Research Article www.acsami.org
Thermal Transport Driven by Extraneous Nanoparticles and Phase Segregation in Nanostructured Mg2(Si,Sn) and Estimation of Optimum Thermoelectric Performance Abdullah S. Tazebay,† Su-In Yi,† Jae Ki Lee,‡ Hyunghoon Kim,§ Je-Hyeong Bahk,∥ Suk Lae Kim,† Su-Dong Park,‡ Ho Seong Lee,§ Ali Shakouri,∥ and Choongho Yu*,† †
Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843, United States Korea Electrotechnology Research Institute, Changwon, Korea § School of Materials Science and Engineering, Kyungpook National University, 80 Daehak-ro, Buk-gu, Daegu 702-701, Korea ∥ Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, United States ‡
S Supporting Information *
ABSTRACT: Solid solutions of magnesium silicide and magnesium stannide were recently reported to have high thermoelectric figure-of-merits (ZT) due to remarkably low thermal conductivity, which was conjectured to come from phonon scattering by segregated Mg2Si and Mg2Sn phases without detailed study. However, it is essential to identify the main cause for further improving ZT as well as estimating its upper bound. Here we synthesized Mg2(Si,Sn) with nanoparticles and segregated phases, and theoretically analyzed and estimated the thermal conductivity upon segregated fraction and extraneous nanoparticle addition by fitting experimentally obtained thermal conductivity, electrical conductivity, and thermopower. In opposition to the previous speculation that segregated phases intensify phonon scattering, we found that lattice thermal conductivity was increased by the phase segregation, which is difficult to avoid due to the miscibility gap. We selected extraneous TiO2 nanoparticles dissimilar to the host materials as additives to reduce lattice thermal conductivity. Our experimental results showed the maximum ZT was improved from ∼0.9 without the nanoparticles to ∼1.1 with 2 and 5 vol % TiO2 nanoparticles at 550 °C. According to our theoretical analysis, this ZT increase by the nanoparticle addition mainly comes from suppressed lattice thermal conductivity in addition to lower bipolar thermal conductivity at high temperatures. The upper bound of ZT was predicted to be ∼1.8 for the ideal case of no phase segregation and addition of 5 vol % TiO2 nanoparticles. We believe this study offers a new direction toward improved thermoelectric performance of Mg2(Si,Sn). KEYWORDS: magnesium silicide, magnesium stannide, titanium dioxide, phase segregation, phonon scattering
■
INTRODUCTION Recent intense demand for sustainable and environmentally friendly energy usage has driven significant efforts to develop new techniques for improving energy utilization. A substantial amount of energy extracted from energy sources is rejected from energy-consuming systems to the environment because of the inefficiency of the systems.1 Considering popular energyconsuming systems have been optimized over many years, the recovery of wasted energy could be a viable option to improve their overall performance. Over the past decade, thermoelectric energy conversion has attracted considerable interest because of the significant performance enhancement of thermoelectric materials.2 In particular, thermoelectric materials whose optimum operating temperatures are 300−500 °C have many application areas such as exhaust manifolds of automobiles2 since this grade of energy, despite relatively large exergy, is not readily usable in many conventional systems such as turbines. For the intermediate temperatures, Mg2(Si,Sn) solid solutions are one © XXXX American Chemical Society
of the best candidates because of nontoxic inexpensive lowdensity raw materials with relatively high performance.3−15 Thermoelectric performance is often indicated by the thermoelectric figure-of-merit (ZT): ZT =
S2σ T ke + kl + k bi
(1)
where S, σ, ke, kl, and kbi are thermopower (or the Seebeck coefficient), electrical conductivity, electronic thermal conductivity, lattice thermal conductivity, and bipolar thermal conductivity, respectively. It has been proven that suppression of lattice thermal conductivity is very effective in increasing ZT.5,6,13,15−18 It has been found that the thermal conductivity values of Mg2(Si,Sn) reported so far are still higher than those of other state-of-the-art thermoelectric materials, which may Received: December 11, 2015 Accepted: February 26, 2016
A
DOI: 10.1021/acsami.5b12060 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Table 1. Parameters Used for the Calculations: Ionized Impurity Concentration (NII), Nonionized Impurity Concentration (NNI), Electron Deformation Potential (Da), Hole Deformation Potential (Dah), Ratio of Segregated Volume to Total Volume (f, 0−1), Specularity Parameter (α) Corresponding to the Boundary Scattering, and a Multiplication Factor (β) in Bipolar Thermal Conductivity to Account for Additional Scattering of the Minority Carriers (holes); Majority Carrier (electron) Concentrations by Hall measurements Are Also Listed Next to the Theoretically Obtained NII for Easy Comparison sample
TiO2 (vol %)
NII (× 1020 cm−3)
Hall carrier concentration (× 1020 cm−3)
NNI (1018 cm−3)
Da (eV)
Dah (eV)
f
α
β
1 2 3 4
0 1 2 5
1.8 1.5 1.4 1.3
1.62 1.67 1.51 1.40
1.0 0.5 0.8 0.7
7 7 7 7
1 1 1 1
0.6 0.6 0.6 0.6
0.65 0.60 0.55 0.50
1 0.8 0.6 0.5
nanoparticle concentrations (0, 0.1, 0.2, 0.5, 1, 2, 5, and 10 vol %). Then, to increase thermopower, we reduced Sb doping to 0.0075 along with the high four concentrations of TiO2 nanoparticles (0, 1, 2, and 5 vol %) because the low concentrations of nanoparticles had negligible influence on thermoelectric properties. Our samples were synthesized by using a two-step solid state reaction process followed by current-assisted sintering and then a densification process. Magnesium (99.0%, Alfa aesar), arsenic-doped silicon (resistivity 1 Based on Mg2Si-Mg2Sn Solid Solutions. ICT: 2005 24th International Conference on Thermoelectrics 2005, 189−195. (11) Zhang, Q.; He, J.; Zhao, X. B.; Zhang, S. N.; Zhu, T. J.; Yin, H.; Tritt, T. M. In Situ Synthesis and Thermoelectric Properties of LaDoped Mg2(Si,Sn) Composites. J. Phys. D: Appl. Phys. 2008, 41, 185103. (12) Zhang, Q.; He, J.; Zhu, T. J.; Zhang, S. N.; Zhao, X. B.; Tritt, T. M. High Figures of Merit and Natural Nanostructures in Mg2Si0.4Sn0.6 Based Thermoelectric Materials. Appl. Phys. Lett. 2008, 93, 102109. (13) Zhang, Q.; Yin, H.; Zhao, X. B.; He, J.; Ji, X. H.; Zhu, T. J.; Tritt, T. M. Thermoelectric Properties of n-Type Mg 2Si0.6‑ySb ySn0.4 Compounds. Phys. Status Solidi A 2008, 205, 1657−1661. (14) Zhang, Q.; Zhu, T. J.; Zhou, A. J.; Yin, H.; Zhao, X. B. Preparation and Thermoelectric Properties of Mg2Si1‑xSnx. Phys. Scr. 2007, T129, 123−126. (15) Chen, L. X.; Jiang, G. Y.; Chen, Y.; Du, Z. L.; Zhao, X. B.; Zhu, T. J.; He, J.; Tritt, T. M. Miscibility Gap and Thermoelectric I
DOI: 10.1021/acsami.5b12060 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces (34) Wang, S.; Mingo, N. Improved Thermoelectric Properties of Mg2SixGeySn1‑x‑y Nanoparticle-In-Alloy Materials. Appl. Phys. Lett. 2009, 94, 203109. (35) Schneider, C. A.; Rasband, W. S.; Eliceiri, K. W. NIH Image to Imagej: 25 Years of Image Analysis. Nat. Methods 2012, 9, 671−675. (36) Bahk, J.-H.; Shakouri, A. Enhancing the Thermoelectric Figure of Merit Through the Reduction of Bipolar Thermal Conductivity with Heterostructure Barriers. Appl. Phys. Lett. 2014, 105, 052106. (37) Bahk, J. H.; Bian, Z. X.; Shakouri, A. Electron Energy Filtering by a Nonplanar Potential to Enhance the Thermoelectric Power Factor in Bulk Materials. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 075204. (38) Yang, H. R.; Bahk, J. H.; Day, T.; Mohammed, A. M. S.; Snyder, G. J.; Shakouri, A. S.; Wu, Y. Enhanced Thermoelectric Properties in Bulk Nanowire Heterostructure-Based Nanocomposites Through Minority Carrier Blocking. Nano Lett. 2015, 15, 1349−1355.
J
DOI: 10.1021/acsami.5b12060 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
