Yoshihiro Hishiyama

Experimental evidence of a single nano-graphene

A single nano-sized graphene sheet is prepared by a combination of electrophoretic deposition (EPD) and heat-treatment of diamond nano-particles on a highly oriented pyrolytic graphite (HOPG) substrate. Heat-treatment at 1600 °C converts diamond nano-particles to single nano-graphenes, where the mean in-plane size and the inter-layer distance from the substrate are estimated at 10–15 and 0.35–0.37 nm, respectively. The considerably large inter-layer distance compared to bulk graphite suggests a large reduction of inter-layer interaction, although a nano-graphene is placed epitaxially on the substrate. The isolated single nano-graphene provides an important model of nano-sized π-electron system, for which recent theory predicts unconventional electronic structure of edge-inherited non-bonding state.

Graphitization of carbon fibre/ glassy carbon composites

Abstract Carbon fibre/glassy carbon composites were prepared by aligning unidirectionally in furfuryl alcohol condensate the PAN-based carbon fibres treated at different temperatures and with different degrees of stretching. The graphitization of the composites was found to start at the boundary between the fibres and glassy carbon matrix, and to proceed into the matrix. This is considered to be due to the stress accumulation at the boundary caused by a large shrinkage of the matrix. The carbon fibres remain nongraphitized even after a heat-treatment at 2800°C. The composites heat-treated at high temperature (2800°C) are found to show a high overall degree of graphitization, unexpected on basis of the known graphitization behavior of carbon fibres and of glassy carbon, and a high degree of uniaxial preferred orientation of crystallites.

Structural Improvement of Carbon Fibers Prepared from Benzene

Carbon fibers have been prepared by thermal decomposition of benzene at temperatures 1050–1080°C. Structural change with stepwise heat treatments up to 3000°C has been studied by X-ray and selected-area electron diffraction. The as-prepared fiber as well as the 1400°C-treated is basically of turbostratic structure, but has a preferred orientation of aromatic carbon planes more or less parallel to the fiber axis. By heat treatment around 2000°C, the preferred orientation is improved enormously; the carbon planes become almost completely parallel to the fiber axis, while the stacking order is still turbostratic. A three dimensional graphite structure is developed by heat treatment above 2400°C, which has qualitatively a similar behavior to that of graphitizing carbon. The fiber heat-treated at 3000°C consists of graphite layers concentrically surrounding the fiber axis.

Structure and properties of highly crystallized graphite films based on polyimide Kapton

Highly crystallized graphite films were prepared by heat treatment of carbonized polyimide films (Kapton) at temperatures of 2700 and 3050° C. Interlayer spacing d002 at room temperature, and electrical resistivity, magnetoresistance and Hall coefficient at room and liquid nitrogen temperatures were measured. All of these data indicate high crystallinity of the graphitized Kapton films obtained. For the graphite films heat treated at 3050° C mean-square mobilities were estimated from the magnetoresistance data at 1 T to be 0.91 m2 V−1 sec−1 at room temperature and 2.3 m2 V−1 sec−1 at liquid nitrogen temperature; the value at liquid nitrogen temperature corresponds to that for a pyrolytic graphite heat treated at 3200° C for 1 h (PG 3200). Magnetic field dependence of Hall coefficient at liquid nitrogen temperature for this sample also agrees well with that for PG 3200. Scanning electron micrographs on the surfaces show that the present graphite films consist of grains of large crystallites, and grain size increases as the crystallinity of the material improves.

Exfoliated graphite from various intercalation compounds

Five different intercalation compounds (GICs) with H2SO4, FeCl3, Na-tetrahydrofuran (THF), K-THF and Co-THF were prepared from flaky natural graphite powder with the average particle size of 400 μm. The exfoliated graphites were prepared from the GICs by rapid heating up to 1000°C and were observed with a scanning electron microscope. A remarkable difference in morphology was observed between the exfoliated graphites prepared from acceptor- and donor-type GICs; the former consists of large balloons across the original graphite flakes, the latter of small balloons with tiny graphite layers. Small particles of Fe2O3 were found on the surface of the exfoliated graphite prepared from FeCl3-GIC. In the exfoliated graphite from Co-THF-GIC, very tiny particles were detected, which seemed to be metallic cobalt. In those from Na- and K-THF-GICs, no alkali metals were detected even by XPS.

Carbonization and graphitization of polyimide film “Novax”

Abstract Polyimide films “Novax” consisting of the imide part with the bridging part of a random mixture of two different structures, were carbonized at various temperatures up to 1100°C and graphitized at 3000°C. The weight change occurred in two steps, as other polyimide films; large weight decrease in a narrow temperature range between 450° and 600°C and a further decrease above 800°C, The former weight decrease could be explained by the evolution of large amount of CO and CO 2 gases, and the latter corresponded to the evolution of N 2 gas and rapid increase in electrical conductivity up to 200 S/cm. The thinnest film gave a graphite film with a high degree of graphitization, high value of magnetoresistance and very low anisotropy ratios.

The chemical bonding state of nitrogen in kapton-derived carbon film and its effect on the graphitization process

Abstract The chemical bonding state of the nitrogen which remains in the carbon films derived from a polyimide Kapton film was studied by X-ray photoelectron spectroscope and related to the changes in structure and transport properties. The nitrogen remaining after heat treatment up to 2200 °C was in a tertiary bonding state, replacing carbon atoms in hexagonal layers, which was supported by theoretical calculation based on ab-initio molecular orbital theory. The departure of substituted nitrogen atoms from hexagonal layers showed a good correspondence with structure development and changes in transport properties, the presence of nitrogen depressing the developments in structure and transport properties. After the departure of nitrogen above 2300 °C graphitization of the films was very remarkable.

Graphite films prepared from carbonized polyimide films

Abstract Graphitizability of various carbonized polyimide films heat treated at 2950°C was studied by X-ray and magnetoresistance measurements and SEM observation. The films used were commercially available polyimide films, Kapton and Upilex, and two kinds of laboratory-prepared polyimide films. Results obtained show experimental evidence that there are three conditions involved in obtaining highly graphitizable carbon films; 1. (1) flatness of original organic molecules, 2. (2) high degree of their orientation and 3. (3) simple release of non-carbon atoms during carbonization.

Carbonization and graphitization of BPDA/PDA polyimide films: effect of structure of polyimide precursor

Poly(amide acid) was prepared from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and p-phenylenediamine (PDA) and was then reacted with NaH and various kinds of alkyl iodides to transform it into its alkyl esters. The cast films were imidized as fixed on glass substrates to give polyimide films. The tensile modulus of polyimide films prepared from the methyl ester precursor was lower than that from poly(amide acid), but it increased with increasing size of the alkyl group. The polyimide films were carbonized by heating to 900°C, and then further heated to 2800°C for graphitization. Their degrees of graphitization and orientation of graphite crystallites as a function of weight loss at imidization were studied by measurements of X-ray diffraction at room temperature and magnetoresistance at liquid nitrogen temperature. Both measurements clearly indicate that the graphitized films prepared from poly(amide ester) have high degrees of graphitization. It was also made clear that the orientation of the graphitized films prepared from poly(amide ester) with small alkyl groups is lower than that from poly(amide acid), but the orientation increased with increase in the size of the alkyl group.

Microtexture and magnetoresistance of glass-like carbons

Abstract Microtextures of glass-like carbons commercially available were investigated through scanning electron microscopy and measurement of their magnetoresistance. A field-emission electron gun type SEM, which is characterized by a specimen chamber equipped in its objective lens, was operated with the acceleration voltage of 2 kV and high fidelity observations were thereby achieved with a resolving power of 2 nm. All the specimens exhibit granular microtextures with the mean grain diameter spanning 6–13 nm. The mean grain diameter correlates to the mean crystallite size La(110) determined by the X-ray powder diffractometry. The density of each sample calculated by using the mean diameter and the X-ray parameters reproduces well its catalog value. Negative magnetoresistance was observed just for the specimens whose average grain sizes are not smaller than 7.3 nm. The crystallite size La(110) of the specimen whose grain size shows the minimum value for appearance of negative magnetoresistance is estimated to be 2.7 nm. The absolute value of negative magnetoresistance under a constant magnetic field increases with increasing the average grain size.