Comparative Study Of Dielectric Properties Of A(fe_0.5Nb（0.5）)O₃（A=Ba, Sr） Ceramics

Considering that giant dielectric constant materials have a greatnumber of application advantages in electronic components integration,intellectualization, miniaturization, thus these materials have beenwidespread concerned. Fe-containing complex perovskites A(Fe0.5B0.5)O3(A=Ba, Sr, Ca; B=Nb, Ta, Sb) materials have attracted considerableattention due to their generally exhibit a giant dielectric constant step (ofthe order of103-105) over a broad temperature and frequency interval.However, dielectric loss of these materials is not small enough to meetthe requirement of the practical application, moreover the giant dielectricconstant step is not spread. So, the key challenge in these materials is toextend its giant dielectric constant step and simultaneously suppress thedielectric loss. In this paper, we have systematically studied BFN andSFN ceramics microstructure and dielectric properties. In order tooptimize the dielectric properties of ceramics as well as deeplyinvestigate the dielectric relaxation, we have synthesized BFN and itssolid solution with SFN in proportion as the corresponding stoichiometricratio.BFN ceramics were synthesized by a conventional solid-statesintered in a temperature range from1300to1400℃. The results showedthat crystal structure no difference between the ceramics sintered atvarious temperatures Ts, and the average grain size increases withincreasing Ts. Dielectric constant ε’ of BFN ceramics increases rapidlyfrom18,868to45,167, while the dielectric loss decreases from0.66to0.43when Tsrises from1300to1400℃. The complex plane impedanceplots revealed that the specimens are electrically heterogeneous,consisting of semiconducting grains and insulating grain boundaries.Based on the IBLC model, the ratio of the average grain size and grainboundary thickness (tg/tgb) decreases, thus ε’ decreases. Microstructureevidence had been provided for the extrinsic origins of the enhanced giant dielectric constant.SFN dense ceramics are achieved by a conventional solid statesintered at1450℃in nitrogen, air and nitrogen atmosphere for3h,respectively. The results showed that crystal structure no differencebetween the ceramics sintered at various atmosphere, and average grainsize and ε’ of SFN ceramics decrease with rising oxygen concentration ofsintered atmospheres. XPS revealed that ε’ of SFN ceramics increaseswith rising oxygen concentration due to space charge polarization weaken.The complex plane impedance plots further demonstrated that SFNceramics are electrically heterogeneous, consisting of semiconductinggrains and insulating grain boundaries, and the difference ofmicrostructure of grain and grain boundary derived from variousdistribution of oxygen vacancy concentration. The average grain sizeincreases and grain thickness decreases with rising oxygen concentrationof sintered atmospheres, using the IBLC model, the ratio of the averagegrain size and grain boundary thickness (tg/tgb) decreases, thus ε’decreases. To arrive at conclusions, the change of ε’ of SFN ceramics thecoefficient results of Maxwell-Wagner and IBLC effect.The dielectric properties of Ba1-xSrx(Fe0.5Nb0.5)O3(x=0.0,0.2,0.4,0.6,0.8) ceramics were presented. It should be noted that ε’(~180,000)was observed for the x=0.2sample at room temperature at1kHz. Thecomplex plane impedance plots of x=0.2sample revealed that the ε’ ismainly due to the grain boundary response. Comparison with pure BFN(x=0.0) and x=0.6revealed that the incorporation of SFN cansignificantly reduce the dielectric loss and improve the frequency andtemperature stabilities of the dielectric properties of BFN. Theexperimental analysis results confirm that strontium incorporated intoA-sites to suppress the formation of mixed-value structure of Fe2+/Fe3+atlow-temperature range and improved electron hopping barrier height athigh-temperature range, the low-and high-temperature dielectricrelaxation of x=0.6sample shift to lower-and higher-temperature,respectively, which induced dielectric constant plateau extended.Two dielectric relaxations with strong frequency dispersion andfollowing a giant dielectric constant step (′~103) were observed in the Ba0.4Sr0.6(Fe0.5Nb0.5)O3ceramics at298-450K and460-700K,respectively, and both dielectric relaxations follow thermal activationprocess of the Arrhenius law. A nearly monodispersive nature of dielectricrelaxation is indicated in the present ceramics at lower temperatures, thefrequency dependent significant drop of dielectric constant at298-393Kwas nearly a Debye relaxation with the intrinsic nature, and this dielectricrelaxation was proposed to originated from an electron hopping processbetween Fe2+and Fe3+ions. The O2annealing can almost completelysuppress the high temperature dielectric relaxation and subsequentlyextends the giant dielectric step, thus the high temperature dielectricrelaxation was attributed to the extrinsic oxygen vacancy. The formationmechanism of the giant dielectric constant step is just a competingbalance result of the low-and high-temperature dielectric relaxations.