Preface

A large number of papers continue to be published in the area of microwave materials and applications; however, there have hitherto been no books in the open literature that treat the different types of microwave materials, such as low-loss dielectric ceramics, glass ceramics, polymer- and elastomer-based composites, cofired ceramics, electromagnetic interference (EMI) shielding materials, tunable dielectrics, dielectric inks, or microwave ferrites. Our purpose here is to provide a comprehensive and self-consistent volume whereby the aforementioned lack of a reference book in the open literature can be remedied.

Microwave dielectric materials play a key role in global society, with a wide range of applications straddling terrestrial and satellite communications, including Internet of Things (IoT), software radio, GPS, DBS TV, environmental monitoring via satellite, etc. The mobile phone is one of the most widely spread technologies on the planet. In many countries the number of mobile subscriptions exceeds the population. The IoT is posed to make an explosive growth in the near future. In this paradigm, many everyday objects will be networked via radio-frequency identification (RFID), printed electronics, and sensor network technologies. Fifth generation (5G) mobile networks or wireless systems represent the next major phase of mobile telecommunication standards beyond the current 4G/IMT-Advanced standards. In addition to providing greater speeds, 5G networks will also need to meet the needs of new uses, such as the IoT as well as broadcast-like services and lifeline communications for times of natural disaster. According to GSMA Intelligence, the revenue for mobile network operators from interconnected devices in the segments of automotive, health, utilities, and consumer electronics will be $1.3 trillion by 2020. The worldwide data volume in mobile communication systems doubles nearly every year. To address this challenge, higher frequency bands will be used for which broadband and multiband equipment is required. The new standard Long Term Evolution (LTE-A) and the standardization process for 5G mobile communications necessitates changes in the technology of antennas and filters. In order to meet the specifications of future systems, new designs and improved or new microwave dielectric components are required. The recent progress in the IoT, microwave telecommunications, satellite broadcasting, and intelligent transport systems (ITSs) has resulted in an increasing demand for low-loss dielectric materials, tunable dielectrics, microwave ferrites, and EMI shielding materials. Low-loss dielectric oxide ceramics have revolutionized the microwave wireless communication industry by reducing the size and cost of filter, oscillator, and antenna components in applications ranging from cellular phones to the IoT. Wireless communication technology demands materials with highly specialized properties. Recently the demand for materials with low sintering temperatures has increased not only to lower the energy cost of devices but also to integrate with polymers and silver-based electrodes. Several polymer-based (polymer-ceramic) composites have also recently been developed for wireless communication technology.

A dielectric resonator is an electromagnetic component that exhibits resonance for a narrow range of frequencies. The resonance is similar to that of a circular hollow metallic waveguide except that the boundary is defined by a large change in permittivity rather than by a conductor. Dielectric resonators (DRs) generally consist of a puck of ceramic that has a high permittivity and a low dissipation factor (tan d). The resonant frequency is determined by the overall physical dimensions of the puck, the permittivity of the material, and its immediate surroundings. The key properties required for a dielectric resonator are high quality factor (Q ~ 1/tan d), high relative permittivity (?r), and near-zero temperature coefficient of resonant frequency (tf). The first chapter describes the different techniques for measuring the microwave properties of materials. Chapter 2 discusses the modeling of relative permittivity of composites using different mixing rules. Developing dielectric resonator materials in which these three properties are simultaneously optimized is difficult. Low-permittivity ceramics are used for millimeter-wave communications and also as substrates for microwave integrated circuits. The medium permittivity ceramics with ?r in the range 25-50 are used for satellite communication and in cell phone base stations. High-?r materials are used in mobile phones where miniaturization of components is very important. For millimeter-wave applications, temperature-stable, low-permittivity, and high-Q (low-loss) substrates are required for high-speed signal transmission with minimum attenuation. Chapters 3 to 6 describes such materials.

The term dielectric resonator (DR) first appeared in 1939 when Richtmeyer showed that a suitably shaped dielectric piece can function as a microwave resonator; however, it took more than 20 years to generate further interest in DRs and to verify Richtmeyer's prediction experimentally. In the early 1960s Okaya and Barash rediscovered DRs while working on rutile single crystals. They measured the permittivity and Q of TiO2 single crystals at room temperature down to 50 K in the microwave frequency range using the commensurate transmission line technique. In the early 1960s Cohn and his co-workers performed extensive theoretical and experimental work on DRs. Rutile ceramics that had an isotropic permittivity of about 100 were used for their experiments, but TiO2 has a poor (+450 ppm/oC) resonant frequency stability that prevented its commercial exploitation. The first microwave filter using TiO2 ceramics was proposed by Cohen in 1968, but this filter was not useful for practical applications because of its high ?r and frequency instability with temperature. A real breakthrough in dielectric resonator ceramic technology occurred in the early 1970s when the first temperature-stable low-loss barium tetratitanate (BaTi4O9) ceramics were developed by the Raytheon Company. Later, barium nanotitanate (Ba2Ti9O20) with improved performance was reported by Bell Laboratories. The next breakthrough came from Japan when Murata Manufacturing Company produced (Zr, Sn)TiO4 ceramics. Commercial production of dielectric resonators started in the early 1980s.

In the last two decades there has been extensive progress in the development of terahertz (THz) technology. This progress has enabled a multitude of potential applications, be it communications, biomedical, imaging, security, quality control of food and agricultural products, matter and light control, or chemical agent detection - in every application it has emerged as one of the most promising areas of interest. The terahertz region of the electromagnetic spectrum, referring roughly to the frequencies from 100 GHz or 0.1 THz to 30 THz, is the bridge between the microwave and infrared spectral bands. The promise of THz radiation resides in its unique properties and characteristics: its non-ionizing nature; its submillimeter spatial resolution; its transmission through dielectric materials that are transparent to it; its interaction with water molecules, making it useful for certain medical diagnoses; its unique signature for molecular and rotational energy levels of many biological and chemical agents; and its potential for wireless communications. Although a large number of materials have been characterized in the microwave frequency range, relatively little attention has been paid to millimeter-wave or terahertz characterization of materials, possibly due to the relatively expensive facilities required. Chapter 5 discusses mm-wave material systems and applications of this technology.

Glasses have good thermal properties compared to polymers and better material homogeneity in comparison to ceramics, and hence they are considered for microwave applications; however, glasses in general have higher dielectric losses and a limited range of dielectric constants. Glass-ceramic composites have the advantage of an increased range of dielectric constants and reduced losses. They can be made with excellent homogeneity to produce sophisticated miniaturized microwave electronic devices. Chapter 7 discusses microwave dielectric properties of important glasses and glass-ceramics.

In the past, microwave devices have been traditionally machined from metal, and coaxial RF connections were made with metallic connectors, generally leading to expensive, heavy, and bulky packages. Moreover, today's electronic circuits for the automotive industry, entertainment electronics, and telecommunications have to handle a steadily increasing level of functionality whilst occupying the smallest volume possible. In the development of complex miniaturized circuits, flexible glass/ceramic or ceramic tapes with sintering aids made of high-temperature cofired ceramics (HTCCs), low-temperature cofired ceramics (LTCCs), and ultralow-temperature cofired ceramics (ULTCCs) play a decisive role as base materials. The choice of HTCC, LTCC, or ULTCC tapes is based on the electrode materials used and cofiring temperature. Cofired ceramics have become crucial in the development of various modules and substrates. In this technology, several thin layers of low-permittivity...