Characteristics of a piezoelectric film at low frequency

Introduction

The characteristics of piezoelectric film in low frequency should be directly to electrical terminology, but often distort |. for any practical application of this technology are almost related to this problem, this paper want to analyze this problem with a certain length, and as far as possible on the non mathematical pattern processing, using language description and case expression concept, hypothesis some readers in transform time domain and frequency domain with the technique of FFT, but is not important.

Connect

In general, a preliminary evaluation of a piezoelectric film is to connect a piezoelectric device with a probe (an oscilloscope probe) to the oscilloscope. Generally speaking, an oscilloscope probe can be regarded as an "infinite impedance". Because of its very large scale, the influence of the probe on the circuit can be ignored. But in many cases, so the piezoelectric film is not access to the oscilloscope probe is almost short, typical of the probe, when the access is effective when the oscilloscope, 1M resistor, or 10M, and there are a lot of convenience in 1M (x1) and 10M (X10 the conversion between physical elements including), impedance 1M is usually the input stage in oscilloscope, not a separate element in the probe. A "X1" probe is actually a shielded cable with a corresponding contact at both ends.

Source capacitance

In order to analyze what will happen after the probe will be connected, we need to consider the characteristics of the piezoelectric film device. Perhaps the most important feature (after the piezoelectricity, of course) is the capacitance of the material. The capacitance is a measure of the storage charge capacity of any of the components, and always exists when the two plates are close to each other. Electrode conductive plate referred to in this article is a piezoelectric film on each side of the printing or metal out of the influence of the capacitance device is mainly affected by the characteristics of insulator electrode separation between the electrodes of the charge storage ability of the insulator measured by expression of its dielectric constant.

Compared with most polymer materials, the dielectric constant of PVDF is very high, about 12 (relative to free space dielectric constant).

Obviously, the capacitance of a component increases with the increase of the area of the conductor, so the capacitance of a large film is larger than the capacitance of the small element. At the same time, the capacitance also increases with the decrease of thickness. Therefore, the capacitance of thin piezoelectric film is larger than that of thick film by the same geometric area.

The relationship can be written as: C= A/t

In the form:

The capacitance of the c~ piezoelectric film

The epsilon Permittivity (also can be expressed as epsilon = R epsilon,

The relative permittivity of the epsilon r~ (about 12 to PVDF), the dielectric constant of the free space in the epsilon

(8.854 * 10-12F/m)

The effective area (overlapping part) of the A ~ PZT electrode

T ~ film thickness

The unit of capacitance is Fala (F), but often encounter is much smaller units; micro method (uF or 10-6F), nanofarad (nF or 10-9F; skin (pF or 10-12F).

The capacitance of any piezoelectric film element can be calculated by formula and can be measured directly with a handheld capacitance meter or instrument (such as a LCR bridge).

The capacitance value should be at a given measurement frequency.

Often determined as 1kHz), a piezoelectric film element is measured.

The value of capacitance generally decreases with the increase of the measuring frequency.

Piezoelectric film equivalent circuit

Let's draw the equivalent of a piezoelectric film el