Complex and isotropic silicide materials based on eco-sustainable elements for thermoelectric devices working in intermediate and high temperature – MASCOTH

A thermoelectric device is able to transform thermal into electric energy (Seebeck effect) and vice versa (Peltier effect). Two challenges concerning materials hinder a large scale production and integration of these devices. The first one is related to the high price and toxicity of the atoms composing the most used materials, the second one arises from their low thermoelectric (TE) efficiency.
The ideal TE material must be a good electric conductor and a bad thermal conductor. In the project MASCOTH, we focus on semi-conductor materials with complex crystallographic structures, the silicon clathrates, where the complexity is at the origin of microscopic mechanisms lowering the thermal conductivity without affecting the electric one.
Clathrates are cage structures where cages of silicon atoms can host guest atoms. As a consequence, the electric properties can be adjusted by means of atoms substitution concerning the atoms of the cage and/or the guest atoms. This way it is possible to produce semi-conductor clathrates with gaps in the 0.5-1 eV range, as well as superconductor clathrates. Surprisingly, these compounds present a very low thermal conductivity (less than 1 W/mK). The mechanisms ruling the energy transfer between the crystalline lattice, the degree of freedom of the guest atoms and those of the host atoms, are not well understood yet and they are the object of recent works of ours. To resume, these compounds are intrinsically nano-structured, on the lengthscale of the unit cell, and their electric and thermal transport properties seem to be uncoupled.
If the crystallographic motif is complex, the unit cell of silicon clathrates is amazingly simple : a simple cubic cell and their electric and thermal properties are isotropes. On the other hand, they are binary systems whose components, barium and silicon, are much more abundant and less toxic than the elements composing the TE materials the most used. However, there are two main obstacles hindering the diffusion of silicon clathrates in this field: their synthesis pressure (several GPa) and their metallic character. The challenge of the project MASCOTH consists in finding silicon clathrates with semi-conductor character, doped « n » or « p », with good thermoelectric properties and synthesized at pressures lower than 1 GPa, if not ambient pressure.
MASCOTH gathers two teams of the GDR thermoélectricité, from the ILM in Lyon and the ICGM in Montpellier, which share renowned expertises on silicon clathrates and on thermoelectricity. This project has the aim of producing silicon clathrates with semi-conductor character at synthesis pressures lower than 1 GPa and with optimized thermoelectric properties. It is the follow-up of recent works of ours, funded by the INSIS, which have allowed us to identify semi-conductive phases of the type III clathrate doped « n », n-Ba24Si100, and the type I clathrate doped « p », p-Ba8Si46. MASCOTH will provide the needed funding for continuing this work by the investigation of these two systems and the optimization of their thermoelectric properties. On the other hand, MASCOTH will allow to fund a more technological work whose aim is to give precise answers concerning the integration and working conditions of silicon clathrates in thermoelectric devices. In this framework, the investigation of aging and oxydation resistance in normal working conditions will be set up for these materials. The aim of this study is to identify the working temperature range, as well as the lifetime of Silicon clathrates when used as branches of a thermoelectric device.