Low-temperature Sintering And Nonlinear Dielectric Properties Of Ba_0.6Sr_0.4TiO₃

Ba_0.6Sr_0.4TiO₃,a typical nonlinear dielectric,has a variety of desirable dielectric characteristics,such as large dielectric constant,low dielectric loss,high electric breakdown strength and strong dielectric nonlinearity.These desirable properties make Ba_0.6Sr_0.4TiO₃ a promising candidate material for microwave dielectric components like resonators,filters,variable capacitors,delay lines and phase shifters,which are dispensable for the next generation telecommunication technology.In view of the current demand of miniaturization,integration and multi-functionalization for microwave components,it is of great importance to improve the sinterability of Ba_0.6Sr_0.4TiO₃ nonlinear dielectric.Among the possible strategies to realize this goal is addition of a lithium sintering additive to promote the sintering of the dielectric.In this work,the ceramic specimens with nominal compositions of Ba_0.6Sr_0.4TiO₃+x wt.%Li₂O?x=0-0.8?were prepared using a citrate precursor method.The sinterability,crystal structure,microstructure and dielectric properties of these ceramic specimens were examined.Particular attention was directed towards the mechanisms responsible for densification of these ceramic specimens.Moreover,the thick film specimens were prepared using a tape casting technique.The sinterability and dielectric properties of these thick film specimens were inspected.The purpose of this work is to work out a feasible strategy to produce dense Ba_0.6Sr_0.4TiO₃ thick films with superior dielectric properties at lowered sintering temperatures.The influence of sintering parameters and lithium addition content on the sinterability,crystal structure,microstructure and dielectric properties of the ceramic specimens were examined.The results indicated the lithium-added ceramic specimens displayed much improved sinterability as compared with the specimen without lithium addition?i.e.x=0?.The ceramic specimen with x=0.4 achieved a relative density of 98.5%when being sintered at 900? for 6h in air,while the ceramic specimen with x=0 attained a comparable densification level?relative density⁹5%?when being sintering at a much higher temperature of 1260?.The ceramic specimen with x=0.4 showed uniform microstructure and good dielectric properties.This specimen exhibited a dielectric constant of 3240 and a dielectric loss of 0.34%at zero bias and 10 kHz couple with a tunability of 34%and a figure of merit?FOM?of100 at 10 kHz and 20 kV/cm,which as a whole are superior over the level for the specimen with x=0.The mechanisms underlying the improved sinterability of the lithium-added ceramic specimens were diagnosed.Transient liquid-phase sintering was believed to be the reason for the low-temperature densification of these specimens.The liquid-phase in these specimens originated from eutectic melting of?Ba,Sr?CO₃ and Li₂CO₃.The?Ba,Sr?CO₃ arose form surface carbonation of synthesized Ba_0.6Sr_0.4TiO₃ powder when being exposed to ambient atmosphere,while the Li₂CO₃was caused by reaction of added lithium with CO₂ released from pyrolysis of the solid citrate precursor during the synthetic process of Ba_0.6Sr_0.4TiO₃ powder.The densification of the lithium-added ceramic specimens was realized via a grain rearrangement behavior occurring in the initial stage of sintering with the aid of the liquid phase formation and then a dissolution-precipitation behavior occurring in the ultimate stage of sintering.The lithium cations precipitated from the liquid phase was found to incorporate into the octahedral site in the perovskite structure of Ba_0.6Sr_0.4TiO₃.On the basis of the above outcomes,free-standing thick films?¹60?m in thickness?with the nominal composition of Ba_0.6Sr_0.4TiO₃+0.4 wt.%Li₂O were prepared using a tape casting technique.The thick film sintered at 1080? for 6 h showed a dense and uniform microstructure,with the relative density being more than96%.This thick film achieved excellent dielectric properties,showing a dielectric constant of 3400 and a dielectric loss of 0.15%at zero bias and 10 kHz along with a tunability of 87%and a figure of merit of 580 at 10 kHz and 160 kV/cm.