Influences of lattice vibration and electron transition on thermal emissivity of Nd³⁺ doped LaMgAl₁₁O₁₉ hexaaluminates for metallic thermal protection system

Normal spectral emissivity of La_1−xNd_xMgAl₁₁O₁₉ (x = 0.05, 0.10, 0.15, 0.25) ceramics has a maximum value at the Nd³⁺ content of x = 0.10 in the wavelength range of 3–6 μm. The difference in emissivity at short wavelengths of 3–6 μm is tightly linked to the absorption transitions from ⁴I_9/2 to ⁴F_3/2, ⁴I_15/2, and ⁴I_13/2 and the intensities of radiation transitions from ⁴F_3/2 to ⁴I_11/2 and ⁴I_13/2 of Nd³⁺ in LaMgAl₁₁O₁₉ ceramics. However, in the wavelength range of 6–14 μm, the emissivity is quite similar for all the samples, which can be ascribed to the stretching vibration of Al-O tetrahedron and Al-O octahedron.