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Oxidation behavior of carbon-carbon composites
Carbon Vol. 17. pp. 407-410 @ Pergimon Press Ltd.. 1979.
ooo8-6223/79/1001-0/~2.~/0 Printed in Great Britain
OXIDATION BEHAVIOR OF CARBON-CARBON
COMPOSITES
HEH-WON CHANGand ROBERT M. RUSNAK Bendix Research Laboratories, Southfield, MI 48076,U.S.A. (Receiued 4 December 1978)
Abstract-This study investigates the oxidation mechanisms of carbon-carbon composite materials of the type used for aircraft brakes. The rate-controlling steps for oxidation at temperatures ranging between 450 and 750°C were determined by measuring activation energies of composite samples in bulk form and in ground form and by measuring oxidation rates as a function of reaction gas flow. Below 65O”C,the two forms of carbon samples showed considerably different activation energies, which suggests that in bulk samples, oxidation is controlled by diffusion through pores, and that as grinding reduces pore length, activation energy increases. At 75O”C,the ratio of oxidation rates was the same as the square root of the ratio of reactant flow rates. It was therefore concluded that the dominant oxidation mechanism at 750°C is diffusion of oxygen through a film of gaseous reaction products at the sample surface. The oxidation mechanism of carbon-carbon composite samples at temperatures between 450 and 650°Cis primarily controlled by the diffusion of oxygen into pores. Between 650 and 750°C.the rate-controlling mechanism undergoes a transition from diffusion through pores to diffusion through stagnant gas film.
1. INTRODUCTION Oxidation of carbon occurs by three processes: (1) chemisorption/desorption, (2) diffusion of oxygen through pores, and (3) diffusion of oxygen through a
stagnant film of reaction products to the surface of carbon[l]. Activation energy depends upon which process predominates. The chemisorption/desorption process, for instance, takes place at low temperatures and yields “true” activation energy. The pore diffusion process takes place at intermediate temperatures and produces an activation energy smaller than the “true” activation energy. The activation energy at high temperatures is even smaller than the activation energy of the pore diffusion process. Carbon-carbon composites have recently attracted considerable interest for friction and heat shield applications. Although much research has been done with various carbons to verify oxidation mechanisms, there is little data for carbon-carbon composites. This investigation was undertaken to determine rate-controlling steps in the oxidation of carbon-carbon composites used for aircraft brakes. In pure carbon, the activation energy produced by the pore diffusion mechanism is much lower than energy produced by chemisorption/desorption. Carbon-carbon composites contain impurities which reduce activation energy; therefore, in carbon-carbon composites the absolute level of activation energy cannot be used to define the controlling mechanism. In this investigation, the oxidation kinetics of carbon-carbon composites in bulk form and in powdered form are compared. The impurity level of a composite remains the same whether the composite is in bulk form, or ground to a powder. Activation energies should be the same for both forms if impurity-enhanced oxidation by chemisorption/desorption is rate-controlling. If pore diffusion is the controlling mechanism; however, then the activation energy should decrease as the ratio of pore radius (r) to pore length (I) decreases[21. Bulk samples are porous throughout the sample thickness. Consequently, grinding the sample CARBON
Vol. 17. No. 5-C
produces a significant reduction in pore length as the bulk size (0.64cm) is reduced to the size of the powder particles (less than 5 pm). If pore diffusion is the ratecontrolling mechanism in the bulk sample, significant activation energy increases should be observed in the oxidation of ground-composite samples. 2. EXPERIMENI’AL
The composite used in this study was composed of 40% carbon binder, 40% high-strength carbon fibers and 20% carbon filler. The composite material was fabricated into disc form, heated in nitrogen to convert the organic binders into carbonaceous materials, and then heattreated at temperatures ranging from 1000 to 2400°C. An induction furnace, continuously purged with 99.998% pure argon, was used for heat-treatment. In order to remove impurities (primarily HZ0 and 02), the argon was passed through a container containing JJrierite and through a container filled with magnesium chips heated to 550°C. Some of the test samples were in bulk form with dimensions of 3.18 x 2.54 x 0.64 cm (henceforth called bulk composite). The rest of the samples were in ground form with particle sizes less than 5 pm (henceforth called ground composite). The oxidation rates of ground composites were measured by thermogravimetric analysis (TGA) on a DuPont 950 analyzer, using 5mg samples. The samples were loaded in the TGA and then purged with nitrogen until the desired reaction temperature was reached. When the temperature reached the preset value, the nitrogen flow was turned off and a NO-cm3/min flow of air was turned on to begin the oxidation process. At the onset of reaction, the reaction chamber was not equilibrated with air; thus, the initial relationship between weight loss and time was nonlinear, and did not become linear until after about 5% oxidation had occurred. Rates used in this study were calculated after 5% oxidation had taken place. In addition to within-particle pores. ground samples placed on a reaction pan have interparticle pores formed
HEH-WON CHANG and ROBERTM.
408
by particte stacking. Inte~~cle
porosity should be coarser than intraparticle porosity, and interparticle porosity should not be a significant factor in oxidation. In order to verify that interparticle porosity was not a factor in oxidation, oxidation rates were measured with
Z-,3- and S-mg samples of ground composite at 6WC, using the same holder for each sample. Three different bed lengths were thus created, changing the effective length of the interp~icle porosity. The reaction kinetics of the three samples would consequently be changed if interparticle porosity is rate-controlling, Results showed that reaction rates were essentially the same for the three powdered samples, indicating that interparticle diffusion had no effect on reaction. The oxidation of bulk composites was performed in a quartz tube furnace. The samples were periodically removed from the furnace and weighed to determine weight loss.
RUSNAK
porosity, active sites and impurities. ~raphitization may be ruled out as a factor in this study because the three samples showed the same d,, spacing: 3.38 A. The effects of pore size distribution and active sites on the oxidation rates were not directly measured. Table 1 shows that ni~ogen BET surface area of the ground sample decreases form Sample A to Sample B to Sample C. Since the three samples were made of the same materials, the number of active sites is apparently propo~ional to the BET surface area, and decreases from Sample A to Sample C. Decrease in the number of active sites as a function of decrease in nitrogen BET surface area is a possible explanation for lower oxidation rates among the samples. The impu~ty content also decreases from Sample A (l.O7wt%) to Sample B (0.93wt%) to Sample C (0.19 wt%). The ashes contain oxidation accelerators such as Fe, Ca, and K as well as decelerators such as Si and Al. The most pronounced change was Fe reduction from 0.535 wt% (Sample A) to
3. RESULTS ANDDISCUSSiON
3.1 Di&sion through pores Tem~rature~e~ndence of oxidation for carboncarbon composites is shown in Fig. 1 for ground samples and in Fig. 2 for bulk samples. Samples for the study differed only in heat-treatment temperature; temperature was increased pro~essively from Sample A to Sample C. Figures 1 and 2 clearly indicate that heat-treatment at higher temperatures increases oxidation-resistance of the composites. Oxidation rates are influenced by ~aphitization,
Table 1. Surface area change with oxidafion (oxidation temperature 650°C) Oxidation level (wt%)
Surface area (m’fg)
A 23.0 198.0 229.0
0 5 IO
TEMPERATURE PC) 7BOmOaBo 6ooBlio t i
I
I-
1 3%
ntd
Fig. 1. Temperature dependence of oxidation rates for ground composites.
LOG RATE lo LOSS/g CARBONAEC)
11.0
12.0
12.0
.-J-x10’ TPK)
Fig. 2. Temperature dependence of oxidation rates for bulk composites.
Sample B 21.6 33.8 35.8
*:9 12.7 12.4
Oxidation behavior of carbon-carbon composites
is the rate-controlling step. By substitution:
Table 2. Activation energies and pre-exponential factors Sample
A B C
rate =
Activation energy (kcal/mole) Ground LogA E 5.28 35.0 5.48 39.1 43.3 6.07
Bulk LogA E IO.5 - 2.41 19.8 - 0.23 29.5 1.99
0.315 wt (Sample B) to 0.037 wt% (Sample C). Therefore
the primary effect of increasing temperature is the reduction of the level of oxidation accelerators. Consistent with observed results, reducing the level of oxidation accelerators decreases oxidation rate. Activation energies and pre-exponential factors (as derived from Figs. 1 and 2) are listed in Table 2. The activation energy of the bulk samples ranges from 10.5 to 29.5 kcal/mole (depending on heat-treatment temperature), while the range for ground samples is between 35.0 and 43.2 kcal/mole. The impurity content for both forms of carbon-carbon composites is the same, but the pore structure of the two forms is different. Thus, the difference in the activation energies between bulk and ground composites can be attributed to alteration in pore structure which affects diffusion through pores. More specifically, in grinding the sample, the length of the interconnected pores is decreased, producing a decrease in oxidation rate which is consistent with the theory[2] that asserts that the oxidation rate varies with the r/l ratio. If the sample is ground finely enough, pore diffusion will produce the same oxidation rate and activation energy as chemisorption (as altered by the presence of impurities). The ratio of activation energies for bulk Sample B to ground Sample B is shown in Table 3. The activation energy in the ground sample is twice higher than in the bulk sample. This provides evidence that diffusion through pores is the rate-controlling step. According to Wheeler121 and Walker [l], the rate for oxidation by diffusion through pores is expressed: rate = C’” +I)‘*(K&t)“*
K, = AemEiRT
(2)
Table 3. Comparison of activation energies for bulk and ground samples at reaction temperatures of 4_5&65O”C
A B C
Ground
Bulk
35.0 39.1 43.3
IO.5 19.8 29.5
(3)
Data for Sample B support the contention that activation energy for diffusion through pores should be one half lower than the level that would be expected if chemisorption/desorption were rate-controlling. There are two sources of evidence that oxidation of Sample A is influenced by oxygen-diffusion through a stagnant film. The ratio of activation energies for Sample A is less than 0.5; moreover, the activation energy for oxidation of Sample A in bulk form is 10.5kcal/mole, close to the activation energy under the gas-diffusion mechanism (less than 8 kcal/mole)[l]. Since the impurity level of Sample A is the highest, it is expected that the transition from pore diffusion to gas-diffusion mechanism occurs at a lower temperature for Sample A than for Samples B and C. The ratio for Sample C is somewhat larger than 0.5. Sample C, in bulk form, also has the lowest oxidation rate (consistent with impurity levels). Since pore diffusion is less important for carbons with low oxidation rates, it is possible that the oxidation of Sample C in bulk form is controlled by a combination of pore diffusion and chemisorption/desorption processes. The possibility that a combination of processes governs oxidation would account for the ratio 0.68 for Sample C. The absolute activation-energy values of ground samples, given in Table 2, are lower than absolute values for carbons and graphites given in the literature. For example, other investigators have recorded values of 64 kcal/mole for spectroscopic grade and polycrystalline graphite[3]; of 63 + 3 kcal/mole for glassy carbon[4]; and of 47-53 kcal/mole for highly oriented stress-recrystallized pyrolytic graphites[S]. Differences in reported values may be attributable to impurities in the samples: impurities are known to lower activation energy. A second possibility is that grinding did not entirely remove the effects of sample porosity.
3.2 Diffusion through stagnant film Wicke[6] has pointed out that as reaction temperature increases, diffusion of oxygen through a stagnant film to the exterior of the carbon sample becomes rate-controlling. In such cases, Walker[l] showed that the oxidation rate is proportional to the square root of the reactant flow rate, thus,
where E is the activation energy when chemical reaction
Activation energy (kcabmole)
c’“+“/ZD:~~AI/~~-NZRT.
(1)
where C is the oxygen concentration, m is the order of the reaction, K, is the intrinsic-rate constant, and D,r is the effective diffusion coefficient. The intrinsic rate constant, K,, is defined
Sample
409
Ratio Bulk/Ground
rate = (V/R)“*
(4)
where V is the linear flow velocity of the reactant, and R is the radius of the solid. In the present experiment, R was a constant; therefore, if diffusion through a stagnant film is rate-controlling, the oxidation rate should vary as P.
0.30 0.51 0.68
In order to define the temperature at which diffusion through stagnant surface layers becomes rate-controlling, the oxidation rates of Sample C in bulk form were measured at temperatures higher than the temperatures
HEH-WON CHANG and
410
ROBERT M. RUSNAK
Table 4. Ratios of oxidation rates and reaetion gas flow rates for bulk composites Oxidation at 650” Rate Rate Rate c F=lOO (%g losslmin)
Flow (cm?min) 100
1.00
500 1000 1500
2.24 3.16 3.87
0.11 0.13 0.15 0.16
which yielded the plots in Figs. 1 and 2. Flow rates of the reacting gas were also varied to determine if the oxidation rates followed the Vi’* relationship. Results are summarized in Table 4. At 75O”C, the relation between oxidation-rate ratio and flow-ratio holds very well, indicating that diffusion of oxygen through stagnant gas film is the dominant oxidation mechanism. At 65o”C, however, the relation does not hold, confirming that pore-diffusion mechanism is dominant at 650”. Since Samples A and B have higher oxidation rates than Sample C, the transition from pore diffusion to gas diffusion occurred at lower temperatures for Samples A and B than for Sample C. In Samples A, B and C, the upper limit for the transition from pore diffusion to gas diffusion is therefore 750°C. The temperature for transition from pore diffusion to gas diffusion would depend upon porosity and upon reaction rate in the chemisorption-controlled zone.
Nevertheless, the transition temperature reported in this study agrees closely with the transition temperature reported by Wallouch and Heintz[7] who found that transition from pore diffusion to diffusion through a stagnant fiIm occurred at 740°C for coarse and fine-grained graphites.
1.0 1.2 1.4 1.5
Oxidation at 750” Rate Rate Rate @ F=lOO (%g loss/mitt) 0.29 0.71 0.97 I .03
1.0 2.4 3.3 3.6
The rate-con~olling oxidation meeha~sm in the carbon composites used for aircraft brakes is mainly pore diffusion, in the temperature range 450-65O”C. At temperatures ranging between 650 and 75O”C, the rate-controlling mechanism shifts to diffusion through a stagnant film. Acknowledgements-The authors thank Mr. A. Simmons for his assistance in the experimental work. The authors also thank Dr. S. K. Rhee of Bendix Research Laboratories as well as Dr. N. Hooton and Messrs. P. Trughber and A. Courtney of Bendix Aircraft Brake and Strut Division for their helpful consultation during the study. 1. P. L. Walker Jr., F. Rusinko Jr. and L. G. Austin, Ado. CM. 11, 133(1959). 2. A Wheeler, Adu. Cut. 3,249 (1951). 3. R. .I. Tyler, H. J. Woutherlood and M. R. R. Mulcahy, Carbon 14, 271 (1976). 4. F. Rodriguez-Reinoso and P. L. Walker Jr., Carbon 13, 7 (1975). 5. F. Rodriguez-Reinoso, P. A. Thrower and P. L. Walker Jr., Carbon 12,63 (1974). 6. E. Wicke, Proc. 5th Symp. on Combustion, p. 245. Reinhold, New York (19.55). 7. R. W. Wallouch and E. A. Heintz, Carbon 12, 243 (1974).
ooo8-6223/79/1001-0/~2.~/0 Printed in Great Britain
OXIDATION BEHAVIOR OF CARBON-CARBON
COMPOSITES
HEH-WON CHANGand ROBERT M. RUSNAK Bendix Research Laboratories, Southfield, MI 48076,U.S.A. (Receiued 4 December 1978)
Abstract-This study investigates the oxidation mechanisms of carbon-carbon composite materials of the type used for aircraft brakes. The rate-controlling steps for oxidation at temperatures ranging between 450 and 750°C were determined by measuring activation energies of composite samples in bulk form and in ground form and by measuring oxidation rates as a function of reaction gas flow. Below 65O”C,the two forms of carbon samples showed considerably different activation energies, which suggests that in bulk samples, oxidation is controlled by diffusion through pores, and that as grinding reduces pore length, activation energy increases. At 75O”C,the ratio of oxidation rates was the same as the square root of the ratio of reactant flow rates. It was therefore concluded that the dominant oxidation mechanism at 750°C is diffusion of oxygen through a film of gaseous reaction products at the sample surface. The oxidation mechanism of carbon-carbon composite samples at temperatures between 450 and 650°Cis primarily controlled by the diffusion of oxygen into pores. Between 650 and 750°C.the rate-controlling mechanism undergoes a transition from diffusion through pores to diffusion through stagnant gas film.
1. INTRODUCTION Oxidation of carbon occurs by three processes: (1) chemisorption/desorption, (2) diffusion of oxygen through pores, and (3) diffusion of oxygen through a
stagnant film of reaction products to the surface of carbon[l]. Activation energy depends upon which process predominates. The chemisorption/desorption process, for instance, takes place at low temperatures and yields “true” activation energy. The pore diffusion process takes place at intermediate temperatures and produces an activation energy smaller than the “true” activation energy. The activation energy at high temperatures is even smaller than the activation energy of the pore diffusion process. Carbon-carbon composites have recently attracted considerable interest for friction and heat shield applications. Although much research has been done with various carbons to verify oxidation mechanisms, there is little data for carbon-carbon composites. This investigation was undertaken to determine rate-controlling steps in the oxidation of carbon-carbon composites used for aircraft brakes. In pure carbon, the activation energy produced by the pore diffusion mechanism is much lower than energy produced by chemisorption/desorption. Carbon-carbon composites contain impurities which reduce activation energy; therefore, in carbon-carbon composites the absolute level of activation energy cannot be used to define the controlling mechanism. In this investigation, the oxidation kinetics of carbon-carbon composites in bulk form and in powdered form are compared. The impurity level of a composite remains the same whether the composite is in bulk form, or ground to a powder. Activation energies should be the same for both forms if impurity-enhanced oxidation by chemisorption/desorption is rate-controlling. If pore diffusion is the controlling mechanism; however, then the activation energy should decrease as the ratio of pore radius (r) to pore length (I) decreases[21. Bulk samples are porous throughout the sample thickness. Consequently, grinding the sample CARBON
Vol. 17. No. 5-C
produces a significant reduction in pore length as the bulk size (0.64cm) is reduced to the size of the powder particles (less than 5 pm). If pore diffusion is the ratecontrolling mechanism in the bulk sample, significant activation energy increases should be observed in the oxidation of ground-composite samples. 2. EXPERIMENI’AL
The composite used in this study was composed of 40% carbon binder, 40% high-strength carbon fibers and 20% carbon filler. The composite material was fabricated into disc form, heated in nitrogen to convert the organic binders into carbonaceous materials, and then heattreated at temperatures ranging from 1000 to 2400°C. An induction furnace, continuously purged with 99.998% pure argon, was used for heat-treatment. In order to remove impurities (primarily HZ0 and 02), the argon was passed through a container containing JJrierite and through a container filled with magnesium chips heated to 550°C. Some of the test samples were in bulk form with dimensions of 3.18 x 2.54 x 0.64 cm (henceforth called bulk composite). The rest of the samples were in ground form with particle sizes less than 5 pm (henceforth called ground composite). The oxidation rates of ground composites were measured by thermogravimetric analysis (TGA) on a DuPont 950 analyzer, using 5mg samples. The samples were loaded in the TGA and then purged with nitrogen until the desired reaction temperature was reached. When the temperature reached the preset value, the nitrogen flow was turned off and a NO-cm3/min flow of air was turned on to begin the oxidation process. At the onset of reaction, the reaction chamber was not equilibrated with air; thus, the initial relationship between weight loss and time was nonlinear, and did not become linear until after about 5% oxidation had occurred. Rates used in this study were calculated after 5% oxidation had taken place. In addition to within-particle pores. ground samples placed on a reaction pan have interparticle pores formed
HEH-WON CHANG and ROBERTM.
408
by particte stacking. Inte~~cle
porosity should be coarser than intraparticle porosity, and interparticle porosity should not be a significant factor in oxidation. In order to verify that interparticle porosity was not a factor in oxidation, oxidation rates were measured with
Z-,3- and S-mg samples of ground composite at 6WC, using the same holder for each sample. Three different bed lengths were thus created, changing the effective length of the interp~icle porosity. The reaction kinetics of the three samples would consequently be changed if interparticle porosity is rate-controlling, Results showed that reaction rates were essentially the same for the three powdered samples, indicating that interparticle diffusion had no effect on reaction. The oxidation of bulk composites was performed in a quartz tube furnace. The samples were periodically removed from the furnace and weighed to determine weight loss.
RUSNAK
porosity, active sites and impurities. ~raphitization may be ruled out as a factor in this study because the three samples showed the same d,, spacing: 3.38 A. The effects of pore size distribution and active sites on the oxidation rates were not directly measured. Table 1 shows that ni~ogen BET surface area of the ground sample decreases form Sample A to Sample B to Sample C. Since the three samples were made of the same materials, the number of active sites is apparently propo~ional to the BET surface area, and decreases from Sample A to Sample C. Decrease in the number of active sites as a function of decrease in nitrogen BET surface area is a possible explanation for lower oxidation rates among the samples. The impu~ty content also decreases from Sample A (l.O7wt%) to Sample B (0.93wt%) to Sample C (0.19 wt%). The ashes contain oxidation accelerators such as Fe, Ca, and K as well as decelerators such as Si and Al. The most pronounced change was Fe reduction from 0.535 wt% (Sample A) to
3. RESULTS ANDDISCUSSiON
3.1 Di&sion through pores Tem~rature~e~ndence of oxidation for carboncarbon composites is shown in Fig. 1 for ground samples and in Fig. 2 for bulk samples. Samples for the study differed only in heat-treatment temperature; temperature was increased pro~essively from Sample A to Sample C. Figures 1 and 2 clearly indicate that heat-treatment at higher temperatures increases oxidation-resistance of the composites. Oxidation rates are influenced by ~aphitization,
Table 1. Surface area change with oxidafion (oxidation temperature 650°C) Oxidation level (wt%)
Surface area (m’fg)
A 23.0 198.0 229.0
0 5 IO
TEMPERATURE PC) 7BOmOaBo 6ooBlio t i
I
I-
1 3%
ntd
Fig. 1. Temperature dependence of oxidation rates for ground composites.
LOG RATE lo LOSS/g CARBONAEC)
11.0
12.0
12.0
.-J-x10’ TPK)
Fig. 2. Temperature dependence of oxidation rates for bulk composites.
Sample B 21.6 33.8 35.8
*:9 12.7 12.4
Oxidation behavior of carbon-carbon composites
is the rate-controlling step. By substitution:
Table 2. Activation energies and pre-exponential factors Sample
A B C
rate =
Activation energy (kcal/mole) Ground LogA E 5.28 35.0 5.48 39.1 43.3 6.07
Bulk LogA E IO.5 - 2.41 19.8 - 0.23 29.5 1.99
0.315 wt (Sample B) to 0.037 wt% (Sample C). Therefore
the primary effect of increasing temperature is the reduction of the level of oxidation accelerators. Consistent with observed results, reducing the level of oxidation accelerators decreases oxidation rate. Activation energies and pre-exponential factors (as derived from Figs. 1 and 2) are listed in Table 2. The activation energy of the bulk samples ranges from 10.5 to 29.5 kcal/mole (depending on heat-treatment temperature), while the range for ground samples is between 35.0 and 43.2 kcal/mole. The impurity content for both forms of carbon-carbon composites is the same, but the pore structure of the two forms is different. Thus, the difference in the activation energies between bulk and ground composites can be attributed to alteration in pore structure which affects diffusion through pores. More specifically, in grinding the sample, the length of the interconnected pores is decreased, producing a decrease in oxidation rate which is consistent with the theory[2] that asserts that the oxidation rate varies with the r/l ratio. If the sample is ground finely enough, pore diffusion will produce the same oxidation rate and activation energy as chemisorption (as altered by the presence of impurities). The ratio of activation energies for bulk Sample B to ground Sample B is shown in Table 3. The activation energy in the ground sample is twice higher than in the bulk sample. This provides evidence that diffusion through pores is the rate-controlling step. According to Wheeler121 and Walker [l], the rate for oxidation by diffusion through pores is expressed: rate = C’” +I)‘*(K&t)“*
K, = AemEiRT
(2)
Table 3. Comparison of activation energies for bulk and ground samples at reaction temperatures of 4_5&65O”C
A B C
Ground
Bulk
35.0 39.1 43.3
IO.5 19.8 29.5
(3)
Data for Sample B support the contention that activation energy for diffusion through pores should be one half lower than the level that would be expected if chemisorption/desorption were rate-controlling. There are two sources of evidence that oxidation of Sample A is influenced by oxygen-diffusion through a stagnant film. The ratio of activation energies for Sample A is less than 0.5; moreover, the activation energy for oxidation of Sample A in bulk form is 10.5kcal/mole, close to the activation energy under the gas-diffusion mechanism (less than 8 kcal/mole)[l]. Since the impurity level of Sample A is the highest, it is expected that the transition from pore diffusion to gas-diffusion mechanism occurs at a lower temperature for Sample A than for Samples B and C. The ratio for Sample C is somewhat larger than 0.5. Sample C, in bulk form, also has the lowest oxidation rate (consistent with impurity levels). Since pore diffusion is less important for carbons with low oxidation rates, it is possible that the oxidation of Sample C in bulk form is controlled by a combination of pore diffusion and chemisorption/desorption processes. The possibility that a combination of processes governs oxidation would account for the ratio 0.68 for Sample C. The absolute activation-energy values of ground samples, given in Table 2, are lower than absolute values for carbons and graphites given in the literature. For example, other investigators have recorded values of 64 kcal/mole for spectroscopic grade and polycrystalline graphite[3]; of 63 + 3 kcal/mole for glassy carbon[4]; and of 47-53 kcal/mole for highly oriented stress-recrystallized pyrolytic graphites[S]. Differences in reported values may be attributable to impurities in the samples: impurities are known to lower activation energy. A second possibility is that grinding did not entirely remove the effects of sample porosity.
3.2 Diffusion through stagnant film Wicke[6] has pointed out that as reaction temperature increases, diffusion of oxygen through a stagnant film to the exterior of the carbon sample becomes rate-controlling. In such cases, Walker[l] showed that the oxidation rate is proportional to the square root of the reactant flow rate, thus,
where E is the activation energy when chemical reaction
Activation energy (kcabmole)
c’“+“/ZD:~~AI/~~-NZRT.
(1)
where C is the oxygen concentration, m is the order of the reaction, K, is the intrinsic-rate constant, and D,r is the effective diffusion coefficient. The intrinsic rate constant, K,, is defined
Sample
409
Ratio Bulk/Ground
rate = (V/R)“*
(4)
where V is the linear flow velocity of the reactant, and R is the radius of the solid. In the present experiment, R was a constant; therefore, if diffusion through a stagnant film is rate-controlling, the oxidation rate should vary as P.
0.30 0.51 0.68
In order to define the temperature at which diffusion through stagnant surface layers becomes rate-controlling, the oxidation rates of Sample C in bulk form were measured at temperatures higher than the temperatures
HEH-WON CHANG and
410
ROBERT M. RUSNAK
Table 4. Ratios of oxidation rates and reaetion gas flow rates for bulk composites Oxidation at 650” Rate Rate Rate c F=lOO (%g losslmin)
Flow (cm?min) 100
1.00
500 1000 1500
2.24 3.16 3.87
0.11 0.13 0.15 0.16
which yielded the plots in Figs. 1 and 2. Flow rates of the reacting gas were also varied to determine if the oxidation rates followed the Vi’* relationship. Results are summarized in Table 4. At 75O”C, the relation between oxidation-rate ratio and flow-ratio holds very well, indicating that diffusion of oxygen through stagnant gas film is the dominant oxidation mechanism. At 65o”C, however, the relation does not hold, confirming that pore-diffusion mechanism is dominant at 650”. Since Samples A and B have higher oxidation rates than Sample C, the transition from pore diffusion to gas diffusion occurred at lower temperatures for Samples A and B than for Sample C. In Samples A, B and C, the upper limit for the transition from pore diffusion to gas diffusion is therefore 750°C. The temperature for transition from pore diffusion to gas diffusion would depend upon porosity and upon reaction rate in the chemisorption-controlled zone.
Nevertheless, the transition temperature reported in this study agrees closely with the transition temperature reported by Wallouch and Heintz[7] who found that transition from pore diffusion to diffusion through a stagnant fiIm occurred at 740°C for coarse and fine-grained graphites.
1.0 1.2 1.4 1.5
Oxidation at 750” Rate Rate Rate @ F=lOO (%g loss/mitt) 0.29 0.71 0.97 I .03
1.0 2.4 3.3 3.6
The rate-con~olling oxidation meeha~sm in the carbon composites used for aircraft brakes is mainly pore diffusion, in the temperature range 450-65O”C. At temperatures ranging between 650 and 75O”C, the rate-controlling mechanism shifts to diffusion through a stagnant film. Acknowledgements-The authors thank Mr. A. Simmons for his assistance in the experimental work. The authors also thank Dr. S. K. Rhee of Bendix Research Laboratories as well as Dr. N. Hooton and Messrs. P. Trughber and A. Courtney of Bendix Aircraft Brake and Strut Division for their helpful consultation during the study. 1. P. L. Walker Jr., F. Rusinko Jr. and L. G. Austin, Ado. CM. 11, 133(1959). 2. A Wheeler, Adu. Cut. 3,249 (1951). 3. R. .I. Tyler, H. J. Woutherlood and M. R. R. Mulcahy, Carbon 14, 271 (1976). 4. F. Rodriguez-Reinoso and P. L. Walker Jr., Carbon 13, 7 (1975). 5. F. Rodriguez-Reinoso, P. A. Thrower and P. L. Walker Jr., Carbon 12,63 (1974). 6. E. Wicke, Proc. 5th Symp. on Combustion, p. 245. Reinhold, New York (19.55). 7. R. W. Wallouch and E. A. Heintz, Carbon 12, 243 (1974).