THE
DEPOSITTON OF PYROLYTIC SILICA REACTORS
CARBON
IN
C. F. CULLIS Dept. of Chemistry, The City University, London, E.C.1, England and
A. C. NORRIS Dept. of Chemistry, Portsmouth
Polytechnic, Portsmouth,
Hants, England
Abstract-When surface carbons are produced by the pyrolysis of gaseous hydrocarbons in a silica tube heated to about lOOO”C,spectrographic analysis shows the presence of up to 1% silicon even when the carbon is deposited on a metal substrate. It seems likely that hydrocarbon radicals react with silica to produce volatile organo-silicon species. As a result, silicon eventually becomes incorporated in the solid carbon, although neither the free element nor silicon carbide appear to be formed. Consideration of the present and previous results indicates that the chemical reactivity of silica surfaces needs to be taken into account in studies of pyrolytic deposition and of catalysis at high temperatures. 1. INTRODUCTION
Surface carbons are frequently prepared by the pyrolysis of gases[l], either by flowing the gas over an electrically-heated element mounted axially in an unheated cylindrical vessel [2,3] or by passing the gas through a tube maintained at a constant high temperature[4,5]. The latter method is more suitable for fundamental studies of the kinetics of carbon deposition[6] and of the formation and properties [7] of surface carbons, since it avoids the complications due to large radial temperature and concentration gradients which are inherent in the former technique. Metals have been employed as substrates for the deposition of carbon in isothermal reactors [8,9], but ceramics are more often used on account of their ability to withstand high temperatures and their supposed chemical inertness. A recent study of the production of pyrolytic carbons, in which surface carbons were deposited in a silica tube at 930-1050°C by the gas phase pyrolysis of a number of
organic compounds [lo], shows that under these conditions, silica is not as inert as has frequently been assumed. 2. EXPERIMENTAL
Pyrolyses were carried out in a conventional flow apparatus constructed of glass. The purified reactant (furan, thiophene or cyclopentadiene) was contained in two thermostatted bubblers and was vaporised by a stream of helium. After dilution with further helium, the combined stream entered a cylindrical reactor (2.5 cm i.d. X 76 cm long) made of transparent fused silica. Plates of the same material (2 cm X 1 cm X 0.1 cm) or of 999% pure titanium, nickel and tungsten (2 cm X 1 cm X 0.15 cm), placed on the floor of the reactor, acted as substrates from which the carbon samples were subsequently removed. A Hilger and Watts 3-metre spectrograph was used to determine the silicon content of carbon samples. The relative error in each determination is -C20%.
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C. F. CULLIS and A. C. NORRIS 3. RESULTS AND DISCUSSION
During a pyrolysis, the flow rates of the carrier and diluent gases and the partial pressure of the reactant were kept constant so that the total gas flow was 180.5 cm3 min-’ at NTP and the mass flow of the reactant was 0.350 & 0.005 m-mole min-‘. Each pyrolysis was allowed to proceed for 4 hr, during which time approximately 0.084 moles of reactant flowed through the reactor. Some typical results relevant to the present discussion are shown in Table 1. In each case the carbon was found to contain appreciable amounts of silicon. Although no detailed systematic study was attempted, the silicon content shows no obvious trend with deposition temperature or with the nature of the reactant or substrate. The presence of silicon in carbons deposited on silica may be attributed to the diffusion of mobile silicon atoms from the substrate surface [ 111. However, the presence of up to 1 *O% silicon in the carbons deposited on metal substrates makes it necessary to assume that the element is removed from the reactor surface to form gaseous siliconcontaining species which later decompose to form silicon. Studies of the deposition of silica by the thermal decomposition of organo-oxysilanes [12,13] support this concept. Since direct reaction of species with silica is thermodynamically unlikely (AH? (SiO,) = - 195
kcal mole-’ at lOOO”C), Klerer[l3] suggests that, above 500°C organo-silicon radicals are chemisorbed onto a hot surface where they react and produce a film of silica. When a monolayer is complete, radicals can link with it to build up the film. These concepts can be applied to the present results i? it is supposed that organic radicals derived from the reactant are chemisorbed onto the silica surface where they react to produce siliconcontaining radicals. The latter radicals are then desorbed into the gas phase and eventually silicon is co-deposited with the carbon. Two further observations made by Klerer are of interest. Firstly, the presence of organic groups (CH,, CzHG etc. are removed on heating) in the silica films deposited below 825°C leads to stress crazing of the film and the surface appears devitrified. Secondly, deposition between 825 and 950°C produces cloudy white and grey films, presumably for the same reason. In the present work, the silica reactor developed a crazed surface upstream of the central deposition zone, the temperature at which crazing occurred lying between 600 and 800°C. Further downstream, but still ahead of the central zone, the reactor surface had the cloudy appearance described above. No crazing or cloudiness appeared either in or after the deposition zone, presumably because all silicon-containing radicals would have already decomposed.
Table 1. Silicon content of pyrolytic surface carbons
Reactant Furan Furan Thiophene Thiophene Cyclopentadiene Cyclopentadiene Cyclopentadiene Cyclopentadiene
Pyrolysis temp. (“C)
Substrate
930 1050 930 1050 990 990 990 990
silica silica silica silica silica titanium nickel tungsten
Silicon content (weight %) 0.50
0.07 0.07 o-30 0.50 0.80 1a00 0.50
THE
DEPOSITION
OF PYROLYTIC
If the above explanation is correct, it would be difficult to prevent or even control the introduction of silicon into pyrolytic carbons formed in the presence of silica. Two recent studies have shown that the incorporation of silicon can enhance the graphitic nature of pyrolytic carbon. silicon Marinkovic Pt a1.[14] pyrolysed tetrachlorideimethane mixtures in a graphite tube over the temperature range 13401630%. It was found that the apparent densities and crystallite heights of the pyrolytic deposits reached maximum values with 0.15-O-20 wt% silicon present; this concentration, it is proposed, corresponds to the maximum solubility of silicon in carbon, any further amounts of the former element being present as p-silicon carbide. Yajima and Hirai]l5] produced carbons from the pyrolysis of silicon tetrachlorideipropane mixtures over a graphite rod heated electrically to 1400-2100°C; these workers believe that silicon and carbon do not form solid solutions to any measurable extent but that the silicon is present only as @silicon carbide, in which form it increases the crystallite dimensions of the deposits and completely eliminates the minimum density normally observed [2,3] in this temperature region. In these two studies, the choice of methane and propane as the reactants probably ensured the presence of a hydrogen-rich atmosphere. Such a strongly reducing environment appears to be conducive to the formation of silicon carbide, for the results of another study[16] show that the latter compound is formed from mixtures of silicon tetrachloride and toluene only when excess hydrogen is present to reduce the halide to the element. In the present work the reactants did not provide a hydrogen-rich atmosphere and the absence of silicon carbide and presumably, therefore, of its precursor, elemental silicon, is indicated by the X-ray diffraction patterns which show only the reflections from graphite[lO]. (Yajima and Hirai[l5]
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519
were able to obtain reflections from /3silicon carbide when this was present in concentrations as low as 0.3%). Thus, it seems likely that the silicon is dispersed throughout the carbon matrix as organo-silicon species. In this form, up to 1% by weight of silicon would probably have little influence on the density and crystal structure of the carbon. However, it has been shown that the introduction of even small quantities of silicon into carbon can produce a proFor example, effect. nounced catalytic contamination of graphite by silicon from a mullite tube has a profound influence on the reaction which occurs between graphite and steam at 900-1300°C [l’i]. It is thus clear that greater attention should be paid to the possible chemical reactivity of the surface of the containing vessel during studies both of the deposition of pyrolytic carbons and of catalytic reactions at high temperatures. authors thank the United Kingdom Atomic Energy Authority for the award
Acknowledgement-The
of a Research Bursary (to A.C.N.) and for valuable financial assistance in the purchase of apparatus.
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9. Robertson S. D., Nature 221, 1044 (1969). 10. Cullis C. F. and Norris A. C., Third Ind. Carbon and Graphite Conf. pp. 235. Sot. Chem. Ind., London (1970). 11. Singer L. S. and Wagoner G., Proc. Fifth Carbon Conf. Vol. 2, pp. 65. Pergamon Press, Oxford (1963). 12. Jordan E. L., J. Electrochem. Sot. 108, 478 (1961). 13. Klerer J., J. Electrochem. Sot. 112, 503 (1965). 14. Marinkovic S., Suznjevic C., Dezarov I.,
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