Michio Inagaki

Raman spectra of ground natural graphite

Abstract Structural changes in Ceylon natural graphite with grinding were studied by Raman spectroscopy along with X-ray diffraction. The natural graphite shows a single Raman band at 1580 cm−1, but the ground graphite samples exhibit two Raman bands at 1360 and 1620 cm−1 in addition to the 1580 cm−1 graphite band. The 1360 cm−1 band increases in intensity with increasing grinding time, and becomes much stronger than the 1580 cm−1 band after 200-hr grinding. Raman results are compared with structural parameters such as effective Debye parameter and C0 spacing obtained from X-ray diffraction measurements, and discussed in terms of structural defects introduced into the crystal lattice of natural graphite. A linear relationship was obtained for the ground graphite when the relative intensity of the 1360 cm−1 band is plotted as a function of effective Debye parameter. The slope of the linear plot is different for the ground graphite from that for the graphitized cokes, indicating a difference in the type of structural defects involved.

Applications of graphite intercalation compounds

The properties of graphite intercalation compounds (GICs) are discussed with respect to possible applications. Five families of intercalates give high electrical conductivity to GICs: pentafluorides leading to high conductivity, 10 8 S/m (higher than metallic copper); metal chlorides; fluorine and alkali metals with bismuth giving relatively high conductivity of the order of 10 7 S/m plus stability in air; and residual halogens showing extremely high stability under severe conditions, though the conductivity is only of the order of 10 6 S/m. Electrodes of different GICs have been tried in primary and secondary batteries, where their characteristics are high electrical conductivity and easy diffusion of electrochemically active species between the graphite layers. Primary lithium batteries of a covalent graphite fluoride are now widely used commercially. Secondary batteries using different host graphites and intercalates give interesting results. Large amounts of hydrogen can be stored in the functional space in alkali metal-GICs. The same GICs show high coefficients of isotope separation of hydrogen at liquid nitrogen temperature. The structure and texture of the host graphite play a decisive role in the absorption and separation behaviors of GICs. Exfoliated graphite prepared by rapid heating of GICs or their residue compounds leads to flexible graphite sheets which have great industrial applications. Some problems connected with the production and use of these sheets are discussed.

Determination of cation distribution in spinels by X-ray diffraction method

Abstract A method for the determination of the cation distribution in spinels by X-ray powder diffraction technique was modified so as to take into consideration the effective temperature factor for integrated diffraction intensities of the sample. The optimum values of the oxygen parameter u and degree of inversion x for the spinel sample were determined as the ones gave the best linearity of the relation between log ( I obs I calc ) and sin 2 θ λ 2 . Eight diffraction lines were used, five of which were attributed to the cations in tetrahedral site and other three to all the cations in the spinel. The modified method was applied to three samples GeCo2O4, CoAl2O4 and NiAl2O4.

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.

Charge‐Discharge Characteristics of Mesophase‐Pitch‐Based Carbon Fibers for Lithium Cells

Mesophase-pitch-based carbon fibers were heat-treated at high temperatures (2,600 or 2,800 C) and examined as anodes for lithium secondary batteries. Four types of carbon fibers were used whose cross-sectional views are: a radial texture with wedge (type A), a radial texture with fine zigzag layers (type B), a double texture (type C), and a concentric texture (type D). Lithium could not be deintercalated after the first lithium intercalation in the type A carbon fiber. The structure of the type A fiber was destroyed during lithium intercalation. The other three types of carbon fibers showed good rechargeability on the first cycle, but demonstrated different behavior after 30 cycles. The highest lithium intercalation and deintercalation capacity was observed for the radially oriented carbon fiber (type B). The x-ray results showed a reversible change in the lattice along the c-axis during the intercalation and deintercalation cycle.

Glass-like carbons

Abstract The preparation, structure, properties and some applications of glass-like carbons are outlined by reviewing the published studies made on these materials.

Characterization by electron microscopy of carbon phases (intermediate turbostratic phase and graphite) in hard carbons when heat-treated under pressure

As hard carbons are heat-treated under pressure (1 h, 5 kbar) an intermediate thermally stable carbon phase (d00.2=3.39Å, turbostratic) appears above 1100° C. Its proportion increases up to 1600 or 1700° C depending in the initial sample (saccharose or glassycarbon). The morphology of this new phase has been determined by HREM to be similar to crumpled sets of paper sheets, it is thus highly porous. When heat-treated under pressure above 1600 or 1700° C, this phase is suddenly transformed into graphite, i.e. the porous texture flattens into lamellae.

Potential change with intercalation of sulfuric acid into graphite by chemical oxidation

Abstract Potential of graphite during the intercalation of sulfuric acid by chemical oxidation with either nitric acid or potassium permanganate in H2SO4 of 18 mol/dm3 was found to increase to positive direction in stepwise. On potential change, a distinct plateau corresponding to the transformation from stage two to one compound and a faint one for the formation of stage three were observed, exactly the same as those observed during galvanostatic oxidation. The intercalation of sulfuric acid into graphite by chemical and electrochemical oxidations was shown to be fundamentally the same processes; potential of graphite increases with oxidation and the intercalation of sulfuric acid proceeds by showing potential plateaux. On the change of potential of graphite by chemical oxidation, there was found to be an upper limit of saturated potential, depending strongly on the oxidizer used and also on the concentration of sulfuric acid even if enough amounts of oxidizer was used. The relation between the threshold potential for stage transformation in H2SO4-GICs, increasing with dilution of sulfuric acid, and the upper limit of saturated potential by chemical oxidation, decreasing with dilution of the acid, governed the intercalation reaction by chemical oxidation in sulfuric acid with various concentrations.

On the formation of solid solution in Co3−xMnxO4 system

The formation of solid solution in the Co3−xMnxO4 system in atmospheres of oxygen, air and argon was examined at a constant temperature of 1000 °C. In oxygen, a small amount of the NaCl-type compound was found to co-exist with the cubic spinel in the composition rangex≤0.1. A single phase of the cubic spinel was found in the range 0.1 to 1.3 and the tetragonal spinel above 1.9. In the rangex=1.3 to 1.9 where the cubic and tetragonal spinels co-exist they both have very broadened diffraction line profiles. In air, the identified phases and the changes in their lattice constants with composition were very similar to those in oxygen, except that the NaCl-type compound and the cubic spinel co-existed over a larger range. In argon, the cubic spinel was not observed over any of the composition range and the NaCI-type compound and the tetragonal spinel co-existed in the wide range of 1.1 to 2.3. The experimental results are discussed with regard to the cation distribution in the spinel and also to the relative stability of Co3+- and Mn3+-ions under the low partial pressure of oxygen.

Fracture toughness and fiber bridging of carbon fiber reinforced carbon composites

Abstract A fracture mechanics study is reported for two different types of carbon fiber reinforced carbon matrix composites, a short-fiber felt composite and a satin weave laminated-composite. It is emphasized prior to discussing their fracture mechanisms that the first matrix cracking is the most important fracture process in these C/C composites. Fiber pull-out and bridging processes in the wake of the propagating crack tip are discussed in relation to experimental R curves. The bridging tractions are estimated by the Dugdale model, from which the fiber bridging-enhanced toughening in lamina composites is demonstrated.