Integrated mesophase injection and in situ transformation in fabrication of high-density carbon-carbon composites

The fabrication of high-density carbon-carbon composite by integrating mesophase injection and in situ transformation methods in different processing cycles was examined. Non-rigidized preform disks 30 mm thick and 68 mm in diameter were rigidized by an initial in situ transformation cycle to an average density of 0.92 g/cm³ after carbonization. The rigidized preforms were subsequently densified by 1-3 cycles of injection with the AR mesophase pitch. After each injection cycle, the flow-oriented mesophase matrix was stabilized and carbonized to 1150 °C. The composites from each injection cycle were further densified by a final in situ cycle. A final density of nearly 2 g/cm³ was attained after three injection cycles and a final in situ cycle. All the cycles except the third injection required only ambient or very moderate pressures.     

Injection and stabilization of mesophase pitch in the fabrication of carbon-carbon composites: Part II. Stabilization process

In the fabrication of carbon-carbon composites by mesophase injection through a fiber preform, it is essential to stabilize the flow-induced microstructure in the flow channels and to prevent relaxation and exudation of the mesophase. Oxidation stabilization studies were conducted on preforms injected with the naphthalene-based AR mesophase pitch. Oxidation mass gain (OMG) curves at 170, 222, and 270 °C were generated for 60°-wedges cut from full size composite disks. The rates of OMG at 170 °C of first- and second-cycle injection wedges and full-size disks were comparable to those using as-spun filaments 30 μm in diameter, and particles sieved to 200 to 340 μm. The results suggest that oxygen is accessible deep into a mesophase matrix and the transport is facilitated by connected array of shrinkage cracks. Oxidation at 170 °C has strong advantage over higher oxidation temperatures by having a higher carbon yield and lower OMG threshold and thus oxidation time required for stabilization. The 60°-wedges could be stabilized at 170 °C after a 25 h oxidation with a 7.2% OMG and attaining a carbon yield above 85%.     

Injection and stabilization of mesophase pitch in the fabrication of carbon-carbon composites. Part III: Mesophase stabilization at low temperatures and elevated oxidation pressures

Micrography and infrared spectroscopy were applied to explore microstructural stabilization of mesophase pitch at oxidation temperatures as low as 130 °C. AR mesophase pitch synthesized from naphthalene was drawn to rods and thick filaments with fine fibrous microstructures that coarsen upon carbonization without adequate stabilization. At 270 °C, the stabilization front advances rapidly to a depth of about 7 μm, after which no further growth is perceptible. At low temperatures, there is a time lag before a stabilized layer can be observed near the surface, but thereafter the stabilization front advances relatively rapidly. For longer oxidation times, the low-temperature stabilization depths can exceed those attained at higher temperatures. FTIR spectra confirmed chemical reactivity and the formation of oxygen-bearing functional groups even at the low oxidation temperatures. Oxidation under a moderate pressure of 0.7 MPa can be effective in raising the oxygen uptake and increasing the stabilization depths significantly. All evidences point to the advantageous use of lower oxidative temperatures: more effective penetration of stabilization depth, lessen destructive effects of over-oxidation, and potentially shorter processing times.     

Microstructural analysis of in situ mesophase transformation in the fabrication of carbon-carbon composites

In this work, we examined the microstructures formed during the pyrolysis of naphthalene mixed with AlCl₃ catalyst, in the critical temperature range of 300-500 °C and at varying pressures. In addition, non-rigidized preforms were densified by multiple cycle in situ transformation and compared the process with impregnation using fully transformed AR mesophase pitch under similar conditions. The process of mesophase formation in the bulk phase and within tightly packed fiber bundles was observed to be similar: spherule nucleation from the isotropic phase, coalescence of spherules forming bulk mesophase, and mesophase flow before hardening. The hardened mesophase displays the coarse, fibrous, and lamellar microstructure observed in needle cokes. The molten naphthalene was observed to evenly penetrate in-depth the large void spaces and fiber bundles. After two in situ cycles, the fiber bundles and the inter-fiber bundle regions were well filled with transformed mesophase. The incremental filling of the larger void spaces reduced the calculated filling efficiencies from 47% in the first cycle to below 15% in the third through fifth cycle. An 8% improvement in densification efficiencies was achieved by applying modest pressures during the pyrolysis. The extent of mesophase penetration with AR mesophase was observed to decrease from the outer to the inner regions of the preform. The results suggest impregnation with naphthalene catalyst mixture is efficient in filling tightly packed fiber bundles but not large void spaces. Multiple cycles are required in order to fill the large void spaces.     

Improving graphitization degree of mesophase pitch-derived carbon fiber by solid-phase annealing of spun fiber

The solid-phase annealing of the mesophase pitch spun fiber was examined between the glass transition (T_g) and softening (T_s) temperatures of the pitch to improve the graphitization degree of the graphitized fiber through recovering or further improving the stacking height of the mesogen molecules in the spun fiber, since the rapid spinning reduced markedly stacking height in the as-spun fiber. A naphthalene mesophase pitch as received carried stacking height of 2.9 nm which was markedly reduced to 1.7 nm by spinning at 230 m/min, giving Lc=40 nm for its graphitized fiber. Annealing at 206 °C improved the stacking height of the spun fiber to 2.4 nm and Lc(002) of the graphitized fiber to 54 nm. Annealing of the methylnaphthalene mesophase pitch fiber at 200 °C was much more effective in improving the stacking height from 3.5 to 5.0 nm and its graphitized fiber to Lc=91 from 40 nm. Such an improved graphitization degree led to improved thermal conductivity and tensile modulus of the graphitized fiber. It must be noted that the annealing of the spun fiber reduced its stabilization rate, indicating densification of molecular stacking in the fiber. The transformation scheme of mesophase pitch into graphite fibers is discussed to clarify the roles of molecular stacking in the clusters and their arrangement in the mesophase pitch fiber during the carbon manufacturing process.     

On the chemistry of the oxidative stabilization and carbonization of carbonaceous mesophase

Two mesophase samples, one derived from a coal-tar pitch (M-A) and the other from a naphthalene-based pitch (M-B), were stabilized with air in a temperature range of 200-300 °C and then carbonized to 1000 °C. Elemental analysis and FTIR spectroscopy were used to monitor the changes produced by oxygen in the chemical composition of the mesophase samples at different stages of stabilization (from 200 to 300 °C) and after carbonization of the stabilized samples (from 300 to 1000 °C). The results show that oxidative stabilization is a dehydrogenative process, where the hydrogen removed is predominantly aliphatic and the oxygen uptake is mainly in the form of C-O-C and CO groups. The more aliphatic character of M-B accelerates the stabilization process with respect to M-A. M-B shows a higher weight gain and also a greater variety of oxygen-containing functional groups. As a result, the plasticity of M-B is more affected by changes in the stabilization temperature than that of M-A. Thus, the stabilization process is easier to control in the case of M-A. On carbonization, oxygen and hydrogen are removed from the stabilized samples and the carbons generated exhibit an increase in interlayer spacing and a decrease in crystallite size as the carbonization temperature increases.     

Preparation of carbon hollow fiber membranes by pyrolysis of polyetherimide

The preparation of carbon membranes by pyrolysis of polyetherimide hollow fibers and the influence of process variables on the final membrane morphology using a statistical experimental design are described in this work. The characterization of polymers and membranes was carried out by thermal analysis and scanning electron microscopy (SEM). The carbonization process was accompanied by mass spectroscopy to monitor the products formed. Similar to carbonization of others polymers, H₂O, CO₂ and CO evolution from 420 to 680 °C, and hydrogen evolution from 450 to 800 °C, indicate the formation of crosslinking of polymeric chains and formation of a graphite-like structure. These experiments permitted the production of thermostable carbon hollow fibers and selection of best treatment conditions. The extent of membrane exposure under oxidizing atmosphere and the maximum temperature of stabilization were decisive in the final membrane morphologic characteristics and properties. When the stabilization temperature was above 500 °C an intensive degradation of the fiber was observed. An initial exposure to an oxidizing atmosphere seems to be fundamental in order to control the final membrane properties. In this atmosphere, heating rates as low as 1 °C min⁻¹ during stabilization reduce cracks in the surface of final membranes.     

Swelling and related mechanical and physical properties of carbon nanofiber filled mesophase pitch for use as a bipolar plate material

Mesophase pitch was investigated as a melt processable precursor to a compression or injection moldable all carbon bipolar plate. After shaping, carbonization to 1000 °C or greater is required to achieve the desired electrical and mechanical properties, but gases evolved during this step lead to swelling. Carbon nanofiber was added to suppress swelling during carbonization and bypass the typical oxidation steps used when processing mesophase pitch. The addition of carbon nanofiber decreased swelling by increasing the viscosity of the melt. Carbonized materials with carbon nanofibers can show strengths (30-50 MPa) and conductivities (20-80 S cm⁻¹) consistent with composite bipolar plate materials. The materials show conductivities below Department of Energy target values at the current carbonization temperatures, which were limited to 1000 °C. The use of glass fibers as a secondary filler led to reduced gas permeability in porous samples.     

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.     

Biopitch-based general purpose carbon fibers: Processing and properties

Eucalyptus tar pitches are generated on a large scale in Brazil as by-products of the charcoal manufacturing industry. They present a macromolecular structure constituted mainly of phenolic, guaiacyl, and siringyl units common to lignin. The low aromaticity (60-70%), high O/C atomic ratios (0.20-0.27%), and large molar mass distribution are peculiar features which make biopitches behave far differently from fossil pitches. In the present work, eucalyptus tar pitches are evaluated as precursors of general purpose carbon fibers (GPCF) through a four-step process: pitch pre-treatment and melt spinning, and fiber stabilization and carbonization. Homogeneous isotropic fibers with a diameter of 27 μm were obtained. The fibers had an apparent density of 1.84 g/cm³, an electrical resistivity of 2 × 10⁻⁴ Ω m, a tensile strength of 130 MPa, and a tensile modulus of 14 GPa. Although the tensile properties advise against using the produced fibers as structural reinforcement, other properties give rise to different potential applications, as for example in the manufacture of activated carbon fibers or felts for electrical insulation.     

Influence of oxidative stabilization on the electrochemical behaviour of coal tar pitch derived carbons in lithium batteries

A commercial coal tar pitch was thermally treated at 430 °C for 4 h and then submitted to hot filtration in order to separate the isotropic phase from the mesophase developed during the treatment. Each phase was then oxidatively stabilized in order to preserve its structure during carbonization and then carbonized at temperatures ranging from 700 to 1000 °C. The effect of the microstructure, particle morphology and chemical composition of the carbons and also the influence of their carbonization temperature on the electrochemical behaviour as electrode materials in lithium cells were studied.Galvanostatic cycling of lithium test cells using the carbon materials as positive electrodes showed the improvement of the electrochemical performance in both isotropic and anisotropic phases by stabilization with air previous to carbonization. More subtle differences between isotropic and anisotropic samples were evidenced and interpreted in terms of their textural properties. Moreover, the electrochemical impedance spectroscopy (EIS) has been demonstrated to be an interesting technique to elucidate the changes occurred in the electrode interfaces when these coal tar pitch based carbons are cycled.     

Activated carbon fibers from electrospinning of polyacrylonitrile/pitch blends

The electrospinnability of pitch was improved by blending in a solution of polyacrylonitrile (PAN) resulting in the reduction of the average fiber diameter from 2000 to 750 nm. The compositions showing good spinnability are proposed within the soluble concentrations in the ternary phase diagram of the PAN-pitch-solvent, which contains lower concentration of the pitch. Activated carbon fibers were derived by stabilization, carbonization and steam activation at 700, 800, 900 and 1000 °C of the PAN/pitch electrospun fibers. The Brunauer, Emmett, Teller (BET) specific surface area ranged from 732 to 1877 m²/g.     

Development of carbon nanofibers from aligned electrospun polyacrylonitrile nanofiber bundles and characterization of their microstructural, electrical, and mechanical properties

Carbon nanofibers with diameters of 200-300 nm were developed through stabilization and carbonization of aligned electrospun polyacrylonitrile (PAN) nanofiber bundles. Prior to the oxidative stabilization in air, the electrospun PAN nanofiber bundle was tightly wrapped onto a glass rod, so that tension existed during the stabilization. We also investigated several carbonization procedures by varying final carbonization temperatures in the range from 1000 to 2200 °C. The study revealed that: (1) with increase of the final carbonization temperature, the carbon nanofibers became more graphitic and structurally ordered; (2) the carbon nanofiber bundles possessed anisotropic electrical conductivities, and the differences between the parallel and perpendicular directions to the bundle axes were over 20 times; and (3) the tensile strengths and Young's moduli of the prepared carbon nanofiber bundles were in the ranges of 300-600 MPa and 40-60 GPa, respectively.     

Preparation and microstructure of hollow mesophase pitch-based carbon fibers

Hollow mesophase pitch fibers with rather thin diameter were successfully prepared by spinning through a C-shaped capillary. Die-swell was found to be the main factor affecting the formation of hollow fiber, and this was controlled by varying the spinning temperature. After carbonization at 1000°C, the hollow fibers possess a relatively small outer diameter of 21 μm and an inner diameter of 6 μm, and show better mechanical properties than solid fibers with similar outer diameter. The higher mechanical properties are attributed to the orientation of mesophase molecules which is related to the shape and dimension of the spinneret. The transverse microstructure of hollow carbon fibers is illustrated by scanning electron microscopy (SEM) observations on fracture sections.     

Short residence time graphitization of mesophase pitch-based carbon fibers

The effects of graphitization time and temperature on the properties of three mesophase pitch-based carbon fibers have been characterized. Graphitization temperatures studied were 2400, 2700, and 3000 °C and residence times ranged from 0.7 to 3600 s. Helium pycnometry, measurements of fiber tow resistance, and X-ray diffraction were employed to study fiber properties. As anticipated, substantial variations in fiber properties were noted for the range of graphitization conditions studied and among the three fiber types. Significant structural evolution and property development occurred even at the shortest furnace residence times. For example, for one of the fibers, a furnace residence time of 0.7 s at 3000 °C resulted in a degree of graphitization value of ∼50%, a density of 1.98 g/cm³, and an electrical resistivity of 6.3 μΩ m (corresponding thermal conductivity ∼200 W m⁻¹ K⁻¹). A simple energy consumption analysis suggests that short residence time graphitization at high temperature may result in both lower costs and substantially higher production rates for fibers prepared from mesophase pitch.     

Study of carbon films from PAN/VGCF composites by gelation/crystallization from solution

A carbon film with a cross-sectional area much larger than that of a commercial carbon fiber (>6000 times) and a thickness of about 0.3 mm was obtained using a new method. In this method, composite materials of polyacrylonitrile (PAN) and vapor-grown carbon fiber (VGCF) prepared by gelation/crystallization from dilute solutions were used as starting materials The gelation/crystallization method was adopted to ensure high orientation of PAN chains. The composite materials were heat-treated at 200-300°C in an oxidizing atmosphere for thermal stabilization and then heat-treated to 1500°C in argon gas to promote carbonization. The tensile modulus and electric conductivity for the carbon materials with cross-sectional areas of about 0.6 mm² (thickness 0.3 mm and width 2 mm) reached 18 GPa and 10 Ω⁻¹ cm⁻¹, respectively. The mechanical and electrical properties of the final carbonized materials were sensitive to the PAN/VGCF composition and the draw ratio. These phenomena were analyzed using Fourier transform IR and X-ray diffraction.     

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.     

Carbon foams with high compressive strength derived from mixtures of mesocarbon microbeads and mesophase pitch

Carbon foam with relatively high compressive strength and suitable thermal conductivity was prepared from mixtures of mesocarbon microbeads (MCMBs) and mesophase pitch, followed by foaming, carbonization and graphitization. The influence of addition amount of MCMB on the properties of as-prepared carbon foams was investigated in detail. Results showed that addition of MCMBs into mesophase pitch could significantly reduce the amount and length of cracks in carbon foams, which results in increase of compressive strength of carbon foams. Carbon foam with high compressive strength of 23.7 MPa and suitable thermal conductivity of 43.7 W/mK, was obtained by adding 50% MCMBs into mesophase pitch, followed by foaming, carbonization and graphitization.     

Carbon fiber from natural biopolymer Bombyx mori silk fibroin with iodine treatment

Carbon fibers were prepared from silk fibers after an iodine treatment and the carbon yield, fiber morphology, structure and mechanical properties were investigated. A single or multi-step carbonization process was used for the preparation. In the single step process, silk fibroin (SF) fibers were heated from 25 to 800 °C with a heating rate of 5 °C min⁻¹ under Ar atmosphere. However, the carbon fiber obtained was partially melted and was too fragile to handle. For better performance, SF fibers were treated with iodine vapor at 100 °C for 12 h and untreated and iodinated SF fibers were heated from 25 to 800 °C by a multi-step carbonization process, which was defined based on the optimum thermal degradation rate of silk. In this multi-step process, the carbon fibers obtained from iodinated SF were structurally intact and stable in appearance, and the carbon yield achieved was ca. 36 wt.%, much higher than the value for untreated SF. X-ray diffraction, Raman spectroscopy and transmission electron microscopic observation revealed that the obtained carbon fibers from both untreated and iodinated SFs had a basically amorphous structure. The strength of carbon fibers prepared from iodinated SF using the multi-step carbonization was considerably increased compared to that of untreated SF. According to viscoelastic measurement, by heating above 280 °C the iodine introduced intermolecular cross-linking of the SF, and its melt flow was inhibited which produced a higher yield and better performance of the carbon fiber.     

Effect of oxidation pre-treatment at 220 to 270 °C on the carbonization and activation behavior of phenolic resin fiber

The effect of oxidation pre-treatment of a phenolic resin fiber was examined from two aspects: one is to examine if the pre-treatment can be a means to increase the yield of carbon fiber and activated carbon fiber (ACF), and the other is to study the effect of the pre-treatment on the carbonization and activation behavior. A phenolic resin fiber was oxidized in air at 220 to 270 °C and it was subsequently carbonized at 900 °C and activated by steam at 900 °C. The oxidation was found to affect significantly the subsequent carbonization process in the way that the yield of the carbonized fiber increased with the severity of the oxidation. On the other hand, the oxidation was found not to affect the chemical and physical properties of the carbonized fiber. The ACF produced from the oxidized fiber had almost same pore structure as the ACF produced from the non-treated fiber when compared at a same activation level. The maximum yield of ACF produced from the oxidized fiber was 1.13 times larger than the yield of ACF produced from the non-treated fiber. Thus we could increase the production yield of ACF significantly without losing its high adsorption performance.