Piezoelectric Materials and the Piezoelectric Transformer
Introduction to Piezoelectrics
The piezoelectric effect describes a relationship between a mechanical stress and
an electric voltage in piezoelectric materials. These permanently polarized materials will
produce an electric field when the materials change dimensions as a result of an imposed
mechanical force. The opposite is also true. When an electric field is applied across
these same materials, the polarized molecules will align themselves with the electric
field, inducing dipoles within the crystal structure. The dipoles change their alignment
and the material changes dimensions. Since the effect goes both ways, it is also called
reversible. [1]
Figure 1. Piezoelectric effect example
The linear piezoelectric theory is based on static parameters including
temperature, porosity, and charging parameters. These considerations are important
when choosing a material for a specific application. Increased porosity increases the
thermal stability which improves the piezoelectricity [2]. Charging parameters such as
charge temperature, charging voltage and time, also have a direct impact of temperature
stability [3]. There are several materials that exhibit this behavior.
Piezoelectric Materials
The materials that exhibit this effect are as diverse as their applications. These
materials include natural crystals, man-made crystals, polymers, and ceramics. Quartz
crystals are prevalent piezoelectrics. There are also instances of biopolymers with
piezoelectric behavior. These bioelectrics, namely poly-L-lactic acid (PLLA) have a
similar uniaxial orientation of molecular dipoles as the crystal piezoelectrics [4]. Bone
also exhibits some piezoelectric properties.
Piezoelectric Material Applications
Piezoelectric materials have been used most regularly for piezoelectric
transducers. A piezoelectric transducer is basically a piece of polarized material with
electrodes attached to two of its opposite faces. Piezoelectric transducers take advantage
of these materials by converting electrical energy to acoustic energy (converse
piezoelectric effect) and vice versa (direct piezoelectric effect) [5]. Thereby,
piezoelectric transducers are used in sound production and detection. Sonar was the
practical application for piezoelectric devices during World War I as ultrasonic
submarine detectors. Electronic frequency generators use quartz crystals, a popular
piezoelectric material. The application of piezoelectric materials in electronic devices
has allowed for the speedy development of today’s electronics. These traditional uses are
still found today, but the most cutting edge use of piezoelectrics is piezoelectric
transformers.
Piezoelectric Transformers
Piezoelectric transformer (PT) technology is not a new idea because it was first
patented in 1954 [6]. However, is only now becoming one of the most promising
alternatives for magnetic transformer. Several Japanese companies decided to develop
the American Rosen Transformer, and have since left their American counterparts in the
dust. The price of these transformers has also been declining, making it them all the
more popular [6].
Rosen piezoelectric transformers are step-up transformers. Rosen-type PTs
operate when an input electrical voltage having a frequency near to the resonance
frequency of the PT. The deformation of the body also affects the output section, which
will develop a proportional voltage between the output electrodes [7].
Figure 2. Basic Rosen transformer structure
In the last 10 years, PTs have been developed for power applications. These
applications include florescent ballasts, battery chargers, DC/DC converters, power
supplies, and automotive applications. The requirements of these applications are very
different and require researchers to overcome manufacturing, EMI, flatness, size, and
efficiently issues. However, PTs have already demonstrated capabilities of up to
40W/cm3. It is that high efficiency potential which has pushed PT development forward.
Most recently, NASA and DARPA are funding research projects for developing PTs to
be applied to satellites [6].
[1] ANSI/IEEE Std, “IEEE Standard on Piezoelectricity,” ANSI/IEEE Standard, vol. 176,
pp. 1-64, March 1987
[2] Z. Xia, S. Ma, X. Qui, Y. Wu, F. Wang, Y. Zhang, “Influency of Porosity on Stability
of Charge Storage and Piezoelectricity for Porous PTFE Film Electrets,” 11th
International Symposium on Electrets, pp. 326-329, October 2002
[3] P. Zhang, Z. Xia, X. Qiu, F. Wang, X.Y. Wu, “Influence of Charging Parameters on
Piezoelectricity for Cellular Polypropylene Film Electrets”, 11th International
Symposium on Electrets, pp. 39-42, October 2002
[4] E. Fukada, “Bioelectrets and Biopiezoelectricity,” IEEE Transactions on Electrical
Insulation, vol. 27, no. 4, pp. 813-819, August 1992
[5] NDT Resource Center, “Piezoelectric Transducers” [Online Document], 28 August
2008, [cited 2 September 2008], Available HTTP://www.ndted.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans/piezotra
nsducers.htm
[6] A. V. Carazo, “50 Years of Piezoelectric Transformers, Trends In The Technology,”
Materials Research Society Symposium, vol. 785, pp. D1.7.1-D1.7.10, 2004
[7] H. Xue, J. Yang, Y. Hu, “Analysis of Rosen Piezoelectric Transformers with a
Varying Cross-Section,” IEEE Transactions on Ultrasonics, Ferroelectrics, and
Frequency Control, vol. 55, pp. 1632-1639, July 2008