Processing of mesocarbon microbeads to high-performance materials: Part I. Studies towards the sintering mechanism

Owing to their exceptional properties, carbon-carbon composites have a variety of important applications. One attractive approach for the efficient and low-cost production of such materials is to utilize the unique sintering ability of mesocarbon microbeads (MCMB). However, the mechanism of MCMB sintering is not fully understood. In this work, detailed studies are made towards this goal, using dilatometric, thermogravimetric, and mass spectrometry techniques along with microstructural analysis. It is shown by independent measurements that significant changes in pycnometric density, mass loss, and shrinkage all occur in the same temperature range (800-1200 K). Based on the obtained results, a new explanation is suggested for the high sinterability of the investigated material, which includes two main stages: (i) neck formation between particles by a viscous phase non-densifying sintering mechanism (<800 K); (ii) rapid sample shrinkage due to crystallographic transformations leading to changes of theoretical particle density in the temperature range 800-1200 K.     

Processing of an aqueous tape casting of mesocarbon microbeads for high-performance carbonaceous laminations

Tape casting is a traditional method for the manufacture of ceramic laminations. In this study, aqueous tape casting was adopted to obtain high-performance carbonaceous laminations with homogeneous density and high strength. For the preparation of a stable and homogeneous slurry of mesocarbon microbeads, the research focuses on the rheological behavior of slurries consisting of a solvent and additives such as a binder, plasticizer and dispersant. After three to five slips of casting tape laminated together under 40 MPa at 85 °C and sinter heated to 1400 °C for 1 h at a given heating ratio, carbonized laminations are obtained with an average density of 1.66 g/cm³, a bending strength of 82.76 MPa and an electrical conductivity of 169.2 S/cm. During sintering of green laminations, the additives are pyrolyzed at 500 °C to form amorphous carbon, which reduces the electrical conductivity and the mechanical strength of the carbonized laminations. However, by controlling the total additive content of the slurries, the influence of the additives can be reduced to a minimum.     

Influence of ferrocene addition on the morphology and structure of carbon from petroleum residue

Nano-sized iron particle/carbon composites have been synthesized from a petroleum residue by heating at 420 °C with ferrocene under pressure. The morphologies and structural features of the composites were investigated using TEM, HREM and XRD measurements. The effect of ferrocene addition on the development of turbostratic carbon from the petroleum residue was discussed. It was found that, by increasing the amount of ferrocene added from 3% to 20%, the size of the nano-iron particles tended to increase from 20–80 nm to 30–180 nm. The iron particles pyrolyzed from ferrocene mainly exist in the forms Fe–O and Fe_1−xS when the ferrocene content was low (3% and 6%), and α-Fe when ferrocene contents were high (10% and 20%). Upon further heat treatment in the range 500 to 900 °C, the iron particles tended to aggregate and Fe–O and Fe_1−xS were transformed into α-Fe and austenite. In comparison with the carbon formed without ferrocene addition, the resulting carbon exhibited a turbostratic structure as shown by HREM, in which the d₀₀₂ spacing decreased with the increase of ferrocene loading and increasing temperature, suggesting the carbonization was promoted by the catalysis of the nano-iron particles.     

Chemistry of the co-pyrolysis of an aromatic petroleum residue with a pyridine-borane complex

Synthesis of boron-doped mesophase by controlled co-pyrolysis of an aromatic petroleum residue and a pyridine-borane complex, PB, as a boron source, was carried out. Pyrolysis was performed at temperatures ranging from 400 to 440 °C, under 1 MPa nitrogen atmosphere. Soaking time was varied between 1.5 and 6 h, and the boron concentration in the pyrolysis mixtures ranged from 0 to 1 wt.%. The effect of the presence of boron upon solid yield, insoluble content, mesophase development and microstructure was studied. PB reacts with molecules of the petroleum residue and creates B-C bonds, B substituting C in the polyaromatic molecules, with sp² hybridisation, and probably also creating B crosslinking between polyaromatic units. The enhanced reactivity caused by B addition is reflected in an increment of viscosity; as a result, the development of mesophase is strongly increased, but the size of the mesophase structures is decreased to a mosaic texture; B also causes an increase of the interlayer spacing and a noticeable enhancement of insoluble material.     

Improving the performance of carbon composites by liquid phase activated sintering of MCMBs doped with Ti/Ni or Fe/Ni

A novel fabrication method for carbon composites using liquid phase activated sintering at 1400 °C in vacuum is described. Specimens for testing were prepared by subjecting mixtures of mesocarbon microbeads (MCMBs), Ti/Ni or Fe/Ni powders, and suitable additives, to aqueous tape casting, and molding the tapes into laminates prior to sintering. After sintering XRD showed that there were some eutectic liquid phases formed in the composites, which were favorable to enhancing the densification and reducing the porosity of the carbon composites. The reduction of the interlayer spacing of graphite crystallites due to the addition of the binary metal particles showed that the formation of the eutectic liquid promoted graphitization. The results showed that both the bend strength and the electrical conductivity of the carbon composites were also improved with the addition of the binary metal mixture.     

Structure of carbon fiber obtained from nanotube-reinforced mesophase pitch

Flow-induced orientation of discotic mesophase precursor typically leads to a center-origin, radial microstructure in the resulting carbon fibers. In the present study, we report the microstructure of carbon fibers that were derived from mesophase pitch that was modified using carbon multi-wall nanotubes (MWNTs). For an MWNT content as low as 0.1 wt.%, the microstructure was found to be fairly random (rather than radial).     

Flow behavior of mesophase pitch

The objective of this work was to provide a comprehensive rheological and structural study of AR mesophase pitch. The shear viscosity of AR mesophase pitch was found to be qualitatively similar to results from prior investigations of mesophase pitch and followed the typical trend for liquid crystalline fluids. However, a distinct hesitation, or kink, was observed in the shear viscosity curve near the shear thinning to constant viscosity transition. This behavior has been seen previously for some low molecular weight and polymeric liquid crystals and was thought to be due to a transition between tumbling and steady alignment of the uniaxial director. Optical studies of mesophase pitch revealed an orientation change of the poly-domain structure at shear rates where the kink was observed. This orientation change results in a high viscosity to low viscosity transition. This viscosity transition, and not tumbling, is responsible for the kink phenomenon. The optical studies also reaffirmed that shear flow reduces the size of the poly domain structure and elongates the domains along the flow direction. Also under quiescent conditions, the poly-domain size increased with increasing relaxation time.     

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.     

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.     

The correspondence between loss of H-NMR signal intensity and mesophase growth during the thermal maturation of a petroleum pitch

Samples of petroleum pitch, Ashland 240, were heated isothermally at 380, 400, 430 and 450°C and the hydrocarbon evolution and mesophase growth monitored simultaneously by the use of high temperature ¹H-nuclear magnetic resonance (NMR) analysis. For all four temperatures the NMR signal indicated no mesophase formation until the signal intensity had dropped by 30%, which corresponded to 20% weight loss from the pitch. The correlation between the onset and increase in mesophase formation with signal intensity loss was found to be independent of temperature. This temperature independence for the petroleum pitch agrees with the work presented previously by Honda et al. (Carbon 1970, 8, 181). who compared weight loss with the growth of quinoline-insoluble material for a coal-derived pitch. Given the correlation between signal loss and mesophase formation, it is suggested that a relative measure of NMR signal intensity could be used to estimate mesophase content of petroleum pitch, during maturation.     

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.     

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.     

Gelcasting of aqueous mesocarbon microbead suspension

Gelcasting, an in situ consolidation colloid forming process, is promising for the manufacture of structural ceramics with various shapes. In this case, the gelcasting process was adopted to obtain carbon artifacts with complex shapes. The mesocarbon microbead (MCMB) that has self-sintering property was chosen as the starting material. In order to optimize the MCMB suspensions for gelcasting, the zeta potentials of MCMB particles dispersing in water and the rheological behaviors of MCMB suspensions were investigated. A stable MCMB suspension with solid loading up to 62.5 wt.% was prepared, from which uniform green body with complex shape was successfully gelcast. The performance of green bodies and sintered samples were measured and their microstructures were observed.     

Processing of mesocarbon microbeads to high-performance materials: Part II. Reaction bonding by in situ silicon carbide and nitride formation

The processing of carbon-ceramic composites by utilizing the unique sintering ability of mesocarbon microbeads (MCMB) is reported. The ceramic constituents (silicon nitride and silicon carbide) are formed in situ by reactions between MCMB and silicon in different atmospheres. In comparison with direct addition of ceramic (SiC, Si₃N₄) phases, in situ formation shows several appealing features. By inducing the reaction of silicon with MCMB, the sintering ability of the composite is enhanced via reaction bonding mechanisms. Similarly, it is demonstrated that composite porosity is limited owing to silicon reaction with nitrogen. The reactive formation of nanoscale ceramic reinforcements via decomposition of the silicon-containing polymer (e.g. poly-carbomethyl-silane) is also reported. This approach results in formation of uniform nanosized (>100 nm) SiC layers strongly bonded to the surface of the carbon particles. The presented results contribute to the development of carbon-ceramic materials with high-operational properties.     

Sintering behaviour of a ZnO waste powder obtained as by-product from brass smelting

The effect of two sintering methods (conventional sintering and two-step sintering) and of alumina addition on the sintering behaviour of a ZnO-rich waste powder (ZnO > 95 wt%), a by-product from brass smelting industry, was studied aiming to improve the sintered density and grain size. Both conventional sintering and two-step sintering methods did not lead to fully dense powder compacts, as densification was conditioned by abnormal grain growth and the particle size of the ZnO-rich residue. When two-step sintering was used the grain growth was reduced comparatively to conventional sintering method. The highest relative sintered density (about 90%) was achieved when samples of ZnO waste and samples of ZnO waste with 2 wt% added Al₂O₃ were processed by two-step sintering and corresponded to a mean grain size of around 18 µm and 7 µm, respectively. XRD and SEM results indicated that alumina addition helped to inhibit grain growth due to the formation of gahnite spinel (ZnAl₂O₄) precipitates in the grain boundaries of zincite (ZnO) grains.     

Injection and stabilization of mesophase pitch in the fabrication of carbon-carbon composites. Part I. Injection process

The fabrication of carbon-carbon composites by injection of low viscosity mesophase pitch through a fiber preform followed by stabilization and carbonization was examined. The fully transformed mesophase MOMP and AR pitches were injected through either soft or rigidized disk preforms 35 mm thick and 68 mm in diameter. Injection provided good even filling of major flow channels and fiber bundles. Flow-induced fibrous microstructures were retained by quenching and preserved by stabilization upon carbonization. A second injection cycle was effective in filling voidage created by thermal densification. A third cycle was applied, but required severe injection conditions and provided only incremental improvement. The carbon-carbon composite reached a density of 1.8 g/cm³ after three injection cycles.