Enhancement of high dielectric constant in   CaCu$_3$Ti$_4$O$_1$$_2$/RuO$_2$ composites in the vicinity of the percolation   threshold
  We observe the large enhancement in the dielectric response near the percolation threshold in a composite nanoparticle system consisting of metallic RuO$_2$ grains embedded into dielectric CaCu$_3$Ti$_4$O$_1$$_2$ (CCTO) matrix and annealed at 1100OC. To understand the nature of the dielectric response, we compare CCTO fabricated by two different techniques, solid state process (CCTO$_S$$_S$) and sol-gel process (CCTO$_S$$_G$) with the intrinsic dielectric constant in both cases found to be on the order of 10$^3$-10$^4$ at 10 kHz. For RuO$_2$/CCTO$_S$$_S$ and RuO$_2$/CCTO$_S$$_G$ composites and composites, an increase of the dielectric constant by factors of 7 and 5 respectively is observed in the vicinity of the percolation threshold of about 0.1, with moderate losses at room temperature. Scanning electron microscopy and Energy Dispersive X-ray spectroscopy indicate that the difference in the size of the effect may arise from the microstructure of the copper oxide enriched grain boundaries in the host CCTO. 
Rupam Mukherjee, Gavin Lawes, and Boris Nadgorny
cond-mat.mtrl-sci