122, piezoceramic electrical

Historically, the first and most important capacitance method is the vibrating capacitor approach implemented by Lord Kelvin in 1897. In this technique (now called the Kelvin probe), the reference plate moves relative to the sample surface at some constant frequency and tlie capacitance changes as tlie interelectrode separation changes. An AC current thus flows in the external circuit. Upon reduction of the electric field to zero, the AC current is also reduced to zero. Originally, Kelvin detected the zero point manually using his quadrant electrometer. Nowadays, there are many elegant and sensitive versions of this technique. A piezoceramic foil can be used to vibrate the reference plate. To minimize noise and maximize sensitivity, a phase-locked... 

Since discovering and making use of the piezoelectric effect in naturally occurring crystals such as quartz and Rochelle salts, scientists have produced a wide range of piezoelectric materials in the laboratoi y. An early example is barium titanate, used in an electrical component called a capacitor. Currently, most piezoelectric materials are oxide materials based on lead oxide, zirconate oxide, and titanium. These very hard piezoelectric materials are termed piezoceramics. 

Kymissis et al. have examined the possibility of generation of electrical power parasitically from devices built in a shoe, a wearable subsystem for the soldier. Merits of three different types of piezoelectric devices are compared. They are a unimorph strip piezoceramic composite, a stave of multilayer laminate of PVDF foil, and a shoe-mounted rotary magnetic generator as a part of technology demonstration a piezoelectric embedded shoe has also been postulated to periodically broadcast a digital radio frequency identification (RFID) signal as the wearer walks. 

For polycr5rstalline piezoceramics to work, the electrically charged dipoles within the entire piezoelectric component must be aligned by placing the piezoceramic within a high electric field—a process known as poling. The ability of piezoceramics to almost instantaneously convert electrical... 

PWAS are made of thin, inexpensive piezoceramic wafers electrically poled in the thickness direction and provided with top and bottom electrodes. Typical PWAS are 7 mm squares or discs with a 0.2 mm thickness (Figure 16.6). PWAS can be bonded to the stmcture with strain gauge installation methodology. They have also been experimentally inserted between the layers of a composite layup, but this option has raised some structural integrity issues that are stiU being examined. The PWAS transducers are called differently by various authors piezos, piezo wafers, piezo wafer transducers or PZTs, etc. In this chapter, we will consistently call them PWAS (same in singular and in plural) or PWAS transducers. ... 

Elastomechanical ultrasonic vibration is generated by the transformation of electric energy in piezoceramic or magnetostrictive sonic converters. A voltage generator serves to convert a low-frequency mains voltage into... 

Electric energy is converted into mechanical vibrations in modem machinery using the piezoelectric effect. It concerivs the reversible property of special ceramic materials to deliver an electric voltage when affected by external forces. This characteristic is used for the generation of ultrasonic vibrations in such a way that the applied voltage is converted into mechanical vibrations. Modern sonic converters usually contain several piezoceramic disks of lead zirconate titanate restricted by two final masses that are mechanically prestressed by a centric screwing. 

Piezoelectric transducers are the most common devices employed for the generation of ultrasound and utilise ceramics containing piezoelectric materials such as barium titanate or lead metaniobate. The piezoceramic element commonly used in ultrasonic cleaners and for probe systems is produced in the form of a disk with a central hole. Ceramic transducers are potentially brittle and so it is normal practice to clamp them between metal blocks. This serves both to protect the delicate crystalline material and to prevent it from overheating by acting as a heat sink. Usually two elements are combined so that their overall mechanical motion is additive (Figure 10.4). Piezoelectric transducers are better than 95% electrically efficient and can operate over the whole ultrasonic range. 

Fine scaled composites can be prepared starting with molded and sintered ceramic arrays made by the soft mold process. Once the piezoceramic pillars have been formed, the remaining spaces are filled with a polymer matrix material. Next the base is removed by grinding. Metal electrodes are then bonded to the ends of the fibers and first used to polarize the piezoceramic at an elevated temperamre and then to apply an electric field or collect developed charges from the material. At this point, the active elements are ready to be used as piezoelectric transducer element. Experiments have shown that high-performance composites can be prepared . 

For speakers or buzzers, audible by humans, devices with a rather low resonance frequency are used (kilohertz range). Examples are a bimorph consisting of two piezoceramic plates bonded together, and a piezoelectric fork consisting of a piezodevice and a metal fork. A piezoelectric buzzer is shown in Figure 4.1.12, which has merits such as high electric power efficiency, compact size and long life. 

A lead containing a piezoceramic disk (e. g. PZT) is reduced on one side by high temperature treatment in direct contact with a carbon block. This reduced layer is no longer piezoelectric but therefore a good electric conductor. Due to the thermal expansion mismatch between the reduced and oxide layers, a curvature develops in the structure, giving it a dome (or rainbow) shape. 

Conventional test equipment like servo-hydraulics or servo-electrics are limited to driving frequencies of typically 50 Hz with respect to working with arbitrary drive signals. Thus, one approach to extending the frequency range can be to combine low and high frequency actuator devices within one test facility whereas the high frequency actuation could be done by piezoceramic interface structures similar to those described before. 

Todoroki, A., 2003. High performance estimations of delamination of graphite/epoxy laminates with electric resistance change method. Compos. Sci. Technol. 63 (13), 1911-1920. Weder, A., et al., 2012. A novel technology for the high-volume production of inteUigent composite structures with integrated piezoceramic sensors and electronic components. Sensors Actuators A 202, 106-110. 

Hao, J.G., Shen, B., Zhai, J.W., Chen, H., 2014. Phase transitional behaviour and electric field-induced large strain in alkaU niobate-modified Bi-0.5(Na0.80K0.20)(0.5(Ti03) lead-free piezoceramics. Journal of AppUed Physics 115, 034101. 

Within a piezoelectric DoD printer, a piezoceramic plate alters its thickness if an electrical field is applied between its opposite surfaces. Subsequentiy, a pressure wave is induced into the fluid that causes a droplet to be emitted from the nozzle. Fluid is normally retained at the orifice due to surface tension at the fluid/air interface hence the action of the pressure wave is to overcome this surface tension and to expel a stream of liquid from the orifice. In order to minimize the internal energies in the liquid, again due to the surface tension of the liquid, droplets will be formed and are ejected from the nozzle. Typical repetition rates, that is, print frequencies, of DoD printers are in the range of 0.1-30 kHz, whereas continuous ink-jet printing print can reach frequencies of 500 kHz or even up to 1 MHz. ... 

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