Ferroelectric solid solutions (Ba,Sr)TiO3 for microwave applications

There is an increasing demand of high permittivity ferroelectric materials in microwave devices. Particularly, the Ba1−XSrXTiO3 system (BST), presents a major interest for electrically controlled devices, such as tunable filters, steerable antennas, phase shifters, varactors, etc. [F. De Flaviis, N.G. Alexopoulos, O.M. Stafsudd, IEEE Trans. Microw. Theory Tech., MTT-45 (6) (1997) 963-969]. Solid solutions with X = 25, 50, 75, 90% were prepared by standard ceramic technology (solid state reaction) and sintered at 1230 and 1260 °C. The temperature dependence of permittivity and of losses at low frequency (1 kHz) has shown interesting results [A. Ioachim, M.I. Toacsen, M.G. Banciu, L. Nedelcu, D. Ghetu, H.V. Alexandru, C. Berbecaru, G. Stoica, Proceedings on the XVII International Conference on Electromagnetic Fields and Materials, Warszawa, May 2004, pp. 78-82]. The unit cell volume depends linearly on the Sr content Ω (Å3) = 61.1 + 3.6X (estimated from Ref. [D. Roy, S. D. Krupandihi, Appl. Phys. Lett. 62 (1993) 1056-1058]). The measured densities of our samples ρ (g/cm3) ≈ 5.50 − 1.00X, represent about 92% of the X-ray estimated density ρ (g/cm3) ≈ 5.99 − 0.99X. Microwave measurements around 1 GHz show substantial decrease of the permittivity from about 1600 to 200 and also of the losses from 0.8% to less than 0.15% with the Sr concentration increase, X = 50-90%. It is concluded that sintering temperature has to be increased for samples of high Sr concentration. Moreover, the addition of MgO and MnO2, 1 wt.% each, improve sintering conditions and decreases the microwave losses. Experimental data show an approximate linear decrease of the Curie point, versus X strontium concentration, according to the formula: TC (°C) ≈ 120 − 360X. Room temperature permittivity and dielectric loss have been measured in microwaves range using the Hakki-Coleman method.