Carbon Whiskers Research Articles

The Apex Structure of Co-Produced Helical Boron Nitride and Carbon Cones

Abstract Conical whiskers of layered compounds such as graphite and hexagonal boron nitride (h-BN)2 are interesting materials because their conical morphology results from the introduction of a local topological defect at the cone apex. In the honeycomb array characteristic of graphite and h-BN basal planes, this topological defect is thought to be a non-hexagonal ring (e.g. a pentagon or a square). These defects are known to form the basis of curved nanostructures like fullerenes and buckytubes. Therefore a comparison of their occurrence in two systems known to harbour nanostructures is worthy of study. The defect structure at the apex of co-produced carbon and boron nitride whiskers was investigated by analytical transmission electron microscopy (TEM). The instrument used was a field-emission JEM3000F high-resolution electron microscope, equipped with a parallel electron energy loss spectrometer (EELS).

Suppression of carbonaceous depositions on nickel catalyst for the carbon dioxide reforming of methane

A new pretreatment of catalyst, deposition–removal (D–R) treatment, is proposed to suppress carbonaceous depositions on Ni/Al 2O 3 catalyst for carbon dioxide reforming of methane. This treatment is based on the hypothesis that active cores forming carbon whiskers are different from surface active sites for the main reaction. In the deposition step, active cores of nickel are detached from the bulk nickel surface as growing cores in carbon whiskers. In the removal step, the carbon whiskers are removed from the nickel surface. These two steps are repeated by flowing pure methane gas and then pure carbon dioxide gas. The thermogravimetric (TG) measurements showed that the repeated D–R treatments reduced the carbonaceous deposition. Carbon monoxide adsorption measurements showed that the nickel surface area was decreased by the treatments. The reaction rate and the turnover frequency were increased by the treatments. The increase in the activity was explained by the development of active sites on newly exposed nickel layers which strongly interacted with the support.

Partial Oxidation of Methane on Supported Nickel Catalysts.

The partial oxidation of methane (POM) reaction is conducted using several nickel catalysts which show high activities in carbon dioxide reforming of methane. Temperature programmed reaction and thermogravimetric analysis show that a reduced state of nickel is required to start the POM reaction, and that carbonaceous deposition occurs on alumina with high nickel contents. The temperature profile of the catalyst bed during the POM reaction suggests that this reaction consists of two consecutive reactions; methane combustion, following by reforming reactions (with steam and carbon dioxide). The heat caused from methane combustion keeps the temperature high enough to progress the reforming reactions.To suppress the carbonaceous deposition which causes catalytic deactivation and plugging of the reactor, carbon deposition-removal (D-R) treatment is applied in the POM reaction. On the basis of the hypothesis that active cores forming carbon whiskers are different from surface active sites for the main reaction, this treatment has been proposed to only remove or deactivate the former core nickel. It was concluded that this treatment makes the catalyst inactive for carbonaceous deposition with only a small influence on the main reaction activity.

Deactivation of alkane oxidative dehydrogenation catalyst by deep reduction in periodic flow reactor

The activity loss of β‐NiMoO4 catalyst has been studied in the oxidative dehydrogenation of propane with an operating periodic flow reactor. Under severe operating conditions (i.e., high temperature and long periods of the pulses), the lattice oxygen depletion is faster than the reoxidation one. After an induction period, carbon whiskers on the catalyst surface were detected together with noticeable amounts of cracking products in the gas phase as CH4, C2 hydrocarbons and H2. Deep reduction dramatically changes the reaction path and irreversibly modifies the catalyst.

Optimization of the Dimensions of Vapor‐Grown Carbon Fiber for Use as Negative Electrodes in Lithium‐Ion Rechargeable Cells

Electrochemical performance examinations of a new generation of carbon whisker were carried out in 1 M , in a 50:50 mixture of ethylene carbonate and diethyl carbonate. The whisker has a high reversible capacity (363 mAh/g), and the coulombic efficiency was 100%. This capacity depends strongly on the diameter of the fiber and the preparation method. When the length of graphite whisker is 23 μm, it gives the highest first cycle coulombic efficiency (96%). We investigated the charge‐discharge characteristics with different current densities. When measuring the slow cyclic voltammogram of the graphite whiskers, five cathodic peaks could be seen. The kinetics of electrochemical intercalation of lithium ions into the graphite whisker was studied by impedance spectroscopy. The diffusion coefficient of lithium in the carbon is larger when the fibers were graphitized and then chopped, than when they were chopped first, and it decreased linearly as x in increased. This is related directly to the microstructure of fiber.

Model for prediction of matrix microstructure for carbon/carbon composites prepared by forced flow-thermal gradient CVI

A model has been developed to predict the microstructure of the carbon matrix deposited during forced flow-thermal gradient chemical vapor infiltration (FCVI). This method employs previous thermodynamic-microstructure and densification models to determine conditions throughout the preform as a function of time. The model was verified by comparison with samples prepared over a range of deposition temperatures, reagent concentrations and flow rates. The model also showed that it should be possible to deposit a matrix of uniformly high thermal conductivity onto conventional size carbon fibers, as well as small diameter, low cost, high thermal conductivity carbon whiskers.

A Hydrogen‐Nitric Oxide Cell for the Synthesis of Hydroxylamine

A ‐NO fuel cell was designed for the synthesis of hydroxylamine in the gas phase. The hydroxylamine produced was trapped in an aqueous solution of held in a silica‐wool disk used as an electrolyte barrier for and NO. Among the cathode electrocatalysts tested, iron‐phthalocyanine (Fe‐Pc) impregnated in graphite was the most favorable one for selective synthesis of hydroxylamine. Active carbon and carbon whiskers used to support the Fe‐Pc enhanced the formation of hydroxylamine remarkably. The carbon itself slightly catalyzed the formation of and . Excluding the effects of the support, Fe‐Pc catalyzed the electrochemical synthesis of hydroxylamine with high selectivity (≈100%). Applied voltage across the cell did not appreciably enhance the formation of hydroxylamine. The reaction under short‐circuit conditions was most favorable for the synthesis of hydroxylamine. It is suggested that the reduction of NO occurs on the site of Fe‐Pc with protons and electrons transferred from the anode. The very selective synthesis of hydroxy‐lamine over Fe‐Pc must be ascribed to an site isolated by phthalocyanine ring. This isolation prohibits both the formation of through the intramolecular elimination of from the adjacent NHO intermediates and the formation of and through the breaking of N‒O bonds.

Radical graft polymerization initiated by azo groups introduced onto the surface of carbon whisker

The radical graft polymerization of vinyl monomers onto carbon whisker, i.e. vapor grown carbon fiber, initiated by azo groups introduced onto the surface has been investigated. The introduction of azo groups onto the surface is achieved by (1) the reaction of 4,4'-azobis(4-cyanopentanoic acid) with isocyanate groups on the surface, which are introduced by the treatment of surface carboxyl and phenolic hydroxyl groups with tolylene-2,4-diisocyanate and (2) the reaction of 2,2'-azobis(2-cyano-n-propanol) with acyl chloride groups on the surface, which are introduced by the treatment of surface phenolic hydroxyl groups with terephthaloyl dichloride. The carbon whiskers having azo groups prepared from the above two methods are abbreviated as CW-Azo 1 and 2, respectively. The azo group content of CW-Azo 1 and 2 is determined to be 0.04 and 0.07 mmol/g, respectively. It is found that the radical polymerization of methyl methacrylate and styrene is initiated by the surface azo groups to give the corresponding p...

Diffusivity of carbon in the copper matrix. Influence of alloying

Abstract The heights of diffusion barriers for copper-based composite materials with carbon whiskers or fibers are calculated. It is shown that the alloying of the matrix by an additional element, substituting copper in an interstitial Cu C solid solution, changes the value of the barrier and the migration entropy. This substitutional impurity influences the diffusion of the carbon-copper matrix. Zirconium as the best alloying element for preventing the diffusion of carbon in Cu C alloy is predicted.

Carbon diffusion in copper-based metal matrix composites

The heights of diffusion barriers for copper-based composite materials with carbon whiskers or fibers are calculated. It is shown that the alloying of the matrix by an additional element, substituting copper in an interstitial Cu-C solid solution changes the value of the barrier and the migration entropy. These substitutional impurities significantly influence the diffusivity of carbon in the copper matrix. Zirconium is predicted as the best alloying element for preventing the diffusion of carbon in Cu-C alloy.

Electrical conduction in thermoplastic elastomer matrix composites containing catalytic chemical vapour deposited carbon whisker

Electrical conduction in carbon whisker/thermoplastic elastomer (TPE) composites was found to be a thermally activated process. The carbon whiskers used were obtained by a catalytic chemical vapour deposition (CCVD) technique conducted at 500 °C and the TPE was a styrene-ethylene butylene-styrene (S-EB-S) block copolymer. The resistivity, (ϱ), versus 1/T curves of the composites exhibited two regions with distinct slopes with an inflection at the glass transition temperature,Tg, of the elastomer, EB; Region I Tg. The thermally activated conduction mechanism of these composites is explained on the basis of electron transport in low-mobility solids with a large number of trap sites, Intra- and intermolecular motion of the polymer chains can result in the electron transport from such trap sites and were correlated to the observed activation energies. Intramolecular motion in region I, was related to the thermally assisted hopping with the activation energy, ΔEA, of 0.067 and 0.030 eV for 33% and 52% whisker volume fraction composites, respectively. Similarly, ΔEA due to the intermolecular segmental chain motions in region II for 33% and 52% whisker volume fraction composites was related to the equilibrium rate of trapping-detrapping of electrons from 0.240 and 0.138 eV deep traps.

Electromechanical and electrothermal behaviours of carbon whisker reinforced elastomer composites

The electromechanical and electrothermal properties of conducting carbon whisker reinforced thermoplastic elastomer (TPE) composites were investigated. The carbon whiskers were derived by a catalytic chemical vapour Deposition (CCVD) process and the TPE was a styrene-ethylene-butylene-styrene (S-EB-S) block copolymer. The electrical resistivity (ϱ) of the composites can be varied either by uniaxial deformation (101–108 Ω cm) or by temperature (101–105 Ω cm). The temperature-resistivity studies indicated that the resistivity of these composites was influenced by the glass transition temperature (Tg) of the TPE. The ϱ versus 1/T curves exhibited two distinct regimes each with a different negative slope which intersected at the Tg of the elastomer. This was correlated to the Tgof the EB segments in the S-EB-S block copolymer (∼ -50°C) by the dynamic mechanical thermal analysis. Further, uniaxial deformation studies at room temperature (20 °C) demonstrated that the resistivity increased exponentially with the deformation. Processing technique considerations and electron micrographs of the morphology of the composites indicated the formation of polymeric film on the carbon whiskers. Thus, the electrical conduction between carbon whiskers in these highly loaded (33 and 52 vol % fraction) composites occurred through the elastomeric film by electron tunnelling. This is explained on the basis of Mott's electron hopping theory, for conduction through several carbon-polymer-carbon (C-P-C) junctions. Further studies by scanning electron microscopy, dielectric thermal analysis and voltage-current characteristics confirmed this observation. Mechanical and electrical properties of the composites indicated that CCVD carbon whiskers can be used to improve the strength and electrical conductivity of TPEs. The change in resistivity (up to five orders of magnitude) of the composites with respect to the deformation or temperature can find use in electromechanical and electrothermal device applications.

A modification in the preparation process of a carbon whisker for the anode performance of lithium rechargeable batteries

In general, a carbon whisker is prepared from hydrocarbons using a vapor-grown method and chopped to a suitable length. Two procedures were examined to prepare the whiskers: in the normal procedure (1A), whiskers were graphitized after the chopping process, and in the other process (2A), they were chopped after the graphitization. The carbon whisker of 2 μm in length by the 1A procedure (2GWH-1A) showed an average capacity of 248 mAh g −1. On the other hand, the whisker of the same diameter by the 2A process (2GWH-2A) showed a much higher capacity of 363 mAh g −1: this value is 1.6 times of that of 2GWH-1A. This modification method may open a new probability to higher performance carbon materials for the anode.

Radical graft polymerization from carbon whisker initiated by peroxyester groups introduced onto the surface

The radical graft polymerization of vinyl monomers onto carbon whiskers, i.e. vapor grown carbon fibers, initiated by tert-butyl peroxyester groups that are introduced onto the surface was investigated. The introduction of tert-butyl peroxyester groups onto the carbon whisker surface was achieved by the reaction of acyl chloride groups on the surface with tert-butyl hydroperoxide. The carbon whisker having acyl chloride groups was prepared by the following three methods: (1) reaction of surface carboxyl groups with thionyl chloride, (2) reaction of surface phenolic hydroxyl groups with succinyl dichloride, and (3) reaction of surface phenolic hydroxyl groups with phthaloyl dichloride. The carbon whiskers having tert-butyl peroxyester groups prepared from these three methods were abbreviated as CW-POE 1, 2, and 3, respectively. The peroxyester group content of CW-POE 1, 2, and 3 was determined to be 0.06, 0.05, and 0.17 mmol/g, respectively. It was found that the radical polymerization of vinyl monomers su...

Gas phase oxidation of benzene to phenol ad hydroquinone by using an H 2O 2 fuel cell system

A reaction system for the oxidation of benzene to phenol and hydroquinone in the gas phase over a carbon whisker cathode was designed by using H 2O 2 fuel cell. The rates of the formation of products increased with increasing the partial pressures of benzene and O 2 in the cathode compartment, but the rates were independent of the partial pressure of H 2 in the anode compartment. The optimum rate of formation was obtained at a cathode potential of −0.1 V (vs. Ag|AgCl) which was positive compared with that of −0.2V observed under short circuit conditions. This result suggests to cogenerate phenol and electricity. The addition of Pd to the carbon whisker cathode improved the oxidation of benzene. However, the adition of Fe 2O 3 to the carbon whisker did not affect the oxidation. Synergism of co-addition of Pd black and Fe 2O 3 to the carbon whisker for enhancing the oxidation which had been reported in the liquid phase was not observed in the gas phase. A reasonable model of the reaction mechanism specific for the oxidation of benzene in the gas phase has been proposed.

Simultaneous Epoxidation of 1-Hexene and Hydroxylation of Benzene during Electrolysis of Water

Abstract Epoxidation of 1-hexene and hydroxylation of benzene to phenol and hydroquinone occurred at the same time on the anode and the cathode, respectively, during electrolysis of water at room temperature. The most favorable anode and the cathode for these reactions were Pt black and the carbon whisker added with Pd black and Fe2O3, respectively.

Surface modification of carbon whiskers by oxidation treatment

To modify the surface characters, carbon whiskers were oxidized with carbon dioxide. Adsorption isotherms were determined by using three kinds of adsorbates: nitrogen, n-butane, and trichloroethylene (TCE). Granular activated carbon was used as the standards of adsorbents. The surface chemical structures were analysed by using NaHCO 3, Na 2CO 3, and NaOH. After CO 2 oxidation treatment of carbon whiskers, micropore structures of pore size distribution, having a sharp peak at a radius of 4 × 10 −10 m were developed, but acidities of acidifie functional group were not increased. The oxidized carbon whiskers were better than the granular activated carbon for adsorption of TCE from aqueous solution.

Modification of carbon whisker surface by the grafting of polyacetals: cationic ring-opening polymerization of cyclic acetals initiated by acylium perchlorate groups introduced onto the surface

The acylium perchlorate groups were successfully introduced onto the surface of a carbon whisker, i.e. a vapor-grown carbon fiber, by the reaction of acyl chloride groups on the surface with silver perchlorate in nitrobenzene at room temperature. The cationic ring-opening polymerizations of 1,3,5-trioxane and 1,3-dioxolane were initiated by the acylium perchlorate groups on the surface and the corresponding polyacetals were grafted onto carbon whisker: the percentage of poly(oxymethylene) grafting increased to 60%. The initiating activity increased with an increase in acylium perchlorate group content. Although the grafted poly(oxymethylene) on the surface was subject to stepwise thermal depolymerization from the chain end, the thermal stability was improved by acetylation of the end group. In addition, the acylium perchlorate groups on the surface have an ability to initiate the cationic copolymerization of 1,3,5-trioxane with styrene, styrene oxide, and 1,3-dioxolane. These polyacetal-grafted carbon whi...

Modification of carbon whisker surface by the grafting of polyethers: Cationic ring-opening polymerization of cyclic ethers initiated by acylium perchlorate groups on the surface

To modify the surface of a carbon whisker, i.e. a vapor-grown carbon fiber, the grafting of polyethers onto the surface by the ring-opening polymerization of cyclic ethers initiated by acylium perchlorate groups introduced onto the surface was investigated. The introduction of acylium perchlorate groups onto the carbon whisker was achieved by the reaction of silver perchlorate with surface acyl chloride groups, which were introduced by the treatment of surface carboxyl groups with thionyl chloride. It was found that the surface acylium perchlorate groups are capable of initiating the cationic ring-opening polymerization of cyclic ethers such as tetrahydrofuran (THF), styrene oxide and propylene oxide, and the corresponding polyethers are effectively grafted onto the carbon whisker, based on the propagation of the polymer from the surface. The rate of the polymerization increased with increasing polymerization temperature, but the percentage of polyether grafting decreased, because the chain transfer reaction of the growing polymer cation is accelerated at higher temperature. Poly(THF)-grafted carbon whiskers dispersed stably in a good solvent for poly(THF), such as THF, but precipitated immediately in a poor solvent. Therefore it is considered that grafted poly(THF) chains on the carbon whisker surface extend outside the surface in THF and interfere with the aggregation of carbon whiskers.

Grafting of polyesters from carbon whisker surface: Copolymerization of epoxides with cyclic acid anhydrides initiated by COOK groups introduced onto the surface

AbstractTo modify the surface of carbon whisker (vapor‐grown carbon fiber) the grafting of polyesters by use of potassium carboxylate (COOK) groups introduced onto the surface was investigated. The introduction of COOK groups onto the carbon whisker was achieved by the treatment of surface carboxyl groups with KOH aqueous solution. Untreated carbon whisker has no ability to initiate the polymerization. It was found that the anionic ring‐opening alternating copolymerization of epoxides with cyclic acid anhydrides is successfully initiated by COOK groups on the carbon whisker surface. The corresponding polyester was grafted onto the surface based on the propagation of polymer from COOK groups introduced on the surface. The percentage of grafting of the polyester from styrene oxide and phthalic anhydride was determined to be 91.0%. The polymerization rate and percentage of grafting increased upon addition of crown ether. Furthermore, the rate of polymerization increased with increasing the dielectric constant of the solvent, but the percentage of grafting decreased. Polyester‐grafted carbon whisker was found to give a stable colloidal dispersion in a good solvent for the grafted polymer. © 1993 John Wiley & Sons, Inc.