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In Pyrolytic Carbons ,\ NATIONAL AERONAUTICS AND SPACE ADMlNlSTRATtON .. in Pyrolytic Carbons ,\ 'I $ h GPO PRICE $ CFSTI PRICE(S) $ -1 I ! Hard copy (HC) 2 -0a Microfiche (MF) <I ff853 July 65 ET PROPULSION LABORATORY $ CALIFORNIA INSTITUTE OF TECHNOLOGY 8 PASAD E N A, C A LI FOR N I A February 1, 1966 NATIONAL AERONAUTICS AND SPACE ADMINISTRATION Technical Report No. 32-532 Kinetics of Graphitization !. !. The Higb - Temperature Structural Transformation in Pyrolytic Carbons E. S. Fischbach (<. (<. oe, Manager Kterials Section JET PROPULSION LABORATORY CALIFORNIA INSTITUTE OF TECHNOLOGY PASADENA.CALIFORNIA February 1, 1966 Copyright @ 1966 Jet Propulsion Laboratory California Institute of Technology Prepared Under Contract No. NAS 7-100 National Aeronautics & Space Administration . JPL TECHNICAL REPORT NO . 32-532 CONTENTS 1 . Introduction ..................... 1 II. Experimental Procedure ................ 3 A . Materials ..................... 3 B . Measurement Techniques ............... 5 C . Heat Treatment ................... 6 111 . Experimental Results ................. 7 A . Unit-Cell Height ................... 7 B . Diamagnetic Susceptibility ............... 9 C . Preferred Orientation (x-ray) .............. 10 D . Apparent Crystallite Diameter .............. 11 E . Microstructure ...................11 F. Dimensional Changes .................13 G . Activation Energy for Graphitization ............ 13 IV. Discussion ...................... 16 V . Summary ...................... 21 Annnnrliv A Una+ Trna+mnnk Ttmn rnrrar+inn . _-9.3 r.rr. ..-.0. ...... I.... I...._... ....._ __ ..-...-.. Appendix B. Sample Size Effects .............. 23 Appendix C . Macroscopic Dimensional Changes ........ 24 Nomenclature ...................... 25 References ....................... 26 1 . Some deposition and structural parameters of the carbons investigated ...................... 3 2 . Correlation between Stage I rate constant kl and apparent crystallite diameter La determined from total diamagnetic susceptibility xT (as deposited) ...............18 JPL TECHNICAL REPORT NO. 32-532 FIGURES 1. As-deposited microstructures of the carbons studied; the substrate side of the deposit is toward the bottom of the photographs .................. .4 2. Unit-cell height as a function of log treatment time at several temperatures for carbon A; points, experimental data; curves, Eq. (1) .............. 8 3. Unit-cell height as a function of log treatment time at 2500OC for several carbons ................... 8 4. Total diamagnetic susceptibility as a function of log treatment time at several temperatures for carbon A; points, experimental data; curves, Eq. (3) .............. 9 5. Total diamagnetic susceptibility as a function of unit-cell height for several carbons ................... 9 6. Diamagnetic-susceptibility anisotropy ratio as a function of log treatment time at several temperatures for carbon A ....... 10 7. Diamagnetic-susceptibility anisotropy ratio as a function of unit-cell height for several carbons .............10 8. Preferred-orientation parameter as a function of log treatment time at several temperatures for carbon A; points, experimental data; curves, Eq. (4) ..............10 9. Preferred-orientation parameter as a function of unit-cell height for several carbons ...................11 10. Apparent crystallite layer diameter (x-ray) as a function of unit-cell height for two carbons heat-treated at various temperatures ..................... 11 1 1. Microstructures after increasing isothermal treatment times at 290OOC for carbon D .................12 12. Microstructures after isochronal (15-35 min) heat treatments at increasing temperature for carbons E and F .........14 13. Fractional change in unit-cell height (logarithmic scale) as a function of treatment time at various temperatures for carbon A ... 15 14. Arrhenius plot of rate constants for carbon A .........15 15. Arrhenius plot, obtained by superposition method, for various carbons ...................16 16. Schematic representation of the degree of layer stacking disorder as a function of apparent crystallite diameter (light line) showing the graphitization path followed by many pyrolytic carbons (heavy line) ...............17 A-1. Heat-treatment time correction: (a) actual (solid line) and ideal (broken line) treatment profiles; (b) effective treatment profile ... 23 C-1. Macroscopic dimensional change as a function of unit-cell height. .. 25 IV JPL TECHNICAL REPORT NO. 32-532 ABSTRACT Under heat treatment at high temperatures, pyrolytic carbons ( pyrolytic graphites ) undergo an extensive structural transformation ( graphitization ) . Layer stacking order, crystallite size, and preferred orientation increase. There are associated changes in macroscopic di- mensions and optical microstructure. The kinetics of this graphitization transformation has been investigated for several pyrolytic carbons in the temperature range 2400-3000°C. The transformation was found to be thermally activated, with an effective activation energy of about 260 kcal/mole. The fundamental graphitization process appeared to be the same for all of the carbons studied, but there were differences in their detailed kinetic behavior associated with differences in as- deposited microstructure. Substrate nucleated carbons deposited under the usual conditions graphitized in a succession of stages of first-order form. In regeneratively nucleated and low-deposition-temperature car- bons these stages were broadened and could not always be resolved. Variations in both the rate and the mode of transformation can be understood in terms of the dependence of the graphitization process on apparent crystallite layer diameter. A simple model of the graphi- tization process consistent with these observations is discussed. The significance or' these Observations ior e1igiiieei iiig applicaiioils is coil- sidered briefly. 1. INTRODUCTION Graphite is the stable crystalline form of carbon at spacing) is 6.708 A; the unit-cell width a (basal-plane ordinary ambient pressure. It has a hexagonal crystal lattice parameter) is 2.462 A. Pregraphitic synthetic car- structure with an ordered ABABAB basal-plane stacking bons have a similar structure, except that the stacking sequence.* The unit-cell height c (twice the interlayer sequence is randomly disordered (turbostratic structure). For a completely turbostratic carbon, ch6.88 A. When 'There is an alternate ordered structure for graphite, the rhom- turbostratic carbons are heated to high temperatures, bohedral form, characterized by an ABCABC stackin se uence. Up to about 30% rhombohedral form can be prducel from graphitization tends to occur, the layer stacking evolving hexagonal graphite by appropriate mechanical treatment. How- toward the ordered sequence of graphite with a conse- ever, it is metastable and reverts to the hexagonal form on heating to temperatures above 1OOO"C (Ref. 1). quent decrease in c. The fractional change in unit-cell 1 JPL TECHNICAL REPORT NO. 32-532 height, g = (6.88 - c),0.172, is a convenient measure of microstructure exhibits characteristic “growth cone” fea- the degree of graphitization. Crystallite growth also tures and may conveniently be approximated by a model occurs, and in many cases an increase in preferred orien- consisting of a stack of wrinkled sheets. When pyrolytic tation is observed in the graphitizing particles. This carbons are heat-treated at temperatures above the graphitization transformation takes place much more deposition temperature, they undergo an extensive readily in certain types of carbon (pyrolytic, petroleum structural transformation somewhat reminiscent of recrys- or pitch coke, etc.) than in others (glassy carbons, carbon tallization in cold-worked metals. Layer ordering occurs, blacks, etc). crystallite size and preferred orientation increase, and the cone microstructure coarsens and may ultimately disappear. As a consequence of these structural changes, The process by which graphitization occurs is complex there are also changes in macroscopic dimensions. and is not well understood despite its fundamental im- portance to the science and technology of carbon and Because of its unique structure (large crystallite size graphite. Information on the kinetics of the process should combined with random layer stacking), high purity, be helpful in determining the detailed mechanism. How- density, and preferred orientation, pyrolytic carbon is a ever, until recently there has been a notable lack of in- much better research material for studying fundamental formation on this subject. In fact, it has been the general processes in carbons than conventional synthetic (or belief that graphitization is relatively insensitive to heat- treatment time and depends primarily on maximum natural) carbons. Relatively little attention has been given heat-treatment temperature. Thus it has been common to the kinetic aspects of the transformation in pyrolytic practice to specify the structure of a soft (graphitizing) carbons. It has been variously reported that there is a carbon in terms of maximum treatment temperature, often saturation partial degree of graphitization which increases with little concern for treatment time or conditions. Dur- with heat-treatment temperature (Ref. 13), that there is ing the last few years there has been increasing evidence a well-defined graphitization temperature (Ref. 14), and that this viewpoint is inadequate. Numerous studies that above 26OOoC the degree of graphitization increases (Refs. 2-12) on conventional soft carbons (petroleum
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