Anomalies of phase transitions during thermal cycling of a Co-C nanocomposite

Abstract

Using differential scanning calorimetry, we have studied the thermal cycling behavior of a Co-C nanocomposite consisting of an array of 3–6 nm cobalt nanoparticles encapsulated in a 3–5-layer graphene shell. The thermodynamic characteristics of the α-Co → β-Co phase transformation in the nanocomposite are shown to differ significantly from those in bulk cobalt samples. In the case of bulk cobalt, the effective activation energy of the phase transition is several times higher than that in nanocobalt, while the thermal effect of the transformation is orders of magnitude lower. A change in the mass of the sample was detected near the temperatures at which thermal effects occur during the α-Co → β-Co transition. These effects were observed only when the sample interacted with air during exposure to room temperature. In the absence of air exposure, thermal cycling of the Co-C nanocomposite in an argon atmosphere revealed no changes in mass or thermal effects of the transformation. Moreover, Co nanoparticles retained the β-phase crystal lattice throughout the studied temperature range of 20–700 °C. The abovementioned behavioral features of Co nanoparticles should be considered a previously unknown specific behavior of a material with polymorphism in the nanoscale.