Microwave ceramics

More detailed recent information on ferroelectric relaxor ceramics can be obtained from Ye (2008). Lead-free BaFeo.5Nbo.5O3 ceramics with multiferroic properties were developed by Bochenek et al. (2009). 

Grain boundary-divided deposits of thickness between 20 A and about Igm consist of an ionic stratification of impurities separated along the grain boundary. As impurities are easily dissolved in the grain boundaries, the composition and crystal phases, respectively, in the grain boundary are very different from those inside the crystal grain. When the amount of impurities increases beyond the Hmit of solid solution, a separate crystalline phase precipitates at the grain boundary. 

Grain boundary segregation (X is the component increased the grain boundary) 

The rapid growth of satellite and mobile radio communications has led to a requirement for narrow band, frequency-stable filters and oscillators. Selectivity and stability are necessary to ensure that signals are confined to closely defined allotted frequency bands and to prevent the intrusion of unwanted signals which would interfere with the satisfactory performance of the system. Antennae are also critical components of any wireless communications system. The need for compactness in satellite and hand-held mobile systems is self-evident. 

The solution to providing stable filters and oscillators in the past lay in bulky coaxial and cavity resonators fabricated from the temperature-stable metal alloy Invar. The dielectric resonator (DR) offers a means of miniaturizing the device. 

In its simplest form a DR is a cylinder of ceramic of relative permittivity 8r sufficiently high for a standing electromagnetic wave to be sustained within its volume because of reflection at the dielectric-air interface. The electric and magnetic field components of the fundamental mode of a standing electromagnetic field are illustrated in Fig. 5.33. 

The wavelength 2d of the standing wave in the dielectric approximates to the diameter, D, of the cylinder, i.e. /.d D. If the resonance frequency is/r then in free space /r = c/2o where c and /.q are respectively the free space velocity and wavelength. In a non-magnetic dielectric medium the velocity pd = c/elJ2 (see Eq. (2.120)) so that 

If the temperature changes, then the resonance frequency f. will also change because of changes in 8r and D. Differentiating Eq. (5.35) with respect to temperature gives 

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This behavior is not yet fully understood. So far, most of the material development has been performed in companies, indicating that the market for microwave dielectrics has been grown tremendously over the last years (as an example, each commercial satellite receiver contains a small microwave ceramic disk as frequency stabilising element for the local microwave... 

The microwave ceramics discussed so far are prepared by sintering pellets of pressed power. Usually, the sintering temperatures are very high (typically around 1200 to 1500°C)... 

The following discussion outlines the historical path taken in the development of microwave ceramics combining high permittivity, high Q and zero tf with long term stability and at an economic cost. Key references are given in the overview by R. Freer [10]. 

Fig. 5.36 The Zr cTiJ,Snz04 (ZTS) system the shaded area indicates the single-phase microwave ceramic field (after [11]).

The development of improved microwave ceramics, and especially research into loss mechanisms, necessitates reliable methods for characterizing properties, particularly Q. A popular method is that described by Hakki and Coleman [14]... 

Fig. 5.38 Microwave ceramic components (a) metallized ceramic engine block for 40 MHz pass band filter at 1.4 GHz (b) 11.75 GHz oscillator incorporating ceramic dielectric resonator together with various resonator pucks.

For high-temperature microwave ceramic processing, the refractory specimen enclosure (the casket) serves as both i) a thermal insulator and ii) a microwave susceptor. The susceptor allows the microwave energy to couple with the material at high temperature. The thermal insulation diminishes the dissipation of thermal energy. On occasion, the caskets are referred to as susceptors, which does not fully depict the dual role of the specimen enclosure or casket as both a thermal insulator and a microwave susceptor (absorber). 

Microwave ceramics Electrical and specific dielectric properties in the microwave range Radomes, microwave-substrates, -filters, workstation windows, insulating beads, rocket heats... 

Gltiss-ceramic composites Bioceramics, electronic ceramics (LTTC carriers, microwave ceramics)... 

For microwave ceramics, the crystal structure should be perfect with good densification in order to get good dielecuic properties. 

Several strategies for the design of dielectric microwave ceramics with high permittivities, high Q-factors, and a close to zero temperature coefficient of the resonance frequency, are Usted in Table 8.5. 

Wersing, W. (1996) Microwave ceramics for resonators and filters. Curr. Opin. Solid State Mater. Sci., 1 (5), 715-731. 

The accurate measurement of sample temperature during heating by a microwave field is seen as one of the major problems to be overcome for the successful exploitation of microwave ceramic processing. Infra-red pyrometry has been used extensively to date, particularly when a single mode applicator forms the basis of the sintering system. However, this technique suffers from the disadvantage that only the surface temperature of the samples is recorded. 

Despite the widespread view discussed above that ceramic materials are essentially transparent to microwave frequencies, perhaps the greatest amount of research effort in the field of microwave ceramic processing is concerned with firing or sintering. Most of this effort, however, has occurred over the last 5 years with a large number of ceramic systems now having received attention. 

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