Differently from fossil pitches, wood tar pitches have been very little studied so far as precursors
of advanced carbonaceous materials (ACM). However, the development of applications for
biopitches is important to increase the revenue of the charcoal manufacturing industry and to
stimulate the use of biomass, thereby answering the appeals of environment preservation. This
work consists of a pioneer study on Eucalyptus tar pitch, its chemical characterization, and
pretreatment aiming toward the production of carbon fibers. The pretreatment is made to adjust
the pitch properties, which are important for the subsequent steps in material processing and
for the final product performance. Fourier transform infrared spectroscopy (FTIR), solid state
13C NMR, and elemental analysis show that biopitches are mainly constituted of interlinked
phenolic rings, which are highly substituted and oxygenated. Pretreatment involved thermal
polymerization. The changes in pitch polymerization degree, structure, and properties during
pretreatments were assessed using differential scanning calorimetry (DSC), gel permeation
chromatography (GPC), thermogravimetry (TG), 13C NMR, and elemental analysis. Moreover,
the softening points (SP) and acetone-insoluble contents (AI) were determined. The results showed
that polymerization was more effective at higher temperatures (about 250 °C) and it was followed
by increases in glass transition temperature (T
g), SP, AI, thermal stability, and coke yield. During
polymerization, side chains were released giving rise to an increase in pitch aromaticity. The
possibility of adjusting wood pitch properties lends them a good perspective as precursors of
ACM.
The electrical breakdown has been investigated for low-pressure argon and nitrogen discharges under the influence of an external longitudinal magnetic field. Plane-parallel aluminum electrodes (5 cm diameter) separated by a variable distance d (4.0 cm < d < 11.0 cm) were sustained with a dc voltage (0 < V < 1 kV). A Helmholtz coil was used to produce an uniform magnetic field(B) parallel to the discharge axis. Paschen curves were obtained and the secondary electron emission coefficient (γ), the first Townsend ionization coefficient (α) and the ionization efficiency(η), were plotted with respect to the variation of the reduced field (E/P). To observe the effect of the magnetic field these curves were plotted for fixed values of B=0 and B=350 Gauss. As consequence of the longitudinal magnetic field, the free paths of the electrons in the Townsend discharge are lengthened and their lateral diffusion is reduced, thus reducing electron losses to the walls. The data presented in this paper give a quantitative description of the B-field effect on the Townsend's coefficients and overall it is concluded that the DC electrical breakdown of the gases is facilitated if a longitudinal magnetic field is applied along the discharge axis.
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