Spatially Directed Pyrolysis via Thermally Morphing Surface Adducts

Combustion is often difficult to spatially direct or tune associated kinetics-hence a run-away reaction. Coupling pyrolytic chemical transformation to mass transport and reaction rates (Damkohler number), however, we spatially directed ignition with concomitant switch from combustion to pyrolysis (low oxidant). A 'surface-then-core' order in ignition, with concomitant change in burning rate,is therefore established. Herein, alkysilanes grafted onto cellulose fibers are pyrolyzed into non-flammable SiO2 terminating surface ignition propagation, hence stalling flame propagating. Sustaining high temperatures, however, triggers ignition in the bulk of the fibers but under restricted gas flow (oxidant and/or waste) hence significantly low rate of ignition propagation and pyrolysis compared to open flame (Linan's equation). This leads to inside-out thermal degradation and, with felicitous choice of conditions, formation of graphitic tubes. Given the temperature dependence, imbibing fibers with an exothermically oxidizing synthon (MnCl2) or a heat sink (KCl) abets or inhibits pyrolysis leading to tuneable wall thickness. We apply this approach to create magnetic, paramagnetic, or oxide containing carbon fibers. Given the surface sensitivity, we illustrate fabrication of nm- and & mu;m-diameter tubes from appropriately sized fibers.

Automated summary

Combustion is often difficult to spatially direct or tune, resulting in run-away reactions. By coupling pyrolytic chemical transformation to mass transport and reaction rates, spatially directed ignition with a switch from combustion to pyrolysis can be achieved. This allows for an "surface-then-core" order in ignition and a change in burning rate. Here, alkysilanes grafted onto cellulose fibers are pyrolyzed into non-flammable SiO2, leading to surface ignition propagation and stalling of flame propagation. Sustaining high temperatures triggers ignition in the bulk of the fibers but with restricted gas flow, resulting in significantly low rates of ignition propagation and pyrolysis compared to open flame.