Chalcogenide glasses/glass-ceramics as thermoelectric materials for room temperature applications – VTG

Thermoelectric materials are able to convert thermal energy into electrical energy (Seebeck effect) and reversibly, electrical energy into thermal energy (Peltier effect). Performances of thermoelectric devices are intimately dependent on that of materials through the figure of merit ZT defined by ZT=(S²ET)/L with T, the temperature; S, the Seebeck coefficient; E and L, respectively the electrical and thermal conductivities and S²E, the power factor.
S, L and E depend on the charge carriers concentration of the material. The power factor S²E is maximal for a concentration of ~10^18–10^21 carriers/cm^3 that corresponds to low gap semiconductors or semimetals. A good thermoelectric material has to present a high ZT, therefore it has to be a good electrical conductor and a bad thermal conductor. In the early 1990’s, Slack presented the concept of « Phonon Glass Electron Crystal » (PGEC) that proposes the study of materials that conduct electricity as a crystal and heat as a glass. This has led to a better understanding of the mechanisms that affect the phonons propagation without altering the electrical charge propagation, and to the development of general rules to increase the thermoelectric systems efficiency.
The PGEC concept together with the use of modern synthesis techniques, has led to the discovery of new improved thermoelectric materials.
The careful analysis of the main general rules to increase the thermoelectric performance points to conducting glasses as one of the best potential systems. Indeed, glasses have complex structure; they can contain heavy atoms weakly linked to the structure and present mass fluctuation, easily allowing high concentrations of inclusions or impurities. Most of semiconductor glasses are based on chalcogenides (S, Se, Te) and/or pnictides (As, Sb, Bi, etc).
This project proposes to study chalcogenide glasses, little studied so far for thermoelectric applications. The targeted range of temperature is between 20°C and 300°C depending on the glass composition and on the glass transition temperature, Tg. Several glass systems based on S, Te, Se, Sb, In, Cu, Ag, Ge, Pb, Ga, etc will be studied. In order to focus on a reasonable number of compositions, a strict selection will be made on the basis of bibliographic data on semiconducting materials and our expertise on glassy systems. One advantage of glasses over intermetallics is the ease of synthesis, processing and shaping. Indeed, glasses are usually obtained by melt quenching or melt spinning (MS). MS technique allows very fast quenching rate leading to amorphous ribbons, which can easily be sintered by Spark Plasma Sintering (SPS) to get amorphous bulks. SPS technique allows fast heating rate and is suitable for the sintering of glasses avoiding the crystallization phenomenon (depending on the difference between Tg and the crystallization temperature, Tc) by an appropriate treatment.
This research project also implies the study of nano-crystals inside the glassy matrix leading to glass-ceramics materials. Indeed, the controlled crystallization of the glassy matrix permits, depending on the nature and the size of crystals, to modify the electrical conductivity without altering the thermal conductivity. These glass-ceramics will be synthesized by conventional technique (ventilated furnace) and non-conventional technique (by SPS), this later technique showing recent interesting results about the reduction of treatment time.
To summarize, our approach is different from the one conventionally used during the last ten years and which consists in reducing subsequently the thermal conductivity from existing materials. In VTG project, we will start from materials with very poor thermal conductivity characteristic of chalcogenide glasses and we will undertake to improve their transport properties through the addition of metallic elements (Cu, Ag, etc) and the creation of nano-crystals through glass-ceramics