Bismuth-Ceramic Nanocomposites with Unusual Thermal Stability via High-Energy Ball Milling

Electrically conducting nanocomposites of bismuth metal and insulating ceramic phases of SiO2 and MgO were generated via high‐energy ball milling for 24 h using zirconia milling media. The resulting nanocomposites contain Bi nanoparticles with sizes down to 5 nm in diameter. The morphology is a strong function of the oxide phase: specifically, the Bi appears to wet MgO while it forms spherical nanoparticles on the SiO2. X‐ray diffraction measurements indicate a nominal bismuth grain size of 50 nm, and peak fitting to a simple bidisperse model yields a mixture of approximately 57 % bulk bismuth and 43 % 27 nm diameter crystallites. Nanoparticles as small as 5 nm are observed in transmission electron microscopy (TEM), but may not constitute a significant volume fraction of the sample. Differential scanning calorimetry reveals dramatic broadening in the temperatures over which melting and freezing occur and a surprising persistence of nanostructure after thermal cycling above the melting point of the Bi phase. When created via high‐energy ball milling, bismuth–silica nanocomposites show unusual thermal stability. The Figure shows a micrograph of a bismuth–silica nanocomposite after cycling four times to a temperature 75 °C above the bulk melting point of bulk bismuth. Even after this thermal treatment, the Bi nanoparticles do not sinter into a bulk structure. Such powders, prepared by ball‐milling, are ∼ 50 vol.‐% bismuth and are electrically conductive.