Densification effects on the electrical behavior of uniaxially compacted bismuth nanowires

Densification of bismuth nanowires following uniaxial compression was employed to form nanostructured bulk pellets. The bismuth nanowires were first synthesized and purified using a modified template-based vapor deposition technique, resulting in nanowire diameters of either 20 or 250 nm, depending on the type of template employed. Characteristics of the compression process were studied with regards to material properties such as electrical conductivity and porosity. To understand the material deformation, pellets were compressed at 100, 500 and 3000 MPa. In comparison, similarly formed microparticle pellets were also examined. The porosity of the pellets was as low as 10% for the microparticle-based samples but at least 30% for the nanostructured materials. This discrepancy was attributed to the difference in mechanical yield strength, shape and aspect ratio of the particles. Hall coefficient measurements revealed that electrons were the dominant carriers with similar concentrations (∼1020 cm−3) across the samples. Electrical conductivity of the pellets was found to be a function of compression level, but the measurements yielded lower values compared to bulk properties. Subsequent analyses revealed that factors such as porosity, particle surface oxide breaching due to deformation and particle-particle interfacial resistance contribute to the observed behavior. Moreover, the nanowire-based pellets exhibit anisotropy in electrical conductivity due largely to the fact that the nanowires align preferentially along a plane perpendicular to the compression process.