Thermo magnetic effects in pyrolytic graphite

Abstract

In this thesis some of the transport coefficients were investigated in pyrolytlc graphites of varying degrees of perfection. The degree of perfection was assessed by comparing the Hall Effect and magnetoresistance with single crystal data. Of the two remaining galvanomagnetic co efficients the Nernst effect (the Peltier effect in a magnetic field) was not measured and the Ettingshausen effect could not be detected. Of the thermomagnetic coefficients the thermal conductivity was not measured and the Righi-Leduc effect could not be detected. The longitudinal and transverse Nernst-Ettingshausen effects were measured over a range of temperature and magnetic induction. The theory of the thermomagnetic coefficients for semi- metals was developed and a computer used to calculate the two latter coefficients for a range of band overlaps and gaps and for various values of the Fermi level, and of the ratio of the mobilities and the densities, assuming acoustic mode lattice scattering. The experimental results on the transverse Nernst- Ettingshausen effect did not agree in magnitude or in its variation with magnetic induction with the theoretical results assuming an overlap of 0.03 eV. The predicted temperature variation was found to be approximately true. An analysis of this coefficient for the case of energy independent scattering is given. This analysis gives results which agree to within 10% in magnitude with the experimental data, and in addition, gives the magnetic induction variation which was found. The longitudinal Nernst-Ettingshausen effect was found theoretically to be very sensitive to the position of the Fermi level, the ratio of mobilities and the ratio of the densities of the carriers. The experimental results, however, exhibited an 'Umkehr' effect, indicating that the computer programme used too simple a band picture. The mean coefficient indicated that the Fermi level was near the centre of the overlap, and that the ratio of the densities of the carriers was nearly one. An investigation of the variation of the properties of the graphite throughout the thickness of a deposit indicated that the best samples had been annealed during the growing process at just over 3000ºC and that there appears to be a critical temperature around 2600ºC for the annealing process. Since some of the properties vary by as much as an order of magnitude across the deposit, care must be taken in specifying exactly where a sample originates in the deposit.