Preparation And Study On The Electrical Properties Of CaCu₃Ti₄O₁₂ Thin Films Doped And Gradient-doped In Different Sites Of The Lattice

In this paper,CaCu₃Ti₄O₁₂ thin films doped and gradient-doped in different sites of the lattice were prepared by modified sol-gel method.The effects of the doping and gradient-doping of Y in A’ sites,Mg in A" sites and Zr in B sites on the microstructure and electrical properties of CCTO thin films were investigated.X-ray diffraction and scanning electron microscopy were used to characterize the phase structure,surface and fracture morphology and microstructure of the films.Dielectric and varistor properties of the films were measured using a precision impedance analyzer,a dielectric temperature spectrum test system,and a varistor DC meter.Firstly,the annealing process of CCTO thin films was studied.It was determined that the annealing temperature of 750℃ was more favorable for the preparation of CCTO thin films.A theoretical model of the dielectric and varistor characteristics of the CCTO thin film prepared at the annealing temperature was successfully performed.It is believed that the dielectric properties of the CCTO thin films are related to the internal conductive grain size and the thickness of the thin insulating grain boundary,and the varistor characteristics are related to the double Schottky barriers existing inside,which makes theoretical preparations for studying the effects of different lattice sites doping and gradient-doping on CCTO thin films.Studies on Y-doped and gradient-doped CCTO thin films in A’ sites showed that Y doping in A’ sites increased the dielectric constant of the CCTO thin film at low frequencies,but at the same time it increased the dielectric loss and reduced its dielectric frequency and temperature stability range.Y doping in A’ sites had little effect on the nonlinearity of the CCTO thin film in that Y doping in A’ sites did not significantly change the nonlinear coefficient of the CCTO film.Y gradient-doping in A’ sites can significantly improve the dielectric properties of the CCTO thin film.Y up-gradient-doping was more conducive to increase the dielectric constant,while Y down-gradient-doping was more beneficial to reduce the loss.Y gradient-doping significantly increased the nonlinear coefficient of the CCTO thin film,where Y up-gradient-doping had the highest nonlinear coefficient of 7.2.Studies on Mg-doped and gradient-doped CCTO thin films in A" sites showed that within the test frequency range,Mg doping in A" sites increased the dielectric constant of the CCTO thin film and broadened the dielectric frequency and temperature stability range.The dielectric loss at the intermediate frequency of the CCTO thin film was significantly reduced,that is,Mg doping in A" sites can significantly improve the dielectric properties of the CCTO thin film.The nonlinear coefficient of the 15mol%Mg-doped CCTO thin film is as high as 7.4.That is to say,Mg doping in A" sites also significantly improved the varistor properties of the CCTO thin film,which indicates that Mg doping in A" sites was beneficial to improve capacitance and varistor properties of the CCTO thin film at the same time.Mg gradient-doping in A" sites increased the dielectric constant of the CCTO thin film and reduced its dielectric loss,but at the same time reduced the nonlinear coefficient of the CCTO thin film.Studies on Zr-doped and gradient-doped CCTO thin films in B sites showed that the 20mol% Zr doping increased the dielectric constant of the CCTO thin film by nearly 2 times,the dielectric loss was significantly reduced,and the dielectric frequency and temperature stability range were significantly improved at the test frequency.At 1kHz and room temperature,the dielectric constant of the thin film was as high as 8124 and the dielectric loss was as low as 0.023,indicating that Zr doping in B sites can significantly improve the dielectric properties of the CCTO thin film.When the doping amount of Zr was 15mol%,the nonlinear constant of the CCTO thin film obtained a maximum value of 5.8.Gradient-doping on B-site Zr increased the dielectric constant of the CCTO thin film and reduced its dielectric loss.Moreover,the nonlinear coefficient of Zr gradient-doping in B sites was as high as 8.3,which indicates that Zr gradient-doping in B sites can increase simultaneously capacitance and varistor properties of the CCTO thin film.