Takuro Tsuzuku

Thermoelectric power of single-crystal graphite at low temperatures

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...

Temperature Dependence of Galvanomagnetic Properties of Graphite between 4.2 K and 298 K

By means of a phase-sensitive detection technique, the basal-plane galvanomagneticp roperties of highly crystalline graphite have precisely been investigated as functions of temperature ( T ) down to 4.2 K and of magnetic field intensity ( H ) up to 6.1 kOe. The average mobility of majority carriers has been found to vary in proportion to T -1.6 in the range 40 K to 80 K and to T -1.2 above it, while a saturating trend due to the defect scattering comes about below it. The Hall coefficient is highly dependent on H at T <200 K, the field dependence being nearly in agreement with Soules observation at 77 K and 4.2 K. The zero-field Hall coefficient behaves in a fashion consistent with Ono-Sugiharas theory over the most part of temperature range examined, but deviates remarkably at T <30 K. The magnetoconductivity tensors are determined as functions of T and H ; both diagonal and off-diagonal components depend considerably on the sample quality especially at T <100 K. Discussions are given with reference t...

Anisotropic electrical conduction in relation to the stacking disorder in graphite

Abstract The in-plane and c-axis conduction behaviors of kish graphite and of hot-worked pyrolytic graphite are discussed in relation to their structural perfection, special interest being focused onto the stacking fault disorder which appears in the form of extended basal dislocation ribbons. Analysis of the two-dimensional magneto-conductivity indicates that the carrier density of faulted specimens increases slowly with temperature (T) even below the degeneracy point of the carrier system, whereas the unfaulted ones do not. The c-axis resistivity (ϱc) has been found to decrease with diminishing stacking disorder for a well-defined specimen group not containing such irregularities as microcracks. This verifies the applicability of the band model to the intrinsic ϱcs, in connection with the success of Onos theory accounting for the wide-range scattering of past data. The discrepancy still remaining between the theoretical and experimental ϱc vs T relationships; as well as the increase of the in-plane conduction carriers density with temperature, seems to be removed by assuming thermal liberation of the localized Tamm-state electrons from the stacking fault planes.

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

Abstract 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.

Thermomagnetic effects in graphite

Abstract Anomalous behaviors of the thermoelectric power with magnetic field and the Nernst-Ettingshausen effect in single crystal graphite, are explained by solving the coupled Boltzmann equation for carriers and phonons. The Fermi surface is approximated by the ellipsoid model instead of the Slonczewski-Weiss model. Anomalies of the thermopower and the Nernst-Ettingshausen coefficient are mainly attributed to the phonon drag component which is strongly enhanced in the presence of a magnetic field. The in-plane vibration plays a dominant role in the phonon drag effect, while the contribution from the out-of-plane mode is negligible because the velocity along the basal plane is very small. The thermoelectric power depends on the carrier concentration difference Ne — Nh, where Ne and Nh denote the electron and hole density, respectively. Therefore, the observed values of the thermopower vary from sample to sample.

Annealing study of electron transport in slightly neutron-irradiated graphite.

A study has been made on the recovery of transport properties of synthetic single-crystal graphite irradiated with fast neutrons to a total dose of 3×1036nvt. Isochronal annealing was conducted stepwise between 100°C and 1600°C. An analysis based on the rigid two-band model gives a consistent account of the diffusion thermoelectricity and the galvanomagnetic effect. In addition to the major normal recovery between 200°C and 300°C, a reverse recovery in the carrier constitution is found to take place in the annealing range from 300°C to 350°C. This seems explicable by considering that divacancies are intermediately transformed to single vacancies during their annihilation process. The phonon-drag thermoelectric power vs. acceptor concentration relationship implies that positive holes are more strongly scattered than electrons by radiation-induced vacancies negatively charged by accepting electrons within the basal plane.

Electrical Conduction in C-Direction of Highly Crystalline Graphites in Relation to Structural Perfection

Electrical resistivity in c-direction (ρ c ) as a function of temperature ( T ) has been investigated of a number of highly crystalline specimens of kish and pyrolytic graphites in relation to their structural perfection evaluated by the basal-plane resistivity ratio between 300 K and 4.2 K ( R T =ρ a (300)/ρ a (4.2)). The ρ c vs R T plot indicates a \(\varLambda\)-shaped curve, and the ρ c vs T relationship becomes gradually metallic-like as R T increases. For pyrolytic graphite, the thinner the specimen the higher the perfection in the range 0.016 to 0.73 mm. Based on these observations, the intrinsic c -axis conduction is concluded to be described within the framework of band theory, while it is impeded by laminar defects and short-circuited sometimes by misaligned basal-planes especially in the pyrolytic specimens. Comparison is made with the theory of Ono who has taken into account the scattering of carriers by stacking faults.

A Simple Two-Band Theory of Galvanomagnetic Effects in Graphite in Relation to the Magnetic Field Azimuth

A simple two-band theory of galvanomagnetic effects in graphite has been developed so as to involve not only the magnetic field intensity (H) but its azimuth (θ) with respect to the c-axis, which successfully accounts for the complicated behavior of transverse magnetoresistance (Δρ/ρ0) in an iron-melt graphite previously found by the authors. The theory verifies the relation νξ when Δρ/ρ0 is expressed in the conventional form Δρ/ρ0∝Hν cosξ θ, and predicts that the more perfect the compensation and/or the lower the mobility of carriers becomes, the closer the magnetoresistance conforms to the H2-law, while a considerable deviation is observed in the intermediate ranges. Discussions are also given on the contributions of minority carriers.

Magneto-seebeck effect in graphite: Giant thermoelectric power due to phonon drag

Abstract The in-plane thermoelectric power () of single-crystal graphite in the presence of a magnetic field applied parallel to the c direction was measured at temperatures (T) from 4.2°K up to 300°K. With an increase of the field intensity (H), the sharp negative dip of the  versus T diagram previously found to be (0) = −11.5μv/°K at 35°K exhibited a slight shift of position to near 30°K and a remarkable growth of depth to (H) ⋍ −80μv/°K for H = 1.7 kG and (H) ⋍ − 2000 μv/°K for H = 8.0 kG. In accordance with Sugiharas theory on the phonon drag effect in semimetals, these anomalies have been ascribed to the strong interaction between carriers and phonons in graphite.

Conduction Electron Spin Resonance of Graphite

The conduction electron spin resonance of graphite has been investigated, using well defined near-ideal crystals and rather two-dimensional pyrolytic graphite specimens at temperatures between 100 K and 300 K. The effects of slight neutron irradiation on the former and of boronation on the latter were also examined. Experimental results were analyzed in accordance with the procedure developed by Feher and Kip. The g -shift data are compared with the band-theoretical calculations by McClure-Yafet and McClure; a decreasing trend with the depression of Fermi level as well as its temperature dependence is noted to be in connection with the spin-orbit splitting of the Landau level at the band degeneracy. The relaxation time is found to be a monotonously increasing function of temperature in qualitative agreement with the Elliott mechanism.