Thermo-Optical Properties of LaMAl11O19 (M = Mg, Mn, Fe) Hexaaluminates for High-Temperature Thermal Protection Applications

LaMAl11O19 (M = Mg, Mn, Fe) ceramics are fabricated by pressureless-sintering method at 1700 degrees C for 10 h in air. The microstructure and thermo-optical properties of LaMAl11O19 ceramics are studied by the X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy measurements. The transition metallic elements such as Mn and Fe at M2+ site can effectively enhance the emissivity of LaMAl11O19 ceramics at short wavelengths. LaFeAl11O19 has the highest emissivity value of more than 0.8 in the wavelength range of 2.5-6 mu m at 500 degrees C. However, the measured emissivity of different LaMAl11O19 ceramics shows a similar trend in the wavelength range of 6-14 mu m. LaFeAl11O19 is a promising protective coating material for metallic thermal protection system due to its high emissivity of above 0.7 over the whole wavelength range of 2.5-14 mu m at temperatures up to 900 degrees C.