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HIGH-TEMPERATURE OXIDATION BEHAVIOUR OF


Curhon. Vol. 31, No. I. pp. I-6. Printed in Great Britain.

1993 Copyright

OOO8-6223/93 $6.00 + 00 0 I993 Pergamon Press Ltd.

HIGH-TEMPERATURE OXIDATION BEHAVIOUR CARBON MATERIALS IN AIR
V. ZH. SHEMET, A. P. POMYTKIN, and V. S. NESHPOR Kiev Polytechnic Institute, pr. Pobedy, 37, Kiev-56,252056, Ukraine
(Received

OF

18March 199 1;accepud in revisedjtirm 27 March 1992)

Abstract-The features ofhigh-temperature oxidation in air ofdifferent home-produced carbon materials (pyrolytic carbon, carbon fibres and coatings and carbon-based composite materials) have been analyzed. It has been determined that the oxidation rates of carbon materials in air at temperatures of up to 800°C can differ rather greatly which is conditioned by peculiarities of their production technology, differences in porosity and density, degree ofcarbon graphitization in the samples, etc. in all cases the oxidation mechanism is determined by the nature of carbon-carbon linkages. The low-temperature region (up to SOVC) is kinetic in which the surface reaction rate of oxygen with carbon is limited. For the high-temperature region of the process, the slight dependence of the rate on temperature is typical. In the case of presence of silicon-bearing protective coatings on the carbon materials, the oxidation process depends on the mass transfer of oxygen and carbon oxides through a barrier layer of SIC and SiOz. Key Words-Oxidation, rials, air. kinetics, isotropic and anisotropic carbon, fibres, carbon-based composite mate-

1. INTRODUCHON

Carbon and graphite/carbon-based composite materials, owing to the rare combination of physicochemical properties, have found a wide field of application. The relatively high heat conduction of graphite and carbon, the low linear expansion coefficient, the high corrosion stability in many media, the electric conduction, the high thermal stability and other properties permit carbon-based materials to be used in the most different branches of technology. At present, carbon fibres, coatings and carbonbased composite materials are most widely used, especially for solving problems in new technology. Among heat-resistant fibres the carbon fibres occupy a special position owing to the combination of high strength and low density. The carbon fibres, manufactured of chemical (synthetic) fibres, possess especially valuable properties. The carbon fibres surpass all known compounds in thermal properties[ l-31. The great concentration of r-conjugations in carbon fibres results in their exclusively high heat conduction. The developed system of reticulate linkages inhibits the course of the processes of thermal degradation. In addition, due to the mobility of such a system, the favourable conditions are created for recombination of the radicals, formed as a result of destruction. Unlike the majority of solid bodies with the temperature rise almost up to 2000°C the mechanical properties of carbon fibre materials become even improved[3,4]. In the inert atmosphere the carbon materials are stable up to about 2 lOo”C[4,5]. They are characterized by a very high stability for the majority of corrosive environments. These materials are very widely used in different branches of technology as structural elements of various devices, especially in advanced missile systems, aerospace vehicles (shuttle, etc.) and

military aircrafts, operating at high temperatures in the air environment. But the sensitivity ofthe carbon materials to oxygen at increased temperatures is rather high and this factor considerably decreases the used temperature limits of such devices applications. The corrosion stability of carbon fibres, coatings and carbon-carbon composite materials to oxygen in the air at high temperatures depends on many factors, which include the chemical composition and physicochemical properties of original artificial and synthetic fibres, the process of production of carbon fibres and their graphitization, the density, specific surface, porosity and presence of defects on the fibre surfaces and materials as well as some other factors. In the present paper the results of high-temperature oxidation of various home-produced carbon materials (pyroltic carbon, carbon fibres and coatings as well as carbon-based composite materials), obtained in our investigations and published earlier[h-91 in open press, are summarized.

2. EXPERIMENTAL

2.1 Materials
The samples for investigation have been cut of homogeneous blocks of the corresponding carbon materials and have been made into plates sized about 10 X 5 X 5 mm. Then the samples have been polished, washed in ethyl alcohol and dried off until constant mass. The samples of fibrous materials have been taken in the form of braided straps of fibres 15-20mm long and mass of up to 200 mg. The outer protective coating of negligible porosity less than 5% and high hardness, consisting of SIC with mass equal to about 2% of the fibres, has been produced by the CVD method[6].

2

V. ZH.SHEMET

etal.

2.2 Procedure
Kinetics of high-temperature oxidation of carbon materials in air at an atmospheric pressure has been investigated by the method of thermobalance. Derivatograph type OD- 103 has been used for oxidation under nonisothermal conditions. In this case, the heating rate of a sample was 10”C/min. The oxidation under isothermal conditions has been accomplished with the aid of balance MTB-10-8 (Setaram). The surface of the samples before and after oxidation has been examined with the aid of scanning electron microscope Mini-Sem-Super. 3. RESULTS AND DISCUSSION One of the most important structure-sensitive characteristics of pyrolytic carbon varieties is its density[6] which can vary within the wide range from 1.22 to 2.25 g/cm3 and reflects the definite differences in the structure (degree of material anisotropy) and substantially influences the physicochemical properties of the material. The experimental data on the influence of the density of isotropic and anisotropic pyrolytic graphite on oxidation rate in air are presented in Figs. 1 and 2[7-91. The rate constants have been calculated as weight loss per unit area of initial geometric surface. The decrease in the density of materials results in friability of its structure, that promotes increasing of active surface area and as a consequence raising of observed oxidation rate. This effect is revealed most brightly at relatively low temperatures (up to 800°C). In this case the oxidation rate is controlled by the surface reaction of oxygen with the active centers of carbon-defects of crystal structure (vacancies of basic plane, edge atoms, etc.) and macrodefects of a sample

(cracks, pores, etc.). At the same time, at high temperatures the degree of preferred orientation and defectiveness of structure exert a lesser influence on the oxidation rate of the material. We should also mention the decrease of the activation energy value for oxidation process in the high temperatures region for both isotropic and anisotropic pyrolytic carbon. This effect has also been observed by other investigators (e.g.,[ 10-141). Among carbon materials, the fibres occupy a special position owing to the low density and special fibril structure inherent only to them which hampers the spreading and growth of cracks. This factor results in their self-strengthening at high temperatures. At the same time the sensitivity of these materials to oxygen in the air at increased temperatures is rather high (Fig. 3). Therefore, the operating life ofthe structural elements made from such fibres is substantially lower than in the case of usage of the other types of carbon materials[ 151. Given in this figure are the results of carbon fibres high-temperature oxidation of different graphitization degree as well as of fibres provided with Sic coating. Somewhat lesser corrosion stability of fibres type BMB (high-modulus, produced from viscose raw material) as compared with material BMH (high-modulus, produced from polyacrylonitrile) is explained by a less perfect structure of the surface, presence of inner macrodefects as well as a lesser graphitization degree of carbon in them. Oxidation of fibre BMH occurs in general uniformly on the whole surface and results in the gradual etching of material. At the same temperatures, the surface of material BMB is covered by great numbers of etching pits[ 15,161. At the same time, the rated kinetic parameters of the oxidation process, such as an activation energy and reaction

isotropic

pyrolytic

graphite

21

E-01

+---_---7--~-----_--T~--~ 7.00 7.50 8.00 8.50 9.00 9.50 10.00 10.50 11 .oo

inverse

temperature

1 /T

/ OK*1 O4

Fig. 1. Dependence of oxidation rate of isotropic pyrolytic graphite on temperature and density of material.

High-temperature oxidation of carbon E+02 6 anisotropic pyrolytic

inverse
Fig. 2. Dependence

temperature

1/T / OK* 1O4

of oxidation rate of anisotropic pyrolytic graphite on temperature and density of material.

order as calculated in[ 171 for these materials possess similar values which is explained by the identical chemical nature of these fibres and the identical mechanism of oxidation[l6]. Dhami et a/.[ 151 report about correlation between microstructure and oxidation behaviour of carbon fibres, as well. To increase the corrosion stability of carbon materials, the protective coatings from refractory com-

pounds are applied to the surface of fibres. Efforts to protect carbon elements from oxidation span more than a half century[3,18,19]. The coating from silicon carbide (Fig. 3)[ 161 substantially increases the corrosion stability of the material in the air which permits to increase the operating temperature of modified fibre at least by 2OOC[ 161. Formation of SiO, layer (oxygen diffusion barrier) during oxidation leads to apparent activation energy increase. It should

1. 00

a



E a (BMH-SE)=128 E, (BMH)

kJ/mol

~ 0.00-j -.
I d

=112 kJ/mol

I Ea(BMB)=lOi! kJ/mol heating rate-lO'C/min

000

900

temperature T / C
Fig. 3. Gasification degree ofcarbon tibres BMH and BMB and fibres BHM provided with silicon carbide coating under conditions of polythermic oxidation in air[ 161,where weight loss degree (Y defined as ratio is of momentary mass loss AM(t) to the initial mass of the sample m,.

4

V.Zn.

SHEMET~T~

al.

be mentioned that due to the difficulties of obtaining the continuous coating of sufficiently high quality on the fibre surfaces, the oxidation process occurs both through points where the continuous protective coating is disturbed and on account of the oxygen diffusion through the coating of SiO,. Therefore, the considerable contribution to the kinetics of material oxidation is made by the processes which occur on the carbon-gas boundary. These conclusions are in accordance with the results of the theoretical paper of Luthra[ 51. The carbon-carbon composite materials (C/C) are of a special interest. The reaction of their oxidation occurs both on the surface of samples and in the voiume of macroscopic spaces, formed as a result of the peculiarities of such materials’ production technology. In the oxidation process, all structural forms of carbon of these materials take part: carbonized carbon fibre; low-tempe~ture pyrolytic carbon which envelops the threads of the braided straps of reinforcing carbon fibre; globular and threadlike pyrolytic carbon which partially fills the inner voids in the materials. Shown in Fig. 4 is a microstructure photograph of three-Dimensions carbon composite material sample. All three structural forms of carbon are well visible. The structure of the emptiness region of small density sample is given in Fig. 5. The globular and threadlike pyrolytic carbon are well visible. The oxidation process of C/C composite materials involves carbon gasi~~tion of reinforcing carbon fibres and all forms of low-temperature pyrolytic carbon. Threadlike carbon formation damaged by oxidation is presented in Fig. 6. Shown in Fig. 7 are the Arrhenius plots for oxidation of “two- and three-dimensional” materials in air. Here one can distinguish the kinetic tem~rature oxidation region (up to -SOo”C), in which the process is limited by oxygen interaction with active centers on carbon surface. That is correlated with the results obtained by other authors[3,12,13,20]. It should be pointed out that the

Fig. 5. Structure of the emptiness region of original least dense carbon composite material (breaking strength7 kg/cm2, magnific-465).

apparent activation energy values for two- and threedimensional C/C composites are different. Influence of macrostructure differences of C/C composites on oxidation process in temperature region over 800°C becomes negligible. Oxidation rate in this case is controlled by oxygen and reaction products mass transfer through relatively immovable film of gases in porous carbon body and over it[ 121. Apparent activation energies are comparable for both types of the materials and are in accordance with data ofother investigators (Table 1). Shown in Fig. 8 is the kinetics of hip-tem~rature oxidation in air of three-dimensional carbon composite material samples of different strength. The dependence of stability to oxidation on breaking strength of the samples of these materials is evidently the dependence of this characteristic on the density of the material (i.e., the corrosion stability of carbon materials somewhat increases with the increase of density). The comparison of obtained data with the results of anisotropic and isotropic pyrolytic graphites oxidation confirms the conclusion.

Fig. 4. Microstructure of the sample of “three-dimensional” carbon composite material (magnific-465): Imaterial of reinforcing fibre; Z-pyrolytic carbon on fibre; 3-globular (threadlike pyrolytic carbon).

Fig. 6. Surface of C/C composite sample in the emptiness region filled with threadlike pyrolytic carbon after oxidation in air at 6Oo’C during 40 min., magnific.-465.

High-temperature oxidation of carbon

5

6.50-

6.40-

1.150

1.250

1.350

1.450

1.550

inverse

temperature

l/T

/

l/(103*‘K)

Fig. 7. Arrhenius plots for oxidation of “two- and three-dimensional” carbon composite materials in air, (Rate constant values are calculated as in[ 171.)

4. CONCLUSIONS

Thus, the carbon materials begin to become oxidized intensivly in air at temperatures over 500°C. The rates of this process for different types of these materials (pyrolytic anisotropic and isotropic graphite, fibres, carbon composite materials) can differ rather greatly which is conditioned by the peculiarities of the production technology of these materials, the degree of carbon graphitization, the differences in porosity and density, and the defectiveness of surface and other factors. But the mechanism of the oxidation process is similar to all carbon materials and is

determined by the nature ofcarbon-carbon linkages. On the basis of the kinetics parameters computation of carbon materials oxidation process (activation energy, reaction order) the low-temperature (kinetic) region of up to about 800°C can be distinguished in which the surface reaction rate of carbon with oxygen is limited. In the high-temperature (over 800°C) region, rather weak dependence of the rate on the temperature is observed. With the presence of silicon protective coatings on the carbon materials, the oxidation process depends on the mass transfer (diffusion) of oxygen and carbon oxides through a barrier layer made of SiO, and SIC.

Table

I. Apparent

activation

energy values for oxidation carbon materials Temperature region (“C) 500-800 400-600 600-900 600-900 below 600 above 800 below 465 above 465 400-600 600-750 750-1000

process of various

Material C/C Composite C/C Sheet Material Graphite Powder C/C Composite Carbon Fibres C/C Composite “Twoand ThreeDimensional”

E,, kJ/mole 140-180 164 8 184 140-180 20 181 79 47123 I16167 21126

Reference 13 14 14 21 22 22 23 23 our data

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V. ZH. SHEMET et aI,

‘: * 0, “bl E+OOc 8-

I+

800

grad

C 1
I

%
L

21
5

L

‘11” 67

I

I

I

I1111

1 E+02

2

3

4

5

67

1 E+03

fracture Fig. Kinetics 8.
of high-temperature

stress

Of

/

kPb+
carbon composite ma-

oxidation in air of the “three-dimensional” terial samples of different strength.

REFERENCES I. A. A. Konkin, In Strong Fibers (Edited by W. Watt and B. Perrow), Vol. 1, p. 253. North-Holland, Amsterdam (1985). 2. D. J. Pysher, K. C. Gove@ R. S. Holder, Jr., and R. E. Tressler, J. Am Ceram. Sot. 72,284 (1989). 3. D. W. McKee. GE Resean :h and Development Center Report, 89CRD 220, p. 53. General Electric Company, New York (1989). 4. E. N. Marmer, Coal graphite materials, p. 2 12. Moscow, (1973). 5. L. Luthra, Carbon 26,2 17 (1988). 6. V. S. Neshpor, Chemical Vapour Deposition of RejiectoryInorganic Materials, p. 32. State Inst. Appl. Chem., Leningrad (1975). V. A. Lavrenko, A. P. Pomytkin, V. Zh. Shemet, and V. S. Neshpor, J. Chem. Solid Fuel (USSR) 4, 139 (1987). V. A. Lavrenko, A. P. Pomytkin, V. S. Neshpor, F. L. Vinokur, and J. Izvest, Inorg. Muter., 16, 873 (1980). (Acad. Nauk USSR). F. L. Vinokur, V. S. Neshpor, E. I. Golovko, R. F. Voitovich, and J. Izvest, Inorg. Mater. 15, 277 (1979). (Acad. Nauk USSR).

10. J. M. Thomas, and E. G. Huges, Carbon 1,209 (1964). 11. C. Rascoe. Carbon 6,365 (1968). 12. E. S. Golovina (Ed.), Reactions ofcarbon with Gases. Moscow (1963). 13. D. W. McKee. Carbon 26.659 (1988). 14. D. W. McKee: Carbon 25; 551(1987). 15. T. L. Dhami, L. M. Manocha, and 0. P. Bahe, Carbon 29,51 (1991). 16. V. A. Lavrenko, V. Zh. Shemet, A. P. Pomytkin, N. P. Yanko, and P. I. Zolkin, High-Temperature Physicochemical Processes on Boundary Solid-Gas. p. 77. Nauka, Moscow, (1984). 17. V. A. Lavrenko, A. P. Pomytkin, and V. Zh. Shemet, J. Chem. Techn. (USSR), 4,34 (1982). 18. H. V. Johnson, U.S. Pat, 1, 948, 382 (February 20, 1934). 19. J. E. Sheehan, Carbon 27,709 (1989). 20. J. Lahaye, F. Louys, and P. Ehrburger, Carbon 28, 137 (1990). 21. D. W. McKee, C. L. Spiro, and E. J. Lamby, Carbon 22, 507 (1984). 22. D. W. McKee, Carbon 24,737 (1986). 23. B. Da%. and S. Marinkovic, Carbon 25,409 (1987).



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