Synthesis and Thermoelectric Properties of Doped Yb 14 MnSb 11-x Bi x Zintls

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1267-DD03-05

Synthesis and Thermoelectric Properties of Doped Yb14MnSb11-xBix Zintls Kurt Star1, Alex Zevalkink2, Chen-Kuo Huang3, Bruce Dunn1and Jean-Pierre Fleurial3 Department of Material Science and Engineering, University of California Los Angeles, Los Angeles, California 90095-1595 2 Materials Science Department, California Institute of Technology, 1200 California Blvd., Pasadena, CA 91125-8099 3 Thermal Energy Conversion Technologies, Jet Propulsion Laboratory/California Institute of Technology, 4800 Oak Grove Dr., Pasadena, CA 91109 1

ABSTRACT Yb14MnSb11 is a very promising thermoelectric material for high temperature applications. This compound is a member of a large family of Zintl phases with a “14-1-11” A14MPn11 stoichiometry (Pn = P, As, Sb, Bi; A = Ca, Ba, La, Sr, Yb, Eu; M = Mn, Al, Cd, Ga, In, Nb, Zn). Yb14MnSb11 exhibits low lattice thermal conductivity values and a p-type semimetallic behavior with values of the non-dimensional figure of merit zT peaking at 1.4 above 1200 K. There is significant interest in investigating how substitutions on any of the atomic sites impact the charge carrier concentration and mobility, band gap and lattice thermal conductivity. Recent reports have studied substitutions on the Yb and Mn sites with the goal of reducing hole carrier concentration and improving carrier mobility values. High energy ball milling has been shown to be a convenient method of synthesis to prepare Yb14MnSb11 and it has been used here to explore the solid solution systems derived from this compound by substituting Sb with Bi or As. High energy ball milling is a non-equilibrium process and not all of the 14-1-11 compounds are easily formed with this method. Characterization of the synthesized compositions was done by X-ray diffraction, electron microprobe, and high temperature measurements of the electrical and thermal transport properties up to 1275 K. The experimental results on undoped and doped solid solution samples are compared to that of pure Yb14MnSb11 samples prepared by the same high energy ball milling technique. INTRODUCTION Yb14MnSb11 is a very promising thermoelectric material for high temperature applications. Materials synthesized via a Sn flux growth method produced material with a reported zT of ~1 above 1200K.1 The relationship between zT, the dimensionless thermoelectric figure of merit, and the Seebeck coefficient (α), temperature (T), thermal conductivity (κ) and electrical resistivity (ρ) is given by equation 1. α 2T zT = (1)

κρ

A discussion of the effects of carrier concentration on each of these parameters and optimizing zT in a classical parabolic band semiconductor can be found in reference 3. Using Zintl–Klemm formalism, the substitution of Mn+2 with Al+3 in the structure will reduce the number of holes in the system. By modifying the carrier concentration in Sn flux-grown Yb14Mn1-xAlxSb11 it was found that zT values could be improved 20-30%.2, 3 Though this method is excellent for growing

high quality crystals, it is not suitable for scale up to the tens of gra