CaCu3Ti4O12 ceramic with a giant dielectric constant was synthesized by sol-gel method and sintered in three different sintering conditions： 1 035 ℃ for 48 h, 1 080 ℃ for 3 h and 48 h. The phase of the ceramics, the element distribution, the valance state of Ti ions at grain boundaries, and the electrical properties were characterized via X-ray diffraction（XRD）, energy dispersive X-ray analysis（EDAX）, X-ray photoelectron spectroscopy（XPS）, electrical conduction and dielectric measurement. The results demonstrate that the grain-boundary microstructure and the electrical properties are influenced by sintering conditions： 1 By raising sintering temperature, the Cu-rich and Ti-poor grain boundary was formed and grain resistivity was decreased. 2 By prolonging sintering time, the content of Ti3＋ near the grain boundary increased, leading to the decrease of the grain-boundary resistivity and the increase of the activation energy at grain boundary. The ceramic, sintering at 1 080 ℃ for 48 h, exhibited a small grain resistivity（60.5 ＊cm）, a large grain-boundary activation energy（0.42 e V）, and a significantly enhanced dielectric constant（close to 1×105 at a low frequency of 1×103 Hz）. The results of electrical properties accord with the internal boundary layer capacitor model for explaining the giant dielectric constant observed in Ca Cu3Ti4O12 ceramics.
The aim of this work was to investigate the size-related magnetism for the single-domain Co Fe2O4 nano-particles synthesized using the hydrothermal method. The effects of the reaction temperature and the reaction time on the lattice constants, particle morphologies, and the room-temperature magnetic properties were studied from the X-ray diffraction, the transmission electron microscope, and the vibrating-sample magnetometer. The experimental results show that the samples are composed of Co Fe2O4 nano-particles with an average crystallite size（D） smaller than 40 nm, and the magnetic properties of the samples can be manipulated in a wide range： the MS values vary from smaller than 50 emu/g to close to 80 emu/g, and the HC values are between about 200 Oe and 2000 Oe. Additionally, the relationship between HC and 1/D^3/2 satisfi es linearship, showing the characteristic of single-domain structure. These results indicate that the single-domain Co Fe2O4 nano-particles with size controlled between the superparamagnetic critical size and single-domain critical size can be easily prepared using this hydrothermal method.
