Bismuth-ceramic Nanocomposites through Ball Milling and Liquid Crystal Synthetic Methods by Timothy

Three methods were developed for the synthesis of bismuth-ceramic nanocomposites, which are of interest due to possible use as thermoelectric materials. In the first synthetic method, high energy ball milling of bismuth metal with either MgO or SiO 2 was found to produce nanostructured bismuth dispersed on a ceramic material. The morphology of the resulting bismuth depended on its wetting behavior with respect to the ceramic: the metal wet the MgO, but did not wet on the SiO 2. Differential Scanning Calorimetry (DSC) measurements on these composites revealed unusual thermal stability, with nanostructure being retained after multiple cycles of heating and cooling through the melting point of the bismuth metal. The second bismuth-ceramic synthesis methodology was based on the use of lyotropic liquid crystals. These mixtures of water and amphiphilic molecules self-assemble to form periodic structures with nanometer-scale hydrophilic and hydrophobic domains. A novel shear mixing methodology was developed for bringing together reactants which were added to the liquid crystals as dissolved salts. The liquid crystals served to mediate synthesis by acting as nanoreactors to confine chemical reactions within the nanoscale hydrophilic domains of the mesophase, and resulting in the production of nanoparticles. By synthesizing lead sulfide (PbS) and bismuth (Bi) particles as proof-of-concept, it was shown that nanoparticle size could be controlled by controlling the dimensionality of the nanoreactors through control of the liquid crystalline phase. Particle size was shown to decrease upon going from three-dimensionally percolating nanoreactors, to two dimensional sheet-like nanoreactors, to one dimensional rod-like nanoreactors. Additionally, particle size could be controlled by varying the iv precursor salt concentration. It was demonstrated that for particle size control, chemistries supporting the partial dissolution of the growing particles was required. Since the nanoparticles did not agglomerate in the liquid crystal immediately after synthesis, bismuth-ceramic nanocomposites could be prepared by synthesizing Bi nanoparticles and mixing in SiO 2 particles which were prepared separately. The liquid crystal environment was advantageous in this regard, as difficulties in finding a common solvent for the two types of nanoparticles were avoided. PbS nanoparticles were characterized using UV-Vis-NIR absorption spectroscopy. An additional synthetic method based on the infusion of ammonia gas to drive pH driven chemistries was also demonstrated for the synthesis of BiOCl and Bi nanoparticles. It is thought that the ability of the ammonium ion to form an uncharged NH 3 molecule allows it to cross the hydrophobic regions of the liquid crystals, …