Microstructure, thermal conductivity, and temperature-dependent infrared emissivity of divalent transition metal ions doped alpha-cordierite ceramics

The pure alpha-cordierite (MAS) and divalent transition metal ions doped alpha-cordierite powders (Mg1.8M0.2)Al4Si5O18 (M=Ni, Co, and Zn) were synthesized by sol-gel technique. Further, the powders were compacted by cold isostatic pressing and then sintered to produce the crystallized bulk ceramics. The relationships between doped ions, microstructure, and thermophysical properties were studied and analyzed. The results show that substituting divalent transition metal ions for a portion of magnesium ions can effectively improve the emissivity of alpha-cordierite attributing to the increasing lattice distortion and the reducing electron-leap energy which is caused by the unique 3d orbital energy level splitting. The average emissivity of alpha-cordierite can be increased by 0.16 through doping Ni2+ at 3-20 mu m at 500 degrees C, and the value decreases with the rise of temperature due to the increase of phonon-phonon scattering. Otherwise, doping with specific ions such as Co2+ or Ni2+ can refine the grains, which can efficaciously decrease the thermal conductivity by enhancing phonon scattering due to the increasing grain interfaces. Coefficient of thermal expansion of alpha-cordierite is increased by doping divalent transition metal ions owing to the decrease in ionic-bond strength caused by the lower electronegativity of doping ions compared with that of magnesium ions.