(Y0.25Yb0.25Er0.25Lu0.25)2(Zr0.5Hf0.5)2O7: A defective fluorite structured high entropy ceramic with low thermal conductivity and close thermal expansion coefficient to Al2O3

Transition metal disilicides are widely used as heating elements and infrared emission coatings. However, the limited intrinsic infrared emissivity and high thermal conductivity are the main limitations to their applications as infrared emission coatings in the thermal protection system. To cope with these problems, four medium and high-entropy transition metal disilicides, i.e., (V0.25Ta0.25Mo0.25W0.25)Si2 (ME-1), (Nb0.25Ta0.25Mo0.25W0.25)Si2 (ME-2), (V0.2Nb0.2Ta0.2Mo0.2W0.2)Si2 (HE-1), and (Cr0.2Nb0.2Ta0.2Mo0.2W0.2)Si2 (HE-2), were designed and synthesized by spark plasma sintering method using transition metal binary disilicides as precursors. The introduction of multi-elements into transition metal disilicides not only improved the infrared emissivity but also reduced the electrical and thermal conductivity. Among them, (Cr0.2Nb0.2Ta0.2Mo0.2W0.2)Si2 (HE-2) had the lowest electrical conductivity of 3789 S cm–1, which is over one order of magnitude lower than that of MoSi2 (50000 S cm–1), and total infrared emissivity of 0.42 at room temperature, which is nearly double of that of TaSi2. Benefiting from low electrical conductivity and phonon scattering due to lattice distortion, the medium and high-entropy transition metal disilicides also demonstrated a significant decline in thermal conductivity compared to their binary counterparts. Of all samples, HE-2 exhibited the lowest thermal conductivity of 6.4 Wm−1K−1. The high-entropy transition metal disilicides also present excellent oxidation resistance at high temperatures. The improved infrared emissivity, reduced thermal conductivity, excellent oxidation resistance, and lower densities of these medium and high-entropy transition metal disilicides portend that they are promising as infrared emission coating materials for applications in thermal protection systems.