Thermoelectric materials are able to convert heat into electrical energy and viceversa. The efficiency of a thermoelectric material is directly linked to its electrical and thermal properties. The scientific challenge is to optimize the three key thermoelectric properties of the materials: Seebeck coefficient (S), electrical (σ) and thermal (κ) conductivities, which determine the figure of merit (zT=S²σT/κ). For maximizing zT, it is necessary to combine high σ, which is typical of metallic compounds, with a high S and low κ, which are associated with non-metallic materials. These thermoelectric properties are highly interconnected by the Boltzmann transport theory and the Wiedemann-Franz law, which make it tough the preparation of high figure of merit materials.
In this work, we have synthesized mayenite ceramic, Ca₁₂Al₁₄O₃₃, by a sol-gel procedure. This is an Earth-abundant oxide, which exhibits low thermal conductivity and electrically insulating behaviour. It crystallizes in a cubic unit cell composed of subnanometer-sized cages and extra-framework oxide anions located within the cages. Its singular structure allows the substitution of extra-framework oxide ions for electrons, forming a mayenite electride and thus leading to a metallic or semiconducting material. This study explores the idea of using Spark Plasma Sintering (SPS) and graphene compositing to modify the electrical properties of mayenite. Neutron powder diffraction data confirms mayenite electride formation, achieving a composition of Ca₁₂Al₁₄O_32.4 for the composite mayenite + 2.0 wt.% of few layer graphene. Compositing results in a reduction in the electrical resistivity of ca. 13 orders of magnitude, while keeping thermal conductivity unchanged. The addition of graphene also affects the system's dominant charge carriers, changing from p-type to n-type semiconducting behavior. The electrical and thermal properties are effectively decoupled in this manner.
Other functional ceramics, particularly high-efficiency thermoelectric materials, may be able to benefit from this approach.