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.     

Formation mechanism of carbon foams derived from mesophase pitch

Carbon foams were prepared from mesophase pitch using foaming, carbonization and graphitization processes. The physical and chemical properties of the mesophase pitch during thermal treatment were studied by Fourier transform infrared spectroscopy, thermogravimetry, mass spectroscopy, rheometry and scanning electron microscopy. The results suggest that gases released from the pitch dissolve, saturate, nucleate and grow in the molten pitch during foaming. Then the resultant bubbles coalesced with the neighboring bubbles driven by the surface tension of the molten pitch. This coalescence generates a shear stress to force aromatic planes of the pitch to arrange regularly and paralleled to the axis of a ligament. The growth of bubbles stopped when the pitch became semi-coke at a temperature above 733 K. The viscosity and surface tension of the molten pitch are major factors that influence the growth of bubbles. After carbonization at 1073 K and graphitization at 2873 K, the well aligned aromatic planes in the foams evolve into highly aligned graphitic structures.     

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.     

Carbonization and liquid-crystal (mesophase) development. 7. Optical textures of cokes from acenaphthylene and decacyclene

Optical textures of cokes prepared by carbonizing acenaphthylene, decacyclene and mixtures thereof at selected values of heat-treatment temperatures and soak time have been compared. Optical textures are assessed using polished surfaces and reflected-polarized-light microscopy in conjunction with a half-wave plate. The acenaphthylene is chemically more reactive than the decacyclene which is itself formed during the carbonization of acenaphthylene. Products of carbonization of acenaphthylene can influence rates of carbonization of the decacyclene. Similar optical textures in cokes cannot be formed by compensating low heat-treatment temperatures with long soak periods. In addition to chemical rate-controlling processes, the physical properties of the system must be acknowledged, in particular the viscosity. Very large non-coalesced growth units of mesophase (800 μm diameter) have been observed. Pre-alignment of growth units of mesophase may occur prior to coalescence.     

Carbonization and liquid—crystal (mesophase) development. 19. Co-carbonization of oxidized coals with model organic compounds

This study examines further the phenomena of the modification of coal carbonizations by organic additives. Anthracene, pyrene and chrysene modify the carbonization in a closed system of coking coals as observed from increases in the size of optical textures of resultant cokes. Weakly caking coals are unaffected. Chrysene is the most efficient modifier probably because of its lowest calculated free valence. The co-additives tetralin and hydrogenated anthracene oil further enhance the modification processes so obviating the necessity to use hydrogenated additives. Co-carbonizations of oxidized coking and caking coals with decacyclene are effective in removing the effects of mild oxidation. Increased rates of carbonization enhance the sizes of optical textures of resultant cokes.     

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.     

Single-walled carbon nanotube buckypaper and mesophase pitch carbon/carbon composites

Carbon/carbon composites consisting of single-walled carbon nanotube (SWCNT) buckypaper (BP) and mesophase pitch resin have been produced through impregnation of BP with pitch using toluene as a solvent. Drying, stabilization and carbonization processes were performed sequentially, and repeated to increase the pitch content. Voids in the carbon/carbon composite samples decreased with increasing impregnation process cycles. Electrical conductivity and density of the composites increased with carbonization by two to three times that of pristine BP. These results indicate that discontinuity and intertube contact barriers of SWCNTs in the BP are partially overcome by the carbonization process of pitch. The temperature dependence of the Raman shift shows that mechanical strain is increased since carbonized pitch matrix surrounds the nanotubes.     

Carbonization of coal blends: mesophase formation and coke properties

Laboratory investigations of strength of cokes from blends of coals incorporating pitch were supported by 7 kg trials. The stronger cokes showed a greater interaction between coal and pitch to produce an interface component of anisotropic mozaics which is relatively resistant to crack propagation. The process whereby coal is transformed into coke includes the formation of a fluid zone in which develop nematic liquid crystals and anisotropic carbon which is an essential component of metallurgical coke. Strength, thermal and oxidation resistance of coke can be discussed in terms of the size and shape of the anisotropic carbon which constitutes the optical texture of pore-wall material of coke. Coals of different rank form cokes with different optical textures. Blending procedures of non-caking, caking and coking coals involve the interactions of components of the blend to form mesophase and optical texture. Petroleum pitches used as additives are effective in modifying the carbonization process because of an ability to participate in hydrogen transfer reactions.     

Carbonization and liquid-crystal (mesophase) development. 5. Importance of eutectic zones formed from nematic liquid crystals

The development of anisotropy in carbons arises from growth of nematic liquid crystals from the plastic phase of carbonization systems. Mixed, nematic liquid crystals form eutectic zones stable at lower temperatures than the single components. This phenomenon may have relevance to co-carbonization of mixtures of organic compounds and to coal blending procedures. A lowering of the temperature of onset of anisotropy may be beneficial to both graphite and coke quality. In the co-carbonization of carbazole and pyromellitic dianhydride to form anisotropic carbons, such a concept may be relevant. However, in the co-carbonization of two coals, although some lowering occurred in the temperature of formation of anisotropy, other interfacial phenomena are observed.     

Carbonization behaviour of modified synthetic mesophase pitches

Stabilization of naphthalene-derived synthetic mesophase pitch was achieved by controlled oxidation or reaction with phenanthrenequinone (PHQ) in order to suppress the swelling during high-temperature treatments. The modified pitches were characterized and their carbonization behaviour was studied by thermogravimetry-mass spectrometry. The results show that naphthenic groups are involved in the stabilization process. PHQ reacts via cycloaddition reactions, yielding oxygen-containing non-planar structures, a feature that accounts for the loss of anisotropic properties of the final material.     

Fabrication of mesophase pitch-derived open-pore carbon foams by replication processing

The replication process, extensively used for the fabrication of metallic sponges, is proposed for the fabrication of mesophase pitch-derived open-pore carbon foams. The mesophase pitch is liquid infiltrated into packed preforms of leachable NaCl particles which are afterwards water dissolved. The resulting material is a pitch-derived foam with an interconnected porous network that replicates the architecture of the salt packed preform. These structures are stabilized by an oxidation treatment and can be subsequently subjected to further treatments of carbonization and graphitization. Replication processing allows precision and uniformity in the pore size and versatility in the volume fraction and shape of pores.     

Laboratory coal carbonization oven

A laboratory coal carbonization oven has been described which simulates the carbonization of coal in a commercial slot-type oven. Essentially a cylinder of packed, crushed coal is inserted at a controlled rate into a long furnace so that a plastic layer moves progressively along the cylinder. Widely varying coal charges and carbonization conditions can be employed. The tensile strength and pore structural data of the cokes produced under standard conditions lie within the range of values obtained for commercial cokes.     

Carbonization and liquid-crystal (mesophase) development. 20. Co-carbonization of a high-volatile caking coal with several petroleum pitches

A high-volatile caking coal and five petroleum pitches were carbonized singly and coal/pitch systems were co-carbonized to 1273 K in the ratio of 75 wt% coal and 25 wt% pitch. Optical textures of cokes from the single carbonizations and co-carbonizations are assessed in terms of modification to the coalcoke by the pitch and unmodified pitch-coke using a point-counting technique. The pitches differ considerably in their carbonization behaviour. Each pitch can be placed into one of three groups defined in terms of their interaction with the high-volatile caking coal. A passive pitch does not modify the coalcoke but apparently carbonizes independently of the coal. An active pitch modifies some of the coalcoke. No pitch-coke can be detected. A super-active pitch modifies the coal-coke extensively beyond the extent expected from a 25% addition. No pitch-coke can be detected. The effects are related to the ability of the pitch to cause depolymerization of the coal. Quinoline-insoluble material in pitch may inhibit modification.     

Model for the coal hydrogenation process and its applications

A coal hydrogenation model has been formulated which incorporates both chemical and microscopic experimental data. In this generalized model, carbonization and hydrogenation are viewed as concurrent processes in the liquefaction of coal. Insufficient hydrogen availability, rapid heating rates and long reaction times at elevated temperatures can promote carbonization reactions. The model describes in detail the reaction pathways involved in the hydrogenation of both inertinite and vitrinite. When vitrinite is hydrogenated in the presence of a hydrogen donor solvent, a plastic material called vitroplast is formed. The vitroplast is either converted to liquid and gaseous products when hydrogen availability is high or becomes mesophase and then semicoke when hydrogen availability is low. Even under favourable hydrogenation conditions, the major reaction pathway in the hydrogenation of inertinite is one of initial mild carbonization followed by hydrogenation. It is evident that the difference in hydrogenation behaviour between vitrinite and inertinite is due, in part, to the ability of the hydrogen donor solvent to penetrate vitrinite but not inertinite particles. The hydrogenation model is useful for explaining various phenomena that occur during hydrogenation, such as the formation of mesophase and semicoke, and the blockage of reactors and preheaters.     

大直径中间相沥青基炭纤维的制备研究

以萘系中间相沥青为原料,通过熔融纺丝和随后的预氧化、炭化以及石墨化处理制备了中间相沥青基圆形炭纤维.研究了热处理温度对纤维导电性能和力学性能的影响,并采用红外光谱仪、元素分析仪、扫描电子显微镜和X射线衍射仪对纤维的组成、形貌和微观结构进行了表征.研究结果表明：纤维在预氧化时形成的羟基、酰基等含氧官能团在随后的炭化、石墨化处理过程中消失;随热处理温度的升高,石墨微晶逐渐发育、长大,并沿纤维轴向高度取向,纤维的电阻率不断降低,力学性能不断增强;3 000℃石墨化纤维电阻率为1.3μΩ·m,对应的强度和模量值为1.6GPa和380 GPa.     

Optical anisotropy of carbonized coking- and caking-coal vitrains

The purpose of this work was to characterize in detail the optical anisotropy formed during carbonization of the range of coals used in the coking industry, the ultimate objective being to attain a better understanding of the coking process. Vitrains hand-picked from a series of coking and caking coals were carbonized to various temperatures between 380 and 1000 °C. The semicokes and cokes so produced were examined by polarized-light microscopy to determine the proportions of the different types of optical anisotropy developed during carbonization. The results demonstrated that coals normally grouped within one class of the coal classification system used by the National Coal Board can lead to cokes which are significantly different in terms of their optical anisotropy. The process of the anisotropic development during carbonization can be explained generally in terms of loss of volatile matter, variations in viscosity of the plastic mass, and distortion of ordered phases by the pressure of evolving gases. Differences in carbonization behaviour as judged by the coke anisotropy can be attributed to differences in the ‘molecular-structure’ of the parent coal. In this respect the oxygen in the coal is considered to be of primary significance.     

中间相沥青基泡沫炭的研究进展

中间相沥青基泡沫炭是一种具有低密度、高强度、高导热、高导电、耐火、耐高温、抗冲击、抗氧化等性能的新型炭材料,具有广泛的应用前景.不同的原料和方法所制备的沥青基泡沫炭的结构、性能和应用也有所不同,通过对以石油系、萘系和煤沥青或改性煤沥青为原料制备中间相沥青基泡沫炭,讨论了制备工艺对泡沫炭的影响,同时对泡沫炭的改性研究及应用进行了概述.     

Changes of pleochroism and extinction contours in carbonaceous mesophase

Reflected polarized-light micrography using crossed polarizers with a gypsum plate has been employed to investigate the microstructure of carbonaceous mesophase formed at the early stage carbonization of pitches. It follows from the changes in pleochroism and in isogyres occurring with the stage rotation, that a simple mesophase spherule is optically a uniaxial positive liquid crystal belonging to the hexagonal system with a straight extinction. Observations of changes in pleochroism and in extinction contours for coalesced and for deformed mesophases, permit to distinguish crosses from nodes and by that to identify four types of linear defects in the stacking of the aromatic layer planes. It is explained schematically how the cross and nodal structures are formed in the coalescence of two simple spherules and in the deformation of such coalesced mesophase.     

Orientation behavior of carbonaceous mesophase spherules having a new molecular arrangement in a magnetic field

Carbonaceous mesophase spherules of a new-type have been found in the heat-treated products of quinoline-soluble portion of coal-tar pitch containing carbon black. Their spherules have molecular orientation different from that proposed by Brooks and Taylor. The effect of a magnetic field is discussed on the orientation of the spherules formed during carbonization processes using a polarization microscope.     

Carbonization of coals to produce anisotropic cokes: 5. Structural factors of coals influencing carbonization properties

The carbonization properties are studied of two particular coals (Zontag Vlei and Metla coals) which are markedly different despite their similar coalification rank, maceral composition, and oxygen and exinite contents. These coals possess different structural features which influence their carbonization. A demineralizing pretreatment improves the properties of Metla coal. However, this is still inferior to the Zontag Vlei coal. O-alkylation of the Metla coal improves fusibility in single carbonizations and susceptibilities, equalling those of the Zontag Vlei coal. Preheat-treatment differentiates between the coals: Metla coal loses its susceptibility at lower temperatures. The chemical analyses of oxygen functionalities of both the original and preheated coals show that their hydroxyl groups behave differently in carbonizations at lower temperatures, indicating that oxygen functionality may be another influential factor. Hydrogen shuttling within the coal may be a third factor as it may remove the oxygen functionality.