Influence Of Annealing Process On Dielectric Properties Of Ba_0.6Sr_0.4TiO₃ Thick Film Prepared By Screen Printing

Barium strontium titanate(Ba_1-xSr_xTiO₃)has strong dielectric nonlinearity under bias electric field,with its dielectric constant obviously declining with increasing bias elelctric field in magnitude.This desired dielectric characteristic renders Ba_1-xSr_xTiO₃a promising candidate material for electrically tunable microwave devices,such as filters,resonators,delay lines,variable capacitors and phase shifters,which are dispensable in the current telecommunication technology.In the past decades,the preparation,structure and dielectric properties of Ba_1-xSr_xTiO₃ have been the subject of extensive researches.Realization of Ba_1-xSr_xTiO₃ in the tunable microwave devices has also been explored.In this work,the effects of oxidizing annealing on dielectric properties of porous Ba_0.6Sr_0.4TiO₃thick films produced by a screen printing technique have been investigated.The evolution of the dielectric properties with performing an oxidizing annealing was explicated in relation with the motion of oxygen vacancies in the porous thick films.The purpose of this work is to offer a viable strategy to improve the nonlinear dielectric properties of the porous thick films,which in turn benefits the utilization of Ba_0.6Sr_0.4TiO₃ in the tunable microwave devices.The fabrication technique of Ba_0.6Sr_0.4TiO₃ thick films by a screen printing method has been investigated.The major technique parameters,including particle graduation of the starting powder,composition of inorganic components in the ink for screen printing,solid loading of the ink and repeated time of screen printing,have been determined based on experimental researches.Employing the screen printing technique,porous Ba_0.6Sr_0.4TiO₃ thick films with a uniform microstructure have been yielded.The microstructure and dielectric properties of the thick films showed a satisfactory reproducibility,which are particularly important to the subsequent researches.The effects of oxidizing annealing on the structure and nonlinear dielectric properties of the porous thick films have been examined with respect to the factors like annealing temperature,time and atmosphere.The results indicated that the cell parameters of the porous thick films became larger after an oxidizing annealing,while no remarkable change in microstructure was detected.Moreover,it was confirmed that an oxidizing annealing was viable in improving the nonlinear dielectric properties of the porous thick films.To look into the mechanisms of the improvement of the nonlinear dielectric properties after an oxidizing annealing,the dielectric behaviors of the porous thick films before and after an oxidizing annealing were inspected as a function of time,temperature,frequency or applied electric field.Meanwhile,to diagnose the nature of carriers in the porous thick films,their complex impedance spectra were checked.The results indicated that the porous thick films displayed numerous dielectric anomalies discrepant from their dense ceramic counterparts,e.g.an aging effect of dielectric properties,an increase dielectric loss,a pronounced Maxwell-Wagner interfacial polarization,a frequency dispersion of dielectric constant at low temperatures and an appreciably narrowed P-E heystersis loop.These dielectric anomalies are related to the motion of oxygen vacancies in the porous thick films.It was found that an oxidizing annealing served to reduce the amount of oxygen vacancies in the lattice and homogenize the distribution of oxygen vacancies in the grains,therefore leading to a depression of the above-mentioned abnormal dielectric behaviors as well as an improvement of the nonlinear dielectric properties.The outcomes of this work demonstrate that an annealing of as-prepared Ba_0.6Sr_0.4TiO₃ thick films in air at 1000? for 2 h is preferred for achieving optimal dielectric properties.At room temperature and 10 kHz,the annealed thick films showed a dielectric constant of 455 and a loss of 1.1%at zero bias electric field together with a tunabiliy of 52%.and a figure of merit of 47 at 66 kV/cm.