Synthesis and Characterization of Solid- Solution Zintl Phases for Thermoelectric Applications

Thermoelectric devices can convert thermal energy into electrical energy or vice-versa. Zintl phases are a class of materials than can be used in such devices because they often possess complex structures necessary for the desired thermoelectric properties (Seebeck, electrical resistivity, thermal conductivity). In 2006, the Zintl phase Yb14MnSb11 was discovered to have a max zT = 1.0 at 1200 K. There are many known compounds of the generic formula A14 MPn11 (A = alkaline earth, Eu, Yb; M = Group 13, Mn, Zn; Pn = P, As, Sb, Bi), however very few of these compounds have been measured for thermoelectric properties. The electronic properties of Yb14MnSb 11 can be tuned through chemical substitution to make solid-solutions. This compound is inherently a p-type material with metallic-like conductivity due to a hole on the [Mn2+Sb412- h+]9- tetrahedral cluster. In order to raise the thermoelectric figure of merit, zT, the carrier concentration needs to be reduced by substituting in elements that donate electrons to the structure and fill the hole. Solid solution series of Yb14Mn1-xAlxSb 11 (x = 0.2, 0.4, 0.6, 0.8, 0.95, 1) and Yb14-xCaxMnSb 11 (x = 2, 4, 6, 8) have been made by Sn-Flux. In a flux reaction, a molten metal (Sn) is used as a solvent. Yb14-xTm xMnSb11 was also made by Sn-flux, but the solubility limit of Tm into the structure is around x = 0.4. All of these elements (Al 3+, Ca2+, and Tm3+) reduce the carrier concentration. The structure and composition were characterized by single crystal X-ray diffraction, powder X-ray diffraction, and electron microprobe analysis. Other compositions including Yb14-xCaxMn (1-y)/2Zn(1-y)/2AlySb11 and Ca 14-xLaxAlSb11 were also explored using a Sn-flux synthetic route. The Sn-flux route is an excellent approach to grow pure-phase single crystals, and single crystals are vital for understanding of the structure and properties. However, mechanical alloying would be a better synthetic method for large scale production of material for thermoelectric devices. A method to make Yb14MnSb11 using mechanical alloying of elemental Yb, and Sb, with a MnSb binary was developed. A similar mechanical alloying approach was used to make Eu14MnSb11. Eu14MnSb 11 single crystals have not been grown from a flux because Eu10 Mn6Sb13 forms as the primary phase. The product from the mechanical alloying synthesis was characterized by powder X-ray diffraction and electron microprobe analysis. Thermoelectric properties up to 1200 K were measured including the Seebeck coefficient, electrical resistivity, and thermal conductivity. Heat capacity measurements of Yb14MnSb11, Yb14Mn1-x AlxSb11 and Eu14MnSb11 were used to reevaluate the thermal conductivity. The thermoelectric properties were compared and contrasted to the data measured on a Sn-flux Yb14MnSb 11 sample that was published in 2006