Tunability of the dielectric constant of Ba 0.1 Sr 0.9 TiO 3 ceramics in the paraelectric state
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Tomoko Kito and Baki Yarar Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, Colorado 80401-1887 (Received 3 January 1994; accepted 13 February 1995)
(x = 0.9) ferroelectric ceramics were prepared successfully using a new wet technique and their structure and dielectric properties compared with those synthesized by the solid-state reaction method. The voltage dependence of the dielectric constant in the paraelectric phase was examined. It was found that the dielectric constants of these materials, prepared by both methods, exhibit large changes with applied voltage in the paraelectric phase. Tunability (the percentage change of the dielectric constant from its zero-bias value in the presence of a dc-biasing electric field) was observed to exceed 30% at only 1.7 kV/cm at 77 K in the samples prepared by the solid-state reaction method. The tunability was found to decrease dramatically as the operating temperatures increased above the Curie point. These observations are interpreted in light of an existing phenomenological theory. The dependence of the dielectric constant in the paraelectric state upon a dc-biasing electric field is also demonstrated as a potential method for the characterization of dielectric nonuniformities in ferroelectric ceramics.
I. INTRODUCTION Ferroelectric materials have received considerable attention recently because of their growing use in electronic, electro-optical, optical, acoustic, and microwave devices.1'2 Potential applications include integrated nonvolatile and dynamic random access memories, pyroelectric detectors, acoustic transducers, and microwave resonators as well as phase shifters.2 Most of the abovementioned applications exploit the properties of the ferroelectric phase. An important characteristic of the ferroelectric and paraelectric phases is the ability to change the dielectric constant and the loss-tangent with an external voltage. However, the dielectric losses are usually higher below the Curie point due to the additional hysteresis losses. Device losses can be reduced by using superconducting instead of metallic electrodes. This idea is being actively pursued to produce novel thin film and voltage-tunable microwave devices such as resonators, filters, and phase shifters.3"5 For such applications, it is desirable that the Curie point of the ferroelectric material be lower than the operating temperature (usually 77 K for high-temperature superconductors) in order to minimize losses. Bai_xSr^TiO3 (x = 0 to 1) is potentially attractive for these applications as its Curie point can be controlled by varying the mole fraction of x of Sr. The Curie point of this material varies monotonically from about 400 K for BaTiO3 (x = 0) to 40 K for x = J. Mater. Res., Vol. 10, No. 6, Jun 1995 http://journals.cambridge.org
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0.95.6 This change is accompanied by a number of dielectric properties which can then be tailored for a specific application at a given operating temperature. Ba^S^TiC^ (x = 0 to 1) ceramics have been
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