Mechanochemical preparation and characterization of nanocrystalline ceramic composites

We produced heterogeneous mixtures of nanocrystalline ceramic materials. An ionic conductor is mixed with an insulator resulting in a heterogeneous material consisting of conducting grains, insulating grains, interfacial regions between equal grains and interfacial regions between different grains. The system studied is (1 - x)Li2O:xBn(2)O(3) with x = 0...1 and it serves as a model system for new materials which could be used as, e.g., solid state electrolytes. The lithium diffusion in this material is being studied with impedance and NMR spectroscopy. The single nanocrystalline components Li2O and B2O3 Were produced in a high energy ball mill (SPEX 8000) by milling the polycrystalline source materials (grain size approximate to 10 mu m). The milling time was varied between 1 and 16 hours which resulted in an average grain size between 40 nm and 20 nm. All preparation steps were performed under argon atmosphere because Li2O is unstable in air. The average grain size was determined by the line broadening of x-ray diffraction patterns. Using the method of Warren and Averbach we were able to determine also the variance of the distribution of grain sizes. The appropriate amounts of the components were then mixed together, milled again for 15 minutes and then pressed to pellets. Density measurements showed that the density of the nanocrystalline samples was reduced by 15% compared to the polycrystalline material. For comparison we also prepared polycrystalline composites. XRD and DSC measurements were used to determine the thermal stability of the samples.