Isotropic Carbon Research Articles

Fabrication and microstructure of boron-doped isotropic pyrolytic carbon

Boron-doped isotropic pyrolytic carbon (pyrocarbon) was prepared by CVD using CH4+BCl3+H2 as gaseous precursors. Microstructure of the deposited pyrocarbon on a graphite substrate was investigated using X-ray diffraction, X-ray photoelectron spectroscopy, polarized light microscopy, and scanning and transmission electron microscopy. It was found that the boron-containing pyrocarbon prepared exhibits a multi-scale structure, different from the pure isotropic pyrocarbon deposited under nearly the same experimental conditions. The multi-scale structure consists of carbon agglomerates of fine wrinkled graphitic sheets which contain substitutional boron and some micrometer-sized boron carbide particles uniformly distributed in the carbon agglomerates. Influence and mechanism of boron co-deposition during CVD process are also discussed.

ChemInform Abstract: Carbon‐Based Liquid Crystals: Art and Science

AbstractReview: 152 refs.

Influence of infiltration pressure on densification rate and microstructure of pyrocarbon during chemical vapor infiltration

Carbon/carbon composites were fabricated by chemical vapor infiltration of needled carbon fiber felts (70% porosity) with natural gas as precursor at different infiltration pressures. The thermodynamics of methane pyrolysis and the densification kinetics were used to investigate the influence of infiltration pressure on the infiltration rate and the microstructure of pyrocarbon. The microstructure of pyrocarbon was examined by polarized light microscopy. Results showed that the initial infiltration rate increased with pressure, whereas it decreased with pressure during the later stages of infiltration, which led to a decrease in the final density of the samples. The pyrocarbon microstructure depended markedly on the infiltration pressure at the given deposition temperature. Rough laminar (RL) carbon is formed as the matrix at the lowest pressure (approximately 1 kPa). At medium pressures (3, 5, and 10 kPa), isotropic (ISO) carbon is formed, followed by RL carbon, in the radial direction of the carbon fiber, with RL being the major matrix. Smooth laminar and ISO structures are the major phases at high pressures (15 kPa).

Optimisation of the melt-spinning of anthracene oil-based pitch for isotropic carbon fibre preparation

This paper demonstrates the suitability of a new environmentally friendly pitch, obtained from anthracene oil, for the preparation of isotropic carbon fibres. The pitch exhibits an adequate thermal behaviour and it is free of solid particles. Green carbon fibres were prepared by means of a melt-spinning process with no filtering step, and subsequent stabilisation and carbonisation. For the optimisation of the melt-spinning process, the influence of the spinning temperature, extrusion pressure, spinneret hole size, winding speed and the interrelationship of these factors upon the microstructure and diameter of the fibres was studied. High winding speeds (250 cm s −1), in combination with a large spinneret hole size (500 μm) and low extrusion pressures (1 bar), led to high quality isotropic carbon fibres with diameters as low as ~ 15 μm and a tensile strength of > 1100 MPa which fulfil the requirements for their application as standard isotropic carbon fibres.

Carbon-based liquid crystals: art and science

Synthetic carbon allotropes such as fullerene, carbon nanotubes and graphene exhibit a liquid crystalline phase behaviour upon appropriate chemical functionalisation and/or physical dispersion in suitable media. This article deals with both thermotropic and lyotropic liquid crystalline phases obtained from these isotropic and anisotropic carbon building blocks along with the carbonaceous mesophase. Different strategies adopted to obtain such ordered fluid phases of carbon are summarised and their subsequent exploitation in the fabrication of carbon-based materials and devices are highlighted. Fluid phase assembly of carbon materials seems to be very promising in the context of high-performance films, fibres and carbon electronics.

Characterisation and feasibility as carbon fibre precursors of isotropic pitches derived from anthracene oil

This work describes the preparation of isotropic carbon fibres from three anthracene oil-derived pitches of different softening points. The total absence of solid particles (e.g., primary quinoline insoluble (QI) particles) and the high aromatic composition turn these precursors into a promising raw material for the preparation carbon fibres, as they can be used without any pre-treatment. The carbon fibres were prepared by a melting process that involved the spinning of the pitches and subsequent oxidative stabilization with air and carbonisation. Scanning electron microscopy was used to monitor the surface appearance and diameter of the fibres.The results showed that softening point by itself cannot explain the spinning behaviour of the pitches. Therefore, detailed characterisation of the samples by means of size exclusion chromatography, UV–fluorescence spectroscopy, and laser desorption time-of-flight mass spectrometry was used to elucidate the composition of the pitches, especially their molecular weight distributions. The combination of the characterisation of the precursors with the characterisation of the fibres provides an overall vision of the carbon fibre preparation from the pitches and will certainly permit a pre-selection of the most appropriate precursor for this application.

Effects of Tolerances on the Structural Behavior of a Bolted Hybrid Joint

In this work, results from a study on bolted joints made of unidirectional, quasi isotropic Carbon Fiber Reinforced Polymer (CFRP) composites, subjected to tensile loads, are reported. CFRP composite materials are widely used in the mechanical industry, such as that of aerospace, where requirements of weight reduction and structural high performances are very compelling. Composite materials generally present a high resistance to fatigue and corrosion; however, the presence of joints produces the major problems and a poor design of joints leads to a drastic reduction of the reliability of structures made of these materials. A hybrid bolted joint involving a metal plate, made of aluminum alloy, and a CFRP composite plate has been considered; the plates are held together by a titanium bolt. Experimental results from literature are compared with those obtained through a numerical analysis developed with Abaqus code. Once the CFRP composite has been analyzed and the numerical model validated through numerical-experimental correlations, other possible configurations have been numerically analyzed in order to ensure the highest strength of the examined hybrid joint. Afterwards the effects of bolt-hole clearance on the stiffness and strength of the same joint have been investigated.

Analysis of the vibrational behavior of the composite cylinders reinforced with non-uniform distributed carbon nanotubes using micro-mechanical approach

Nanocomposites are a promising new class of structural materials for the aerospace structural components. This paper presents a detailed theoretical investigation of dynamic characteristics of cylinders made of carbon nanotube-reinforced composites. The cylinders are modeled as a cylindrical shell consisting of an isotropic matrix reinforced with transversely isotropic carbon nanotubes. Two different types of carbon nanotube reinforcements are considered: single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs). The effects of carbon nanotube aspect ratio, dispersion, alignment and volume fraction on the elastic modulus are analyzed using the Eshelby–Mori–Tanaka theory. Mass and stiffness matrices are obtained via Ritz method and natural frequencies of the structure are derived through solving the eigenvalue problem. Finally, the effects of the CNT distribution on mode shapes and natural frequencies are discussed.

Isotropic Pyrolytic Carbon Fabricated by Developing Self-Heating Plates Chemical Vapor Deposition

Isotropic pyrocarbon was deposited on self-heating plate at 1100°C and atmospheric pressure via a novel deposition reactor. Two graphite papers were mounted horizontally in chemical vapor deposition furnace, which were used as self- heating plates with alternating current flowing through them. And the upper surface of the lower self-heating plate also was used as substrate of pyrocarbon deposition. The microstructure of the deposit was investigated by polarized light microscopy, scanning electron microscope, transmission electron microscopy and laser Raman spectra, respectively. The results showed that there were some irregular clusters of globular structure and onion-like structure in the deposits. The thickness and density of deposit were13-15 mm and 1.88 g/cm3 in 30h. A novel deposition model of electromagnetic effect on intermediate product has been developed in order to establish a better description of the deposition process.

Platelet Adhesion on Silicon Modified Carbon Nanotubes Films

Owing to its high mechanical strength, chemical inertness, low frictional coefficient, high wear and corrosion resistance properties and so on, carbon nanotubes (CNTs) films have emerged as a potential material for biomedical applications. In order to investigate the blood compatibility properties of CNTs films synthesized on the silicon (Si) substrate using thermal chemical vapor deposition, the blood compatibility was evaluated in vitro by the platelet adhesion and the activated partial thromboplastin time (APTT). The experimental results demonstrated that the Si-CNTs films reduced thrombus formation by minimizing the platelet adhesion, activation , aggregation and had a tendency to retard the intrinsic coagulation activities of blood due to a higher APTT compared to Si, lower temperature isotropic pyrolytic carbon (LTIC) and Polydimethylsiloxane (PDMS) materials. Further, we found that hydrophobic surfaces are more prone to direct cellular motility in comparison with hydrophilic surfaces. It is concluded that CNTs films on the Si substrate were effective for improvement of blood compatibility.

Single Pd atoms in activated carbon fibers and their contribution to hydrogen storage

Palladium-modified activated carbon fibers (Pd-ACF) were synthesized by melt-spinning, carbonization and activation of an isotropic pitch carbon precursor premixed with an organometallic Pd compound. The hydrogen uptake at 25 °C and 20 bar on Pd-ACF exceeded the expected capacity based solely on Pd hydride formation and hydrogen physisorption on the microporous carbon support. Aberration-corrected scanning transmission electron microscopy (STEM) with sub-Ångstrom spatial resolution provided unambiguous identification of isolated Pd atoms occurring in the carbon matrix that coexist with larger Pd particles. First principles calculations revealed that each single Pd atom can form Kubas-type complexes by binding up to three H 2 molecules in the pressure range of adsorption measurements. Based on Pd atom concentration determined from STEM images, the contribution of various mechanisms to the excess hydrogen uptake measured experimentally was evaluated. With consideration of Kubas binding as a viable mechanism (along with hydride formation and physisorption to carbon support) the role of hydrogen spillover in this system may be smaller than previously thought.

Numerical modeling of carbon/carbon composites with nanotextured matrix and 3D pores of irregular shapes

A multiscale modeling approach is utilized to evaluate the contribution of irregularly shaped three-dimensional pores to the overall elastic properties of carbon/carbon composites. The degree of anisotropy of a carbon matrix depends on nanotexture, which is defined by manufacturing conditions. Elastic properties of the matrix are predicted assuming a Fisher distribution of orientations of graphene planes with respect to the pyrolytic carbon deposition direction. X-ray computed microtomography is employed to identify pores in a sample of carbon/carbon composite. The pores have highly irregular shapes so that micromechanical modeling based on the analytical solutions of elasticity becomes inapplicable. Thus, the cavity compliance tensor of an individual pore is found numerically by finite element method, and then used in a micromechanical modeling procedure. Examples of pores in isotropic and transversely isotropic pyrolytic carbon matrices are considered. The accuracy of pore approximation by ellipsoidal shapes is evaluated.

Naphthalene and o-Xylene Adsorption onto High Carbon Fly Ash

Adsorption is an effective remediation technique for petroleum hydrocarbons because of its ease of use and high efficiency. The utilization of high-carbon content industrial by-products in such applications can present significant economic and environmental advantages. In this study, batch adsorption tests and petrographic analyses were used to investigate the adsorption of two nonpolar petroleum contaminants, naphthalene and o-xylene, onto seven fly ashes with varying carbon contents, with powdered activated carbon (PAC) as a control. Six equilibrium isotherm models were used to evaluate the batch data. The results yielded nonlinear sorption isotherms characterized by high sorption capacity at low concentrations. The naphthalene and o-xylene adsorption capacity of the fly ashes was correlated with the unburned carbon content, specific surface area of the sorbent, and the percentage of the anisotropic and isotropic carbon content of the ash. On the basis of the Polanyi-Dubinin-Manes model, a pore-filling mechanism is the dominant mechanism for the adsorption of the nonpolar organic chemicals onto PAC, whereas the adsorption onto fly ash is likely to be governed by the unburned carbon content and the specific surface area of the ash.

The Pyrocarbon Deposition Techniques of Mechanical Heart Valve Prostheses

The structure and property of pyrocarbon varies widely with different deposition conditions. The isotropic carbon which can only been deposited in the bed of fluidized particles is very important in biomedical fields, for instance, it is often used as the coating of artificial heart valve components. The deposition of isotropic pyrocarbon containing silicon is experimented in fluidized bed over a wide range of deposition conditions. The results show that bed temperature influences strongly average coating rate, coating density, silicon content and coating micro-hardness. Propane concentration has a much effect on coating density, carbon matrix density and isotropic characteristics. Total gas flow rate and inlet dimension of fluidized bed affect the formation of fluidized bed.

THE EFFECT OF PLY ORIENTATION ON THE PERFORMANCE OF ANTENNAS IN OR ON CARBON FIBER COMPOSITES

In this paper, the anisotropic conductivity efiect of quasi- isotropic carbon flber laminates on conformal load-bearing antenna structures (CLAS) is presented. The conductivity of a quasi-isotropic IM7/977-3 CFRP laminate is measured using waveguide techniques. The results show that orientation of the surface ply relative to the polarization of the incident E-fleld has a major in∞uence on the re∞ectivity. This difierence is attributed to the fact that carbon flbres oriented parallel to the E-fleld plies behave as good conductors, while ofi-axis plies present as lossy dielectric layers with a flnite conductivity. This anisotropic behavior of the ply layers is shown to have a distinctive in∞uence on the operation of both microstrip patch and slot antennas.

Formation of Isotropic Carbon Matrix in Carbon/Carbon Composites Derived from Pitch

To manufacture a carbon/carbon composite the coal tar pitch was used as the matrix precursor and the PAN (polyacrylonitrile)-based carbon fiber was used as the reinforcing material to weave 3-directional preform. For pressure carbonization HIP equipment was used to produce a maximum temperature of 1000oC and a maximum pressure of 100 MPa. The carbonization was induced by altering the dwell temperature between 250oC and 420oC, which is an ideal temperature for the moderate growth of the mesophase nucleus that forms within the molten pitch during the pressure carbonization process. The application of high pressure during the carbonization process inhibits the mesophase growth and leads to the formation of spherical carbon particles that are approximately 30 nm in size. Most particles were spherical, but some particles were irregularly shaped. The spread of the carbon particles was larger on the surface of the carbon fiber than in the interior of the matrix pocket.

Intensity‐dependent Direct Solar Radiation‐ and UVA‐induced Radical Damage to Human Skin and DNA, Lipids and Proteins

Skin can be exposed to high-intensity UV-radiation in hot countries and during sunbed use; however, the free-radical damage at these intensities is unknown. We used electron spin resonance spectroscopy to measure free-radical generation in ex vivo human skin/substitutes +/- the spin-trap 5,5 dimethyl-1-pyrroline N-oxide (DMPO) exposed to solar-irradiation equivalent to Mediterranean sunlight. Skin-substitutes, model DNA-photosensitizer systems, lipids and proteins were also irradiated with low-intensity UVA/visible light. Without DMPO a broad singlet was detected (using both irradiations) in skin/substitutes, nail-keratin, tendon-collagen, phospholipid and DNA+melanin or riboflavin. In addition to lipid-derived (tentatively tert-alkoxyl/acyl-) and protein radicals detected with DMPO at lower intensities, isotropic carbon-, additional oxygen- and hydrogen-adducts were detected in solar-irradiated skin/substitutes at higher intensities. Carbon-adducts were detected in UVA-irradiated human skin cells, DNA+melanin or riboflavin and soybean-phospholipid. Anisotropic protein-adducts, comparable to adducts in solar-irradiated tendon-collagen, were absent in UVA-irradiated skin fibroblasts suggesting the trapping of extracellular collagen radicals. Absence of hydrogen-adducts in fibroblasts implies formation in the extracellular compartment. We conclude damage at high intensities is part cellular (carbon- and oxygen-radicals) and part extracellular (protein- and hydrogen/H(+)+e(-) ), and skin substitutes are suitable for sunscreen testing. While UVA absorption and lipid-oxidation is direct, DNA and protein-oxidation require photosensitisation.

<i>In Vivo</i> Study of Carbon Artificial Femoral Head

Hip hemiarthroplasty is a popular method for curing hip joint diseases. Comparing with the total hip replacement (THR), hip hemiarthroplasty has some advantages, such as simpler operation, lower cost and less injury. However, inevitable acetabular cartilage wear, which leads to ultimately conversion to THR, has been reported by many authors. That limits its applications. To solve this problem, more suitable biomaterial should be chosen to make the femoral head. In this research, a kind of carbon femoral head, which was made of graphite coated with low temperature isotropic pyrolytic carbon, was studied in vivo. Nineteen New Zealand adult white rabbits were divided into 3 groups. Every rabbit was taken the replacement operation and time-dependently killed after certain periods. X-ray photographs of the hip joint, macroscopic apperarance and histological morphometry of the neocartilage around the prosthesis, were examined. The results proved that the coating material of the femoral head was biocompatible and the neocartilage tissue around the prothetic head might protect the acetabulum from wear. However, because of the complicated physiological environment, further research is needed.

Wettability and bloodcompatibility of a-C:N:H films deposited by PIII-D

Abstract Wettability of amorphous carbon films can be adjusted by doping with other elements, including F, Si, Ti, O and N. In this study, nitrogen-doped hydrogenated amorphous carbon (a-C:H:N) films were deposited on silicon wafers (100) by plasma immersion ion implantation–deposition (PIII-D) using C2H2 + N2 gas mixtures. Film composition and bonding structure were analyzed by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). Surface morphology of the films was characterized using an extended multimode nanoscope atomic force microscope (AFM). Wettability examination was performed using the sessile drop method. Bloodcompatibility of the films was assessed by in vitro platelet adhesion test. The results suggest that a-C:N:H film synthesized at the highest N2/C2H2 flux ratio of 2/1 presents better anti-thrombotic property than the low temperature isotropic pyrolytic carbon (LTIC) and this improvement of bloodcompatibility can owe to the increase of hydrophilicity of the film caused by N incorporation in the carbon network and additional increase of surface roughness.

Manufacturing and high heat-flux testing of brazed actively cooled mock-ups with Ti-doped graphite and CFC as plasma-facing materials

In the frame of the EU project ExtreMat new Ti-doped isotropic graphites and carbon fibre-reinforced carbons (CFCs) with high thermal conductivity and reduced chemical erosion were brazed to a CuCrZr heat-sink to produce flat-tile actively cooled mock-ups (MUs). Brazing was done using a low CTE interlayer to shift the stresses to the metal–metal interface. These MUs were exposed to high heat-fluxes in the electron beam facility JUDITH. Screening tests were conducted increasing the heat load stepwise up to 15 MW m−2, followed by 100 cycles at 15 MW m−2, subsequent screening up to 20 MW m−2 and 100 cycles at 20 MW m−2. All MUs withstood screening at 15 MW m−2 and most of them survived screening at 20 MW m−2. Ti-doped CFC MUs showed a significant improvement compared with the undoped reference CFC, surviving several cycles at 20 MW m−2 on all tiles. One of the Ti-doped graphite MUs withstood 100 cycles at 20 MW m−2 on one tile, representing a promising result.