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

The pure α-cordierite (MAS) and divalent transition metal ions doped α-cordierite powders (Mg_1.8M_0.2)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 α-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 α-cordierite can be increased by 0.16 through doping Ni²⁺ at 3–20 µm at 500 °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 α-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.