Naphthalene-derived Mesophase Pitch Research Articles

Accelerating the oxidative stabilization of pitch fibers and improving the physical performance of carbon fibers by modifying naphthalene-based mesophase pitch with C9 resin

A spinnable mesophase pitch with 95 vol.% optical anisotropy and 267 °C softening point was prepared through co-polymerization of 95 wt.% naphthalene pitch and 5 wt.% C9 resin. The effects of introducing C9 resin on the formation, structure and properties of naphthalene-derived mesophase pitch and the oxidative stabilization of resulting pitch fibers as well as the final physical properties of carbon fibers were systematically investigated. The results suggest that C9 resin could effectively facilitate the generation and development of liquid crystals in synthetic naphthalene pitch during the heat soaking process and introduce a certain amount of methyl side chains linking in polycyclic aromatic hydrocarbon molecules, so as to reduce the melt viscosity of modified mesophase pitch and improve its flow-spinnability and oxidative reaction capability. The pre-oxidation process of the modified pitch-spun fibers could be completed ahead of 4 h at 240 °C in an air atmosphere, compared with unmodified pitch fibers. This shows a possibility to decrease the production cost of mesophase pitch-based carbon fibers. Although the tensile strength of 1000 °C carbonized fibers prepared from the modified mesophase pitch (1.23 GPa) is slightly higher than that of original carbon fibers (1.18 GPa), the axial electrical resistivity and the thermal conductivity of corresponding graphite fibers after 3000 °C graphitization are 2.0 μΩ m and 677 W/(m K), respectively, which are significantly enhanced in comparison with those of original graphite fibers (3.2 μΩ m and 473 W/(m K).

Enhanced oxidation performance of pitch fibers formed from a heterogeneous pitch blend

The oxidation performance of heterogeneous pitch fibers formed by blending a coal tar-derived isotropic pitch (IPc) and a naphthalene-derived mesophase pitch (MPn) was studied. The role of IPc in affecting the oxidation performance of such heterogeneous fibers was illustrated. The IPc spherical domains distributed in the MPn matrix facilitate the reduction of oxidization time and the decrease of the gradient of oxygen distribution. As such, the MPn/IPc blends have been demonstrated to be a feasible approach for reducing the oxidation difficulties in producing large diameter carbon fibers. Further improvement on the oxidation efficiency and the suppression of deep stabilization in large diameter pitch fibers were observed, when the blend precursors were oxidized in pure oxygen atmosphere.

The structure and properties of ribbon-shaped carbon fibers with high orientation

Using a naphthalene-derived mesophase pitch as a starting material, highly oriented ribbon-shaped carbon fibers with a smooth and flat surface were prepared by melt-spinning, oxidative stabilization, carbonization, and graphitization. The preferred orientation, morphology, and microstructure, as well as physical properties, of the ribbon-shaped carbon fibers were characterized. The results show that, the ribbon-shaped fibers possessed uniform shrinkage upon heat treatment, thereby avoiding shrinkage cracking commonly observed in round-shaped fibers. As heat treatment progressed, the ribbon-shaped graphite fibers displayed larger crystallite sizes and higher orientation of graphene layers along the main surface of the ribbon-shaped fiber in comparison with corresponding round-shaped fibers. The stability of the ribbon-shaped graphite fibers towards thermal oxidation was significantly higher than that of K-1100 graphite fibers. The longitudinal thermal conductivity of the ribbon fibers increased, and electrical resistivity decreased, with increasing the heat treatment temperatures. The longitudinal electrical resistivity and the calculated thermal conductivity of the ribbon-shaped fibers graphitized at 3000°C are about 1.1μΩm and above 1100W/mK at room temperature, respectively. The tensile strength and Young’s modulus of these fibers approach 2.53 and 842GPa, respectively.

Preparation and Characterization of Highly Oriented Ribbon Shape Pitch-based Carbon Fibers

Using a naphthalene derived mesophase pitch as starting material, high oriented ribbon-shaped carbon fibers with smooth and flat surface were prepared by melt-spinning, oxidation stabilization as well as further carbonization and graphitization processes. The composition, morphologies and microstructure of the ribbon-shaped carbon fibers treated at various conditions were characterized by elemental analyses, IR spectroscopy, XRD, Raman spectroscope, scanning electron microscope and polarized-light microscope. The results show that the carboxyl, carbonyl and ether functional groups formed during the oxidizing stabilization process are removed by subsequent carbonization treatment. The width and thickness of the ribbon-shaped fibers at the transverse section decrease from 1.6mm and 18μm of pitch fiber to 1.2mm and 9μm of graphitized carbon fibers, respectively. The relative intensity of the diffraction peak at about 2θ = 26° corresponding to (002) crystal plane of hexagonal graphite increases with the heat-treatment temperature increasing. This indicates that the crystal size of graphite in this carbon fibers also increases in this process. In comparison with the carbonized fibers, the carbon fibers after graphitization treatment display more perfect crystal orientation along the main surface of ribbon-shaped carbon fibers.

Chemical, microstructural and thermal analyses of a naphthalene-derived mesophase pitch

A detailed characterisation of a synthetic naphthalene-derived mesophase pitch, in its as-received state and during pyrolysis, has been performed. The study has been conducted by means of various techniques and with a particular attention to Raman microspectroscopy. The Raman spectra show features in common with the naphthalene precursor, i.e., a broad and complex band at 1150–1500 cm −1 and a multicomponent G band at 1600 cm −1. These features correspond to the vibration modes of the molecules of the pitch and more especially to the non-aromatic C–C bonds involved in alkyl groups, aryl–aryl bonds or naphthenic rings. The pyrolysis of the pitch into coke takes place within a narrow temperature range (480–500 °C) through the elimination of hydrogen and light alkanes resulting from the breaking of homolytic C–H bonds and naphthenic cycles, respectively. This process initiates a swelling of the pitch. The analysis of the Raman features shows that the structure of the pitch is only slightly affected within this temperature range. Conversely, significant structural changes of the material (as shown by the vanishing of the multicomponent bands at 1600 and 1150–1500 cm −1) are evidenced beyond 750 °C, simultaneously with a hydrogen release and an increase of the true density. This phenomenon corresponds to the extension of the graphene layers of the coke and the formation of a distorted carbon network.

Large diameter carbon fibres from mesophase pitch

Large diameter carbon fibres of 15–60 μm were produced from naphthalene-derived mesophase pitch. In the as-spun state, preferred orientation increases with fibre diameter, confirming a previous study. However, the sheath–core structure that develops during carbonisation of large diameter fibres results in a decrease in preferred orientation. As a consequence, the strength, modulus and electrical resistivity also decrease. The coarse optical texture of the partially relaxed core allows a higher degree of graphitisation. With extended stabilisation, the core can be eliminated and the mechanical properties improve whilst the electrical resistivity increases.

Effect of Stabilization Conditions on the Microstructure and Electrochemical Properties of Melt-blown Graphite Fibers Prepared from NMP

Naphthalene derived mesophase pitch WP) was spun into short fibers by using melt-blown technology. The pitch fibers oxidative stabilization were carried out heating rates of . The heating rate was a key factor to maximate the capacity of the Li-ion secondary battery through controlling the morphology of the graphitized fiber. The diameters of the melt-blown fibers prepared were in the range of with functions of air jet speed, air temperature and the temperature of the nozzle. The graphitized fibers of diameters showed various morphological structure with heating rate of the stabilization. Radial, radial-random and skin-core cross-sectional structure of the fibers were observed at the respective heating rate of . Most crystalline structure of graphite was obtained from the fiber stabilized at heating rate of exhibiting the best anode performance with 400 mAh/g of capacitance and of charge/discharge efficiency.

Mesophase pitch and phenolic resin blends as binders for magnesia–graphite bricks

Naphthalene-derived mesophase pitch (NMP) and phenolic resins of novolac type and resol type were applied for the binder of MgO–C brick. NMP provided a brick of high strength at over 1000°C and high oxidative resistivity, however, the apparent porosity of the brick was high because of NMP's expansion at the carbonization. Both types of resins provided bricks of high strength and low apparent porosity at low temperature, the strength seriously decreasing over 1000°C. A blend of NMP and the resins provided bricks of excellent performance with high strength and low apparent porosity in a wide temperature range and high oxidative resistivity. The thermosetting resin bound MgO around 100°C and surrounded the dispersed NMP particles to prevent expansion at the latter carbonization, while NMP-originating carbon moderated the strong shrinkage of the resin over 1000°C, maintaining the high strength and density of the brick in a wide temperature range.

Densification of carbons prepared from mesophase pitch and phenolic resin blend

Co-carbonization of naphthalene-derived mesophase pitch (NMP) and novolac-type phenolic resin (NV) was investigated to densify the carbon derived from the mesophase pitch. Blending both types of phenolic resin to NMP suppressed the expansion during carbonization to densify the resultant carbon. Thermosetting NV before the carbonization of NMP fixed the molten pitch in the crosslinked network, suppressing the expansion of NMP. Blending in THF before the carbonization was effective in making the region of NMP smaller, leading to higher bulk density and finer mosaic texture of the resultant carbon. The carbon yields of the blend were lower than those expected from the additivity line. A large amount of naphthenic hydrogen of NMP is released to react with phenolic OH to prevent crosslinking and decompose phenolic resin. The air blowing of NMP drastically decreased its aliphatic hydrogens, increasing the carbon yields and the bulk density of the blend.

Modification of naphthalene-derived mesophase pitch with benzoquinone

Naphthalene-derived mesophase pitch (NMP) was chemically modified with p-benzoquinone (BQ) at relatively low temperature, then carbonized and graphitized. An activated Diels-Alder reaction between NMP molecules and BQ occurred at 115 °C followed by a dehydration reaction at 158 °C. The modified NMP changed its texture from anisotropic to isotropic with increasing BQ concentration. The carbon yield of modified NMP also increased with BQ concentration up to 81.4 from 74.6%. No coalescence of the modified NMP powders was observed during carbonization and graphitization when modified at BQ NMP ratios higher than 10 10 . The bulk density of the disks molded and carbonized at 1000 °C was a maximum at a 10 10 composition among the four compositions examined. Molecules in NMP were found to increase in molecular size and be substituted by carbonyl groups through the Diels-Alder reaction of BQ. Such molecules lose stacking order and fusibility, forming a more isotropic texture, while the carbon yield increases.

The oxidative stabilization and carbonization of a synthetic mesophase pitch, part I: The oxidative stabilization process

The transformation of mesophase pitch to an ordered carbon by high-temperature heat treatment in an inert atmosphere must be preceded by a suitable period of thermosetting or oxidative stabilization. The chemical process of oxidative stabilization is not well characterized, although research has clearly indicated that the oxidation history of the mesophase has a profound impact on the mechanical qualities of the resultant carbon product. In this study, we oxidized samples of a commercially available, synthetic mesophase pitch to determine the effect of different time/temperature profiles of oxidation on the quantity and type of introduced functionality. Fourier transform infrared spectroscopy (FT-IR) was used to characterize and quantify oxygen functionality present in this oxidized pitch. The oxidation process was characterized by periods of weight gain and weight loss. It was concluded that weight gain during oxidation involves the loss of aliphatic content in the pitch, with a concurrent increase in oxygen content as predominately ester and anhydride functionality. Weight loss is characterized by a loss of aromatic carbon content from the pitch and a further increase in the relative oxygen content in the pitch. It was discovered that changing the time/temperature profile of oxidation affects the pitch primarily via the degree of weight gain and weight loss experienced, rather than through the introduction of specific functionality with weight gain dominating at lower oxidation temperatures and shorter time periods, and weight loss dominating at higher temperatures and longer periods of oxidation. We conclude by suggesting mechanisms of derivatization to explain the oxidative stabilization of this synthetic, naphthalene-derived mesophase pitch in an oxygen-rich atmosphere.

Carbon disc of high density and strength prepared from heat-treated mesophase pitch grains

Carbonaceous grains for binderless moulding were prepared by control of the preheat-treatment of the naphthalene-derived mesophase pitch. The carbonized and graphitized discs were evaluated in terms of physical properties and textures. Carbon disc of strikingly high bulk density, 2.04 g/cm 3, high compressive strength, 1.94 tf/cm 2, and large shrinkage ratio, 15.9%, was obtained through the grinding of the pitch preheat-treated at 753 K, their cold moulding, carbonization at a heating rate of 12 K/h, and successive graphitization at 2173 K. A homogeneous fine-mosaic texture of optical anisotropy was observed over the whole region in the artifact, indicative of sufficient adhesion among grains during the carbonization. X-ray analyses suggested a graphitizing nature of the grains in their moulds. Grains preheat-treated under 753 K exhibited excess fusibility for adhesion, leading to partial flow texture, where many cracks were easily induced during the calcination, decreasing the strength of the disc. In contrast, preheat-treatment over 753 K caused insufficient adhesion among grains, decreasing markedly both density and strength of the disc. A slight but significant presence of the pyridine-insoluble/quinoline-soluble fraction in the preheattreated pitch system is indicated to play a major role in adhesive ability of the grains preheat-treated at 753 K.

The oxidative stabilization and carbonization of a synthetic mesophase pitch, part II: The carbonization process

The carbonization behaviors of a series of synthetic naphthalene-derived mesophase pitch powders subjected to different time and temperature profiles of oxidative stabilization were studied by thermogravimetric analysis (TGA) and thermal volatilization analysis/sub-ambient thermal volatilization analysis (TVA/SATVA). Qualitative and quantitative analysis of the carbonization products was made using a combination of gravimetric and FT-IR techniques. It was concluded that an “ideal” oxidation profile could be defined alternatively in terms of the relative magnitude of the product fractions from carbonization, rather than in terms of a specific time/temperature profile of oxidation. Mechanisms are suggested to explain the carbonization behavior of both unoxidized and oxidized mesophase pitches. It was also concluded that the melting and fusion associated with the carbonization of previously unoxidized or insufficiently oxidized pitch is accompanied by the production and subsequent rearrangement, migration, and recombination of relatively large quantities of oxygen free aromatic radicals. In contrast, the dimensional stability associated with sufficiently oxidized pitches can be attributed to concerted decarboxylations of ester and anhydride crosslinks, which result in “caged” aromatic radicals that are advantageously positioned for recombination without migration or rearrangement.

Some factors for the high performances of mesophase pitch based carbon fibre

The tensile strength and Young's modulus of the carbon fibres prepared from naphthalene derived mesophase pitch were studied by varying the spinning temperature, heating rate and final temperature of the stabilization and graphitization temperatures to find the best properties obtainable from the particular pitch. The heating rate was very influential on the tensile strength of the fibre; a slow heating of 0.5° C min−1, provided the highest strength, as high as 5 GPa at the optimum final temperature of stabilization. A higher or lower final temperature reduced the strength. Insufficient oxygen uptake or decomposition of oxygen groups at the surface of the fibre could induce defects, reducing the strength. In contrast, Young's modulus of the fibres was rather insensitive to preparation conditions. The carbonization and graphitization temperature influenced the mechanical properties as follows: the strength increased stepwise with the carbonization temperature, whereas the modulus increased sharply with the graphitization temperature. The structural factors most influential differed with these properties.

Reorganization of molecular alignment in naphthalene and methyl-naphthalene derived pitches

Molecular alignment in methyl-naphthalene and naphthalene-derived mesophase pitch materials was studied at different levels of resolution by means of optical microscopy, X-ray, and TEM using the 002 lattice fringe technique, to determine the influence of different solidification procedures on the molecular alignment in the solidified pitches. Rapid resolidification procedures —quenching in liquid nitrogen or extraction —were found to reduce domain size and molecular alignment in the solidified pitch materials, whereas annealing followed by a slow cooling step produced better alignment, larger domains, or Brooks and Taylor type spheres, depending upon the composition of the pitch. Such rearrangement can be compared to a rate-controlled process such as recrystallization. Quite different molecular rearrangement behavior was found for naphthalene and methyl-naphthalene pitches, reflecting variations in their molecular stacking heights. The degree of liquid crystal alignment and the limitations of TEM in examining these types of structures are discussed.

Self-adhesive carbon grains oxidatively prepared from naphthalene-derived mesophase pitch for mould of high density

Self-adhesive carbonaceous grains for binderless mould were prepared through the oxidation treatment of the ground mesophase pitches catalytically synthesized from naphthalene with HF/BF3. The oxidation conditions strongly influenced the carbonization behaviors of the mesophase pitch grains and the strength of the resultant carbon artifacts (diameter: 20 mm, thickness: 4 mm). The oxidation at 493 K for 1–2 h in air flow was optimum to balance the fusibility and the thermosetting property of the mesophase pitch grains. The properly oxidized grains were proven to show self-adhesive ability, shape stability, and higher carbon yield, providing a carbon artifact of strikingly high density, 1.88 g/cm 3, high compressive strength, 2.9–2.6 tf/cm 2, and large shrinkage ratio, 33–42 vol% by the calcination at 1573 K. The observation using a polarized light optical microscope and SEM indicated fine-mosaic texture, dense packing, and sufficient adhesion of the grains in the carbonized artifact.