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Kinetics of the graphitization-induced dimensional changes of artificial carbons
Cubon,
1976, Vol. 14, pp. 267-270.
Pergamon Press.
Printed in Great Britain
KINETICS OF THE GRAPHITIZATION-INDUCED DIMENSIONAL CHANGES OF ARTIFICIAL CARBONS K. J. HUNGERS and U. ROSENBLA~T &hunk & Ebe GmbH., D-63Giegen, West Germany (Received
24 November
1975)
Abstract-The graphitization shrinkage of seven molded artit%% carbons made from a broad spectrum of filler materials with thermoplastic or thermosetting binders and baked to 1200°Cwas measured pe~ndic~~ to the grain orientation. Heat treatments were ~~orrned isotherm~y between 1500and 2900°Cfor times between 0.5 and 4 hr. It is shown that the shrinkage rates of all the carbons can be described by a general kinetic law. The total shrinkage after infinite isothermal time is found to be temperature dependent. The characteristic temperature dependence of the shrinkage is determined by the binder system. The filler and other manufacturing parameters only influence the amount of shrinkage.
Graphitization in “soft” carbons now seems to be accepted as a thermally activated kinetic transformation of an imperfect graphitic lattice to a three-dimensionally ordered graphitic structure [l]. Given enough time this transformation process isothermally proceeds to completion, at least at tempe~tures above about 2000°C. However, the morphology may impose practical limits on the development of structural perfection. For example, in synthetic carbons, the filler coke in the bulk carbon does not reach the same degree of graphitization as isolated coke particles graphitized under the same conditions [2]. hlacroscopic bulk shrinkage causes problems in the technological ~p~tization of artificial baked carbon materials. It is well known from industrial experience that this shrinkage depends to a large extent on the filler used. For example, shrinkage is much greater with carbon black than with petroleum coke fillers. The linkage between the filler particles and the binder may also be impor~nt. Little information is available on the influence of binder coke shrinkage. In practice, the rate as well as the total amount of shrinkage occurring at constant heating rate is important. Temperature gradients occur, especially in the initial range of heating when the carbon is imperfect and the thermal diffusivity is low, and the resulting thermal stresses increase with increasing shrinkage rate. Therefore, a better understanding of shrinkage behaviour can contribute to understanding of thermal stresses failure of large carbon blocks as well as to determination of an optimum graphitization temperature program. For the above reasons, the kinetics of the graphitization-induced shrinkage of several carbons comprising either thermoplastic or thermosetting binders and a wide variety of filler materials (pitch and raw coke, carbon black coke, electrographite and carbon black) has
The seven different types of molded carbons that were studied are listed in Table 1. All were baked to 1200°C. Previous measurements of graphitization-induced shrinkage with a high temperature, recording, vertical push-rod dilatometer [3] have been reported elsewhere [4]. Good curves may be obtained by this method, especially at constant heating rate. However, quantitative evaluation of the kinetics is difficult because of corrections required for the continuously changing thermal expansion contribution. In the present study, micrometer measurements of sample length were made at room temperature before and after isothermal heat treatment in a graphite tube furnace which could be heated at an average rate of about 2OO“C/min.Treatment temperatures were between 1500 and 29qO”Cwith residence times of 0.5 to 4 hr. Samples 7 x7 x 30mm’ were cut perpendicular to the grain orientation from large blocks. 3. REsULrsANDANAtysrs Typical results are given in Figs. 1 and 2 where sample length is plotted against treatment time at various temperatures for materials B (thermoplastic pitch binder) and E (thermosetting binder) respectively. Similar length change curves were found for all of the carbons tested. In general, there is a rapid initial Iength decrease, but the shrinkage
Type A B C D E F G
267
to reach
a saturation
value
Table 1. Composition of the artiticial carbons
been investigated.
@resent address: Institut ftir Chemische Technik der UniversitSt, D-75 Karlsruhe, Kaiserstr. 12, West Germany,
appears
Filler Coke (pitch) Coke (raw) Electrographite Electrographite t Coke Electrographite + Carbon black Coke (carbon black) Coke (pitch)
Binder Thermopiastic Thermoplastic Thermosetting Thermosetting Thermosetting ~ermoplastic Thermoplastic
for
268
K. J. I
‘10
H~TTINGER and
U.
ROSENBLATI
Plots of 1, vs t-“’ are straight lines with intercept 1, at t = 00 in accordance with this equation, as shown for carbon E in Fig. 3. Similar plots were found for all of the
I
carbons studied. The final shrinkage values after infinite treatment time are seen to be temperature dependent and may evidently be determined by linear extrapolation from data for t ~Shr. Isothermal shrinkage curves measured at various temperatures may be superimposed by normalizing them to the total shrinkage at that temperature, (IO- I& This is shown in Fig. 4 for material E by the curve labeled f(t), referred to the linear time axis at the bottom. The new rate equation is:
29.5 E I I
29.0
L 0
2
L
6
d[G - Im)/(lo- L)l/dt = - k’[(lt - L)/(l, - LX.
t.h
Fig. 1. Length change of carbon type B after graphitization at constant temperatures. 30.0
I
\ 10
f
Integration between the limits (1,- I.&lo - I_) = 1 at time zero and (1 - L)/(L,- I,) at time t yields:
I Type
‘. -._ ‘0 \ io-._ l.-.-
(4)
(1,- lJ2-(lo-L-*=
E 17LO%
This may be simplified in the same manner as eqn (2) and plots of (I, - l,)/(l,- L) vs t-l’* are linear through the origin as shown in Fig. 4 by the curve labeled f(t-I”), referred to the scale at the top.
1675 2000
29. I I
(9
2k’t(I,- lJ2.
2270 2LOO 2160
‘I.
2610 2780 2900
29.C I
Type E .-
-A_-0-o_.-I-
I
2
17L0°C
o/ . /
1875 2000
-!+ t,h Fig. 2. Length change of carbon type E after graphitization at constant temperatures.
treatment times above 4 hr. The observed behaviour can
be described by a third order rate law in terms of the parameter (I, - L,) where l, and I, are the sample lengths after treatment times f and infinity, respectively:
29.0
1
44 - I,)/dt = - /c(&- l-)3.
(1)
t .h 03
(2)
1
I
I
112
114
t ,h Fig. 3. Results of Fig. 2 plotted against t-“2 according to eqn (4).
Integration between the limits (IO- I,) at time zero and (It - L) at time r, where I, is the initial length, yields: 1I = 1m+ [2kt + (lo - Im)-*I-“* f
1
II
L2
co
43 21.5 I,,!
1
1
OS
f
The results showed that (lo- l_)-*<2kt, even for short treatment times (see Table 2), so eqn (2) may be simplified to: 1 = I, + (2&r)-“*.
(3)
Table 2. Examples for (1, - I,)-’ and 2kt on carbon type Eat 2400°C k
2.1
2kt
(mm-* hr-‘)
(b:)
(mm-‘)
46
0.1 0.5 1
9.2 46 92
Fig. 4. Superposition of the isothermal shrinkage curves of carbon type E. tr”* scale for f(r-I”) at top; linear t scale for f(r) at bottom.
269
Kinetics of the graphitization-induced dimensional changes of artificial carbons
The temperature dependence of the kinetics of the shrinkage may be calculated as a consequence of the superposition of the isothermal curves. According to Fig. 3, (I, - L) is temperature dependent. Writing eqn (2) in the form (1 - Im)-’-(IO - La-* = 2kt
(24
it may be combined with eqn (5) to give k’lk = (I, - I,)’ = (kl,/kd exp (- AEIRT)
(6)
where the right hand term comes from the Arrhenius equation for the rate constant. The apparent activation energy AE may be obtained from the slope of a plot of 2ln(I,- I-) vs l/T. For all carbons with thermosetting binders a single activation energy of 33 kcal/mole is found for the entire temperature range 1700-29OOY, as illustrated in Fig. 5. For carbons with thermoplastic binders, Fig. 6, two regions with different slopes are found. For all of these carbons the activation energy is 16kcal/mole at low temperatures and 5 kcal/mole in the high temperature range. Due to different filler materials, the total shrinkage of type G (pitch coke) is quite different from that of type F (carbon black coke). Nevertheless, the break in the Arrhenius plot occurs at the same temperature of about
-"0_
_t
0.5 103/T, OK-'
Fig. 5. Arrhenius plot according to eqn (9) for two carbons with a thermosetting binder system.
0.1
2300°C for these two carbons. The meaning of this break needs further investigation. Carbon B with a raw coke filler is an exception, apparently because the filler and binder are not distinctly different phases. considerations demonstrate that the These graphitization-induced bulk shrinkage of various types of conventional baked carbons can be.described by a general self-consistent kinetic analysis. This may be taken as a confirmation that the L-values are really temperature dependent. This important result is shown in Fig. 7 which summarizes the L-values as a function of treatment temperature for all of the carbons tested so far. Two different types of behaviour can be recognized: (a) for all of the carbons with a thermosetting binder, total shrinkage causes a nearly linear decrease of length with temperature; (b) for carbons with a thermoplastic binder, most of the shrinkage occurs in the lower temperature range, below about 2ooo”C. This latter behaviour makes such carbons difficult to graphitize, especially large blocks, because thermal stresses resulting from small temperature gradients may cause failure. However, knowledge of the shrinkage behaviour offers the possibility of optimizing the temperature program during heating. 4. CONCLUSIONS
The shrinkage of a broad spectrum of molded artificial carbons perpendicular to grain orientation has been measured after isothermal heat treatments between 1500 and 2900°C. The shrinkage behatiuLof all of these carbons can be represented by a self-consistent empirical description. This offers a possibility to determine the total (infinite time) isothermal shrinkage of each carbon from data for treatment times not exceeding 5 hr. Total shrinkage is found to be temperature dependent. Isothermal shrinkage curves for the same carbon at different temperatures may be superimposed if the shrinkage values (1,-I& are divided by the total shrinkage (lo- I& A rate analysis based on this observation revealed that the shrinkage characteristics of a carbon are determined by the binder system used, i.e. thermosetting or thermoplastic. The shrinkage kinetics of carbons with a thermosetting binder may be characterized by a single apparent activation energy of 33 kcal/mole over the entire range 1700-2900°C.For carbons made with 30.0
- -0.6
- -0.8
t
\
I
I
1500
2000 -
I
10
E -6
29.5
--1.0
I
29.0
-0.8
-1.6
L
^_
0.3
04 -
0.5 lO'/T."K-'
Fig. 6. Arrhenius plot according to eqn (9)for three carbons with a thermoplastic binder system.
2500 T,"C
Fig. 7. Final length of all artificial carbons after isothermal heattreatment at increasing temperatures (from plots according to Fig. 3).
270
K. J. H~ITINGER and U. ROSENBLATT
a thermoplastic binder, two temperature ranges are found with different apparent activation energies, namely 16 kcal/mole up to 23OOTand 5 kcal~mole between 2300 and 29OOT. The dominant influence of the binder system on shrinkage is also shown by the temperature dependence of the total shrinkage. This parameter increases linearly with temperature over the range studied for all carbons with a thermosetting binder. In contrast, for carbons with a ~e~oplastic binder most of the shrinkage is completed below about 2OWC. Filler quality, grain size and other manufacturing parameters influence only the amount of bulk shrinkage. Although these preliminary results on the graphitization-induced dimensional changes of carbons need theoretical expl~ation, they may be imme~tely
useful for controlling technical graphitization processes. It appears possible to calculate and optimize graphitization schedules on the basis of very simple meas~ements. This will be the topic of another paper. Acknowledgement--The authors wish to thank the editors for helpful assistance.
1. D. B. Fisch~ch. In Chemistryand Fhysics of Carbon (Edited by P. L. Walker, Jr.), Vol. 7. Marcel Dekker, New York (1970). 2. F. Roberts, Chem.Ing. Techn. 36, 849 (PM). 3. C. Busche and K. J. Htittinger, &hunk & E&e-Hefte 27, 13 (1974). 4. K. J. Hiittinger and U. Rosenblatt, Extended Abstracts, 12th Biennal Conference on Carbon, Pittsbur~, Penn. (1975).
1976, Vol. 14, pp. 267-270.
Pergamon Press.
Printed in Great Britain
KINETICS OF THE GRAPHITIZATION-INDUCED DIMENSIONAL CHANGES OF ARTIFICIAL CARBONS K. J. HUNGERS and U. ROSENBLA~T &hunk & Ebe GmbH., D-63Giegen, West Germany (Received
24 November
1975)
Abstract-The graphitization shrinkage of seven molded artit%% carbons made from a broad spectrum of filler materials with thermoplastic or thermosetting binders and baked to 1200°Cwas measured pe~ndic~~ to the grain orientation. Heat treatments were ~~orrned isotherm~y between 1500and 2900°Cfor times between 0.5 and 4 hr. It is shown that the shrinkage rates of all the carbons can be described by a general kinetic law. The total shrinkage after infinite isothermal time is found to be temperature dependent. The characteristic temperature dependence of the shrinkage is determined by the binder system. The filler and other manufacturing parameters only influence the amount of shrinkage.
Graphitization in “soft” carbons now seems to be accepted as a thermally activated kinetic transformation of an imperfect graphitic lattice to a three-dimensionally ordered graphitic structure [l]. Given enough time this transformation process isothermally proceeds to completion, at least at tempe~tures above about 2000°C. However, the morphology may impose practical limits on the development of structural perfection. For example, in synthetic carbons, the filler coke in the bulk carbon does not reach the same degree of graphitization as isolated coke particles graphitized under the same conditions [2]. hlacroscopic bulk shrinkage causes problems in the technological ~p~tization of artificial baked carbon materials. It is well known from industrial experience that this shrinkage depends to a large extent on the filler used. For example, shrinkage is much greater with carbon black than with petroleum coke fillers. The linkage between the filler particles and the binder may also be impor~nt. Little information is available on the influence of binder coke shrinkage. In practice, the rate as well as the total amount of shrinkage occurring at constant heating rate is important. Temperature gradients occur, especially in the initial range of heating when the carbon is imperfect and the thermal diffusivity is low, and the resulting thermal stresses increase with increasing shrinkage rate. Therefore, a better understanding of shrinkage behaviour can contribute to understanding of thermal stresses failure of large carbon blocks as well as to determination of an optimum graphitization temperature program. For the above reasons, the kinetics of the graphitization-induced shrinkage of several carbons comprising either thermoplastic or thermosetting binders and a wide variety of filler materials (pitch and raw coke, carbon black coke, electrographite and carbon black) has
The seven different types of molded carbons that were studied are listed in Table 1. All were baked to 1200°C. Previous measurements of graphitization-induced shrinkage with a high temperature, recording, vertical push-rod dilatometer [3] have been reported elsewhere [4]. Good curves may be obtained by this method, especially at constant heating rate. However, quantitative evaluation of the kinetics is difficult because of corrections required for the continuously changing thermal expansion contribution. In the present study, micrometer measurements of sample length were made at room temperature before and after isothermal heat treatment in a graphite tube furnace which could be heated at an average rate of about 2OO“C/min.Treatment temperatures were between 1500 and 29qO”Cwith residence times of 0.5 to 4 hr. Samples 7 x7 x 30mm’ were cut perpendicular to the grain orientation from large blocks. 3. REsULrsANDANAtysrs Typical results are given in Figs. 1 and 2 where sample length is plotted against treatment time at various temperatures for materials B (thermoplastic pitch binder) and E (thermosetting binder) respectively. Similar length change curves were found for all of the carbons tested. In general, there is a rapid initial Iength decrease, but the shrinkage
Type A B C D E F G
267
to reach
a saturation
value
Table 1. Composition of the artiticial carbons
been investigated.
@resent address: Institut ftir Chemische Technik der UniversitSt, D-75 Karlsruhe, Kaiserstr. 12, West Germany,
appears
Filler Coke (pitch) Coke (raw) Electrographite Electrographite t Coke Electrographite + Carbon black Coke (carbon black) Coke (pitch)
Binder Thermopiastic Thermoplastic Thermosetting Thermosetting Thermosetting ~ermoplastic Thermoplastic
for
268
K. J. I
‘10
H~TTINGER and
U.
ROSENBLATI
Plots of 1, vs t-“’ are straight lines with intercept 1, at t = 00 in accordance with this equation, as shown for carbon E in Fig. 3. Similar plots were found for all of the
I
carbons studied. The final shrinkage values after infinite treatment time are seen to be temperature dependent and may evidently be determined by linear extrapolation from data for t ~Shr. Isothermal shrinkage curves measured at various temperatures may be superimposed by normalizing them to the total shrinkage at that temperature, (IO- I& This is shown in Fig. 4 for material E by the curve labeled f(t), referred to the linear time axis at the bottom. The new rate equation is:
29.5 E I I
29.0
L 0
2
L
6
d[G - Im)/(lo- L)l/dt = - k’[(lt - L)/(l, - LX.
t.h
Fig. 1. Length change of carbon type B after graphitization at constant temperatures. 30.0
I
\ 10
f
Integration between the limits (1,- I.&lo - I_) = 1 at time zero and (1 - L)/(L,- I,) at time t yields:
I Type
‘. -._ ‘0 \ io-._ l.-.-
(4)
(1,- lJ2-(lo-L-*=
E 17LO%
This may be simplified in the same manner as eqn (2) and plots of (I, - l,)/(l,- L) vs t-l’* are linear through the origin as shown in Fig. 4 by the curve labeled f(t-I”), referred to the scale at the top.
1675 2000
29. I I
(9
2k’t(I,- lJ2.
2270 2LOO 2160
‘I.
2610 2780 2900
29.C I
Type E .-
-A_-0-o_.-I-
I
2
17L0°C
o/ . /
1875 2000
-!+ t,h Fig. 2. Length change of carbon type E after graphitization at constant temperatures.
treatment times above 4 hr. The observed behaviour can
be described by a third order rate law in terms of the parameter (I, - L,) where l, and I, are the sample lengths after treatment times f and infinity, respectively:
29.0
1
44 - I,)/dt = - /c(&- l-)3.
(1)
t .h 03
(2)
1
I
I
112
114
t ,h Fig. 3. Results of Fig. 2 plotted against t-“2 according to eqn (4).
Integration between the limits (IO- I,) at time zero and (It - L) at time r, where I, is the initial length, yields: 1I = 1m+ [2kt + (lo - Im)-*I-“* f
1
II
L2
co
43 21.5 I,,!
1
1
OS
f
The results showed that (lo- l_)-*<2kt, even for short treatment times (see Table 2), so eqn (2) may be simplified to: 1 = I, + (2&r)-“*.
(3)
Table 2. Examples for (1, - I,)-’ and 2kt on carbon type Eat 2400°C k
2.1
2kt
(mm-* hr-‘)
(b:)
(mm-‘)
46
0.1 0.5 1
9.2 46 92
Fig. 4. Superposition of the isothermal shrinkage curves of carbon type E. tr”* scale for f(r-I”) at top; linear t scale for f(r) at bottom.
269
Kinetics of the graphitization-induced dimensional changes of artificial carbons
The temperature dependence of the kinetics of the shrinkage may be calculated as a consequence of the superposition of the isothermal curves. According to Fig. 3, (I, - L) is temperature dependent. Writing eqn (2) in the form (1 - Im)-’-(IO - La-* = 2kt
(24
it may be combined with eqn (5) to give k’lk = (I, - I,)’ = (kl,/kd exp (- AEIRT)
(6)
where the right hand term comes from the Arrhenius equation for the rate constant. The apparent activation energy AE may be obtained from the slope of a plot of 2ln(I,- I-) vs l/T. For all carbons with thermosetting binders a single activation energy of 33 kcal/mole is found for the entire temperature range 1700-29OOY, as illustrated in Fig. 5. For carbons with thermoplastic binders, Fig. 6, two regions with different slopes are found. For all of these carbons the activation energy is 16kcal/mole at low temperatures and 5 kcal/mole in the high temperature range. Due to different filler materials, the total shrinkage of type G (pitch coke) is quite different from that of type F (carbon black coke). Nevertheless, the break in the Arrhenius plot occurs at the same temperature of about
-"0_
_t
0.5 103/T, OK-'
Fig. 5. Arrhenius plot according to eqn (9) for two carbons with a thermosetting binder system.
0.1
2300°C for these two carbons. The meaning of this break needs further investigation. Carbon B with a raw coke filler is an exception, apparently because the filler and binder are not distinctly different phases. considerations demonstrate that the These graphitization-induced bulk shrinkage of various types of conventional baked carbons can be.described by a general self-consistent kinetic analysis. This may be taken as a confirmation that the L-values are really temperature dependent. This important result is shown in Fig. 7 which summarizes the L-values as a function of treatment temperature for all of the carbons tested so far. Two different types of behaviour can be recognized: (a) for all of the carbons with a thermosetting binder, total shrinkage causes a nearly linear decrease of length with temperature; (b) for carbons with a thermoplastic binder, most of the shrinkage occurs in the lower temperature range, below about 2ooo”C. This latter behaviour makes such carbons difficult to graphitize, especially large blocks, because thermal stresses resulting from small temperature gradients may cause failure. However, knowledge of the shrinkage behaviour offers the possibility of optimizing the temperature program during heating. 4. CONCLUSIONS
The shrinkage of a broad spectrum of molded artificial carbons perpendicular to grain orientation has been measured after isothermal heat treatments between 1500 and 2900°C. The shrinkage behatiuLof all of these carbons can be represented by a self-consistent empirical description. This offers a possibility to determine the total (infinite time) isothermal shrinkage of each carbon from data for treatment times not exceeding 5 hr. Total shrinkage is found to be temperature dependent. Isothermal shrinkage curves for the same carbon at different temperatures may be superimposed if the shrinkage values (1,-I& are divided by the total shrinkage (lo- I& A rate analysis based on this observation revealed that the shrinkage characteristics of a carbon are determined by the binder system used, i.e. thermosetting or thermoplastic. The shrinkage kinetics of carbons with a thermosetting binder may be characterized by a single apparent activation energy of 33 kcal/mole over the entire range 1700-2900°C.For carbons made with 30.0
- -0.6
- -0.8
t
\
I
I
1500
2000 -
I
10
E -6
29.5
--1.0
I
29.0
-0.8
-1.6
L
^_
0.3
04 -
0.5 lO'/T."K-'
Fig. 6. Arrhenius plot according to eqn (9)for three carbons with a thermoplastic binder system.
2500 T,"C
Fig. 7. Final length of all artificial carbons after isothermal heattreatment at increasing temperatures (from plots according to Fig. 3).
270
K. J. H~ITINGER and U. ROSENBLATT
a thermoplastic binder, two temperature ranges are found with different apparent activation energies, namely 16 kcal/mole up to 23OOTand 5 kcal~mole between 2300 and 29OOT. The dominant influence of the binder system on shrinkage is also shown by the temperature dependence of the total shrinkage. This parameter increases linearly with temperature over the range studied for all carbons with a thermosetting binder. In contrast, for carbons with a ~e~oplastic binder most of the shrinkage is completed below about 2OWC. Filler quality, grain size and other manufacturing parameters influence only the amount of bulk shrinkage. Although these preliminary results on the graphitization-induced dimensional changes of carbons need theoretical expl~ation, they may be imme~tely
useful for controlling technical graphitization processes. It appears possible to calculate and optimize graphitization schedules on the basis of very simple meas~ements. This will be the topic of another paper. Acknowledgement--The authors wish to thank the editors for helpful assistance.
1. D. B. Fisch~ch. In Chemistryand Fhysics of Carbon (Edited by P. L. Walker, Jr.), Vol. 7. Marcel Dekker, New York (1970). 2. F. Roberts, Chem.Ing. Techn. 36, 849 (PM). 3. C. Busche and K. J. Htittinger, &hunk & E&e-Hefte 27, 13 (1974). 4. K. J. Hiittinger and U. Rosenblatt, Extended Abstracts, 12th Biennal Conference on Carbon, Pittsbur~, Penn. (1975).