Piezoelectric Transducers

Both the nature and the magnitude of any piezoelectric effect in a crystalline substance are dependent on the direction of the applied force or electric field with respect to the crystallographic axes of the substance. Numerous crystalline materials do exhibit piezoelectric characteristics to some marked extent, but for all practical purposes choice is limited to some half dozen “natural” crystals and an even more limited range of polycrystalline ceramic materials that exhibit piezoelectric characteristics only after polarizing. The word “natural” is used in the sense that both piezoelectric substances were, and still are in certain cases, cut from the naturally occurring substances rather than a synthesized product of the same chemical formula. Quartz, for instance, is an outstanding ex ample of a natural piezoelectric substance, and although the fabrication of piezoelectric plates and elements from natural quartz crystals still survives, synthetic quartz crystals may now be produced on a practical, controlled scale.

DISTORTION and ELECTRICITY

Basically, the piezoelectric effect is an interrelationship between mechanical distortion and electrical effects peculiar to certain crystalline materials. If such materials are distorted by mechanical loading, they generate electricity; conversely, if charged with electricity, they undergo a dimensional change. The piezoelectric crystal, in other words, is a simple form of transducer for converting one form of energy into another, and vice versa.

PLATE ACTION

A general analysis of the performance of piezoelectric material is usually related to its behavior in plate form. With respect to the orientation of a specimen plate with respect to the axes of the crystal from which it’s cut, emphasis may be given to one particular “plate” action. Plate action for various piezoelectric substances is given in Table 7-1.

Table 7-1. Plate Action of Typical Piezoelectric Materials.

SELECTION OF MATERIAL

Selection of the piezoelectric element best suited to any particular duty is only part of the overall problem. The ultimate performance depends also, to a considerable degree, on the method of mounting and driving, and to a lesser degree, on the service conditions (Table 7-2). It’s particularly important that the method of anchorage and /or the driving mechanism used don’t interfere with the dimensional changes involved.

Limiting factors for piezoelectric elements are maximum ser vice temperature and safe valves of relative humidity. Maximum service temperatures are given in Table 7-3. Problems with relative humidity are only present with a limited number of piezoelectric materials, notably Rochelle salt, where the safe range of use is 40-70 percent relative humidity, and dipotassium tartate, the behavior of which is affected by relative humidities around 70 percent. See Table 7-4.

CHARGE and VOLTAGE MODES

Most modern types of piezoelectric transducers are available in either charge-mode or voltage-mode versions. Charge-mode transducers require external charge amplifiers and low-noise cable connections. Voltage-mode transducers may incorporate built-in microelectronic amplifiers capable of being coupled directly to the readout instrument or analyzer via ordinary coaxial cable.

Table 7-2. Typical Applications of Plate Elements.

Table 7-3. Maximum Service Temperatures.

Table 7-4. Typical Piezoelectric Applications.

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Fundamentals of Transducers (all articles)