Petroleum Derived Carbons

20 Mesophase: The Precursor to Graphitizable Carbon

Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0021.ch020

HARRY MARSH and CHRISTOPHER CORNFORD Northern Coke Research Laboratories, School of Chemistry, University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, England

The great majority of organic materials carbonize, that is they thermally decompose in an inert atmosphere, to provide volatile compounds and black carbonaceous residues variously called chars, charcoals, soots, carbons, semi-cokes or cokes. Carbonization systems either pass through a fluid phase as with petroleum and coal-tar pitch and some coals, or remain entirely in the solid phase as with cellulose and coconut shell. Carbons formed in the latter process are invariably non-graphitizing because on heating to 3300 Κ they do not develop, to any signifi cant degree, an extensive, crystalline, three-dimensional, graphite lattice. However, carbons formed in the former process, with few exceptions such as from sugar, are graphitizing, because on heating to 3300 K, they develop an extensive crystalline structure and are essentially graphitic. The non-graphitizing carbons are described as hard carbons, isotropic in bulk properties and possess appreciable surface area and pore volume within a microporous network. The graphitizing carbons are des cribed as soft carbons, anisotropic in properties and possess low surface areas and little porosity. Clearly, there must exist important differences between the structures of these two classes of carbon which will explain these major differences in properties. All, soft, graphitizing carbons are not identical but exhibit significant differences in the extent of development of their graphitic properties, for example in the size and relati ve orientations of the crystal(lite) components. These differences are intimately associated with the chemical properties of the parent material being carbonized, and are dependent on the con ditions of carbonization. This paper is concerned, in detail, with the mechanism of formation of graphitizable, anisotropic carbons, and the factors which influence the final, overall properties of these carbons. In the studies of both classes of carbon, the optical microscope has played a major role. Optical microscopy, using polarized light and preferably a phase-sensitive plate, distinguishes these two classes of carbon: the anisotropic structures rotate the 266

In Petroleum Derived Carbons; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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plane of the p o l a r i z e d l i g h t and e x h i b i t e x t i n c t i o n contours, bands, nodes e t c . With " t i n t e d p o l a r i z e d l i g h t " , obtained by using a gypsum p h a s e - s e n s i t i v e p l a t e , the a n i s o t r o p i c carbon e x h i b i t s , from i t s p o l i s h e d surface, yellow, blue and red areas which interchange colour on r o t a t i o n of the specimen. Each colour represents a s e c t i o n of a volume element of a given c r y s t a l orientation. I f the s i m i l a r l y o r i e n t a t e d areas are of small diameter ( < 5 urn) and appear with a s e n s i t i v e - t i n t as a m u l t i coloured mozaic, they are termed mozaics": l a r g e r areas (5 t o 100 μπι) are termed "domains" and appear as l a r g e isochromatic areas. The i s o t r o p i c carbons e x h i b i t a purple colour at a l l o r i e n t a t i o n s of the specimen. I t i s the shape, s i z e and s t r u c t u r e of these a n i s o t r o p i c areas which e s s e n t i a l l y c o n t r o l the p r o p e r t i e s of the h e a t - t r e a t e d carbons. These shapes, s i z e s and s t r u c t u r e s are themselves functions of the chemical and p h y s i c a l p r o p e r t i e s of the c a r b o n i z a t i o n system. I t i s the d e t a i l e d a n a l y s i s of these functions which c o n s t i t u t e s c u r r e n t research themes and provide i n s i g h t i n t o a unique mechanism of formation of a n i s o t r o p i c carbon v i a the nematic l i q u i d c r y s t a l and mesophase, as o u t l i n e d below.


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Liquid Crystals. The o p t i c a l , a n i s o t r o p i c p r o p e r t i e s e x h i b i t e d by carbons which g r a p h i t i z e f l l imply that the l a m e l l a r s t r u c t u r e of g r a p h i t e i s already e s t a b l i s h e d i n low temperature carbons (800 K), a l b e i t i n a h i g h l y imperfect form. In order t o understand how such imperfect, l a m e l l a r s t r u c t u r e s form from an i s o t r o p i c , f l u i d phase of c a r b o n i z a t i o n ( p y r o l y s a t e ) , i t i s necessary, f i r s t of a l l * t o consider the p r o p e r t i e s of (nematic) l i q u i d c r y s t a l s . I t was Brooks and Taylor f 2 l , of CSIRO, A u s t r a l i a who, by examining a n i s o t r o p i c development of cokes i n coal seams metamorphosed by igneous i n t r u s i o n s , suggested an explanation of the growth of anisotropy i n terms of l i q u i d c r y s t a l s . L i q u i d c r y s t a l systems Γ31 have been recognized s i n c e 1888 f4l. The term was o r i g i n a l l y used t o describe systems obtained by m e l t i n g such substances as c h o l e s t e r y l benzoate which had unusual f l u i d p r o p e r t i e s and which were a n i s o t r o p i c when viewed i n thin s e c t i o n s between the c r o s s e d - p o l a r i z e r s of an o p t i c a l microscope. Such systems possessed more s t r u c t u r a l order than found i n normal ( i s o t r o p i c ) l i q u i d s , but were not genuinely crystalline. In a debate on nomenclature, F r i e d e l [5~\ suggested that the term mesophase" (intermediate s t a t e ) would obviate the apparent i n c o n s i s t e n c y o f a c r y s t a l l i n e - l i q u i d " . But, mesophase i s r a t h e r too imprecise a term i t s e l f . However, i t s use as a d e s c r i p t i o n of an intermediate phase i n carbon formation i s now e s t a b l i s h e d and t h i s current usage i s discussed below. Of the several categories i n t o which l i q u i d c r y s t a l s may be placed, i t i s the nematic ( t h r e a d - l i k e ) rather than the smectic ( s o a p - l i k e ) or c h o l e s t e r i c system which has relevance t o fl

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c a r b o n i z a t i o n . The nematic l i q u i d c r y s t a l may be d i s t i n g u i s h e d from others i n the way i t separates from the i s o t r o p i c l i q u i d as s p h e r i c a l droplets which e v e n t u a l l y coalesce t o g i v e a nematic domain. These d r o p l e t s are a n i s o t r o p i c . In the o p t i c a l microscope, such m a t e r i a l s form domains o f p r e f e r r e d o r i e n t a t i o n of molecules, the domains being d i s t i n g u i s h e d by boundaries which appear as b l a c k l i n e s (threads) i n the f i e l d of view of the microscope. The r e t i c u l a r net of e x t i n c t i o n contours i s the m a n i f e s t i o n of changes i n the d i r e c t i o n of molecule alignment and such s t r u c t u r a l d i s c o n t i n u i t i e s , termed " d i s i n c l i n a t i o n s " [3] are of c o n s i d e r a b l e relevance t o d i s c u s s i o n s of carbon formation. The arrangement of molecules, e.g. 6-methoxy-2-naphthoic a c i d , i n the nematic l i q u i d c r y s t a l i s u s u a l l y envisaged as one i n which the molecules l i e p a r a l l e l t o one another, but with no order i n the s t a c k i n g sequence (Figure 1). Containing surfaces exert a considerable i n f l u e n c e , with c o n s t i t u e n t molecules near t o the s u r f a c e / i n t e r f a c e l y i n g p a r a l l e l t o the surface (Figure 1). Molecules which form nematic l i q u i d c r y s t a l s u s u a l l y possess features of common geometry. The molecules are u s u a l l y elongated or r e c t i l i n e a r . F l a t segments such as benzene r i n g s enhance l i q u i d c r y s t a l U n i t y ; the molecules are r i g i d along the long axis and the existence of strong dipoles and e a s i l y p o l a r i z a b l e groups i s important. Mesophase. That nematic l i q u i d c r y s t a l s could be formed from the f l u i d phase of c e r t a i n c a r b o n i z a t i o n systems was suggested by Brooks and T a y l o r The immediate i m p l i c a t i o n i s that the s t a c k i n g sequence of molecules i n such l i q u i d c r y s t a l s correspond t o the p a r a l l e l imperfect s t a c k i n g of molecules/lamellae i n p r e g r a p h i t i c carbons. Thus, the nematic l i q u i d c r y s t a l formed during carboni z a t i o n processes came t o be termed the mesophase (but see below). I t was Taylor [6~\ who made the relevant i n i t i a l observations i n the Wongawilli coal seam of New South Wales. An igneous dyke, a tongue of molten magma, had penetrated the coal seam and e s t a b l i s h e d a p o s i t i v e thermal gradient through t h e coal seam t o wards the dyke. The e f f e c t , over some hundreds of metres, was t o e s t a b l i s h a very gradual thermal gradient reaching maximum temperatures which passed through and exceeded the temperatures of formation of a n i s o t r o p i c coke from c o a l . I t was the slow r a t e s of heating and the i d e n t i f i c a t i o n of a very narrow temperature zone of formation of a n i s o t r o p i c carbon which provided the f i r s t clues of the dependence of a n i s o t r o p i c carbon formation upon a l i q u i d c r y s t a l type p r e c u r s o r . Previous l a b o r a t o r y experiments had a l l used t o o r a p i d a h e a t i n g r a t e and the d e t a i l of formation of a n i s o t r o p i c carbon was missed. T a y l o r f 6 ] observed t h a t , on approaching the dyke, small spheres, i n i t i a l l y micrometer s i z e , were observed i n the v i t r i n i t a . Figure 2 shows s i m i l a r spheres growing i n l a b o r a t o r y carbonized



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acenaphthylene. The spheres, i n p o l i s h e d s e c t i o n , e x h i b i t e d r a t h e r unusual o p t i c a l p r o p e r t i e s which were c o n s i s t e n t with a n i s o t r o p i c s t a c k i n g of the constituent lamellae p a r a l l e l to an e q u a t o r i a l plane Γ 6]· An a l t e r n a t i v e p o s s i b i l i t y of circum f e r e n t i a l stacking, as i n an onion, was discounted. These spheres, on f u r t h e r heating, grew at the expense of the v i t r i n i t e and coalesced to form the mozaic s t r u c t u r e s . T a y l o r f o l recognized that t h i s a n i s o t r o p i c sphere was p r o v i d i n g the c l u e to the development of g r a p h i t i z i n g carbons. The molecular arrange ments w i t h i n the sphere, deduced from o p t i c a l p r o p e r t i e s and confirmed by e l e c t r o n d i f f r a c t i o n , c l o s e l y resembled that of the nematic phase i n substances which form l i q u i d c r y s t a l s . Hence, the mechanism of formation of g r a p h i t i z a b l e carbons(via the mesophase) came t o be a s s o c i a t e d with p r o p e r t i e s of l i q u i d crystals. L i q u i d C r y s t a l s and Mesophase. The o r i g i n a l suggestion by Brooks and Taylor f 2] and taken up by other workers Γ7-181 promoted considerations of the p o s s i b i l i t y of c l o s e s i m i l a r i t y i n growth processes and p r o p e r t i e s between the mesophase and nematic l i q u i d c r y s t a l s . I t i s now g e n e r a l l y accepted that the mesophase i s a p l a s t i c , a n i s o t r o p i c phase which grows from the f l u i d p y r o l y s a t e and s o l i d i f i e s t o give the a n i s o t r o p i c coke. The p l a s t i c p r o p e r t i e s of the mesophase , i t s high degree of anisotropy ["2"1, the e f f e c t of magnetic f i e l d s which can a l i g n the constituent molecules of mesophase £l9j_, the o r i e n t a t i o n of molecules p a r a l l e l to contained or c o n f i n i n g surfaces Γ 13"| and of e u t e c t i c e f f e c t s [*17l a l l f i t i n w e l l with the known p r o p e r t i e s of l i q u i d c r y s t a l s . But, there e x i s t s other evidence which i n d i c a t e s that t h i s intermediate phase between the i s o t r o p i c p y r o l y s a t e (e.g. p l a s t i c coal or c o a l - t a r p i t c h ) and a n i s o t r o p i c coke i s NOT a nematic l i q u i d c r y s t a l as such. For example, f o r almost a l l systems the mesophase i s formed i r r e v e r s i b l y . It i s r e l a t i v e l y insoluble i n benzene and i n p y r i d i n e f 2 0 l . A c t i v a t i o n energies of formation of 140 t o 180 kJ m o l " correspond to chemical rather than to p h y s i c a l processes Γ2ΐ"1· Observed increases i n molecular weight [2~\ and C/H r a t i o s I 20[ a l s o i n d i c a t e the chemical nature of the processes l e a d i n g t o mesophase formation. The spacings between c o n s t i t u e n t lamellae of the mesophase are known t o decrease with time at constant temperature Γ2l~l, t h i s a l s o being i n d i c a t i v e of continuing chemical processes o c c u r r i n g w i t h i n the mesophase a f t e r i t s formation. The above d i s c u s s i o n suggests that the s i m i l a r i t i e s with nematic l i q u i d c r y s t a l s mainly concern the ordering or s t a c k i n g processes of mesophase growth, w h i l s t the d i s s i m i l a r p r o p e r t i e s are i n d i c a t i v e of chemical processes of mesophase formation and change w i t h i n the mesophase. I t can be argued t h a t , although mesophase has s t r u c t u r a l s i m i l a r i t i e s to nematic l i q u i d c r y s t a l s , 1


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i t s formation and maturation i s a chemical process* Hence, i t i s suggested as d e f i n i t i o n s of the two terms, that mesophase i s a s t r u c t u r a l analogue and p o s s i b l y a pseudomorph of an i n i t i a l , t r a n s i e n t , nematic l i q u i d c r y s t a l phase i n which only Van der Waals and d i p o l e i n t e r a c t i o n s o r i e n t a t e the molecules. Thus, a •model p r o c e s s f o r formation of nematic l i q u i d c r y s t a l s and then mesophase can be described as f o l l o w s . During the c a r b o n i z a t i o n process of a parent substance, e.g. c o a l , c o a l - t a r p i t c h , petroleum p i t c h e t c . , the v a r i o u s p y r o l y t i c r e a c t i o n s of the c a r b o n i z a t i o n r e s u l t i n an i n c r e a s e i n the average molecular weight of the f l u i d , c a r b o n i z a t i o n system. T h i s i s i l l u s t r a t e d i n the c a r b o n i z a t i o n of anthracene i n a c l o s e d system which leads t o the formation of d i a n t h r y l and t r i a n t h r y l f l O l which f u r t h e r co-condense t o produce l a r g e , planar molecules c o n t a i n i n g nine t o about t h i r t y hexagonal r i n g s . With i n c r e a s i n g c a r b o n i z a t i o n temperature, i n c r e a s i n g f l u i d i t y of the system increases the d i f f u s i v i t y of the constituent molecules l e a d i n g t o an enhanced p r o b a b i l i t y of molecule-«iolecule i n t e r a c t i o n s . That an a n i s o t r o p i c s t r u c t u r e subsequently develops suggests that s u r f a c e - t o - s u r f a c e i n t e r a c t i o n s come f i r s t , with the edge-to-edge i n t e r a c t i o n s coming l a t e r . T h i s s u r f a c e - t o - s u r f a c e i n t e r a c t i o n could resemble that of a p h y s i c a l adsorption process. Here, the l a r g e r i s the adsorbed molecule, the higher i s the molar enthalpy of adsorption. A s s o c i a t e d with these higher enthalpies of adsorption are the longer residence times, or duration of stay, of the adsorbed molecule on the adsorbent sur face [ 2 2 % In the c a r b o n i z a t i o n systems, t h i s p h y s i c a l inter?a c t i o n r e s u l t s i n the formation of the nematic l i q u i d c r y s t a l from the planar (aromatic) molecules. I f such a t t r a c t i v e forces can withstand the d i s r u p t i v e forces of molecular c o l l i s i o n s , then a r e l a t i v e l y s t a b l e , stacked, molecular s t r u c t u r e can e x i s t w i t h i n the p y r o l y s a t e . A d d i t i o n a l molecules can be adsorbed, i n c r e a s i n g the thickness and length of what i s now an •embryonic nematic liquid crystal. At t h i s stage, the l i q u i d c r y s t a l may be r e v e r s i b l y d i s s o c i a t e d f 2 3 l . But, i f the duration of i t s existence i s s u f f i c i e n t l y long, perhaps m i l l i s e c o n d s as d i s t i n c t from nanoseconds (the duration of molecular c o l l i s i o n s ) , then chemical bonding may occur between the c o n s t i t u e n t molecules of the'embryonic l i q u i d c r y s t a l which now loses i t s a b i l i t y to d i s s o c i a t e r e v e r s i b l y i n t o i t s c o n s t i t u e n t molecules. The nematic l i q u i d c r y s t a l then becomes a polymeric, a n i s o t r o p i c s t a b l e phase with a s t r u c t u r e based upon that of the t r a n s i t o r y liquid crystal. This polymeric phase i s the mesophase, which with few exceptions Γ23] i s detected and s t u d i e d i n the o p t i c a l microscope. Mesophase Growth and Coalescence.


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The l i q u i d c r y s t a l and mesophase thus o r i g i n a t e by a process of homogeneous n u c l e a t i o n . Once formed, the mesophase spheres (shapes) continue to grow at the expense of the i s o t r o p i c , f l u i d



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phase of the c a r b o n i z a t i o n system u n t i l they e v e n t u a l l y touch each other* What happens at t h i s stage i s a f u n c t i o n of the p l a s t i c i t y of the mesophase i t s e l f * Should i t have r e t a i n e d s u f f i c i e n t p l a s t i c i t y during i t s growth p e r i o d (and the carboni z a t i o n c o n d i t i o n s are not too t u r b u l e n t ) then the mesophase shape w i l l have responded t o the requirements of minimum s u r f a c e energy and w i l l have become s p h e r i c a l * When two s p h e r i c a l mesophase meet, then t h i s f a c i l i t y t o respond towards minimum s u r f a c e energy again d i c t a t e s that these two spheres adopt a s i n g l e s p h e r i c a l shape, i . e . they c o a l e s c e . T h i s coalescence probably occurs by a process s i m i l a r t o growth. The s u r f a c e of a mesophase sphere i s made of molecules with t h e i r planes at r i g h t angles t o the sphere s u r f a c e (Figure 3). During coalescence, the molecules i n i t i a l l y overlap and a d s o r p t i o n / i n t e r a c t i o n i s possible. The degree of p l a s t i c i t y possessed by the mesophase at temperatures where coalescence occurs i s a f u n c t i o n of two p r o p e r t i e s of the mesophase. I t i s a f u n c t i o n of the degree of polymerization or c r o s s - l i n k a g e which e x i s t s w i t h i n the mesophase. The higher the degree of p o l y m e r i z a t i o n , the lower i s the p l a s t i c i t y of the mesophase. The degree of p o l y m e r i z a t i o n i s dependent upon the chemical r e a c t i v i t y of the constituent molecules. I f t h i s r e a c t i v i t y i s high, then at comparatively low temperatures the degree of p o l y m e r i z a t i o n i s extensive and the mesophase has l i t t l e p l a s t i c i t y . The degree of p l a s t i c i t y i s , secondly, a f u n c t i o n of the degree of n o n - p l a n a r i t y of c o n s t i t uent molecules. Such s t r u c t u r e s as "arm-chair" or "boat", while not preventing l i q u i d c r y s t a l formation could i n h i b i t the f a c i l i t y of r e l a t i v e movement ( s l i p ) w i t h i n the mesophase, t h i s movement being necessary t o respond t o the forces of coalescence. Whatever the r e l a t i v e importance of chemical or p h y s i c a l e f f e c t s , mesophase of low p l a s t i c i t y w i l l , consequently, grow with a nons p h e r i c a l shape and when contact i s made between mesophase shapes, then coalescence i s s e v e r e l y r e s t r i c t e d . As a r e s u l t , only mozaics of f i n e - g r a i n e d a n i s o t r o p i c m a t e r i a l are formed, and NOT the l a r g e domains of a n i s o t r o p i c carbon. Generally, the more aromatic i s the parent substance, the greater i s the p l a s t i c i t y of the r e s u l t a n t mesophase, and the greater i s the temperature range over which the mesophase r e t a i n s this plasticity. T h i s i s determined by the r e l a t i v e l y low chemical r e a c t i v i t y of aromatic molecules and t h e i r greater p l a n a r i t y when compared with, e.g. the h e t e r o c y c l i c and s u b s t i tuted, unsaturated, non-planar molecules found i n some petroleum d e r i v a t i v e s and low-rank c o a l s . Thus, i n i n d u s t r i a l s i t u a t i o n s , the coalesced mesophase g e n e r a l l y r e t a i n s s u f f i c i e n t p l a s t i c i t y t o respond t o the shear forces set up by thermal convection currents and v o l a t i l e bubble movement through the systems and thus creates the flow-type anisotropy of coking coals or the needle-coke anisotropy of a r o m a t i c - r i c h petroleum sources. The needle-coke anisotropy from petroleum sources can be l a r g e r , by an order of



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SURFACE Figure 1. Diagram of molecular arrangements within nematic liquid crystals

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Figure 2. Optical micrograph showing mesophase growth and coalescence from acenaphthylene \ \

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Figure 3.

Diagram of the initial stages of coalescence of mesophase spheres



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magnitude, than the flow-type anisotropy from c o a l s . For i n d u s t r i a l s i t u a t i o n s , P a t r i c k f24"| has described meso phase growth during the c a r b o n i z a t i o n of B r i t i s h coals and White Γ7] has e x t e n s i v e l y examined mesophase from many petroleum sources. Marsh and co-workers f 8 - 1 7 l carbonized model compounds as well as coals and pitches i n attempts to formulate the b a s i c p r i n c i p l e s which u n d e r l i e mesophase growth c h a r a c t e r i s t i c s i n the industrial situation. To study the growth processes u s u a l l y r e quires the p r e p a r a t i o n of p o l i s h e d samples for o p t i c a l microscopy, and t h i s can be a tedious process. However, an observation by Marsh, Walker et al_. |"l6l that the mesophase coalescence can be r e s t r i c t e d by c a r b o n i z a t i o n under pressures greater than about 70 MPa (10,000 p . s . i . ) means that the shape and s i z e of mesophase can be monitored by the more convenient technique of scanning e l e c t r o n microscopy (Figure 4). The d e t a i l of the many carbon i z a t i o n s , of s i n g l e substances and mixtures of g r a p h i t i z a b l e and n o n - g r a p h i t i z a b l e model compounds i s p u b l i s h e d f8-161 from which the f o l l o w i n g conclusions can be made e x p l a i n i n g the wide v a r i a t i o n i n s i z e (0.3 to 100 μπι) and shape ( i r r e g u l a r , sphere, n e e d l e - l i k e ) f l 3 l of mesophase. These conclusions i n v o l v e con s i d e r a t i o n s of c a r b o n i z i n g systems where processes other than those discussed above may be o p e r a t i n g , e . g . the presence of i n e r t s o l i d s and n o n - g r a p h i t i z i n g compounds: A. Flow-type or coalesced anisotropy r e s u l t s

from:-

(i) m o b i l i t y of the developed mesophase a n i s o t r o p i c s t r u c t u r e s over surfaces of s o l i d s ( i n e r t s ) or at l i q u i d / g a s i n t e r f a c e s , r e s u l t i n g i n t h e i r coalescence and flow-*type appear ance, often associated with convection currents w i t h i n the p l a s t i c phase. (ii) the growth, p r e f e r e n t i a l l y , of mesophase at s u r f a c e s , r a t h e r than i n the bulk of the i s o t r o p i c l i q u i d . (iii) the enhanced p l a s t i c i t y of mesophase a s s o c i a t e d with molecular a r o m a t i c i t y as i n c o a l - t a r p i t c h , or with the presence of oxygen i n h e t e r o c y c l i c , p o l y c y c l i c systems which are known to enhance the growth of mesophase [ l 4 , 1 5 l . A decrease i n the com p l e x i t y of mixtures of molecular species a s s o c i a t e d with an increase i n the homogeneity of chemical r e a c t i v i t y a l s o favours coalesced a n i s o t r o p y . B. The non-coalescence attributable t o : -

of the mesophase s t r u c t u r e s can be

(i) c o l l o i d a l s o l i d i m p u r i t i e s w i t h i n the c a r b o n i z i n g system being adsorbed on surfaces of mesophase. (ii) r e s i d u a l compounds, not forming l i q u i d c r y s t a l s , being adsorbed on surfaces of growing l i q u i d c r y s t a l s . (iii) p h y s i c a l separation by formation of i s o t r o p i c carbon. (iv) low m o b i l i t i e s of molecules of high molecular weight


274


and low p l a s t i c i t y of mesophase r e s t r i c t i n g growth and coalescence during the c a r b o n i z a t i o n process*


C. The smaller s i z e s of a n i s o t r o p i c s t r u c t u r e s can be able t o :

attribut-

(i) d i l u t i o n of those compounds, which form l i q u i d c r y s t a l s , i n an i s o t r o p i c l i q u i d composed p r i n c i p a l l y of compounds capable of forming only i s o t r o p i c carbon* (ii) the existence of a range of temperature over which c o n s t i t u e n t molecules of the c a r b o n i z i n g system s e p a r a t e l y form l i q u i d crystals* T h i s can be a s s o c i a t e d with extensive r e s t r u c t u r i n g of molecules i n the c a r b o n i z a t i o n process. (iii) slow growth r a t e s of l i q u i d c r y s t a l s r e s u l t i n g from the high molecular weight of c o n s t i t u e n t molecules. V a r i a t i o n s i n c a r b o n i z a t i o n c h a r a c t e r i s t i c s between, e.g. petroleum and c o a l - t a r feedstocks can be a t t r i b u t e d t o s i g n i f i c a n t d i f f e r e n c e s i n chemical composition of these m a t e r i a l s . A more comprehensive understanding of these v a r i a t i o n s i s dependent upon an improved knowledge of the chemistry of these m a t e r i a l s . However, the complexity of petroleum feedstocks, i n p a r t i c u l a r , must not be underestimated. I t has r e c e n t l y been observed by Isaacs [ 2 5 ] and Marsh et a l . [ l 7 ] that mixtures of organic compounds, when co-carbonized, form a n i s o t r o p i c carbon, despite the observation that when carbonized s i n g l y , the organic compounds form only i s o t r o p i c carbon, e.g. c o - c a r b o n i z a t i o n of carbazole and p y r o m e l l i t i c dianhydride* Perhaps i n t e r m o l e c u l a r r e a c t i o n s between these two compounds produce a new s e r i e s of molecules capable of forming l i q u i d c r y s t a l s f25~l. On the other hand, i t may be that mixed nematic l i q u i d c r y s t a l s form e u t e c t i c s £l7j s t a b l e at temperatures lower than the temperatures of formation of l i q u i d c r y s t a l s from s i n g l e components. There i s a relevance of these observations t o c a r b o n i z a t i o n s of blends or mixes of f u s i b l e components. Mesophase S t r u c t u r e and G r a p h i t i z a b i l i t y . In the above d i s c u s s i o n s , the argument i s made that the s t a c k i n g sequences of c o n s t i t u e n t molecules i n nematic l i q u i d c r y s t a l s formed i n the p y r o l y s a t e are maintained i n the mesophase and e s t a b l i s h e d i n the a n i s o t r o p i c carbon. Thus, the a n i s o t r o p i c carbons possess an imperfect, p r e g r a p h i t i c s t r u c t u r e which i s p r o g r e s s i v e l y p e r f e c t e d , w i t h i n c r e a s i n g temperature, towards g r a p h i t e . Two f a c t o r s s i g n i f i c a n t l y i n f l u e n c e t h i s g r a p h i t i z a t i o n process; f i r s t , the s i z e of the anisotropy, (mozaic or domain) i s important s i n c e boundary c o n d i t i o n s l i m i t c r y s t a l growth processes; second, the d e t a i l of the s t a c k i n g sequences i n f l u e n c e s g r a p h i t i z a t i o n which e s s e n t i a l l y i s the removal of the s t a c k i n g defects i n the o r i g i n a l carbon. S t a c k i n g defects are a s s o c i a t e d , not only with the r e l a t i v e p o s i t i o n s of c o n s t i t u e n t



20.

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AND

CORNFORD

Mesophase

275

molecules, but a l s o with the presence of heteroatoms, vacancies and l a c k of p l a n a r i t y i n these l a r g e stacked molecules. Consequently, the p r o p e r t i e s of a g r a p h i t i z e d m a t e r i a l must depend, c l o s e l y , upon the s t r u c t u r e of the mesophase ( i t s extent of order and molecular p e r f e c t i o n ) t h i s being dependent upon the chemical and s t r u c t u r a l p r o p e r t i e s of the c o n s t i t u e n t molecules of the p y r o l y s a t e and the c a r b o n i z a t i o n c o n d i t i o n s . To c o n t r o l the s t r u c t u r e of the mesophase o f f e r s a method of c o n t r o l l i n g the p r o p e r t i e s of the graphitfc m a t e r i a l . Marsh, Cornford and Crawford [26,27] use two techniques to analyze s t r u c t u r e i n mesophase and p r e g r a p h i t i c carbons. O p t i c a l microscopy, with q u a n t i t a t i v e r e f l e c t a n c e measurements from p o l i s h e d surfaces, using p o l a r i z e d l i g h t provides information about the extent of conjugated bonding w i t h i n c o n s t i t u e n t molecules of the pyrolysate/mesophase/carbon and about the s t a c k i n g sequences of the molecules i n e s t a b l i s h i n g the a n i s o t r o p i c s t r u c t u r e . High r e s o l u t i o n , phase-contrast e l e c t r o n microscopy [28] of carbons can resolve the " l a t t i c e " of the stacked, constituent molecules as f r i n g e images, so p r o v i d i n g a d i r e c t v i s u a l d e s c r i p t i o n of s t r u c t u r e w i t h i n mesophase. Marsh, Cornford and Crawford [29] using these two comple mentary techniques, examined s t r u c t u r a l changes o c c u r r i n g during the c a r b o n i z a t i o n and g r a p h i t i z a t i o n of p o l y v i n y l c h l o r i d e and a c o a l - t a r p i t c h . Both techniques c l e a r l y support the model of mesophase growth and g r a p h i t i z a t i o n o u t l i n e d above. The growth of conjugated molecules i n the l i q u i d p y r o l y s a t e i s monitored, no marked conjugation i s detected as mesophase i s developed, but i n c r e a s e d conjugation occurs as the p l a s t i c mesophase i s converted t o s o l i d carbon. The e l e c t r o n microscope r e v e a l s the p a r a l l e l s t a c k i n g of molecules i n mesophase from p o l y v i n y l c h l o r i d e (Figure 5) so p r o v i d i n g d i r e c t evidence of the s t r u c t u r a l o r i e n t a t i o n suggested by Brooks and T a y l o r [2] and others [ 7 ] . The p e r f e c t i o n of s t a c k i n g i n the g r a p h i t i z e d m a t e r i a l (2673 K) i s seen i n Figure 6. S t r u c t u r e w i t h i n mesophase of d i f f e r e n t s i z e s has a l s o been examined [26]. Mesophase mozaics, about 1 μιπ diameter, can be formed from the s t r u c t u r a l l y complex compound 2-(o-hydroxyl-phenyl) -benzo-thiazole ( I ) , ( F i g u r e 7) and mesophase, about 50 μα diameter can be formed from the planar molecules of a c r i d i n e ( I I ) , ( F i g u r e 8) P r e l i m i n a r y r e s u l t s from a phase-contrast, e l e c t r o n microscope study of these two mesophase m a t e r i a l s are as Figures 9 and 10, which a l s o contain t r a c i n g s from the f r i n g e images. The s t a c k i n g sequences and length of f r i n g e i n the mesophase prepared from (I) are l e s s developed than i n the mesophase prepared from a c r i d i n e ( I I ) . This would suggest ( a l b e i t from an i n i t i a l i n v e s t i g a t i o n ) that d i f f e r e n c e s i n stacking development and molecular shape may be a s s o c i a t e d with mesophase of d i f f e r e n t s i z e s , and i l l u s t r a t e s how c o r r e l a t i o n s between mesophase c h a r a c t e r i s t i c s ( p l a s t i c i t y , coalescence and ordering) and g r a p h i t i z a b i l i t y may be e s t a b l i s h e d .



276


Figure 4. Stereoscan micrograph of botryoidal, non-coalesced mesophase produced by carbonizing a coal tar pitch under pressure

Figure 5.

Phase-contrast electron micrograph showing lattice-fringes from carbon (polyvinyl chloride, HTT 923 K)



MARSH AND CORNFORD

MeSOphùSe

Figure 6. Phase-contrast electron micrograph showing lattice-fringes from carbon (polyvinyl chloride, HTT 2673 K)

Figure 7. Stereoscan micrograph of mesophase from 2-(o-hydroxylphenyl)benzothiazole, HTT 850 Κ



278


Figure 8. Stereoscan micrograph of mesophase from acridine, HTT 850 Κ

Figure 9. Phase-contrast electron micrograph showing lattice-fringes from carbon (2-(o-hydroxylphenyl)benzothiazole, HTT850K)



MARSH

A N D CORNFORD

MeSOphdSe

Figure 10. Phase-contrast electron micrograph showing lattice-fringes from carbon (acridine, HTT 850 K)


PETROLEUM

280


Industrial

DERIVED

CARBONS

Relevance.

Studies of mesophase c o n s t i t u t e a good example of "an improvement i n the q u a l i t y of d i s c u s s i o n of the p r o c e s s " l e a d i n g , i t i s hoped "to an improvement i n the q u a l i t y of the product of the p r o c e s s " . Examples where d i s c u s s i o n s of mesophase have i n d u s t r i a l relevance i n c l u d e : A. A l l aspects of production of commercial g r a p h i t e ; the processes w i t h i n the delayed coker and other cokers used i n production of petroleum coke; the production of s p e c i a l i z e d forms of coke; the c a r b o n i z a t i o n of c o a l - t a r p i t c h ; the i n t e r a c t i o n of mesophase with the g r i s t during the c a r b o n i z a t i o n of b l e n d s ; the p r o p e r t i e s of baked e l e c t r o d e s . B. The i s o l a t i o n of mesophase, and i t s subsequent manipulation to produce g r a p h i t i c m a t e r i a l of c o n t r o l l e d s t r u c t u r e and properties e . g . i n d u s t r i a l construction u n i t s . C. The production of c e r t a i n carbon blacks which possess considerable order of l a m e l l a r stacking p a r a l l e l to surfaces [ 3 0 , 3 l ] . D. Aspects of p r o d u c t i o n of carbon f i b r e s from petroleum and c o a l - t a r p i t c h , and coal e x t r a c t s . E. Aspects of p r o d u c t i o n of b l a s t furnace coke from s i n g l e coals and coal blends (important i n countries with shortages of coking c o a l s ) ; coke strength could be r e l a t e d to growth and o r i e n t a t i o n s of mesophase r e l a t i v e to surfaces of components of blends and v e s c i c l e s [32]. Acknowledgements. The fundamental studies which are described i n t h i s paper form part of the research programme of the B r i t i s h Carbonization Research A s s o c i a t i o n and the authors are g r a t e f u l to the D i r e c t o r of the A s s o c i a t i o n f o r permission to p u b l i s h . The Science Research C o u n c i l , U . K . , provided h i g h - r e s o l u t i o n , e l e c t r o n microscope f a c i l i t i e s to Dr. H . Marsh and the scanning e l e c t r o n microscope to the School of Chemistry, U n i v e r s i t y of Newcastle upon Tyne. Summary; Mechanisms are discussed of the formation of g r a p h i t i zable a n i s o t r o p i c carbons from f l u i d p y r o l y s a t e s i n terms of nematic l i q u i d c r y s t a l s and the mesophase. A l l graphitizable carbons are not i d e n t i c a l but e x h i b i t s i g n i f i c a n t d i f f e r e n c e s i n the extent of development of g r a p h i t i c p r o p e r t i e s ; examples are the s i z e and r e l a t i v e o r i e n t a t i o n of the c r y s t a l components. Explanations are advanced f o r the v a r i a t i o n s i n s i z e , shape, coalescence and o r i e n t a t i o n s of a n i s o t r o p i c mozaics and domains i n g r a p h i t i z i n g carbons. The importance of the chemical composition of parent substances i n determining mesophase growth c h a r a c t e r i s t i c s i s emphasized. The i n d u s t r i a l relevance of mesophase i n carbon and graphite production i s summarized.



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Literature Cited. 1. Fischbach D.B., 'Chemistry and Physics of Carbon', Ed. P.L. Walker Jr., (1971) 7, 1, Edward Arnold, New York. 2. Brooks J.D. and Taylor G.H., 'Chemistry and Physics of Carbon', Ed. P.L, Walker Jr., (1968) 4. 243, Edward Arnold, New York. 3. Brown G.H., Doane J.W. and Neff V.D., 'Review of Structure and Properties of Liquid Crystals', Butterworths, 1971. De Gennes P.G., 'The Physics of Liquid Crystals', Clarendon, Oxford, 1974. 4. Reinitzer F., Monatsh, (1888) 9, 421. 5. Friedel G., An. Physique, (1922) 18, 273. 6. Taylor G.H., Fuel, London, (1961) 40, 465. 7. White, J . L . , Progress in Solid State Chem., (1974) 9, 59. 8. Marsh H., Akitt J.W., Hurley J.M., Melvin J.M. and Warburton A.P., J. Applied Chemistry, (1971) 21, 251. 9. Marsh H., Dachille F., Melvin J.M. and Walker P.L., Carbon, (1971) 9, 159. 10. Evans S. and Marsh Η., Carbon, (1971) 9, 733. 11. Evans S. and Marsh H., Carbon, (1971) 9, 747. 12. Marsh Η., Foster J.M. and Hermon G., Carbon (1973) 11, 424. 13. Marsh Η., Fuel, London, (1973) 52, 205. 14. Marsh Η., Foster J.M., Hermon G. and Iley Μ·, Fuel, London, (1973) 52, 234. 15. Marsh H. et al,, Fuel, London, (1973) 52, 243. 16é Marsh H. et al., Fuel, London, (1973) 52, 253. 17. Marsh H., Cornford C. and Hermon G., Fuel, London, (1974) 53, 168. 18. Goodarzi F. et al., Fuel, London, (1975) 54, 105. 19. Sanada Y., Furuta T. and Kimura Η., Carbon, (1972) 10, 644. 20. Huttinger K.J., Bitumen, Teere, Asphalte, (1973) 24, 255. 21. Honda H. et al., Carbon, (1970) 8, 181. 22. De Boer J.H., 'The Dynamical Character of Adsorption', Oxford University Press, London, 1968. 23· Lewis R.T., Abstracts of 12th Biennial Conf. on Carbon, Amer. Carbon Soc., Pittsburgh, 1975. 24. Patrick J.W, et al., Fuel, London, (1973) 52, 198. 25. Isaacs L.G., Carbon, (1968) 6, 765. 26. Marsh H., Augustyn D., Cornford C., Crawford D. and Hermon G., Abstracts of 12th Biennial Conf. on Carbon, Amer. Carbon Soc., Pittsburgh, 1975. 27. Marsh H. and Cornford C., Abstracts of 12th Biennial Conf. on Carbon, Amer. Carbon Soc., Pittsburgh, 1975. 28. Ban L.L., 'Surface and Defect Properties of Solids', (1972) 1, 54. The Chemical Society, London. 29. Marsh Η., Cornford C. and Crawford D., Unpublished results. 30. Marsh H., Carbon, (1973) 11, 254. 31. Donnet J.B. et al., Carbon, (1974) 12, 212. 32. Cornford C. and Marsh H., Unpublished results.


Petroleum Derived Carbons

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