Disordered Carbon Material Research Articles

Li/Na Storage Properties of Disordered Carbons Synthesized by Mechanical Milling

Impact of structural disorder induced by mechanical milling on electrochemical properties of carbonaceous materials is systematically examined. Carbonaceous materials with different structural disorder are prepared by mechanical milling with different rotation speeds. Thus prepared carbons show sloping voltage profiles in Li/Na cells, which resemble those of soft carbons. The disordered carbon material prepared at 600 rpm shows a reversible capacity of >500 mA h g−1 in a Li cell. In addition, the disordered carbon also delivers 200 mA h g−1 in a Na cell with high Coulombic efficiency for continuous cycles. Heat treatment of the disordered carbon results in the formation of hard carbon with nanopores, and thus Li/Na storage properties are significantly changed.

The composition, structure and properties of four different glassy carbons

Glassy carbon (GC) is a class of disordered carbon materials that is known to be superelastic and non-graphitizing up to 3000 °C. The maximum heat treatment temperature is often used as a proxy to denote structure and physical properties. GC synthesised at low temperatures (~1000 °C) is often classified as Type I GC which has advantages of higher elastic modulus, resistance to oxidation, and lower permeability to gases. Type II GC is synthesised at higher temperatures (>2000 °C), has fewer impurities, is more electrically conductive, and is rated to a higher service temperature. Here Type I and II GC samples sourced from two suppliers are investigated using Rutherford backscattering spectrometry and elastic recoil detection analysis for composition, Raman spectroscopy, transmission electron microscope imaging, X-ray and neutron diffraction for structure determination, nanoindentation for mechanical properties, and Van der Pauw measurements for resistivity. The results show that the broad classifications of Type I or Type II do not correlate with the physical properties of the samples. We conclude that the quoted maximum heat treatment temperature alone is not sufficient to specify the properties of GC and that a careful microstructural examination of the material should be used to inform materials selection.

Modeling and predicting total hydrogen adsorption in nanoporous carbon materials for advanced nuclear systems

The retention of hydrogen in a variety of disordered carbon materials including graphene nanoplatelets and amorphous carbon was measured by volumetric adsorption and modeled using thermodynamic methods. To date, high temperature hydrogen adsorption research in carbon has predominately focused on graphite used as plasma-facing materials in fusion systems, moderators in high temperature gas reactors, and fuel components in molten-salt cooled reactors. Tritium, a hydrogen isotope that is produced in these systems, adsorbs on carbon surfaces that could be used effectively as sinks to remove tritium. Since hydrogen has chemical similarity to tritium, it can be used as a surrogate to simplify experimental studies, greatly expediting materials exploration. Thus, an understanding and prediction of hydrogen behavior on carbon materials is essential for evaluation of performance and safety of present and future reactor designs. Chemisorption experiments were conducted on 9 different materials with a range of surface areas, pore volumes, and pore size distributions. The results showed that carbons with higher surface area and pore volume exhibit higher hydrogen adsorption, which is attributed to larger quantities of active sites. Further, hydrogen adsorption was found to increase in materials with greater micropore (<10 Å) surface area, which was interpreted as being a result of in-pore trapping. In contrast to previous studies that assumed a single isotherm, four isotherms were examined including two- and three-parameter methods and homo- and heterogeneous surface methods—the Langmuir, Temkin, Freundlich and Sips isotherms. Analyses of non-linear models by ordinary least squares and Marquardt's percent standard deviation minimization were performed. It was found that constant order homogeneous models that have been used in the past lack the ability to describe pathways and reactions steps that occur. Further, it was found that Sips isotherm consistently provided the best minimization of error and most accurate prediction.

Structural and morphological peculiarities of needle-like diamond crystallites obtained by chemical vapor deposition

Polycrystalline (100) textured diamond films containing mixture of micrometer scale crystallites and surrounding them smaller size diamonds and disordered carbon were obtained by chemical vapor deposition from methane‑hydrogen gas mixture activated by a direct current discharge (DC CVD). The films were oxidized during exposure in air at temperature providing selective volatilization of the smallest fraction of diamond crystallites and disordered carbon material. Structural peculiarities of the material remaining after film oxidation were analyzed by electron microscopy and compared with results of computer simulations. The simulations were performed using two dimensional empirical model designed to describe formation of {111} and {100} facets of the diamond crystallites during CVD. The porous and disordered structures at {111} growing facets predicted by the simulation are in excellent agreement with the experimental observations.

Single‐Walled Carbon Nanotubes Shock‐Compressed to 0.5 Mbar (Phys. Status Solidi B 11/2017)

Single-walled carbon nanotubes (SWCNTs) have been dynamically (shock) compressed to 0.5 Mbar, above the limit of their structural integrity. Two distinct types of material are identified by high-resolution transmission electron microscopy (HRTEM) and multi-wavelength Raman spectroscopy in the sample recovered after shock: multi-layer graphene (MLG) and a two-phase material composed of nano-clustered graphene and amorphous carbon whereas no diamond-like carbon or carbon nano-onions are found. Peak decomposition of the Raman spectra was used to estimate the coherent scatterers (clusters) size in MLG at 36 nm from the D- to G-band intensity ratio dependence on the photon excitation energy. Botella et al. (article no. 1700315) propose the peak fitting model for decomposition of the Raman spectra of highly disordered carbon material containing graphene nano-clusters and stress the importance of accounting for heptagonal- and pentagonal-ring defects in graphene layers for the analysis of such spectra. The cover image shows HRTEM images and the correspondent Raman spectra of the two types of material along with peak decomposition of the two-phase material with the peaks assigned to heptagons (a) and pentagons (b). Particulars of the SWCNTs transformation to other structural forms of carbon at high pressure/temperature are discussed.

Rethinking Coal: Thin Films of Solution Processed Natural Carbon Nanoparticles for Electronic Devices.

Disordered carbon materials, both amorphous and with long-range order, have been used in a variety of applications, from conductive additives and contact materials to transistors and photovoltaics. Here we show a flexible solution-based method of preparing thin films with tunable electrical properties from suspensions of ball-milled coals following centrifugation. The as-prepared films retain the rich carbon chemistry of the starting coals with conductivities ranging over orders of magnitude, and thermal treatment of the resulting films further tunes the electrical conductivity in excess of 7 orders of magnitude. Optical absorption measurements demonstrate tunable optical gaps from 0 to 1.8 eV. Through low-temperature conductivity measurements and Raman spectroscopy, we demonstrate that variable range hopping controls the electrical properties in as-prepared and thermally treated films and that annealing increases the sp(2) content, localization length, and disorder. The measured hopping energies demonstrate electronic properties similar to amorphous carbon materials and reduced graphene oxide. Finally, Joule heating devices were fabricated from coal-based films, and temperatures as high as 285 °C with excellent stability were achieved.

On the nature of cracks and voids in nuclear graphite

Microcracks in neutron-irradiated nuclear grade graphite have been examined in detail for the first time using a combination of transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), energy dispersive X-ray (EDX), and energy filtered TEM (EFTEM). Filler particles from both unirradiated Pile Grade A (PGA) and three irradiated British Experimental Pile ‘O’ (BEPO) graphite specimens were investigated with received doses ranging from 0.4 to 1.44 displacements per atom (dpa) and an irradiation temperature of between 20 and 120 °C. We suggest that the concentration and potentially the size of microcracks increase with increasing neutron irradiation and show that disordered carbon material is present in a range of microcracks (of varying size and shape) in all specimens including unirradiated material. EFTEM and EELS data showed that these cracks contained carbon material of lower density and graphitic character than that of the surrounding bulk graphite. The presence of partially filled microcracks has potentially significant implications for the development of microstructural models for the prediction of radiation-induced dimensional and property changes in nuclear graphite.

Variable-range hopping transport: crossovers from temperature dependence to electric field dependence in disordered carbon materials

Variable-range hopping (VRH) is an important conduction mechanism in disordered conductors. One example of such a disordered conductor is reduced graphene oxide in which VRH dominates the temperature dependence of electronic conduction. Electronic transport is generally by electric field-assisted, thermally-driven VRH, following the theory of Mott, and Pollak and Riess. However, with the increase of electric field and decrease of temperature, we identify a surprisingly smooth crossover to solely field-driven VRH described by the theory of Shklovskii. We give the analytic expression for the crossover field E C as a function of temperature and coefficients from thermally-driven and field-driven VRH. Besides reduced graphene oxide, we show in this work that our crossover scenario can also account for the experimentally measured conductivity data of three-dimensional (3D) carbon networks as well as that of quasi-1D highly-doped conducting polymers, illustrating the wide validity of our proposed physical scenario. Our crossover scenario has the advantage of combining two distinct regimes of VRH conduction yet remaining within the currently established theoretical framework.

Catalytic graphitization of carbon/carbon composites by lanthanum oxide

Graphitized carbon/carbon composites were prepared by the process of catalytic graphitization with the rare-earth catalyst, lanthanum oxide (La2O3), in order to increase the degree of graphitization and reduce the electrical resistivity. The modified coal tar pitch and coal-based needle coke were used as carbon source, and a small amount of La2O3 was added to catalyze the graphitization of the disordered carbon materials. The effects of La2O3 catalyst on the graphitization degree and microstructure of the carbon/carbon composites were investigated by X-ray diffraction, scanning electron microscopy, and Raman spectroscopy. The results showed that La2O3 promoted the formation of more perfect and larger crystallites, and improved the electrical/mechanical properties of carbon/carbon composites. Carbon/carbon composites with a lower electrical resistivity (7.0 μΩ·m) could be prepared when adding 5 wt.% La2O3 powder with heating treatment at 2800 °C. The catalytic effect of La2O3 for the graphitization of carbon/carbon composites was analyzed.

All-Carbon Molecular Tunnel Junctions

This Article explores the idea of using nonmetallic contacts for molecular electronics. Metal-free, all-carbon molecular electronic junctions were fabricated by orienting a layer of organic molecules between two carbon conductors with high yield (>90%) and good reproducibility (rsd of current density at 0.5 V <30%). These all-carbon devices exhibit current density-voltage (J-V) behavior similar to those with metallic Cu top contacts. However, the all-carbon devices display enhanced stability to bias extremes and greatly improved thermal stability. Completed carbon/nitroazobenzene(NAB)/carbon junctions can sustain temperatures up to 300 °C in vacuum for 30 min and can be scanned at ±1 V for at least 1.2 × 10(9) cycles in air at 100 °C without a significant change in J-V characteristics. Furthermore, these all-carbon devices can withstand much higher voltages and current densities than can Cu-containing junctions, which fail upon oxidation and/or electromigration of the copper. The advantages of carbon contacts stem mainly from the strong covalent bonding in the disordered carbon materials, which resists electromigration or penetration into the molecular layer, and provides enhanced stability. These results highlight the significance of nonmetallic contacts for molecular electronics and the potential for integration of all-carbon molecular junctions with conventional microelectronics.

Nanostructured carbide-derived carbon synthesized by chlorination of tungsten carbide

Nanostructured carbide-derived carbons were synthesized from α-tungsten carbide (WC-CDC) powder via gas phase chlorination within the temperature range from 700 to 1100 °C. Analysis of X-ray diffraction results showed that WC-CDC are mainly amorphous consisting of relatively small graphitic crystallites and the apparent crystallite size along the a- and c-directions of graphite structure L a ≈ 4 nm and L c ≈ 1.5 nm were calculated. The first-order Raman spectra showed the graphite-like absorption peak at ∼1590 cm −1 and the disorder-induced peak at ∼1350 cm −1. The low-temperature N 2 sorption experiments were performed and a specific micropore surface area up to 1550 m 2 g −1 and total pore volume up to 0.89 cm 3 g −1 were obtained for WC-CDC synthesized at T = 1100 °C. High-resolution transmission electron microscopy and electron energy loss spectroscopy studies revealed that WC-CDC prepared at 800 °C correspond to the highly disordered carbon material but WC-CDC prepared at 1100 °C showed partial graphitization.

Effect of pyrolysis temperature on the microstructure of disordered carbon nanowires

Without any catalyst or pressure, bulk non-graphitizing carbon does not exhibit any change in its atomic order (or graphitize) even at 3000 °C. We propose a route for significant graphitization in polymer-derived disordered carbon materials at moderate temperatures by pre-aligning the precursor polymer chains. Accordingly, we employ a template-based fabrication process, where the capillary forces align the polyfurfuryl alcohol chains in the form of nanowires with diameter from 100–200 nm. The nanowires are pyrolyzed at temperatures ranging from 600 °C to 2000 °C and characterized with transmission electron and Raman microscopy as well as electrical conductivity measurements. The observed significant change in atomic rearrangement is explained by the alignment of precursor chains during the templated synthesis.

Electrochemical Oxidation of Carbon for Electric Power Generation: A Review

Published research is reviewed for purposes of establishing baseline performance expectations for the direct conversion of carbon in a molten carbonate fuel cell. Examination of data from several sources supports the possibility of direct carbon conversion at a total efficiency of 80% (HHV) and peak power of at least 0.11 W/cm2 using atomically disordered carbon materials derived from fossil or biological sources The net anode reaction results in CO2 product at 0.1-0.2 V polarization, while parasitic corrosion of carbon by the reverse Boudouard reaction may be significant in parts of the carbon bed remote from the current collector. Also important are an understanding of the mechanism and extent of carbon wetting by the electrolyte. A cell configuration allowing simultaneous measurement of electrode potentials and off-gas composition is suggested for use in producing the data necessary for scale-up

Fabrication and Characterization of Graphitic Carbon Nanostructures with Controllable Size, Shape, and Position

The incorporation of carbon materials in micro- and nanoscale devices is being widely investigated due to the promise of enhanced functionality. Challenges in the positioning and addressability of carbon nanotubes provide the motivation for the development of new processes to produce nanoscale carbon materials. Here, the fabrication of conducting, nanometer-sized carbon structures using a combination of electron beam lithography (EBL) and carbonisation is reported. EBL is used to directly write predefined nanometer-sized patterns in a thin layer of negative resist in controllable locations. Careful heat treatment results in carbon nanostructures with the size, shape, and location originally defined by EBL. The pyrolysis process results in significant shrinkage of the structures in the vertical direction and minimal loss in the horizontal direction. Characterization of the carbonized material indicates a structure consisting of both amorphous and graphitized carbon with low levels of oxygen. The resistivity of the material is similar to other disordered carbon materials and the resistivity is maintained from the bulk to the nanoscale. This is demonstrated by fabricating a nanoscale structure with predictable resistance. The ability to fabricate these conductive structures with known dimensions and in predefined locations can be exploited for a number of applications. Their use as nanoband electrodes is also demonstrated.

Early stages of the carbon nanotube growth by low pressure CVD and PE-CVD

The early stages of single-walled (SWCNT) and multi-walled carbon nanotube (MWCNT) growth were investigated by chemical vapour deposition (CVD) and plasma enhanced CVD (PE-CVD). SWCNT and MWCNT nuclei were observed using low pressure and short duration synthesis process. Microscopic evidences of the quite long duration, and hence complexity, of the MWCNT nucleation compared to SWCNT growth was provided. Besides, Raman spectroscopy pointed out significant differences in terms of disorder between the CVD and PE-CVD samples at the early stage of the growth process. The CVD process was found to be more selective for carbon nanotube growth, without significant disordered carbon material deposited on the substrate.

Hollow porous carbon microspheres obtained by the pyrolysis of TiO 2/poly(furfuryl alcohol) composite precursors

Disordered carbon materials with high porosity were prepared through the pyrolysis of TiO 2/poly(furfuryl alcohol) composites, obtained by the sol–gel method. The composites were prepared starting from titanium tetra-isopropoxide (TTIP) and furfuryl alcohol (FA) as precursors. Two different synthetic procedures for our composites were carried out, based on the addition of furfuryl alcohol (FA) before or after the TiO 2 nanoparticles formation. Also, different TTIP/FA ratio was tested. The hybrid materials obtained by both synthetic routes were pyrolyzed, under argon flow, at 900 °C producing novel TiO 2/carbon composites. All samples were characterized by XRD, FT-IR, DR-FTIR, Raman spectroscopy and TEM. Results indicated the effective FA polymerization on TiO 2 (anatase) nanoparticles, and polymer conversion to disordered carbon after the pyrolysis, simultaneously with TiO 2 anatase–rutile phase transition. The resulting TiO 2/carbon composites were treated with HF solution aiming the oxide dissolution, yielding an extremely porous carbon material as insoluble fraction. The morphology of these porous carbon materials is strongly dependent on the synthetic route adopted for the composite precursor, varying from carbon foam to highly ordered hollow microspheres.

Structure and electrical resistivity of nano-carbon materials

The electrical resistivity and microstructure of nano-carbon materials, such as anisotropic coal tar pitch (pitch C) and polyparaphenylene (PPP), were investigated. The resistivity of the samples measured under applied pressure decreased as the packing density increased. The change ratio, ρ/PD (resistivity/packing density), for pitch C was larger than that for well-known petroleum pitch (Ashland 240), and the resistivity value of the former was about 30% smaller than that of the latter at the highest packing density. The difference in the resistivity under pressure is thought to be caused not only by the contact resistance of the grains but also by the difference in the domain structure of the two kinds of samples. The packing density of PPP, which is a disordered carbon material, is scarcely changed by applied pressure. The resistivity of PPP was found to be affected strongly by the heat treatment temperature.

Textural and Structural Analysis of Carbon Materials by Transmission Electron Microscopy and Image Processing

Image analysis techniques have been applied to TEM (transmission electron microscopy) observation of carbon materials, in order to evaluate the texture and structure of carbons visually and quantitatively. Although X-ray diffraction, various spectroscopy, and transport measurements have been widely used as techniques for quantitative analyzing carbonbased materials, it is difficult to attain atomic resolution from these results. Whole and partial images of materials can be observed in various magnifications by means of electron microscopy. The microscopic observation, however, is not quantitative by itself. In the present paper, structure and defects of ordered and disordered carbon materials are analyzed quantitatively by TEM combined with image analysis, which can provide a powerful tool to characterize the carbons from another viewpoint.

Lithium Storage Mechanism of Disordered Mesophase Carbon Fibers Studied by [sup 7]Li-Nuclear Magnetic Resonance

To clarify the lithium storage states in disordered carbon materials for Li-ion secondary batteries 7 Li-nuclear magnetic resonance (NMR) spectra and cyclic voltammograms (CVs) of mesophase carbon fibers were measured. The CV obtained with mesophase carbon fibers fired at 800°C showed two peaks positioned in different potential regions. A sharp peak appearing at about 0.03-0.2 V vs. Li/Li + is attributed to lithium accommodation in the crystallites having a graphite structure and a broad one in the range of 0.2 to 1.2 V is attributed to the disorder site. In contrast, 7 Li-NMR spectra of the 800°C sample gave only a single peak, which shifted from 0 ppm at 0.8 V to 16 ppm at 30 mV depending on the charging level of lithium. Similar aspects of the spectra shift were also observed in a series of fully charged mesophase carbon fibers prepared at different firing temperatures. These results indicate that the rate of the exchange of lithium between the two different sites, the edge/surface site and intercalated site, is too fast to differentiate the different states. This suggests that there is a possibility of realizing a high capacity and high power anode for Li-ion secondary batteries based on a low-temperature mesophase carbon.

First-Principles Study of the Lithium Interaction with Polycyclic Aromatic Hydrocarbons

We have performed first-principles calculations in order to understand the binding mechanism of Li atoms in disordered carbon materials that are used for negative electrodes of rechargeable lithium batteries. We used pyrene, anthracene, and phenanthrene molecules as parts of disordered carbon. We examined several binding sites for two Li atoms in these aromatics and found that they are bound with substantial negative binding energies. The most negative one was −142.8 kJ/mol for Li-containing pyrenes, −211.0 kJ/mol for anthracenes, and −146.2 kJ/mol for phenanthrenes at the B3LYP/6-31G*//HF/6-31G* level of calculation. Li atoms are bound to interstitial (ring-over) and edge sites. In addition to these binding mechanisms, we found that Li atoms could be bound, forming a Li dimer in anthracene and phenanthrene. Their binding energies are −200.5 and −146.2 kJ/mol, respectively, being larger in magnitude than Li2 dissociation energy. These aromatics lose their planarity when they accommodate Li atoms. We found...