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A study of the interaction of atomic oxygen with various carbonaceous materials
A STUDY OF THE OF ATOMIC OXYGEN CARBONACEOUS
INTERACTION WITH VARIOUS MATERIALS
P. PA~~BI~A~AN, N.M. RODRIGUEZ. B.Z. JAM;” and R.T.K. BAKER Departments of Chemical Engineering and ~M~chanical Engineering. Auburn University. Auburn. (Received
19 April
Alabama
3hK4Y. U.S.A. in
1990: nccepred
rer?sed
,form
1 I Mu!
1YYO)
Abstract-A variety of techniques have been used to examine the manner by which atomic oxygen species interact with various carbonaceous solids. In addition. the effect of such a treatment on the physical and surface chemical properties of the carbons has been determined. In situ electron microscopy studies have enabled ub to establish that graphite is probably the most reactive form of carbon for this reaction. This behavior is rationalized according to the notion that atomic oxygen reacts preferentially with the delocalized m electrons in the graphite basal plane to create micro-pits. It is shown that careful control of the reaction of atomic oxygen with a carbonaceous surface can lead to certain beneficial effects when such structures are used in a reinforcement application. Key Words-Atomic
oxygen. electron microscopy. carbon fibers, graphite. XPS.
1. INTRODUCTION
At altitudes between 200 and 700 kilometers, a region designated “low earth orbit” (LEO), one of the major constituents of the atmosphere is atomic oxygen[ l]. A combination of factors such as low pressure and intense solar activity favor the existence of these species[2]. Past experiences with shuttle flights orbiting in the LEO have shown that the interaction of atomic oxygen with various constructional materials can have a profound effect on their durability and performance[3-S]. In this context the behavior of various carbonaceous materials during exposure to atomic oxygen is of tremendous technological importance because carbon in the form of composites is used extensively in space applications[6]. Some early studies by Hennig and coworkers[7] indicated that the reaction of atomic oxygen with carbonaceous materials proceeded rapidly at room temperature, there being practically no activation energy. They also reported that, in atomic oxygen, natural graphite pitted freely at cleaved surfaces. Marsh and coworkers[S-101 confirmed these findings and in addition claimed that the rate of carbon gasification was independent of the structure of the carbonaceous solid. Otterbien and Bonnetain[ll.12] studied the interaction of graphite with atomic oxygen and found that the reaction proceeded in three steps, which they claimed to be associated with desorption of a surface oxide: (25-100°C) with an apparent activation energy of 5 kcalfmole, a transition region (loo-200°C) having an activation energy of 30 kcalimole, and a high temperature region (200700°C) the rate of which appeared to be governed by the supply of atomic oxygen. More recently, Yang and coworkers[l3,14] have used gold decoration inconjuction with transmission electron microscopy to
probe the origin of the pits produced in the basal plane surfaces of graphite following exposure to atomic oxygen at 150°C. From these studies they made some significant conclusions: Carbon atoms were preferentially attacked by atomic oxygen in the direction of the CIaxes, rather than the direction of the c axes. This action resulted in the creation of monolayer pits, which emanated from lattice vacancies continuously formed by atomic oxygen. It was significant that gold decoration of the surface was only possible after treatment of the graphite in high purity nitrogen at 900°C. indicating that the chemisorbed oxide species prevented the diffusion of gold atoms across the graphite basal plane. A very comprehensive kinetic study of the oxidation of soot by atomic oxygen has been performed by Wicke and coworkers[lS-181. They used a microbalance in combjnation with a mass spectrometer to record continuously the loss in weight and changes in gas phase products when soot samples were reacted in atomic oxygen at temperatures ranging from 23 to 600°C. At 23”C, atomic oxygen attack on soot was governed by a large net adsorption process. which resulted in a modification in carbon-carbon bonding characteristics in the soot. At slightly higher temperatures. the chemisorbed oxygen was found to desorb entirely as CO and CO?, and this process became very rapid at temperatures above 200°C. From their kinetic data they concluded that at 500°C the rate of gasification of soot by atomic oxygen was estimated to be at least five orders of magnitude faster than that observed with molecular oxygen. SEM post-reaction examination of carbon fiber from different precursors was carried out by Barnet and Norr[ 191in an attempt to correlate the reactivity of the fibers to an oxygen plasma with various structural parameters. in these studies fibers were ex867
P. PATTABIRAMANet (11.
868
posed to severe conditions that produced extensive burn-off of the fibers and the conclusions regarding their structure were based on micrographs of the remaining part of the fiber. In the current study we have used a combination of novel and conventional electron microscopy techniques to observe the morphological chages produced in various carbonaceous materials during exposure to a beam of atomic oxygen. In particular, in situ studies of the reaction taking place between graphite and atomic oxygen were followed to elucidate the detailed features that accompany the reaction and how such behavior depends on structural parameters. This type of experiment was also extended to cover the reactivity of filamentous carbon with atomic oxygen. Filamentous carbon is a type of material formed by the catalytic decomposition of certain hydrocarbons on small metal particles. Extensive studies of this material have been performed by controlled atmosphere and high resolution electron microscopy[20,21]. The combination of these techniques have shown that the filaments consist of a unique structure having a relatively well organized external structure surrounding a disorderd core. The thickness and orientation of the external region have been found to be dependent on the nature of the catalyst used and the conditions of the experiment[22]. In this regard, carbon filaments can be viewed as a micro-version of a conventional carbon fiber, and as such provide an ideal electron transmission analog, thereby overcoming the need for thinning of a bulk fiber, which causes modifications and damage to the surface. In addition, two other sets of experiments have been carried out, one designed to ascertain modifications in certain physi~o/chemical properties of treated samples and the other being initial attempts at the development of protective coatings for carbon against attack by atomic oxygen. 2. EXPERIMENTAL
2. I. Ekctron microscopy studies In situ electron microscopy studies of transmission specimens undergoing reaction with atomic oxygen were performed in a specially designed environmental cell that replaced the objective pole piece in the microscope[23]. A closed circuit tele~sion/video recording system was used to continuously record changes in the appearance of a specimen as the reaction proceeded at magnifications over 3 x lo6 times and a resolution better than 0.4 nm. In these experiments atomic oxygen was produced in signi~cant concentrations by passing a mixture of N,O and N, (1:l) through a microwave discharge cavity at 2.0 Torr, a procedure that has been found to create an atomic oxygen flux relatively free of molecular oxygen[24,25]. The oxygen atom reactor system was linked to the environmental cell within the microscope via a brass coupling unit carefully
lined with a treated quartz tube in order to minimize the loss of oxygen atoms due to recombination[ 151. Treatment of bulk specimens in an atomic oxygen environment was accomplished in a larger scale apparatus using a gas mixture of Nz and NO. In this apparatus the conditions were selected so that only ground state (O’P) oxygen atoms were produced. Samples were mounted on a metal heating plate connected to a thermocouple for temperature control and measurement. With this arrangement it was possible to expose samples to atomic oxygen for several hours at temperatures up to 275°C. Changes occurring during reaction were determined from a comparison of the appearance of treated and untreated specimens. These examinations were carried out in a JEOL 840 scanning electron microscope at a resolution of 3.5 nm. In a complementary series of experiments scanning electron microscope examinations combined with weight loss measurements were used to evaluate the protection against atomic oxygen attack obtained by coating carbonaceous materials with a vapor deposited film of silica. Samples used for weight loss measurements were completely covered with a uniform layer of silica, whereas those prepared for SEM examination had only half the surface exposed to the silica source. Under these circumstances it was possible to locate the boundary between coated and uncoated regions and assess the degree of protection offered by the deposited silica layer. 2.2. Studies of surface properties Surface energy measurements of treated and untreated carbon fibers were made using the technique devised by Donnet and coworkers[26]. In this method the carbon fiber (-20 mm in length) was bound to a thin plastic template with a hole through which it could be hung to the arm of a CAHN 2000 microbalance. A micro-elevator was modified to gradually raise a beaker containing two immiscible liquids up to the fiber. Changes in the interaction of the fiber with the respective liquids were recorded as weight changes by the microbalance. X-ray photoelectron spectroscopy was used to determine the changes in chemical composition of carbon fibers following exposure to an atomic oxygen flux of 1.26 x lo*” atoms/cmZ for one hour at 70°C. The flux was calculated from photolyti~ titration measurements pe~ormed with known amounts of NO. Finally, the effect of treating carbon fibers in atomic oxygen on the interfacial bonding with an epoxy resin was estimated according to a modified fiber pull out method developed by Miller and coworkers[27]. A fiber with a droplet of epoxy was attached to a plastic template, which was hung from the fixed end of an Instron tensile tester. The free end was positioned between two glass slides with the aid of a microvice, in a similar fashion to the apparatus described in reference[28]. The shape characteristics of the curves obtained from this mi~robond test provides infor-
Fig.
1 Electron micrograph of a graphite specimen undergoing attack in atomic oxygen at 100°C.
Fig. 2. Electron micrograph of a different region of the sample shown in Fig. 1, taken 10 min later Preferential attack along a twin boundary is indicated by “x.”
Fig. 3. Electron micrograph of a further region of the same sample taken 20 min later
870
P. PATTABIRAMAN ef al.
Fig. 4. Sequence of photographs taken from the video display showing the attack of filamentous
Interaction
of atomic oxygen with carbon
carbon in atomic oxygen at 100°C. The time interval between each frame is 5 mm
871
872
P. PATTABIRAMAN
mation on the degree of the interfacial interaction between the fiber surface and the polymeric matrix. The graphite specimens used in the transmission electron microscopy studies were prepared from natural single crystals (Ticonderoga, N.Y.) according to a standard method(291. Batches of carbon filaments were prepared from the metal catalyzed decomposition of ethylene, the details of which can be found elsewhere[22]. Samples of AUX-12K (unsized yarn) and AS4-3K (fabric) were provided by Hercules Co., the former was used for tests involving single fiiaments and the composite was prepared from the fabric and Epon 828,‘(Shell Chemicals). Coating of a selected number of materials by silica to a thickness of about 100 nm was carried out by vacuum evaporation of a 50150 mixture of SiOJSi. Following deposition the samples were given a mild oxidation treatment in air at 350°C to ensure that complete conversion to SiO? was achieved.
3. RESULTS
AND DiSCUSSIONS
3.1. Controlled atmosphere electron microscopy
studies
When transmission specimens of single crystal graphite were treated in 2.0 Torr of a gas mixture containing atomic oxygen at room temperature no apparent changes were observed over short periods of time; nevertheless erosion of the surface took place very rapidly when the temperature was raised to 100°C. This took the form of the creation of irregular shaped pits over the entire basal plane region. It was significant that this same pattern of behavior was observed with specimens exposed to the atomic oxygen beam at room temperature and subsequently heated to 100°C under vacuum conditions; it appears that atomic oxygen adsorbs and reacts with graphite at room temperature (i.e., forms bonds with carbon) without significant loss of mass. Additional energy is then required to break the bonds and release the gases, which we assume are the same as those identified by Wicke and coworkers in their experiments with soot mentioned earlier[ 15-181. The characteristics of these pits were quite different from those formed by certain metal and metal oxide particles in graphite in the presence of molecular oxygen at appreciably higher temperatures~30,31]. In this case attack took place at isolated imperfections in the basal plane to produce well defined hexagonal shaped pits, which over a period of time expanded to become more circular in outline and progressively deeper as the catalyst penetrated through the structure. Figures l-3 are electron micrographs of three different regions of a specimen undergoing attack in atomic oxygen at 100°C. The time delay between each micrograph is approximately 10 min. Continuous observatron of the reaction revealed a number of other features of the system. (a) Attack appeared to be confined to the basal plane regions; there was little evidence of widespread edge reces-
et al.
sion. In this regard it is worth noting that prior to treatment in atomic oxygen, specimens were not outgassed in high vacuum at elevated temperature and as a consequence edge regions might contain adsorbed species (b) Detailed examination of the pitting action showed that individual pits did not appear to expand with time but increased their boundaries by coalescence with other adjacent pits. Eventually pitting in a given multilayer proceeded to such an extent that an entire multilayer was removed, leaving a lacy unreacted residue on the surface. Furthermore. the depths of pits, as estimated by the difference in contrast with surrounding unattacked regions, tended to be uniform. (c) The indiscriminate nature of the reaction made it impossible to identify any dynamic events that could be followed for the purpose of estimating rates of oxidation. (d) An exception to this general behavior was the observation that atomic oxygen tended to display preferential attack along twin bands. This aspect is clearly evident in Fig. 2 at the location marked “x”. (d) The number of fresh pits produced per unit area increased with both time and temperature. The preferential attack of atomic oxygen on graphite, which was confined to the basal plane with no evidence of corrosion at the edges. clearly indicates that the adsorption and bonding of oxygen and the subsequent removal of carbon is restricted to areas where there is a significant availability of electrons (i.e., rr electrons). This finding demonstrates that the reactivity of carbons towards atomic oxygen is very much dependent on structural parameters, which contradicts previous reports by Marsh et al. [8-lo], who suggested that the rate of carbon gasification was independent of the nature of the carbonaceous solid. It is then expected that materials containing sp* or sp bonds, or those containing graphitic structures are expected to be much more reactive than those with saturated bonds, and that materials possessing saturated bonds such as silica should remain inert in the presence of atomic oxygen and thus could be used as protective coatings. In addition, the in situ experiments confirmed previous observations reported by several investigators that the erosion of graphite does not occur as a direct consequence of the interaction of atomic oxygen with graphite at room temperature (in which case the surface will be saturated with bonded oxygen), but by the addition of a small amount of energy, or prolonged outgassing of the sample. In a second series of experiments, carbon filaments supported on a carbon film were reacted in atomic oxygen under the same conditions as those used for the graphite specimens. Some features of the unreacted material are apparent on the electron micrograph, Fig. 4A, where the fish bone structure of the ordered external component is apparent. When carbon filaments were heated in an atomic oxygen environment, the first signs of gasification were observed at 50°C. The progressive change in appearance of this material at IbOY can be seen in the
~nteraetion of atomic oxygen with carbon
Fig. 5. Scanning electron micrograph of a carbon fiber before reaction in atomic oxygen
Fig. h. Scanning electron micrograph of the same fiber as in Fig. 5, following reaction in atomic oxygen at 75°C for 2.5 h.
uel nce of stills taken from the TV monitor at 5 _ ~~ -. mm Intervals, Figures 4A to 4D. Under these conditions attack took place preferentially at the graphitic skin regions with subsequent oxidation of the disordered inner core structure. This behavior is to be contrasted with that found with molecular oxygen where the inner core gasified at 625°C followed by
removal of the graphitic skin at 72S”C[20], It was that in the presence of atomic oxygen, oxidation of the amorphous carbon film support coincided with attack of the inner core component of the filaments. These results confirm our belief that there is selectivity in the reaction of atomic oxygen with carbonaceous solids, signi~cant
P. PATTABIRAMAN etal.
Fig. 7. Scanning
electron
micrograph
of a carbon
fiber epoxy composite
before
reaction
in atomic
reaction
in atomic
oxygen.
Fig. 8. Scanning electron
micrograph
of the same composite as in Fig. 7, following oxygen at 75°C for 2.5 h.
Post-reaction scanning electron microscopy examinations McKee and Mimeault[32] summarized the developments in surface treatments of carbon fibers in a review article published in 1973. Since that time the major emphasis on the surface chemical modification
3.2.
of the carbon fiber has tended to follow procedures of either plasma etching[26,33], electrochemical techniques[34-371, or catalysis in a molecular oxygen environment[38,39]. With the latter approach, selected catalysts could be chosen to produce pitting, but in this case weakening of the fiber can occur by
Interaction
of atomic oxygen with carbon
875
Fig. 9. Scanning electron micrograph of a carbon fiber epoxy composite that has been heated in atomic oxygen at 100°C for 4.0 h. Region (A) has been coated with a l(H) nm thick film of silica. and region (B) is uncoated.
the extensive reaction caused by the catalyst. In a recent report it is claimed that when silver is used as the catalyst for the gasification of carbon fiber in molecular oxygen. this treatment offers advantages over other methods of oxidation including plasma etching in argon and oxygen[38]. In the present studies we have attempted to apply the knowledge gained from our in situ electron microscopy experiments with atomic oxygen-graphite and atomic oxygen-carbon filaments to the behavior of carbon fibers when treated under similar conditions. Figures 5 and 6 are scanning electron micrographs showing the appearance of a carbon fiber before and after treatment in atomic oxygen at 75°C for 2.5 hours. Widespread and evenly distributed surface pitting is clearly evident in the treated specimen. Although these observations appear to be somewhat different from those reported by other authors[33] who performed the experiment under conditions
where prolific attack of the structure occurred. it is worth noting that there are significant differences in the conditions and nature of the material used in the two investigations. The conditions of our experiments were selected according to the observations of the in situ gasification studies of graphite and carbon filaments, from which it is possible to identify the parameters under which the onset of attack by atomic oxygen occurred, and in this way control the extent of the reaction. The exposure time (2.5 h) used in our experiments was selected to achieve sutficient pitting to be observable by SEM. In addition, the samples used in our experiments were conventional PAN carbon fibers that had not been graphitized. In other experiments, the effect of reacting a carbon fiber epoxy composite in atomic oxygen at the same conditions as those stated above was investigated. From a comparison of Figs. 7 and 8 it is ap-
Table 1. Weight loss measurements
I ;;;W& MYLAR I
MATERIAL
UNCOATED (% LOSS)
FIBER/EPOXY COMPOSITE :3 2:3
P. PATTABIRAMAN Table
2. Atomic
concentration
of carbon
ei ul
fiber surfaces
detcrmincd
from XPS analysis
FIBER EXPOSED TO ATOMIC OXYGEN
parent that whiie the fibrous component has undergone extensive pitting, attack of the epoxy matrix has been far less severe, being limited to a “peeling action” in certain regions. In an attempt to evaluate the degree of protection offered by a silica coating, SEM examinations were performed on the carbon fiber/epoxy composite samples in which the exposed surface was partially covered with a 100 nm thick film of silica. These specimens were treated in an atomic oxygen environment for 4.0 hours at 100°C. Figure 9 shows the startling contrast in appearance of the surface at the boundary between the silica coated (A) and the uncoated (B) regions. The silica coated region appears to be unchanged while the exposed area shows evidence of widespread attack. Although no tests were performed to evaluate the degree of adhesion of the silica film on the composite, the lack of pitting of the fibers or peeling action of the matrix suggests that good adhesion was achieved A quantitative estimate of the protective effect of the silica treatment was made from weight loss measurements in which uniformly coated and uncoated specimens were reacted side by side in the atomic oxygen flux for 4.0 h at 100°C. The data obtained for the composite samples is presented in Table 1 and compared with that found for two polymeric materials, Kapton and Mylar, which were reacted under the same conditions. From these experiments it is clear that the silica coating afforded a very effective protection for all three materials. 3.3. ~e~~ure~ents of surface properties XPS was used to ascertain the change in relative atomic concentration of carbon and oxygen following exposure to atomic oxygen. The data for treated and untreated specimens is presented in Table 2. The significant increase in oxygen atom concentration on the surface is consistent with the notion that species derived from atomic oxygen are adsorbed on the fiber. This leads to the possibility of modification in the wetting properties of the fiber and a consequent change in the strength of an interaction with a polymeric matrix material. The surface energy measurements consisted of determining the values for the polar and dispersive components of the surface energy of treated and untreated fibers. The polar component is the result of short range interactions and is relatively insensitive towards crystalline structure, being dependent
on the density of polar atoms per unit area and their intrinsic polarity. The dispersive component results from long range interactions and is a function of the density and mass of the underlying material. In the current experiments. the polar and dispersive components were determined from the interaction of fibers with water and either pentane or heptane respectively. Figure 10 illustrates the typical curves obtained when carbon fibers are immersed in the hydrocarbon-water system, which are the result of the interfacial reaction of carbon fibers with liquids of different polarity. The untreated fiber (A) exhibits a strong affmtty for the hydrocarbon (section b), but shows little tendency to interact with the polar solvent (section c). This is because of the total absence of polar groups on the carbon fiber surface. In contrast, when the fiber is treated with atomic oxygen at 75°C for 2.5 hours, the surface properties change dramatically. This aspect can be seen very clearly from the strong interaction of the treated fiber with water, where there is a substantial increase in the wetting properties over that exhibited with the hydrocarbon. The values presented in Table 3 were obtained by analysis of the experimental data according to the procedures given in ref. 26. Finally, an estimate of the strength of the inter-
IA1
II -Cd
Untreated Fiber
b
a
Lr
-9-c
C
[Bl FiberTreatedinAtomicOxygen
FIBER
DISPLACEMENT
Fig. 10. Variation of the wetting characteristics of carbon fibers in various media. (A) untreated and (B) treated in atomic oxygen: (a) air. (b) pentane or heptane, and (c) water.
Interaction Table 3. Surface
of atomic
oxygen
877
with carbon
energy characteristics of carbon fibers
MATERIAL
UNTREATED flBER FIBER EXPOSED TO ATOMIC OXYGEN
facial bonding between fibers and an epoxy resin was obtained by the microband test. The values reported in Table 4 are the mean of ten measurements for each type of fiber. It should be noted that the fibers treated in an atomic oxygen environment were very brittle and required careful handling while the epoxy droplet was allowed to cure and while positioning the sample on the Instron. Examination of the data presented in Tables 3 and 4 shows that treatment of carbon fibers in an atomic oxygen environment has extremely beneficial effects. On one hand. the roughness of the surface will enhance the adhesion between the matrix and the fiber to prevent delamination. In the case of carbon fibers interacting with highly polar matrices (i.e., oxygen rich structures), the presence of oxygenated groups at the surface of the fiber will tend to increase the strength of the interaction between fiber and matrix. Nevertheless, because the extreme reactivity of carbon towards atomic oxygen, extensive treatment may lead to weakening of the structure due to enhanced pitting. In contrast, a well controlled oxidation of the fiber in such an environment should result in both an enhancement in the physical interaction caused by the micro-pitting action and an increase in the chemical interaction established between the oxygenated groups on the fiber and the matrix. The experiments carried out using graphite as a model demonstrated that it should be possible to adsorb and subsequently bond atomic oxygen to carbon at room temperature without damage to the structure provided the treatment is short and performed out at low temperature. Based on present observations it is apparent that the approaches adopted for measurement of surface chemical properties must take into consideration the fact that graphite surfaces treated in atomic oxygen at relatively low temperature contain a significant
Table 4. Interfacial
strength
32~4 66+7
$;:,
number of oxygenated groups that could prevent the adsorption of other nonpolar gas molecules. Indeed, as demonstrated by Yang et a1.[13,14], in order to produce reactive sites on the surface it was necessary to heat graphite in nitrogen at a temperature of 900°C in order to remove adsorbed oxygenated species produced on the surface during exposure to atomic oxygen. It is clear, therefore, that unless such a procedure is followed, one might obtain misleading data for the estimate of surface areas from gas adsorption methods. One possible solution might be to examine the interaction of these treated carbon surfaces with selected polar liquids.
4. SUMMARY
The results of this study confirm previous findings that atomic oxygen is extremely active for the gasification of carbon. However, the current in situ electron microscopy experiments have demonstrated that atomic oxygen selectively attacks different carbon structures, with graphite being the most reactive material due to the presence of delocalized rr electrons, which act as centers for reaction. This behavior contrasts with the reactivity pattern found between various carbons and molecular oxygen at elevated temperatures and therefore leads to the possibility of the development of some interesting applications. The treatment of a carbonaceous solid in an atomic oxygen environment under well controlled conditions such as exposure time and temperature can produce physical and chemical modifications to the surface of the solid. which can be tailored to improve the performance of the material in a number of applications. In the case of carbon fibers, for use as reinforcement agents with epoxy resins, a careful treatment in atomic oxygen should produce an enhancement in surface area coupled with the forma-
determinations
from microbond
test
MATERIAL
INTERFACIAL STRENGTH
UNTREATED FIBER FIBER EXPOSED TO ATOMIC OXYGEN
49.7 f 5 87.9 k 9
P. PATTABIRAMAN etal.
878
tion of surface species that can form bridges between the fiber and the matrix, resulting in improved bonding.
Acknowiedgements-This
work was supported by the Department of Energy, Basic Energy Sciences Grant No. DEFGOS-89ERl4076. The in situ transmission electron microcopy facilities were built under NSWC Contract No. 60921~86-C-A226. The authors are indebted to Dr. Brian Wicke of General Motors Co. for invaluable discussions, Dr. Charles W. Neely for use of the atomic oxygen ex situ facilities and Messrs W.B. Downs and M.S. Kim for performing some of the experiments.
REFERENCES
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10. H. Marsh, R. Reed. E. O’Hair. and W. F. K. WynneJones, Trans. -Farad. Sot. 61, 274 (1965). 11. M. Otterbein and L. Bonnetain. Compt. Rend. 258(9), 2563 (1964).
12. M. Otterbein and L. Bonnetain, Compt. Rend. 259 (4). 791 (1964). 13. R. T. Yang and C. Wong, J Chem. Phys. 75, 4471 (1981). 14. C. Wong. R. T. Yang, and B. L. Halpern. J Chem Phys. 78, 3325 (1983).
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17. B. G. Wicke and K. A. Grady, 22”” Intern Combust. Symp. Seattle, Washington. Paper 42 (1988).
18. B. G. Wicke and K. A. Grady. Extended Abstracts, IYlh Biennial Conf. Carbon. Pennsylvania State University, P562 (1989). 19. F. R. Barnet and M. K. Norr, Carbon 11,281 (1973). 20. R. T. K. Baker, M. A. Barber, P. S. Harris, F. S. Feates. and R. J. Waite, J. Catal. 26, 51 (1972). 21. M. Audier. M. A. Coulon. and A. Oberlin. Carbon 18. 73 (1980). 22 N. M. Rodriguez, M. S. Kim, W. B. Downs, and R. T. K. Baker. In Carbon Fibers Filumenrs and Composites (edited by J. L. Figueiredo. C. A. Bernardo, k. T. K. Baker; and K. J. Huttinger), NATO ASI Series E. Applied Sciences. Vol. 177. v. 541. Kiuwer Acad. Pub.: Dordrecht (1989). ’ 23 N. M. Rodriguez, S. G. Oh, W. B. Downs, P. Pattabiraman, and R. T. K. Baker, Rev. Sci. Instrum., (In press). 24. P. D. Francis. Brit. J. AppilPhys.. Ser 2.2, 1717 (1969). 25. A. Y. M. Ung, Chem. Phys. Letters 32, 351 (1975). 26‘ J. B. Donnet, M. Brendle. T. L. Dhami. and 0. P. Bahl. Carbon 24, 757 (1986). 27. 8. Miller, P. Muri. and L. Rebenfeld. Composite Sci. and Tech. 28, 17 (1987). 28. M. R. Piggott. Carbon 27, 657 (1989). 29. G. R. Hennig in Chemisrr!, and Physics of Carbon (Edited by P. I_. Walker, Jr.) Vol. 2, p. 121 Marcel Dekker. New York. (1965). 30. J. M. Thomas in Chemistry and Physics of Carbon (Edited by P. L. Walker, Jr.) Vol. 1, p. 121 Marcel Dekker. New York. (1965). 31. R. T. K. Baker, J. A. France, L. Rouse, and R. J. Waite, J Catal. 41. 22 f 1976). 32. D. W. McKee and V. J. Mimeault. in Chemistry and Physics of Carbor~(Edited by P. L. Walker, Jr. and P. A. Thrower) Vol. 8. v. 151 Marcel Dekker. New York (1973). ’ 33. 1. K. Ismail and M. D. Vangsness, Carbon 26, 749 (1988).
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INTERACTION WITH VARIOUS MATERIALS
P. PA~~BI~A~AN, N.M. RODRIGUEZ. B.Z. JAM;” and R.T.K. BAKER Departments of Chemical Engineering and ~M~chanical Engineering. Auburn University. Auburn. (Received
19 April
Alabama
3hK4Y. U.S.A. in
1990: nccepred
rer?sed
,form
1 I Mu!
1YYO)
Abstract-A variety of techniques have been used to examine the manner by which atomic oxygen species interact with various carbonaceous solids. In addition. the effect of such a treatment on the physical and surface chemical properties of the carbons has been determined. In situ electron microscopy studies have enabled ub to establish that graphite is probably the most reactive form of carbon for this reaction. This behavior is rationalized according to the notion that atomic oxygen reacts preferentially with the delocalized m electrons in the graphite basal plane to create micro-pits. It is shown that careful control of the reaction of atomic oxygen with a carbonaceous surface can lead to certain beneficial effects when such structures are used in a reinforcement application. Key Words-Atomic
oxygen. electron microscopy. carbon fibers, graphite. XPS.
1. INTRODUCTION
At altitudes between 200 and 700 kilometers, a region designated “low earth orbit” (LEO), one of the major constituents of the atmosphere is atomic oxygen[ l]. A combination of factors such as low pressure and intense solar activity favor the existence of these species[2]. Past experiences with shuttle flights orbiting in the LEO have shown that the interaction of atomic oxygen with various constructional materials can have a profound effect on their durability and performance[3-S]. In this context the behavior of various carbonaceous materials during exposure to atomic oxygen is of tremendous technological importance because carbon in the form of composites is used extensively in space applications[6]. Some early studies by Hennig and coworkers[7] indicated that the reaction of atomic oxygen with carbonaceous materials proceeded rapidly at room temperature, there being practically no activation energy. They also reported that, in atomic oxygen, natural graphite pitted freely at cleaved surfaces. Marsh and coworkers[S-101 confirmed these findings and in addition claimed that the rate of carbon gasification was independent of the structure of the carbonaceous solid. Otterbien and Bonnetain[ll.12] studied the interaction of graphite with atomic oxygen and found that the reaction proceeded in three steps, which they claimed to be associated with desorption of a surface oxide: (25-100°C) with an apparent activation energy of 5 kcalfmole, a transition region (loo-200°C) having an activation energy of 30 kcalimole, and a high temperature region (200700°C) the rate of which appeared to be governed by the supply of atomic oxygen. More recently, Yang and coworkers[l3,14] have used gold decoration inconjuction with transmission electron microscopy to
probe the origin of the pits produced in the basal plane surfaces of graphite following exposure to atomic oxygen at 150°C. From these studies they made some significant conclusions: Carbon atoms were preferentially attacked by atomic oxygen in the direction of the CIaxes, rather than the direction of the c axes. This action resulted in the creation of monolayer pits, which emanated from lattice vacancies continuously formed by atomic oxygen. It was significant that gold decoration of the surface was only possible after treatment of the graphite in high purity nitrogen at 900°C. indicating that the chemisorbed oxide species prevented the diffusion of gold atoms across the graphite basal plane. A very comprehensive kinetic study of the oxidation of soot by atomic oxygen has been performed by Wicke and coworkers[lS-181. They used a microbalance in combjnation with a mass spectrometer to record continuously the loss in weight and changes in gas phase products when soot samples were reacted in atomic oxygen at temperatures ranging from 23 to 600°C. At 23”C, atomic oxygen attack on soot was governed by a large net adsorption process. which resulted in a modification in carbon-carbon bonding characteristics in the soot. At slightly higher temperatures. the chemisorbed oxygen was found to desorb entirely as CO and CO?, and this process became very rapid at temperatures above 200°C. From their kinetic data they concluded that at 500°C the rate of gasification of soot by atomic oxygen was estimated to be at least five orders of magnitude faster than that observed with molecular oxygen. SEM post-reaction examination of carbon fiber from different precursors was carried out by Barnet and Norr[ 191in an attempt to correlate the reactivity of the fibers to an oxygen plasma with various structural parameters. in these studies fibers were ex867
P. PATTABIRAMANet (11.
868
posed to severe conditions that produced extensive burn-off of the fibers and the conclusions regarding their structure were based on micrographs of the remaining part of the fiber. In the current study we have used a combination of novel and conventional electron microscopy techniques to observe the morphological chages produced in various carbonaceous materials during exposure to a beam of atomic oxygen. In particular, in situ studies of the reaction taking place between graphite and atomic oxygen were followed to elucidate the detailed features that accompany the reaction and how such behavior depends on structural parameters. This type of experiment was also extended to cover the reactivity of filamentous carbon with atomic oxygen. Filamentous carbon is a type of material formed by the catalytic decomposition of certain hydrocarbons on small metal particles. Extensive studies of this material have been performed by controlled atmosphere and high resolution electron microscopy[20,21]. The combination of these techniques have shown that the filaments consist of a unique structure having a relatively well organized external structure surrounding a disorderd core. The thickness and orientation of the external region have been found to be dependent on the nature of the catalyst used and the conditions of the experiment[22]. In this regard, carbon filaments can be viewed as a micro-version of a conventional carbon fiber, and as such provide an ideal electron transmission analog, thereby overcoming the need for thinning of a bulk fiber, which causes modifications and damage to the surface. In addition, two other sets of experiments have been carried out, one designed to ascertain modifications in certain physi~o/chemical properties of treated samples and the other being initial attempts at the development of protective coatings for carbon against attack by atomic oxygen. 2. EXPERIMENTAL
2. I. Ekctron microscopy studies In situ electron microscopy studies of transmission specimens undergoing reaction with atomic oxygen were performed in a specially designed environmental cell that replaced the objective pole piece in the microscope[23]. A closed circuit tele~sion/video recording system was used to continuously record changes in the appearance of a specimen as the reaction proceeded at magnifications over 3 x lo6 times and a resolution better than 0.4 nm. In these experiments atomic oxygen was produced in signi~cant concentrations by passing a mixture of N,O and N, (1:l) through a microwave discharge cavity at 2.0 Torr, a procedure that has been found to create an atomic oxygen flux relatively free of molecular oxygen[24,25]. The oxygen atom reactor system was linked to the environmental cell within the microscope via a brass coupling unit carefully
lined with a treated quartz tube in order to minimize the loss of oxygen atoms due to recombination[ 151. Treatment of bulk specimens in an atomic oxygen environment was accomplished in a larger scale apparatus using a gas mixture of Nz and NO. In this apparatus the conditions were selected so that only ground state (O’P) oxygen atoms were produced. Samples were mounted on a metal heating plate connected to a thermocouple for temperature control and measurement. With this arrangement it was possible to expose samples to atomic oxygen for several hours at temperatures up to 275°C. Changes occurring during reaction were determined from a comparison of the appearance of treated and untreated specimens. These examinations were carried out in a JEOL 840 scanning electron microscope at a resolution of 3.5 nm. In a complementary series of experiments scanning electron microscope examinations combined with weight loss measurements were used to evaluate the protection against atomic oxygen attack obtained by coating carbonaceous materials with a vapor deposited film of silica. Samples used for weight loss measurements were completely covered with a uniform layer of silica, whereas those prepared for SEM examination had only half the surface exposed to the silica source. Under these circumstances it was possible to locate the boundary between coated and uncoated regions and assess the degree of protection offered by the deposited silica layer. 2.2. Studies of surface properties Surface energy measurements of treated and untreated carbon fibers were made using the technique devised by Donnet and coworkers[26]. In this method the carbon fiber (-20 mm in length) was bound to a thin plastic template with a hole through which it could be hung to the arm of a CAHN 2000 microbalance. A micro-elevator was modified to gradually raise a beaker containing two immiscible liquids up to the fiber. Changes in the interaction of the fiber with the respective liquids were recorded as weight changes by the microbalance. X-ray photoelectron spectroscopy was used to determine the changes in chemical composition of carbon fibers following exposure to an atomic oxygen flux of 1.26 x lo*” atoms/cmZ for one hour at 70°C. The flux was calculated from photolyti~ titration measurements pe~ormed with known amounts of NO. Finally, the effect of treating carbon fibers in atomic oxygen on the interfacial bonding with an epoxy resin was estimated according to a modified fiber pull out method developed by Miller and coworkers[27]. A fiber with a droplet of epoxy was attached to a plastic template, which was hung from the fixed end of an Instron tensile tester. The free end was positioned between two glass slides with the aid of a microvice, in a similar fashion to the apparatus described in reference[28]. The shape characteristics of the curves obtained from this mi~robond test provides infor-
Fig.
1 Electron micrograph of a graphite specimen undergoing attack in atomic oxygen at 100°C.
Fig. 2. Electron micrograph of a different region of the sample shown in Fig. 1, taken 10 min later Preferential attack along a twin boundary is indicated by “x.”
Fig. 3. Electron micrograph of a further region of the same sample taken 20 min later
870
P. PATTABIRAMAN ef al.
Fig. 4. Sequence of photographs taken from the video display showing the attack of filamentous
Interaction
of atomic oxygen with carbon
carbon in atomic oxygen at 100°C. The time interval between each frame is 5 mm
871
872
P. PATTABIRAMAN
mation on the degree of the interfacial interaction between the fiber surface and the polymeric matrix. The graphite specimens used in the transmission electron microscopy studies were prepared from natural single crystals (Ticonderoga, N.Y.) according to a standard method(291. Batches of carbon filaments were prepared from the metal catalyzed decomposition of ethylene, the details of which can be found elsewhere[22]. Samples of AUX-12K (unsized yarn) and AS4-3K (fabric) were provided by Hercules Co., the former was used for tests involving single fiiaments and the composite was prepared from the fabric and Epon 828,‘(Shell Chemicals). Coating of a selected number of materials by silica to a thickness of about 100 nm was carried out by vacuum evaporation of a 50150 mixture of SiOJSi. Following deposition the samples were given a mild oxidation treatment in air at 350°C to ensure that complete conversion to SiO? was achieved.
3. RESULTS
AND DiSCUSSIONS
3.1. Controlled atmosphere electron microscopy
studies
When transmission specimens of single crystal graphite were treated in 2.0 Torr of a gas mixture containing atomic oxygen at room temperature no apparent changes were observed over short periods of time; nevertheless erosion of the surface took place very rapidly when the temperature was raised to 100°C. This took the form of the creation of irregular shaped pits over the entire basal plane region. It was significant that this same pattern of behavior was observed with specimens exposed to the atomic oxygen beam at room temperature and subsequently heated to 100°C under vacuum conditions; it appears that atomic oxygen adsorbs and reacts with graphite at room temperature (i.e., forms bonds with carbon) without significant loss of mass. Additional energy is then required to break the bonds and release the gases, which we assume are the same as those identified by Wicke and coworkers in their experiments with soot mentioned earlier[ 15-181. The characteristics of these pits were quite different from those formed by certain metal and metal oxide particles in graphite in the presence of molecular oxygen at appreciably higher temperatures~30,31]. In this case attack took place at isolated imperfections in the basal plane to produce well defined hexagonal shaped pits, which over a period of time expanded to become more circular in outline and progressively deeper as the catalyst penetrated through the structure. Figures l-3 are electron micrographs of three different regions of a specimen undergoing attack in atomic oxygen at 100°C. The time delay between each micrograph is approximately 10 min. Continuous observatron of the reaction revealed a number of other features of the system. (a) Attack appeared to be confined to the basal plane regions; there was little evidence of widespread edge reces-
et al.
sion. In this regard it is worth noting that prior to treatment in atomic oxygen, specimens were not outgassed in high vacuum at elevated temperature and as a consequence edge regions might contain adsorbed species (b) Detailed examination of the pitting action showed that individual pits did not appear to expand with time but increased their boundaries by coalescence with other adjacent pits. Eventually pitting in a given multilayer proceeded to such an extent that an entire multilayer was removed, leaving a lacy unreacted residue on the surface. Furthermore. the depths of pits, as estimated by the difference in contrast with surrounding unattacked regions, tended to be uniform. (c) The indiscriminate nature of the reaction made it impossible to identify any dynamic events that could be followed for the purpose of estimating rates of oxidation. (d) An exception to this general behavior was the observation that atomic oxygen tended to display preferential attack along twin bands. This aspect is clearly evident in Fig. 2 at the location marked “x”. (d) The number of fresh pits produced per unit area increased with both time and temperature. The preferential attack of atomic oxygen on graphite, which was confined to the basal plane with no evidence of corrosion at the edges. clearly indicates that the adsorption and bonding of oxygen and the subsequent removal of carbon is restricted to areas where there is a significant availability of electrons (i.e., rr electrons). This finding demonstrates that the reactivity of carbons towards atomic oxygen is very much dependent on structural parameters, which contradicts previous reports by Marsh et al. [8-lo], who suggested that the rate of carbon gasification was independent of the nature of the carbonaceous solid. It is then expected that materials containing sp* or sp bonds, or those containing graphitic structures are expected to be much more reactive than those with saturated bonds, and that materials possessing saturated bonds such as silica should remain inert in the presence of atomic oxygen and thus could be used as protective coatings. In addition, the in situ experiments confirmed previous observations reported by several investigators that the erosion of graphite does not occur as a direct consequence of the interaction of atomic oxygen with graphite at room temperature (in which case the surface will be saturated with bonded oxygen), but by the addition of a small amount of energy, or prolonged outgassing of the sample. In a second series of experiments, carbon filaments supported on a carbon film were reacted in atomic oxygen under the same conditions as those used for the graphite specimens. Some features of the unreacted material are apparent on the electron micrograph, Fig. 4A, where the fish bone structure of the ordered external component is apparent. When carbon filaments were heated in an atomic oxygen environment, the first signs of gasification were observed at 50°C. The progressive change in appearance of this material at IbOY can be seen in the
~nteraetion of atomic oxygen with carbon
Fig. 5. Scanning electron micrograph of a carbon fiber before reaction in atomic oxygen
Fig. h. Scanning electron micrograph of the same fiber as in Fig. 5, following reaction in atomic oxygen at 75°C for 2.5 h.
uel nce of stills taken from the TV monitor at 5 _ ~~ -. mm Intervals, Figures 4A to 4D. Under these conditions attack took place preferentially at the graphitic skin regions with subsequent oxidation of the disordered inner core structure. This behavior is to be contrasted with that found with molecular oxygen where the inner core gasified at 625°C followed by
removal of the graphitic skin at 72S”C[20], It was that in the presence of atomic oxygen, oxidation of the amorphous carbon film support coincided with attack of the inner core component of the filaments. These results confirm our belief that there is selectivity in the reaction of atomic oxygen with carbonaceous solids, signi~cant
P. PATTABIRAMAN etal.
Fig. 7. Scanning
electron
micrograph
of a carbon
fiber epoxy composite
before
reaction
in atomic
reaction
in atomic
oxygen.
Fig. 8. Scanning electron
micrograph
of the same composite as in Fig. 7, following oxygen at 75°C for 2.5 h.
Post-reaction scanning electron microscopy examinations McKee and Mimeault[32] summarized the developments in surface treatments of carbon fibers in a review article published in 1973. Since that time the major emphasis on the surface chemical modification
3.2.
of the carbon fiber has tended to follow procedures of either plasma etching[26,33], electrochemical techniques[34-371, or catalysis in a molecular oxygen environment[38,39]. With the latter approach, selected catalysts could be chosen to produce pitting, but in this case weakening of the fiber can occur by
Interaction
of atomic oxygen with carbon
875
Fig. 9. Scanning electron micrograph of a carbon fiber epoxy composite that has been heated in atomic oxygen at 100°C for 4.0 h. Region (A) has been coated with a l(H) nm thick film of silica. and region (B) is uncoated.
the extensive reaction caused by the catalyst. In a recent report it is claimed that when silver is used as the catalyst for the gasification of carbon fiber in molecular oxygen. this treatment offers advantages over other methods of oxidation including plasma etching in argon and oxygen[38]. In the present studies we have attempted to apply the knowledge gained from our in situ electron microscopy experiments with atomic oxygen-graphite and atomic oxygen-carbon filaments to the behavior of carbon fibers when treated under similar conditions. Figures 5 and 6 are scanning electron micrographs showing the appearance of a carbon fiber before and after treatment in atomic oxygen at 75°C for 2.5 hours. Widespread and evenly distributed surface pitting is clearly evident in the treated specimen. Although these observations appear to be somewhat different from those reported by other authors[33] who performed the experiment under conditions
where prolific attack of the structure occurred. it is worth noting that there are significant differences in the conditions and nature of the material used in the two investigations. The conditions of our experiments were selected according to the observations of the in situ gasification studies of graphite and carbon filaments, from which it is possible to identify the parameters under which the onset of attack by atomic oxygen occurred, and in this way control the extent of the reaction. The exposure time (2.5 h) used in our experiments was selected to achieve sutficient pitting to be observable by SEM. In addition, the samples used in our experiments were conventional PAN carbon fibers that had not been graphitized. In other experiments, the effect of reacting a carbon fiber epoxy composite in atomic oxygen at the same conditions as those stated above was investigated. From a comparison of Figs. 7 and 8 it is ap-
Table 1. Weight loss measurements
I ;;;W& MYLAR I
MATERIAL
UNCOATED (% LOSS)
FIBER/EPOXY COMPOSITE :3 2:3
P. PATTABIRAMAN Table
2. Atomic
concentration
of carbon
ei ul
fiber surfaces
detcrmincd
from XPS analysis
FIBER EXPOSED TO ATOMIC OXYGEN
parent that whiie the fibrous component has undergone extensive pitting, attack of the epoxy matrix has been far less severe, being limited to a “peeling action” in certain regions. In an attempt to evaluate the degree of protection offered by a silica coating, SEM examinations were performed on the carbon fiber/epoxy composite samples in which the exposed surface was partially covered with a 100 nm thick film of silica. These specimens were treated in an atomic oxygen environment for 4.0 hours at 100°C. Figure 9 shows the startling contrast in appearance of the surface at the boundary between the silica coated (A) and the uncoated (B) regions. The silica coated region appears to be unchanged while the exposed area shows evidence of widespread attack. Although no tests were performed to evaluate the degree of adhesion of the silica film on the composite, the lack of pitting of the fibers or peeling action of the matrix suggests that good adhesion was achieved A quantitative estimate of the protective effect of the silica treatment was made from weight loss measurements in which uniformly coated and uncoated specimens were reacted side by side in the atomic oxygen flux for 4.0 h at 100°C. The data obtained for the composite samples is presented in Table 1 and compared with that found for two polymeric materials, Kapton and Mylar, which were reacted under the same conditions. From these experiments it is clear that the silica coating afforded a very effective protection for all three materials. 3.3. ~e~~ure~ents of surface properties XPS was used to ascertain the change in relative atomic concentration of carbon and oxygen following exposure to atomic oxygen. The data for treated and untreated specimens is presented in Table 2. The significant increase in oxygen atom concentration on the surface is consistent with the notion that species derived from atomic oxygen are adsorbed on the fiber. This leads to the possibility of modification in the wetting properties of the fiber and a consequent change in the strength of an interaction with a polymeric matrix material. The surface energy measurements consisted of determining the values for the polar and dispersive components of the surface energy of treated and untreated fibers. The polar component is the result of short range interactions and is relatively insensitive towards crystalline structure, being dependent
on the density of polar atoms per unit area and their intrinsic polarity. The dispersive component results from long range interactions and is a function of the density and mass of the underlying material. In the current experiments. the polar and dispersive components were determined from the interaction of fibers with water and either pentane or heptane respectively. Figure 10 illustrates the typical curves obtained when carbon fibers are immersed in the hydrocarbon-water system, which are the result of the interfacial reaction of carbon fibers with liquids of different polarity. The untreated fiber (A) exhibits a strong affmtty for the hydrocarbon (section b), but shows little tendency to interact with the polar solvent (section c). This is because of the total absence of polar groups on the carbon fiber surface. In contrast, when the fiber is treated with atomic oxygen at 75°C for 2.5 hours, the surface properties change dramatically. This aspect can be seen very clearly from the strong interaction of the treated fiber with water, where there is a substantial increase in the wetting properties over that exhibited with the hydrocarbon. The values presented in Table 3 were obtained by analysis of the experimental data according to the procedures given in ref. 26. Finally, an estimate of the strength of the inter-
IA1
II -Cd
Untreated Fiber
b
a
Lr
-9-c
C
[Bl FiberTreatedinAtomicOxygen
FIBER
DISPLACEMENT
Fig. 10. Variation of the wetting characteristics of carbon fibers in various media. (A) untreated and (B) treated in atomic oxygen: (a) air. (b) pentane or heptane, and (c) water.
Interaction Table 3. Surface
of atomic
oxygen
877
with carbon
energy characteristics of carbon fibers
MATERIAL
UNTREATED flBER FIBER EXPOSED TO ATOMIC OXYGEN
facial bonding between fibers and an epoxy resin was obtained by the microband test. The values reported in Table 4 are the mean of ten measurements for each type of fiber. It should be noted that the fibers treated in an atomic oxygen environment were very brittle and required careful handling while the epoxy droplet was allowed to cure and while positioning the sample on the Instron. Examination of the data presented in Tables 3 and 4 shows that treatment of carbon fibers in an atomic oxygen environment has extremely beneficial effects. On one hand. the roughness of the surface will enhance the adhesion between the matrix and the fiber to prevent delamination. In the case of carbon fibers interacting with highly polar matrices (i.e., oxygen rich structures), the presence of oxygenated groups at the surface of the fiber will tend to increase the strength of the interaction between fiber and matrix. Nevertheless, because the extreme reactivity of carbon towards atomic oxygen, extensive treatment may lead to weakening of the structure due to enhanced pitting. In contrast, a well controlled oxidation of the fiber in such an environment should result in both an enhancement in the physical interaction caused by the micro-pitting action and an increase in the chemical interaction established between the oxygenated groups on the fiber and the matrix. The experiments carried out using graphite as a model demonstrated that it should be possible to adsorb and subsequently bond atomic oxygen to carbon at room temperature without damage to the structure provided the treatment is short and performed out at low temperature. Based on present observations it is apparent that the approaches adopted for measurement of surface chemical properties must take into consideration the fact that graphite surfaces treated in atomic oxygen at relatively low temperature contain a significant
Table 4. Interfacial
strength
32~4 66+7
$;:,
number of oxygenated groups that could prevent the adsorption of other nonpolar gas molecules. Indeed, as demonstrated by Yang et a1.[13,14], in order to produce reactive sites on the surface it was necessary to heat graphite in nitrogen at a temperature of 900°C in order to remove adsorbed oxygenated species produced on the surface during exposure to atomic oxygen. It is clear, therefore, that unless such a procedure is followed, one might obtain misleading data for the estimate of surface areas from gas adsorption methods. One possible solution might be to examine the interaction of these treated carbon surfaces with selected polar liquids.
4. SUMMARY
The results of this study confirm previous findings that atomic oxygen is extremely active for the gasification of carbon. However, the current in situ electron microscopy experiments have demonstrated that atomic oxygen selectively attacks different carbon structures, with graphite being the most reactive material due to the presence of delocalized rr electrons, which act as centers for reaction. This behavior contrasts with the reactivity pattern found between various carbons and molecular oxygen at elevated temperatures and therefore leads to the possibility of the development of some interesting applications. The treatment of a carbonaceous solid in an atomic oxygen environment under well controlled conditions such as exposure time and temperature can produce physical and chemical modifications to the surface of the solid. which can be tailored to improve the performance of the material in a number of applications. In the case of carbon fibers, for use as reinforcement agents with epoxy resins, a careful treatment in atomic oxygen should produce an enhancement in surface area coupled with the forma-
determinations
from microbond
test
MATERIAL
INTERFACIAL STRENGTH
UNTREATED FIBER FIBER EXPOSED TO ATOMIC OXYGEN
49.7 f 5 87.9 k 9
P. PATTABIRAMAN etal.
878
tion of surface species that can form bridges between the fiber and the matrix, resulting in improved bonding.
Acknowiedgements-This
work was supported by the Department of Energy, Basic Energy Sciences Grant No. DEFGOS-89ERl4076. The in situ transmission electron microcopy facilities were built under NSWC Contract No. 60921~86-C-A226. The authors are indebted to Dr. Brian Wicke of General Motors Co. for invaluable discussions, Dr. Charles W. Neely for use of the atomic oxygen ex situ facilities and Messrs W.B. Downs and M.S. Kim for performing some of the experiments.
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