Preparation Of Rare Earths-modified Ba_（1-x）(Na_0.5Bi_0.5)_xTiO₃Powders And Its Conductivity Properties

The conductive powders have attracted considerable attention in recent yearsbecause of their very important applications in many fields especially in staticelimination. Rare earth modified method can improve the conductivity and thedielectric properties of powders, and become the hot research of powder modified.In this study, BaTiO₃, Na_0.5Bi_0.5TiO₃and Ba_（1-x）(Na_0.5Bi_0.5)_xTiO₃powders wereprepared by the sol-gel method. The process conditions of preparing Na_0.5Bi_0.5TiO₃and Ba_（1-x）(Na_0.5Bi_0.5)_xTiO₃powders such as pH, reaction temperature and heattreatment temperature were studies. The effects of the process conditions on powderstructure, morphology and resistivity, dielectric constant effects of Na_0.5Bi_0.5TiO₃and Ba_（1-x）(Na_0.5Bi_0.5)_xTiO₃powders were discussed to find the optimum processconditions. The optimum process conditions are as follows: pH=3.5，reactiontemperature of20℃, drying temperature of150℃, warm-up temperature of80℃,calcining temperature of700℃. The Na_0.5Bi_0.5TiO₃powders with an average grainsize of50nm，has the room temperature resistivity of3.71×10⁶Ω m, the frequency of500Hz, the dielectric constant εr393.21, and its cut dielectric loss in the highfrequency region is relatively stable. The optimum process conditions of preparingBa_（1-x）(Na_0.5Bi_0.5)_xTiO₃powders with citric acid as a chelating agent are as follows:pH=4.5，reaction temperature of40℃, drying temperature of120℃, warm-uptemperature of5000℃, calcining temperature of750℃. The resistivity of theBa_（1-x）(Na_0.5Bi_0.5)_xTiO₃powders decreases with the increases of x, and reached thelowest point of3.24×10⁷Ω m with the values of x was0.8.Based on obtained pure BaTiO₃and Na_0.5Bi_0.5TiO₃powders, the liquid-phasedoping technique was adopted to improve the conductivity of the BaTiO₃andNa_0.5Bi_0.5TiO₃powders. The resistivity of Sm-dopped BaTiO₃and Na_0.5Bi_0.5TiO₃powders are much lower than the other rare earths dopped powders. When theconsistency of Sm is0.3at%, the resistivity of Sm-dopped BaTiO₃powder is thelowest, which drops from4.30×10⁹Ω m to2.38×10⁶Ω m. When the consistency ofSm is0.5at%, the resistivity of Sm-dopped Na_0.5Bi_0.5TiO₃powder is the lowest,which drops from3.71×10⁶Ω m to2.41×10⁵Ω m. The result of dielectric testsshowed that Sm-doping method improved the ceramic dielectric constant, and thedielectric loss versus frequency of Sm-dopped Na_0.5Bi_0.5TiO₃and pure Na_0.5Bi_0.5TiO₃have essentially the same changing tendency. The result of XRD analysis showedthat Gd, Pr and Sm were incorporated into the A site. The result of XPS analysisshowed that the powder surface after rare earth doping enriched Na elements, and non-stoichiometric surface layer was formed.Combined with electrical properties and morphology analysis of theexperimental test results, the gaseous penetration experiments was done by usingSm elements as diffusion agent。The composition, structure and conductivity of theSm-penetrated BaTiO₃, Na_0.5Bi_0.5TiO₃and Ba_（1-x）(Na_0.5Bi_0.5)_xTiO₃powders beforeand after doping were studied. The results reveals that the powder resistivity dropswith the increase of penetration temperature. The resistivity of Sm-penetratedBaTiO₃powder drops from4.30×10⁹Ω m to4.27×10⁴Ω m. The resistivity ofSm-penetrated Na_0.5Bi_0.5TiO₃powder drops from3.71×10⁶Ω m to4.01×10⁵Ω m.The resistivity of Sm-penetrated Ba0.6（Na0.5Bi0.5）0.4TiO3powder drops from5.76×10⁸Ω m to3.67×10³Ω m.The first-principles calculation was carried out with Materials Studio softwarepackage for Gd, Pr and Sm-modified BaTiO₃and Na_0.5Bi_0.5TiO₃system. The resultsshow that dopant elements render crystal cell distorted. After comparison ofsingle-point energy between A-site substitution and B-site substitution, it is foundthat rare earth elements are relatively easy to substitute A site. Impurity levels areintroduced within the band gap or overlap with the band edge of the BaTiO₃andNa_0.5Bi_0.5TiO₃structures and leading to band gap narrowing, which results in thereduction of the resistivity of the rare earth elements-modified BaTiO₃andNa_0.5Bi_0.5TiO₃system. By the analysis of calculation results and experimental data,it is found that they match with each other and will be helpful for future research.