Naphthalene-based Mesophase Research Articles

Molecular origin of viscoelasticity and influence of methylation in mesophase pitch

The viscoelastic and thermomechanical properties of pitches are responsible for their melt-spinning behavior, a critical step for manufacturing high-performance pitch-based carbon fibers. Here, we systematically explore the impact of methyl group modifications on the viscoelastic and thermal properties of mesophase pitches. We employ a range of atomistic modeling approaches, including Density Functional Theory (DFT), Density Functional Tight Binding (DFTB), and Classical Molecular Mechanics (MM), to provide detailed insights into the molecular interactions and structural changes. Our results revealed the molecular mechanisms that promote layered structures leading to the anisotropic nature of the viscoelastic behavior of mesophase pitch. Furthermore, we propose a modified molecular representation of naphthalene-based mesophase pitch based on the analysis of x-ray diffraction measurements. This study provides fundamental insights into the molecular structures of mesophase pitch and the role of methyl groups controlling its viscosity, which offer valuable insights into mesophase-based carbon fiber production.

The effect of the molecular structure of naphthalene-based mesophase pitch on the properties of carbon fibers derived from it

The effect of the molecular structure of naphthalene-based mesophase pitch on the properties of carbon fibers derived from it

The effect of the molecular structure of naphthalene-based mesophase pitch on the properties of carbon fibers derived from it

AbstractMesophase pitch-based carbon fibers (MPCFs) have a high modulus of elasticity, low electrical resistivity and high thermal conductivity, so can be used in many fields. Carbon fibers were prepared from two naphthalene-based mesophase pitches, one synthesized by a HF/BF3 catalytic one-step method (AR-MP) and the other by an AlCl3 catalytic two-step method (N-MP). The mesophase pitches, spun pitch fibers, pre-oxidized fibers, carbonized fibers and graphitized fibers produced from them were characterized by TG-MS, FT-IR, 13C-NMR, MALDI-TOF-MS, XRD, SEM and elemental analysis. The molecular structures and properties of mesophase pitches were compared, and the effects of molecular structures on the structures and properties of the carbon fibers produced from them were measured. In comparison to N-MP, AR-MP has a rod-like semi-rigid molecular configuration containing more naphthenic structures and methyl side chains. The pre-oxidized fibers derived from AR-MP have a better carbon layer orientation, so that their graphitized fibers have a higher thermal conductivity of 716 W/(m·K). N-MP has a higher aromaticity with a disc-like rigid molecular configuration, so that the graphitized fibers prepared from it have a higher tensile strength of 3.47 GPa due to fewer defects being formed during preparation. The molecular structures of AR-MP and N-MP have an obvious influence on the structures and properties of their graphitized fibers.

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).

Study on spinning stability and microstructure of mesophase pitch

The effect of spinning temperature on phase stability, rheological stability, and thermal stability of mesophase pitch (MP), naphthalene-based mesophase pitch (AR) and microstructure of pitch filament were investigated in this study. Results showed that the interface stratification of anisotropic and isotropic phases, local enrichment of isotropic phases and extensive enrichment of isotropic phases existed in the spinning process for mesophase pitch. The local enrichment of isotropic phase was observed in MP and AR at the spinning temperature of 320 °C. The interface stratification of anisotropic and isotropic phases and extensive enrichment of isotropic phases were generated in MP when the spinning temperature reached 340 °C. This phase separation reduced the rheological stability and thermal stability of mesophase pitch and then affected the spinning ability. The inhomogeneous pores were also formed in the MP and AR mesophase pitch filament at the spinning temperature of 340 °C.

Preparation of mesophase pitch from refined coal tar pitch using naphthalene-based mesophase pitch as nucleating agent

By adding naphthalene-based mesophase pitch (NBMP) into refined coal tar pitch (RCTP), pyrolysis products were obtained by dry-blending and co-carbonization method. The properties of pyrolysis products were characterized by polarizing microscope, thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), X-ray diffractometer (XRD) and Raman spectroscopy. The result shows the NBMP can obviously change the mesophase development of RCTP because it can promote the formation of mesophase spheres during the pyrolysis process. The carbon yield and aromaticity of pyrolysis products were increased with the increase of NBMP content. The anisotropic texture was wide-area structure when the NBMP content was 5 wt%. However, when the content of the additive is increased to 10 wt%, pyrolysis products will appear coking. The main role of this additive is to accelerate the reaction rate as the “nucleating agent” of the thermal condensation reaction.

Insights into the flow behaviour of the AR24 mesophase pitch and nanoparticle-doped AR24 mesophase pitch during infiltration into particulate porous preforms

Synthetic mesophase pitches, extensively used as graphite precursors, manifest particular fluid-dynamic properties given their liquid crystal nature. Their recent use in infiltration processes to manufacture graphite-based monolithic and porous materials demands the study of their physical properties, mainly viscosity and contact angle, under dynamic conditions. In this work a new infiltration device was conceived and constructed to measure the infiltration kinetics of the AR24 naphthalene-based mesophase pitch and TiC nanoparticle-doped AR24 mesophase pitch into preforms of packed NaCl particles. The experiments allowed activation energies for viscosity to be obtained, along with pseudo-equilibrium and advancing contact angles.

De-ashing of naphthalene-based mesophase pitch synthesized by the AlCl3-catalyzed method

De-ashing of naphthalene-based mesophase pitch synthesized by the AlCl3-catalyzed method

Influence of fiber content on the structure and properties of short carbon fiber reinforced carbon foam

A novel carbon foam with high strength was produced from mixtures of short carbon fibers and Mitsubishi naphthalene-based mesophase pitch. These mixtures subsequently underwent foaming, carbonization and graphitization. Investigations were carried out to determine the effect on the structure and properties of the carbon foam through the addition of fibers to carbon foam. The results have shown that compressive strength increased linearly with fiber fraction, but decreased at higher fiber fractions. Thermal conductivity increased at lower fiber fraction, but dropped with more fibers addition to carbon foam. The predominant strengthening mechanism was attributed to the pulling-out effect based on the fiber–matrix interface.

Preparation of Pitch Based Mesoporous Carbon as Catalyst Supporting Material for DMFC

Pore-size controlled porous carbons were prepared for the catalyst-supporting materials of direct methanol fuel cell (DMFC) from the naphthalene-based mesophase pitch. Porous carbon was synthesized by etching the silica sphere from the carbon/silica mixture. The pore size of the porous carbon was controlled by the size of silica sphere. 60wt% Pt-Ru/porous carbons catalysts were prepared by using impregnation method. Physical and electrochemical properties of the catalyst were characterized by using XRD, cyclic voltammetry (CV) and unit cell test.

Preparation of highly crystalline carbon nanofibers from pitch/polymer blend

High crystalline carbon nanofibers were prepared by using polymer blend technique. Naphthalene-based mesophase pitch (AR pitch) was dispersed finely in polymethylpentene matrix, spun by using a melt-blown spinning machine, stabilized at 160 °C in an oxygen atmosphere and carbonized at 900 °C in a nitrogen atmosphere. Bundles of the carbon nanofibers with ca. 100 nm in diameter were obtained after removal of polymethylpentene at the carbonization process. No impurity carbon was observed. The carbon nanofibers consisted of fine carbon crystallites with preferred orientation along the fiber axis. After heating to 3000 °C, the carbon crystallites grew drastically to have an interlayer spacing of 0.3367 nm and a crystallite thickness of 56.9 nm, respectively, with remarkable improvement of the preferred orientation of the crystallites. Advantages and disadvantages of the present method were discussed briefly.

The influence of processing temperature on the structure and properties of mesophase-based polygranular graphites

Two polygranular graphites were prepared by sintering a coal-tar pitch based mesophase and a naphthalene-based mesophase. The influence of temperature on the structural (density, porosity and light texture), mechanical (flexural strength) and electrical (electrical resistivity) properties of the carbons was studied at different stages of carbonization/graphitization (400–2600°C). The results show that the density of the materials increases continuously with temperature, pores mainly appearing below 800°C, during the low-sintering phase and at the initial stages of the solid-sintering process. Above this temperature, porosity decreases due to the densification of the materials. Densification is clearly evidenced by a reduction in interlayer spacing and an increase in crystallite size. Flexural strength and electrical resistivity follow a different trend depending on the temperature range. Thus, flexural strength reaches its maximum value at 1000°C, while electrical resistivity decreases continuously with temperature. Any variation in these properties is mainly related with porosity and crystallographic order. The different composition of the raw materials causes structural, mechanical and electrical changes to occur to different extent in both materials.

Pyrolysis behaviour of stabilized self-sintering mesophase

A coal-tar pitch-based mesophase and a naphthalene-based mesophase were stabilized with air, using a multi-step temperature/time program from 200 to 300 °C. The extent of the stabilization was monitored by elemental analysis. The pyrolysis behaviour of the stabilized samples was studied by thermogravimetric analysis and differential scanning calorimetry. The results showed that the different chemical composition of parent mesophases affected the degree and effectiveness of stabilization, and consequently, the plasticity of the stabilized samples and the intensity of the exothermic effects during their pyrolysis. Naphthalene-based mesophase is more aliphatic and oxygen is taken more rapidly and in a larger amount than in coal-tar pitch-based mesophase. Consequently, the naphthalene-based mesophase stabilized more easily but stabilization was more difficult to control. Moreover, the variations in oxygen uptake and weight gain during stabilization reveal that in this sample, above 250 °C, degradation competes with stabilization. The pyrolysis behaviour of the mesophases is extremely sensitive to the changes produced by stabilization. In the coal-tar pitch-based mesophase the weight loss decreased, and the temperature of maximum rate of weight loss and temperature of initial weight loss increased with the severity of stabilization, while in the naphthalene-based mesophase this tendency was not observed. The competition between stabilization and degradation seems to be the responsible factor for this different behaviour. It was also found that the sinterability of the stabilized samples was mainly governed by their plasticity.

Ribbon fibres from naphthalene-based mesophase: Surface studies and fibre/matrix interactions in polycarbonate composites

Naphthalene-based mesophase ribbon fibres were studied in the present work. The fibres were surface treated in a plasma reactor and their mechanical and surface characteristics determined before and after the surface oxidation. The dispersive and non-dispersive components of the surface energy were obtained by contact angle measurements using a tensiometric method, and the surface oxygen content was measured by XPS. The mechanical characterisation was made by single filament tensile tests. The interactions between the ribbon fibres and a polycarbonate matrix were assessed by means of fragmentation tests. The results were compared with those obtained with pitch-based fibres with circular cross-sections.

Microstructure and texture of pitch-based ribbon fibers for thermal management

The textures and microstructures of three different experimental mesophase pitch-based ribbon fibers were studied using the complementary techniques of scanning and transmission electron microscopy and wide angle X-ray diffraction. The effect of mesophase pitch precursor and capillary entry design on the structure and heat transfer capabilities of these ribbon-shaped fibers was investigated. It was found that ribbon fibers melt-spun from a synthetic, naphthalene-based mesophase had higher degrees of preferred orientation and graphitization, larger crystallite sizes and lower electrical resistivities than those of fibers melt-spun from a petroleum-based, heat-soaked mesophase. However, capillary entry design proved to be critical to the development of a more linear transverse texture and a structure that one might expect to be more conducive to axial heat transfer.

High thermal conductivity ribbon fibers from naphthalene-based mesophase

Their extremely high thermal conductivity, combined with their relatively low density, make mesophase pitch-based carbon fibers attractive for many applications where heat transfer is critical. Although many thermal management applications could create large markets for mesophase fibers, the current high cost of these fibers makes their use uneconomical. The objective of the present research is to produce low-cost, mesophase pitch-based carbon fibers with high mechanical and thermal properties. In the current study, circular and ribbon fibers were produced from a naphthalene-based mesophase. After stabilization and carbonization, their mechanical and electrical properties were compared to fibers produced at similar conditions, but using a heat-soaked mesophase precursor. The ribbon fibers produced from the naphthalene-based mesophase exhibited higher moduli and electrical resistivities than round fibers formed from the same precursor. Also, the mechanical and electrical properties of the naphthalene-based ribbon fibers were superior to ribbon fibers previously produced using a heat-soaked mesophase and heat treated at equivalent conditions. At carbonization temperatures of only 2250°C, the ribbon fibers produced from naphthalene-based mesophase developed electrical resistivities as low as 2.68 μohm · m, a factor of three lower than those previously produced from the heat-soaked mesophase. Thus, ribbon fibers formed from naphthalene-based mesophase should exhibit higher thermal conductivities than either round fibers formed from the same precursor or ribbon fibers formed from a heat-soaked precursor. An additional benefit is that fibers formed from naphthalene-based mesophase develop excellent properties at relatively low carbonization temperatures.

An Evaluation of Naphthalene-Based Mesophase as a Carbon Fiber Precursor

Naphthalene-based mesophase, supplied by Mitsubishi Gas Chemical Co., Inc., (under the commercial name “AR” mesophase) and a heat-soaked mesophase pitch were melt-spun into fiberform using a pilot-scale extruder. The oxidation conditions of the as-spun fibersformed from each mesophase were optimized (based on tensile strength), and all fibers were graphitized at 2400°C. The naphthalene-based mesophase was found to be more spinnable, despite the fact thatits viscosity was more temperature-dependent than that of the heat-soaked mesophase. Also, fibers formed from the naphthalene-based mesophase were found to stabilize more rapidly than those formed from the heatsoaked mesophase. After carbonization, the naphthalene-based mesophase fibers exhibited much higher tensile strengths and moduli than equal-sized fibers produced from heat-soaked mesophase. The naphthalene-based mesophase fibers also exhibited lower electrical resistivities. Wide angle X-ray diffraction showed that the naphthalene-based mesophase fibers developed a smaller average interplanar spacing of the graphite basal planes and a larger average crystallite size than the heat-soaked mesophase pitch-based fibers.