The Preparation And Characterization Of Nano-composites And Their Doped X7R-type BaTiO₃-based Dielectric Ceramics

The X7R-type BaTiO3-based Multilayer ceramic capacitor (MLCC) is well known due to its perfect temperature stability of dielectric constants. EIA X7R specifications demand that the dielectric constant change is less than±15% from its value at 25℃over the temperature range of-55℃to 125℃. The development tendency of X7R-type ceramics are lowing the sintering temperature, increasing the permittivity and miniaturization, which requires the precise control of compositions and microstructure of ceramics. The BaTiO3-Nb2O5-Co2O3 system shows a high dielectric constant and Nb2O5-Co2O3 composite oxides are beneficial for promoting the dielectric properties. Nano-scale composite dopants and ultra-fine BaTiO3 powders are necessary to go well with the development tendencies of X7R-type ceramics. In this paper, the Nb-Co-Nd-Mn-O (NCNM) nano-composites and Nb-Co-Nd-Mn-Zn-B-Si-O (NCNMZBS) nano-composites were synthesized via sol-gel combustion method. Then the BaTiO3 powders prepared via sol-gel method were used as main materials to prepare the X7R-type ceramics by adding nano-composites, with the goals of obtaining the high-performance dielectric materials as well as lowing the sintering temperature.1. The NCNM nano-composites and NCNMZBS nano-composites were synthesized via sol-gel combustion method. The reaction process and the influence of preparing conditions on phase compositions, microstructure were investigated by means of Thermogravimetric, Fourier-transform infrared, X-ray diffraction methods and transmission electron microscopy. The as-burnt powders should be calcined at 500℃again to obtain nano-composite powders, with low aggregation and uniform grains size. When the ratio of citrate to nitrate radicals was equal to 1, the microstructures was improved. The CoNb2O6 and NdNbO4 composite oxides formed in NCNM powders. CoNb2O6,ZnNb2O6,Zn(BO2)2,(Co,Zn)2SiO4 and NdNbO4 composite oxides formed in NCNMZBS powders.2. The X7R-type ceramics were prepared by adding NCNM nano-composites into BaTiO3 powders. The effect of calcining temperature of nano-composites, ratio of citrate to nitrate radicals in sol precursor, doping amount and sintering temperature on the phase compositions, microstructure and dielectric properties were studied. The results indicated that by doping 1.40 wt% NCNM nano-composites (calcined at 500℃), the sintering temperature of X7R-type ceramics could be reduced to 1220℃, with a room temperature dielectric constant of 4930. Further increasing the sintering temperature to 1260℃, the room temperature permittivity was as high as 5350. Comparing with the samples by traditional solid-doping method, the dielectric properties of ceramics were improved obviously and their sintering temperature was reduced effectively.3. The X7R-type ceramics were prepared by adding NCNMZBS nano-composites into BaTiO3 powders. The effect of calcining temperature of nano-composites, doping amount, Zn-B-Si-O content of nano-composites and sintering temperature on the phase compositions, microstructure and dielectric properties were studied. By adding 2.20 wt% NCNMZBS nano-composites (Zn-B-Si-O was 0.60 wt% and NCNM was 1.60 wt%), the ceramics sintered at 1180℃also had a relatively high dielectric constant of 3940. The results indicated that NCNMZBS nano-composites played a role in lowing the sintering temperature further, and the dielectric properties were also excellent.