Temperature‐stable, medium‐permittivity dielectric ceramics have been used as resonators in filters for microwave (MW) communications for several decades. The growth of the mobile phone market in the 1990s led to extensive research and development in this area. The main driving forces were the greater utilization of available bandwidth, that necessitates extremely low dielectric loss (high‐quality factor), an increase in permittivity so that smaller components could be fabricated, and, as ever in the commercial world, cost reduction. Over the last decade, a clear picture has emerged of the principal factors, that influence MW properties. This article reviews these basic principles and gives examples of where they have been used to control microwave properties and ultimately develop new materials.
High unloaded quality factor (Qu), zero temperature coefficient of resonant frequency (τf) and high relative permittivity (εr) microwave dielectric ceramics have been fabricated based on BaZn1/3Nb2/3O3. Properties have been optimized for the composition, 0.9Ba([Zn0.60Co0.40]1/3Nb2/3)O3–0.1Ba(Ga0.5Ta0.5)O3 for which Qu=32 000 @ 3.05 GHz, εr=35, and τf=0. The new compounds are disordered according to x-ray diffraction (XRD) and may be indexed using a simple perovskite unit cell, a=4.09 Å. Small peaks (e.g., d≈3.01 Å, relative intensity, 4.5) attributed to a barium niobate second phase are also present in XRD patterns. These ceramics are suitable in terms of cost and performance for base stations supporting third generation architecture.
The dielectric response of ferroelectric-dielectric composites is theoretically addressed. Dielectric permittivity, tunability (relative change of the permittivity driven by dc electric field), and loss tangent are evaluated for various composite models. The analytical results for small dielectric concentration and relative tunability are obtained in terms of the traditional electrostatic consideration. The results for large tunability are obtained numerically. A method is proposed for the evaluation of the tunability and loss at large concentrations of the dielectric. The basic idea of the method is to reformulate the effective medium approach in terms of electrical energies stored and dissipated in the composite. The important practical conclusion of the paper is that, for random ferroelectric-dielectric composite, the addition of small amounts of a linear dielectric into the tunable ferroelectric results in an increase of the tunability of the mixture. The loss tangent of such composites is shown to be virtually unaffected by the addition of moderate amounts of the low-loss dielectric. The experimental data for (Ba,Sr)TiO3 based composites are analyzed in terms of the theory developed and shown to be in a reasonable agreement with the theoretical results.
The ceramic Ba8ZnTa6O24 has been synthesized in isolation and its dielectric and crystallographic properties characterized. The material affords excellent dielectric properties, with a high unloaded quality factor Qu=20 800 at 3.28 GHz, high relative permittivity εr=29 and a temperature coefficient of resonant frequency τf=29.4 ppm/°C. The crystal structure adopted is complex, comprising mixed cubic and hexagonal perovskite subunits, and contains cation vacancies on the octahedral sites. A second phase with a closely related structure is identified, demonstrating the existence of a family of materials. This structural complexity offers diverse opportunities for substitutions calculated to enhance the figures of merit reported.
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