L.S. Singer

The electronic and structural characteristics of carbon fibers from mesophase pitch

Abstract The electronic properties (resistivity, magnetoresistance, and electron spin resonance) of mesophase pitch-based carbon fibers have been studied in relation to the fiber structure and processing conditions. Reproducible correlations between the electronic properties and the structure demonstrate that the electronic properties are a sensitive indicator of the degree of graphite order in the fibers. Fibers having radial transverse structure are more easily graphitized than fibers with random transverse structure. The effect of differences in the thermosetting procedure is relatively unimportant. The ultimate degree of graphitization attainable by these fibers is comparable to that of similarly heat-treated pyrolytic carbons, whereas PAN-based carbon fibers are ultimately capable only of properties comparable to pitch-based fibers treated in the 1700–2300°C range.

The mesophase and high modulus carbon fibers from pitch

Abstract The evolution of the basic carbonization studies which led to the development of carbon fibers from mesophase pitch is described. The structure and properties of these fibers and comparisons with carbon fibers from other precursors are also discussed. Several basic areas related to fibers, the mesophase, and the mechanism of carbonization require further investigation.

ESR study of the kinetics of carbonization

Abstract Electron spin resonance (ESR) data have been obtained for a well-graphitizing and a poorly-graphitizing pitch as a function of heat-treatment time at reaction temperatures between 400° and 490°C. Measurements of ESR spin concentration and linewidth and of atomic C H ratio have been used to delineate the kinetics of the transformation of pitch to coke. Correlations between the various ESR and constitutional parameters have been used to obtain information concerning the mechanism of carbonization.

Constitution of coexisting phases in mesophase pitch during heat treatment: Mechanism of mesophase formation

Abstract The molecular constitutions of the coexisting anisotropic and isotropic phases of a heat-treated petroleum pitch were studied as a function of heat treatment time. The results show that the coexisting phases, which were isolated by high temperature centrifuging, contain different proportions of similar size molecules. The molecular weight distribution (MWD) of the mesophase did not change during the transformation, but the average MW of the isotropic phase increased significantly. Although the smaller molecules (400–1100 amu) present in the coexisting mesophase polymerize during the transformation, the constant MWD of the mesophase is explained by the stability of the larger pitch molecules (>1100 amu) and by the selective transfer of the smaller molecues (400–1100 amu) from the isotropic phase to the mesophase via stirring and generated gas percolation mechanisms. These transferred smaller molecules, which had been enriched in the isotropic phase, restore molecular equilibrium to the mesophase and result in the growth of mesophase regions. The nucleation of mesophase spheres in the coexisting isotropic phase is due to polymerization of the 400 to 1100 MW molecules in the isotropic phase and to the selective transfer of the 400 to 1100 MW molecules from the isotropic phase to the mesophase.

An electron spin resonance study of the carbonization of the aromatic hydrocarbon acenaphthylene

Abstract Electron spin resonance measurements have been made on carbonized acenaphthylene in the pure state and in solutions in inert liquids. Well-resolved nuclear hyperfine structure is observed in the liquid solutions. Isotopic substitution experiments prove that nuclear hyperfine interaction is the main source of line broadening in the ESR of these low-temperature chars. The identities and reactions of the free radicals are discussed.

Electron spin resonance of stable aromatic radical intermediates in pyrolysis

Abstract This paper presents the results of electron spin resonance measurements on several types of stable aromatic free radicals suspected as intermediates in carbonization. The radicals studied include phenalenyl, 1-methylphenalenyl, 1-phenylphenalenyl, fluorenyl, 9-phenylfluorenyl, and biphenyleneallyl. The radicals all show remarkable stability at high temperatures. For at least a limited group of compounds, a correlation appears to exist between the planarity of free radical intermediates formed during carbonization and the degree of order of the final graphites.

Further electron spin resonance studies of the pyrolysis of aromatic hydrocarbons

Abstract Electron spin resonance studies of the pyrolysis of aromatic hydrocarbons in liquid media have been continued. The method applied previously to the compound acenaphthylene has been extended to additional aromatic hydrocarbon systems including 9,9′-bifluorene, Δ 9,9′ -bifluorene, periflanthene, and derivatives of anthracene. Hydrocarbon concentration and temperature of measurement were important factors in obtaining the resolved proton hyperfine spectra needed to identify the radical intermediates. The role of these intermediates in the initial reactions of carbonization has been clarified by additional chemical studies.

High-temperature centrifugation: Application to mesophase pitch

Abstract A laboratory high-temperature centrifuge capable of operating up to 500°C at centrifugal accelerations of 600 g has been designed and constructed. The apparatus has been particularly useful in separating the isotropic and anisotropic phases in mesophase pitches and quantitatively determining phase contents. The efficiency of phase separation was determined for a petroleum mesophase pitch as a function of temperature and centrifugal acceleration. Effects of gas evolution, sphere coalescence and intraphase precipitation were also observed.

Electron spin resonance and the structure of carbon fibers

Abstract The ESR properties of 3000°C heat treated fibers derived from mesophase pitch and PAN have been measured and compared with computer-simulated spectra. Conduction electron motional averaging was observed in the PAN- and pitch-based fibers with domains small in the transverse direction. When perpendicular to the applied field, pitch-based fibers with large transverse domains exhibited a complex spectrum indicating no motional averaging by the conduction electrons. Computer-simulated spectra were in agreement with experiment and were useful in the determination of “single crystallite” ESR parameters. The relationship between ESR behavior and fiber structure is discussed.

Physical properties of a magnetically-oriented carbonaceous mesophase

Abstract Diamagnetic susceptibility, electron spin resonance, and specific heat measurements have been made on a magnetically-oriented mesophase pitch. The diamagnetic anisotropy (χ⊥ − χ∥) was found to be −0.69 × 10−6emu CGSg−1 and the average susceptibility \ gc = (2 χ∥ + χ⊥) 3 = −0.73 × 10 −6 emu CGS g−1. ESR measurements of the g-factor and linewidth confirmed the anistropic behavior and indicated free radical concentrations of ~1.2× 1019g−1. The low-temperature specific heat results showed evidence of an anomalous linear term. A consideration of the magnitude of the susceptibility and atomic C H ratio suggests that the molecules consist of relatively small aromatic regions connected by aliphatic and aryl bridges.