High frequency applications

These applications can be divided into those which demand high power (e.g. ultrasonic cleaner) intermediate power (e.g. Tweeter ) and signal power (e.g. Delay line). Some examples are briefly described below. 

The structure of a typical composite power transducer for ultrasonic cleaning is illustrated in Fig. 6.32. The PZT toroids are kept in compression (stress 25 MPa) by the bolt to reduce the risk of their fracturing under the high drive fields. The PZT will be of the hard variety ( acceptor -doped see Section 6.3.2) so as to minimize the risk of depolarization under the high mechanical stresses experienced. The associated high Qm value and low electrical tan 3 ensure that the losses are kept within acceptable limits. The structure has the added advantage that any heat developed in the ceramic can be dissipated through the massive metal end pieces. 

The device must be dimensioned so that at resonance a half-wavelength is accommodated in the distance L. The amplitude of motion at the outer metal surface will be related to that of the ceramic through the ratio q of their acoustic 

A final benefit of this type of structure is that the acoustic behaviour is primarily governed by the properties and dimensions of the metal parts and only to a minor extent by those of the ceramic, so that small variations in the ceramic properties become unimportant. 

For efficient transfer of power from the generator to the medium, usually water, the two must be acoustically matched. The discontinuity can be smoothed by fixing a 2/4 thick layer of material having an acoustic impedance intermediate between that of the radiating surface material and water, and polymers having impedances of about 3.5 Mrayl are readily available. The velocity of sound in them is approximately 2500 ms-1 so that the thickness required at 50 kHz is about 12 mm. In practice the transducer is often bonded to an ultrasonic cleaning tank and then the tank and water become a complicating part of the transducer. 

A recently published and interesting market study by Yole Developpement estimates the high-frequency market for SiG MESFETs to be 400 million in 2007 [65]. This also takes into account a dramatic annual price reduction of the components. 

A GaN substrate would be a help in this respect but it would need to be semi-insulating. In addition, GaN has a poor thermal conductivity and is not very suitable due to this negative material property. Aluminum nitride substrates may become the substrate of choice for GaN high-frequency applications. It has a reasonable thermal conductivity and is intrinsically semi-insulating but only time will tell. 

Unlike SiC, GaN is still not mature for the market. There is no doubt that the technological issues will be overcome but it will take some time. And finally, since money rules in the ruthless commercial world, will the manufacturing cost of GaN HEMTs be low enough to capture a lion s share or just a niche portion of the market  

By far the biggest application for SiC technology is the high-power electronics market. It may not be as glamorous as the high-frequency or the optoelectronic markets but it is big. The current size of the power-device market is 16 billion [65] and it is growing at a rate of almost 10% per year [66]. The question on everyone s mind is how big will the SiC share be  

if we begin substituting the old-fashioned Si devices with fresh new ultra-fast SiC unipolar devices, more than 75% of the power will go through an SiC device. 

---

If the connectors are to be used in high frequency applications, they must be made of plastics with low dielectric loss to avoid either damage to the part or signal loss in the circuit. 

The inversion procedure is most straightforward when attenuation in the coupling fluid is ignored. This may present problems in high-frequency applications. 

Soft magnetic ferrites are oxides and they are electrical insulators. Because of their exceptionally higher resistivities, ferrites are particularly suitable for high frequency applications, of about 100.000 cycles 110 kHz). 

Plasma oxide has found utility in high-frequency applications for dual-layer isolation,8 because of its low dielectric constant and high breakdown voltage. Also, it is in compression when deposited, so that it can be used as the dielectric when thick films (2 to 5 microns) are needed. Such thick films when deposited by thermal CVD (which is deposited in tension) tend to crack. One final advantage to the use of plasma oxide rather than plasma nitride is that... 

Magneisum ferrite spinel. MgFe204 is one a representative of soft ferrites extensively used for high-frequency applications. Fully dense bulk ferrites have been synthesized by low-temperature sintering of fine powders, by thermal decomposition and by co-precipitation. Preparation from ultra-fine powder is more advantageous since the composition can be more easily controlled, and the electrical and thermal properties are improved as a result of the reduced grain size. 

Substrates The substrates in microelectronics are mainly Si wafers. For mobile applications, silicon-on-insulator (SOI) wafers increasingly replace bulk Si wafers and for very specific high-frequency applications, III-V compound semiconductors (e.g., GaAs) are used. The majority of substrates in microfabrication are Si wafers, but metal, glass, and ceramic substrates are also common. Particularly when using glass, quartz, and ceramic wafers in CMP processes, it has to be taken into account that they are brittle and easy to break. The situation is worse when the material is also under stress induced by deposited layers. For applications where the backside of the wafer has to be structured (e.g., in bulk micromachining), double-side polished substrates are employed. 

The capacitance of the aerogels can be improved via thermal and electrochemical activation procedures. For high frequency applications the first activation procedure is to... 

High frequency applications in which the wavelength is comparable to the scale of the composite macrostructure, show the full potential of composite structures. Impedance, bandwidth, and radiation pattern can be controlled in such systems in a sophisticated manner impossible in single-phase systems. By prepoling PZT fibers or ribbons before the assembly of the composite, it is possible to construct polar solids of new type for use in complex transducer arrays operating in scanning and focusing modes. 

The magnitude of the applications for polymeric substrates has been estimated (in tons) for 1987 on a worldwide basis as phenolic resin, 78K epoxy resin, 130K polyester fiber, 1,010 polyimide film, 235 molding compounds, 330 polymers for high-frequency applications, 300 and high-temperature polymers, 1,440 (4). 

PTFE Teflon Politef Tetrafluoroethylene Resin Fluor Tetran. Versatile, chemically inert thermoplastic homopolymer. Used as tubing or sheeting for chemical laboratory and process work gaskets and pump packings as electrical insulators especially in high frequency applications, filtration fabrics, protective clothing, prosthetic aids. White solid usable between -270° and 265°. DuPont Janssen Chimica. 

---