Effects of additives on the microstructure and dielectric properties of Ba2Ti9O20 microwave ceramic

Preparation of dense and phase-pure Ba2Ti9O20 is generally difficult to achieve using a solid-state reaction, due to the presence of several thermodynamically stable compounds in the vicinity of the desired composition. This work investigated the effects of various additives (TiO2, MnO, and ZrO2) on the densification, microstructural evolution, phase stability, and dielectric properties of Ba2Ti9O20. Ceramics with theoretical density of ≥95% were achieved in all cases after sintering at 1300 °C. A pure Ba2Ti9O20 phase was obtained by treating the material with TiO2 additions (≤5.6 wt.%) and sintering at temperatures ranging between 1200 and 1350 °C. Ba2Ti9O20 is a nonstoichiometric compound that can accommodate an excess amount of TiO2. As the temperature was increased, pure Ba2Ti9O20 partially decomposed and formed a mixture of BaTi4O9 and Ba2Ti9O20. The ceramic with excess TiO2 sintered at 1390 °C possessed a higher permittivity and a lower quality factor due to the larger grain size and lower density. For ceramic with the addition of ZrO2 (≤6 wt.%), pure Ba2Ti9O20 phase was obtained after sintering between 1200 and 1390 °C, and the quality factor was improved. The decomposition temperature of the Ba2Ti9O20 phase was greater than 1390 °C. For sintering temperatures ≥1350 °C, the extent of Ba2Ti9O20 phase decreased with MnO additions. As the MnO content reached 0.5 wt.%, only BaTi4O9 and TiO2 phases existed, suggesting a decrease in the decomposition temperature of Ba2Ti9O20 with the addition of MnO. The microwave properties of the ceramics degraded significantly at the sintering temperature of 1390 °C.

Synthesis of nanocrystalline Cu2 – xSe at room temperature by γ-irradiation

Ten nm nonstoichiometric compound Cu2 – xSe was firstly synthesized at room temperature by γ-irradiation, using sodium selenosulfate as the selenium source. The influence of the coordination of copper ions and the pH value of the solutions on the formation of Cu2 – xSe has been studied. The Cu2 – xSe was prepared here for x = 0.13.Key words: copper selenide, nanocrystalline, γ-irradiation.

Dielectric Loss in Vanadyl Pnictates

The results of audio frequency complex impedance studies for vanadyl phosphate and its hydrates (VOPO4*nH2O;n=0,1,2) are reported. Measurements were made at seventeen frequencies between 10 and 105 Hz over the temperature range 5.5-380K. Two lowtemperature features were observed and are correlated with particular water species between the VOPO4 layers. In light of chemical evidence and compleximpedance measurements on related compounds, it is speculated that VOPCO4*nH2O (Pc = P, As) can be thought of as a mixed nonstoichiometric compound [VO(HPcO4)]x[VOPcO4O4]1-x*nH2O and the solid Brϕnsted acid nature of the compound is related to proton movement between layer pnictates and intralayer waters.