A novel compound that exhibits poor thermal conductivity in the 25-425 degree C range but shows good electrical conductivity has been developed by a team of researchers led by Dr Kanishka Biswas from Bengaluru’s Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR). The compound, silver copper telluride (AgCuTe), shows promise as a thermoelectric material for converting waste heat into electricity.
Since nearly 65% of utilized energy is wasted as heat, the focus is on developing materials that exhibit good thermoelectric property with both glass- and metal-like properties. Potential applications of the thermoelectric technology are in automobile industry, chemical, thermal and steel power plants where large quantities of heat are wasted.
Due to the low thermal conductivity of the material developed by JNCASR, one end of the 8 mm-long rod that is contact with waste heat remains hot while the other end maintains cold temperature. The temperature difference is essential for the generation of electrical voltage. At the same time, the material exhibits good electrical conductivity like metal. The results were published in the journal Angewandte Chemie.
In the AgCuTe material, the silver atoms (cation) are weakly bound, giving rise to poor thermal conductivity due to the slow vibration of the lattice (soft lattice). At high temperatures, copper in the material further lowers the thermal conductivity along with silver. “Since the silver lattice vibrates slowly, it provides record low thermal conduction of 0.35 W per metre per kelvin, which is actually close to the glass,” Dr Biswas says.
“Both cations [silver and copper] contribute to low thermal conductivity but silver contributes more. Over 170 degree C, both silver and copper ions flow like liquid within the rigid tellurium sublattice, thereby reducing the thermal conductivity to the level of glass without affecting the hole (electrical carrier) transport,” says Subhajit Roychowdhury from JNCASR and first author of the paper.
Tellurium lattice
In contrast, the tellurium atoms (anion) are strongly bound and the lattice is very rigid. The strongly bound tellurium provides a conduction channel for holes thus rendering good electrical conductivity as seen in metals.
“By combining silver and copper with tellurium we have made our material as a combination of glass and metal — poor thermal conductivity and good electrical conductivity,” Dr Biswas says.
“Silver telluride does not have good thermoelectric property because it has higher thermal conductivity than our material,” says Roychowdhury.
It is a challenging task to have glassy and metallic properties in a single material, which is the fundamental challenge in the field of thermoelectrics. “We addressed this challenge through structural chemistry by creating a bonding hierarchy in the material,” Dr Biswas says.
The calculated efficiency to convert heat into electricity is 14% for the new material developed by JNCASR researchers. The lead telluride (PbTe) has higher efficiency of 18%. “But unlike lead telluride that contains lead, which is toxic, our material is lead-free,” he adds. The theoretical calculation to know the electronic structure was done in collaboration with Prof. Umesh V. Waghmare of JNCASR and coauthor of the paper. “We are trying to increase the efficiency by doping with different cations and anions,” Dr Biswas says.
