S.L. di Vittorio

New characterization techniques for activated carbon fibers

Abstract Recent work on activated carbon fibers with specific surface area SSA ≥ 1000 m2/g is reviewed. Because of their heterogeneity, it is necessary to characterize activated carbon fibers by multiple characterization techniques, such as x-ray diffraction, Raman scattering, transport properties, magneto-resistance, photoconductivity, and electron spin resonance. Further insight into the structure-property relationships in activated carbon fibers is achieved through their study as a function of heat treatment temperature, using these same characterization techniques. Whereas transport-related properties are most sensitive to the specific surface area, the structure and Raman spectra are most sensitive to the state of graphitization.

Magnetic properties of activated carbon fibers

Abstract Magnetic susceptibilities and ESR spectra were measured for pitch-based activated carbon fibers (ACF) having huge specific surface areas (≈3000m2/g). These fibers consist of ca. 10A micro-pores with which dangling bond spins are associated. Under an atmospheric condition, a broad weakly temperature dependent ESR signal (ΔH≈800G) was observed, while evacuation of the sample generates a narrow ESR signal (ΔH≈40G) with Curie law behavior superimposed upon the broad signal. From the experimental results for gas adsorption and heat-treatment effects, the narrow signal and the broad signal are respectively assigned to isolated dangling bond spins and conduction electron spins interacting with dangling bond spins directly attached to graphitic skeletons.

Image analysis of TEM pictures of fluorine-intercalated graphite fibers

A digitization of the TEM pictures of fluorine-intercalated graphite fibers has been used to carry out quantitative measurements of the defect structure of this material. Emphasis is given to both the computer analysis technique and to the characterization of the defects. The amount of intercalation-induced disorder increases with increasing fluorine concentration. The fast Fourier transform of the digitized TEM image exhibits two diffuse spots, corresponding to the c -axis repeat distance of the intercalation compound. The length and width of the spots are a measure of the out-of-plane and in-plane disorder present in the fibers. From the fast Fourier transform, the distribution of interlayer repeat distances and the fraction of unintercalated graphite regions throughout the material is obtained. By selecting a small range of repeat distances and carrying out an inverse fast Fourier transform, the spatial distribution of material with a given repeat distance is determined. Regions with the same repeat distance are found to form islands. This particular feature of fluorine graphite intercalation compounds, as well as the nature of the microscopic defects and the staging behavior of fluorine-intercalated graphite fibers, are discussed in connection with the dual covalent and ionic nature of the carbon-fluorine bond in fluorine-intercalated graphite.

A model for disorder in fluorine-intercalated graphite

The structural and electronic properties of fluorine- and bromine-intercalated graphite fibers and HOPG are summarized. In contrast to the bromine intercalate, which is purely ionic for any experimentally attainable intercalate concentration, fluorine has a dual ionic and covalent behavior in graphite. Furthermore, whereas bromine-intercalated graphite is ordered, fluorine-intercalated graphite is disordered. The stiff graphene planes are buckled and islands of various fluorine concentrations are formed. A thermodynamic model is proposed that accounts for the differences between fluorine- and bromine-intercalated graphite materials. The model describes the competition between ionically bonded and covalently bonded intercalate phases of fluorine in graphite. Covalent bonding is more favorable energetically, but an important nucleation barrier exists due to strain and to the destruction of the conjugation of the double bonds.

ESR study of activated carbon fibers: preliminary results

We have carried out Electron Spin Resonance (ESR) measurements on activated carbon fibers (ACF) with specific surface areas (SSA) of 3000 and 2000 m[sup 2]/g. The ESR spectrum of ACF fibers in air is extremely broad (500 to 1000 Gauss), and the spin susceptibility decreases rapidly with decreasing specific surface area. Also measured was the ESR signal of the desorbed fibers in vacuum. As a result of desorption, the broad peak decreases slightly in intensity, and a narrow ([approx]65 Gauss at room temperature) peak appears. We report results on the temperature dependence of both peaks. The narrow peak is interpreted as due to spins associated with dangling bonds, whereas we attribute the broad peak to the conduction carrier spins which is broadened by the boundary scattering process ([ital T][sub 1] contribution) and the dipolar broadening process ([ital T][sub 2] contribution) associated with the dangling bond spins.