Carbon 1967, Vol. 5, pp. 613-617. Pergamon Press Ltd. Printed in Great Britain

PYROLYTIC-CARBON COATINGS DEPOSITED

FROM METHANE FLUID BEDS”

J. CHIN

General Atomic Division of General Dynamics Corporation,

John Jay Hopkins Laboratory for Pure and Applied Science, San Diego, California

(Received 7 April 1967)

Abstract-Pyrolytic-carbon deposits were produced from 100% methane fluid beds at 1400-22OO’C. The microstructurcs of the carbons deposited were anisotropic-columnar, isotropic, or grainy-isotropic. Formation of the anisotropic-columnar structures occurred at high methane flows, large bed areas, and low-to-intermediate temperatures. As the deposition temperatures increased, the apparent crystallite sizes increased with accompanying decreases in layer spacings.The metallographic features of the result- ing structures are those of isotropic pyrolytic carbons. Decreasing the methane flow rate and the surface area of the fluid bed at intermediate-to-high temperatures (1800-2200°C) resulted in structures which show typical isotropic features when viewed with polarized light but show ordered grainy textures when observed with bright-field optical micrographs. The relations between deposition conditions and struc- ture are discussed.

1. INTRODUCTION

DEPOSITION of pyrolytic carbons in fluidized beds has been intensely studied at General Atomic for the past five years. (lV6) When deposited as coat- ings on oxide and carbide fuel particles, these carbons are of special technological importance for high-temperature gas-cooled reactors in the retention of fission products and the protection of fuel during fuel-element fabrication.

The structure of the pyrolytic carbon deposited is known to be dependent on the mechanism by which the solid carbon is formed. For a given coater configuration, the independent variables which are important in determining this structure are :

1. Bed temperature.

2. Chemical composition of the fluidizing gas.

the volume of the fluidized bed divided by the total gas flow at a given bed temperature.

Pyrolytic carbons deposited from mixtures of l-4Oo/o methane-in-helium at temperatures from 1300 to 2400°C with limited variation in bed area and geometry have been examined by BOKROS et al (4-C)

Production usage demands evaluation of gases cheaper than methane-helium and methods of increasing coating rates. A logical system that fits these requirements would be one that utilized a 100% hydrocarbon fluidizing gas. The purpose of the study described herein was to determine what structural changes in the deposited carbon occur as a result of systematic changes in bed geometry, bed temperature, and contact time when 100% methane is used as the input gas.

3. Bed surface area.

4. Contact time. 2. EXPERIMENTAL

The contact time is the average time the hydro- The carbon was deposited in an induction-

carbon gas is in contact with particles in the fluid heated 3.5 cm dia. graphite fluidized bed. The

bed. In this paper, contact time is calculated from methane used for these experiments was spccitied to contain99*9+% CH4,<0.01°/o 0,, <O-1Soj N,,

Gas from the methane *Work performed as part of the 1964-65 HTRDA

and <0.021°h CO,.

Fuel and Fuel Cycle Development Program under the cylinder was fed directly into the coaters without

sponsorship of twenty-three U.S. investor-owned additiona1 Purification* electric utility companies. Coatings were simultaneously deposited at

613 8

614 J. CHIN

atmospheric pressure on spherical 150-250~ (Ths.s3Ua.r,)Cz particles and small graphite disks. Bright and polarized light optical micro- graphs of each coating run were taken and com- pared. Pyrolytic carbon coatings were stripped from disks for density, mechanical, and structural property measurements. Evaluation of deposits was made from the appearance of the optical micrographs, density, layer spacing, and crystel- lite size measurements. Apparent crystallite sizes L, were obtained directly from the coated particles using X-ray diffractometer measurements of a single layer of particles mounted on double-faced tape. The (002) line broadening using CuK radia- tion was used to calculate Lcc7) from

0.891 0

Lc=pcosA where il=wavelength,

b=half-height (002) line &Bragg angle.

3. RESULTS

width,

The effect of changing process variables on the coating efficiencies is shown in Fig. 1. Efficiencies were calculated from the differential weight of the particles before and after coating divided by the

0 2000 4000 6000

BED AREA (CM’)

FIG. 1. Three-dimensional plot of coating efficiency vs. coating process variables. Efficiency is determined by the carbon recovered on the coated particles relative to the

carbon introduced into the coater.

weight of carbon passed through the coater as methane. The efficiency is this fraction multiplied by 100. High, moderate and poor refer to coating efficiencies of >50%, lo-SO% and (10% respectively.

Densities for the methane-deposited structures were found to be between l-6 and 2.1 g/cm3. This range is similar to that obtained from methane- helium mixtures. Densities were generally highest where coating efficiency was poorest, and lowest where coating efficiency was highest.

Most of the structures had crystallite sizes (Lc) in the range of 26-40 A. Increased crystallite size appeared to be a function of higher temperature, as is shown in the plot of apparent crystallite size vs. deposition temperature (Fig. 2). These data are for a small bed (150 cm2) and a long contact time (0.23-0.31 set). Layer spacings, as determined by a measure of the da02 line, were in the range 3.44- 3.51 A. The general trend is one of decreasing layer spacing with increasing temperature (see Fig. 3).

Observations of the coating microstructures can best be illustrated by a three-dimensional plot of observed structures vs. coating process variables shown in Fig 4. Methane flow is used as the

90 ( I

FIG. 2. Plot of apparent crystallite size vs. temperature for a small bed (150 cm*) and long contact time (0.23-

0.31 set).

PYROLYTIC-CARBON COATINGS DEPOSITED FROM METHANE FLUID BEDS 615

FIG. 3. Plot of deposition temperature vs. layer spacing (&w).

ordinate as a measurable process parameter instead of contact time. It should be remembered that contact time is approximately inversely pro- portional to gas flow but is also dependent on bed temperature and bed geometry. Although there are gradual changes in appearance, three general structures may be noted: (1) isotropic, (2) grainy- isotropic, and (3) anisotropic-columnar.

The isotropic structures are typical of those observed when coatings were deposited from methane-helium mixtures.(4) These structures are not optically active under polarized light and appear featureless under bright field examination (see Figs. 5 and 6). Isotropic structures are obtained in all methane fluidized beds at inter- mediate combinations of bed areas, methane flows, and temperatures.

The grainy-isotropic region is similar to the transitional region observed with methane-helium deposits. Coatings deposited in this region show a lack of reactivity under polarized light similar to that of conventional isotropic structures. Bright field optical micrographs show structure-related polish- ing defects that give the surface a porous or grainy

jO.000

20,000

CH4 FLOW (CC/MIN)

10.000

(“C) 0 2000 4000 6000

BED AREA (CM2)

FIG. 4. Three-dimensional plot of observed structures vs. coating process variables.

616 J. CHIN

texture (Fig. 7). Densities of these deposits are typically greater than 1.9 g/cm3, which is too high for a porous pyrolytic-carbon structure. Coating rates for these deposits were the highest observed.

Anisotropic-columnar structures are typified by strong optical activity under polarized light, show- ing the characteristic cross pattern of the ordered pyrolytic-carbon structure (Fig. 8). This structure differs from the laminar anisotropic structures (Fig. 9) deposited from methane-helium mixtures in that the microstructure suggests columnar grain growth. Coating efficiencies in this region are very low. Stresses built into these structures during coating made removal from the graphite disks, and consequently density measurements, impossible in most cases. Densities, when they could be obtained, were found to be above 2-O g/cm3.

4. DISCUSSION

The range of coating parameters studied for 100% methane fluid beds can be divided into three regions according to the type of structure produced :

1. A region of low methane flow, small bed area, and intermediate-to-high temperature where moderately high-density grainy-isotropic structures are formed.

2. A region of high flow, large bed area, and intermediate-to-low temperature where high- density anisotropic-columnar pyrolytic car- bon is produced.

3. An intermediate region where isotropic pyrolytic carbon can be formed at all temper- atures by adjustment of methane flow and bed area.

The division of the isotropic structures into grainy-isotropic and isotropic is arbitrary. For the temperature range studied, the surface areas used, and the contact times available, variations in gas phase reactions were reflected in structural differences in the pyrolytic carbons deposited. Thus, there are not just three structures but count- less shades of structural differences between the extremes described by the three zones.

To help clarify possible reasons for the struc- tural and physical property changes observed in this work, some results of earlier, related studies will now be discussed. EGLOFF@) rep,orts that

methane is almost totally decomposed above 1000°C and that the products of this decomposi- tion may react in many ways, depending on the thermal history of the gas and the presence of other materials which can act as catalysts. One important catalyst in the deposition of pyrolytic carbon is the carbon itself. Condensed carbon catalyzes the de- composition of methane to its elements and, under select conditions, the polymerization of its compo- nents to higher aliphatic and aromatic hydro- carbons. Since this effect is proportional to the carbon concentrations, abrupt structural changes from an isotropic to an anisotropic-columnar structure (as shown in Fig. 8) might be attributed to the catalytic effect of carbon.

The high gas flow of the larger area fluid beds reported here controlled the contact time, which was critical to the formation of the structures formed. The contact time is dependent on the heat transfer to the particles(“) and therefore depen- dent on both the bed geometry and gas flow. The rates of the reactions to produce higher hydro- carbons or polymers are dependent on component concentration and temperature.@-‘O) Thus, the production of particular hydrocarbons during the decomposition of methane in fluid beds is neces- sarily dependent on contact time and bed tempera- ture. Therefore, by altering temperature, bed area, and gas flows, changes in density, crystallite size, and layer spacing can be effected.

Another important parameter in the gas-phase reactions is the ratio of hydrogen to methane. The decomposition of methane has been shown to be inversely proportional to the fourth power of the hydrogen partial pressure.(i2) Carbon growth during the decomposition of methane has also been shown to be inversely proportional to the hydrogen partial pressure. (i2) Hydrocarbon formation and carbon growth are thought to be inhibited by the preferential adsorption of hydrogen.(s*‘2) Surface adsorptions of hydrocarbons on carbon are also influenced by surface activities, which are depen- dent in part on lattice spacing at the surface.‘i3’ This geometric dependency of active site deposi- tion is translated to the crystal growth behavior of vapor-deposited carbons.(i4)

In the present study, altering the bed area changed both the material density and structure. Thus, surface activities or reaction-controlled adsorption-desorption. processes perhaps change

(b) FIG. 5. Comparative bright field photomicrographs of isotropic coatings prepared from: (a) a loo”:, methane fluid bed at 16Oo’C; (h) a 9% methane-helium fluid bed at 1600-C.

(b) FIG. 6. Comparative polarized light photomicrographs of isotropic coatings prepared from: (a) a 100% methane fluid bed at 1600°C; (b) a 97’ 0 methane-helium fluid bed at 1600°C.

FIG. 7. Bright field optical micrograph of a typical grainy-isotropic structure

(4

6) FIG. 8. Bright field and polarized light photomicrographs of an anisotropic-columnar

structure prepared at 1600°C.

FIG. ri. Microstructure of a typical anisotropic-laminar pyrolytic carbon prepared at 1350°C from a 35”,, methane-helium mixture.

PYROLYTIC-CARBON COATINGS DEPOSITED FROM METHANE FLUID BEDS 617

with changes in fluid bed areas. blather these ~~ERENC~

changes cause local or gross changes in the gas- 1. ENGLE G. B., LUBY C. S. and BOKROS J. C., Evalua- phase reactions is not known. A further possibility tion of (Th,U)C, Carbon-Coated (Th.U)C* Par- is that the larger fluid beds affect the gas-phase ticles, and Carbon Coatings, USAEC Report GA-

nucleation rate by changing the heat transfer to 3067. General Atomic Division. General Dynamics

the gas. Corporation, April 26, 1962.

2. 5. SUMMARY

GOEDD~L W. V., USAEC Report GA-3.588, General Atomic Division, General Dynamics Corporation

It has been shown that most structures deposited (1962).

from methane-helium fluid beds can be denosited 3.

from methane fluid beds. Altering the deposition GOEDDFX W. V. and ZUMWALT L. R., USAEC Report GA-4260, General Atomic Division, General

conditions by altering the gas-phase reactions pro- Dynamics Corporation (1963).

duced three general structures: (1) isotropic, (2) 4. BOKROS J. C., Carbon 3, 17 (1965).

grainy-isotropic, and (3) ~isotropic-columnar. 5. BOKROS I. C. et al., USAEC Report GA-5016, Part 1,

The following coating parameters were varied to General Atomic Division, General Dynamics

produce these changes: (1) bed temperature, (2) Corporation (1964).

6. bed area, and (3) methane flow. Where bed temper-

BOKROS J. C., Carbon 3,201 (1965).

7. ature was the only variable, increasing the tempera- ture decreased the layer spacing and increased the

TAYLOR A., X-ray MetatZography. Wiley (1961).

8. EGLOFF G., The Reactions of Pure Hydrocarbons. Reinhold Publishing Corp. (1937).

apparent crystallite size of the deposits. Densities 9. GRISDALE R. O., J. Appl. Phys. 24,1082 (1953).

showed a characteristic minimum in the tempera- 10. DIEFBNDORF R. J., General Electric Research ture range 1600-1800°C. Coating efficiencies were Laboratory Report GE-60-RL-2432M (1960).

highest where coating densities were lowest. 11. BOTZ‘ERILL J. S. M. et al. Paper presented at the

Increasing bed area and methane flow increased Symposium on Developments in Fluid-Particle

the apparent anisotropy of the carbons deposited. Technology, 57th Meeting, American Institute of Chemical Engineers, Boston (1964).

Ack~a~~ed~~e~t~Tho author would like to express his 12. VON BOGDANDY L., RUTSCH W. and STRANSKI L. N.,

gratitude to J. C. BOKROS, W. V. GOEDDEL, C. S. LUBY, 2. ~lektrache~. 66, 661-666 (1962).

R. J. PRICE and L. R. ZUMVVALT for their constructive 13. LAIDLER K. J., Chemical Kiszetics, pp. 174-176.

comments and helpful suggestions on the presentation McGraw-Hill (19.50).

of this work. . , 14. SEARS G. W.,J. C&m. Z%ys. 31,358 (1959).