Carbonaceous Mesophase Research Articles

Multiscale theory and simulation for carbon fibre precursors based on carbonaceous mesophases

An integrated multiscale computational materials modelling toolbox developed to predict product and processing properties of carbonaceous mesophases is presented and used to simulate linear and non-linear rheology, processing flow induced textural transformation and carbon fibre textures. The spatial resolution of the predictions spans from the nanoscale to the microscale. The nanoscale processes that lead to defect nucleation are linked to process induced structuring. Modelling the linear rheology reveals the role of polydomain textures on the elasticity modulus and can be used to predict domain sizes from simple small amplitude oscillatory flows. Geometric modelling provides insights on fibre texture and elucidates the role of surface anchoring on the fibre structure. The shown ability of the integrated model to predict product structure, flow processing and rheology in a linked sequence demonstrates the utility of computational material science as an efficient and economical innovation tool.

Structure and high temperature frictional behavior of a coal pitch-derived carbonaceous mesophase (CM) treated by high-energy ball milling

A coal tar pitch-derived carbonaceous mesophase (CM) was treated in a high-energy ball mill apparatus. The structures for the raw and the as-milled CMs were characterized by X-ray diffraction and laser-Raman spectroscopic techniques, and the frictional behaviors for the CMs were investigated by using a SRV high temperature friction and wear tester. The results have shown that, high-energy ball milling leads to a drop in the crystallinity of the CMs and a decrease in the size of graphite planar micro-crystals, implying a higher structural amorphism caused by the high-energy ball milling. In addition, the CMs display a high temperature lubrication effect. High-energy ball milling is supposed to be beneficial to the graphitization of the CMs induced by friction mechanical action, and, therefore, facilitate the high temperature lubrication effect to some extent.

Structures and high temperature frictional behaviors of carbonaceous mesophases doped with metallic element nickel (Ni) through mechanical alloying

Incorporation of metallic element Ni into a coal tar pitch-derived carbonaceous mesophase (CM) was performed through mechanical alloying in a high energy ball mill apparatus. The structures for the raw and Ni-doped carbonaceous mesophases were characterized by X ray diffraction and Laser-Raman spectroscopic techniques, and the frictional behavior for the carbonaceous mesophases were investigated by using a SRV high temperature friction and wear tester. The results have shown that, the Ni-doped carbonaceous mesophase through mechanical alloying shows a drop in the crystallinity and a decrease in the size of graphite planar micro-crystals, implying a transition to the amorphous structures caused by the mechanical alloying. In addition, the carbonaceous mesophases display a high temperature lubrication effect, and, compared with the carbonaceous mesophases without mechanical alloying, the Ni-doped carbonaceous mesophase from mechanical alloying can provide more evident and longer lasting time high temperature lubrication effect. It can be assumed the there probably exists some correlation between the high temperature lubrication effect for the carbonaceous mesophases and the variation in microcrystalline planar size (La) of the tribo-induced structures from the CM.

Structure development and texture formation in carbonaceous mesophase fibers

This paper studied the thermal relaxation phenomena after melt-extrusion of a rigid discotic uniaxial nematic mesophase pitch using mathematical modeling and computer simulation. The Ericksen and Landau–de Gennes continuum theories were used to investigate the structure development and texture formation across mesophase pitch based carbon fibers. It was found that during the thermal relaxation after the cessation of extensional flow the discotic nematic molecules stored elastic free energy decays. The distorted nematic molecules reorient to release the stored elastic free energy. The difference in time scales for molecular reorientation and thermal relaxation resulted in different transverse textures. The rate at which the fibers are cooled is the main factor in controlling the texture development. A slow cooling rate would permit the nematic discotic molecules to reorient to a well developed texture. The random texture is a result of rapid quenching. The numerical results are cconsistent with published experimental observations.

Texture modeling in carbon–carbon composites based on mesophase precursor matrices

A computational modeling study of texture formation in carbon–carbon composites based on carbon fibers and carbonaceous mesophase precursors is presented. The modeling predictions on texture formation and disclination structures are quantitatively validated with extensive experimental data. The number and type of disclinations displayed by the carbonaceous mesophase matrix is shown to be governed by the elasticity of the mesophase, the carbon fiber–mesophase interfacial energy, the size of the fibers, and positional arrangement of the fibers. The simulations provide new insights on the fundamental principles that govern texturing and disclination nucleation, and on how to control the structure of carbon–carbon composites through fiber concentration, fiber cross-section, and fiber–matrix interaction.

Multiscale simulation of flow-induced texture formation in polymer liquid crystals and carbonaceous mesophases

This paper presents theory and simulation of flow-induced structures in liquid crystalline materials, useful to the creation of synthetic material structures and to the biomimetics of natural fibers. A multiscale theory and simulation of hydrodynamic texture formation is presented; it provides fundamental principles for control and optimization of structures in liquid crystal polymers and carbonaceous mesophases. In thermotropic flow-aligning nematic polymers it is found that as the shear-rate increases, the pathway between an oriented non-planar state and an oriented planar state is through meso-texture formation and coarsening, with temperature and shear rate being efficient fields to control the grain size of the texture. For capillary flow of carbonaceous mesophases, the simulations predict the emergence of macroscopic ring patterns whose thickness and density can be controlled by the applied pressure drops. The results provide insight on microstructure formation and control in liquid crystalline materials.

Optical and structural modeling of disclination lattices in carbonaceous mesophases

An integrated microstructural and optical model for carbonaceous mesophases is developed and used to explain the principles that govern the formation and stability of experimentally observed disclination lattices. The model is able to capture the orientation features of disclination lattices, including the type and location of disclination lines, and the orientation field in the mesophase matrix. The optical model based on reflection polarized optical microscopy is able to replicate all the details observed in actual observations. The typical brush figures have the proper distribution, orientation, and intensity. The computational predictions offer science-based routes to create and control desirable material architectures based on carbonaceous mesophase-carbon fiber composites.

Improvement of the thermo-mechanical properties of isotropic graphite, produced from carbonaceous mesophase, by the addition of different carbides

This work demonstrates the possibilities for optimization of doped fine grain graphites with high thermal conductivity and high thermal shock resistance for application as plasma-facing materials at those areas of the vessel wall in fusion devices receiving the highest particle and power loads. A mixture of carbonaceous mesophase powder and different fine grain carbides (B 4 C, TiC, VC, ZrC and WC) was used as starting material. These carbides act as catalysts, decreasing the graphitization temperature. VC acts as catalyst of the graphitization at the lowest temperature, and ZrC is the most effective catalyst of all investigated carbides. With increasing graphitization temperature, the open porosity of all doped materials becomes closed, suggesting the existence of a diffusion mechanism responsible for both the catalytic effect and the closing of the open porosity. Carbides addition does not strongly influence the mechanical properties of pure graphite.

Computational modeling in processing flows of carbonaceous mesophases

Carbonaceous mesophases are liquid crystalline precursor materials that can be spun into high performance carbon fibers using the melt spinning process, which is a flow sequence consisting of capillary, diverging, porous media, converging, and extensional flows that modifies the precursor molecular orientation structure. Carbon fiber property optimization requires a better understanding of the principles that control the structure development during the fiber formation processes and the rheological processing properties. This paper presents the elastic and continuum theory of liquid crystals and computer simulations of structure formation for pressure-driven capillary flow of carbonaceous mesophase precursors used in the industrial carbon fiber spinning process. The simulation results capture the non-Newtonian rheology of mesophase and the formation of characteristic fiber macro-textures.

Computational modeling in processing flows of carbonaceous mesophases

Carbonaceous mesophases are liquid crystalline precursor materials that can be spun into high performance carbon fibers using the melt spinning process, which is a flow sequence consisting of capillary, diverging, porous media, converging, and extensional flows that modifies the precursor molecular orientation structure. Carbon fiber property optimization requires a better understanding of the principles that control the structure development during the fiber formation processes and the rheological processing properties. This paper presents the elastic and continuum theory of liquid crystals and computer simulations of structure formation for pressure-driven capillary flow of carbonaceous mesophase precursors used in the industrial carbon fiber spinning process. The simulation results capture the non-Newtonian rheology of mesophase and the formation of characteristic fiber macro-textures.

Computational rheology of carbonaceous mesophases

Mesophase pitches are multicomponent discotic nematic liquid crystals (DNLCs), whose characteristic molecular weight is intermediate between low molar mass and polymeric nematic liquid crystals. Flow modelling of these fluids is performed using a previously formulated mesoscopic viscoelastic rheological theory [J. Non-Newtonian Fluid Mech. 94 (2000) 87] that takes into account flow-induced texture transformations. A complete extra stress tensor equation is developed from first principles for liquid crystal materials under non-homogeneous arbitrary flow. This mesoscopic viscoelastic model has been adapted to describe the rheology of flow-aligning thermotropic DNLCs as models of mesophase pitches. We develop a fundamental understanding of the relations between rheology and flow of carbonaceous mesophases using theory and simulation by characterizing the steady and transient shear rheological material functions of flow-aligning DNLCs. Predictions for simple shear flow (under non-homogeneous conditions) for the apparent shear viscosity and first normal stress differences are presented. The predicted relations among rheological properties, shear-induced microstructure, processing conditions and material parameters of discotic mesophases are characterized and discussed.

Transient rheology of discotic mesophases

This paper presents an analysis of the role of orientation on the rheology of discotic mesophases subjected to slow shear start-up flows, using a projection of the Landau-de Gennes equations of nematodynamics. Analysis of the shear stress surface as a function of tilt and twist orientation with respect to the shear plane shows that the stress surface is dense in well-oriented and periodically located sets of maxima and minima. Thus overshoots and undershoot stress responses to shear-start up are predicted to be the rule rather than the exception. In-plane (within the shear plane) shear start-up stress responses can exhibit multiple, single, or no overshoots, depending on the number of maxima traversed on the way to steady state. Responses originating from orientations close to the vorticity axis lead to stress undershoots. Complex stress responses, such as a weak overshoot-strong undershoot sequence, are found for intermediate tilt-twist initial states. In-plane modes lead to amplitude and strain scaling. Out-of-plane modes do not display amplitude or strain scaling. These results provide will be useful to interpret and use transient shear rheological data of carbonaceous mesophases and highly filled suspensions of disc-like particles.

SnNi-deposited carbonaceous mesophase spherule as anode material for lithium ion batteries

Carbonaceous mesophase spherule (CMS) is a commercial anode material for rechargeable lithium batteries. A composite anode material of SnNi deposited carbonaceous mesophase spherule was prepared by co-precipitation method. The structural and electrochemical characterization of the SnNi/CMS composite anode material was studied. According to the measurement of its electrochemical characterization, the prepared SnNi/CMS composite anode material shows much better electrochemical performance than CMS. The first discharge capacity of 360 mA h g −1 was obtained for the SnNi/CMS composite anode material, and its discharge capacity maintained at 320–340 mA h g −1 in the following cycles. It indicates that the modification of CMS with SnNi alloy can further improve the intercalation performance of CMS. SnNi/CMS composite material shows a good candidate anode material for the commercial rechargeable lithium batteries.

Computational modeling of ring textures in mesophase carbon fibers

Carbon fibers are widely used in many industrial applications due the fact of their excellent properties. Carbonaceous mesophases are liquid crystalline precursor materials that can be spun into high performance carbon fibers using the melt spinning process, which is a flow cascade consisting of pressure driven flow-converging die flow-free surface extensional spinline flow that modifies the precursor molecular orientation structure. Carbon fiber property optimization requires a better understanding of the principles that control the structure development during the fiber formation processes and the rheological processing properties. This paper presents the elastic and continuum theory of liquid crystalsand computer simulations of structure formation for pressure-driven flow of carbonaceous liquid crystalline precursors used in the industrial carbon fiber spinning process. The simulations results capture the formation of characteristic fiber macro-textures and provide new knowledge on the role of viscous and elastic effects in the spinning process.

On the chemistry of the oxidative stabilization and carbonization of carbonaceous mesophase : Fanjul, F. et al. Fuel, 2002, 18, (16), 2021–2070

The ethanol utilization pathway (alc system) of Aspergillus nidulans requires two structural genes, alcA and aldA, which encode the two enzymes (alcohol dehydrogenase and aldehyde dehydrogenase, respectively) allowing conversion of ethanol into acetate via acetyldehyde, and a regulatory gene, alcR, encoding the pathway-specific autoregulated transcriptional activator. The alcR and alcA genes are clustered with three other genes that are also positively regulated by alcR, although they are dispensable for growth on ethanol. In this study, we characterized alcS, the most abundantly transcribed of these three genes. alcS is strictly co-regulated with alcA, and encodes a 262-amino acid protein. Sequence comparison with protein databases detected a putative conserved domain that is characteristic of the novel GPR1/FUN34/YaaH membrane protein family. It was shown that the AlcS protein is located in the plasma membrane. Deletion or overexpression of alcS did not result in any obvious phenotype. In particular, AlcS does not appear to be essential for the transport of ethanol, acetaldehyde or acetate. Basic Local Alignment Search Tool analysis against the A. nidulans genome led to the identification of two novel ethanol- and ethylacetate-induced genes encoding other members of the GPR1/FUN34/YaaH family, AN5226 and AN8390.

Morphology-stable alloy/C composites for lithium insertion

Composite materials, obtained by depositing sub-micron size particles of SnCu x and SnSb x on the surface of carbonaceous mesophase spherules (CMS), can endure the large volume effect of alloy host for lithium insertion and extraction. The alloy/C composites deliver large reversible capacities (430 mAh g −1 for SnSb x /CMS, 390 mAh g −1 for SnCu x /CMS); the capacity retention at the 30th cycle is 90% for SnSb x /CMS and 95.2% for SnCu x /CMS, respectively. They are promising anode materials for lithium-ion batteries.

Simulation of texture formation processes in carbonaceous mesophase fibres

Carbon fibres are spun from carbonaceous mesophases using standard melt spinning techniques. These melt spun carbon fibres exhibit a set of distinct cross-sectional textures. Two widely reported textures in literature are the planar radial (PR) and planar polar (PP). This work uses a mesoscopic model, based on the classical Landau-de Gennes theory of liquid crystals adapted to carbonaceous mesophases, to elucidate the principles that control the texture formation processes. The model is able to capture the microstructure and the formation of the PR and PP textures. A phase diagram for classical PR and PP textures has been constructed in terms of temperature and fibre radius, thus establishing the processing conditions and geometric factors that lead to the selection of these textures. The multipath formation process of the planar polar texture through defect splitting, direct planar polar formation, and defect annihilation has been thoroughly characterized. The results of this work provide new knowledge for optimization and control of mesophase carbon fibre textures.

Modeling elastic and viscous effects on the texture of ribbon-shaped carbonaceous mesophase fibers

Carbonaceous mesophases are discotic nematic liquid crystals that are spun into high performance carbon fibers using the melt spinning process. The spinning process produces a wide range of different fiber textures and cross-sectional shapes. Circular planar polar (PP), circular planar radial (PR), ribbon planar radial (RPR), and ribbon planar line (RPL) textures are ubiquitous ones. This paper presents, solves, and validates a model of mesophase fiber texture formation based on the classical Landau-de Gennes theory of liquid crystals, adapted here to carbonaceous mesophases. The effects of fiber cross-sectional shape and elongational flow on texture formation are characterized. Emphasis is on qualitative model validation using existing experimental data [1]. The role of elasticity and flow-induced orientation on texture selection mechanism on ribbon-shaped mesophase fibers is characterized. The model is able to predict the formation of the commonly observed line texture, and the fine structure of the line is reproduced and explained in terms of classical liquid crystal defect physics. The results provide additional knowledge on how to optimize and control mesophase fiber textures.

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.

Texture formation in carbonaceous mesophase fibers.

Carbonaceous mesophases are discotic nematic liquid crystals that are spun into high performance carbon fibers using the melt spinning process. The spinning process produces a wide range of different fiber textures. Planar polar (PP) and planar radial (PR) textures are two ubiquitous ones. This paper presents theory and simulation of the texture formation process using the Landau-de Gennes mesoscopic theory for discotic liquid crystals. The computed PP and PR textures phase diagram, given in terms of temperature and fiber radius, is presented to establish the processing conditions and geometric factors that lead to the selection of these textures. Thin fibers adopt the PR texture, while thicker fibers and higher temperatures adopt the PP texture. The influence of elastic anisotropy to the formation of textures and structure is thoroughly characterized.