Anomalous behaviour of a small laboratory acheson graphitization furnace

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contrast electron micrograph of a thermal black[ll] while Fig. 2 shows a typical furnace black[ 111 (HAF, N 330) exhibiting numerous growth centers, in contradiction to the oversimplified picture of Marsh. The existence of these growth centers proves that in nearly all carbon blacks the orientation of the planes beyond two or three peripheral layers is not parallel to the surface. The exception of therma1 blacks must be due to the time element in particle formation, two or three orders of magnitude Ionger than in the case of other blacks. Marsh’s liquid crystal hypothesis would require all blacks to have a structure similar to that of thermal blacks. Thus, the significance of an extended liquid crystal phase in the carbon black structure, if any, is of necessity limited to the unique case of thermal blacks. In conclusion, if Marsh’s statement “the liquid-gas phase interface of the droplet dictates that the orientation of the liquid crystal molecules should be parallel to the surface of the droplet and transferred to surfaces of coalescing droplets” is correct, we must answer his provocative title with “no” when referring to the majority of blacks.

1. 2. 3. 4. 5.

6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Centre de Recherches sur la Physic0 -Chimie des Surfaces Solides 24, Avenue du President Kennedy 68200 ~ulho~e, France Co&on, 1974, Vol. 12, pp. 214-216.

Anomalous

Behaviour

tory Acheson

Pergamon

J. B. DONNET J. LAHAYE A. VOET G. PRADO

Press.

Printed

of a Small

Graphitization

16.

17.

EDITOR

REFERENCES Marsh H., Carbon 11, 254 (1973). Hall C. E., J. A@l. Phys. 19, 271 (1948). Boehm H. P., Z. An. Allg. Chem. 297,315 (1958). Akamatu H. and Kuroda H., Proc. Fourth Conf. Carbon. Pergamon Press p. 355 (1960). Kasatotshkin V. I., Lukianovitch V. M., Popov N. M. and H. V. Tshmutov H. V., J. Chem. Phys. 57, 822 (1960). Donnet J. B. and Bouland J. C., Rev. G&t. &out. 41, 407 (1964). Kaye G., Carbon 2, 413 (1965). Heckman F. A. and Harling D. F., Rubber C&m. Technol. 39, 1 (1966). Ergun S., Carbon 6, 141 (1968). Ban L. L., Surface and Defect Properties of Solids Vol. 1, p. 54. Chem. Sot. (1972). Marsh P. A., Voet A., Mullens T. J. and Price L. D., Carbon 9, 797 (1971). Donnet J. B., Schultz J. and Eckhardt A., Carbon 6, 781 (1968). Marsh P. A., Voet A., Mullens T. J. and Price L. D., Rubber Chem. T@ch~o~.43, 470 (1970). Sweitzer C. W. and Heller G. L., Rubber World 134, 855 (1956). Prado G. and Lahaye J., CR Acad. Sci. Paris 274, 569 (1972). Lahaye J., Prado G. and Donnet J. B., Tenth Bienial Conf. Carbon, Summary of Papers, p. 102 (1971). Prado G., Thesis Un. du Haut-Khin, France, p. 103 (1972).

in Great Bntain

Labora-

Furnace

(Received 25 June 1973) Acheson graphitization furnaces [ 1,4] are no doubt of an old design but they are still playing an important role for industrial purposes. The construction of such furnace and the process of graphitization are quite simple, i.e. it mainly consists of passing a heavy current through the charge under graphitization. In spite of its simplicity, the loading of the furnace is by itself an art and requires a lot of skill and careful planning. A defective loading may cause certain irregularities in the degree of graphitization etc of the product. In the furnace, the charge is usually protected and supported by a resistor material which, in most cases, is a calcined granular petroleum coke

of (-40+ 70) mesh size. Sometimes to the resistor material are added small amounts of natural graphite [2] as well. The volume of the resistor material depends, to some extent, on the size of the charge[2]. The charge and resistor material are then surrounded by a thick layer of insulating material. Different insulating materials may be used, and some of them are silicon carbide, sellimite, thoria, zirconia, silica, etc. The dimensions of the furnace used by the authors are 3’ X 2’ X 2’. This is a small laboratory scale furnace as compared to the commercial furnaces. Standard loading procedure was followed, i.e. the material was packed in such a way that the current was passed in the direction perpendicular to the extrusion direction in the case of extruded stock and in the direction parallel to the moulding direction in the case of moulded stock. The charge capacity of our furnace is about

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TO THE

2.5 kg. A mixture of quartz powder and calcined petroleum coke powder (1: 1 by weight) was used as insulating material. First of all, the furnace was preheated at a low temperature for a period of about lo-15 hr. and thereafter it was run at maximum rating. In the initial stages, since the resistance of the charge is very much lower than that of the surrounding petroleum coke and of the insulating material, most of the current passes through the charge and heats it. At the same time, the surrounding resistor material (protecting layer) will also get heated up. Thus the overall voltage across the furnace will be high to start with. As the heating continues, gradually the current passing through the charge not only lowers its resistance but also heats the surrounding material and therefore, the voltage continuously decreases for some time until the charge has attained a sufficiently high temperature. It may be noted that this high temperature will also cause heating of the surrounding resistor and insulating material. After a few hours of passing the current, the temperature of the insulating layer, which is adjacent to the resistor material, does increase. In a case, when the insulation material is made of mixture of quartz powder (SiOJ and calcined petroleum coke the following reaction may begin to take place after it has attained a temperature of about 1750%: SiO,+3C

l;i110(‘

SiC+2COf.

EDITOR

Variation of overall resistance of furnace with time

0 150-

(Anamolaus

( Pre-heatmg

behawour) time

* 16 hr)

0 090-

0 070-

c:

0054-

%-

0050-

6 f

0046-

B 0.042-

I I 11

I I I I 11

I

9

3

5

7

II

Time,

11

11

13 15 I7 19 21 23 25 27 hr

Fig. 1.

I Variation furnace

0090-

(1)

The gaseous product goes out from the furnace walls and a net product which is a hard and high resistance layer of SIC is left behind. Another consequence of the reaction given by equation (1) can be the creation of voids. As the temperature further increases more insulation material heats up and more silicon carbide and voids can be formed. Since Sic is a hard and abrasive material and a very good insulator, it opposes the Ilow of current and naturally the total resistance of the furnace will increase. Therefore, there can be an increase in voltage occurring at a constant current as a result of the combined effect of Sic and of voids. This is the stage at which our furnace has shown an ‘anomalous’ behaviour. As the time passes, more insulating material within the zone of about 1750°C becomes changed to Sic (according to equation (1) and this increases the overall resistance of the furnace. This change of overall resistance with time is shown in the graph (Fig. 1) by a minimum at the time when the reaction given by equation (1) gets initiated. It is noted further that after some time, the furnace resistance becomes

215

of overall resistonce with time (Normal

behavlour) 0.060

-

0 070-

0 060-

c

0056-

: 0 t;

0052-

5 0048Lz

0 044

pre_ heatq

0.040t’me i

Time,

Fig. 2.

hr

of

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constant and does not change thereafter. The silicon carbide and voids now act as a cold blanket for the heat from the central core. The residence time which has an appreciable effect on the resistivity of the final product[3] should be counted only after the maximum current is passing through the charge at constant voltage, i.e., constant power input and this is indicated by the flat portion in the graph (Fig. 1). When the furnace was unloaded, it has been observed that the insulating material that was in contact with the resistor material has changed to blue. green hexagonal cvrstals of Sic. The next layer of the insulating material were the green irregular shaped platy crystals of Sic. The insulation material next to this layer was found to be hard. If this material was not replaced and further runs were performed under similar conditions, the anomalous behaviour disappears as shown by graph Fig. 2. The furnace resistance goes down from run to run because some of the Sic near to the central zone may get decomposed (equation No. 2) with the formation of graphite. Sic

s

Carbon, 1974, Vol. 12, pp. 216-218.

Si + C (graphite). Pergamon

Press.

Printed

(2)

EDITOR

Acknowledgements-The authors are thankful to Prof. A. R. Varma, Director, National Physical Laboratory for his permission to publish this note. The authors are also grateful to Dr. G. C. Jain for his interest in the work. 0. P. BAHL B. S. CHAUHAN Materials and Industrial Physics Group National Physical Laboratory Hillside Road New Delhi-110012 (India)

REFERENCES 1. Kirkothmer, Encyclopedia of Chemical technology (II Edn.), 4, 183-189 (1967). 2. Hader R. N., Gamson B. W. and Bailey B. L., Modern Chemical Progress, Vol. 4, p. 7-16 Reinhold Publ. N.Y. (1956). 3. Fair F. B. and Collins F. M., Proc. 5th Conf. Carbon 1, 503 Pergamon Press, N.Y. (1962). 4. Mantel1 C. L., Carbon and Graphite Handbook. Interscience Publ. p. 323 (1968).

m Great Bntam

The Influence of Chlorine on the Formation and Properties of Pyrolytic Carbon (Received 24 April 1973) Several investigations [llr] have been made of the effect of chlorine on the properties of pyrolytic carbons but few conclusions have been drawn as to the mode of action of the halogen. This letter summarises the results of these studies and suggests how chlorine produces the observed effects. Finally, the influence of chlorine is contrasted with that of sulphur. The first systematic study of the effect of chlorine was carried out by Cullis, Manton, Thomas and Wilman[l] who pyrolysed methane and the four chloromethanes at temperatures within the range 840-930°C. The replacement of hydrogen by chlorine in the reactant molecule decreased both the preferred orientation and the heights and diameters of the carbon crystallites. In a later study, Higgs, Finicle, Bobka, Seldin and Zeitsch[2] formed carbons by the pyrolysis of methane in the presence of 0.06-24 volume %

chlorine at temperatures from 1300-2000°C. With 3.6% chlorine present, the densities of the carbons deposited above 1700°C were higher than those formed from methane alone but, with higher concentrations of the additive, the densities decreased and the formation of gas-phase carbon was enhanced. Cullis and Norris [3] subsequently investigated the formation of carbon from mixtures of cyclopentadiene and hydrogen chloride in which the concentration of the additive was from 1.5 to 15 times (by volume) that of the fuel. Within the temperature range 900-1050°C, the additive had no significant effect on either the density or the crystallite dimensions of the deposited carbons but it did increase dramatically the formation of gas-phase carbon. More recently, Kobayashi, Ikawa and Iwamoto[4] reported the formation of pyrolytic carbons from propane in the presence of 12 volume % carbon tetrachloride at temperatures from 800-1100°C. The additive increased the rate of carbon deposition but produced no systematic changes in the density and crystallite height of the deposits. Kobayashi et al.[4] proposed that the effective-