Piezoelectricity is the appearance of a voltage across the sides of a crystal when it is subjected to mechanical stress or squeezing. A piezoelectric sensor is a device that uses the piezoelectric effect to measure changes in pressure, acceleration, temperature, strain, or force by converting them to an electrical change.
Recently, piezoelectric polymer sensors have become one of the fastest growing technology areas in the global sensor market. Currently, they are used as versatile tools for the measurement of various processes.
Let's understand what makes piezoelectric films suitable for numerous applications and the possible solutions to further improve their performance:
Key Features of Piezoelectric Film
There are numerous situations where a piezoelectric film has been considered for use in the sensor solution. Among the areas that piezoelectric sensors are currently used are, in:
− Medical
− Aerospace
− Nuclear instrumentation, and
− Touch parts of mobile phones as a pressure sensor
In the automotive industry, piezoelectric elements are used to monitor combustion when developing internal combustion engines. Since the discovery of piezoelectric polymers, the technology has advanced significantly and is being adopted on a larger scale than previously.
In general, piezoelectric films that are used in piezoelectric sensors are strong, lightweight and flexible. They come in a wide range of thicknesses and areas. Piezoelectric films can usually be molded into unique designs and can be glued with commercial adhesives. Some general features of piezoelectric films include:
− High dielectric strength
− High voltage output
− High elastic compliance
− Low acoustic impedance
− Large dynamic range
− Wide frequency range
− High impact resistance
− High mechanical strength and stability
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Polymers Exhibiting Piezoelectricity
Four critical elements exist for all polymers that possess piezoelectric features, regardless of the morphology that is present in the polymers. These essential elements are:
− The presence of permanent molecular dipoles
− The ability to orient or align the molecular dipoles
− The ability to sustain the dipole alignment once it is achieved, and
− The ability of the material to undergo large strains when it is mechanically stressed
Among common polymers, poly vinylidene fluoride (PVDF), PVC and nylon are examples of materials that exhibit piezoelectric behavior.
PVDF to Have Won the Race
For the materials that were already mentioned, PVDF has been found to exhibit high levels of piezoelectricity. The other polymers, such as PVC and nylon, do not match the high piezoelectric effect that is observed with PVDF and its copolymers.
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The manufacturing process of piezoelectric PVDF material consists of stretching the PVDF sheet at a temperature that is close to the melting point of about 175°C. This causes a polymer chain packing of the molecules into a piezoelectric crystalline phase. The stretched polymer is exposed to a high electric field to generate the piezoelectric properties in the material.
− PVDF generates an electrical charge in response to temperature variations.
− Also, it absorbs infrared energy in the 7-20 micron wavelength and can also function as a human motion sensor.
− It can also be used as a pyroelectric sensor and laser beam profile sensor in more advanced applications.
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Piezoelectric Potential - Polymer v/s Ceramics
In addition to polymers, certain ceramic materials can be used to produce piezoelectric films. Some naturally occurring crystals, such as: quartz and topaz exhibit piezoelectricity. Lead zirconate titanate or PZT, a man-made ceramic material, is the most common ceramic piezoelectric material in use today.
Both the ceramics and piezoelectric polymers offer certain advantages and disadvantages when used in piezoelectric films.
− Piezoelectric polymer films have low acoustic impedance, which is closer to that of human tissue, water and organic materials than ceramic films. Such a close match of the impedance allows for the efficient transduction of acoustic signals in both tissue and water.
− Piezoelectric polymer films also display excellent sensitivity and low density.
− When piezoelectric polymers are extruded into thin films, they can be joined to a structure without affecting the mechanical motion.
In general, piezoelectric films are ideal for use in strain sensing applications, which demand high sensitivity and wide bandwidths.
However, when compared to ceramic films, polymer piezoelectric films make for a relatively weak electrochemical transmitter, especially in low frequency and resonance applications. Also, PVDF is not suitable for usage or storage at temperatures above about 100°C.
This limits its performance in relatively high temperature applications. In addition, when the electrodes of the film are exposed, the sensor has a tendency to be quite sensitive to electromagnetic radiation.
» Continue reading to know more about piezoelectric films. Find out the major application sectors and the solutions to the challenges faced by piezo films.