| Currently, materials with high dielectric constant are one of the hot research topics of microelectronics industry. Materials with high dielectric constant are the core materials of electronic components, such as dynamic random access memory (DRAM) and multilayer ceramic capacitors (MLCC).CaCu3Ti4O12(CCTO) materials with similar perovskite-type structure exhibited high dielectric constant about12000at low frequency, and the dielectric constant was temperature independent from room temperature to300℃. Although CCTO materials with good temperature and frequency stability have a great potential application in the electronic components industry, the dielectric loss of CCTO ceramics is also too high for practical application. Usually, higher calcining and sintering temperatures are essential for CCTO ceramics when using the solid-state method due to the non-uniformity of mixing raw materials. Consequently, lots of researchers devote themselves to improve the dielectric properties of CCTO ceramics through doping, substituting and other wet chemical methods.In this paper, effects of different reaction conditions on the phase formation of CaCu3Ti4O12(CCTO) powders and Nao.5Bi0.5Cu3Ti4O1(NBCTO) powders were studied in detail by sol-gel method. Through sol-gel method, CCTO and NBCTO precursor powders and ceramics were successfully obtained. The phase structure, microstructure, electrical properties and reaction mechanism of CCTO and NBCTO ceramics were then further studied by comparison between the solid-state method and sol-gel process. Finally, the CCTO and NBCTO ceramics obtained by sol-gel process exhibit much more excellent dielectric properties than those obtained by the solid-state method.Firstly, CCTO precursor powders were prepared by sol-gel method, the effects of different Ti concentration of reaction solution, pH values, aging time and the kinds of reaction solvent on the phase formation of CCTO precursor powders were studied in detail. The optimum reaction conditions were determined as follows:[Ti]=0.6mol/L, pH=1.58, aging time was6h and the best solvent was ethanol. CCTO precursor powders with good crystallization were obtained. Through systematically study the effect of different sintering temperatures and holding times on phase structure, microstructure and dielectric properties of CCTO ceramics, it was found that when the precursor powders were pre-calcined at700℃and sintered at1100℃for15h, the CCTO ceramics had a pure perovskite structure, clearer grain boundary, higher density and less porosity. The dielectric constant was about35000and the dielectric loss was0.014at1kHz. The samples had good temperature stability from room temperature to120℃. Obviously, a wide dielectric relaxation peak was found near300℃. Complex impedance results showed that the grain was semiconducting and the grain boundaries was insulating. The grain resistance (Rg) was3.50Ω and the grain boundary resistance (Rgb) was0.87MQ, respectively. The high permittivity can be explained by the internal barrier layer capacitance (IBLC) model.Secondly, the optimum reaction conditions for NBCTO precursor powders were determined as follows:[Ti]=0.53mol/L, the volume of glacial acetic acid was3mL, aging time was14h, and the best solvent was ethanol. NBCTO precursor powders with good crystallization were obtained by sol-gel method. Through systematically study the effect of different calcining and sintering temperatures on phase structure, microstructure and dielectric properties of NBCTO ceramics, it was found that when the precursor powders were pre-calcined at730℃and sintered at1020℃for7.5h, the NBCTO ceramics had a pure perovskite structure, clearer grain boundary, higher density and less porosity. The dielectric constant was about18110and the dielectric loss was0.058at1kHz. Complex impedance results showed that the grain was semiconducting and the grain boundaries was insulating. The grain resistance (Rg) was7.50Ω and the grain boundary resistance (Rgb) was0.24MΩ, respectively. The high permittivity can also be explained by the internal barrier layer capacitance (IBLC) model.Lastly, CCTO and NBCTO precursor powders obtained by sol-gel process and solid-state method were used to fabricate CCTO and NBCTO ceramics, respectively. The comparison of the phase structure, microstructure and electrical properties were studied. The results showed that the temperatures for the formation of crystal phase for the sol-gel method was lower at least100℃than that for the solid-state method. The grain boundary resistance (Rgb=0.87MΩ) of CCTO ceramics obtained by sol-gel method was higher than that (Hgb=0.039MΩ) of CCTO ceramics obtained by the solid-state method. Additionally, the grain boundary resistance (Rgb=0.24MΩ) of NBCTO ceramics obtained by sol-gel method was higher than that (Rgb=0.045MΩ) of NBCTO ceramics obtained by the solid-state method. The frequency stability and temperature stability of the CCTO and NBCTO ceramics obtained by sol-gel method were better than those obtained by the solid-state method. The calculated conduction activation energies of grain boundaries for the samples obtained by sol-gel method were lower than that by the solid-state method through the imaginary part of impedance, indicating the sol-gel process is an useful method to improve the dielectric properties of the materials. |
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