Pyrolytic Carbon Deposition Research Articles

An innovative wood derived carbon-carbon nanotubes-pyrolytic carbon composites with excellent electrical conductivity and thermal stability

The functionality of wood has evolved with time to adapt to the emerging needs of society. Carbonized wood-based composites have attracted tremendous interest in the fields of aerospace, military power, electric power, and system electronic devices, especially at high temperatures. Nevertheless, their electrical conductivity and thermal stability characteristics are still far from satisfactory. Herein, an innovative wood-derived carbon-carbon nanotubes-pyrolytic carbon composites (WDC-CNTs-PyCs) is successfully fabricated by chemical vapor deposition and chemical vapor infiltration. The combination of wood-derived carbon (WDC), carbon nanotubes (CNTs), and pyrolytic carbon (PyC) has never been reported in any previous work. We have innovatively introduced PyC into the WDC by chemical vapor infiltration. CNTs promote the continuous deposition of PyC to form dense structures. WDC-CNTs-PyC demonstrates significant compressive strength (85.4 MPa) and excellent electrical conductivity (632 S cm–1). The weight loss rate of WDC-CNTs-PyC is 6% after heating at 500 °C for 10 min in the air atmosphere. Furthermore, WDC-CNTs-PyC could resist oxyacetylene ablation above 2300 °C for 15 s. With excellent electrical conductivity, outstanding thermal stability, and mechanical properties, WDC-CNTs-PyC opens up a surprising strategy for efficiently fabricating various high-performance electronic device composites that could be used in high-temperature fields.

Fabrication of electron tunneling probes for measuring single-protein conductance.

Studying the electrical properties of individual proteins is a prominent research area in the field of bioelectronics. Electron tunnelling or quantum mechanical tunnelling (QMT) probes can act as powerful tools for investigating the electrical properties of proteins. However, current fabrication methods for these probes often have limited reproducibility, unreliable contact or inadequate binding of proteins onto the electrodes, so better solutions are required. Here, we detail a generalizable and straightforward set of instructions for fabricating simple, nanopipette-based, tunnelling probes, suitable for measuring conductance in single proteins. Our QMT probe is based on a high-aspect-ratio dual-channel nanopipette that integrates a pair of gold tunneling electrodes with a gap of less than 5 nm, fabricated via the pyrolytic deposition of carbon followed by the electrochemical deposition of gold. The gold tunneling electrodes can be functionalized using an extensive library of available surface modifications to achieve single-protein-electrode contact. We use a biotinylated thiol modification, in which a biotin-streptavidin-biotin bridge is used to form the single-protein junction. The resulting protein-coupled QMT probes enable the stable electrical measurement of the same single protein in solution for up to several hours. We also describe the analysis method used to interpret time-dependent single-protein conductance measurements, which can provide essential information for understanding electron transport and exploring protein dynamics. The total time required to complete the protocol is ~33 h and it can be carried out by users trained in less than 24 h.

Morphological and Optical Transformation of Gas Assisted Direct Laser Written Porous Silicon Films.

Direct laser writing (DLW) of mesoporous porous silicon (PS) films is shown to selectively create spatially separated nitridized and carbonized features on a single film. Nitridized or carbonized features are formed during DLW at 405nm in an ambient of nitrogen and propane gas, respectively. The range of laser fluence required to create varying feature sizes while avoiding damage to the PS film is identified. At high enough fluence, nitridation using DLW has been shown as an effective method for laterally isolating regions on the PS films. The efficacy in preventing oxidation once passivated is investigated via energy dispersive X-ray spectroscopy. Changes in composition and optical properties of the DL written films are investigated using spectroscopic analysis. Results show carbonized DLW regions have a much higher absorption than as-fabricated PS, attributed to pyrolytic carbon or transpolyacetylene deposits in the pores. Nitridized regions exhibit optical loss similar to previously published thermally nitridized PS films. This work presents methods to engineer PS films for a variety of potential device applications, including the application of carbonized PS to selectively engineer thermal conductivity and electrical resistivity and of nitridized PS to micromachining and selective modification of refractive index for optical applications.

Bithiophene as a Sulfur-Based Promotor for the Synthesis of Carbon Nanotubes and Carbon-Carbon Composites.

We assess bithiophene (C8H6S2) as a novel sulfur-based promotor for the growth of single-walled carbon nanotubes (SWCNTs) in the aerosol (floating catalyst) CVD method. Technologically suitable equilibrium vapor pressure and an excess of hydrocarbon residuals formed under its decomposition make bithiophene an attractive promoter for the production of carbon nanotubes in general and specifically for ferrocene-based SWCNT growth. Indeed, we detect a moderate enhancement in the carbon nanotube yield and a decrease in the equivalent sheet resistance of the films at a low bithiophene content, indicating the improvement of the product properties. Moreover, the relatively high concentrations and low temperature stability of bithiophene result in non-catalytical decomposition, leading to the formation of pyrolytic carbon deposits; the deposits appear as few-layer graphene structures. Thus, bithiophene pyrolysis opens a route for the cheap production of hierarchical composite thin films comprising carbon nanotubes and few-layer graphene, which might be of practical use for hierarchical adsorbents, protective membranes, or electrocatalysis.

Enhancing inward and outward mass transport during chemical vapor deposition of pyrolytic carbon for better synthesis of ZTC

Zeolite-templated carbon (ZTC) can be prepared by chemical vapor deposition (CVD), which involves the coupled mass transport and chemical reaction of carbon precursors. Combining theoretical and experimental approaches, this work demonstrated that reducing the particle size of the template enhanced both the inward and outward mass transport during zeolite-templated CVD, which, in turn, improved the structural properties of the resulting ZTC. Specifically, the inward diffusion of the carbon precursor was modelled quantitatively by fitting the deposition mass of pyrolytic carbon measured in zeolite templates of different particle sizes (d = 4, 0.8 and 0.002 mm). This model revealed that the inward diffusion of the carbon precursor was enhanced in smaller zeolite particles, allowing more uniform deposition of pyrolytic carbon inside the zeolite and better replication of the zeolite's pore structure in the resulting ZTC. For the outward mass transport, semi-quantitative analysis was conducted on the intermediate components that were responsible for the external deposition of pyrolytic carbon. In particular, the concentration of the intermediate components was predicted to increase with the particle size of the zeolite. This prediction was confirmed by the experimental characterization results, where ZTC samples prepared with larger zeolite particles showed thicker external layers of quasi-crystalline graphitic carbon.

Radial growth kinetics of epitaxial pyrolytic carbon layers deposited on carbon nanotube surfaces by non-catalytic pyrolysis

Chemical vapor deposition of pyrolytic carbon (PyC) onto carbon nanotube (CNT) surfaces by hydrocarbon pyrolysis can thicken the nanotube diameter until the desirable size. Vapor-induced thickening of CNTs may require no metal catalysts because the CNT-templated growth can easily convert PyC deposits into epitaxial graphene shells. However, little work has been done on the kinetics of the vapor-induced CNT thickening process during the non-catalytic pyrolysis. In this study, we discuss the growth mechanism of epitaxial PyC layers on CNT surfaces achieved by the low-pressure (≤1.5 kPa) pyrolysis of ethylene, which is distinct from previous studies using much higher gas pressures (4–100 kPa) for the PyC deposition. The average thickness of deposited PyC layers is calculated by analyzing the transmission electron microscopy images of PyC-coated CNTs. The crystallinity of PyC-coated CNTs is evaluated by Raman spectroscopy and X-ray diffraction. The thickness of PyC layers on CNT surfaces can be adjusted by simply changing the growth time under typical pyrolysis parameters (800 °C-1.5 kPa of ethylene). We find a two-stage nonlinear behavior to describe the growth kinetics of PyC layers, indicating that the radial growth rate of PyC layers can be influenced by the number of surface active sites. This study reveals the self-assembly mechanism of graphitic layers on the CNT template, which is expected to promote the diameter-controlled synthesis of CNTs.

Porous nanographene formation on γ-alumina nanoparticles via transition-metal-free methane activation.

γ-Al2O3 nanoparticles promote pyrolytic carbon deposition of CH4 at temperatures higher than 800 °C to give single-walled nanoporous graphene (NPG) materials without the need for transition metals as reaction centers. To accelerate the development of efficient reactions for NPG synthesis, we have investigated early-stage CH4 activation for NPG formation on γ-Al2O3 nanoparticles via reaction kinetics and surface analysis. The formation of NPG was promoted at oxygen vacancies on (100) surfaces of γ-Al2O3 nanoparticles following surface activation by CH4. The kinetic analysis was well corroborated by a computational study using density functional theory. Surface defects generated as a result of surface activation by CH4 make it kinetically feasible to obtain single-layered NPG, demonstrating the importance of precise control of oxygen vacancies for carbon growth.

Effect of preparation method on the mechanism for oxidation of C/C-BN composites

C/C-BN composites and C f /BN/PyC composites exhibiting different structures for pyrolytic carbon (PyC) and boron nitride (BN) were studied comparatively to determine their oxidation behavior. This study used five types of samples. Porous C/C composites were modified with silane coupling agents (APS) and then fully impregnated in water-based slurry of hexagonal boron nitride (h-BN); the resulting C/C-BN preforms were densified by depositing PyC by chemical vapor infiltration (CVI), resulting in three types of C/C-BN composites. The other two C f /BN/PyC composites were obtained by depositing a BN interphase and PyC in carbon fiber preforms by CVI; one was treated with heat, and the other was not. This study was focused on determining how the PyC deposition mechanism, morphology and pore structure were affected by the method of BN introduction. In the 600–900 °C temperature range, the C f /BN/PyC composites and C/C composites underwent oxidation via a mixed diffusion/reaction mode. The C/C-BN composites had a different pore structure due to the formation of nodules comprising h-BN particles; both interfacial debonding and cracking were reduced, resulting in higher resistance to gas diffusion, lower oxidation rate and larger activation energy (Ea) in the temperature range 600–800 °C. In addition, the mechanism for oxidation of C/C-BN composites gradually exhibited diffusion control at 800–900 °C because the formation of h-BN oxidation products healed the defects. The oxidation mechanism was more dependent on pore structure than on BN structure or content.

Unveiling the existence and role of a liquid phase in a high temperature (1400 °C) pyrolytic carbon deposition process

Based on experiments aiming at depositing pyrolytic carbon onto carbon nanotubes by means of a chemical vapor deposition (CVD) process at 1400 °C, the work we report here demonstrates that the deposition involves the transient formation of a liquid phase (here organic), and that wetting physics is still able to apply in spite of the high temperature and the nanoscale, i.e., far beyond the condition range usually investigated. This was unexpected, because the high temperature makes all the physical and chemical processes involved transitory, including the fact that the liquid turns itself into solid carbon because of the ongoing carbonization process. The observations provide an estimate of the time scales of the involved processes, as they have to be short enough to complete in spite of the high temperature conditions. From a practical point of view, as the resulting material is a solid, all-graphenic carbon, the work demonstrates that using wetting physics in high temperature CVD can be a way to dramatically modify the surface energetics and the nano/microscale morphology of carbon nanofilaments. The statement is assumed to also apply to other chemical systems.

Catalytic activity of K10 montmorillonite in the chemical vapor deposition of multi-wall carbon nanotubes from methane

The catalytic activity of K10 montmorillonite (K10Mt) as a substrate during the chemical vapor deposition (CVD) of carbon nanotubes (CNT) was investigated. The influence of free iron present in the structure of K10Mt during CNT synthesis was investigated. The synthesis was carried out both on samples containing free iron and after its chemical extraction. The structural and microstructural parameters of the samples were characterized by scanning electron microscopy with X-ray dispersion spectroscopy (SEM/EDS), energy dispersive transmission electron microscopy (TEM/EDS), X-ray fluorescence spectroscopy, X-ray diffraction (XRD), infrared spectroscopy (FTIR), and Raman spectroscopy, and X-ray photoelectron spectroscopy. It was found that K10Mt, before Fe removal, was covered with coarser forms of carbon with rounded shapes characteristic for pyrolytic carbon deposits, while after chemical purification the K10Mt particles were covered with regular forms of CNT. During the CVD process at 850 °C, multi-walled carbon nanotubes about 1 μm in length were formed on the surface of pure K10Mt. Under the conditions of synthesis, the K10-Mt/CNT nanocomposites were formed. After extraction of free iron from K10Mt, the most likely nucleation sites for CNT growth were Mg- and Fe-containing compounds structurally integrated with the K10Mt substrate, such as iron phosphides, magnesia, and magnesium aluminate.

Synthesis and microwave absorption properties of multilayer SiC/C foam with alternating distribution of C and SiC

The multilayer SiC/C foam with alternating distribution of C and SiC (MSCF) was designed, which was planned to be a outstanding microwave absorber. The melamine-derived light carbon foam (CF) was determined to be the initial porous template. The multilayer SiC/C structure was synthesized by depositing chemical vapor deposition (CVD) of SiC, pyrolytic carbon (PyC) and SiC coatings sequentially on CF skeletons. SEM and XRD results showed that the multilayer SiC/C structure with alternating distribution of C and SiC was successfully prepared. A large number of heterogeneous interfaces formed between the carbon and SiC layers, which was the key to improving the microwave absorbing performance. As expected, the resulting MSCF showed satisfactory impedance matching ratio (Zr) and an optimal reflection loss value of −49.15 dB at 15.6 GHz with the thickness of 1.4 mm. Moreover, the corresponding effective absorption band covered the frequency range of 13.98–17.78 GHz. Furthermore, the microwave absorbing mechanism based on the multilayer SiC/C structure was proposed. Under the guarantee of the high value of Zr, the multiple reflections and scattering, dipole polarization, and interface polarization played a synergistic effect in the multilayer SiC/C structure.

Influence of Pyrocarbon Deposition in the Porous Structure of Activated Carbons

Deposition of pyrolytic carbon from the vapor phase onto the surface of various activated carbons, i.e., industrial (SKT and AG-3) and specially prepared via addition of KOH to a composition of carbon black with petroleum pitch followed by carbonization and washing, was studied. The deposition was carried out by passing benzene vapor in a stream of N2 through heated activated carbon. The obtained modified samples were studied using molecular probes by measuring adsorption isotherms of benzene and CCl4 at 20°C on the same samples. The results showed that the weight gain increased from 3% to 15% depending on the sample and the volume of micropores of width 0.4-0.6 nm increased to 0.07-0.15 cm3/g as the deposition time increased from 60 to 450 min. The modified activated carbons obtained in this manner could be used further to obtain carbon molecular sieves with pore sizes from 0.3 to 0.4 nm.

Physical and Mechanical Properties of Pyro-Compacted Carbon-Carbon Material on a Fabric Warp

The work showed that a change in the length of the samples within 60–100 mm and a width within 12–25 mm, as well as the shape of the samples, did not have a significant effect on the change in the mechanical characteristics when tested in compression in both directions of the UPA-4 material obtained by deposition of pyrolytic carbon from the gas phase on the warp made of graphite viscose fabric URAL-T-22. The carbon matrix of this material was formed as a result of the gradual deposition of pyrolytic carbon from the gas phase at a temperature of 980°C for ~150 h and at 1070°C for ~100 h. The paper presents the results of tests at different temperatures. Tests of flat specimens for bending, compression, and stretching were performed at room temperature. To assess the heat resistance of the material, samples were tested for bending in a neutral argon medium at a speed of active capture of 5 mm/min in a temperature range of 20–2800°C. The tests have shown that the developed material is structural and heat-resistant, since in the temperature range up to 2200°C it retains its bending strength, and as the temperature rises to 2500°C, it deforms with plasticity without breaking.

Synthesis and (some) applications of carbon-nanotube-supported pyrolytic carbon nanocones

Graphene-based cones with nanosized apex can be obtained by means of a high temperature pyrolytic carbon depositionprocess using methane and hydrogen as gaseous feedstock and single carbon nanotubes as deposition substrates. Aside thecones, micrometer-sized carbon beads or fibre segments are deposited meanwhile which are a key morphological componentfor allowing handling and mounting the carbon cones and then using them for various applications. Based on both theliterature dealing with pyrolytic carbon deposition processes and experimental observations, a peculiar depositionmechanism is proposed, involving the transient formation of pitch-like liquid phase droplets which deposit onto theindividual carbon nanotubes. In this picture, it is believed that a key parameter is the diameter ratio for the droplets and thenanotubes, respectively. The cone concentric texture and perfect nanotexture are shown by high resolution transmissionelectron microscopy, which allows interesting mechanical and conducting properties to be predicted. Correspondingly,applications of the carbon nanocones as electron emitters for cold-field electron sources on the one hand, and as probes forvarious modes of near-field microscopy on the other hand, have been tested.

Pyrolytic Carbon Coating Effects on Oxide and Carbide Kernels Intended for Nuclear Fuel Applications

The coating of nuclear fuel kernels with pyrolytic carbon (PyC) is a well-understood practice dating back over half a century. In spite of decades of studies related to these coatings, no study has yet investigated the effect of the PyC deposition coating process on the kernels themselves. In this study, the composition and crystallographic phase of kernel materials were observed to change after exposure to the thermal and chemical environment of the PyC coating process. Specifically, the coating process increased the fraction of high carbon content phase within carbide microsphere kernels, with W2C containing microspheres driven toward WC, and UC containing microspheres driven toward UC2. Oxide microspheres consisted of a mixture of two crystalline phases. The monoclinic phase within yttria-stabilized zirconia microspheres was eliminated by the coating process resulting in a purely tetragonal phase. Hafnium oxide microspheres were more stable showing no detectable change in composition or crystal structure after coating.

Methane pyrolysis in preparation of pyrolytic carbon: Thermodynamic and kinetic analysis by density functional theory

The density functional theory has been successfully applied in analyzing pyrolytic carbon deposition by methane pyrolysis from the view of thermodynamics and kinetics based on a total number of 39 elementary reactions. M06-2X/def2-TZVP level was employed to optimize species structures and locate the transition states. The enthalpy changes and Gibbs free energy changes of all the reactions in the temperature range of 298.15–1800 K were derived with optimized species. Results show that the reacting temperature should be above 1200 K based on the equilibrium constant analysis, which is consistent with the typical reaction temperature adopted in experiments. Potential energy surface profiles report that radical attacking reactions have lower energy barriers than those direct decomposition reactions, especially hydrogen radical attacking reactions. The energy barriers of the first steps, dehydrogenations of methane and ethylene, are 272.4 kJ·mol−1 and 288.9 kJ·mol−1 at 1200 K, which are very close to the experimental activation energy for methane pyrolysis. The most favorable decomposition reaction path is the path of hydrogen radical attacking reactions. The highest energy barrier of the path at 1200 K is 185.7 kJ·mol−1 presented by the C–H bond breaking in ethynyl attacked by hydrogen radical.

Oxidation behavior of carbon/carbon-boron nitride composites fabricated by additives and chemical vapor infiltration

Carbon/carbon-boron nitride (C/C-BN) composites were manufactured by adding hexagonal boron nitride (h-BN) powders into carbon fiber preform and a subsequent chemical vapor infiltration (CVI) process for deposition of pyrolytic carbon (PyC). Microstructure and oxidation behavior of carbon/carbon composites with 9 vol% h-BN (C/C-BN9) were studied in comparison to carbon/carbon (C/C) composites. Results showed that with the addition of h-BN powders, a regenerative laminar (ReL) PyC with higher texture was achieved. Note that the introduction of h-BN powder make great contributes to graphitization degree of PyC, leading to larger oxidation activation energy. Moreover, under an air atmosphere, h-BN started to oxidize above 800 °C, and generated molten boron oxide (B2O3) which prohibited oxygen diffusion by filling in pores, cracks and other defects. As these reasons mentioned above, after oxidation tests under an air atmosphere, mass losses of C/C-BN9 composites were lower than that of C/C composites at all test temperatures (600–900 °C), indicating that the oxidation resistance of C/C-BN9 composites is better than that of C/C composites.

Microwave synthesis of carbon onions in fractal aggregates using heavy oil as a precursor

In this work we report a method for preparing carbon onions through microwave heating of heavy oil. It was shown that microwave heating of heavy oil mixed with a carbon catalyst at 300 W leads to the growth of a several-centimetre long fractal carbon structure in just 60 s. Scanning electron microscopy (SEM) images showed that the structure is predominantly made of networks of small particles with diameters ranging from tens to hundreds of nanometres. High-resolution transmission electron microscopy (HRTEM) images revealed the presence of disordered graphitic structures, including carbon onions with diameters as small as 30 nm. This was supported by the Raman spectroscopy which showed typical spectra for carbon onions with disordered graphitic structure. CO2 gas sorption results revealed a specific surface area of up to 164 m2/g. The carbon onions are believed to form through pyrolysis of the oil into light hydrocarbons followed by nucleation and growth of concentric graphitic layers. The carbon tree, subsequently, grew through aggregation of the carbon onions and further deposition of pyrolytic carbon, with the electric field configuration favoured the longitudinal growth of thin branches. This study demonstrates that carbon onions can be prepared from cheap unrefined liquid precursors.

Preparation of Ultramicroporous Materials by Fluidized-Bed Pyrolytic Deposition of Carbon in Pores of a Carbon Matrix

Samples of microporous materials were prepared by pyrolytic deposition of carbon from divinyl in a fluidized bed reactor onto carbon supports obtained from coconut and pine nut shells. The influence exerted on the adsorption capacity for CO2 by the divinyl pyrolysis temperature (in the range 600–750°С) and degree of densification (in the range 6–22%) was studied. Carbon deposition conditions ensuring the specific surface area (SBET) lower than 5 m2 g–1 at the adsorption capacity for СО2 higher than 1 mmol g–1 for both samples were found.

High textured carbon from chemical vapor infiltration with ethanol precursor and its rate of pyrolytic carbon deposition

Preparation of C/C composites with ethanol precursor was studied and the role of ethanol in obtaining high textured (HT) carbon was confirmed. HT carbon was obtained more with ethanol precursor than with propane precursor as reported by Ren et al. The pyrolytic carbon deposition rate constant from ethanol precursor, which has never been reported before in any other research, was obtained. It was confirmed that a proper mixture precursor of ethanol and propane could be used in the process of temperature gradient chemical vapor infiltration (TG-CVI) on behalf of the uniform deposition throughout the preform and the deposition with more HT carbon,. The pyrolytic carbon deposition rate constant for the CVI with propane precursor obtained in this research was 2.2-times greater than that reported by Vaidyaraman.