Formation of graphitic rods in carbon/carbon composites reinforced with carbon nanotubes

The formation of graphitic rods with a carbon nanotube (CNT) in the center was observed in CNT-reinforced phenolic resin-based carbon/carbon composites heat treated at 2000 °C. TEM characterization indicated that the carbon surrounding the CNT has a much better degree of graphitization compared to the carbon in most of the matrix. The formation temperature (2000 °C) of the graphitic rod is lower than for stress graphitization and normal graphitization of phenolic resin.     

Facile synthesis of biosourced graphitic nanoparticle anchored silica nanoparticle hybrid for the development of functional polyurethane composite

The present work demonstrates a facile approach for the generation of functional polyurethane coatings using a biosourced graphitic nanoparticle anchored silica nanoparticle hybrid. Hence, 3-aminopropyl triethoxysilane was reacted with silica nanoparticles to obtain amino groups on the surface and was further reacted with a carboxyl terminated graphitic nanoparticle obtained from the incineration of camphor. The formation of hybrid structure was established through the electron microscopy images and other spectroscopic techniques. The infrared spectroscopic measurements reveal the successful formation of carbon–silica nanohybrid through amide linkages. The synthesized hybrids were dispersed in different weight percentages into a polyether polyol and then reacted with diisocyanate to form polyurethane nanocomposite. The presence of unreacted amino groups in the carbon–silica nanohybrid is helpful in urea linkage formation, which leads to uniform dispersion in the polymer matrix. The prepared polyurethane composite possess exceptional physico-chemical properties owing to the presence of nanoparticulates. Interestingly, the resulting composite showed shape recovery behavior. The shape recovery behavior of the obtained coating under temperature of 60 °C was found to correlate with the increase in the nanomaterial content. It is also found that storage modulus of the composite at room temperature increases from 183 MPa to 432 MPa in the case of neat and 1.5% carbon-silica nanohybrid incorporated polyurethane respectively.     

The role of carbon in microstructure evolution of SiBCO ceramics

The composition of polymer derived ceramics could be readily tuned through controlling the structure and element content of the polymer precursors, and investigation on the effect of the element on microstructure evolution is important to the design of advanced ceramics. In this article, the effect of carbon content in SiBCO polymer precursors was systematically investigated. The polymer network and thermal stability of polymer precursors and the carbon content of pyrolyzed SiBCO ceramic could be readily tuned by controlling the DVB amount used. Carbon contributed to the formation of graphitic carbon in SiBC_xO ceramics and inhibited the growth of β–SiC and SiO₂ crystals at 1600 °C, but lead to an increase in the graphitic carbon phase at 1800 °C.     

High resolution transmission electron microscopy study on polyacrylonitrile/carbon nanotube based carbon fibers and the effect of structure development on the thermal and electrical conductivities

Polyacrylonitrile (PAN) and PAN/carbon nanotube (CNT) based carbon fibers at various CNT content have been processed and their structural development was investigated using high resolution transmission electron microscope (HR-TEM). In CNT containing carbon fibers, the CNTs act as templating agents for the graphitic carbon structure development in their vicinity at the carbonization temperature of 1450 °C, which is far below the graphitization temperature of PAN based carbon fiber (>2200 °C). The addition of 1 wt% CNT in the gel spun precursor fiber results in carbon fibers with a 68% higher thermal conductivity when compared to the control gel spun PAN based carbon fiber, and a 103% and 146% increase over commercially available IM7 and T300 carbon fibers, respectively. The electrical conductivity of the gel spun PAN/CNT based carbon fibers also showed improvement over the investigated commercially available carbon fibers. Increases in thermal and electrical conductivities are attributed to the formation of the highly ordered graphitic structure observed in the HR-TEM images. Direct observation of the graphitic structure, along with improved transport properties in the PAN/CNT based carbon fiber suggest new applications for these materials.     

Atomistic process of electron-stimulated structural ordering in tetrahedral amorphous carbon

To have insight to the atomistic process of graphitic ordering in tetrahedral amorphous carbon films which is induced by irradiation of high-energy electrons, the role of temperature in the graphitization was experimentally studied. The change of electron diffraction patterns before and after irradiation at room temperature indicated graphitic ordering, but irradiation at − 170 °C resulted in disordering. Systematic measurements of the temporal evolution of electron energy loss spectra with irradiation at various elevated temperatures and data analysis based on the Johnson–Mehl–Avrami model yielded the activation energy for electron-stimulated ordering of as small as ∼ 0.09 ± 0.01 eV. The most plausible model to account for all the experimental facts is the dissociative diffusion that is triggered by the electron-stimulated displacement of carbon atoms at the initial sites.     

Highly porous graphitic materials prepared by catalytic graphitization

Ultrathin graphitic nanostructures are grown inside solid activated carbon particles by catalytic graphitization method with the aid of Ni. The graphitic nanostructures consist of 3–8 graphitic layers, forming a highly conductive network on the surface of disordered carbon frameworks. Owing to the ultrathin characteristic of the produced graphitic nanostructures, the resulted porous graphitic carbons show a high specific surface area up to 1622 m²/g. A detailed investigation reveals that the features of the growing graphitic nanostructures are strongly associated with the catalytic temperature as well as the state of Ni nanoparticles. Some well-dispersed fine Ni particles with diameter below 15 nm are found to be the key to form the ultrathin graphitic nanostructures at appropriate catalytic temperature. Also, a novel mechanism is proposed for the catalytic formation of the ultrathin graphitic nanostructures. As the electrode material of electrochemical capacitors, the porous graphitic carbon exhibits much higher high-rate capacitive performance compared to its activated carbon precursor.     

Characterization of free carbon forms in β-SiC nanopowders by temperature-programmed oxidation and Raman spectroscopy

Free carbon appears in β-SiC nanopowders as two-dimensional and ultrathin structures exposed predominantly on particle surfaces. These structures range from graphitic to lamella and amorphous, with more than one type being present at once. Their relative proportions and total percentage are quantified based on CO₂ evolution traces resulting from temperature-programmed oxidation (TPO). The TPO peak parameters are representative of the carbon nanostructure. The temperature of the peak maximum relates to the degree of graphitic order, whereas an apparent activation energy to structure homogeneity. Among the Raman parameters (for a 514-nm excitation wavelength), the area ratio of the D and G + D′ lines closely correlates with the relative proportion and structural perfection of the graphitic form. Furthermore, the area ratio of the β-SiC and carbon Raman lines is a strong function of the free carbon content in the range 0.1 to 5 wt%. The established correlations provide a guide to the consistent implementation of both techniques to characterize mixed carbon forms in β-SiC nanopowders.     

Electron microscopy profiling of ion implantation damage in diamond: Dependence on fluence and annealing

The doping of diamond by ion implantation has been feasible for 25 years, but with the proviso that low dose implants can be annealed whereas high dose implants “graphitize”. An understanding of the types of defects, and their depth profiles, produced during the doping/implantation of diamond remains essential for the optimization of high-temperature, high-power electronic applications. This study focuses on investigating the nature of the radiation damage produced during the implantation of carbon ions into synthetic type Ib and natural diamonds using a spread of 4 energies, corresponding to typical doping energies, according to the CIRA (Cold-Implantation-Rapid-Annealing) routine, as well as a single energy implantation at room temperature. Both conventional and high resolution cross-sectional electron microscopies were achieved and used to analyze the implanted diamonds in conjunction with electron energy loss spectroscopy (EELS) and selected area diffraction (SAD). The cross sections were obtained using two different preparation methods.For low fluence implantations, using the CIRA routine, it is confirmed that the damaged diamond regains its crystallinity after annealing at 1600 K. However, above the amorphization threshold fluence, followed by rapid annealing at 1600 K, the whole implanted layer consisted of primarily amorphous carbon. High resolution TEM shows that the implanted layer consists of nano-regions with bent (002) graphitic planes and regions of amorphous carbon. The interface between the implanted layer and the diamond substrate near end of range shows diamond nanocrystallites, interspersed between regions of amorphous carbon and with bent (002) graphitic planes. There is no evidence for epitaxial regrowth. For high dose single energy ion implantation at room temperature, the unannealed layer shows a high degree of disorder at the maximum ion range, with some alignment of basal planes related to graphitic carbon, but with some of the diamond structure still partially intact. The implanted range included a diamond layer above the damaged region. This diamond layer showed no evidence of amorphous carbon.     

Calcium oxide and potassium hydroxide catalysed low temperature methane production from graphite and water Comparison of catalytic mechanisms

An experimental study was carried out to obtain information on the catalytic mechanisms involved in the methanation of graphite using, separately, potassium and calcium as catalysts, and water and/or hydrogen as reactants. The mechanisms for the potassium-catalysed graphite—water reaction appear to be the same in the wide temperature range from 473 to 873 K as indicated by the constant activation energy, 46 kJ mol^?1, found for methane production. The intercalation of potassium into the graphite as a possible step in the methane synthesis has been investigated and ruled out. XPS studies indicate the formation of an active form of more positively charged carbon from graphite when graphite is heated at low temperature in the presence of a calcium catalyst and water vapour. The activation energy for this carbon depolymerization reaction is 68.1 kJ mol^?1. Methane formation occurs only in the presence of hydrogen due to its reaction with the active carbon with an activation energy of 106.6 kJ mol^?1.     

Low-temperature catalytic oxidation of multi-walled carbon nanotubes

When in a pure form, carbon nanotubes are known to be stable in air up to ∼800 K making them attractive for a large variety of applications. In this work, we report a significant decrease of ignition temperature (in some cases occurring at ∼500 K) and a reduction in the apparent activation energy for oxidation in air as a result of impregnation with nanoparticles (<2 nm) of metal (Pt, Pd, Ni and Co) acetylacetonates or by decoration with corresponding oxides. Surprisingly, defects introduced by partial oxidation of the carbon nanotubes do not in practice have any influence on the enhancement of further oxidation. Reduction temperatures of metal oxides with H₂ were close to those of other carbon supported catalyst materials. However, the carbon nanotubes showed a tendency for low temperature gasification in the presence of hydrogenation catalyst metals (Pt, Pd).     

Nitrogen‐doped carbons derived from poly(ionic liquid)s with various backbones and cations

In this study, poly(ionic liquid)s (PILs) with various backbone/cation pairings (backbones – ethyl methacrylate, styrene; covalently attached cations – butylimidazolium, trimethylammonium, butylpyrrolidinium) were successfully synthesized as carbon precursors. Pyrolysis of PILs produced carbons with sheet‐like structures with a metallic luster in contrast to the powder form of carbons produced from polyacrylonitrile. The results also show that cation type has a significant impact on surface area, graphitic content, graphitic nitrogen content, nitrogen retention, carbon yield and surface chemistry of the PIL‐derived carbons. The design of nitrogen‐doped carbons based on a diverse set of PILs with various cation and polymer backbone chemistries may be an effective strategy to optimize carbon properties for subsequent application in energy storage devices. © 2019 Society of Chemical Industry     

High-Pressure Raman Study on the Decomposition of Polycrystalline Molybdenum Hexacarbonyl

High pressure Raman scattering on Mo(CO)₆ up to 37.7 GPa at ambient temperatures are recorded inside the Diamond-Anvil-Cell (DAC). The Raman spectra of the molybdenum hexacarbonyl completely vanish at 23.3 GPa, signifying novel pressure induced decomposition. The decomposed Raman spectrum recovered at ambient conditions has the characteristic features of a highly dense polymer with δ(OCO) units possibly attached to metal centers, C=O functional group, and graphitic carbon; and also CO adsorbed to molybdenum metal centres. The so formed highly dense metal mediated polymer upon relaxation from high pressure conditions and upon exposure to 514.5 nm argon ion laser outside the DAC transforms to MoO₃ in the presence of disordered carbon as characterized by the Raman modes.     

Biodiesel Production by Amorphous Carbon Bearing SO3H, COOH and Phenolic OH Groups, a Solid Brønsted Acid Catalyst

A solid Brønsted acid of amorphous carbon bearing SO₃H, COOH and phenolic OH groups has been studied as a catalyst for biodiesel production. The carbon material functions as a stable and efficient catalyst for the formation of biodiesel from oleic acid at 353 K; the catalytic performance is 70–80% that of sulfuric acid. The carbon material also exhibits remarkable catalytic performance for transesterification of triolein at 403 K, maintaining high catalytic activity even in the presence of water. These results suggest that this catalyst can directly convert crude vegetable oils composed of triglycerides, free higher fatty acids and water into biodiesel with minimal energy consumption.     

High-yield production of N-doped graphitic platelets by aqueous exfoliation of pyrolyzed chitosan

Full exploitation of the properties of N-doped graphene and few-layers graphitic platelets will require a method for the mass production of this highly conductive material. Herein the preparation of few-layers graphitic platelet dispersions at concentrations up to 0.16 mg ml⁻¹ is reported. These suspensions are produced by dispersion and exfoliation of N-doped graphitic carbons obtained from the chitosan biopolymer. This high concentration derives from the morphology of pyrolyzed chitosan constituted by a graphitic carbonaceous residue with loose stacking of the graphene sheets that is prone to undergo easy exfoliation. The presence of few layers graphitic sheets was confirmed by Raman spectroscopy, transmission electron microscopy, electron diffraction and atomic force microscopy. Our method results in the formation of N-doped graphitic platelet suspensions with a yield of 40 wt.% of the initial graphitic carbon, which could potentially be improved to 90 wt.% by four consecutive sonication–centrifugation cycles of the same pyrolyzed chitosan residue. The absence of defects or oxides is confirmed by X-ray photoelectron, infrared and Raman spectroscopies. Solution processing of graphene and few-layers graphitic flakes opens up a range of potential large-area applications, from device and sensor fabrication to liquid-phase chemistry.     

Morphology,structure and density evolution of carbon nano-structures deposited by N-IR pulsed laser ablation of graphite

Carbon films were deposited by pulsed laser ablation on Si <100> substrates, heated at temperatures increasing from RT to 800 °C, from a pure graphite target, operating in vacuum (~ 10^− 4 Pa). The laser ablation was performed by an Nd:YAG laser, operating in the near IR wavelength (λ = 1064 nm).Micro-Raman and grazing incidence X-ray diffraction analysis (GI-XRD) established the progressive formation of ordered nano-sized graphitic structures, increasing substrate temperature. The surface morphology is characterised by macroscopic roughness (SEM, AFM) while the low temperature samples are characterised by very smooth surface. The film density, evaluated by X-ray reflectivity measurements, is also affected by the substrate temperature. This structural property modification induces relevant variation on the emission properties of carbon films, as evidenced by Field Emission measurements. The film structure and texturing is also strongly related to laser wavelength: the low energy associated to the IR laser radiation (1.17 eV) causes an early aromatic cluster formation at T = 400 °C associated to a sensible increase in the aromatic plane stacking distance (d₀₀₂ ~ 0.39 nm), compared to graphite. These density decrease shows a direct correlation with the electron emission properties. Roughness and presence of voids play a negative role both on the threshold electric field E_th and enhancement factor (β) The density decreasing and graphitic layer widening are notably to be ascribed to the very fast out-of-equilibrium growth and to the presence of large activated carbon species in the “plume”.     

Evolutions of rod diameter,molten zone and temperature gradient of oxide eutectic ceramics during laser floating zone melting

Laser floating zone melting (LFZM) is an important directional solidification (DS) technique in oxide ceramic fabrication owing to its very high temperature gradient and rapid cooling rate. In this work, variations and evolutions of rod diameter, molten zone, and temperature gradient are explored in fabrication of Al₂O₃-based eutectic ceramics. At low solidification rate of 4 μm/s, periodical oscillations with almost the same period (~275 s) are produced in melting zone shape/size, as-grown rod diameter, temperature distribution, and temperature gradient. The temperature gradient fluctuates between 3.6 × 10³ K/cm and 6.9 × 10³ K/cm with an average value of about 5.3 × 10³ K/cm. As increasing the solidification rate, the temperature gradient decreases and the periodical oscillation weakens. Based on the evolutions of the temperature field, interface location and molten zone shape, the formation mechanism of periodical oscillation during LFZM processing is evaluated.     

Nanoscopic observations of stress-induced formation of graphitic nanocrystallites at amorphous carbon surfaces

The formation of graphitic nanocrystallites at the surface of amorphous carbon under large mechanical stresses was examined by using micro-Raman spectrometry, transmission electron microscopy and in-situ compressions. In the Raman analyses of severely deformed (above a strain energy density criterion of 5.9 J/m²) surface regions of nanoscratched and nanoindented amorphous carbon films, two additional sharp and narrow peaks, D_Gr and G_Gr at 1330 and 1580 cm⁻¹, appeared from the main unchanged broad spectra, revealing the transformation of some small-range amorphous carbon to nanocrystalline graphite. Transmission electron microscopic images presented the formation of surface shear layer within which dispersed graphitic nanocrystallites (a size of about 3 nm) were formed in the remaining amorphous matrix. The in-situ nanoscopic observation of amorphous carbon nanopillars under compressions confirmed the formation of graphitic nanocrystallites at pillar edge surfaces. The formed graphite (0 0 1) and (1 0 0) lattices were well oriented along maximum resolved shear stresses, being an evidence of lattice reconstruction and suggesting a possibility of stress-induced graphitization of amorphous carbon in the absence of heat.     

Synthesis of nitrogen-doped porous graphitic carbons using nano-CaCO3 as template,graphitization catalyst,and activating agent

Nitrogen-doped porous graphitic carbons (NPGCs) with controlled structures were synthesized using cheap nano-CaCO₃ as template, melamine-formaldehyde resin as carbon precursor, and dilute HCl as template removing agent. In addition to its use as a template, the nano-CaCO₃ acted as an internal activating agent to produce micro- and mesopores, as an adsorbent to remove the released hazardous gases (i.e. HCN, NH₃), and as a mild graphitization catalyst. The obtained NPGCs with hierarchical nanopores contained as high as 20.9 wt% of nitrogen, had surface areas of up to 834 m² g^–1, and also exhibited high thermal stability with respect to oxidation. Using carbohydrate or phenolic resin as the carbon precursor, this simple approach was also capable of producing hierarchical porous graphitic carbons with high surface area (up to 1683 m² g^–1) and extremely large pore volumes (>6 cm³ g^–1). X-ray diffraction and infrared spectroscopy suggested that the intermediate CaCN₂ or CaC₂ generated during the carbonization plays a critical role in the formation of the graphitic structure.     

Facile synthesis of graphitic carbons decorated with SnO2 nanoparticles and their application as high capacity lithium-ion battery anodes

A facile and potentially scalable synthesis route to obtain SnO₂–carbon composites was developed. SnO₂ nanoparticles were deposited on the surface of two types of graphitic carbon: (a) commercial porous graphite (HG) and (b) graphitic carbon nanostructures. The synthesis procedure consists of two simple steps: (i) room temperature formation/deposition of SnO₂ nanocrystals and (ii) thermal treatment at 350 °C to generate SnO₂ nanoparticles (size ~3.5 nm) over the carbon surface. The electrochemical performance of the graphitic carbons and the SnO₂–carbon composites as anode materials in Li-ion rechargeable batteries was investigated. In all cases, tape casting electrode fabrication allowed almost full active material utilization. Good cyclabilities were achieved, with HG and HG–SnO₂ showing capacities of 356 and 545 mAh g⁻¹, respectively after 50 cycles.