Patent Description:
Piezoelectric behavior has been known for years. Piezoelectric ceramics are often blended into polyvinylidene difluoride ("PVDF") based materials to create a piezoelectric composite film. Typically, such films are developed to be integrated with devices, such as sensors, transducers, actuators or wearables. This is done either by spin-coating a solution onto the device with electrodes, or adhering the freestanding piezoelectric film to the device by various techniques like cold pressing, hot pressing, etc. Thus, it is critical for films to have good surface properties along with adherence to device surfaces.

<CIT> relates to a vibration-isolating article which is composed of a composite material comprising a powdered piezoelectric material and a high polymer, said composite material having electrical leakage paths. The electrical leakage paths may be formed by an electrically conductive powdered material incorporated into the composite material, or the powdered piezoelectric material per se, or a matrix of the high polymer.

<CIT> provides an energy conversion composition which has an excellent capability of absorbing and damping energy such as dynamic, thermal, optical, or electric energy. The composition can be made into a very thin and small but excellent damping product. The energy conversion composition is characterized in that its base material contains a moment activator which can increase the amount of dipole moments in the base material. The energy conversion composition can be utilized as a vibration damping, sound absorptive, impact absorptive, vibration-proof, electromagnetic wave absorptive, piezoelectric, and endothermal material as well as a polarity liquid.

<NPL>) investigates dielectric properties of the blends of poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer and poly(vinylidene fluoride-chlorotrifluoroethylene) copolymer. The high field (><NUM> MV/m) polarization response of the blends with a small amount of copolymer (<NUM> and <NUM> wt % copolymer) is higher than that of the neat terpolymer due to the interface contribution. At the same time the breakdown field was improved by mixing a small amount of the copolymer with the terpolymer. Consequently, a higher energy density of about <NUM> J/cm<NUM> is obtained in these blends in contrast to about <NUM> J/cm<NUM> in the terpolymer.

In <NPL>) enhancement of weak and high field dielectric responses was observed in poly(vinylidene fluoride) terpolymer/ZrO<NUM> nanocomposites. A frequency-dependent interfacial polarization mechanism (related to the interfacial area between polymer and nanoparticles) is responsible for the enhancement of the dielectric constant at the low field. Due to the improvement of chain mobility and stabilization of the polar phase in the polymer/nanoparticle interface regions, the high field dielectric response of nanocomposites was also enhanced in the certain particle loading. Based on the high-field polarization saturation phenomenon, the dimension of interface regions in the nanocomposites was estimated as <NUM>-<NUM>.

The present invention provides a piezoelectric composition and a device according to the independent claims. Polyvinylidene fluoride triflouroethylene chlorotrifluoroethylene terpolymer ("PVDF-TrFE-CFE"), is one of the fluorinated polymers from the PVDF family with excellent piezoelectric properties, having excellent transfer of charge from polymer to piezoelectric ceramic during poling, thereby maximizing d33 values. Ethylene vinyl acetate ("EVA") is a very soft polymer with flexibility, impact/puncture resistance, excellent adhesion, and hot-melt adhesive glue properties. The current invention utilizing a blend of these two, joining the well-established advantages of PVDF-TrFE-CFE with those of EVA to create a polymer matrix. This polymer matrix is combined with a piezoelectric additive and then turned into a film, the film having piezoelectric properties.

The piezoelectric charge constant, d33, for PVDF-TrFE-CFE blended with <NUM> wt. % PZT is around 55pC/N, while for compositions with <NUM>-5wt. % EVA added to the polymer matrix, the d33 is in the range of <NUM>-50pC/N. Thus, addition of EVA is not seen to compromise the d33 value seen in PVDF-TrFE-CFE composite films.

The inventive films created showed a combination of benefits over the prior art from having the two different polymers (i.e., EVA and PVDF-TrFE-CFE) blended into a polymer matrix, to which is added piezoelectric additives like PZT, BaTiO<NUM>, etc. at greater than <NUM> wt. To wit, the inventive film delivers improved surface smoothness, scratch resistance and adhesion.

The piezoelectric additive can include one or more from the group comprising PZT, BaTiO<NUM> and KNLN. The piezoelectric additive can comprise <NUM>-90wt. % of the blend, or more preferably <NUM>-<NUM> wt. The EVA can comprise <NUM>-<NUM> wt. % of the polymer matrix, and more preferably <NUM>-<NUM> wt. % or any amount (e.g., <NUM> wt. %, <NUM> wt. %, <NUM> wt. %, <NUM> wt. %, or <NUM> wt. %) or range (e.g., <NUM> to <NUM> wt. %, <NUM> to <NUM> wt. %, <NUM> to <NUM> wt. %, <NUM> to <NUM> wt. %, <NUM> to <NUM> wt. %, <NUM> to <NUM> wt. %, <NUM> to <NUM> wt. %, <NUM> to <NUM> wt. %, or <NUM> to <NUM> wt. %) therein.

In another embodiment, a device comprises a piezoelectric composition, the composition comprising a fluorinated polymer, ethylene-vinyl acetate (EVA), the EVA and fluorinated polymer comprising a polymer matrix, and, a piezoelectric additive, wherein the fluorinated polymer is a member of the polyvinyl difluoride (PVDF) family, wherein the PVDF is PVDF-TrFE-CFE, and wherein the device is selected from a group comprising a sensor, a transducer, an actuator or a wearable device.

In one non-limiting embodiment, the terms are defined to be within <NUM>%, preferably within <NUM>%, more preferably within <NUM>%, and most preferably within <NUM>%.

The terms "wt. %", or "mol. %" refers to a weight percentage of a component, a volume percentage of a component, or molar percentage of a component, respectively, based on the total weight, the total volume of material, or total moles, that includes the component. In a non-limiting example, <NUM> grams of component in <NUM> grams of the material is <NUM> wt. % of component.

The terms "inhibiting" or "reducing" or "preventing" or "avoiding" or any variation of these terms, when used in the claims and/or the specification includes any measurable decrease or complete inhibition to achieve a desired result.

The use of the words "a" or "an" when used in conjunction with any of the terms "comprising," "including," "containing," or "having" in the claims, or the specification, may mean "one," but it is also consistent with the meaning of "one or more," "at least one," and "one or more than one.

The inventive compositions can "comprise," "consist essentially of," or "consist of" particular ingredients, components, compositions, etc. disclosed throughout the specification. With respect to the transitional phrase "consisting essentially of," in one non-limiting aspect, a basic and novel characteristic of the compositions and films of the present invention is that they have piezoelectric properties as well as improved surface smoothness, scratch resistance, and/or adhesion characteristics.

Other objects, features and advantages of the present invention will become apparent from the following figures, detailed description, and examples. It should be understood, however, that the figures, detailed description, and examples, while indicating specific embodiments of the invention, are given by way of illustration only and are not meant to be limiting on the scope of the claims. Additionally, it is contemplated that changes and modifications within the scope of the claims will become apparent to those skilled in the art from this detailed description. In further embodiments, features from specific embodiments may be combined with features from other embodiments. For example, features from one embodiment may be combined with features from any of the other embodiments. In further embodiments, additional features may be added to the specific embodiments described herein.

In one aspect, the present invention concerns a mechanically flexible piezoelectric composite film based on a PVDF-TrFE-CFE/EVA blend that exhibits retention of piezoelectric performance with improved surface/mechanical properties.

In one embodiment, the invention includes blending EVA copolymer with PVDF-TrFE-CFE and using it as a base matrix for mixing with piezoelectric ceramic filler to prepare a piezoelectric composite film. This allows for better mechanical and piezoelectric properties of the piezoelectric composites, wherein the EVA polymer can play a role in enhancing the mechanical properties while retaining higher piezoelectric properties that seen before addition of EVA. The amount of EVA in the blend can be anywhere from <NUM>-<NUM> weight%, more preferably from <NUM>-<NUM> weight%. A blend of just the two polymers could be manufactured and sold as a product, ready for addition of a piezoelectric additive. A sellable blend could also already have the ceramic added.

The piezoelectric additive could include BaTiO<NUM>, PZT, and other piezoelectric additives known in the art to have piezoelectric properties of interest. One could also use a mixture of more than one different piezoelectric additive. The total amount of piezoelectric additives could comprise <NUM>- <NUM> wt. % of the final product, more preferably <NUM>-<NUM> wt. Once the piezoelectric additives are blended into the polymer blend, this blend of polymers and piezoelectric additives could be used or sold in this form.

In a preferred embodiment, the blend of polymers and piezoelectric additive is then processed into a thin film. This can be done in any number of ways that are known in the art, including but not limited to solution casting, melt processing and spin coating. A final thickness of the film can be <NUM>-<NUM>, though thickness can of course be higher or lower than this range.

It was found in testing that the addition of <NUM>-<NUM> wt. % of EVA to PVDF-TrFE-CFE did not reduce the d33 values significantly. It was also found that the addition of EVA provided better wettability of the piezoelectric additives into the blend, as well as improving surface properties (like scratch, mar, etc.), flexibility and adhesion. This improvement in adhesion can help in easier integration of freestanding films with the final device.

The present invention will be described in greater detail by way of specific examples. The following examples are offered for illustrative purposes only, and are not intended to limit the invention in any manner. Those of skill in the art will readily recognize a variety of noncritical parameters which can be changed or modified to yield essentially the same results.

Non-limitiing details of fabrication, characterization and performance of the PVDF-TrFE-CFE based piezoelectric films are provided below.

Solution cast films were made by using PVDF-TrFE-CFE & EVA as polymers and PZT as ceramic filler with THF as solvent. Desired amount of PVDF-TrFE-CFE and EVA was weighed and dissolved in THF to make <NUM> wt/wt. % solution with constant stirring. PZT was added to this polymer solution and stirred further. This solution was then casted with a doctor blade as a thin film on a glass plate.

The obtained films were annealed at <NUM> for <NUM> hours followed by poling as usually done for piezoelectric films. The samples were poled at <NUM> for <NUM> minutes under <NUM> KV/mm electric field strength. A list of samples prepared for testing is shown below in Table <NUM>.

SEM study of the samples was carried out using ZEISS EVO-<NUM> scanning electron microscope. SEM images were taken under secondary electron mode with operating voltage of <NUM> kV. All the samples were air cleaned and coated with <NUM> gold prior to imaging. The images obtained in SEM shows a clear difference in the morphology by adding EVA in the composite film. The addition of EVA shows a network structure on the surface which was not present in piezoelectric films without EVA. This network structure may assist with better mechanical properties.

The piezoelectric constant (d33 - pC/N) is measured by Piezotest piezometer with operating frequency of <NUM>. The dynamic force applied during measurement was <NUM> N. Piezometer measurements are given in Table <NUM>.

The values of d33 (pC/N) for the samples measured with the piezometer do not reflect a negative effect on piezoelectric properties of the composite due to addition of EVA.

Nanoindentation Results:
Mechanical properties (modulus and hardness) was measured by using the Nano-Indenter® XP(Keysight Technologies, Inc. , Santa Rosa, CA). On each sample, <NUM> indents were made and average values were calculated as per Olivier and Pharr method. The average value and it's standard deviation is given in Table <NUM>.

The hardness and modulus of the composite film decreased after addition of EVA as shown above in table <NUM>.

Tests were conducted on scratch resistance on three sample of film. The first was PVDF-TrFE-CFE (<NUM> wt. %) + <NUM> wt. The second was CFE (<NUM> wt. %) + <NUM> wt. % PZT + <NUM> wt. The third sample comprised PVDF-TrFE-CFE (<NUM> wt. % PZT + 5wt. These tests showed that the scratch depth was reduced by as much as <NUM>%.

The piezoelectric charge constant, d33, for PVDF-TrFE-CFE blended with <NUM> wt. % PZT is around 55pC/N, while for compositions with <NUM>-5wt. % EVA added to the blend, the d33 is in the range of <NUM>-50pC/N. Thus, addition of EVA is not seen to compromise the d33 value seen in PVDF-TrFE-CFE composite.

The final films developed showed a combination of benefits over the prior art from the two different polymers (i.e., EVA and PVDF-TrFE-CFE) blended as a polymer matrix with the piezoelectric fillers like PZT, BaTiO<NUM>, etc. added up to greater than <NUM>-<NUM> wt.

While it has generally been shown that the EVA is first combined with the fluorinated polymer before adding the piezoelectric additive, the three components can in fact be combined in any order, or all at once.

Claim 1:
A piezoelectric composition, comprising:
a fluorinated polymer, wherein the fluorinated polymer is a member of the polyvinyl difluoride, PVDF, family, wherein the PVDF is polyvinylidene fluoride trifluoroethylene chlorotrifluoroethylene terpolymer, PVDF-TrFE-CFE;
ethylene-vinyl acetate, EVA, the EVA and fluorinated polymer creating a polymer matrix, and,
a piezoelectric additive.