Preparation Of CaCu₃Ti₄O₁₂ Ceramics With Colossal Permittivity Under Reducing Atmosphere And Research Of Reoxidation Effect

In recent years,the base metalization technology of the inner electrode has been developing rapidly in related fields such as multilayer chip ceramic capacitors（Multi-layer Ceramic Capacitors,MLCC）,which is one of the most effective ways to realize the low cost of MLCC and other chip components.The co-fire technology based on reduction sintering is the key to realize the base metalization of the inner electrode.The giant dielectric-piezoresensitive properties of Ca Cu₃Ti₄O₁₂（CCTO）ceramics have great potential in the direction of achieving miniaturization,high capacity,and multifunctionality of MLCCs.However,reduction sintering leads to the decomposition of CCTO materials,and the base metalization of the inner electrodes of CCTO ceramic-based chip components is difficult to achieve.Thus this thesis is centered on the implementation of reduction sintering scheme for CCTO ceramics,the regulation of electrical properties based on the reduction-reoxidation process,and the exploration of the reoxidation mechanism.The details of the study are as follows:The researchers found that severe copper volatilization often accompanies the decomposition process of CCTO ceramics.For this reason,this thesis first developed the effect of copper partial pressure on the phase composition of CCTO ceramics during reduction sintering.It was found that sealed sintering and reducing the sintering temperature during reduction sintering could effectively inhibit the decomposition of CCTO ceramics by weakening the volatilization of copper elements.In combination with hermetic sintering,ceramics of Ca Cu3Ti_4-xGe_xO₁₂+3%BN（x=0,0.05,0.10,0.20）system with low-temperature sintering characteristics were developed.After sintering at 1025℃with high-purity N₂ atmosphere,the samples all showed giant dielectric properties but the dielectric loss deteriorated significantly.For this reason,this paper combined reoxidation treatment to promote grain boundary oxidation and reduce dielectric loss.The base metal inner electrode sheet element must limit the reoxidation temperature to prevent the inner electrode from oxidizing,so the reoxidation temperature is limited to 800℃.Optimize Ge⁴⁺doping amount,when x=0.1,after 1025℃high purity N₂ atmosphere sintering,800℃air atmosphere reoxidation,the sample dielectric loss was reduced to only 0.07（1 k Hz）and then the giant dielectric characteristic was maintained.The relative dielectric constant was up to 1.2×10⁴（1 k Hz）.In order to further reduced the dielectric loss of CCTO ceramics,based on the effect of reduction-oxidation process(Ni²⁺-Ge⁴⁺)co-doped CCTO ceramics,Ca Cu_3-xNi_xTi_3.9Ge_0.1O₁₂+3%BN（CCNTGO）（x=0,0.05,0.10）was formulated.At the doping concentration of x=0.05,the 1025℃sintering 800℃reoxidized ceramic sample achieved good dielectric loss of 0.05 and relative dielectric constant of 9.3×10³.The variable temperature reimpedance analysis showed that the resistivity and activation energy of the ceramic sample with x=0.05 were greatly improved.The increase of the resistivity of the grain boundary at room temperature could reduce the leakage current.The increase of the activation energy of the grain boundary can enhance the ability of the grain boundary to block the electron transition,which together leads to the reduction of the dielectric loss.Reoxidation is a key step in grain boundary oxidation,and for this reason,the effects of reoxidation temperature and time on the electrical properties of CCTO-based ceramics are investigated in this paper.The dielectric frequency spectrum test at room temperature showed that the relative dielectric constant and dielectric loss of all CCTO-based ceramic samples will decrease as the reoxidation temperature increases and the time increases,for example.The relative dielectric constant of ceramic samples with Ge⁴⁺doping concentration（x=0.1）decreased from 19584（1 k Hz）at the reoxidation temperature of700℃to 7638（1 k Hz）at 900℃,and the dielectric loss decreased from 0.45（1 k Hz）to0.03（1 k Hz）.The relative dielectric constant of the ceramic sample with Ge⁴⁺doping concentration x=0.1 decreased from 13688（1 k Hz）with a reoxidation time of 0.5 h to7805（1 k Hz）at 8 h,and the dielectric loss decreased from 0.25（1 k Hz）at 0.5 h to 0.05（1k Hz）at 8 h.The gradual decrease in the relative permittivity of CCTO-based ceramics may be caused by the gradual increase in the thickness of the ceramic depletion layer with the increase in the reoxidation temperature and time,and the gradual decrease in the dielectric loss of CCTO-based ceramics may be caused by the gradual increase in the grain boundary resistivity and grain boundary activation energy at room temperature of CCTO-based ceramics with the increase in the reoxidation temperature and time.