RESUMO
The growth of long carbon fibers was investigated using hyperbaric-pressure laser chemical vapor deposition (HP-LCVD). Precursors included the unbranched alkenes with linear structure 1-C(x)H(2x) (where x = 2,4,5,6,7,8), the unbranched alkynes, i.e., 1-C(x)H(2)(x-2) (where x = 3,4,5,6,8), and benzene. Rate constants, reaction orders, and apparent activation energies were derived for each precursor over a range of experimental conditions. Axial growth rates from the alkenes were 1-2 orders of magnitude greater than for the alkynes, while growth rates for benzene exceeded 10 mm s(-1). Generalized expressions for the growth rate vs molecular weight were determined. For the alkenes, the growth rate was directly proportional to the square root of the precursor molecular weight, while the alkynes exhibited an inverse relationship. Two regions of differing reaction order were identified for the alkynes; at pressures less than 2.0-2.5 bar, the average reaction order was 3.07, while above 2.0-2.5 bar, reaction orders diverged. Expressions were derived for the fraction of carbon atoms deposited per alkyne molecule transported; the deposition efficiency decreased with molecular weight for the alkynes, due in part to the Soret effect. In contrast, the reaction order for the alkenes was 1.65, and for benzene was 2.25. A phase change in the deposit was observed for both the alkenes and alkynes, with the exceptions of pentene and pentyne. Complete axial rate equations for the alkenes and alkynes were derived, as well as volumetric growth equations for the alkynes. It was shown that the volumetric rate increases nonlinearly with laser power at sufficiently high pressures.
