Piezo Films

BY KUREHA

About Piezo Films

1969 Dr. Heiji Kawai discovered the piezoelectric properties of polyvinylidene fluoride (PVDF). He was the first to demonstrate that polarized PVDF exhibits a large piezoelectric coefficient of 6-7pC/N a value that was roughly 10 times any other polymer examined at that time.

The spatial disposition of hydrogen and fluorine atoms along the polymer chain provides PVDF a unique structure for effective polarization. Today’s Kureha’s PVDF “Piezo Film” achieves charge sensitivity (d31) of 25 pC/N.

Piezoelectric Polymers

Like piezoelectric quartz and ceramic materials, Piezo Film produces voltage or charge proportional in both amplitude and frequency to applied dynamic strain. While the (d31) strain constant of Piezo Film is somewhat lower than PZT ceramics, its lower dielectric constant results in 10 times the voltage output versus ceramics for the same force input (g31 stress constant). And, like other piezoelectric materials, Piezo Film changes dimensions in response to applied voltage, again in direct proportion to the amplitude and frequency of the applied voltage. It can withstand strong fields (75V/µm) where most ceramics depole. Finally, Piezo Film is also pyroelectric, generating a voltage proportionate in frequency to changing temperature, about 10V/°C. It absorbs strongly in the same infrared wavelengths (5-20um) as heat emitted from the human body.

Ideal for transducer design, Piezo Film is broad band, operating across a wide frequency range (0.01Hz to GHz) and through an expansive dynamic range. PVDF’s low acoustic impedance, about the same as human tissue and water, makes it an excellent sensor in applications as diverse as medical vital signs monitoring to underwater acoustic sonar. PVDF films have excellent impact resistance and are also are resistant to moisture and most

It has low mass and low mechanical Q, so it does not exhibit self resonance, which can be a source of noise for more brittle piezo materials. Being a fluoropolymer, it is chemically inert and has very low moisture absorption.

PVDF requires the mechanical orientation of the polymer chains to facilitate polarization. PVDF is stretched 5:1 to achieve optimal sensitivity. The process temperature at which PVDF is oriented and annealed determines its maximum temperature withstanding capability. Piezo film is most sensitive when processed and annealed at 65-75°C. When annealed above this temperature range, the resulting piezoelectric sensitivity is lower, but stable.

Practically speaking, 85-90°C is the upper range for Piezo Film (storage or operating temperature), with sensitivity falling to about 50% of the 65-75°C annealed films. Even though sensitivity is lower at these elevated temperatures, the high voltage sensitivity of Piezo Film still delivers an impressive signal to noise ratio. For higher temperature sensors, customized copolymers of PVDF can be fabricated without the required mechanical orientation, and these specialty films can withstand 125-150°C, again depending on process conditions.

Piezo Film is mechanically tough and impact resistant, yet flexible, conforming well to the sensing surface. It is manufactured in 350mm widths in a continuous process, producing rolls of 1000m or longer. Subsequently, conductive screen printed inks or sputtered electrodes can then be applied. The film is easily die or laser cut to desired sensor dimensions, and a variety of interconnections can be affixed.

Finally, Piezo Film is inexpensive, so even disposable applications are very practical.

The KF Piezo Advantage

Kureha pioneered the development and commercialization of the first Piezoelectric films. Kureha’s KF Piezo utilizes KF Polymer, a uniquely crystalline PVDF that yields the most sensitive film in the market today. This allows designers to solve the most challenging OEM applications with low cost, broad band transducers offering high signal to noise across a wide dynamic range.

Comparisons

Properties

Sheets and Rolls

KF Piezo Films Properties vs Other Materials Properties

in Length Expansion Mode

Typical Piezo Films Properties

Sheets and Rolls

Copolymer (PVDF/TrFE) can be supplied as powder, or as ultrathin cast films up to thick elements of 200um or more.

KF Copolymer

Supplied in either powder form, or in a variety of piezoelectric polymer configurations, KF-2200 is also based on Kureha’s unique PVDF processing technology. Copolymer of vinylidene fluoride & trifluoroethylene (P(VDF/TrFE)) exhibits excellent piezoelectric and pyroelectric properties. The copolymers very unique properties allow the material to be solvent cast, extruded, sprayed, and otherwise fabricated in a variety of shapes (ie tubes, cylinders, fiber, wire or cable, or as piezoelectric conformal coatings).

Unlike homopolymer, the PVFD/TrFE Copolymer does not require chain orientation through a stretching process to achieve high polarization. The lack of mechanical orientation makes copolymer an excellent solution for high temperature applications (up to 125 - 150°C). It also exhibits a high electromechanical coupling at ultrasonic frequencies (10X PVDF) thus providing very high transducer efficiencies.

Flexible and durable, KF copolymers can easily be applied to surfaces via solvent. They are excellent for Printable Electronics. Like PVDF, copolymer has an acoustic impedance is close to water, making it suitable for applications to include hydrophones, ultrasonic probes, and conformal coatings on catheters for in vitro applications.

Custom Sensor Configurations

Kureha is dedicated to flexible support for its KF Piezo customers, providing unmetallized piezoelectric PVDF films in sheet or roll form. We also provide copolymer powder. Through our value add partners, we can supply Piezo Film elements and custom sensor components.

Conductors, Coatings, Interconnection

Value added sensor configurations include Piezo Films having conductive electrodes of evaporatively deposited metals (i.e. Ni, Ag, Au, ITO), or with screen printed electrodes (silver and silver carbon inks, PEDOT). Electrode patterns from simple to complex geometries are easily fabricated during screen printing or etching sputtered conductors wby laser or chemical etchants.

EMI shielding, protective laminates and conformal coatings, die cutting, and lead attach with flexible film crimp connectors or a variety of other interconnect solutions are also available.