Piezoelectric constant:

The most widely used piezoelectric constants d3n and G3n, that is, charge

And the voltage, respectively with two subscripts. The former refers to the electric axis,

The latter refers to the mechanical axis. Because the piezoelectric film is thin, the electrode is only

It can be on the upper and lower surfaces. As the charge or voltage is always passed through

The thickness of the membrane (n=3) is transmitted, so the electric axis is always "3".

As shown in Figure 28, the mechanical stress can be added to any axial direction.

So, the mechanical axis can be 1, 2, or 3.

In general, the mechanical axial 1 of the piezoelectric film is used for low frequency sensing and driving (<100kHz), while the mechanical axial 3 is used for high frequency ultrasonic sensing and driving (>100KHz).

Directional characteristics:

The piezoelectric material is anisotropic, that is, the electrical and mechanical responses are different and depend on the axial direction of the electric field or the axial stress or strain added. In the calculation of the piezoelectric effect, this direction must be taken into account.

Example 1:

In a long 2.54cm, width 2.54cm and thickness of piezoelectric thin film switch 110 m, applying a 1.45 pounds per square inch (- 10000N/m) load. There is a rigid support behind the switch element. The force is used in the direction of the thickness (that is, the G33 mode). In this case, the load is acted on the long - to - width area of the piezoelectric film. The open circuit voltage produced by the thickness direction is:

In the form:

V/m = voltage output of a piezo film thickness per meter

L = N/m membrane associated area of applied stress, the PSI - conversion to N/m - about 7000.

Example two:

The piezoelectric film and a switching element in the same cases, but the force is (10000N/m - * 0.0254m - =6.45 Newton), and the structure form of flexible support membrane. The force is acted on the cross section of the thickness (WT). The piezoelectric film under load is stretched, so the g model as fand.

The force is added to a much smaller cross section, resulting in a sharp increase in the output voltage. Small area produces higher stress.

dynamic range

The piezoelectric film has a large dynamic range, it has been used to sense the space of a mass of 10's high-speed objects 12 grams of impact; while in other extreme conditions, it can also be 300000 shock waves generated in the atmospheric pressure measurement in the process of weapon test. A recent study carried out by an area of 155.5mm * 18.5mm, the thickness of the maximum output energy of 52 m thick film. The film is subjected to a force of about 350MPa (in the direction of tensile, or "n=1") without failure. The charge produced is very linear, and the following are the results of the maximum stress.

Maximum charge: 20 C, namely 6.95 mC/m2

Maximum measured voltage: 1600 V, that is 30.8 x 106 V/m

Maximum transformation energy: 30.9 mJ, that is 207 kJ/m3

A later experiment shows that a piezoelectric film device can endure about 10% of the above energy for a long time without any measurable damage.

Motor transformation

When a piezoelectric film is affected by the voltage, the size of the film is changed due to the attraction or rejection of the internal dipole in the electric field. With the addition of a voltage polarity, the piezoelectric film becomes thinner, longer and broadened. In addition to the opposite polarity, the length and width of the piezoelectric film shrink and thicker, and the AC voltage can make the piezoelectric film "vibrate".

The size of the deformation can be calculated according to the "d3n" constant of piezoelectricity.

Changes in