Abstract
Bulk bismuth has moderate thermoelectric properties. The reduction of its dimensionality (for example Bi nanowires) can improve its thermoelectric properties, in other words, this reduction can improve the Seebeck coefficient, and decrease the electrical resistivity and thermal conductivity. These nanowires could be used for Peltier refrigeration. Micro- and mesoporous silica? have channels that allow to confine Bi atoms and thus to create nano-sized Bi wires inside. Molten Bi at high pressure was compressed under high pressure in order to insert it into the pores. High pressure and high temperature are generated by a diamond anvil cell (DAC) and a CONAC28 large volume press. The lattice dynamics and the stability of the different phases of Bi were first studied. The experimental results are consistent with those calculated and the Raman spectrum of Bi III was observed for the first time. The Bi/silica micro- and meso-porous composite samples were characterized by XRD, Raman, SEM, TEM, and NPD. Some characterization techniques were suitable to confirm the synthesis. In the case of Bi/MFI, Bi chains of 6Å were observed with TEM. The average number of Bi atoms per cell (14 Bi) was determined by the Rietveld refinement using the calculated model (24 Bi/cell). DFT calculations have been performed for Bi/MFI nanocomposites to predict their physical properties. They show that the composites with 14 Bi would have a gap energy between 0.4eV and 1.69eV. In the case of Bi/nanotubes, only TEM could be used to characterize the nanotubes filled with 4nm diameter Bi nanowires prepared in the DAC or Bi rods of diameter around 10nm produced in CONAC28. In addition, the nanotubes in the Bi/nanotube composite transformed into crystalline quartz. Physical measurements were also performed in order to check if the thermoelectric properties are improved. Using the Van der Pauw method, an increase of electrical resistivity was observed (10 times and 3.3 times higher than in bulk Bi). However, the Seebeck coefficient measured has the same order of magnitude for both bulk Bi and the composites. Therefore, the measurements of isolated Bi nanowires will be necessary.