Oxides ferroelectric (Ba, Sr)TiO3 for microwave devices

Barium-strontium titanate (BST), ternary ceramic compounds with the molar formula (Ba1-xSrx)TiO3, were prepared by solid-state reaction from raw materials. Three compositions with x = 0.25, 0.50 and 0.75, have been checked. Samples with two type of thermal treatment at 1230 and 1260degreesC have been manufactured and analyzed. Perovskite type, polycrystalline structure of the BST ceramics was revealed by the X-ray diffraction data at room temperature. Strontium ion (Sr2+) enters substitutionally on the Ba2+ site. A small decrease in the unit cell volume with the Sr content increase was observed. Dielectric measurements have been performed at low and high frequencies. The temperature dependence of low frequency permittivity shows large interval of ferroelectric transition, which appears to be merely due to dispersion of crystallites dimensions. The increasing concentration of Sr ion leads to a shift of the Curie point below room temperature. Experimental data show a linear decrease of the Curie point T-C (degreesC); approximate to 120-360x, versus x Strontium fraction. Room temperature permittivity and dielectric loss have been measured in microwaves by using the Hakki-Coleman method. BST samples prepared using water as a milling medium exhibit lower porosity and lower dielectric loss in microwaves than samples prepared using acetone as a milling medium. Dielectric loss Of 10(-3) at 1 GHz shows that this ceramic material is quite suitable for high frequency device applications. (C) 2003 Elsevier B.V. All rights reserved.