Carbonization and liquid-crystal (mesophase) development. Part 4. Carbonization of coal-tar pitches and coals of increasing rank

Coal-tar pitches, from coals of different rank and with various quinoline-insoluble contents, were carbonized under pressure (67 to 200 MN m⁻²) to maximum temperatures of 923 K. The resultant cokes were examined by optical and scanning electron microscopy in terms of size and shape of anisotropic structures within the coke. Natural quinoline-insolubles and carbon blacks both destroyed growth of the mesophase and development of anisotropy. Graphite particles (<10 μm) promoted growth and coalescence of the mesophase. Fourteen coals, of carbon content 77 to 91 wt%, VM 41 to 26%, were similarly carbonized under pressure. In the lower-rank coals no microscopically resolvable anisotropic mesophase was produced, but at a carbon content of 85% anisotropic units 1–2 μm in diameter were detected, increasing in size at a carbon content of 90% to 5 μm diameter. Results are discussed in terms of the origins of anisotropic mosaics observed in cokes, their variation in size with coal rank, and their significance in the carbonization of coal.     

Structural study of cokes using optical microscopy and X-ray diffraction

Cokes exhibiting a range of optical texture from isotropie to anisotropic domains > 60 μm diameter were examined by X-ray diffraction. The variation of an optical texture index (OTI) with crystallite height and interlayer spacing was studied. The OTI varies little with the X-ray parameters for cokes whose optical texture is larger than medium-grained (1.5–5.0 μm) mosaic anisotropy. For cokes of smaller optical texture there is a sharp decrease in crystallite height and an increase in interlayer spacing. These results are discussed in terms of fluid mesophase removing defects in cokes of optical texture of size of coarsegrained mosaics and larger. The cokes of smaller optical texture are formed from less fluid mesophase which does not coalesce. Defects therefore remain in this anisotropic carbon of the coke so reducing crystallographic order.     

Carbonization and liquid-crystal (mesophase) development. 13. Kinetic considerations of the co-carbonizations of coals with organic additives

Five coals, of rank from an anthracite to a non-caking coal, have been carbonized singly and also cocarbonized with decacyclene, mixing ratio 7:3, in the temperature range 648 K to 823 K, heating at 10 K min^?1, with various soak times. The objective of the study is to derive the basic factors which influence the kinetics of formation of mesophase and anisotropic coke. Accordingly, resultant cokes were polished and surfaces examined by reflected polarized light in an optical microscope. The size, shape and extent of anisotropic development is discussed in terms of the conditions of carbonization and the rank of coal. In these systems a somewhat larger optical texture results in cokes produced at the higher carbonization temperatures. The temperature of onset of growth of anisotropic carbon in co-carbonizations was below that of either the coal or the decacyclene. Reactivities are evidently modified. The origins, growth and coalescence of growth units of anisotropic carbon in these cocarbonizations of coals with decacyclene are demonstrated.     

Carbonization and liquid-crystal (mesophase) development. Part 3. Co-carbonization of aromatic and heterocyclic compounds containing oxygen, nitrogen and sulphur

A detailed investigation is reported of the effects upon size and shape of anisotropic mesophase structures in resultant semi-cokes of co-carbonizing eighteen oxygen-, nitrogen- or sulphur-containing compounds with fluorene, carbazole and acenaphthylene. Carbonizations were carried out under pressures of 130 to 320 MN m⁻² to a maximum heat-treatment temperature of 873 K, the mesophases being examined by optical and scanning electron microscopy. Additions of phthalimide, phthalic anhydride or pyromellitic dianhydride to fluorene and carbazole caused development of anisotropic carbon where none was formed on carbonization of the single compounds, or enhanced existing mesophase growth processes. Improvements in mesophase growth result in improved graphitizability of the semi-coke. Of the oxygen-containing compounds, the polycyclics with quinone groupings, or monocyclic molecules with several functional oxygen groupings, assist the growth of anisotropy. Phenol severely retards mesophase growth. Reasons are advanced which incorporate mechanisms of liquid-crystal formation. The implication for coal carbonization is that as the oxygen content in coals, on coalification, becomes increasingly attached to aromatic systems, then the oxygen in prime coking coals may actually enhance the growth of the mesophase during its carbonization. Nitrogen- and sulphur-containing compounds on co-carbonization with acenaphthylene at nitrogen or sulphur contents greater than 3% by weight may cause deterioration of mesophase growth. Such compounds do not significantly affect the carbonization of prime coking coals, but may contribute to the smallness of anisotropic structures in the carbons from coals of lower rank.     

Carbonization and liquid crystal (mesophase) development. 14. Co-carbonization of coals with A240 Ashland petroleum pitch: effects of conditions of carbonization upon optical textures of resultant cokes

This study examines the effect of pitch concentration, rate of heating, soak temperature and time of soak upon the optical texture of cokes prepared from the co-carbonizations of a coal (Oxcroft-Clowne, NCB Rank 802) and three vitrains of NCB Rank 204, 801, 902 with Ashland A240 petroleum pitch. Using the coal (Rank 802) with 10 wt % and 25 wt % additions of pitch caused progressive penetration of the pitch into the coal with a resultant development of a mozaic anisotropy in the coke to replace partially the original coke isotropy. With 50 wt % addition of pitch almost all of the coal particles, 600 to 1100 μm in size, were modified during carbonization. Some pitch coke was formed. For the coal and three vitrains with increasing rates of co-carbonization from 0.5–10 K min^?1 to 1200 K, using 25 wt % of A240 pitch, resultant cokes showed progressively increased extents of modification. For the two vitrains (Rank 801, 902) soaking at temperatures of 650–690 K caused a decrease in the extent of modification of isotropic coke when compared with the coke of HTT 1200 K. Evidently fast heating rates create the conditions of fluidity necessary for the pitch to modify the coal leading to growth of mesophase and anisotropic coke.     

Carbonization and liquid-crystal (mesophase) development: Part 1. The significance of the mesophase during carbonization of coking coals

Recent concepts of the growth processes of liquid-crystal structures, also called mesophase systems, during the carbonization of pitch substances is extended to coal carbonization. A basic model is formulated to explain the coal and coal-blend carbonization processes leading to metallurgical coke. This model explains the differences observed by optical microscopy in the size and shape of anisotropic structures seen in cokes in terms of liquid-crystal growth processes. These processes are considered to be restricted by chemical heterogeneity within the plastic phase, or to be influenced by the presence of solid surfaces (inerts) within the carbonizing system.     

Carbonization of hydrogenated ethylene tar pitch; study of the pitch molecular compactness factor and coke optical texture

Compactness factors of aromatic molecules in hydrogenated ethylene tar pitch were calculated as a parameter to relate to properties of mesophase of the carbonization system. Compactness factors, φ, derived from structural analyses of hydrogenated ethylene tar pitch were also related to the size and shape of optical textures of resultant cokes. Hydrogenated ethylene tar pitches having values of φ 〉 0.5 gave cokes with flow type anisotropy and relate to formation of peri-condensed structures. The spin-lattice relaxation times, T₁, for the cokes derived from hydrogenated ethylene tar pitch, are related to their optical texture.     

Carbonization and liquid-crystal (mesophase) development. 16. Carbonizations of soluble and insoluble fractions of coal-extract solution

A coal-extract solution prepared by extraction of a coking coal (CRC 301a) with anthracene oil by the National Coal Board is separated into fractions using solvents of increasing solvent power. These fractions are carbonized to 823 K and the optical textures of resultant cokes are assessed. The objective of the study is to examine the role of the molecular components of the coal-extract solution including the residual anthracene oil in mechanisms of formation of the optical texture of the anisotropic coke. Generally, the low-molecular-weight fractions of the coal-extract solution produce cokes with larger sized optical textures than the coke from the parent coal-extract solution. The higher-molecular-weight fractions produce cokes with smaller sized optical textures. Isotropic coke is produced from material which is not soluble in benzene and tetrahydrofuran. Within this parent-coal-extract solution it would appear that the dominant partner effect is influential over the size of the optical texture of coke from the coal-extraction solution, that is the minor component of smaller molecules controls the necessary growth of liquid crystals. Also, the presence of anthracene oil augments the size of optical texture of resultant cokes by providing the necessary physical fluidity of the system and possibly some chemical stability.     

Carbonization and liquid-crystal (mesophase) development. 9. The co-carbonization of vitrains with Ashland A200 petroleum pitch

Vitrains from a wide range of ranks of coals were carbonized singly and also co-carbonized (HTT 1273 K) with 25% of Ashland A200 petroleum pitch. Polished surfaces of the resultant cokes were examined for optical texture in a polarizing-light optical microscope using a half-wave retarder plate to produce interference colours. For the anthracites, there is no modification of either component during co-carbonization. The growth of optical texture from the A200 pitch is not affected. For all caking vitrains the optical texture of coke from the blend system is extensively modified when compared to the optical texture of coke from the vitrain. For the low-rank non-caking vitrains the isotropic coke becomes totally or partially anisotropic in co-carbonization. The mechanism of modification of the optical texture of resultant cokes is related to the formation of nematic liquid crystals, mesophase and the semi-coke. It is not considered that the chemistry of pyrolysis is modified on cocarbonization of the vitrain and pitch.     

Carbonization and liquid-crystal (mesophase) development. 11. The co-carbonization of low-rank coals with modified petroleum pitches

Coals of rank ranging from medium quality coking to non-caking, non-fusible, have been co-carbonized with Ashland petroleum pitches A170, A240 and A200 as well as pitches modified by heat-treatment with aluminium chloride using A170, and by reductive hydrogenation of the A200. The mixing ratio was 7:3, the final HTT was 873 K, heating at 10 K min^?1 with a soak time of 1 h. The optical texture of the resultant cokes is assessed using polished surfaces and a polarized-light microscope using reflected light and a half-wave plate. The changes in optical texture are studied from the point of view of using coals of low rank in the making of metallurgical coke. The optical texture of resultant cokes is modified by co-carbonization and the mechanism involves a solution or solvolysis of the non-fusible coals followed by the formation of nematic liquid crystals and mesophase in the resultant plastic phase. The modified A170 pitch is more effective in modifying optical texture than the A170 because of an increase in molecular weight. The hydrogenated A200 is a very reactive additive probably because of an increased concentration of naphthenic hydrogen. The hydrogenated A200 can modify the optical texture of cokes from the organic inerts of coals and from oxidized, non-fusible coals.     

Carbonization and liquid-crystal (mesophase) development. 17. Co-carbonization of a solvent-refined coal with petroleum pitches and hydrogenated coal-extract solution

This study characterizes the optical textures of cokes prepared by the carbonization of Ashland petroleum pitches, of non-hydrogenated and hydrogenated coal-extract solutions (CES) and of blends of non-hydrogenated CES materials with the petroleum pitches and hydrogenated CES materials. At an HTT of 823 K petroleum pitches produce cokes with large sized optical texture of flow-type anisotropy characteristic of needle-cokes. The non-hydrogenated CES materials produce cokes with optical textures of mozaics, 2–10 μm. However, following hydrogenation the CES materials carbonized to cokes all of which possess considerable large sized optical textures which for some materials resemble that of needle-cokes by possessing strong flow-type anisotropy, > 100 μm. Hydrogenation of the CES materials evidently facilitates the physical and chemical requirements for growth and coalescence of lamellar nematic liquid-crystals and mesophase from the fluid phase of carbonization leading to anisotropic carbon. Co-carbonizations of the non-hydrogenated and hydrogenated CES materials exhibit the dominant partner effect and are comparable in behaviour with Ashland petroleum pitches which are known to produce needle-cokes on carbonization.     

Formation of graphitisable carbons from gilsonite pitch and polyvinyl chloride—a mass spectrometric and N.M.R. study

Gilsonite pitch and polyvinyl chloride (PVC) were carbonised at slow rates of 0.5°c min^?1 and the products of carbonisation examined by chromatography, mass spectrometry, nuclear magnetic resonance spectroscopy, optical microscopy, scanning electron microscopy and electron spin resonance spectroscopy. The original Gilsonite pitch contains ～ 5% of aromatic protons, rising to 24% in soluble material just prior to formation of the graphitisable semi-coke at 410°c. The carbonised PVC melts (389°c) to form a black isotropic liquid with 23% aromatic protons, rising to 41% at the onset of the coalescence of the mesophase spheres (420–440°c). The constituent molecules are considered to be hydrogenated cyclic structures possessing a great variety of saturated and unsaturated side-groups. During carbonisation, there is loss of side-groups, but it is the essentially aliphatic cyclic structures which co-condense to form the mesophase. Further aromatisation must occur within the mesophase and semi-coke. The presence of heteroatoms within the Gilsonite pitch is considered to restrict the growth of the mesophase structures and their ability to coalesce to form the much larger units of anisotropic character observed in the case of PVC. Electron spin resonance spectroscopy suggests that free radicals are not necessary to form the mesophase, the free radical character being developed after formation of the mesophase, probably as the aromaticity is developed.     

The role of liquid crystals in the formation of graphitizable carbons (cokes)

The formation of cokes and graphites proceeds via the creation from the isotropic fluid phase of carbonization of pitch and coal, of lamellar nematic liquid crystals or mesophase. This anisotropic fluid, deformable mesophase, develops as spheres within which constituent molecules are stacked parallel to an equatiorial plane. This type of structure facilitates coalescence to a coherent mass which eventually forms a graphitisable carbon. The ‘onion-skin’ structure of mesophase spheres cannot so coalesce. Different optical textures of cokes and graphites owe their origin to different chemical reactivities and fluidities of mesophase, the lower the fluidity the smaller the size of the optical texture. Mesophase from lameller molecules is compared with conventional rod-like nematic liquid crystals. Structures in needle-cokes, metallurgical coke, coke from solvent refiend coal and carbon fibre from pitch are discussed in terms of formation and properties of lamellar nematic liquid crystals.     

Modification of the carbonization properties of coal-tar pitch by supercritical fluid extraction

The carbonization properties of coal-tar pitch were modified by supercritical fluid (SCF) extraction. Pitches extracted with supercritical toluene (SCFE pitch) contained none of the quinoline insoluble (QI) matter responsible for anisotropic structures with small unit sizes. The size of anisotropic structures from SCFE pitches was closely related to the β-resin (toluene insoluble and quinoline soluble fraction) content. Anisotropic structures from blended pitches prepared to have the same β-resin content as SCFE pitches (by blending toluene soluble (TS) and β-resin fractions obtained through a conventional liquid solvent extraction) were smaller. Extraction of the β-resin fraction with supercritical toluene could be interpreted by the co-solvent effects of the TS fraction dissolved in the extract phase. The high concentration of TS fraction in the extract phase enhanced the solubility of the β-resin fraction into this phase. In the raffinate phase, the heavier β-resin components, which are unsuitable for the development of mesophase structures, coagulated and formed unextractable QI matter through reduction in the concentration of the TS fraction. Since the TS concentration in both the phases depends on pressure and the ratio of the amounts of supercritical toluene and pitch, the control of the β-resin content in the SCFE pitches is possible through the adjustment of these two parameters.     

Unusual structures in mesophase from a petroleum pitch

Cokes from the carbonization of a petroleum pitch possess an optical texture containing anisotropic spherules with an unusual structure. These spherules (10 to 20 μm diameter) were found to be present in the original pitch and could be removed as a quinoline-insoluble fraction. This suggests that the spherules are formed during the modification of the pitch after initial fractionation of the petroleum feed-stock. The optical texture of these spherules, examined by polarized light optical microscopy, reveals a complicated ‘rosette’ structure quite unlike the structure of the spheres of mesophase now known as ‘Brooks and Taylor’ spheres. An EDAX analysis of polished segments of the spherules indicated no unusually high concentration of inorganic matter, e.g. vanadium, suggesting that the spherules did not originate from asphaltenes. The influence of such spherules or inclusions in the parent pitch upon properties of resultant cokes is briefly commented upon.     

Catalytic graphitization of cokes by indigeneous mineral matter

Indigeneous mineral matter in coals acts catalytically towards graphitization during heat treatment of coals to 2273 K. Nineteen coals of a wide range of rank were demineralized by acid extraction. Original and demineralized coals were carbonized in the range 1073–2273 K, and the resulting cokes examined by optical microscopy, X-ray diffraction and phase-contrast high resolution electron microscopy. Optical microscopy indicated the extent of formation of anisotropic carbon in the resultant cokes. The (002) X-ray diffraction profiles indicated three types of catalytic effect, for which electron microscopy demonstrated different crystallite structures and interrelations. The importance of catalytic graphitization in metallurgical cokes in relation to their strength and reactivity is discussed.     

Carbonization and liquid-crystal (mesophase) development. 8. The co-carbonization of coals with acenaphthylene and decacyclene

Several coals of different rank have been carbonized singly and also co-carbonized with acenaphthylene and decacyclene. The resultant cokes were mounted in resin and polished surfaces were examined for optical texture using a polarized-light optical microscope fitted with a half-wave retarder plate. The optical texture can be assessed qualitatively (visually) or quantitatively by a point-counting technique in terms of size and shape of constituent isochromatic anisotropic units. Some cokes from coals were Isotropic. Acenaphthylene was only able to exert a smaller influence than decacyclene on the optical texture of the resultant cokes from co-carbonizations. Decacyclene was able to modify the optical texture for both the low-rank non-fusible and the caking coals. The effects of changing the proportions of coal to additive were examined. Results are interpreted in terms of ‘depolymerization’ of the coal by the action of the additive (as solvent) and also by the action of the additive in modifying the processes of formation of semi-coke via nematic liquid crystals.     

Field emission of polycrystalline diamond films grown by microwave-plasma chemical vapor deposition. I. Effects of surface morphology of diamond

The effects of surface morphology on the field emission of non-doped polycrystalline diamond films with thicknesses ranging from 5 to 55 μm were studied. Diamond films grown by a microwave-plasma chemical vapor deposition technique had both the diamond and non-diamond components with pyramidal and angular crystalline structures. Although the average crystallite size increased with increasing the film thickness (d), the volume fraction of the non-diamond components in the films was insensitive to the film thickness. However, the turn-on electric field, F_T, (defined as the low-end electric field to emit electrons) showed a U-shape dependence on the film thickness. This U-shape dependence was explained by a model in which the emission current was controlled by Fowler–Norheim tunneling of electrons at surface of the pyramids when d was thinner than 20 μm and by carrier transport in the polycrystalline diamond film when d was thicker than 20 μm. The lowest field of 4 V/μm was obtained in the film with 20 μm thick.     

Carbonization and liquid-crystal (mesophase) development. 18. carbonization of coal-extract solutions (non-hydrogenated): influence of digestion conditions upon optical texture of cokes

Coal-extract solutions have been produced by the dissolution of a prime coking coal in anthracene oil followed by the removal of the undissolved solids. A range of coal-extract solutions prepared under different conditions was carbonized and the optical texture of the polished surfaces of the resultant cokes were assessed. The coal-extract solution prepared with the longest digestion time and at the highest temperature produced a coke with the largest anisotropic domains with some flow structure. Removal of the anthracene oil component of the coal-extract solution by extraction with selected solvents modified the carbonization behaviour such that although the coke yield increased substantially there was a significant decrease in the size of the anisotropic domains of the resultant cokes.