l218 M L. Greene et al. Carbon 40(2002)1217-1226 ch as thermal conductivity were developed for three measure 18 inches longx9 inches wideX875 inches in different carbonized pitch precursors. Although the specific height. The heating element of the furnace and all internal nature of these precursors was not revealed by the supplier parts are graphite and the hot zone length was estimated at BP Amoco), we believe these results are of interest 5 inches. The temperature inside the hot zone was because there are few reports on the development of measured with an Ircon Mirage dual wavelength pyrometer carbon fiber properties at short graphitization times. Previ- [10, 11]. Argon was used as the purge gas usly, Richardson and Zehms [7 reported on heat treat Graphitization of the fibers was achieved by one of tw ment of pyrolytic carbon at temperatures between 2600 methods: continuous or batch processing. Continuous and 3000C for times ranging from 15 s to 10 min. processing was used to obtain hot zone residence times Time-dependent changes in gross dimensions and inter- rom 0.7 to 58.5 s, while batch processing was used to heat layer spacing were reported, although further analysis was the fibers for residence times ranging from 900 to 3600 s limited. Pandic [8] studied graphitization of a petroleum Graphitization temperatures of 2400, 2700 and 3000C coke and measured the interlayer spacing, stack height, and were evaluated coherence length for fibers processed at furmace residence For continuous mode operation, the fiber tow was times of 20 and 40 s, and 1, 3, and 4 min. The results threaded through the furnace from a supply reel at one end suggested that carbon passed through a rhombohedral form to a take-up winder at the opposite end using two pulleys during graphitization since the measured interlayer spacing at the entrance and exit of the furnace. Only sufficient was greater than the hexagonal form. Fischbach [9 tension was applied to the tows to eliminate any slack; it is erformed heat treatments on petroleum and coal tar pitch estimated that the pulling force was less than 50 g. Six cokes(similar to pitch mesophase) to study the kinetics of residence times: 0.7. 2.0.5.4. 12.5.. and 585s were graphitization for time periods lasting from 2 to 1000 min studied. These times were determined by measuring at temperatures of 2200 to 2900C Effective reaction rates time required for the fiber to travel through the hot zone increased by a factor of -10 over this temperature range and carrying out a rudimentary heat transfer calculation and the variation in the unit cell parameter was reported This analysis was performed using approximate heat hese earlier results indicate that graphitizable carbons transfer equations, assumptions regarding the thermal (such as mesophase) should transform rapidly at high conductivities of the fibers, modeling the fiber tow as a temperature. The results of the present study confirm that uniform infinite cylinder, and calculating the centerline significant graphitization may be attained within very short temperature. It was determined that the fiber tows attained times. We also show that fibers with comparatively higl the hot zone temperature nearly instantaneously [10]. The thermal conductivities may be produced using graphitiza cooling rate of the fibers heated by the continuous method tion times of <1 s. An energy analysis of the manufactur- was also very rapid; i.e., the fibers were essentially ing costs associated with the preparation of carbon fibers 'quenched from elevated temperature. Although the ef- under these conditions is presented, and we suggest that fects of cooling rate on fiber properties were not character- fiber production under these conditions appears econom- ized, the rapid cooling rate employed may also infuence cally favorable the structural properties of the fibers Batch mode processing was car wrapping approximately 8 m of the fiber tow around a 2. Experimen small piece of carbon felt which was then placed in the hot zone. All batch-mode samples were heated at a rate of-20 2. 1. Precursor fibers oC/min with an intermediate hold at 500C. The cooling rate of the furnace following the graphitization hold was The three pitch-based precursor fibers supplied by BP also estimated to be 20C/min. Heating and cooling rates Amoco(Alpharetta, GA, USA) were simply designated A, were obtained by using the manual control mode of the B, and C. The only information provided was that the furnace. Due to the high thermal activation energy of the fibers were carbonized, but not graphitized, implying that graphitization process [9, 12, time at temperatures beloy they had been heat treated to temperatures less than 1800 the final heat treatment temperature was neglected in the C. Fibers a and c were difficult to handle and would eat treatment analysis and was not included in the break if manipulated too harshly. Fiber B, in contrast, was isothermal residence times reported quite easy to handle without difficulty. Fiber diameter, shape, and microstructure were not characterized 2.3 Fiber characterisation 2.2. Fiber heat treatment in batch and continuous modes The densities of the precursor and heat treated fibers were measured by He pycnometry using a Micromeretics A water-cooled resistance furnace (Bethlehem Ad- AccuPyc 1330. To increase precision, the sample cell was vanced Materials, Knoxville, TN, USA) was used for fiber purged 10 times prior to each measurement and each heat treatment. The exterior dimensions of the furnace sample was run 25 times. The calculated standard devia-1218 M.L. Greene et al. / Carbon 40 (2002) 1217 –1226 such as thermal conductivity were developed for three measure 18 inches long39 inches wide38.75 inches in different carbonized pitch precursors. Although the specific height. The heating element of the furnace and all internal nature of these precursors was not revealed by the supplier parts are graphite and the hot zone length was estimated at (BP Amoco), we believe these results are of interest 3.5 inches. The temperature inside the hot zone was because there are few reports on the development of measured with an Ircon Mirage dual wavelength pyrometer carbon fiber properties at short graphitization times. Previ- [10,11]. Argon was used as the purge gas. ously, Richardson and Zehms [7] reported on heat treat- Graphitization of the fibers was achieved by one of two ment of pyrolytic carbon at temperatures between 2600 methods: continuous or batch processing. Continuous and 3000 8C for times ranging from 15 s to 10 min. processing was used to obtain hot zone residence times Time-dependent changes in gross dimensions and inter- from 0.7 to 58.5 s, while batch processing was used to heat layer spacing were reported, although further analysis was the fibers for residence times ranging from 900 to 3600 s. limited. Pandic [8] studied graphitization of a petroleum Graphitization temperatures of 2400, 2700 and 3000 8C coke and measured the interlayer spacing, stack height, and were evaluated. coherence length for fibers processed at furnace residence For continuous mode operation, the fiber tow was times of 20 and 40 s, and 1, 3, and 4 min. The results threaded through the furnace from a supply reel at one end suggested that carbon passed through a rhombohedral form to a take-up winder at the opposite end using two pulleys during graphitization since the measured interlayer spacing at the entrance and exit of the furnace. Only sufficient was greater than the hexagonal form. Fischbach [9] tension was applied to the tows to eliminate any slack; it is performed heat treatments on petroleum and coal tar pitch estimated that the pulling force was less than 50 g. Six cokes (similar to pitch mesophase) to study the kinetics of residence times: 0.7, 2.0, 5.4, 12.5, 33.1, and 58.5 s were graphitization for time periods lasting from 2 to 1000 min studied. These times were determined by measuring the at temperatures of 2200 to 2900 8C. Effective reaction rates time required for the fiber to travel through the hot zone 5 increased by a factor of |10 over this temperature range and carrying out a rudimentary heat transfer calculation. and the variation in the unit cell parameter was reported. This analysis was performed using approximate heat These earlier results indicate that graphitizable carbons transfer equations, assumptions regarding the thermal (such as mesophase) should transform rapidly at high conductivities of the fibers, modeling the fiber tow as a temperature. The results of the present study confirm that uniform infinite cylinder, and calculating the centerline significant graphitization may be attained within very short temperature. It was determined that the fiber tows attained times. We also show that fibers with comparatively high the hot zone temperature nearly instantaneously [10]. The thermal conductivities may be produced using graphitiza- cooling rate of the fibers heated by the continuous method tion times of ,1 s. An energy analysis of the manufactur- was also very rapid; i.e., the fibers were essentially ing costs associated with the preparation of carbon fibers ‘quenched’ from elevated temperature. Although the ef￾under these conditions is presented, and we suggest that fects of cooling rate on fiber properties were not character- fiber production under these conditions appears econom- ized, the rapid cooling rate employed may also influence ically favorable. the structural properties of the fibers. Batch mode processing was carried out by loosely wrapping approximately 8 m of the fiber tow around a 2. Experimental small piece of carbon felt which was then placed in the hot zone. All batch-mode samples were heated at a rate of |20 2 .1. Precursor fibers 8C/min with an intermediate hold at 500 8C. The cooling rate of the furnace following the graphitization hold was The three pitch-based precursor fibers supplied by BP also estimated to be 20 8C/min. Heating and cooling rates Amoco (Alpharetta, GA, USA) were simply designated A, were obtained by using the manual control mode of the B, and C. The only information provided was that the furnace. Due to the high thermal activation energy of the fibers were carbonized, but not graphitized, implying that graphitization process [9,12], time at temperatures below they had been heat treated to temperatures less than 1800 the final heat treatment temperature was neglected in the 8C. Fibers A and C were difficult to handle and would heat treatment analysis and was not included in the break if manipulated too harshly. Fiber B, in contrast, was isothermal residence times reported. quite easy to handle without difficulty. Fiber diameter, shape, and microstructure were not characterized. 2 .3. Fiber characterization 2 .2. Fiber heat treatment in batch and continuous modes The densities of the precursor and heat treated fibers were measured by He pycnometry using a Micromeretics A water-cooled resistance furnace (Bethlehem Ad- AccuPyc 1330. To increase precision, the sample cell was vanced Materials, Knoxville, TN, USA) was used for fiber purged 10 times prior to each measurement and each heat treatment. The exterior dimensions of the furnace sample was run 25 times. The calculated standard devia-