Radiative thermal diode driven by the synergy between isotope engineering and lattice thermal expansion

Recently, the application of isotope engineering to design and optimize near-field radiative thermal diodes based on polar materials has emerged as an appealing and promising technique. Besides the isotope effect, the lattice thermal expansion in those polar materials with light atoms impacts appreciably their phonon and optical properties. In this work, we present a strategy for designing efficient thermal diodes through the synergy between isotope engineering and lattice expansion in light polar materials. The isotope-induced phonon frequency shift as well as the phonon softening with temperature thus give rise to match and mismatch of surface phonon polaritons (SPhPs) resonant frequency under forward and reverse temperature bias, respectively. Using cubic boron nitride (cBN) as a material platform, we obtain its dielectric function from first-principles calculation, including the four-phonon scattering and phonon frequency shift. This computational design allows us to achieve an ultrahigh thermal rectification ratio of 7.01 at a 10 nm gap, which is 75% higher than that with only isotope engineering. This study thus paves a way for effective design of radiative thermal diodes that may promote the thermal management of electronics and optoelectronics.