ESR IN HEAT-TREATED CARBONS FROM THE ENDOCARP OF BABASSU COCONUT

The results of an electron spin resonance (ESR) study for the endocarp of babassu coconut with heat treatment temperature up to 2,200-degrees-C are presented. Correlations between the transition range (700 less-than-or-equal-to HTT < 1,300-degrees-C) with the low and high HTT ranges are emphasized. Similar to other heat treated carbons, free radicals were observed in the low HTT range, and free charge carriers in the high HTT range. In the transition range, the usual broadening of the linewidth was observed for the macroscopic samples (phi congruent-to 1 mm, where phi is the particle size). On the other hand, the ground samples (phi < 37-mu-m and phi < 74-mu-m) showed two well-defined resonances (g less-than-or-equal-to 2.004; DELTA-H less-than-or-equal-to 2 G and g greater-than-or-equal-to 2.000; DELTA-H greater-than-or-equal-to 10 G). The temperature dependence of the ESR spectra show that the narrow line is associated to remainder free radicals and the broad one to free charge carriers that dominate in the high HTT range. Although the electrical conductivity becomes appreciable about 700-degrees-C HTT, when the material experiences a nonmetal-metal percolation transition, the resonance of the charge carriers for the macroscopic samples is clearly observed only above 1,300-degrees-C HTT. This fact is explained by the silicon effect, which plays an important role in this material.