PhD 2015-18 (Matthieu Aumand)

Study of the elementary plastic mechanisms of nanostructured thermo-electrical materials

Context:

Thermoelectric (TE) materials can convert the waste heat dissipated into electrical power. Without moving part nor mechanisms, the thermoelectric generators (TEG) are within time, more environmental friendly and less costly. The last generation of thermoelectric materials uses the property of a small scale microstructure in order to enhance the global output. Today, a few families of these “nanostructured” TEs allows to cover a wide temperature range, from room temperature to above 1000°C. Each of them presents varieties in both TE and mechanical properties, essential parameters for application suitability. For instance, a possible application is to convert the waste rejected heat of a car exhaust gas, for supplementing the alternator or even replace it. Although these alloys are presented as extremely brittle materials, plasticity mechanisms are supposed to influence their physical properties. This study aims to induce plasticity in these materials, and study their relationship with the TE properties.

Results:

Classical compression test conducted on TE materials suggests that, when tested under ambient pressure, the experiments are untimely stopped by the presence of native cracks that propagate in the samples well before the onset of plastic deformation. An alternative is using an apparatus using a isostatic confining pressure: the Paterson machine. Originally designed for geophysics studies (by mimicking the hydrostatic pressure existing inside the terrestrial crust), this machine enables the simultaneous application of hydrostatic pressure and a uniaxial compression force on a sample of macroscopic size (several cm3): the maximum pressure is 450 MPa, the maximum uniaxial force is 100kN and a furnace allows to work up to 1100°C. With this setup, it is possible to impose plastic deformation in brittle materials, thanks to the closure of cracks by the hydrostatic pressure.

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