Power factor enhancement in a magnetic field using polycrystalline bismuth microwire arrays

Measurements of the magneto-Seebeck coefficient and magnetoresistivity of two bismuth microwire arrays having different diameters and a bulk sample were performed for applied magnetic fields between 0 and 2 T and temperatures between 50 and 300 K. The magneto-Seebeck coefficients were always enhanced as a result of controlling the phonon scattering process and the magnetoresistivity also invariably increased. The power factors in an applied magnetic field were estimated, and the power factors of the microwire arrays were found to be enhanced over the whole measurement range. By contrast, the power factor of the bulk sample was not enhanced at temperatures under 200 K. The difference in the responses of the bulk sample and the microwire arrays is strongly related to suppression of the magnetoresistivity; the suppression ratios of the magnetoresistivity of the bulk sample, which has a low aspect ratio, are smaller than those of the microwire arrays. It was experimentally demonstrated that there is an optimum magnetic field that maximizes the power factor and that the enhancement ratio of the power factor is much greater for structures that have a high aspect ratio, such as microwire arrays. This suggests that microwire arrays are suitable for enhancing the power factor by applying a magnetic field.