Faculty of Physics - University of Bucharest

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2024-11-23 18:33

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Conference: Bucharest University Faculty of Physics 2002 Meeting

Section: Solid State Physics and Materials Science

Title:

Ferroelectric Ceramics For Microwave Applications

Authors:

C.Berbecaru, H.V.Alexandru, A.Ioachim *, L.Nedelcu, M.Lisca

Affiliation:

Faculty of Physics, University of Bucharest.

* National Institute of Materials, Bucharest

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Keywords:

Abstract:

Ferroelectric ceramics have extensive applications in the microwave domain, including mobile phone communications, sterable antenna, military devices, etc. In the GHz frequency range, lower Curie point ceramics are preferred of higher permitivity values (2090). Several ceramic compositions and heating treatments we have tested for optimum qualities, on a large temperature range (–120  120 oC), at lower (1 kHz) and higher frequencies (GHz). Temperature stability of parts per million are required for permitivity of such ferroelectric ceramics, in the usual temperature range. In view of this fact, lower temperature characterization of permitivity and losses, were necessary in order to discern the nature and peculiar features of ferroelectric transitions. Earlier known ceramic compositions, based on barium titanate, using new dopants are intensively studied in the literature. Characterization of three such compositions we shall present. 1/ BNT–barium titanate 30 % doped with Neodim (2 samples, heating treatment 1340 and 1360 oC). Ceramics shows high permitivity values of ~ 80 up to 120 oC, at 1 kHz and 3 GHz. Ferroelctric transition could not be tested down to –100oC at 1 kHz, although a sort of Curie-Weiss law was partly tested (pyroelectric coefficient was measured down to – 150 oC, in the Institute of Materials). 2/ BST–barium strontium titanate ~ 1/1 ratio (2 samples of slightly different compositions). Both ceramics show peak permititivity values at – 60 (sample 1.1) and – 50 oC (sample 2.2) and diffuse transitions. In the ferroelectric phase, a secondary maxima appears around –105 oC and – 88 oC respectively. A minor secondary phase, of other composition, was confirmed by X-ray analysis. Typical permitivity hysteresis extends more than 60 oC in the ferroelectric phase. 3/ Barium titanate, 20 % Cerium doped samples, show also diffuse transitions around – 20oC and high (68)103 peak permitivity values.