryogenic application in comparison to other heavy cooling devices. The power generation efficiency or coefficient of performance of thermoelectric devices are determined by the dimensionless figure of merit, ZT = [(S 2 σ)/κ]T, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the absolute temperature. 4-6 All of these quantities are related to each other and changing one affects the others, so increasing ZT is really challenging. In the recent years, the development of new techniques for controlling the material properties through nanostructuring, 7 modulation doping, 8,9 resonant doping, 10,11 and band engineering near Fermi level 12,13 have helped to enhance ZT significantly. The rapid development of technologies enabled to increases ZT above 1 for cooling applications at around room temperature and power generation at high temperatures but at the low- temperature (cryogenic) range, the existing ZT is far below the application requirement. For low-temperature thermoelectric materials, most of the focus is toward narrow band gap semiconductors and Kondo insulators. In different temperature ranges below room temperature, there are many materials that are being investigated. The well-known low-temperature thermoelectric material is single crystal Bi 1-x Sb x with ZT ∼ 0.5 at ∼150 K. 14 Encapsulating Ce in clathrate has enhanced ZT to 0.1 at 150 K. 15 Doping on extremely high mobility materials CuAgSe has enhanced ZT to 0.1 at 100 K. 16 Doped FeSi have been reported to have peak ZT of 0.12 at 120 K. 17 However, the operating temperature for these materials is above 100 K, which is above liquid nitrogen temperature, 77 K. For temperature below 77 K, ZT is very low because the temperature T is very small. The current trend for cryogenic thermoelectric materials involves mostly Kondo insulators like FeSb 2 , 18-20 CrSb 2 , 21 and some rare earth Kondo systems like YbAl 3 , 22 CeCu 6 , 23 Ce 0.5 La 0.5 Al 3 , 24 YbCuAl, 25 CeAl 3 , 25 CePd 3 , 26 and so forth. Rare earth metallic heavy Fermions like YbAl 3 , CePd 3 26 have been investigated to the temperature range of 150 K and a peak ZT ∼ 0.23 have been reported for both n- and p-type. Our focus in this work is to study materials having good ZT at below 77 K. FeSb 2 was studied for its giant Seebeck coefficient below 77 K by Bentein. 18 Much effort have been made on that materials but the optimized ZT is not more than 0.026. 18-20,27 In phonon drag systems like FeSb 2 28 and CrSb 2 , it is very difficult to decouple the electrical and phonon part and the value of ZT remains low. There are some rare earth Kondo systems with good power factor at temperature below 77 K, but they have high thermal conductivity so the overall ZT is low. Maintaining that high power factor and reducing thermal conductivity is really challenging. Received: April 17, 2014 Revised: July 17, 2014 Published: July 31, 2014 Letter pubs.acs.org/NanoLett © 2014 American Chemical Society 5016 dx.doi.org/10.1021/nl501436w | Nano Lett. 2014, 14, 5016-5020