Structure and nanophase evolution of single-crystalline Ni carbide nanoparticles

Nickel's ability to incorporate carbon into its lattice makes it a key catalyst for the synthesis of advanced carbon materials like graphene and carbon nanotubes. The conventional Ni–C phase diagram delineates three primary phases: a liquid state above the eutectic temperature, cubic Ni₄C_1–x phase below this temperature, and hexagonal Ni₃C compound below 1100 °C. However, under the extreme conditions of the electrical explosion of wire process, Ni carbide nanoparticles exhibit complex nanophase behaviors that surpass bulk phase predictions. These nanoparticles exhibit three distinct liquid nanodroplet states, corresponding to the eutectic point, a solid solution, and an unsaturated hexagonal close-packed Ni₃C_1–x nanophase. A key discovery is the transition from the unsaturated Ni₃C_1–x phase, formed via top-down cooling, to the saturated Ni₃C₁ phase, which is formed through bottom-up heating. This highlights the irreversible nature of the Ni–C nanophase diagram. The nanophase evolution from hexagonal to cubic structures is influenced by unidirectional and multidirectional carbon diffusion in ellipsoidal and spherical nanoparticles, respectively. This leads to intermediate nanostructure variations associated with phase separation and particle segregation. These phenomena markedly differ from those observed in thin-film processes. This study, including an analysis of magnetic properties, presents a preliminary Ni–C nanophase diagram and bridges the gap between bulk phases and theoretical 1–3 nm nanophases.