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Electrical resistivity of pitch during heat treatment
Electrical resistivity treatment*
of pitch during heat
John A. Sharp Tar industries Services, Mill Lane, Wingerworth, (Received 31 March 7987)
Chesterfield,
UK
The d.c. electrical resistivity of coal-tar pitch has been measured from ambient temperature to 450°C. The change in resistivity with temperature up to 200°C has been found to be consistent with viscositycontrolled diffusion of current carriers. In an isothermal study at 45O”C, the start of mesophase formation was found to be accompanied by a rapid rise in the pitch resistivity, which subsequently levelled off and remained nearly constant until the onset of mesophase coalescence and phase inversion. The resistivity of the resultant mesophase pitch was found to be less temperature susceptible than that of the parent pitch. (Keywords: pitch; viscosity; electrical resistivity)
Sakai et al.’ reported the measurement of the d.c. electrical conductivity of some molten pitches from 6&16o”C and drew attention to the strong similarities that exist in temperature susceptibility between the conductivity and the fluidity of the pitches they examined. Using the Nernst-Einstein equation for diffusion and the Stokes-Einstein viscosity equation, an expression was derived which indicated the existence of a rectilinear relationship between the conductivity and the viscosity of a medium in which current passed by diffusion of carrier Arrhenius-type plots of log species. Rectilinear (conductivity) against inverse absolute temperature were produced and were compared with the results of earlier viscosity work by the same group to show that the activation energy terms for both properties were similar’. Although Sakai et al. found rectilinear Arrhenius-type plots for the viscosity and conductivity of pitches, there is considerable evidence that, particularly over a wider range of temperatures, such viscosity plots show appreciable curvature, indicating deviation from the simple exponential mode13. These deviations have been associated particularly with measurements at higher temperatures4, and they may well be connected with the structural viscosity properties of primary quinolineinsoluble materials found in most coal-tar pitches4m6. Measurement of the resistivity of some coal-tar pitches over the temperature range from ambient to 450°C has now shown that the wide-range Arrhenius plots of resistivity against inverse temperature also show marked curvature, and a comparison has been made with the viscosity curve for one pitch over a similar range. The change in resistivity of the pitch during isothermal heating at 450°C has also been examined. EXPERIMENTAL Resistivity measurements were made on samples of pitch held in borosilicate tubes (650 x 20mm i.d.), heated in a fluidized sand bath. The resistivity was measured using a simple probe having two parallel electrodes 10 x 20 mm, * This paper was presented future material’,
Newcastle
at the conference ‘Pitch: the science of a upon Tyne, UK, 24-26 March 1987.
001~2361/87/111487-04$3.00 0 1987 Butterworth & Co. (Publishers)
Ltd.
and 1Omm apart, constructed in platinum with a borosilicate glass supporting structure. An applied d.c. potential of lO.OOV was used, and the current in the circuit was measured using a Keithley 61OC Electrometer having a maximum f.s.d. sensitivity of lo-l4 A. The results reported here were measured on a sample of cokeoven pitch of softening point 103.3” (R & B), 4.2% quinoline-insolubles, 34.6 % toluene-insolubles and a density of 1.302g cmp3 at 38°C. The viscosity and other properties of this material have been reported in detail elsewhere under the code name ‘Pitch F’5*6. In an experiment, the tube containing the resistivity probe, and four similar tubes containing parallel pitch samples were heated in approximately 50K steps from ambient to 45o”C, with an isothermal period at each stage during which the current flowing through the sample was measured. On reaching 45o”C, the bath temperature was maintained k 2 K for a period approaching 5OOmin, during which time the current was monitored continuously. Readings were noted at 5 min intervals where significant changes were occurring, and at points of particular interest one of the tubes containing the parallel pitch samples was withdrawn from the heating bath. The withdrawn samples were polished and examined microscopically by reflected polarized light. After the isothermal period at 45o”C, the assembly was allowed to cool and the resistivity of the heat-treated sample was again measured on a stepwise rising cycle to 45o”C, similar to that used in the initial stage. During the measurements of the sample resistance at lower temperatures, where currents less than about 10m7A were involved, fluctuating static charges caused by the motion of the fluidized sand made it impossible to obtain steady readings. Under these conditions, the air supply to the bath was turned off and the charge was allowed to dissipate before making the reading. At higher temperatures this effect was minimal, and up to about 380°C steady continuous readings could be obtained. Above 38o”C, evolution of bubbles of volatiles and/or gases led to fluctuations in the current readings, and the high temperature data for the curves presented in Figures I&4 were obtained by observing the fluctuations for a period of about 2 min, to ascertain a central current value
FUEL, 1987, Vol 66, November
1487
Electrical resistivity of pitch during heat treatment: J. A. Sharp Figure 2 indicates that although there is a rectilinear relationship between log (resistivity) and log (viscosity) over the upper part of the plot, the lower part is markedly curved. The rectilinear portion corresponds to
2 3
2
Inverse temperature, Figure 1 pitch: -,
Arrhenius plots of resistivity resistivity; ---, viscosity
x lo3 (K) and viscosity
for a coke-oven
4
2
that was then used to calculate the resistivity. The fluctuations were usually up to about + 2 % of the central value.
Log viscosity Figure 2 Relationship oven pitch
between
6
(mPa)
resistivity
and viscosity
for a coke-
RESULTS Figure 1 contains two Arrhenius plots, the upper curve relating the logarithm of the resistivity of the pitch (lefthand ordinate scale) to the inverse absolute temperature, while the lower curve relates the logarithm ofthe viscosity of the pitch (right-hand ordinate scale) to the inverse absolute temperature, using data published previously5x6. The interrelation of the resistivity and viscosity of the pitch is presented in logarithmic form in Figure 2, which is constructed from the smoothed data from the two regression lines shown in Figure I. The change in resistivity of the pitch sample during an extended isothermal ‘soak’ at 450°C is shown in Figure 3, the three marked points (a, b and c) on the curve corresponding to the times at which a parallel sample was withdrawn from the heating bath for use in subsequent microscopic examination. Figure 4 shows the variation in the logarithm of the sample resistivity with temperature when the heat-soaked pitch was reheated from room temperature to 450°C and the corresponding changes for the original pitch prior to heat treatment. Figures 5-7 are photomicrographs showing the state of mesophase development at points a, b and c, respectively, in Figure 3. DISCUSSION
FUEL,
1
2.0 ’
Temperature variation of resistivity The Arrhenius plots of resistivity and viscosity in Figure I show a basic similarity to each other, but also some differences in detail. The derived plot shown in
1488
3.5
1987,
Vol 66, November
0
a
I
I
100
200
I
300
I
400
Time (min) Figure 3 Variation oven pitch
ofresistivity
with soaking time at 450°C for a coke-
Electrical
2
1
I
0
100
I
I
300
400
I 200
Temperature
(‘C)
Figure 4 Variation of resistivity with temperature for a coke-oven pitch: ---, original sample; --, sample after heat treatment at 450°C
Figure 5
Microscopic
appearance
of pitch at point a (Figure 3)
measurements made at temperatures up to about 200°C which corresponds roughly to the regime used by Sakai et al.‘. However, if the rectilinear relationship that these authors propose between the conductivity and fluidity of pitches were obeyed precisely, the gradient of the rectilinear part of the log/log line in Figure 2 should be unity, instead of which a value of 0.936 is observed. Whether this deviation from unity is sufficiently great to be significant is not yet known, but the general concept of a viscositycontrolled diffusion mechanism for conduction in pitches over this temperature range appears to be supported by these data. It is thought that the curvature of the left-hand part of the plot in Figure 2, corresponding to measurements made at temperatures above 200°C is probably caused by non-Newtonian behaviour of the pitch due to the presence of dispersed
resistivity
of pitch
during
heat treatment:
J. A. Sharp
solid matter. If this were the case, the structural component of the viscosity must be assumed to have little influence on the movement of current-carrying species through the pitch hydrocarbon matrix, because of the presence of dispersed solids. It had been expected that during the prolonged heat treatment of the pitch at 45O’C, the gradual increase in the volume fraction of the more conductive mesophase in the sample would lead to a concomitant decrease in the sample resistivity ‘A . However, reference to Figure 3 shows that on holding the sample isothermally at 450°C the resistivity began to rise quite sharply over the first 30min, and then levelled out and remained fairly constant for the next 250min. A parallel sample of pitch withdrawn from the heating bath at point a (Figure 3) showed the presence of 17 ~01% of mesophase, present as spheres of mean diameter 6 pm surrounded by clusters of primary quinoline insolubles with a matrix of continuous isotropic phase (Figure 5). During the period between points a and b (Figure 3) the mesophase continued to grow, and a parallel pitch sample withdrawn at point b showed a mesophase content of 51 vol%, present as chains and clusters of spheres ofmean diameter 45 pm, together with some larger coalesced areas (Figure 6). The relative independence of the sample resistivity on the mesophase content during the growth of the anisotropic units was unexpected, and it was only after about 300min that a small well-defined increase in resistivity was followed by the expected gradual fall. A
(Figure 3)
Figure 6
Microscopic
appearance
of pitch at point b
Figure 7
Microscopic
appearance
of pitch at point c (Figure 3)
FUEL,
1987,
Vol 66, November
1499
Electrical resistivity of pitch during heat treatment: J. A. Sharp
third parallel sample, withdrawn at point c (Figure 3) after 480min at 450°C showed the presence of 75 ~01% of mesophase, virtually all of which was coalesced (Figure 7). It seems fairly clear that during this stage, between samples Band C, the fall in resistivity was associated with the coalescence of the mesophase into large continuous anisotropic areas. The small peak in the resistivity trace at about 300 min may be connected with the small peak that Collett and Rand’ observed in the viscosity traces of similar pitches during heat treatment at temperatures of 46&475”C, and which they associated with the formation of an interlocked structure prior to phase inversion. Experiments with other pitches of coke-oven origin have confirmed the general form of the curve shown in Figure 3, with initial sharp rises in resistivity over a period of about 30min at 450°C levelling off to an extended section which then showed a small maximum followed by a gradual decline. The mesophase contents at the shoulder, corresponding to point a in Figure 3, were about 18 vol ‘A, and those after the maximum had been passed, corresponding to point b in Figure 3, were 49-52 vol %, although the mesophase particle size distributions were influenced by the amount and nature of the dispersed solids in the original pitches. Nature of current carriers
As a consequence of the use of direct current in the measuring circuit, there is some possibility of polarization effects occurring, or of the depletion of ionic current carrying species by discharge at the electrodes. Such effects would be expected to result in an increase in the measured resistivity of the samples, and could explain the rapid rise observed in the early stages of the heat-soaking stage at 450°C. However, despite appreciable differences in the times taken to reach 450°C during the warming-up stages, there was a close similarity in the behaviour of the three coke-oven pitches examined, and it seems more probable that the resistivity rise is connected with some aspect of the early stages of mesophase formation. One explanation involves the abstraction of current-carrying species from the matrix as part of the nucleation or early growth of the mesophase. Although the nature of the current carriers is unknown, some possibilities will be considered. Zander and Palm” have observed that when coronene is doped with iodine up to a content of about 0.5 wt %, the solid-state resistance falls virtually exponentially with increasing iodine concentration. These authors have also found that when chloroform-insoluble fractions of pitches were doped with 1 wt ‘A of iodine, a corresponding reduction in resistivity was observed. In these cases, the enhanced conductivity was ascribed to the formation of charge-transfer complexes between the iodine and the
1490
FUEL, 1987,
Vol 66, November
polynuclear aromatic hydrocarbons (PAH). Speight and Penzes’ I have examined the electrical conductivity of 2 wt % nitrobenzene solutions of some aromatic and heteroaromatic compounds, and have found that in most cases the conductivity was markedly enhanced when 2 wt % of mercuric chloride was added to the solution. The enhancement was most noticeable for nitrogen and sulphur heterocyclic compounds, and in some cases exceeded one order of magnitude. It seems possible that in the experiments reported here, similar charge-transfer complexes could be formed between aromatic or heteroaromatic component molecules of the pitch and some of the inorganic impurities. These complexes could be responsible for a significant proportion of the charge transport, and in addition would probably be more reactive than the parent molecules under the thermal polymerization conditions of the measurements. Under these circumstances, the progressive immobilization of such complexes by incorporation into the growing mesophase spheres would ultimately return the conduction process to the level of the unmodified hydrocarbons. The resistivity of the material that had been treated at 450°C for 480min was found to have a more nearly rectilinear dependence on the measurement temperature over the range from room temperature to 450°C than had the parent pitch (Figure 4). This may indicate that the viscosity-controlled carrier diffusion conduction process, which appears to hold for the original material at temperatures up to about 200°C is modified in the heattreated product, perhaps to a more solid-state semiconduction type of process. ACKNOWLEDGEMENT The author thanks the European Coal and Steel Community for financial assistance in this work, which was part of Research Project 7220-EC-822. REFERENCES
5 6 I 8 9 10 11
Sakai, M., Hirota, S. and Inagaki, M. Carbon 1984,22,187-189 Sakai, M. and Inagaki, M. Carbon 1981, 19, 3743 Okazaki, H. Ind. Eng. Chem. Prod. Res. Deu. 1982,21, 130-134 Collett, G. W., Shepherd, P. M. and Rand, B. Proc. 5th London Carbon Conf. 1978, Vol. 1, pp. 280-293 Brizes. D. K. H. Fuel 1980.59. 201-207 Br& Carbonization Research Association: Carbonization Research Report No. 33 MacDougall, F. H. and Green, R. G. J. infectious Diseases 1924, 34, 192-202 Fricke, H. J. Gen. Physiol. 1924, 6, 3755384 Collett, G. W. and Rand, B. Fuel 1978, 57, 162-170 Zander. M. and Palm. J. Erdiil Kohle Erdaas Petrochem 1985.38. 162-164 Speight, J. G. and Penzes, S. Chem. Ind. (London) 1978,729-731 ”
of pitch during heat
John A. Sharp Tar industries Services, Mill Lane, Wingerworth, (Received 31 March 7987)
Chesterfield,
UK
The d.c. electrical resistivity of coal-tar pitch has been measured from ambient temperature to 450°C. The change in resistivity with temperature up to 200°C has been found to be consistent with viscositycontrolled diffusion of current carriers. In an isothermal study at 45O”C, the start of mesophase formation was found to be accompanied by a rapid rise in the pitch resistivity, which subsequently levelled off and remained nearly constant until the onset of mesophase coalescence and phase inversion. The resistivity of the resultant mesophase pitch was found to be less temperature susceptible than that of the parent pitch. (Keywords: pitch; viscosity; electrical resistivity)
Sakai et al.’ reported the measurement of the d.c. electrical conductivity of some molten pitches from 6&16o”C and drew attention to the strong similarities that exist in temperature susceptibility between the conductivity and the fluidity of the pitches they examined. Using the Nernst-Einstein equation for diffusion and the Stokes-Einstein viscosity equation, an expression was derived which indicated the existence of a rectilinear relationship between the conductivity and the viscosity of a medium in which current passed by diffusion of carrier Arrhenius-type plots of log species. Rectilinear (conductivity) against inverse absolute temperature were produced and were compared with the results of earlier viscosity work by the same group to show that the activation energy terms for both properties were similar’. Although Sakai et al. found rectilinear Arrhenius-type plots for the viscosity and conductivity of pitches, there is considerable evidence that, particularly over a wider range of temperatures, such viscosity plots show appreciable curvature, indicating deviation from the simple exponential mode13. These deviations have been associated particularly with measurements at higher temperatures4, and they may well be connected with the structural viscosity properties of primary quinolineinsoluble materials found in most coal-tar pitches4m6. Measurement of the resistivity of some coal-tar pitches over the temperature range from ambient to 450°C has now shown that the wide-range Arrhenius plots of resistivity against inverse temperature also show marked curvature, and a comparison has been made with the viscosity curve for one pitch over a similar range. The change in resistivity of the pitch during isothermal heating at 450°C has also been examined. EXPERIMENTAL Resistivity measurements were made on samples of pitch held in borosilicate tubes (650 x 20mm i.d.), heated in a fluidized sand bath. The resistivity was measured using a simple probe having two parallel electrodes 10 x 20 mm, * This paper was presented future material’,
Newcastle
at the conference ‘Pitch: the science of a upon Tyne, UK, 24-26 March 1987.
001~2361/87/111487-04$3.00 0 1987 Butterworth & Co. (Publishers)
Ltd.
and 1Omm apart, constructed in platinum with a borosilicate glass supporting structure. An applied d.c. potential of lO.OOV was used, and the current in the circuit was measured using a Keithley 61OC Electrometer having a maximum f.s.d. sensitivity of lo-l4 A. The results reported here were measured on a sample of cokeoven pitch of softening point 103.3” (R & B), 4.2% quinoline-insolubles, 34.6 % toluene-insolubles and a density of 1.302g cmp3 at 38°C. The viscosity and other properties of this material have been reported in detail elsewhere under the code name ‘Pitch F’5*6. In an experiment, the tube containing the resistivity probe, and four similar tubes containing parallel pitch samples were heated in approximately 50K steps from ambient to 45o”C, with an isothermal period at each stage during which the current flowing through the sample was measured. On reaching 45o”C, the bath temperature was maintained k 2 K for a period approaching 5OOmin, during which time the current was monitored continuously. Readings were noted at 5 min intervals where significant changes were occurring, and at points of particular interest one of the tubes containing the parallel pitch samples was withdrawn from the heating bath. The withdrawn samples were polished and examined microscopically by reflected polarized light. After the isothermal period at 45o”C, the assembly was allowed to cool and the resistivity of the heat-treated sample was again measured on a stepwise rising cycle to 45o”C, similar to that used in the initial stage. During the measurements of the sample resistance at lower temperatures, where currents less than about 10m7A were involved, fluctuating static charges caused by the motion of the fluidized sand made it impossible to obtain steady readings. Under these conditions, the air supply to the bath was turned off and the charge was allowed to dissipate before making the reading. At higher temperatures this effect was minimal, and up to about 380°C steady continuous readings could be obtained. Above 38o”C, evolution of bubbles of volatiles and/or gases led to fluctuations in the current readings, and the high temperature data for the curves presented in Figures I&4 were obtained by observing the fluctuations for a period of about 2 min, to ascertain a central current value
FUEL, 1987, Vol 66, November
1487
Electrical resistivity of pitch during heat treatment: J. A. Sharp Figure 2 indicates that although there is a rectilinear relationship between log (resistivity) and log (viscosity) over the upper part of the plot, the lower part is markedly curved. The rectilinear portion corresponds to
2 3
2
Inverse temperature, Figure 1 pitch: -,
Arrhenius plots of resistivity resistivity; ---, viscosity
x lo3 (K) and viscosity
for a coke-oven
4
2
that was then used to calculate the resistivity. The fluctuations were usually up to about + 2 % of the central value.
Log viscosity Figure 2 Relationship oven pitch
between
6
(mPa)
resistivity
and viscosity
for a coke-
RESULTS Figure 1 contains two Arrhenius plots, the upper curve relating the logarithm of the resistivity of the pitch (lefthand ordinate scale) to the inverse absolute temperature, while the lower curve relates the logarithm ofthe viscosity of the pitch (right-hand ordinate scale) to the inverse absolute temperature, using data published previously5x6. The interrelation of the resistivity and viscosity of the pitch is presented in logarithmic form in Figure 2, which is constructed from the smoothed data from the two regression lines shown in Figure I. The change in resistivity of the pitch sample during an extended isothermal ‘soak’ at 450°C is shown in Figure 3, the three marked points (a, b and c) on the curve corresponding to the times at which a parallel sample was withdrawn from the heating bath for use in subsequent microscopic examination. Figure 4 shows the variation in the logarithm of the sample resistivity with temperature when the heat-soaked pitch was reheated from room temperature to 450°C and the corresponding changes for the original pitch prior to heat treatment. Figures 5-7 are photomicrographs showing the state of mesophase development at points a, b and c, respectively, in Figure 3. DISCUSSION
FUEL,
1
2.0 ’
Temperature variation of resistivity The Arrhenius plots of resistivity and viscosity in Figure I show a basic similarity to each other, but also some differences in detail. The derived plot shown in
1488
3.5
1987,
Vol 66, November
0
a
I
I
100
200
I
300
I
400
Time (min) Figure 3 Variation oven pitch
ofresistivity
with soaking time at 450°C for a coke-
Electrical
2
1
I
0
100
I
I
300
400
I 200
Temperature
(‘C)
Figure 4 Variation of resistivity with temperature for a coke-oven pitch: ---, original sample; --, sample after heat treatment at 450°C
Figure 5
Microscopic
appearance
of pitch at point a (Figure 3)
measurements made at temperatures up to about 200°C which corresponds roughly to the regime used by Sakai et al.‘. However, if the rectilinear relationship that these authors propose between the conductivity and fluidity of pitches were obeyed precisely, the gradient of the rectilinear part of the log/log line in Figure 2 should be unity, instead of which a value of 0.936 is observed. Whether this deviation from unity is sufficiently great to be significant is not yet known, but the general concept of a viscositycontrolled diffusion mechanism for conduction in pitches over this temperature range appears to be supported by these data. It is thought that the curvature of the left-hand part of the plot in Figure 2, corresponding to measurements made at temperatures above 200°C is probably caused by non-Newtonian behaviour of the pitch due to the presence of dispersed
resistivity
of pitch
during
heat treatment:
J. A. Sharp
solid matter. If this were the case, the structural component of the viscosity must be assumed to have little influence on the movement of current-carrying species through the pitch hydrocarbon matrix, because of the presence of dispersed solids. It had been expected that during the prolonged heat treatment of the pitch at 45O’C, the gradual increase in the volume fraction of the more conductive mesophase in the sample would lead to a concomitant decrease in the sample resistivity ‘A . However, reference to Figure 3 shows that on holding the sample isothermally at 450°C the resistivity began to rise quite sharply over the first 30min, and then levelled out and remained fairly constant for the next 250min. A parallel sample of pitch withdrawn from the heating bath at point a (Figure 3) showed the presence of 17 ~01% of mesophase, present as spheres of mean diameter 6 pm surrounded by clusters of primary quinoline insolubles with a matrix of continuous isotropic phase (Figure 5). During the period between points a and b (Figure 3) the mesophase continued to grow, and a parallel pitch sample withdrawn at point b showed a mesophase content of 51 vol%, present as chains and clusters of spheres ofmean diameter 45 pm, together with some larger coalesced areas (Figure 6). The relative independence of the sample resistivity on the mesophase content during the growth of the anisotropic units was unexpected, and it was only after about 300min that a small well-defined increase in resistivity was followed by the expected gradual fall. A
(Figure 3)
Figure 6
Microscopic
appearance
of pitch at point b
Figure 7
Microscopic
appearance
of pitch at point c (Figure 3)
FUEL,
1987,
Vol 66, November
1499
Electrical resistivity of pitch during heat treatment: J. A. Sharp
third parallel sample, withdrawn at point c (Figure 3) after 480min at 450°C showed the presence of 75 ~01% of mesophase, virtually all of which was coalesced (Figure 7). It seems fairly clear that during this stage, between samples Band C, the fall in resistivity was associated with the coalescence of the mesophase into large continuous anisotropic areas. The small peak in the resistivity trace at about 300 min may be connected with the small peak that Collett and Rand’ observed in the viscosity traces of similar pitches during heat treatment at temperatures of 46&475”C, and which they associated with the formation of an interlocked structure prior to phase inversion. Experiments with other pitches of coke-oven origin have confirmed the general form of the curve shown in Figure 3, with initial sharp rises in resistivity over a period of about 30min at 450°C levelling off to an extended section which then showed a small maximum followed by a gradual decline. The mesophase contents at the shoulder, corresponding to point a in Figure 3, were about 18 vol ‘A, and those after the maximum had been passed, corresponding to point b in Figure 3, were 49-52 vol %, although the mesophase particle size distributions were influenced by the amount and nature of the dispersed solids in the original pitches. Nature of current carriers
As a consequence of the use of direct current in the measuring circuit, there is some possibility of polarization effects occurring, or of the depletion of ionic current carrying species by discharge at the electrodes. Such effects would be expected to result in an increase in the measured resistivity of the samples, and could explain the rapid rise observed in the early stages of the heat-soaking stage at 450°C. However, despite appreciable differences in the times taken to reach 450°C during the warming-up stages, there was a close similarity in the behaviour of the three coke-oven pitches examined, and it seems more probable that the resistivity rise is connected with some aspect of the early stages of mesophase formation. One explanation involves the abstraction of current-carrying species from the matrix as part of the nucleation or early growth of the mesophase. Although the nature of the current carriers is unknown, some possibilities will be considered. Zander and Palm” have observed that when coronene is doped with iodine up to a content of about 0.5 wt %, the solid-state resistance falls virtually exponentially with increasing iodine concentration. These authors have also found that when chloroform-insoluble fractions of pitches were doped with 1 wt ‘A of iodine, a corresponding reduction in resistivity was observed. In these cases, the enhanced conductivity was ascribed to the formation of charge-transfer complexes between the iodine and the
1490
FUEL, 1987,
Vol 66, November
polynuclear aromatic hydrocarbons (PAH). Speight and Penzes’ I have examined the electrical conductivity of 2 wt % nitrobenzene solutions of some aromatic and heteroaromatic compounds, and have found that in most cases the conductivity was markedly enhanced when 2 wt % of mercuric chloride was added to the solution. The enhancement was most noticeable for nitrogen and sulphur heterocyclic compounds, and in some cases exceeded one order of magnitude. It seems possible that in the experiments reported here, similar charge-transfer complexes could be formed between aromatic or heteroaromatic component molecules of the pitch and some of the inorganic impurities. These complexes could be responsible for a significant proportion of the charge transport, and in addition would probably be more reactive than the parent molecules under the thermal polymerization conditions of the measurements. Under these circumstances, the progressive immobilization of such complexes by incorporation into the growing mesophase spheres would ultimately return the conduction process to the level of the unmodified hydrocarbons. The resistivity of the material that had been treated at 450°C for 480min was found to have a more nearly rectilinear dependence on the measurement temperature over the range from room temperature to 450°C than had the parent pitch (Figure 4). This may indicate that the viscosity-controlled carrier diffusion conduction process, which appears to hold for the original material at temperatures up to about 200°C is modified in the heattreated product, perhaps to a more solid-state semiconduction type of process. ACKNOWLEDGEMENT The author thanks the European Coal and Steel Community for financial assistance in this work, which was part of Research Project 7220-EC-822. REFERENCES
5 6 I 8 9 10 11
Sakai, M., Hirota, S. and Inagaki, M. Carbon 1984,22,187-189 Sakai, M. and Inagaki, M. Carbon 1981, 19, 3743 Okazaki, H. Ind. Eng. Chem. Prod. Res. Deu. 1982,21, 130-134 Collett, G. W., Shepherd, P. M. and Rand, B. Proc. 5th London Carbon Conf. 1978, Vol. 1, pp. 280-293 Brizes. D. K. H. Fuel 1980.59. 201-207 Br& Carbonization Research Association: Carbonization Research Report No. 33 MacDougall, F. H. and Green, R. G. J. infectious Diseases 1924, 34, 192-202 Fricke, H. J. Gen. Physiol. 1924, 6, 3755384 Collett, G. W. and Rand, B. Fuel 1978, 57, 162-170 Zander. M. and Palm. J. Erdiil Kohle Erdaas Petrochem 1985.38. 162-164 Speight, J. G. and Penzes, S. Chem. Ind. (London) 1978,729-731 ”