Reduction-Reoxidation Preparation Of Giant Dielectric CaCu₃Ti₄O₁₂ Ceramics And Its Doping Modification Study

CaCu₃Ti₄O₁₂（CCTO）ceramics have unique advantages in the miniaturization and Multifunctionality of chip components by combining giant dielectric effect and pressure-sensitive effect,which are often sintered in the air atmosphere,resulting in the use of oxidation-resistant precious metal Ag/Pd alloy as the inner electrode of the chip components,which is not conducive to the cost reduction of the components.The application of the base metal inner electrode,represented by Ni,is the key to the cost reduction of the chip element,and the Ni electrode must be reduction sintered to avoid oxidation,however,the reduction sintering can lead to serious decomposition of CCTO ceramics,and the giant dielectric and pressure sensitive properties are lost.In order to realize the co-fire of CCTO ceramics and Ni electrodes,this thesis focuses on the preparation of reduction-resistant and decomposition-resistant CCTO ceramics and the regulation of their electrical properties.The main research contents are as follows:Based on literature reports,reduction of sintering causes CCTO ceramics to decompose and is accompanied by severe loss of copper elements.Therefore,the inhibition of copper volatilization is the entry point to address the decomposition of CCTO in this thesis.For this purpose,CCTO ceramics were placed in alumina crucibles and sealed with lids to reduce Cu volatilization during reduction sintering;secondly,BN was introduced to form a B-containing liquid phase to reduce the sintering temperature to weaken Cu element volatilization.After sintering with high-purity N₂at 1050℃,XRD and EDS analysis showed that the decomposition of CCTO was effectively suppressed.The reduced CCTO ceramics still have giant dielectric properties with a high dielectric constant of 2.8×10⁴（1 k Hz）.However,the reduction sintering resulted in difficult oxidation of grain boundaries,and the reduced CCTO ceramics had severely deteriorated dielectric losses up to 1.56（1 k Hz）.To reduce the dielectric loss,a reoxidation process was used to reduce the dielectric loss to 0.14（1 k Hz）,while the sample still maintained the giant dielectric properties with a dielectric constant of 9.9×10³.Accordingly,a reduction-reoxidation preparation scheme for CCTO ceramics is proposed in this thesis.It should be noted that the reoxidation temperature must be strictly limited in order to avoid oxidation of the Ni inner electrode,therefore,the reoxidation temperature in this thesis is limited to 800°C only.The lower reoxidation temperature makes it difficult to fully oxidize the grain boundaries and reduce the dielectric loss of CCTO ceramics prepared by reduction-reoxidation.For this reason,this thesis aims to promote grain boundary oxidation by refining the grain size,increasing the number of grain boundaries,and improving the oxygen permeability at the grain boundaries.It is found that Bi³⁺ion doping can effectively inhibit grain growth,increase grain boundary resistivity,and reduce dielectric loss.Among them,the grain size was reduced from 13.4μm to 7.6μm when the doping amount of Bi³⁺was increased from x=0（CBCTO-00 sample）to x=0.1（CBCTO-10 sample）,and the dielectric loss was reduced from 1.05 to 0.14（1 k Hz）after oxidation at 800°C,while the sample still maintained the giant dielectric properties with a high dielectric constant of 9.9×10³.On this basis,the CBCTO-10 sample with the best electrical properties was selected to study the effect of reducing atmosphere sintering temperature on the microstructure and electrical properties of the sample.The results showed that increasing the sintering temperature increased the decomposition of CCTO and formed the second phase at the grain boundaries.The second phase at the grain boundary can effectively increase the grain boundary resistivity,which has a significant effect on the reduction of dielectric loss.In addition,this thesis further optimized the temperature reduction rate and reoxidation time.Finally,the best electrical properties of the ceramics were obtained by sintering the CBCTO-10 samples at 1050°C with a 300°C/h temperature reduction rate and a 2-h reoxidation time.In order to avoid the oxidation of the Ni inner electrode,the reoxidation temperature must be strictly limited,but the lower reoxidation temperature will make it difficult to fully oxidize the CCTO ceramic grain boundary and reduce the dielectric loss,therefore,the reoxidation temperature in this thesis is limited to 800℃.At1000℃sintering,the addition of Sn⁴⁺can effectively reduce the grain size and promote grain refinement.When the Sn⁴⁺doping amount was 0.2（CCTSO-20sample）,the grain boundary resistivity of the oxidized ceramic was 7.4×10⁶Ωcm,which was 2 orders of magnitude higher than that of the undoped CCTO,and the dielectric loss was only 0.05.On this basis,the best electrical properties of the CCTSO-20 sample were selected to study the effect of reduction on the basis of this,the sample with the best electrical properties,CCTSO-20,was selected to study the effect of reduction atmosphere sintering temperature on the microstructure and electrical properties of the sample.The results show that low temperature sintering helps fine crystallization of grains,increases the grain boundary resistance and reduces the dielectric loss.Therefore,the optimal sintering temperature for CCTSO ceramics is 1000℃.