Effect of calcining temperature on structural and dielectric properties of Li0.30Cr0.02Ni0.68O ceramics

Li0.30Cr0.02Ni0.68O (abbreviated as LCNO) ceramics were prepared by sol-gel process and calcined at various temperatures of 800, 900, 1000, 1100 and 1200 °C for 3 h, followed by sintering at 1300 °C for 5 h. Phase composition and microstructure of the sintered pellet samples were characterized using X-ray diffraction analysis, scanning electron microscopy and transmission electron microscopy, and dielectric properties were also investigated. All LCNO pellet samples showed the single phase of cubic rock-salt NiO structure and their crystallite sizes in nanoscale (∼31-48 nm) were obtained. The microstructure of surfaces and fracture surfaces of the sintered pellets calcined at 800-1000 °C exhibited dense, uniformly distributed and polyhedral grains with grain size of ∼60-120 μm. Some pores and shapeless grains with the smaller size of about 10-60 μm were observed in the sintered pellets calcined at above 1000 °C. All LCNO samples showed giant dielectric constants (∼104-105) at room temperature and over the measured frequency that might be attributed to the boundary layer capacitors (BLCs) mechanisms. The dielectric constant increased with an increase of calcining temperature from 800 °C to 1000 °C, corresponding to denser and larger grains. For the sintered samples calcined at above 1000 °C, the dielectric constant decreased probably due to the presence of some pores and shapeless grains. The sintered sample calcined at 1000 °C had the highest dielectric constant (4.54 × 105 at 1 kHz) at room temperature.