Nano-domains in thermoelectric Half-Heusler CoTi 0.5 Sc 0.5 Sb alloys
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Nano-domains in thermoelectric Half-Heusler CoTi0.5Sc0.5Sb alloys Joaquin Miranda Mena, Heiko G. Schoberth, Thomas Gruhn, and Heike Emmerich Lehrstuhl für Material- und Prozesssimulation, Universität Bayreuth, Universitätsstraße 30, D-95447 Bayreuth, Germany ABSTRACT Nano-phase separation is of great relevance for functional materials like thermoelectrics. Indeed, nano-domains in CoSb-based half-Heusler thermoelectrics have been found to reduce the lattice heat conductivity, which increases the figure of merit. Within this context, we studied the configurational energy in the alloy CoTi0.5Sc0.5Sb by means of first-principle calculations. We consider structures formed by Ti (Sc) nano-domains. In recent publications we have showed that these domains are the most stable atomic configurations. In this work we found that for a given concentration the electronic density of states is considerably modified as the volume of the domains are increased. INTRODUCTION The search of thermoelectrics that can efficiently operate at mid- and high- temperatures has become a priority in the material science community [1,2]. Due to the fact that many of the energy combustion occur in the range of 400 K to 700 K, one would like to find materials that can harvest energy within this range. For instance, the automotive industry has already put considerable efforts to engineer devices that can convert heat into electrical currents. Although we have witnessed improvement in the last years, it is necessary to explore other families of materials, which can go beyond the traditional Te-based thermoelectrics. In this regard, halfHeuslers present features that put them among the best candidates [3]. Just to mention a few, they can be prepared from environmental friendly elements and rather not expensive. Their structure is similar to the traditional semi-conductors. As a matter of fact, from the point of view of manufacturing, this close relation could be an advantage for manufacturing modules within the semi-conducting industry. The figure of merit (ZT) determines the quality of a thermoelectric and it encompass electronic and heat transport properties. Formally, it is defined by ZT=S2σΤ/(κe+κl), where S is the Seebeck coefficient, σ the electrical conductivity, while κe and κk are the electronic and lattice thermal conductivity respectively. During the last 10 years the half-Heuslers have shown steady improvement in their figure of merit. Thus, the alloying of CoTi and NiTi based halfHeusler have allowed to achieve ZT = 1.0 for Ti0.2Hf0.8CoSb0.8Sr0.2[4] and Ti0.25Zr0.25Hf0.5NiSn0.99Sb0.01 [5]. Apparently one of the reasons of the enhanced ZT is the introduction of nano-phase separation; indeed there have been reports of reduction of lattice thermal conductivity in phase separated phases of Mn and Ti in CoTi1-xMnxSb [6-8]. The experimental results point out that an increased of the nano-domains damp the thermal conductivity carried out by the lattice vibrations.
In the past, the lattice and electronical structures have been extensively investig
