Tree-like carbon grown from camphor

Macroscopic multi-branched tree-like carbon has been prepared by the chemical vapor deposition of camphor in argon using ferrocene as catalyst. The results show that the building blocks forming the carbon trees are carbon microspheres. Both the diameter of the tree branches and the size of the carbon microspheres become smaller with increasing ferrocene content. The ratio of tree branch diameter to carbon microsphere size is about 1.35, which is independent of the ferrocene content. Increasing the argon flow rate in the range of 500-2500 ml/min benefits the growth of carbon trees, and the most favorable argon flow rate is 2000-2500 ml/min. Increasing the reaction temperature in the range of 1000-1150 °C can enhance the coalescence of the carbon microspheres, and thus result in the carbon trees with smooth and straight branches.     

Morphology and microstructure of spring-like carbon micro-coils/nano-coils prepared by catalytic pyrolysis of acetylene using Fe-containing alloy catalysts

Three-dimensional (3D) spring-like carbon nano-coils were obtained in high purity (nearly 100%) and high yield (20%) by the catalytic pyrolysis of acetylene at 750-790 °C using an Fe-based catalyst; 54Fe-38Cr-4Mn-4Mo or 71Fe-18Cr-8Ni-3Mo (SUS513). The morphologies and microstructure were examined in detail, and the growth mechanism is also discussed. The diameter of carbon fiber, from which the carbon nano-coils was formed, was 50-200 nm, the coil diameter 100-1000 nm, and the coil pitch 10-500 nm. The nano-coils were generally grown by a mono-directional growth mode, and laces with various morphologies were very commonly observed on the surface. It was observed that the spring-like carbon nano-coils are actually composed of two fused nano-coils.     

The influence of formation conditions on the growth and morphology of carbon trees

Carbon trees, quite different from those previously reported, have been produced by the catalytic chemical vapor deposition of toluene using ferrocene as the catalyst precursor. The influences of formation conditions such as catalyst mass, toluene flow rate, and reaction time on the tree growth and morphology have been investigated. The yield of carbon trees is greatly affected by catalyst quantity. A lower toluene flow rate (50-100 ml/min) or shorter reaction time (10-30 min) leads to trees with thinner (several microns) and filamentous branches, while a higher toluene flow rate (greater than 200 ml/min) or longer reaction time (60-120 min) produces thicker (tens of microns) and spherical branches. Results suggest that the morphology of the carbon trees can be adjusted by varying the reaction conditions.     

Solid-phase transformation of glass-like carbon nanoparticles into nanotubes and the related mechanism

Multi-walled carbon nanotubes have been synthesized through the solid-phase transformation of metal-containing glass-like carbon nanoparticles by heating at temperatures of 800-1000 °C. From microscopic observations on the morphologies and structures of the nanotubes and various intermediate objects, it is shown that the transformation occurs by nanoparticles first assembling into wire-like nanostructures, and then transforming into nanotubes via particle-particle coalescence and structural crystallization.     

Carbon infiltration of carbon-fiber preforms by catalytic CVI

Experimental studies were conducted to assess catalytic chemical vapor infiltration processing for preparing carbon/carbon composites as a potential improvement to conventional one. The catalyst was introduced into the carbon fiber preforms by wet impregnation. Using C₃H₆/Ar/H₂ as the original gas, catalytic carbon was formed at 500-1000 °C for 1-3 h. It was found that carbon filaments were formed as the preparing temperatures were 500-700 °C, and carbon particles could be obtained at 800-1000 °C. The increasing rate of density was up to 0.916 g/ml/h when the sample was formed at 600 °C for 1 h with the catalytic of 0.7 wt.% Ni, and the carbon yield arrived to 90 wt.% . According to the micrographs of catalytic carbon, the forming mechanism of carbon filaments agreed with that of carbon filaments due to a metal catalyst. The weighted average interlayer spacing of C/C composites with catalytic carbon decreased to 0.341.     

Synthesis of ZnO nanoparticles by an aerosol process

Spherical nano-sized zinc oxide (ZnO) particles were produced by a spray pyrolysis method using the aerosol technique described in this study. The effects of reaction temperatures of 600, 800 and 1000 °C and collection locations of the particles, such as the flask collector and the tube exit, on the morphology and crystal structure of the ZnO particles were investigated. X-ray diffraction (XRD) studies showed that the crystallinity of the particles was increased by increasing the reaction temperature from 600 °C to 1000 °C. Fourier transform infrared spectroscopy (FTIR) measurements revealed that the particles were pure and similar to each other. Scanning electron microscopy (SEM) revealed that the synthesized nanoparticles had sizes between 200 nm and 400 nm, with uniform morphologies. A computational fluid dynamics (CFD) model of the horizontally positioned tube reactor was developed. Simulation results provided information about the residence time and the temperature distribution along the tube, which were found to be correlated to the particle morphology.     

Thermodynamic analysis of glycerol partial oxidation for hydrogen production

Thermodynamics of glycerol partial oxidation for hydrogen production has been studied by Gibbs free energy minimization method. The optimum conditions for hydrogen production are identified: reaction temperatures between 1000 and 1100 K and oxygen-to-glycerol molar ratios of 0.4-0.6 at 1 atm. Under the optimal conditions, complete conversion of glycerol, 78.93%-87.31% yield of hydrogen and 75.12%-87.97% yield of carbon monoxide could be achieved in the absence of carbon formation. The glycerol partial oxidation with O₂ is suitable for providing hydrogen-rich fuels for Molten Carbonate Fuel Cell and Solid Oxide Fuel Cell. The carbon-formed and carbon-free regions are found, which are useful in guiding the search for suitable catalysts for the reaction. Inert gases have a positive effect on the hydrogen and carbon monoxide yields.     

The transformation of acetylene black into onion-like hollow carbon nanoparticles at 1000 °C using an iron catalyst

Acetylene carbon blacks were heat treated at 1000 °C in the presence of ferric nitrate. The morphologies and structural features of original carbon blacks and the carbonized product were investigated using TEM, HRTEM, XRD and nitrogen sorption measurements. It was found that, with the increase of ferric nitrate loading from 4:1, 8:1, 12:1 to 16:1 (the weight ratio of ferric nitrate to acetylene carbon black), the yield of carbonized product decreased from 64.6%, 61.8%, 46.2% to 28.6%. Onion-like hollow carbon nanoparticles with diameter of 40 to 100 nm were generated, implying that the discontinuous short fragments of carbon black were reconstructed and transformed into turbostratic concentric graphitic layers by the iron-based catalytic graphitization. The mechanism was discussed briefly.     

Carbonization and thermal expansion of glassy carbon derived from bis-ortho-diynylarenes

Bis-ortho-diynylarene (BODA) monomers form rigid polynaphthalene networks via thermal Bergman cyclizations at 300-400 °C. Upon further heating to 1000 °C, glassy carbon with high yield (>80%) is formed from the polynaphthalene precursor. Dilatometry was used to determine the linear coefficient of thermal expansion (CTE) of various BODA-derived glassy carbon. The measured CTE was similar to other glassy carbon-based systems ranging from 3.20 to 6.92 × 10⁻⁶ °C⁻¹ over 20-1000 °C. An increase in short-range order was apparent when polynaphthalene networks were carbonized to 1500 °C; the CTE observed for such thermal cycling was 2.85-2.93 × 10⁻⁶ °C⁻¹ over 20-1000 °C. Using dilatometry also provided insight on the carbonization mechanism to provide optimization of glassy carbon yields during thermal cycling.     

Swelling and related mechanical and physical properties of carbon nanofiber filled mesophase pitch for use as a bipolar plate material

Mesophase pitch was investigated as a melt processable precursor to a compression or injection moldable all carbon bipolar plate. After shaping, carbonization to 1000 °C or greater is required to achieve the desired electrical and mechanical properties, but gases evolved during this step lead to swelling. Carbon nanofiber was added to suppress swelling during carbonization and bypass the typical oxidation steps used when processing mesophase pitch. The addition of carbon nanofiber decreased swelling by increasing the viscosity of the melt. Carbonized materials with carbon nanofibers can show strengths (30-50 MPa) and conductivities (20-80 S cm⁻¹) consistent with composite bipolar plate materials. The materials show conductivities below Department of Energy target values at the current carbonization temperatures, which were limited to 1000 °C. The use of glass fibers as a secondary filler led to reduced gas permeability in porous samples.     

Kinetic study of carbon nanotube synthesis over Mo/Co/MgO catalysts

The kinetics of carbon nanotube (CNT) synthesis by decomposition of CH₄ over Mo/Co/MgO and Co/MgO catalysts was studied to clarify the role of catalyst component. In the absence of the Mo component, Co/MgO catalysts are active in the synthesis of thick CNT (outer diameter of 7-27 nm) at lower reaction temperatures, 823-923 K, but no CNTs of thin outer diameter are produced. Co/MgO catalysts are significantly deactivated by carbon deposition at temperatures above 923 K. For Mo-including catalysts (Mo/Co/MgO), thin CNT (2-5 walls) formation starts at above 1000 K without deactivation. The significant effects of the addition of Mo are ascribed to the reduction in catalytic activity for dissociation of CH₄, as well as to the formation of Mo₂C during CNT synthesis at high temperatures. On both Co/MgO and Mo/Co/MgO catalysts, the rate of CNT synthesis is proportional to the CH₄ pressure, indicating that the dissociation of CH₄ is the rate-determining step for a catalyst working without deactivation. The deactivation of catalysts by carbon deposition takes place kinetically when the formation rate of the graphene network is smaller than the carbon deposition rate by decomposition of CH₄.     

High power supercapacitors using polyacrylonitrile-based carbon nanofiber paper

Porous carbon nanofiber paper has been obtained by one-step carbonization/activation of PAN-based nanofiber paper at temperatures from 700 to 1000 °C in CO₂ atmosphere. The paper was used as supercapacitor electrode without any binder or percolator. At low temperature, e.g., ?900 °C, nitrogen enriched carbons with a poorly developed specific surface area (S_BET ? 400 m²/g) are obtained. In aqueous electrolytes, these carbons withstand high current loads without a noticeable decrease of capacitance, and the normalized capacitance reaches 67 μF/cm². At 10 s time constant, the values of energy and power densities are 3-4 times higher than for activated carbons (AC) presenting higher specific surface area. By carbonization/activation at 1000 °C, subnanometer pores are developed and S_BET = 705 m²/g. Despite moderate BET specific surface area, the capacitance reaches values higher than 100 F/g in organic electrolyte. At high power densities, the nanofiber paper obtained at 1000 °C outperforms the energy density retention of ACs in organic electrolyte. The high power capability of the carbon nanofiber papers in the two kinds of electrolytes is attributed both to the high intrinsic conductivity of the fibers and to the high diffusion rate of ions in the opened mesopores.     

The oxidation of aniline with silver nitrate to polyaniline-silver composites

Silver nitrate oxidizes aniline in the solutions of nitric acid to conducting nanofibrillar polyaniline. Nanofibres of 10-20 nm thickness are assembled to brushes. Nanotubes, having cavities of various diameters, and nanorods have also been present in the oxidation products, as well as other morphologies. Metallic silver is obtained as nanoparticles of ∼50 nm size accompanying macroscopic silver flakes. The reaction in 0.4 M nitric acid is slow and takes several weeks to reach 10-15% yield. It is faster in 1 M nitric acid; a high yield, 89% of theory, has been found after two weeks oxidation of 0.8 M aniline. The emeraldine structure of polyaniline has been confirmed by FTIR and UV-vis spectra. The resulting polyaniline-silver composites contain 50-80 wt.% of silver, close to the theoretical expectation of 68.9 wt.% of silver. The highest conductivity was 2250 S cm⁻¹. The yield of a composite is lower when the reaction is carried out in dark, the effect of daylight being less pronounced at higher concentrations of reactants.     

Deposition mechanism and microstructure of pyrocarbon prepared by chemical vapor infiltration with kerosene as precursor

Large-size carbon/carbon composites (Φ 450 × Φ 230 × 15 mm³) have been produced by chemical vapor infiltration with kerosene as precursor. The microstructure of pyrocarbon was examined by polarized light microscopy and scanning electron microscopy. The infiltration kinetics was analyzed to investigate the infiltration rate limitation by parameters such as temperature. The results show that rough laminar carbon constitutes the majority of the matrix at a medium temperature (about 1100 °C), while smooth laminar and isotropic structures occur at temperatures lower than 1000 °C and higher than about 1200 °C, respectively. The apparent activation energy of kerosene decomposition in the temperature range 900-1200 °C is about 125.6 kJ/mol.     

Overcoming the barrier to graphitization in a polymer-derived nanoporous carbon

A new pathway to synthesize a carbon with both nanoporosity and pre-graphitic structures has been discovered by annealing at 2000 °C a CO₂ activated, non-graphitizing, nanoporous carbon originally derived from polyfurfuryl alcohol. The activation process with CO₂ overcomes the barrier to graphitization normally present in this carbon even when treated at high temperature. Gas adsorption analysis, skeletal density measurements, X-ray diffraction, and transmission electron microscopy are utilized to probe the structure of both the non-activated and the activated carbons at 800, 1200, 1800, and 2000 °C. The influence of activation time is also examined. Prior to activation the nanopore walls are comprised of several layers of disordered graphenes. Activation eliminates the barrier to graphitization by reducing the number of layers below the limit of detection and by removing carbon material highly susceptible to oxidation. Annealing at 2000 °C of the carbon activated to 84% burnoff induces the formation of pre-graphitic domains amongst the nanoporous carbon. The (0 0 2) bands corresponding to 2θ = 24.3°, 26°, and 26.5° are identified and assigned to amorphous, turbostratic, and graphitic morphologies. A pore volume of 0.50 cm³ g⁻¹ localized in pores below 2 nm in size is preserved after annealing.     

Low-temperature solution synthesis of carbon nanoparticles, onions and nanoropes by the assembly of aromatic molecules

Graphitic carbon nanostructures were prepared in solution by two methods: solvothermal synthesis and hot injection. Small carbon nanoparticles with uniform diameters of 3-6 nm, carbon onion particles with larger diameters of 30-80 nm, and carbon nanoropes with a length of hundreds of nm and a width of 3-20 nm, were formed using commercial mesophase pitches as a carbon precursor through solution-phase synthesis below 200 °C. In the solvothermal synthesis, organic-organic assemblies of aromatic molecules from the pitches could be constrained into different stacking arrangements directed by varying the concentration of the block copolymer P123 template in toluene solution at 200 °C. In the hot injection method, when oleic acid was used as a solvent at 180 °C, the assemblies of the aromatic building blocks were controlled by varying the reaction time (5-30 min) or the concentration of H₂SO₄ catalyst (0.015-0.061 mol L⁻¹) in the nucleation and growth process.     

EELS studies of carbide derived carbons

Carbide derived carbons (CDCs) have been synthesized from VC, WC, TaC, NbC, HfC and ZrC at T = 1000 °C, and from TiC at T = 700-1200 °C via a chlorination reaction. The CDCs have been studied by means of high resolution transmission electron microscopy and electron energy-loss spectroscopy (EELS). These studies show that the structures of CDCs are strongly dependent upon the nature of the starting carbide and the synthesis temperature. The structures range from very disordered nanoporous carbon, consisting of randomly curved graphene layers, to compact spherical entities of compactly packed graphitic layers. EELS studies of carbon core-loss spectra show that the relative sp² content in these carbons varies between 83% and 98%. The low-loss part of the EEL spectra shows that the positions of the π-plasmons and the bulk plasmons are more dependent on the carbide precursor, than the synthesis temperature. The densities of the CDC particles have been estimated using the bulk plasmon positions as well as the sp³ content determined from the C-K edge spectra. The densities calculated from sp³ are close to the pycnometric ones, while the densities calculated from bulk plasmon positions are lower and reflects the nanostructure of CDCs.     

Vertically aligned carbon nanotube arrays grown on a lamellar catalyst by fluidized bed catalytic chemical vapor deposition

Large amount of vertically aligned carbon nanotube (CNT) arrays were grown among the layers of vermiculite in a fluidized bed reactor. The vermiculite, which was 100-300 μm in diameter and merely 50-100 μm thick, served as catalyst carrier. The Fe/Mo active phase was randomly distributed among the layers of vermiculite. The catalyst shows good fluidization characteristics, and can easily be fluidized in the reactor within a large range of gas velocities. When ethylene is used as carbon source, CNT arrays with a relatively uniform length and CNT diameter can be synthesized. The CNTs in the arrays are with an inner diameter of 3-6 nm, an outer diameter of 7-12 nm, and a length of up to several tens of micrometers. The as-grown CNTs possess good alignment and exhibit a purity of ca. 84%. Unlike CNT arrays grown on a plane or spherical substrate, the CNT arrays grown in the fluidized bed remain their particle morphologies with a size of 50-300 μm and the good fluidization characteristics were preserved accordingly.     

Experimental study on sawdust air gasification in an entrained-flow reactor

Experiments were performed in an entrained-flow reactor to better understand the processes involved in biomass air gasification. Effects of the reaction temperatures (700 °C, 800 °C, 900 °C and 1000 °C), residence time and the equivalence ratio in the range of 0.22-0.34 on the gasification process were investigated. The behavior of biomass gasification was discussed in terms of composition of produced gas. Four parameters, i.e. the low heating value, fuel gas production, carbon conversion and cold gas efficiency were used to evaluate the gasification. The results show that CO, CO₂ and H₂ are the main gasification products, while hydrocarbons (CH₄ and C₂H₄) are the minor ones. With the increase of the reaction temperature, the concentration of CO decreases, while the concentrations of CO₂ and H₂ increase. The concentrations of CH₄ and C₂H₄ reach their maximum value when the reaction temperature is 800 °C. The optimal reaction temperature is considered to be 800 °C and the optimal equivalence ratio is 0.28 in that the low heating value of the produced gas, carbon conversion and cold gas efficiency achieve their maximum values. The kinetic parameters of sawdust air gasification are calculated basing on the Arrhenius correlation.     

Carbon coating of anatase-type TiO2 through their precipitation in PVA aqueous solution

Fine particles of photocatalytic anatase-type TiO₂ prepared through hydrolysis of titanium-tetraisopropoxide were coated by carbon through their precipitation in poly(vinyl alcohol) (PVA) aqueous solution, followed by heat treatment at high temperatures of 400-1000 °C in a flow of high purity Ar. Without carbon coating, the phase transformation from anatase to rutile started above 600 °C, but it was suppressed up to 800 °C with carbon coating. Suppression of the phase transformation depended on the amount of carbon coated, apparent suppression being observed with carbon content above 5 mass%. The amount of carbon coated on anatase was controlled by changing the concentration of PVA in the solution. In order to have a carbon content of about 5 mass%, a PVA solution with more than 2 mass% had to be used.