Piezoelectric disc

Fig. 6.5 A typical impedance-frequency characteristic for a piezoelectric disc specimen having diameter 25 mm and thickness 1 mm and poled through the thickness.

The measuring system is the oscillating bubble tensiometer, described in detail elsewhere (Loglio et al. 1996, 1998b, 2001, Kovalchuk et al. 2001, Miller et al. 2000). Figure 1 shows a schematic drawing of the measurement assembly, which is composed of three parts 1) the sample (20 mm x 20 mm x 40 mm) pyrex vessel, 2) the capillary (2.00 mm internal diameter stainless-steel tube with flat tip ), 3) an air chamber with deformable walls (made with piezoelectric discs). Possible fluidodynamic instability of the bubble has been taken into account in the design of the bubble pneumatic circuit. 

Guo, N. Cawley, P. Hitchings, D. (1992). The Finite Element Analysis of the Vibration Characteristics of Piezoelectric Discs, Journal of Sound and Vibration, Vol..l59, No.l, pp.115- 38, ISSN 0022-460X... 

Fig. 7.4 Equivalent circuit of a piezoelectric disc (A), embedded in a structure (B).

To characterize the behavior of a piezoelectric disc it is helpful to use the complex impedance Z, ie, the ratio of voltage to current. By varying the exciting frequency, a typical frequency response curve of such elements can be obtained with a minimum amount of the impedance Z in the case of series resonance and a maximum in the case of parallel resonance. The two frequencies also represent the respective proportion of energy forms, the mechanical and electrical energy (Fig. 7.5). 

Krause (Ref 16) describes a piezoelectric device, called "carbon piezometer , in which a carbon disc, similar to the one used in telephones, is utilized in lieu of quartz or tourmaline crysts... 

A more common geometry is a thin disc of diameter d electroded over both faces and poled in a direction perpendicular to the faces. The resonance on which attention is focused is that of a radial mode, excited through the piezoelectric effect across the thickness of the disc. In this case the route from the resonant frequencies to the coefficients d and g is the same as in the case of the rod, although the expressions are more complex. 

The attractive features of a piezoelectric/electrostrictive actuator are mentioned above, but a penalty is the small displacements generated. For a d33 value of 500 pCN-1 (or, more appropriately in the present context, pmV-1) a voltage of 1000 Y applied across a 1mm thick poled disc would produce a... 

Fig. 1. Schematic representation of the main features of the interferometric surface forces apparatus. Crossed mica sheets (1) are glued onto semi-cylinder optically polished silica discs (2). One of the discs is attached to a piezoelectric crystal tube (3), and another to the force measuring double cantilever spring (4). White light passes through the window positioned in the bottom of the apparatus and reflects between two silver mirrors. Constructive interference occurs for some wavelengths and the fringes of equal chromatic order are passed through the upper silver mirror to the spectrometer where they can be viewed and their wavelengths determined. Adapted from Ref. [9]. 1996, with permission from Elsevier.

A compact disc-based microarray system was developed by Kido et and used for immunoassays. They used a piezoelectric inkjet applicator to deposit the proteins onto a polycarbonate disc. Recently, Clair et al. reported the attachment of small molecules to a polycarbonate compact disc (CD) surface via phosphodiester bonds. Molecular interactions between analytes and some of these molecules can be detected with a conventional CD player. ... 

Piezoelectric ceramic discs, whioh are the heart of the transducer. As a voltage is applied, the discs expand and contract along the transducer axis. Several discs (usually 2 or 4) are used to increase the movement produced. Using an even greater number of discs ensures that high voltage will be applied within transducer only. 

The upper glass disc was rigidly attached to a piezoelectric crystal tube. The lower glass disc was mounted on a weak cantilever spring whose force constant had been previously measured. The distance between the two surfaces could be controlled coarsely (to 1 pern) by an upper micrometer rod. Fine control (to 0-1 nm) was achieved through two movements the piezoelectric crystal moved the upper mica surface whereas the lower mica surface was moved by a differential spring mechanism driven by a micrometer. These mechanisms were hypersensitive, allowing accurate measurement of weak forces down to a few nanonewtons (10 N)t. 

The ultrasonic transducer was a PZ26 piezoelectric ceramic disc (Ferroperm Piezoceramics A/S, Kvistgard, Denmark). The tranducer should be designed to operate in the region of the resonance frequency of the resonator channel. 

With its low acoustic impedance, extreme bad width, high piezoelectric coefficient, and low density (only one-quarter the density of ceramic materials), PVDF is ideally suited as a transducer for hroad hand rmdenvater receivers in hghtweight hydrophones. The softness and flexibiHty of PVDF give it a comphance 30 times greater than ceramic. PVDF can thus he utilized in a hydrophone structure using various device configurations, such as compliant tubes, rolled cylinders, discs, and planar stacks of laminated material. 

The sensor consists of a small disc of piezoelectric coated on both sides with silver to make electrical contact. This disc is mounted in a metal can behind a ceramic faceplate for mechanical protection (Figure 3). 

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