Piezoelectric ceramics electromechanical coupling factor

For piezoelectric ceramic thin disk with larger (diameter/thick) ratio, in the lower frequency range, the major vibration mode is radial vibration mode. In the radial vibration mode, besides the fundamental mode, there are some overtone modes with inharmonic frequency separation. The planar effective electromechanical coupling factor kp is decreased with the increasing in overtone mode order. The relation between planar effective electromechanical coupling factor and overtone mode order can be fit with second order polynomial. 

In general, elastic coefficients, piezoelectric constants and electromechanical coupling factors are determined by the transportation line method, which was recommended as IRE standard (1957, 1958, 1961). This method is a dynamic method, because the measurement sample is made as a vibrator and measiued in a vibration state at a frequency near its intrinsic resonance frequency. When a small AC voltage is applied to a piezoelectric sample (poled single crystal, or poled bulk ceramic) having a pair of electrodes on opposite surfaces, the sample vibrates at a frequency near its resonance frequency, fr. Figure 22-12 shows a sample inserted in a transportation network. By changing the... 

After a piezoelectric ceramic is poled by a DC field higher than the coercive field Ec, the sample has symmetry of 00mm. For this piezoelectric ceramic, a typical process chart for the determination of dielectric constant, piezoelectric constant, elastic coefficient and electromechanical coupling factor is shown in Figure 22-13. 

The elearical and mechanical properties of piezoelectric polymers make them a suitable alternative to conventional ferroelectric ceramics (such as PZT) to transducer design, particularly as sensing probes. The lower electromechanical coupling factor makes the polymer inferior as a transmitttog material and has limited its use as a projector or pulse-echo transduction material. 

The value of tand for PVDF is typically 0.1. This is 10 times greater than the value of 0.01 for ceramics (PZT 5). As discussed earlier, it is this high mechanical loss or damping that prevents measurement of the electromechanical coupling factors using the piezoelectric resonance method. 

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