Electromigration-Biased Carbon Transport and Isotope Fractionation in High-Carbon-Solubility Metals

Electromigration, usually studied as a failure mechanism can also guide materials formation. We demonstrate that direct current drives interstitial carbon transport in solid cobalt and nickel leading to graphite precipitation, isotope fractionation, and distinct thermal properties of the recovered phase. Carbon supplied centrally accumulates as graphite 8-10 mm away on the cathode side, whereas alternating-current control experiments under comparable heating produce only thin deposits near the source. Raman mapping and ToF-SIMS profiling reveal ¹²C enrichment across source compositions from natural abundance to ∼88 at% ¹³C, with Raman-derived enrichment factors of 1.29-1.62. The composition dependence of these factors, with increased enrichment toward the ¹³C-rich end, supports isotope-sensitive transport coupled to localized precipitation. Finite-bias calculations demonstrate directional migration-barrier asymmetry, while isotope-dependent hopping and a concentration-threshold model predict preferential ¹²C enrichment in both hosts. Frequency-domain thermoreflectance (FDTR) measurements of both thermal conductivity components reveal a pronounced in-plane and a weaker out-of-plane composition dependence. These results show current-directed graphite formation in solid metals accompanied by isotope fractionation and composition-dependent thermal properties. This approach may extend to other mobile interstitial solutes in suitable metal hosts.