Production of natural graphite particles with high electrical conductivity by grinding in alcoholic vapors

Natural graphite particles with a high crystallinity, which were sieved to obtain particles less than 63 μm, were ground with a ball mill under a dry atmosphere and various alcoholic vapors such as i-C₃H₈OH, n-C₃H₈OH, C₂H₅OH, and CH₃OH. The size and flakiness of the ground products and the electrical conductivity of the films made from the ground products were experimentally examined. Grinding the particles under alcoholic vapors slowly reduced the particle size and was similar to grinding in dry air, but grinding in alcoholic vapors produced flakier products. The graphite films, which were composed of flakier particles and were ground in alcoholic vapor, displayed higher electrical conductivities than the feed graphite particles. The products ground in C₂H₅OH, i-C₃H₇O and n-C₃H₇OH vapors had 50% or less of the specific resistance of the feed particles.     

Pinhole formation and weight loss during oxidation of industrial graphite and carbon

The performance of graphite or carbon above 400°C depends to a large degree on its resistance to oxidation. In this work oxidation data of graphite and carbon was analyzed using the number of created pinholes for short-term oxidation and weight loss for long-term oxidation. The effect of grain size, purification, and the amount of inorganic impurities was investigated. The migration of vanadium, the formation and morphology of pinholes, and the weight loss were followed in different samples. Short-term oxidation is diminished by the presence of calcium salts and enhanced by the presence of V₂O₅, but the latter effect is diminished by the presence of Ca₃V₂O₈. Both calcium salts and V₂O₅ enhance the long-term oxidation.     

Composite graphitic nanolayers prepared by self-assembly between finely dispersed graphite oxide and a cationic polymer

Chemical flaking of graphite has been performed by reacting natural graphite with a strong oxidizing agent, NaClO₃/HNO₃. The formed hydrophilic, negatively charged graphite oxide (GO) colloids can be dispersed in water which allows the deposition of thin GO/cationic polymer (poly(diallyldimethylammoniumchloride, PDDA) multilayer films on a glass substrate by wet-chemical self-assembly. The feasibility of the charge-regulated layer-by-layer deposition is demonstrated by mutual charge titrations of the film-forming species. Visible-light spectroscopy revealed progressive growth of the film thickness with the number of deposition of steps, while XRD and AFM showed that partially exfoliated, highly anisometric (aspect ratio >50) graphite oxide platelet aggregates were deposited with an average thickness of the stacked graphite oxide platelets of 10 carbon layers (7.4 nm). Reduction of multilayer assemblies of GO and PDDA on glass yielded a non-conductive turbostratic carbon nanofilm. The original, conductive graphite-like structure was restored by reduction with N₂H₄ and annealing at 400 °C which, by gradual ordering of the carbon crystallites, caused a significant decrease in the resistivity.     

Effect of iodine incorporation on the electrical properties of amorphous conducting carbon films

In this work, the influence of iodine incorporation on the electrical properties of amorphous conducting carbon films, prepared by the vapor phase pyrolysis of maleic anhydride, is reported and discussed. X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies reveal structural changes in the system. The anomalous behavior in the electrical properties of the intercalated system at low temperatures is investigated. The system shows a positive temperature coefficient of resistance (TCR) at low temperatures, which suggests reasons for the induced ordering of the system at low temperatures with the iodine incorporation. Also, a systematic increase in the conductivity of the sample is observed. The crossover temperature depends on the disorder in the system. The results indicate the possibility of metal-insulator (M-I) transition as a function of preparation temperature, iodine concentration and magnetic field.     

Ball-milling: the behavior of graphite as a function of the dispersal media

The ball-milling in liquid media leads to well organized, thin and highly anisometric graphite (HAG) crystals. The presence in the milling container of a liquid, which acts as a lubricant and decreases the violence of the shocks, is relevant. Two liquids are used: n-dodecane and water. With dodecane, inert towards graphite and the metal of the milling tools, the powder consists of pure graphite whereas with water, the graphite particles are covered with nanocrystallites (15 nm) of a magnetic compound: the maghemite (γ Fe₂O₃). The electrochemical properties of those powders are interesting. The highly anisometric graphite leads to an irreversible capacity around half of that for the initial graphite powder, in contradiction with previous results claiming that higher the surface area, the higher the irreversible capacity. In fact, milling in the presence of dodecane provokes essentially a cleavage, which increases the global area, but does not drastically change the number of edge carbon atoms, responsible for the increase of the large irreversible capacity. The graphite–maghemite composites present a high capacity, partly reversible by oxidation–reduction between iron and wustite (FeO). This reaction is made possible by the nanometric size of the particles, and therefore their high reactivity.     

Effect of atmosphere on the mechanical milling of natural graphite

Natural graphite was ball milled in a vibratory and a planetary mill under different atmospheres. Its microstructure was found to evolve very differently when milled in oxygen and in an inert atmosphere. This phenomenon is most pronounced when it was milled in a planetary mill where the deformation forces are mainly shear in nature. Mechanically induced oxidation on the surface, probably along the edges of the graphene planes, is responsible for suppressing the fracture rate and preserving the crystallinity of natural graphite milled in oxygen. Understanding this mechanochemical process may open up a new route for the synthesis of carbonaceous materials suitable for lithium ion battery applications.     

Formation of encapsulated molybdenum carbide particles by annealing mechanically activated mixtures of amorphous carbon with molybdenum

The investigation of morphological and structural changes during high-energy ball milling and thermal annealing of mixtures of amorphous carbon and molybdenum demonstrated that the activation leads to the formation of a nano-size carbide Mo₂C phase. Morphological characteristics of the annealed, mechanically activated amorphous carbon-molybdenum samples depend on the time of preliminary mechanical activation. Annealing of amorphous carbon-molybdenum samples after preliminary mechanical activation, at a temperature of 860 °C, caused the formation of nano-sized encapsulated particles of molybdenum carbide Mo₂C. The surface of the Mo₂C nanoparticles was coated with a shell composed of hexagonal polyaromatic carbon 5-20 nm thick. The size of the encapsulated particles varies within a rather broad range from 10 to 20 nm to several hundred nanometers.     

Ultra high thermal conductivity polymer composites

Epoxy composites based on vapor grown carbon fiber (VGCF) were fabricated and analyzed for room temperature thermophysical properties. An unprecedented high thermal conductivity of 695 W/m K for polymer matrix composites was obtained. The densities of all the composites are lower than 1.5 g/cc. In addition the high value of coefficient of thermal expansion (CTE) of the polymer material was largely reduced by the incorporation of VGCF. Also, unlike metal matrix composite (MMC), the epoxy composite has an electrically insulating surface. Based on the composite thermal conductivities, the room temperature thermal conductivity of VGCF, heat-treated at 2600°C, was estimated to be 1260 W/m K. Furthermore, the longitudinal CTE of the heat-treated VGCF was determined, for the first time, to be −1.5 ppm/K.     

Preparation of an activated carbon artifact: oxidative modification of coconut shell-based carbon to improve the strength

Coconut shell-based activated carbon was oxidized in aq. H₂SO₄, HNO₃ and H₂O₂ to induce surface oxygen functional groups on its surface and to increase the mechanical strength of the resultant activated carbon artifact with PVB as a binder. Although all oxidation was confirmed to significantly increase the strength, aq. H₂O₂ was found to be most effective, giving strength as high as 6000 kPa, which is believed to be sufficient for the electrode of an electric double layer capacitor (EDLC). The increase of CO₂ evolving groups induced on the surface of activated carbon appears to be responsible for the strength increase. There was an optimum extent of oxidation for the strength as well as the performance of the electrode. Too much oxidation reduces the electrical conductivity of the activated carbon. Facile oxidation by aq. H₂O₂ can be recommended as a practical modification of the surface since it takes place safely below 100°C without releasing any harmful gas.     

Deodorization performance of charcoal particles loaded with orthophosphoric acid against ammonia and trimethylamine

A deodorant was prepared by drying charcoal particles after dipping in aqueous H₃PO₄ solutions. The deodorization performances of the samples against NH₃ and (CH₃)₃N odor gases were examined by a detection test tube method and compared with those of a conventional coconut shell-derived active carbon loaded with H₃PO₄ in the same manner. The charcoal particles with H₃PO₄ exhibited higher performances than those of the other against both the odor gases. Ammonia gas was caught on the sample surface through reaction with the loaded H₃PO₄ to form NH₄H₂PO₄ and further (NH₄)₂HPO₄ but the deodorization mechanism for (CH₃)₃N could not be decided. The high performances of the charcoal particles loaded with H₃PO₄ were due to its characteristic porous structure consisting of large pores, i.e., such pores were suited for loading a large amount of H₃PO₄ and were not apt to be blocked by the loaded H₃PO₄. Also large pores were not blocked by expansion of H₃PO₄ on the pore surface through the deodorization reactions. The active carbon being composed of a large number of micropores did not exhibit these advantages.     

Size control and characterization of spherical carbon aerogel particles from resorcinol-formaldehyde resin

Spherical resorcinol-formaldehyde (RF) aerogel particles were synthesized by emulsion polymerization of resorcinol with formaldehyde in a slightly basic aqueous solution, followed by supercritical drying with carbon dioxide. RF carbon aerogel particles were prepared by carbonizing of the RF aerogel particles at a high temperature under a nitrogen atmosphere. By changing the viscosity of the RF sol added to the cyclohexane containing a surface-active agent for preparation of the spherical RF hydrogels, we investigated the influence of the apparent viscosity of the RF sol on the size of the generated RF carbon aerogel particles. We could successfully prepare the RF carbon aerogel particles with a truly spherical shape and control their size in the range from about 10 to 500 μm by changing the apparent viscosity of the RF sol. The spherical RF carbon aerogel particles with an average diameter of 20 μm have a BET surface area of about 800 m²/g and a uniform mesopore radius of 1.78 nm.     

Fracture toughness of PAN-based carbon fibers estimated from strength-mirror size relation

Fracture toughness (K_IC) of representative high-strength type PAN (polyacrylonitrile)-based carbon fibers, Torayca™ T300 and T800H, with or without artificial surface defects, were estimated to be ca. 1 MPam^1/2 from the tensile strength vs. fracture mirror size relation, assuming a constant crack-to-mirror size ratio. The corresponding critical energy release rate (Γ) was ca. 7.4 J m⁻², which was close to the value derived from the reported surface energies for a graphite crystal. Similar K_IC values were obtained for the old-type PAN-based carbon fibers from the reported data by the use of the present estimation procedure.     

Production and structural characterization of carbon soot with narrow UV absorption feature

In the present study, by a simple method, carbon soot with a narrow absorption features at around 220 nm was produced by pyrolysis of methane gas at a hot (1400-2800 °C) filament. A detailed correlation between the structure and the optical spectra of carbon soot was carried out using a high-resolution transmission electron microscope. It appears that the origin of the narrow absorption peak is an onion-like graphitic structure. Interestingly, we found that the absorption peak position and the size of the soot can be controlled by the production temperature. The absorption peak is located at 217.5 nm, i.e., at the position of an intense interstellar absorption, when the onion structure has about 5 nm diameter. Infrared spectroscopy was applied to further elucidate the chemical composition of the obtained soot.     

Physical and chemical analyses of unburned carbon from oil-fired fly ash

In the hydrometallurgy process of extracting vanadium and nickel products from oil-fired fly ash, a large amount of unburned carbon remains as a by-product. Because of insufficient knowledge of the physical and chemical properties and related applications of the unburned carbon, the residue has been disposed to landfill or incinerated. In order to explore the utilization of unburned carbon, this preliminary study analyzed its morphology, particle size distribution, specific surface area, pore size distribution, density and chemical composition. The results indicate that the chemical composition is 73-91% carbon, 5-19% ash, 2.5-11.5% volatile substances, and 0.7-1.9% water, it consists of porous spherical particles and some crumbled particles with particle sizes mainly in the range 1-100 μm, the specific surface area is 16-33 m²/g, the pore size ranges between 0.02 and 10 μm, the real density is about 2.05-2.16 g/cm³, the apparent density about 0.15 g/cm³ and it can be characterized as a puffy, readily wind-blown powder.     

Fabrication of toughened B4C composites with high electrical conductivity using MAX phase as a novel sintering aid

MAX phase Ti₃AlC₂ was chosen as a novel sintering aid to prepare electrically conductive B₄C composites with high strength and toughness. Dense B₄C composites can be obtained at a hot-pressing temperature as low as 1850 °C with 15 vol% Ti₃AlC₂. The enhanced sinterability was mainly ascribed to the in situ reactions between B₄C and Ti₃AlC₂ as well as the liquid phase decomposed from Ti₃AlC₂. Both the Vickers hardness and fracture toughness increase with increasing Ti₃AlC₂ amount, and high hardness and toughness values of 28.5 GPa and 7.02 MPa m^−1/2 respectively were achieved for B₄C composites sintered with 20 vol% Ti₃AlC₂ at 1900 °C. Crack deflection by homogenously distributed TiB₂ particles was identified as the main toughening mechanism. Besides, B₄C composites sintered with Ti₃AlC₂ show significantly improved electrical conductivity due to the percolation of highly conductive TiB₂ phase, which could enhance the machinability of B₄C composites largely by allowing electrical discharge machining.     

Effect of preparation conditions on the characteristics of exfoliated graphite

Exfoliated graphite (EG) was prepared from graphite intercalation compounds (GICs) synthesized electrochemically with different electricity consumption from 10.83 to 40.00 A h/kg. Effects of electricity consumption on the synthesis of GICs and of exfoliation temperature on different parameters of EG, i.e. exfoliation volume, volatile content, specific surface area and pore volume measured by mercury porosimetry, length and width of worm-like particles, and distance between neighboring balloons based on the zigzag model for worm-like particles of EG, were studied. These parameters were found to depend strongly on the electricity consumption and also on exfoliation temperature. Exfoliation volume, volatile content, specific surface area and pore volume on EG prepared at 1000 °C increased with increasing electricity consumption, but the distance between neighboring balloons was found to decrease. These results reveal marked development of pores in EG samples. Raising exfoliation temperature increased exfoliation volume, specific surface area and pore volume up to 800 °C. Above this temperature these parameters tended to be stable.     

Well-aligned polyaniline/carbon-nanotube composite films grown by in-situ aniline polymerization

Multiwalled carbon nanotubes (MWNTs) encapsulated by polyaniline (PANI) of nanometer size have been synthesized by in-situ polymerization, and are found to be orientationally ordered by the aligned MWNTs. Procedures are demonstrated for the preparation of nanocomposite-tube (NCT) films with controlled organization. Changes in the dimensions of the nanocomposites were measured using SEM and TEM techniques. The interaction between PANI and MWNTs and the nature of chain growth have been investigated and explained according to the results of FT-IR analysis. The improvement of thermal stability and crystallinity of the nanocomposites have been evaluated by using TGA and XRD. The mechanism of charge transport in these composites has also been studied by measuring the DC conductivity of all samples and examining the temperature-conductivity relations. MWNT alignment should be possible with other nanometer-sized building composites films, offering a general route for controlled assembly of organized nanomaterials and devices.