Influence of manganese dioxide on the phase composition and microstructure of C-Al materials

In order to avoid the generation of Al₄C₃ and AlN during heat treatment of carbon composite re-fractories with the addition of aluminum powder, this paper probes the effects of MnO₂ on the phase composition and microstructure of C-Al materials. C-Al and C-Al-MnO₂ specimens were prepared by using graphite flake, aluminium powder, and MnO₂ fines as main starting materials, thermosetting phenolic resin as a binder, and being fired in a sag-ger filled with graphite powder at various temperatures. The phase composition and microstructure of the specimens were investigated by means of X-ray diffraction and scanning electron microscopy. Manganese dioxide was studied by thermogravimetric-differential thermal analysis. It is found that for C-Al materials without MnO₂, Al powder first generates granular aluminium carbide, and then transforms to fibrous AlN and Al₂O₃. For C-Al materials with MnO₂ addition, since MnO₂ gradually decomposes with the evolution of oxygen, aluminum powder is directly oxidized to form Al₂O₃ followed by generating MnAl₂O₄ spinel. Thus, Al₄C₃ and AlN fail to form. The formed Al₂O₃ is present as granular at 800 to 1200℃, fibrous at 1400℃, and fully-developed prismatic at 1600℃.