- Email: [email protected]
Electrical d.c. conductivity of molten pitches
187
Letters to the Editor It follows, therefore that the formation energy on the surface is less than the formation energy in the bulk. A the~ochemical consideration may also be given to the abstraction of a carbon from the basal plane by an 0 atom. The following hypothetical path may be used for the abstraction process: C(solid) 2 C(g) + O(g) f: CO(g) At 7OO’C,E, is 171Sl kcal/mole for heat of sublimation and E2 = -258.& kcal/mole for heat of reaction between C and 0 atoms. The overall energy E is E, -t E, = - 86.54 kcal/mole, or 3.76 eV per ato; This energ; is qiite close to the Ek values in Table 1. Ackno&&emenr-This work is supported by the National Science Foundation under Grant CPE-8120569.
FEILINC CEN RALPH
T.
YANG
Deportment of Chemicul ~ngine~ing State University of New York at Buflaio Amherst NY 14214 U.S.A. REFERENCES 1. G. R. Hennig, J. Chem. Phys. 40, 2877 (1964). 2. E. L. Evans and J. M. Thomas, 3rd Co& Carbon and Graphite, DD. 3-9. Society Chemical Industry, London (19?1). -3. K. L. Yang and R. T. Yang, AZChE J. in press (1984). 4. C. Baker and A. Kelly, Nature, 193, 235 (1962). 5. C. A. Coulson and M: D. Poole, Carbon i, 275 (1964). 6. P. A. Thrower and R. M. Mayer, Phys. Status Soiidi 47, 11 (1978).
om-6223/U 163.00+ .Ga Q 1984 Pergamon Press Ltd.
Curbon Vol. 22. No. 2, PP. 187-189, 1984 ~nnted in Great Britain.
Electricaf
d.c. conductivity of molten pitches
The fluidity & = inverse viscosity, l/q) of molten pitches is discussed in conjunction with the interrelation of Cpwith the electrical d.c. conductivity K. It will be shown that K is an indicator of the fluidity of molten pitch. The electrical method we proposed relies on the detection of an electrical voltage drop in an electrical cell, and it is simpler and more convenient than the usual rheological methods for studying the fluidity of molten pitches; especially in the high temperature region where the quantitative measurement of rheological parameters is difficult. in general, pitch is known as a typical insulator and its electrical conductivity has been considered to be very small. Our recent investigation on the dielectric loss of pitches[l], however, revealed that molten pitches show sizeable values of electrical conductivity in the low frequency region of an applied sinusoidal electrical field, and that the conductivity K increases with increasing temperature. Although, at the present, we do not know the exact nature of the carriers, it can be presumed that the fluidity of molten pitch would have a strong influence on the mobility of the carriers under electrical field. Hence, the electrical conductivity might be used as a convenient indicator for pitch fluidity. Such an electrical method would be effective for practical purposes and en~n~~ng uses. The electrical d.c. conductivity K was measured over a wide range of temperature (70-i6O”C) for three kinds of petroleum pitches (PP-1. PP-2 and PP-C) and for four kinds of coal tar pitches (CP-1, CP-2, CP-j and CP-4) by using double disk plate electrodes 40.0mm in dia., which were made of gilded brass. The thickness of the sample inserted between the electrodes was kept at 1.0 mm during the measurement of K. The electrodes with the pitch sample were placed in a stainless steel chamber which was immersed in an oil bath thermocontrolled within kO.02”C using a mercury temperature regulator. The temperature of the sample was measured by a CA thermocouple attached to the electrode disk. The reproducibility of the measured values of ti was within & 5% for all of the samples in the temperature ranges. This is a good reproducibility, especially considering the possible disturbances because of the evolution of tow molecular weight pitch components during the measurements.
8-
j-
t-
,__
I--
160 0
(“Cl
Fig. 1. The temperature dependences of electrical d.c. conductivity K for the petroleum pitches.
188
Letters to the Editor
The temperature dependences of K are shown in Fig. 1 for petroleum pitches and in Fig. 2 for coal tar pitches. The values of K for all of the pitch samples increase with increase in temperature. In order to ascertain the conductivity mechanism, the values of K for all sample pitches are plotted semilogarithmically in Fig. 3 against the inverse Kelvin temperature l/T. The linear relations in Fig. 3 imply that the rate processes would be dominated by the electrical d.c. conductivity in molten pitches, the equation of which is expressed as K=
Kg.
exp ( - AHJRT).
-
rl
18
'E Lb - 16
1
(1)
The experimental parameters, K~ and AH,., for each pitch sample are listed in Table 1. The values of AH, range from 30 to 50 kcal/mol for almost all of the samples. However, the temperature dependence of K for PP-C sample showed a quite different feature, having a very small value of AH,. Note that the values of AH,,30 N 50 kcal/mol) are comparable to those of the activation energy AH,, of molten pitch viscosity[46]. Moreover, the softening point listed in Table 1, a measure for pitch fluidity, shows a qualitative interrelation with the temperature dependence of K; In Figs. 1 and 2, for each group of petroleum and coal tar pitches, the sequence of the conductivity curves from left to right is in good accordance with increasing softening point, although there is an exception between CP-2 and CP-3. Those experimental facts suggests that the electrical d.c. conductivity might be intimately related to the fluidity of molten pitch. The molecular consideration for the interrelation between 4 and K is provided as follows. The electrical d.c. conductivity in rate processes can be generally related to the diffusion coefficient D of the carriers by the microscopic Nernst-Einstein equation[2]
10
0
3
,” x y
8
6
0 60
80
(2) where n and q are the concentration and charge of the carrier, respectively, and k is the Boltzman constant. In eqn (2), the diffusion coefficient D for a simple spherical carrier can be expressed by the Stokes-Einstein equation[3]:
D=kT
6nqa'
(3)
100
120 0
140
(“C)
Fig. 2. The temperature dependences of electrical d.c. conductivity K for the coal tar pitches.
Table 1. Empirical parameters (K,,, AH,) for electrical d.c. conductivity and softening point (Ring & Ball method)
where r~is the shear viscosity of pitch and a is the radius of the spherical carrier. Combining eqn (2) with eqn (3) gives
K,(Sal
-1
)
AHK(kcal/mol)
softening point(OC)
PP-1
1.58 x lO=
51.3
116.6
PP-2
1.76 x ld4
41.4
79.5
PP-c
1.29 x 102
22.5
92.3
CP-1
8.84 x 18
31.5
84.5
CP-2
1.00 x lff
31.2
109.0
CP-3
1.19 x l&l
37.6
101.0
Equation (4) is significant for expressing the interrelation of K with pitch fluidity 4.7 Namely, K is proportional to the
CP-4
1.76 x ld4
41.4
90.5
tin eqns (3) and (4) Newtonian fluid flow is assumed. This is a reasonable assumption for isotropic pitches, especially at infinitely low shear rate regions. In the context of this paper, the processes of diffusion and conduction always take place at negligibly low rates of shear. Therefore, the Newtonian fluid flow assumption in eqns (3) and (4) is valid for molten isotropic pitches. This assumption would not hold, however, for molten mesophase pitches which show a marked non-Newtonian fluid flow even at low rates of shear.
fluidity. In other words, the rapid increase in K in Figs. 1 and 2 means the rapid increase in the fluidity, and hence the rapid decrease in n with increasing temperature. By comparing eqn (1) with eqn (4), AH, is considered to be identical with AH,,, as supported by the present experimental results[46]. In conclusion, the electrical d.c. conductivity of molten pitch has a high potential for studying pitch fluidity, for example, the establishment of a novel electrical technique for determining softening point, the quality control of
KZ--
nq2
6nqa
=-VI2.4. 6na
Letters
to the Editor
molten pitch in engineering processes through measuring the electrical voltage drop of a probe inserted into the container of molten pitch, and the rheological studies of the mechanisms of mesophase formation in pitches at elevated temperatures, etc. Although, in this investigation, we studied the temperature dependence of K for only isotropic pitches, it will also be interesting to investigate the ~-0 relation for mesophase pitches because of their intrinsic properties in an electric field. An investigation to characterize the carriers causing the electrical conductivity of molten pitches is now in progress in our laboratory by measuring the relaxation time of the carriers in the conduction process.
-18
_‘5 2 -20
-22
y -24
c
School of Materials Science Toyohashi University of Technology Tempaku-cho Toyohashi, Aichi 440, Japan
MOTOTSUGLJ SHINA MICHIO
SAKAI
d
HIROTA INAGAKI -26
-28
REFERENCES I. M. Sakai, Y. Ushijima and M. Inagaki, The 7th Annual Meeting of Japan Carbon Society. Tokyo (1980). 2. Physics of Electrolytes (Edited by J. Hladik), Vol. 1, Chap. 5, Academic Press, New York (1972). 3. A. Einstein, Investigations on The Theory of The Brownian Movement. Dover Publications, New York (1965). 4. M. Sakai and M. Inagaki, Carbon 19, 37 (1981). 5. F. F. Nazem, Fuel 59, 851 (1980). 6. F. F. Nazem, Carbon 20, 345 (1982).
-30 2.3
2.4
2.5
2.6
2.7
2.9 (K-l)
1000/T
Fig.
2.8
3. The semilogarithmic plots of the electrical conductivity K vs inverse temperature l/T.
Carbon Vol. 22. No. 2, pp. 1X9-191, 1984 Printed in Great Britain.
C
d.c.
ooO8-6223/84 $3.00+ .OO 1984 Pergamon Press Ltd.
Soot derived from the detonation of a trinitrotoluene charge (Received 11 August 1983) Since 2,4,6_trinitrotoluene (TNT) is an oxygen-deficient explosive, a detonation of the TNT charge permits derivation of a kind of soot. Regardless of this well-known fact, little is reported about the features of the soot. In the present letter we describe the morphological features of the soot derived from the detonation of the TNT charge which are distinct from those of so-called carbon blacks produced by usual techniques. The TNT charge was prepared by pressing finely powdered TNT (melting point: 79.680.5”C; particle size: less than 0.15mm). The detonation of the TNT charge was carried out in a semi-closed vessel using the assembly illustrated in Fig. I. The amount of soot which could be recovered in one detonation was so small that we had to prepare the amount of soot required for the present experiment in several runs. The detonator fragments in the raw soot were removed by treating with acetone. The soot was further boiled with 20% hydrochloric acid to dissolve some impuritites occluded in the soot; it was then washed repeatedly with distilled water and was finally dried at 100°C. A transmission electron micrograph of the soot derived from the detonation of the TNT charge is shown in Fig. 2(A) and a high magnification micrograph in Fig. 2(B).
Figure 2(B) indicates that the soot is mainly made up of stacks consisting of 5-10 carbon layers. The appearance of the carbon layers is not flat but is curved irregularly to such an extent that the existence of graphite crystallites is not appreciable. X-Ray examination indicates that the profile corresponding to the (002) diffraction peak of the soot was extremely broad. On the other hand, it is well known that, by employing processes of thermal decomposition or incomplete burning of hydrocarbons in the gaseous phase, the soots derived become spherical particles. The appearance of the spherical particles has generally been explained in terms of droplet formation based on the views of hydrocarbon polymerization[l, 21. It was therefore decided to examine whether the soot formed by burning the TNT charge in air consist of spherical particles or not. A transmission electron micrograph of the soot is illustrated in Fig. 2(C). In Fig. 2, comparing (A) with (B), it can be confidently stated that there is a substantial difference in the soot formatlonprocesses of detonation and burning. Therefore, the unspherical appearance of the soot derived from the detonation is not explicable by the droplet formation mechanism.
Letters to the Editor It follows, therefore that the formation energy on the surface is less than the formation energy in the bulk. A the~ochemical consideration may also be given to the abstraction of a carbon from the basal plane by an 0 atom. The following hypothetical path may be used for the abstraction process: C(solid) 2 C(g) + O(g) f: CO(g) At 7OO’C,E, is 171Sl kcal/mole for heat of sublimation and E2 = -258.& kcal/mole for heat of reaction between C and 0 atoms. The overall energy E is E, -t E, = - 86.54 kcal/mole, or 3.76 eV per ato; This energ; is qiite close to the Ek values in Table 1. Ackno&&emenr-This work is supported by the National Science Foundation under Grant CPE-8120569.
FEILINC CEN RALPH
T.
YANG
Deportment of Chemicul ~ngine~ing State University of New York at Buflaio Amherst NY 14214 U.S.A. REFERENCES 1. G. R. Hennig, J. Chem. Phys. 40, 2877 (1964). 2. E. L. Evans and J. M. Thomas, 3rd Co& Carbon and Graphite, DD. 3-9. Society Chemical Industry, London (19?1). -3. K. L. Yang and R. T. Yang, AZChE J. in press (1984). 4. C. Baker and A. Kelly, Nature, 193, 235 (1962). 5. C. A. Coulson and M: D. Poole, Carbon i, 275 (1964). 6. P. A. Thrower and R. M. Mayer, Phys. Status Soiidi 47, 11 (1978).
om-6223/U 163.00+ .Ga Q 1984 Pergamon Press Ltd.
Curbon Vol. 22. No. 2, PP. 187-189, 1984 ~nnted in Great Britain.
Electricaf
d.c. conductivity of molten pitches
The fluidity & = inverse viscosity, l/q) of molten pitches is discussed in conjunction with the interrelation of Cpwith the electrical d.c. conductivity K. It will be shown that K is an indicator of the fluidity of molten pitch. The electrical method we proposed relies on the detection of an electrical voltage drop in an electrical cell, and it is simpler and more convenient than the usual rheological methods for studying the fluidity of molten pitches; especially in the high temperature region where the quantitative measurement of rheological parameters is difficult. in general, pitch is known as a typical insulator and its electrical conductivity has been considered to be very small. Our recent investigation on the dielectric loss of pitches[l], however, revealed that molten pitches show sizeable values of electrical conductivity in the low frequency region of an applied sinusoidal electrical field, and that the conductivity K increases with increasing temperature. Although, at the present, we do not know the exact nature of the carriers, it can be presumed that the fluidity of molten pitch would have a strong influence on the mobility of the carriers under electrical field. Hence, the electrical conductivity might be used as a convenient indicator for pitch fluidity. Such an electrical method would be effective for practical purposes and en~n~~ng uses. The electrical d.c. conductivity K was measured over a wide range of temperature (70-i6O”C) for three kinds of petroleum pitches (PP-1. PP-2 and PP-C) and for four kinds of coal tar pitches (CP-1, CP-2, CP-j and CP-4) by using double disk plate electrodes 40.0mm in dia., which were made of gilded brass. The thickness of the sample inserted between the electrodes was kept at 1.0 mm during the measurement of K. The electrodes with the pitch sample were placed in a stainless steel chamber which was immersed in an oil bath thermocontrolled within kO.02”C using a mercury temperature regulator. The temperature of the sample was measured by a CA thermocouple attached to the electrode disk. The reproducibility of the measured values of ti was within & 5% for all of the samples in the temperature ranges. This is a good reproducibility, especially considering the possible disturbances because of the evolution of tow molecular weight pitch components during the measurements.
8-
j-
t-
,__
I--
160 0
(“Cl
Fig. 1. The temperature dependences of electrical d.c. conductivity K for the petroleum pitches.
188
Letters to the Editor
The temperature dependences of K are shown in Fig. 1 for petroleum pitches and in Fig. 2 for coal tar pitches. The values of K for all of the pitch samples increase with increase in temperature. In order to ascertain the conductivity mechanism, the values of K for all sample pitches are plotted semilogarithmically in Fig. 3 against the inverse Kelvin temperature l/T. The linear relations in Fig. 3 imply that the rate processes would be dominated by the electrical d.c. conductivity in molten pitches, the equation of which is expressed as K=
Kg.
exp ( - AHJRT).
-
rl
18
'E Lb - 16
1
(1)
The experimental parameters, K~ and AH,., for each pitch sample are listed in Table 1. The values of AH, range from 30 to 50 kcal/mol for almost all of the samples. However, the temperature dependence of K for PP-C sample showed a quite different feature, having a very small value of AH,. Note that the values of AH,,30 N 50 kcal/mol) are comparable to those of the activation energy AH,, of molten pitch viscosity[46]. Moreover, the softening point listed in Table 1, a measure for pitch fluidity, shows a qualitative interrelation with the temperature dependence of K; In Figs. 1 and 2, for each group of petroleum and coal tar pitches, the sequence of the conductivity curves from left to right is in good accordance with increasing softening point, although there is an exception between CP-2 and CP-3. Those experimental facts suggests that the electrical d.c. conductivity might be intimately related to the fluidity of molten pitch. The molecular consideration for the interrelation between 4 and K is provided as follows. The electrical d.c. conductivity in rate processes can be generally related to the diffusion coefficient D of the carriers by the microscopic Nernst-Einstein equation[2]
10
0
3
,” x y
8
6
0 60
80
(2) where n and q are the concentration and charge of the carrier, respectively, and k is the Boltzman constant. In eqn (2), the diffusion coefficient D for a simple spherical carrier can be expressed by the Stokes-Einstein equation[3]:
D=kT
6nqa'
(3)
100
120 0
140
(“C)
Fig. 2. The temperature dependences of electrical d.c. conductivity K for the coal tar pitches.
Table 1. Empirical parameters (K,,, AH,) for electrical d.c. conductivity and softening point (Ring & Ball method)
where r~is the shear viscosity of pitch and a is the radius of the spherical carrier. Combining eqn (2) with eqn (3) gives
K,(Sal
-1
)
AHK(kcal/mol)
softening point(OC)
PP-1
1.58 x lO=
51.3
116.6
PP-2
1.76 x ld4
41.4
79.5
PP-c
1.29 x 102
22.5
92.3
CP-1
8.84 x 18
31.5
84.5
CP-2
1.00 x lff
31.2
109.0
CP-3
1.19 x l&l
37.6
101.0
Equation (4) is significant for expressing the interrelation of K with pitch fluidity 4.7 Namely, K is proportional to the
CP-4
1.76 x ld4
41.4
90.5
tin eqns (3) and (4) Newtonian fluid flow is assumed. This is a reasonable assumption for isotropic pitches, especially at infinitely low shear rate regions. In the context of this paper, the processes of diffusion and conduction always take place at negligibly low rates of shear. Therefore, the Newtonian fluid flow assumption in eqns (3) and (4) is valid for molten isotropic pitches. This assumption would not hold, however, for molten mesophase pitches which show a marked non-Newtonian fluid flow even at low rates of shear.
fluidity. In other words, the rapid increase in K in Figs. 1 and 2 means the rapid increase in the fluidity, and hence the rapid decrease in n with increasing temperature. By comparing eqn (1) with eqn (4), AH, is considered to be identical with AH,,, as supported by the present experimental results[46]. In conclusion, the electrical d.c. conductivity of molten pitch has a high potential for studying pitch fluidity, for example, the establishment of a novel electrical technique for determining softening point, the quality control of
KZ--
nq2
6nqa
=-VI2.4. 6na
Letters
to the Editor
molten pitch in engineering processes through measuring the electrical voltage drop of a probe inserted into the container of molten pitch, and the rheological studies of the mechanisms of mesophase formation in pitches at elevated temperatures, etc. Although, in this investigation, we studied the temperature dependence of K for only isotropic pitches, it will also be interesting to investigate the ~-0 relation for mesophase pitches because of their intrinsic properties in an electric field. An investigation to characterize the carriers causing the electrical conductivity of molten pitches is now in progress in our laboratory by measuring the relaxation time of the carriers in the conduction process.
-18
_‘5 2 -20
-22
y -24
c
School of Materials Science Toyohashi University of Technology Tempaku-cho Toyohashi, Aichi 440, Japan
MOTOTSUGLJ SHINA MICHIO
SAKAI
d
HIROTA INAGAKI -26
-28
REFERENCES I. M. Sakai, Y. Ushijima and M. Inagaki, The 7th Annual Meeting of Japan Carbon Society. Tokyo (1980). 2. Physics of Electrolytes (Edited by J. Hladik), Vol. 1, Chap. 5, Academic Press, New York (1972). 3. A. Einstein, Investigations on The Theory of The Brownian Movement. Dover Publications, New York (1965). 4. M. Sakai and M. Inagaki, Carbon 19, 37 (1981). 5. F. F. Nazem, Fuel 59, 851 (1980). 6. F. F. Nazem, Carbon 20, 345 (1982).
-30 2.3
2.4
2.5
2.6
2.7
2.9 (K-l)
1000/T
Fig.
2.8
3. The semilogarithmic plots of the electrical conductivity K vs inverse temperature l/T.
Carbon Vol. 22. No. 2, pp. 1X9-191, 1984 Printed in Great Britain.
C
d.c.
ooO8-6223/84 $3.00+ .OO 1984 Pergamon Press Ltd.
Soot derived from the detonation of a trinitrotoluene charge (Received 11 August 1983) Since 2,4,6_trinitrotoluene (TNT) is an oxygen-deficient explosive, a detonation of the TNT charge permits derivation of a kind of soot. Regardless of this well-known fact, little is reported about the features of the soot. In the present letter we describe the morphological features of the soot derived from the detonation of the TNT charge which are distinct from those of so-called carbon blacks produced by usual techniques. The TNT charge was prepared by pressing finely powdered TNT (melting point: 79.680.5”C; particle size: less than 0.15mm). The detonation of the TNT charge was carried out in a semi-closed vessel using the assembly illustrated in Fig. I. The amount of soot which could be recovered in one detonation was so small that we had to prepare the amount of soot required for the present experiment in several runs. The detonator fragments in the raw soot were removed by treating with acetone. The soot was further boiled with 20% hydrochloric acid to dissolve some impuritites occluded in the soot; it was then washed repeatedly with distilled water and was finally dried at 100°C. A transmission electron micrograph of the soot derived from the detonation of the TNT charge is shown in Fig. 2(A) and a high magnification micrograph in Fig. 2(B).
Figure 2(B) indicates that the soot is mainly made up of stacks consisting of 5-10 carbon layers. The appearance of the carbon layers is not flat but is curved irregularly to such an extent that the existence of graphite crystallites is not appreciable. X-Ray examination indicates that the profile corresponding to the (002) diffraction peak of the soot was extremely broad. On the other hand, it is well known that, by employing processes of thermal decomposition or incomplete burning of hydrocarbons in the gaseous phase, the soots derived become spherical particles. The appearance of the spherical particles has generally been explained in terms of droplet formation based on the views of hydrocarbon polymerization[l, 21. It was therefore decided to examine whether the soot formed by burning the TNT charge in air consist of spherical particles or not. A transmission electron micrograph of the soot is illustrated in Fig. 2(C). In Fig. 2, comparing (A) with (B), it can be confidently stated that there is a substantial difference in the soot formatlonprocesses of detonation and burning. Therefore, the unspherical appearance of the soot derived from the detonation is not explicable by the droplet formation mechanism.