Improved thermal stability and infrared emissivity of high-entropy REMgAl11O19 and LaMAl11O19 (RE=La, Nd, Gd, Sm, Pr, Dy; M=Mg, Fe, Co, Ni, Zn)

Abstract LaMgAl11O19 (LMA), characterized by high melting point, low density and thermal conductivity as well as good infrared emissivity, is regarded as a potential candidate for the thermal protection of hypersonic vehicles. Nevertheless, the unsatisfied phase stability at high temperature results in declining of the emissivity below 6 μm, which limits the extensive applications of LaMgAl11O19. In order to overcome this obstacle, three dense bulk high-entropy ceramics, (La0.2Nd0.2Gd0.2Sm0.2Pr0.2)MgAl11O19 (HE LMA-1), (La0.2Nd0.2Gd0.2Sm0.2Dy0.2)Mg Al11O19 (HE LMA-2) and La(Mg0.2Fe0.2Co0.2Ni0.2Zn0.2)Al11O19 (HE LMA-3), were designed and successfully prepared through solid state reaction at 1700 °C for 4 h in one step. XRD analyses show that the phase compositions of HE LMA-1, HE LMA-2 and HE LMA-3 are single-phase solid solutions with the relative density of 95.61 %, 95.49 % and 94.31 %, respectively. Heat treatment experiments demonstrate that the phase composition of HE LMA-1 remains a single phase after high-temperature heating, while second phase appears in other two samples. The stability of HE LMA-1 is attributed to small average size difference δ (∼4 %) of constitute elements. Intriguingly, the average emissivity of HE LMA-1 in the range of 3–6 μm reaches 0.9, which is significantly higher than that of LMA and other two HE LMA samples. The emissivity of all samples remains above 0.95 from 6 to 10 μm. In the far infrared region (10–14 μm), although the emissivity of these specimens decreases slightly, it still exceeds 0.85. The UV-Vis absorption spectra indicate that the formation of many discrete impurity energy levels with small intervals in HE LMA-1 promotes the f electrons to transit between adjacent impurity energy levels and conduction band, which enhances the infrared emission of HE LMA-1 below 6 μm. In a word, with improved phase stability and thermal emissivity in infrared range, high-entropy REMgAl11O19, especially (La0.2Nd0.2Gd0.2Sm0.2Pr0.2)MgAl11O19 (HE LMA-1), is a promising candidate in thermal protection coatings of hypersonic vehicles.