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.

Microwave Dielectric Ceramics for Resonators and Filters in Mobile Phone Networks

Dielectric ceramic materials have been studied for decades due
to both their application in important technologies and the fundamentally
interesting relationships among their crystal chemistry, crystal structures,
and physical properties. Recent dramatic changes in microelectronics and in
particular wireless communications technologies have made the importance of
materials with the unusual combination of high dielectric constant, low dielectric
loss and low temperature dependence of dielectric constant of great interest.
In this review, the present state of knowledge of the chemistry, structure,
and dielectric properties of these materials, as well as examples of our own
work, are described.

Dielectric materials for applications in microwave communications

The dielectric loss (tan δ) of titanium dioxide (TiO2) disks has been measured at a frequency of 3 GHz High-purity TiO2 sintered to almost-full density exhibits a very high tan δ, which is interpreted to be due to oxygen deficiency To counter this, doping with stable divalent and trivalent cations, such as Mg and Al, leads to a low tan δ, probably by preventing Ti4+ reduction The tan δ of polycrystalline TiO2 doped with divalent and trivalent ions with ionic radii in the range of 05–095 A at 3 GHz can be very low: 6 × 10−5 (Q∼ 17 000) at a temperature of 300 K The tan δ of undoped pure TiO2 disks increases when the disks are cooled from 300 K to ∼100 K At temperatures <100 K, the tan δ decreases rapidly, which is interpreted as carrier freeze-out The tan δ for all the high-Q doped TiO2 polycrystalline samples smoothly decrease to ∼5 × 10−6 (Q∼ 200 000) at 15 K, comparable to that of single crystals

Microwave Dielectric Loss of Titanium Oxide

Ferroelectric materials are the best dielectric and piezoelectric materials known today. Since the discovery of barium titanate in the 1940s, lead zirconate titanate ceramics in the 1950s and relaxor-PT single crystals (such as lead magnesium niobate-lead titanate and lead zinc niobate-lead titanate) in the 1980s and 1990s, perovskite ferroelectric materials have been the dominating piezoelectric materials for electromechanical devices, and are widely used in sensors, actuators and ultrasonic transducers. Energy losses (or energy dissipation) in ferroelectrics are one of the most critical issues for high power devices, such as therapeutic ultrasonic transducers, large displacement actuators, SONAR projectors, and high frequency medical imaging transducers. The losses of ferroelectric materials have three distinct types, i.e., elastic, piezoelectric and dielectric losses. People have been investigating the mechanisms of these losses and are trying hard to control and minimize them so as to reduce performance degradation in electromechanical devices. There are impressive progresses made in the past several decades on this topic, but some confusions still exist. Therefore, a systematic review to define related concepts and clear up confusions is urgently in need. With this objective in mind, we provide here a comprehensive review on the energy losses in ferroelectrics, including related mechanisms, characterization techniques and collections of published data on many ferroelectric materials to provide a useful resource for interested scientists and engineers to design electromechanical devices and to gain a global perspective on the complex physical phenomena involved. More importantly, based on the analysis of available information, we proposed a general theoretical model to describe the inherent relationships among elastic, dielectric, piezoelectric and mechanical losses. For multi-domain ferroelectric single crystals and ceramics, intrinsic and extrinsic energy loss mechanisms are discussed in terms of compositions, crystal structures, temperature, domain configurations, domain sizes and grain boundaries. The intrinsic and extrinsic contributions to the total energy dissipation are quantified. In domain engineered ferroelectric single crystals and ceramics, polarization rotations, domain wall motions and mechanical wave scatterings at grain boundaries are believed to control the mechanical quality factors of piezoelectric resonators. We show that a thorough understanding on the kinetic processes is critical in analyzing energy loss behavior and other time-dependent properties in ferroelectric materials. At the end of the review, existing challenges in the study and control of losses in ferroelectric materials are analyzed, and future perspective in resolving these issues is discussed.

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Losses in ferroelectric materials

A dielectric ceramic comprised of (Zn1-xMgx)TiO3 (x= 0 to x= 0.5) with low sintering temperature and promising microwave properties was prepared by applying a semichemical synthesis route and a microbeads milling technique. X-ray diffractometry and thermal analyses results indicated that the phase stability region of the hexagonal (Zn,Mg)TiO3 extended to higher temperatures as the amount of magnesium increased. The dielectric properties in this system exhibited a significant dependence on the sintering conditions, especially near the phase decomposition temperature. From 950°C, the temperature compensation characteristics occurred as the phase composition changed from hexagonal (Zn,Mg)TiO3 to two phases: (Zn,Mg)2TiO4 and rutile. The magnesium content for zero temperature coefficient (tauf) was ~3 mol% at 950°C; however, tauf increased with the sintering temperatures because of the shift of the decomposition temperature.

Low‐Fired (Zn,Mg)TiO3 Microwave Dielectrics

Ceramics based on BaTi 4 O 9 and Ba 2 Ti 9 O 2 0 have been largely ignored over the past decade by manufacturers of high Q, high dielectric constant, temperature compensated materials for commercial wireless communications at elevated frequencies. When processed correctly, these ceramics yield properties that rival if not exceed those of ZrTiO 4 -based products currently used. Materials incorporating ZnO/Me 2 O 5 (Me = Ta, Nb) and their properties are described.

N Coats

Papers

BaTi4O9/Ba2Ti9O20-based ceramics resurrected for modern microwave applications