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-XSr XTiO3 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 deg. 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 (A3) = 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: T C ( deg. C) ∼ 120 - 360X. Room temperature permittivity and dielectric loss have been measured in microwaves range using the Hakki-Coleman method