Carbon filaments of macroscopic length may be produced by heating transition metal catalyst particles in a hydrocarbon-hydrogen mixture. Because the nanometer-sized catalyst particles contain relatively many surface atoms, they melt at a temperature considerably below that of bulk material; this depression of the melting point is difficult both to calcu-late and measure. This work addresses the fundamental question of whether the catalyst particles are in the liquid or solid state during filament growth.

We present measurements of the average number of filaments grown per unit area of substrate as a function of temperature which show that the the activity of the catalyst particles drops precipitously above 1030°C and nearly disappears above 1120°C. This compares with the melting point of the Fe-C eutectic of 1154°C. TEM observations show that the average dia-meter of an active catalyst particle is 15±7 nm. Calculations based on the model of Ross and Andres [1] for the melting temperature of small clusters of atoms predict a drop in activity above 1050°C similar to our observa-tions.