Where Should We Look For High Zt Materials: Suggestions From Theory.
- PDF / 232,569 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 119 Downloads / 159 Views
WHERE SHOULD WE LOOK FOR HIGH ZT MATERIALS: SUGGESTIONS FROM THEORY.
M. Fornari [1,2] , D. J. Singh [1], I. I. Mazin [1] and J. L. Feldman [1] [1] Naval Research Laboratory, Washington DC [2] George Mason University, Fairfax VA. ABSTRACT The key challenges in discovering new high ZT thermoelectrics are understanding how the nearly contradictory requirements of high electrical conductivity, high thermopower and low thermal conductivity can be achieved in a single material and based on this identifying suitable compounds. First principles calculations provide a material specific microscopic window into the relevant properties and their origins. We illustrate the utility of the approach by presenting specific examples of compounds belonging to the class of skutterudites that are or are not good thermoelectrics along with the microscopic reasons. Based on our computational exploration we make a suggestion for achieving higher values of ZT at room temperature in bulk materials, namely n-type La(Ru,Rh)4Sb12 with high La-filling. INTRODUCTION The quest for a material with high thermoelectric efficiency consists of optimizing both thermal and electronic properties in order to balance the lattice thermal conductivity κL with the electronic conductivity σ, the Seebeck coefficient S and the electronic thermal conductivity κe. The goal is to obtain a figure of merit ZT=σS2T /(κL+κe) as high as possible at the operating temperature of the device, i.e. 300 K or below for coolers.
Figure 1 Half of the BCC conventional cell of the skutterudite structure. Three octants are occupied by pnictogen “square” rings. The remaining octant is filled with a rare earth. The metal atoms occupy a cubic sublattice.
Z6.3.1 Downloaded from https:/www.cambridge.org/core. Columbia University Libraries, on 11 Jun 2017 at 10:04:34, subject to the Cambridge Core terms of use, available at https:/www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-626-Z6.3
The quantities involved in Z are strongly coupled to each other. A good thermoelectric material should be a high effective mass semiconductor (high S) with considerable carrier density after doping (high σ) and with complex unit cell in order to make low κL possible. Filled skutterudites are among the promising materials. They satisfy the above criteria and indeed La(Fe,Co)4Sb12 and CeFe4Sb12 have shown p-type high figures of merit [1,2], though unfortunately not at temperatures applicable to cooling devices. On the other hand, the skutterudite structure is very stable upon chemical substitutions and alloying and new interesting compounds with low thermal conductivity and optimal electronic properties at lower temperature could be found. Using first principles calculations we performed a systematic computation of the electronic properties of La-filled skutterudites, LaM4Pn12 (M=Fe, Ru, Os and Pn=P, As, Sb). The comparison between band structures of different compounds emphasized trends and pointed out a particularly promising composition La(Ru,Rh)4Sb12 for which a ZT possibly exceeding unit
Data Loading...
