Multi-component hexaborides with high infrared emissivity and low thermal conductivity for ultra-high temperature thermal protection applications

As the flight speed of hypersonic vehicles increases, the airframe temperature rises sharply, which puts forward greater demands for ultra-high temperature thermal protection materials with high infrared emissivity and low thermal conductivity. In this work, LaB₆, (La_1/3Eu_1/3Ca_1/3)B₆ (MEB₆), and (La_1/5Eu_1/5Ca_1/5Ba_1/5Sr_1/5)B₆ (HEB₆) were prepared by boron-carbon thermal reduction and spark plasma sintering. To regulate the thermal conductivity and infrared emissivity, alkaline earth metal elements are introduced into the design of multi-component hexaborides. Among them, the infrared emissivity of HEB₆ in the 1.28-5 μm wavelength range increased from 81.487% to 95.662% compared to LaB₆. Compared with LaB₆, the thermal conductivity of HEB₆ is reduced significantly from 57.640 W·m^-1·K^-1 to 17.041 W·m^-1·K^-1. Among all samples, HEB₆ exhibited the lowest electrical conductivity of 2952.667 S·cm^-1, which was much lower than that of LaB₆ (75826.000 S·cm^-1). This strategy improves the infrared emissivity and reduces thermal conductivity. On the one hand, the introduction of alkaline earth metals reduces the electron density of the material and modulates the electronic band structure, thereby reducing the conductivity and increasing the infrared emissivity. On the other hand, large atomic mass and size differences aggravate the asymmetry of crystal structural units, resulting in enhanced phonon scattering and reduced thermal conductivity. Hence, the increase of emissivity and the decrease of thermal conductivity are simultaneously regulated, providing a new strategy for the development of thermal protection materials for hypersonic vehicles.