High-entropy (Mg,Co,Ni,Zn)Ti2O5 pseudobrookite with improved phase stability and emissivity for thermal-protection coatings on alumina–silica fiber fabrics

As high emissivity emitters, traditional pseudobrookite ceramics are limited in thermal protection applications due to their structure degradation at high temperatures. In this work, a high-entropy pseudobrookite, (Mg,Co,Ni,Zn)Ti 2 O 5 , was designed, and CoTi 2 O 5 was selected as a single-component counterpart to evaluate the high-entropy effect on phase stability and emissivity. After heating at 1400 °C for 15   h, (Mg,Co,Ni,Zn)Ti 2 O 5 retained a single pseudobrookite phase with nearly unchanged emissivity, whereas CoTi 2 O 5 exhibited obvious phase degradation and emissivity deterioration. The higher 2.5–7 μm emissivity of (Mg,Co,Ni,Zn)Ti 2 O 5 was attributed to its narrower band gap (2.66   eV) and a higher oxygen vacancy fraction (56.87%). When coated on alumina-silica fiber fabric, the (Mg,Co,Ni,Zn)Ti 2 O 5 coating exhibited improved thermal-protection performance owing to its high infrared emissivity and reduced heat conduction, while maintaining good mechanical properties after heat treatment. These results demonstrate that (Mg,Co,Ni,Zn)Ti 2 O 5 is a promising high-emissivity ceramic for extreme thermal environments.