Simple rules for the understanding of Heusler compounds

The demand for graphites with specific properties and for an improvement in the economics of graphite production in recent years has led to an increased interest in catalytic graphitisation. At the present time, however, there is no review available of published studies and this paper attempts to make good this deficiency. Mechanisms whereby inorganic additives can catalyse graphitisation are considered, followed by a discussion of the mechanisms by which inorganic additives promote the graphitisation process. A comprehensive survey of the relevant literature to date is presented which describes in more detail the type of carbon used, the experimental techniques employed and the improvement obtained in graphite quality.

Catalysis of graphitisation

Influence of alkali on the carbonization process—I: Carbonization of 3,5-dimethylphenol-formaldehyde resin with NaOH

In the semicontinuum model of lattice vibrations in graphite proposed by Komatsu and Nagamiya, the expressions of the dispersion relation contain the elastic constants ${C}_{11},$ ${C}_{12},$ ${C}_{13},$ ${C}_{33},$ ${C}_{44},$ and \ensuremath{\kappa} and the interlayer spacing c as parameters, where $c\ensuremath{\rho}{\ensuremath{\kappa}}^{2}$ is the bending elastic constant of a graphite layer and \ensuremath{\rho} is the density. We improve this semicontinuum model by taking these parameters as a function of temperature. For the parameters except \ensuremath{\kappa}, we use the experimental data already known and the relations derived from them. \ensuremath{\kappa} is determined by fitting the calculated specific heat to the experimental one. The experimental specific heat for single-crystal graphite is derived by evaluating the data reported in the literature. With the value of \ensuremath{\kappa} thus determined, the improved semicontinuum model can explain the experimental specific heat well in the temperature range below 350 K. Then, \ensuremath{\kappa} decreases more rapidly with increasing temperature than the other elastic constants; this result means that softening of the out-of-plane vibrations occurs. It is suggested that the softening is closely related to the negative thermal expansion parallel to the layer planes. The calculated Gr\"uneisen constants are compared with the experimental ones. The second derivative of the specific heat curve with respect to temperature gives information on the frequency distribution function of lattice vibrations. From the analysis of the low-temperature specific heat, the value of ${C}_{44}$ at room temperature is determined to be $0.425\ifmmode\times\else\texttimes\fi{}{10}^{11}{\mathrm{d}\mathrm{y}\mathrm{n}/\mathrm{c}\mathrm{m}}^{2}.$

Temperature dependence of lattice vibrations and analysis of the specific heat of graphite

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.

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

The thermoelectric power (TEP) along the basal plane of a synthetic single-crystal graphite has been measured in the temperature range from 300°K down to 7°K. The TEP versus temperature diagram exh...

Thermoelectric power of single-crystal graphite at low temperatures

Electrical resistance, magnetoresistance and magnetization were measured for Heusler-type alloys, Fe 2+ x V 1- x Ga. An anomalous giant magnetoresistance (GMR) near 50% was observed for Fe 2 VGa which was a weak and inhomogeneous ferromagnet. For alloys with x >0, which show clear ferromagnetism, the electrical resistance showed an anomaly below the Curie temperature. For alloys with x <0, the temperature dependence of the resistance indicated behavior like that of a semiconductor. The energy gap was tentatively estimated to be 0.03 eV for x =-0.10. The origin of the present GMR is discussed in terms of the metal-insulator transition arising from ordering of the magnetic moment.

Anomalous giant magnetoresistance in Heusler-type alloys Fe2+xV1-xGa

The low-temperature thermoelectricity has been studied extensively for single-crystal graphite and highly oriented pyrolytic graphites (PG) annealed at 3600°c, 3200°c and 2800°c respectively. In the thermoelectric power versus temperature diagram for these specimens, the characteristic phonondrag dip is found to deepen stepwise in the order of structural imperfection as follows : - 11.5 μv/°k for single crystal, - 29.5 μv/°k for 3600°c-annealed PG and - 39 μv/°k for 3200°c-annealed PG. For the 2800°c-annealed, however, the dip becomes as shallow as - 8.5 μv/°k again. The deepening is appropriately ascribed to the perturbation of band structure which takes place so as to bring about relative increase of conduction electrons with the increase of imperfection, whereas it is ultimately cancelled by the decline of the phonon-drag itself. A phenomenological analysis made of galvano-magnetic properties of the same specimens verifies such an interpretation.

A. Ono

Papers

Thermoelectric power of single-crystal graphite at low temperatures

Anomalous giant magnetoresistance in Heusler-type alloys Fe2+xV1-xGa

The phonon-drag thermoelectricity of graphite in relation to structural imperfection

Variable-range hopping conduction and negative magnetoresistance of disordered carbons at low temperature

Thermomagnetic effects in graphite