Ultralow Thermal Conductivity and Novel Thermoelectric Materials
Pöhls, Jan-Hendrik Mr.
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More than half of the energy produced worldwide is lost as heat and even recovering a fraction of that would be beneficial for global climate change. Thermoelectric materials can recover waste heat and convert it to useful energy. However, thermoelectric materials are not commercially applied in many areas due to low efficiency. The search for high-performance thermoelectric materials is challenging because thermoelectrics require enhanced electronic properties and low thermal conductivity. A potential route to discover novel high-performance thermoelectric materials can be provided by first-principles calculations. While the electrical properties can be calculated with a high accuracy, an accurate prediction of the heat transport is currently not feasible. However, insight into the heat transport can be given by computing the lowest limit of heat transport. In the present study, a new model for minimum thermal conductivity was developed in which the thermal energy is transported between entities of phonons oscillating in a range of frequencies and limited by the phonon mean speed. This model was motivated by understanding the lowest experimental thermal conductivity reported to date for a fully dense solid, measured here for PCBM (κ = 0.07 W m−1 K−1 at 300 K) which agrees with the present model. Slightly higher thermal conductivities were determined herein for porous ZnO tetrapod composites. The latter experimental results were confirmed with finite element calculations. In a high-throughput screening within The Materials Project the electronic properties of ∼48,000 inorganic compounds were calculated and two novel high-performance thermoelectric classes, XYZ2 (X,Y : rare earth or transition metals; Z: Group VI elements) and metal phosphides, show promise. A variable relaxation time was developed using a semi-empirical approach to accurately calculate the temperature-dependent electronic properties. Three compounds of the XYZ2 class were synthesized and their thermoelectric properties were analyzed in both computational and experimental studies. All compounds exhibit extremely low thermal conductivity and a maximum figure of merit of ∼0.73 was found. As an example of metal phosphides, NiP2 was synthesized and the experimental thermoelectric properties agree well with computation. The low thermal conductivity of the thermoelectrics was confirmed with the present model.