F LU0ROCAR BON PY R 0LYT IC GRAPH ITE D.
H.
LEEDS'
A N D
J U L I A N
HEICKLEN'
Aerospace Corp., El Segundo, Calif. 90045 An attempt was made to prepare high density pyrolytic graphite by the decomposition of C2F4 in a flow reactor and to obtain the parameters of the deposition. Sooting pressures, important in defining optimum deposition conditions, were determined for the C Z F ~system as a function of deposition temperature and flow rate. Deposition rates were measured, examined as a function of deposition pressure and temperature, and noted as being relatively temperature insensitive above 1750OC. Deposition rates with C2F4 were up to 50% lower (225OOC.) than CHI depositions under similar circumstances, and the densities of the CzF4 deposits were lower. Consequently, CH4 has superior deposition characteristics. I t was desired to characterize the properties of the cZF4 pyrolytic graphite compared to CH4 pyrolytic graphite prepared under similar conditions. The densities of the deposits from both systems were shown to rise with increase i n deposition temperature and decrease in deposition pressure. Photomicrographs of the deposits contained characteristic microstructures. An attempt was made to produce high density pyrolytic graphite from C2F4 at temperatures below 1500' C. to circumvent some of the residual interlaminar cooldown stresses acquired when the material is produced at 2000' to 225OOC. Production of high-density deposits was not feasible because of the low deposition rates encountered.
HIGHdensity
(2.25 grams per cc.) pyrolytic graphite is commercially produced in the flow pyrolysis of methane. However, many hydrocarbons give essentially the same results. Presumably acetylene is always an intermediate, irrespective of the starting hydrocarbon, although some investigators (Cole and Minkoff, 1957) do not believe it is a critical factor in the reaction. The details of the deposition are different if a halocarbon is used as the starting gas. Other work in this laboratory has indicated that CZF, carbonizes easily a t temperatures in excess of 600°C. The thermodynamically favored process is
The initial step in the decomposition is (Modica and LaGraff, 1965)
CzF4 -+ 2CF2 The CF2 fragments can further decompose or add fluorine to form C or CFI, respectively. I t was of interest to determine whether a useful form of pyrolytic graphite could be prepared from the pyrolysis of a fluorocarbon, in order to shed light on the mechanism of graphite decomposition and assess the effect on structures. Another reason for considering a fluorocarbon rather than a hydrocarbon as a starting material concerns the nature of the impurities. Pyrolytic graphite from hydrocarbons retains some hydrogen, an impurity which cannot come from halocarbons. Residual hydrogen in many crystalline solids is known to have a dramatic effect on strength.
' Present address, Super-Temp Co., Los Angeles, Calif.
* Present address, Department of Chemistry, Pennsylvania State University, University Park, Pa.
16802
This paper shows the feasibility of producing pyrolytic graphite from C2F4. This graphite does not have hydrogen as an impurity, and therefore might have different properties than graphites produced from hydrocarbons. Some of the properties of the graphites produced from C2F4 were compared with those from graphites produced from CH4 under comparable conditions. Experimental Procedure
The reactor assembly has been described in detail (Diefendorf, 1956). Deposition was effected by flowing the reactant gas (either C2F4 or CH4) through a nozzle and along an AGOT graphite (a commercial product of Graphite Products Division, Union Carbide Co., New York, N. Y.) tube surrounded by a resistance-heated deposition tube. The deposition tube was 1% inches in outside diameter and 9% inches long with a 200 RMS microfinish surface. Prior to and after deposition, the tube and nozzle were weighed. The increase in weight gave the amount of carbon deposited. During the deposition, the flow rate and pressure were monitored continually. A Leeds & Northrup Co. optical pyrometer was used to obtain the brightness temperature of the tube by sighting the pyrometer through a quartz window onto the brightest portion of the reaction tube. The brightness temperatures were not corrected for the quartz window. For some runs the pressure a t which soot formation began to occur was found by visual determination of sooi cone formation just above the nozzle. After deposition, the tube was sectioned and the thickest areas of the deposit were allocated for measurement. One area of freshly separated deposit was measured and weighed to determine bulk density. Another sample was immersed in xylene to obtain "true" densities by displacement. A third sample was sent to R. J. Diefendorf a t Rensselaer Polytechnic Institute for independent true density determination by the density gradient method VOL. 8 NO. 3 S E P T E M B E R 1969
233
using miscible liquids. Elemental analysis was done on a fourth portion. A final portion was mounted and polished for metallographic analysis. Photomicrographs a t 200 and 500 magnifications were obtained. The brightness temperature may be in error by 200" to 300°C. from the true temperature (including an 80°C. quartz window correction). T o make valid comparisons, comparable runs were done with CH, and C2F4. Results and Discussion
Soot Formation. Tetrafluoroethylene was passed through a hot tube and pyrolytic graphite was formed. As the C2F4 pressure was raised, soot formed in the gas phase. The onset of this soot formation occurred a t a sharply defined pressure (sooting pressure) for a given temperature and flow rate. If the pressure was lowered slightly, the soot disappeared. The precision of the sooting pressure measurements is about &lo% (Figure 1). Sooting pressure is important because the optimum pressure for the highest deposition rate of high-density pyrolytic graphite is 1 to 2 Torr below the sooting pressure (Diefendorf, 1964). At brightness temperatures below 1000" C ., the sooting pressure is very much larger than a t higher temperatures. Undoubtedly this reflects the fact that the decomposition rate of the gas falls rapidly as the temperature is reduced below 1000°C. The results in Figure 1 can be summarized as follows: For a given flow rate, the sooting pressure is not significantly altered by brightness temperature changes between 1260" and 1760" C.; above 1760"C., the sooting pressure rises with temperature a t a given flow rate; at any temperature the sooting pressure exhibits a minimum flow rate of about 0.3 standard cubic foot per hour; at
NO SOOT SEEN BELOW 30 mm ANY MASS FLOW
n I
c
0 v, 0
' t
O
W
1000
2000
1
750
3000
4000" F
I I I I l l 1000 1250 1500 1750 2000 2250'C TEMPERATURE
Figure 1. Variation of sooting pressure with deposition temperatures for C2F4 at various flow rates 234
any flow rate the sooting pressure exhibits a minimum a t a brightness temperature of 1500°C. These trends will probably be relatively independent of the furnace geometry, though the absolute sooting pressures will not. The initiation of soot formation is a gas-phase process and indicates that the gas-phase reaction has become important relative to wall deposition. An increase in pressure retards diffusion to the wall and thus promotes gasphase carbon growth. Consequently, if the pressure is sufficiently large, soot appears. At very low flow rates, decomposition of the gas is complete and the carbon deposit is mainly near the entrance of the tube. Apparently under these conditions a larger pressure-i.e., a larger deposition rate-than at somewhat higher flow rates is needed to produce enough carbon to see the onset of soot, although the reason for this is not clear. At very high flow rates, the residence time is smaller than a t lower flow rates, so that the particles do not have time to reach their full size; the tendency to soot is reduced and a higher pressure is required for soot formation. From Figure 1 it can be deduced that the optimum flow rate is about 0.5 to 1.0 standard cubic foot per hour, even though the sooting pressure is near its minimum. For lower flow rates the deposition rate is reduced, whereas at higher flow rates much of the gas is either unreacted or the carbon is carried out the stack. Theoretical studies (Heichlen et al., 1969) have shown that for flow rates above 1 standard cubic foot per hour, the maximum rate of carbon production is not reached in this reactor until the gas has left the reactor. For brightness temperatures above about 1760"C., the sooting pressure rises with temperature for a given flow rate. A possible explanation is that above 1760"C. particle growth becomes reversible, the decomposition of large carbon species becomes important, and soot formation is retarded. Another possibility is that the accommodation coefficient for particle deposition rises with temperature, and thus carbon particles are more rapidly removed from the gas phase. Carbon Deposition. The deposition rate and density of the carbon deposit are listed in Table I for various temperatures and gas pressures for both C2F4and CH,. Under the experimental conditions about 15 grams per hour of available carbon flowed through the tube. At about 15 Torr of C2F4, the deposition rate is about 3.5 grams per hour for temperatures of 1750" to 2250" C. but the density of the deposit drops drastically with temperature; for CH4 a t about the same pressure, the deposition rate and density are, respectively, 4.5 grams per hour and 2.17 grams per cc. at both 2000" and 225OoC., while a t 1750°C. both parameters are reduced; as the pressure drops a t 1750"C., the deposition rate diminishes and the density rises, for both CH4 and CZFI;at 15OO0C., the deposition rates for both gases were too small to obtain reliable data a t low pressures. For comparable conditions the results with C2F4 and CH4are similar, but both the deposition rates and densities are somewhat lower with the C2F4 system. The formation of pyrolytic graphite has been envisoned as occurring through two possible routes. The early work was reviewed by Bone and Coward (1908). More recent reviews were by Porter (1955) and Gaydon and Wolfhard (1960). The last two articles came to opposite conclusions. One school suggests that small decomposition fragments
l & E C P R O D U C T R E S E A R C H A N D D E V E L O P M E fU T
~~~~
Rate
Displace-
of Depo-
Temp., F'ressure Torr c. 0
1300 1000
20.0 15.4 13.5 9.5 4.8 3.3 1.0 11.0 1.0 8.0 45.0
2250 2000 1750 1750 1750 1500 1500
20.0 15.4 13.1 3.1 1.0 9-14 1.0
2250 2000 1750 1750 1750 1750 1750 1500 1500
Bulk
sition, Density, G.IHr. G.ICc. From CzF, 1.82
1.2 1.5 1.25 0.4 0.5 0.4
Gradient
ment
Density,
Densify,
G.fCc.
G.f Cc.
...
2.17 1.71 1.02 1.13 1.57 1.67
1.78 1.90 1.50
1.82 2.13
...
1.48
... ...
... ...
...
...
...
~
Table II. Pyrolytic Graphite Composition Flaw rate = 1.0 standard cu. footihour Deposition time = 3 to 7 hours
Table 1. Pyrolytic Graphite Deposition Flow rate = 1.0 standard CU. footiholir Deposition time = 3 to 7 hours
...
Temp.,
Pressure, Torr
2250 2000 1750
20.0 15.4 13.5
c.
Impurity: %
wt.
Impurity," Mole %
From C2F. 0.15 0.11 0.13
0.10 0.01 0.08
From CH, 2250 2000 1750
20.0 15.4 13.1
0.1 >0.1 0.2
1
>I 2
"Fluorinefmm C,F, and hydmgen from CH,.
...
...
Fmm CH, 4.8 4.3 2.4 1.8 1.45
1.2 0.4
2.07 2.01
...
1.98 2.06 1.30
...
... 2.18 1.60 2.18 2.16
...
...
2.16 2.18 1.63 2.35 2.27 1.28
...
(such as Cs or C2H,) deposit on the wall (Porter, 1955). The other school suggests that the C2Hs intermediate polymerizes to larger and larger aromatic molecules, which condense easily on precipitated carbon skeletal structures near and a t the wall (Gaydon and Wolfhard, 1960). Since the thermal decomposition of C,F, is known to yield CF, radicals (Modica and LaGraff, 19651, it is likely that deposition from CaF, occurs via small fragments. The similarity in the deposition between CH, and C9F4supports the small fragment deposition hypothesis for CH,. The process of sooting, however, supports the other hypothesis. Both mechanisms must occur, the conditions determining the microstructure. The desorption of H or F from the solid is prohahly a critical step. Chemical Analysis. The per cent of fluorine or hydrogen impurity in the carbon as determined by combustion and wet chemical analysis is listed in Table 11. With CzF, as a reactant gas, only a very small amount of fluorine is deposited. On the other hand, the deposit is about 0.1 to 0.2 weight % hydrogen, with CH, as the starting gas. The latter result is considerably lower than the value of about 1 weight % hydrogen found by Brown and Watt (1958) for samples deposited a t 1600" to 1700°C. and is in agreement with the values of 0.05 to 0.25 weight % hydrogen found by Grisdale et d. (1951) for deposits at 900- to 1200°C. Our value corresponds to about 1 to 2 mole % hydrogen. Metallography. A General Electric Model XRD5 x-ray diffractometer was used to ohtain the d spacings by measuring the x-ray angle a t which displacement occurs. For C,F, pyrolytic graphite made a t 2250°C. the C-C interlayer spacings were within the experimental accuracy of that measured for CH, pyrolytic graphite-3.43 A. This value should be reliable to better than 0.01 A,, as our samples were only 0.01 to 0.02 inch thick, and errors due to penetration effects are negligible (Woken, 1967). Figure 2 contains four 200 x photomicrographs of cross
Figure 2. Cross sections of pyrolytic graphite deposits Pressure. 10 to 15 mm. of Hg
Flow rate. 1 rtondord CU. foot per hour
Upper leh. From CHI, 175O'C. Upper righl. From CHI, 2000' C. lower
leh.
From Cd,, 1750" C.
Lower right.' From GF,,
2000"C.
sections of pyrolytic graphite deposits. The upper photographs show deposits made from decomposing CH, on graphite tubes a t 1750" and 2000°C.; the lower show deposits made from decomposing C,F, on graphite tubes under similar conditions. The upper photographs show a banded structure, which a t upper left runs parallel to the delamination shown. The banding appears to he caused hy a mixture of soot particles and pyrolytic graphite depositing a t the hot wall (Williams, 1967). T h e delamination shown is not necessarily caused by the soot codeposition (which may have weakened the interlayer bonding), hut may have been caused by grinding and polishing. The observation of banding indicates that the CH, systems in the upper photographs are a t the verge of sooting. The lower photographs do not have a banded structure. This indicates that for the same conditions the CzF, system is further from sooting. Therefore, the gas-phase reaction does not become as important relative to wall deposition as in the CH, system. VOL. 8 N O , 3 S E P T E M B E R 1 9 6 9
235
The deposition rates of the upper photographs (2.4 and 4.3 grams per hour, respectively) are higher than the lower photographs (1.7 and 4.0 grams per hour, respectively). The fine-grained structure a t lower left is similar to boron-modified pyrolytic graphite from methane (Campbell et al., 1966). The structure a t lower right is indicative of a continuously renucleated pyrolytic graphite, whereas that a t lower left is substrate-nucleated pyrolytic graphite. Campbell et al. concluded that the highest rupture pressure of pyrolytic graphite tubes was obtained with a continuously renucleated microstructure. The structures in both lower photographs contain extremely fine nucleating particles. The fineness of the nucleating particles shown in these structures may reflect the higher stability of fluorocarbon compounds compared to hydrocarbons. Conclusions
Pyrolytic graphite prepared from a fluorocarbon source gas has low fluorine impurity and no hydrogen impurity. The deposition parameters of graphite from C2F4 were measured. Large amounts of residual hydrogen were found in hydrocarbon pyrolytic graphite produced under conditions identical to the fluorocarbon deposition. Although deposition rates are lower with C2F4than with CHI, it is now possible to study the effect of hydrogen (or its absence) on the strength and other properties of pyrolytic graphite. The clean-substrate nucleated and continuously renucleated microstructure, hitherto available only from a two-complement gas system (source gas plus diluent), may be obtained from a single-component system, the C2F4 system.
Diefendorf for some of the density measurements, Gerald Wolten for the x-ray data and interpretation, and William Barry and J. D. McClelland for helpful suggestions. Literature Cited
Bone, W. A., Coward, H. F., J . Chem. SOC.93, 1197 (1908). Brown, A. R. G., Watt, W., “Preparation and Properties of High Temperature Pyrolytic Carbon, ” Conference on Industrial Carbon and Graphite, p. 86, Society of Chemical Industry, London, September 1957. Campbell, J. G., Haas, M. L., Coulbert, C. D., “Free Standing Pyrolytic Graphite Thrust Chambers for Space Operation and Attitude Control Phase I. Analysis and Preliminary Design,” Air Force Rocket Propulsion Laboratory, TR-66-95,75, 165, 182 (June 1966). Cole, D. J., Minkoff, G. J., Proc. Roy. SOC. London A239, 230 (1957). Diefendorf, R. J. (to General Electric Co.) U.S. Patent 3,138,435 (June 23, 1964). Diefendorf, R. J., U.S. Patent 3,213,177 (Oct. 19, 1956). Gaydon, A. G., Wolfhard, H. G., “Flames,” p. 206, Macmillan, New York, 1960. Grisdale, R. O., Pfister, A. A., Van Roosbroeck, W., Bell System Tech. J . 30, 271 (1951). Heicklen, J., Hudson, J. L., Armi, L., Carbon 7,365 (1969). Modica, A. P., LaGraff, .J. E., J . Chem. Phys. 43, 3383 (1965). Porter, G., Combustion Res. Rev. 1955, 108. Williams, R.,private correspondence with Super-Temp Co., January 1967. Wolten, G., “Accuracy of Lattice Parameters in a Small Crystallographic Laboratory,” Aerospace Corp. TR 0158(3250-10)-8(1967).
Acknowledgment
The authors thank Richard Williams and William Smith, Super-Temp Co., for assistance with some of the experiments and for useful discussions and suggestions, R. J.
RECEIVED for review June 27, 1968 ACCEPTED May 1, 1969 Financial support received through Air Force Contract 04 (695)1001.
PREPARATION OF SODIUM FLUOTANTALATES FROM ORGANIC SOLUTIONS R O B E R T
E.
E B E R T S ’
Norton Co., Newton, Mass. 02164
THEextractive
metallurgy of tantalum and niobium usually involves opening the tantalite (or columbite) ore with hydrofluoric acid. A commonly practiced method of purifying tantalum and niobium involves the extraction of fluorocomplexes of these elements into a waterimmiscible organic ketone-e.g., methyl isobutyl ketone (MIBK). Various liquid-liquid extraction schemes (Werning et al., 1954; Werning and Higbie, 1954; Koerner et al., 1958; Pierret, 1964) have been devised for purifying these elements from impurities and from one another. Present address, Arthur D. Little, Inc., Acorn Park, Cambridge, Mass. 236
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
The purified metal compound can be recovered from the organic phase by back extraction into water and subsequent precipitation of hydroxide with ammonia, or by crystallization of potassium fluotantalate or fluoniobate by addition of a potassium salt. Patents (Brethel et al., 1960; Foos and Greenberg, 1962) have been issued on processes for crystallizing potassium fluotantalates from the organic extract. Work in these laboratories confirmed that high purity KZTaF7can be made in this way. The work reported here was performed to determine whether sodium octafluotantalate could be recovered directly from the organic phase, s ~ c its e high solubility in water makes recovery from aqueous solutions
