Microstructure and composition engineering Yb single-filled CoSb_(3) for high thermoelectric and mechanical performances

A broad tunability of the thermoelectric and mechanical properties of CoSb_(3) has been demonstrated by adjusting the composition with the addition of an increasing number of elements.However,such a strategy may negatively impact processing repeatability and composition control.In this work,singleelement-filled skutterudite is engineered to have high thermoelectric and mechanical performances.Increased Yb filling fraction is found to increase phonon scattering,whereas cryogenic grinding contributes additional microstructural scattering.A peak zT of 1.55 and an average zT of about 1.09,which is comparable to the reported results of multiple-filled SKDs,are realized by the combination of simple composition and microstructure engineering.Furthermore,the mechanical properties of Yb single-filled CoSb_(3) skutterudite are improved by manipulation of the microstructure through cryogenic grinding.These findings highlight the realistic prospect of producing high-performance thermoelectric materials with reduced compositional complexity.

Inherent Anharmonicity of Harmonic Solids

Atomic vibrations,in the form of phonons,are foundational in describing the thermal behavior of materials.The possible frequencies of phonons in materials are governed by the complex bonding between atoms,which is physically represented by a spring-mass model that can account for interactions(spring forces)between the atoms(masses).The lowest-order,harmonic,approximation only considers linear forces between atoms and is thought incapable of explaining phenomena like thermal expansion and thermal conductivity,which are attributed to nonlinear,anharmonic,interactions.Here,we show that the kinetic energy of atoms in a solid produces a pressure much like the kinetic energy of atoms in a gas does.This vibrational or phonon pressure naturally increases with temperature,as it does in a gas and therefore results in a thermal expansion.Because thermal expansion thermodynamically defines a Grüneisen parameterγ,which is a typical metric of anharmonicity,we show that even a harmonic solid will necessarily have some anharmonicity.A consequence of this phonon pressure model is a harmonic estimation of the Grüneisen parameter asγ≈(3/2)(3−4x^(2))/(1+2x^(2)),where x=vt/vl is the ratio of the transverse and longitudinal speeds of sound.We demonstrate the immediate utility of this model by developing a high-throughput harmonic estimate of lattice thermal conductivity that is comparable to other state-of-the-art estimations.By linking harmonic and anharmonic properties explicitly,this study provokes new ideas about the fundamental nature of anharmonicity,while also providing a basis for new material engineering design metrics.