Non-uniformity of temperatures appears as a general rule in hot nanofiber
synthesis reactors, where local variations of the balance between long-range
radiative heat exchanges and short-range conductive heat exchanges are
driven by size and composition of structures. Metal particles larger than a
few tens of nanometer are radiation captors, when fiber bodies thinner than
10 nm are transparent. We note that typical gradient lengths correspond
remarkably well to usual length of nanotubes (micron range), and increase
with fiber radius. For anisotropically radiating reactors as well as for
furnace-type reactors, difference between radiative and conductive
environments allows temperature intervals as high as several hundred
degrees. Practically, axial thermal gradients arise by fiber attachment to a
metallic particle, eventually at each tip (ideally with different sizes or
compositions), or by attachment to a hot wall (electrode, supporting
substrate, or target). The resulting thermal gradients are unusually stiff,
typically 10 K µm−1. We show that local temperature evolution
in early stage of nucleation is triggered by dimension of attached
particle(s). We show that a diffusion flux of adatoms induced by the axial
thermal gradient is quantitatively consistent with a feeding flux, as
measured for different reactors. In addition, we notice that the spontaneous
minimization of free energy at fiber tip is such that a temperature drop
favors axial extension.