on the oxidation resistance of carbon-carbon...

Carbon Vol. 30. No. 3. pp. 365-374. 1992 ~8~~23/92 f5.W + .(X1 Printed in Great Britain. copyright 0 1992 Pergamon Press Ltd.

ON THE OXIDATION RESISTANCE OF CARBON-CARBON COMPOSITES OBTAINED BY CHEMICAL VAPOR INFILTRATION OF

DIFFERENT CARBON CLOTHS

T. CORDERO,* P. A. THROWER and L. R. RADOVIC Fuel Science Program, Department of Materials Science and Enginee~ng,

The Pennsylvania State Unive~ity, University Park, PA 16802, U.S.A.

(Received 15 October 1991; accepted 23 October 1991)

Abstract-Materials of widely differing properties were used as substrates for the preparation of carbon- carbon composites by chemical vapor deposition of carbon from a propylene-nitrogen mixture at at- mospheric pressure and 7509-850°C. The pyrolytic carbon yields were found to be independent of the surface area of the carbon-fabric substrate. The oxidation behavior of the composites and their individual components was studied over the entire burn-off range (0%-100%). The structure of the starting and partially reacted composites was investigated by X-ray diffraction and scanning electron microscopy. No synergistic effects were observed when the reactivities of the individual componentswere significantly different. It is thus concluded that a straightforward analysis of the complete burn-off profile of a composite, together with the knowledge of the structure and reactivity of the individual components, is necessary and can be sufficient for a fundamental understanding of its oxidation resistance.

Key Wo~Oxidation, carbon-carbon composites, CVD

1. INTRODUCTION

Oxidation protection of carbon-carbon composites is one of the most important unresolved issues in the quest for a new generation of materials with high specific-strength properties. In the past decade, a number of publications[l-121 and a brief review[l3] have appeared on this topic, stimulated by increasing interest in using these materials in structural as well as ablative applications. In particular, extensive studies have been conducted, and some of them pub- lished[l4], on the effects of a large number of oxidation inhibitors, protective coatings, and sealants. However, at a recent review of the state of knowledge in this area[lS], a consensus emerged that a fundamental understanding of oxidation kinetics, in- hibition, and protection of C-C composites still does not exist. It is probably safe to state that the “base case,” that of the oxidation resistance of unprotected composites seems to have been somewhat “hastily” analyzed. Even a superficial analysis of the relevant literature shows orders of magnitude differences, largely left unexplained, in reported oxidation rates for the various composites used. A case in point, for example, is the lower limit of intrinsic (i.e., chem- ically controlled) carbon oxidation reactivity, de- fined on the basis of data for a “single crystal” carbon. In a theoretical analysis, Luthra[lO] shows it to be about 10m3 kg/m2/s at 800°C and lo-” kg/m*/s at 600°C. These values are based on the work of

-. *Permanent address: Department of Chemical Engi-

neering, University of Malaga, 29071 MBlaga, Spain.

Thomas[lti], in which the overall rate is estimated on the basis of the rate of recession of etch pits in the basal plane of graphite. However, the reported oxidation rates of, for example, SP-1 graphite[l7] and pitch-based carbon fibers[l8] are significantly lower than this “theoretical minimum”[lO].

The objective of our study was to begin to (re)examine the oxidation resistance behavior of C- C composites, having in mind the (arguably insuf- ficiently exploited) existence of excellent qualitative, and in some instances very good quantitative, correlations between processing conditions (e.g., heat- treatment temperature), structure (e.g., crystalo- graphic parameters) and reactivity of carbons (see, for example, ref. [ 171). In particular, we address the issue of possible synergistic (or simple additivity) effects[l l] of fiber and matrix reactivities on the oxidation behavior of C-C composites prepared by chemical vapor deposition (CVD) of carbon on carbon cloths of widely differing structural properties.

2. EXPERIMENTAL SECTION

The composites were prepared in a conventional horizontal tubular furnace (ID, 5 cm> by chemical vapor infiltration of carbon at 750”850°C. Propyl- ene (2.5 vol. %), in a stream of nitrogen at atmo- spheric pressure, was used as the pyrolytic carbon precursor. Typically, a piece of carbon cloth (cu. 3 cm x 1 cm) was placed in a ceramic boat inside the isothermal zone of the furnace. The temperature was maintained to within ?l”C. Three commercial carbon cloths having widely differing properties were primarily used as substrates (see Table 1). Some ex-

345

366 T. CORDERO et al.

Table I. Selected properties of carbon cloths used in the preparation of C-C composites

Prouertv ACC* VCLt WCAt

Tw

Surface density, g/m2

Surface area, m2/g

Density (He,Hg), g/cm3

% Carbon % Ash RH

Active Carbon carbon cloth

110 269

1300 (N2) 1 (Kr)

9z2 (“zY4) 3.8 0:75 - 8.6

Graphite cloth

242

1 (Kr)

(1.42, 1.42) 99.9 0.01 7.9

*Activated charcoal cloth (Charcoal Cloth Limited, Maidenhead, Berkshire, U.K.).

tVCL = carbon cloth (Thornel, Amoco Performance Products, Inc.). WCA = graphite cloth (Thornel, Amoco Performance Products, Inc.).

periments were also performed using as substrates a zeolite powder (cracking catalyst Z-14 US, Dav- ison Chemical), a natural diamond powder (Aesar, Johnson-Matthey Inc., Catalog #13402), and a ceramic cloth (Nextel 312, 3M).

The structure of both the cloths and the composites was studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). For XRD studies, the samples were ground, placed inside a glass-supported hollow aluminum holder and examined using a Rigaku diffractometer with mono- chromatic (40 kV, 20 mA) CuKo radiation. The SEM photographs were obtained using an acceler- ating voltage of 25 kV, with no conducting metallic layer applied. The structure of the pyrolytic carbon matrix was investigated by recovering the carbon after demineralizing the zeolite-carbon composite. Standard demineralization procedure, involving HCl and HF treatment, was used.

Oxidation resistance studies (1 atm 02, 60 cm3(STP)/min) were performed by both nonisothermal and thermogravimetric analysis (Cahn mi-

0.8 -

1 3

E

OS- -. - ACC

-- VCK

0.4 - . . ..a..* “CL

0.2 ---a- “c**o

-.-‘*-.’ WCA

-I 200 400 600 800 1000

Temperature (“C)

Fig. 1. Nonisothermal TGA reactivity profiles of the fibers in 1 atm O2 (heating rate, 5°C mitt-‘).

crobalance, model li3X). The complete “burn-off profile” (i.e., carbon conversion vs. time curve) was investigated. The oxidation process was interrupted at selected conversion levels, and the partially reacted samples were analyzed by scanning electron microscopy.

Fig. 2. Rate vs. conversion plots for the VCL cloth at 1 atm O1 and different temperatures (given in “C).

Table 2. Experimental conditions used and deposit yields obtained during the preparation of C-C composites (1 atm,

2.5 vol. % propylene in Nz; 24 h)

Flow rate Temperature Cloth (cm3fmin) (“C)

ACC 200 750 ;: 750 800

200 850 VCL 200 750

400 750 200 800 200 850

WCA 200 750 400 750 200 800 200 850

%CVD

7.13 19.03 7.31

39.37 6.98 7.33

18.84 35.07 7.04 7.39

20.62 38.51

Table 3. Comparison of deposit yields on different carbonaceous and ceramic substrates (24 h; 750°C; 200

cm3 min-‘)

Substrate Surface area (m’/g)

ACC cloth -1300 (NJ VCL cloth -1 (Kr) WCA cloth -1 (Kr) Nextel 312 cloth <I Zeolite n.a. Diamond n.a.

n.a. = data not available.

% CVD

7.13 6.98 7.04 1.43

33.5 1.33

Oxidation resistance of carbon-carbon composites 361

3. RESULTS

Figure 1 compares the oxidation resistance of the carbonaceous cloth substrates that were screened for the present investigation. On the basis of these results, substrates with the lowest, highest, and inter- mediate reactivity were selected for infiltration and a more detailed oxidation study. As expected, the high-surface-area activated carbon cloth with a relatively low carbon content, and thus a high H/C ratio and high (re)active surface area[l9], is the most reactive, whereas the low-surface-area WCA graphite cloth, having a high carbon content (and thus a low H/C ratio and a relatively low (re)active surface area) is the least reactive. This order of reactivity is in qualitative agreement with their degree of

crystalline order, determined by X-ray diffraction (shown in Table 4): the average d-spacing decreases and the average crystallite size increases in the expected order: ACC, VCL, WCA.

Figure 2 shows the isothermal reactivity profiles for the VCL cloth in 1 atm 0,. It is seen that the reaction rate (expressed in g of C reacted per unit time per g of residual carbon) monotonically increases in the entire conversion range. The same behavior was exhibited by the other two carbon- cloth substrates examined in this study. Indeed, this is a common feature for the uncatalyzed gasification of many carbon materials[l9]. It can be related to the fact that, as gasification proceeds, the ratio of edge (potentially reactive) sites to the total number of carbon atoms monotonically increases[20]. In all

8

Fig. 3. SEM photographs of the ACC fibers (a) and ACCICVD850 composite (b)

368 T. CORDER0 et al.

cases, the apparent activation energy of the process was found to be independent of conversion, suggesting the absence of intrapore diffusional limitations.

Table 2 summarizes our chemical vapor infiltration results obtained after 24 hours under different conditions of temperature and flow rate. In all three cases, it is observed that doubling of the flow rate has no major effect on the amount of pyrolytic carbon deposited (CVD carbon) (i.e., the CVD carbon yield does not depend on the residence time of the gas in the reactor). The higher the deposition temperature and the longer the deposition time, the larger the CVD carbon yield. Furthermore, there is no relationship between the substrate’s surface area and pyrolytic carbon yield (Table 1). Attempts to verify the existence of a surface-area effect were

made by reducing the partial pressure of the carbon precursor (2.5 x 10m5 atm C,H,); however, no deposition took place on any of the substrates, even after 48 hours at 850°C. Deposition on non-carbonaceous substrates (Table 3) gave results that are at least qualitatively more in line with the corresponding surface areas, being greatest for the microporous zeolite and quite low for the nonporous diamond powder (particle size, 40-60 pm) and the low-surface-area ceramic cloth.

Figures 3-5 show representative SEM photographs of ACC, VCL, and WCA cloths and their respective composites prepared at 850°C. In all cases, the deposit appears to completely cover the fiber surface. It is interesting to note, however, that its morphology appears most homogeneous on the ACC cloth and least homogeneous on the WCA

a

Fig. 4. SEM photographs of the VCL fibers (a) and VCL/CVD850 composite (b).


Fig. 5. SEM photographs of the WCA fibers (a) and WCAICVD850 composite (b)

0 ACC

A ACc?KX’D760(6h)

n AtXXVD760(24h)

A AccKNDBoo(24h)

Ii34 1.26-2 1.4.A 1.1

l/T (l/IO

a - ACC

- ACUCVD760

- ACUCVDOCm

c!onwmioll

Fig. 6. Arrhenius plot of ACC and ACUCVD composites (at 50% conversion).

Fig. 7. Rate vs. conversion plots for the ACC cloth and its composites at 460°C and 1 atm 02.

CAR 30:3-c


v-

0.0 oi a-4 ail oi

Fig. 8. Rate vs. conversion plots for the VCL cloth and its composites at 550°C and 1 atm OZ.

cloth. This could be related to the large number of nucleation centers for CVD carbon on the very reactive ACC cloth and the small number of these centers on the unreactive WCA cloth.

Figure 6 shows the Arrhenius plots of the microporous ACC cloth and its composites. The apparent activation energy is seen to decrease as the CVD carbon content increases, from -35 kcal/mol for the substrate to -21 kcallmol for the composite with 19% CVD carbon. This could be due to increasing diffusional limitations as the deposited matrix carbon decreases the effective size of the pores.

Figures 7 and 8 clearly show the oxidative-protection effect of the pyrolytic carbon deposited on the ACC and VCL carbon cloths: the reactivity of the composite decreases with increasing CVD carbon content. Similar results were obtained by Mar- inkovic and coworkers usinr! as substrates a cellulose powder[21] and a PAN-dehved, phenolic-resin-im-

Fig. 9. SEM photograph of the ACCICVD850 composite after 60% conversion at 460°C in 1 atm OZ.

Fig. 10. SEM photograph of the VCLICVDSSO composite after 40% conversion at 550°C in 1 atm 02.


remain and the essentially intact hollow tubes of pyrolytic carbon are clearly seen. In contrast, both Fig. 8 and Fig. 10 suggest that no such preferential oxidation occurs for the VCL composite; in addition to the disappearance of most of the matrix after 40% conversion, the representative SEM photograph (Fig. 10) contains visible signs of fiber oxidation.

. 1 *

0.2 a4 ds da 1:o

-on

Fig. 11. Rate VS. conversion plots for the WCA cloth and its composites at 650°C and 1 atm Oz.

pregnated carbon-fiber preform[8]. An important distinction should be noted, however, between Figs. 6 and 7: while for the VCL composite the reactivity profile does not change appreciably with the increase in CVD carbon content, for the ACC composite significant changes are observed. These can be ex- plained by the vast reactivity differences between the fibers and the matrix (see below) in the latter case. The comparison of the matrix content data in Table 2 with the reactivity profiles in Fig. 7 suggests that the ACC fibers are completely consumed first; the reaction temperature is not high enough for the gasification of the matrix and so the reaction essentially stops after -93%, -81%) and -61% conversion for the ACCICVD750, ACCICVD800, and ACCICVD850 composites, respectively, in excellent agreement with the data in Table 2. Figure 9 is a direct confirmation of such behavior of the ACCI CVD850 composite: after 60% conversion, no fibers

Figures 11 and 12 show that the oxidation behavior of the WCAlCVD composites is exactly opposite to that of the ACClCVD composites: the CVD carbon matrix is more reactive than the fibers and its complete consumption occurs before the onset of oxidation of the cloth substrate. The discontinuities in the reactivity plots (Fig. 11) correspond to the matrix contents of the composites (i.e., -7%, -21% and -39% (Table 2) for the WCA/CVD750, WCA/ CVD800, and WCA/CVD850 composites, respectively. The scanning electron micrograph of the WCA/CVD850 composite (Fig. 12) confirms this: after 40% conversion, the fibers remain essentially intact and no signs of the CVD carbon matrix are visible.

Figure 13 places in perspective the reactivities of the fabrics and composites obtained in this study with respect to representative values from the literature. If typical surface areas of 0.1-l m2/g are assumed for the least-reactive carbons, it is easily concluded that these reactivities are lower than the “theoretical minimum”[ 10,121.

4. DISCUSSION

While the objective of this investigation was not to analyze the kinetics of chemical vapor deposition of carbon and the relationship between substrate nature and deposit structure, several interesting ob- servations can be made on these issues. The most striking result is the same extent of carbon deposition on the ACC and WCA (and VCL) cloths, whose

Fig. 12. SEM photograph of the WCAKVDSO composite after 40% conversion at 650°C and 1 atm 0,.

372 T. CORDERO etal.

surface areas differ by three orders of magnitude (Table 3). Under the conditions employed (relatively high pressure), the nature of the surface and the number of active sites on it clearly do not govern the deposition process[26], even though the coverage of ACC cloth corresponds to less than a monolayer. Similar results were recently reported in a detailed study by Ismail et al.[27]: steady-state carbon deposition rates (using 10% CH, in Ar at 1 atm) on Saran char (853 m2/g) and WCA cloth (0.66 m2/g) were very similar. This was shown to be the result of a drastic loss of micropore surface area by the Saran char substrate after only 2%-3% of carbon deposition. It may well be that at the high pressures employed in our study, in addition to the total surface area[23] or the active surface area[26,27] of the carbon substrate, gas-phase reactions play a major role in the kinetics of carbon deposition, as suggested by McAllister et al. [28].

The average crystallographic parameters of the fibers, matrix, and their composites, shown in Table 4, together with the reactivity data, do suggest an influence of the substrate and reaction conditions on the nature of the deposit. For each substrate, as expected, higher deposition temperatures result in composites of increasing structural order. Deposi- tion at low temperatures results, however, in composites whose structure is more disordered than that of the corresponding substrates. The structure of the matrix carbon (deposited on a zeolite substrate; see below) is more ordered than that of ACC carbon, and less ordered than that of the WCA carbon, in agreement with the reactivity data.

The reactivity profiles in Figs. 7, 8 and 11 show

Table 4. Crystallographic parameters obtained for the fibers (cloths), matrix (deposited carbon) and the

C-C composites

Sample d, (nm)

Deposited carbon 0.363 ACC 0.419 ACClCVD750 0.423 ACCICVD800 0.364 ACUCVDBSO 0.344 VCL 0.389 VCLI CVD750 0.404 VCL/CVD800 0.360 VCL/CVD850 0.345 WCA 0.353 WCAiCVD750 0.343 WCA/CVD800 0.343 WCAICVD850 0.341

L, (nm) L, (nm)

1.7 4.4 0.9 0.9 :::

3.9 :.: 3.7 1:3 1.0 0.9 3.8 1.1 4.1 1.5 4.3

7.0 ;:: 2.5 :.: 2.6 617

that, in two out of three cases, the oxidation behavior of a carbon-carbon composite can be understood based on knowledge of the structure and behavior of the individual components of the composite, as discussed above. This, in turn, can be estimated using qualitative correlations between carbon’s thermal history (or crystallinity) and C/H ratios and parameters, such as active[l7,11] and reactive[l9] surface area. In other words, no synergism[ll] is observed in the gasification reactivity of the composites. When a smaller difference exists between the reactivities of the individual components of the composite, the situation is less clear, as discussed below.

In an attempt to determine separately the reactivity of the matrix material, a composite was pre-

9.0E-4 l.lE-3 1.3E3 l.SE-3

Fig. 13. Arrhenius plots of reactivity (maximum rate of change of bumoff) for selected carbons and C-C composites (1 atm air): 0, SP-1 graphite[l7]; 0, graphitized carbon black (V3G)[17]; v, carbon black (Monarch 700)[17]; A, activated carbon cloth (ACC), 1 atm 02; X , P-inhibited C-C composite[22]; I, 3DHTC/C composite[ll]; + , VSB- 32 graphitized fibers[23]; *, WCA cloth[23]; 0, WCA cloth, 1 atm 0,; +, CYOOC-C composite[24]; H, P55 carbon

fibers, 1 atm 0,[25].

I I I I I I I I I I 5 IO 15 20 25 30 35 40 45 50 55 60

Wt=)

Fig. 14. X-ray diffraction patterns of (a) zeolite substrate, (b) zeolite/CVD composite and (c) demineralized zeolite/

CVD composite.

Oxidation resistance of carbon-carbon composites

pared by chemical vapor infiltration of the zeolite powder (Table 3). Subsequently, the substrate ma-

373

synergistic effect cannot be evaluated. This is in agreement with the findings of Jones et a/.[51 and Lahaye et al.[ll], who concluded that the nature of the substrate (fibers) influences the structure and the reactivity of the matrix (and therefore of the composite). Work is in progress to further investigate this issue using C-C composites prepared by im- pregnation/carbonization.

terial was removed by acid leaching of the composite. Figure 14 shows the X-ray diffraction patterns of the substrate, the composite, and the recovered matrix material. Clearly, the disappearance of all zeolite peaks and the appearance of broad C(O02) and C(10) peaks in Figure 14 (c) indicate that this procedure was successful. Table 4 includes the crystallographic parameters obtained for the matrix and Fig. 15 shows its isothermal reactivity profiles. It is interesting to observe two distinct regions in the oxidation behavior of the matrix carbon, perhaps suggesting that two different structures are formed on the zeolite substrate. This result is consistent with the finding[29] that at relatively low temperatures and high partial pressures of the hydrocarbon, both laminar and isotropic carbon can form on the substrate surface. It is also consistent with the behavior observed during regeneration of coked zeolite cat- alysts[30]. (However, its implications have not been pursued further in the present study.)

The fact that the matrix provides some oxidation protection to the VCL fibers (Fig. 7) means that it is less reactive than the VCL substrate. However, a comparison of the data in Fig. 15 with the reactivity profiles for VCL fibers alone, shown in Fig. 2, in- dicates the contrary. For example, at 500°C in the range O-50% burn-off, the reactivity of VCL cloth does not surpass 1 h-l, whereas that of the matrix material (deposited on the zeolite) reaches values as high as 2 h-l in most of this range. Evidence for the higher reactivity of the “isolated” matrix component (with respect to its reactivity in the composite) can also be found by comparing, for example, the reactivities of ACUCVD750 and WCA/CVD750 composites, in Figs. 7 and 11. A more disordered carbon structure thus results by depositing carbon on the zeolite (as evidenced also by the data shown in Table 4). and in this case the possibility of existence of a

-48)

ho -490

-6U.l

4.0 -BdD

50

2.0

1.0

Cl0 0.0 OS! a4 01) 0.8 1.0

canwwian

Fig. 15. Rate vs. conversion plots (1 atm 02; T in “C) for the deposited matrix material recovered after acid leaching

of the zeolite/CVD composite.

5. CONCLUSIONS

A straightforward analysis of the complete burn- off profile of a carbon-carbon composite (O-100% conversion), together with the knowledge of the structure and reactivity of the individual components, can be sufficient for achieving a fundamental understanding of the oxidation resistance of C-C composites.

Acknowledgments-This study was made possible, in part, by a grant (to TC) from the Ministry of Education and Sciences of Spain. We thank Dr. Roger Bacon for kindly providing the samples of WCA and VCL carbon cloths. Donation of the ACC sample by Charcoal Cloth Ltd. is also gratefully acknowledged. Discussions with Dr. Jose Rodriguez-Mirasol are appreciated.

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