| The search for new high performance dielectric materials, exhibiting temperature-and frequency-stable, colossal permittivity (ε’> 1000) as well as sufficiently low dielectric loss, continues to arouse considerable interest motivated by their myriad device miniaturization and high-energy-density storage applications. In 2002, Subramanian reported that the CaCu3Ti4O12 (CCTO) ceramics exhibited a colossal permittivity and good temperature stability. Furthermore, in recent years, in order to obtain giant dielectric material of excellent performance and good practicability, the researches mainly focus on in two aspects:One is the ACu3Ti4O12 ceramics with similar structures to CCTO, was obtained by replacing Ca2+ in CCTO with different ions, so the giant dielectric properties of CCTO was improved. Second, ACu3Ti4O12 giant dielectric materials with excellent performance were prepared with adopting new technology, optimizing parameters. But so far, there are limited literatures reporting the Y2/3Cu3T14O12 (YCTO) and (1-x) Y2/3Cu3Ti4O12-xSrTiO3 ceramics, and the microstructure and electrical properties of the YCTO ceramics prepared by the sol-gel method have not yet been reported. The Y2/3Cu3Ti4O12and(1-x)Y2/3Cu3Ti4O12-xSrTiO3 ceramics prepared by sol-gel method as the main object of the study. The effects of sol and sintering conditions on the ceramic microstructure and electric properties were detailed researched. The dielectric materials with superior properties were prepared and choosed by optimizing the preparation technology. Through the above research work, the main conclusions obtained from experimental results and analyses were as follows:1. Y2/3Cu3Ti4O12 (YCTO) ceramics were successfully synthesized by sol-gel method (SG) and solid-state method (SS), respectively. The optimized processing parameters for the syntheses of precursor powder by sol-gel process were determined as follows: the Ti(OC4Hg)4 concentration was 0.50 mol/L, the CH3COOH volume was 8 mL and the volume percentage of H2O was 11.2%. Particularly, on the basis of XRD and TG-DSC analyses, the phase formation temperature of YCTO-SG was at least 100℃ lower than that of YCTO-SS. YCTO-SG ceramics sintered at 1060℃ for 25 h showed fine-grained microstructure, and higher dielectric constant (ε’≈5.24×104) at 1 kHz compared to YCTO-SS ceramics (ε’≈0.93×104). The higher dielectric constant of the YCTO-SG ceramics was attributed to the grain size effect. Furthermore, the YCTO-SG ceramics showed a distinct high-temperature (>300℃) relaxor-like behavior. According to the calculated activation energy value, the single ionization of oxygen vacancies was responsible for the conduction and dielectric anomaly behaviors of YCTO-SG ceramics.2.0.85Y2/3Cu3Ti4O12-0.15SrTiO3(0.15STO) ceramics ceramics were successfully synthesized by sol-gel method and solid-state method. The optimized processing parameters for the syntheses of precursor powder by sol-gel process were determined as follows:the Ti(OC4H9)4 concentration was 0.45 mol/L, the CH3COOH volume was 12 mL and the water-bath temperature was 30℃. Through optimizing the sintering process of 0.15STO-SG ceramics, it was found that 0.15STO ceramics sintered at 1050℃ for 20 h exhibited higher dielectric constant(2.03-1.04×105,0.1-10 kHz). Furthermore, the effect of SrTiO3 content on the phase structure, microstructure and dielectric properties of (1-x) Y2/3Cu3Ti4O12-xSrTiO3 ceramics were discussed. For x= 0.15, it was found the solid solution limit for this series. The formation of SrTiO3 secondary phase was evident in the XRD patterns for x> 0.15. And for x> 0.15, two dielectric constant plateaus were observed in a lower frequency (40 Hz-100 kHz) range in the dielectric constant plots. According to the activation energies and resistances of grains and grain boundaries, two dielectric constant plateaus could be attributed to the effects of grains and electrode.3. YCTO and 0.15STO precursor powder were prepared by sol-gel method, respectively. Effect of Al content on the phase structure, microstructure and dielectric properties of YCTO and 0.15STO ceramics were investigated in detail, respectively. The excellent dielectric properties of the 0.5% (wt.%) Al doping YCTO ceramics (ε’=1.9×104, tanδ=0.049,1 kHz) and 1% (wt.%) Al doping 0.15STO ceramics (ε=2.8×105, tanδ=0.17,1 kHz) were obtained. The complex impedance results revealed that the grain was semiconducting and the grain boundaries was insulating. And the higher grain boundaries resistance, the lower the low-frequency dielectric loss is. |
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