Thermal Stability of MAX Phases

The susceptibility of MAX phases to thermal dissociation at 1300-1550 °C in high vacuum has been studied using in-situ neutron diffraction. Above 1400 °C, MAX phases decomposed to binary carbide (e.g. TiC x ) or binary nitride (e.g. TiN x ), primarily through the sublimation of A-elements such as Al or Si, which results in a porous surface layer of MX x being formed. Positive activation energies were determined for decomposed MAX phases with coarse pores but a negative activation energy when the pore size was less than 1.0 μm. The insights for tailor-design of MAX phases with controlled thermal stability and intercalated MXenes for energy storage are addressed.