Patent Description:
The present invention is directed to the field of piezoelectric composite having polymer coated piezoelectric fillers and in particular to films containing such piezoelectric composite.

Piezoelectric elements find application in a variety of electro-mechanical devices such as resonators, sensors, buzzers, transducers, ultrasonic receivers and generators. As a result, in recent years there has been a sharp increase in the demand for such piezoelectric elements with several industry practitioners looking at improving the performance of the piezoelectric elements. Traditionally, one of the criteria for a suitable piezoelectric material is to have a balance of low dielectric loss (denoted by (tan δ)) while retaining a sufficiently high piezoelectric charge coefficient (d<NUM>). A material with high piezoelectric charge coefficient (d<NUM>) is indicative of its excellent piezoelectric property and signifies the development of suitable electric potential, when a mechanical stress is applied to the material.

On the other hand, dielectric loss (denoted by (tan δ)) in piezoelectric materials, is indicative of energy loss in the form of heat, when a piezoelectric material is used in electro-mechanical devices. Dielectric loss causes internal heating in electro-mechanical devices, affecting both performance efficiency as well as long term durability of such electro-mechanical devices. In other words, minimizing dielectric loss would render the piezoelectric material to be more efficient in its performance. Yet another consideration has been to obtain piezoelectric materials that have low dielectric constant or low relative permittivity for certain application. Relative permittivity determines the ability of a material to polarize in response to an applied electric field. It is often preferred to have a material whose relative permittivity is low so as to enable the piezoelectric material to have a high piezoelectric voltage constant (g), and rendering such materials to have improved sensing performance.

Further, it has been observed in few instances that high relative permittivity, may cause dielectric loss to increase, which affects both output and performance efficiency of a device (e.g. an actuator). Therefore, for a piezoelectric material, a suitable balance of piezoelectric charge coefficient (d<NUM>), relative permittivity and dielectric loss (tan δ), would provide the desired piezoelectric performance suited for sensing applications.

However, it has been reported in several research publications, that the performance of piezoelectric material is particularly susceptible to fluctuation in a high moisture environment, as has been reported in several publications such as (i) <NPL>, (ii) <NPL>. In fact, it has been observed by industry practitioners that in certain instances piezoelectric materials, upon exposure to high moisture environment, experience increased dielectric loss (tan δ), which in turn adversely affects the performance of an electro-mechanical device. As described in the patent <CIT>, direct exposure of a piezoelectric material to moisture can damage a device, where moisture on contact with the piezo material, may induce unwarranted electrical discharge or short circuiting. In the past, surface functionalized piezoelectric materials have been used to shield the piezoelectric material from direct moisture contact. However, such technical solutions have had limited success. The patent <CIT> illustrates a polymeric piezoelectric composite comprising a polymer matrix formed from one or more of a monomer or a precursor polymer; and an in-situ dispersion of a piezoelectric ceramic filler and an ionic additive within the polymer matrix. The patent <CIT> concerns a lead-free piezoelectric composite that includes a polymeric matrix. The polymeric matrix (preferably a PVDF-based copolymer like PVDF-TrFE-CFE terpolymer) is loaded with greater than <NUM> vol. % of lead-free piezoelectric particles. The publication in the <NPL>et al discloses composites of aligned (K,Na,Li)NbO<NUM> (KNLN) piezoceramic particles in a PDMS polymer matrix.

Thus, for the foregoing reasons, there remains a need for developing piezoelectric materials that demonstrate low dielectric loss (tan δ) even when such piezoelectric materials are exposed to humid conditions or high moisture environment. In particular, there is a need to develop piezoelectric materials that demonstrate a desired balance of low dielectric loss (tan δ) and suitable piezoelectric charge coefficient (d<NUM>), even when such materials are exposed to humid environment.

The invention relates to a piezoelectric composite, comprising: (a) a polymer matrix; and (b) a plurality of coated piezoelectric filler particles, wherein each of the plurality of coated piezoelectric filler particles is dispersed in the polymer matrix, and further wherein each of the plurality of coated piezoelectric filler particles comprises a piezoelectric material having at least a portion of its outer surface coated with a polymeric material having at least one polar functional group.

In some embodiments of the invention, the polymeric material comprises at least one polar functional group selected from hydroxyl group, carboxylate group, ester group, sulfonic acid group, amide group, siloxane group, amino group, sulfhydryl, phosphate, ether group, halogen group, and a nitrile group.

In some embodiments of the invention, the plurality of coated piezoelectric filler particles are present in an amount ≥ <NUM> volume% and ≤ <NUM> volume%, with regard to the total volume of the piezoelectric composite.

In some embodiments of the invention, the piezoelectric material is selected from hydroxyapatite, apatite, lithium sulfate monohydrate, sodium bismuth titanate, quartz, tartaric acid, poly(vinylidene difluoride), barium titanate, potassium sodium niobate (KNaNb)O<NUM> (KNN), lead zirconate titanate (PZT), lead niobium titanate (PNT), lead scandium niobium titanate (PSNT), lead metaniobate, lithium doped potassium sodium niobate (KNLN) represented by the formula (K<NUM>Na<NUM>)<NUM>-tLitNbO<NUM>, wherein the variable 't' ranges from greater than <NUM> to less than <NUM> (<NUM><t<<NUM>).

In some embodiments of the invention, the polymeric material is selected from polydimethylsiloxane (PDMS), polyvinylpyrrolidone (PVP), grafted polyolefins, acrylate and methacrylate polymers, polyethylene glycol, vinyl acetate polymers polycarbonate, vinylidene fluoride (VDF) polymers, cyano polymers, poly(phenylene oxide) or combinations thereof.

Preferably, in some embodiments of the invention, the polymeric material is selected from polydimethylsiloxane (PDMS), polyvinylpyrrolidone (PVP), grafted polyolefins, polyethylene glycol, vinyl acetate polymers, cyano polymers, or combinations thereof. More preferably, in some embodiments of the invention, the polymeric material is selected from polydimethylsiloxane (PDMS), or polyvinylpyrrolidone (PVP).

In some embodiments of the invention, the polymeric material is selected from polyvinylpyrrolidone (PVP) or polydimethylsiloxane (PDMS) and the piezoelectric material is a lithium doped potassium sodium niobate (KNLN) represented by the formula (K<NUM>Na<NUM>)<NUM>-tLitNbO<NUM> wherein 't' is <NUM> (KNLN3) or 't' is <NUM> (KNLN6).

In some embodiments of the invention, the polymeric material is polyvinylpyrrolidone (PVP) and the piezoelectric material is a lithium doped potassium sodium niobate (KNLN) represented by the formula (K<NUM>Na<NUM>)<NUM>-tLitNbO<NUM> wherein 't' is <NUM> (KNLN3) or 't' is <NUM> (KNLN6).

In some embodiments of the invention, the polymeric material is polydimethylsiloxane (PDMS) and the piezoelectric material is a lithium doped potassium sodium niobate (KNLN) represented by the formula (K<NUM>Na<NUM>)<NUM>-tLitNbO<NUM> wherein 't' is <NUM> (KNLN3) or 't' is <NUM> (KNLN6).

According to the invention, the polymer matrix and the polymeric material coating of piezoelectric material are of different polymer composition.

In some embodiments of the invention, the polymer matrix is a polymer selected from vinylidene fluoride (VDF) polymer, polycaprolactone, polysiloxane, polydimethylsiloxane (PDMS), polyolefin co-polymers, polysiloxane-polycarbonate copolymers, acrylate and/or methacrylate copolymers, polyethylene terephthalate (PET), polycarbonate (PC), polybutylene terephthalate (PBT), poly(<NUM>,<NUM>-cyclohexylidene cyclohexane-<NUM>,<NUM>-dicarboxylate) (PCCD), glycol modified polycyclohexyl terephthalate (PCTG), poly(phenylene oxide) (PPO), polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC), polystyrene (PS), polyethyleneimine or polyetherimide (PEI) and their derivatives, thermoplastic elastomer (TPE), terephthalic acid (TPA) polymers, poly(cyclohexanedimethylene terephthalate) (PCT), polyethylene naphthalate (PEN), polyamide (PA), polysulfone sulfonate (PSS), sulfonates of polysulfones, polyether ether ketone (PEEK), acrylonitrilebutadiene styrene (ABS), polyether ketone ketone (PEKK), polyphenylene sulfide (PPS), or blends or combinations thereof.

Preferably, the polymer matrix is a polymer selected from vinylidene fluoride (VDF) polymer, polycaprolactone, polyolefin co-polymers, polysiloxane-polycarbonate copolymers, acrylate and/or methacrylate copolymers, polyethylene terephthalate (PET), polycarbonate (PC), polybutylene terephthalate (PBT), poly(<NUM>,<NUM>-cyclohexylidene cyclohexane-<NUM>,<NUM>-dicarboxylate) (PCCD), glycol modified polycyclohexyl terephthalate (PCTG), poly(phenylene oxide) (PPO), polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC), polystyrene (PS), polyethyleneimine or polyetherimide (PEI) and their derivatives, terephthalic acid (TPA) polymers, poly(cyclohexanedimethylene terephthalate) (PCT), polyethylene naphthalate (PEN), polyamide (PA), polysulfone sulfonate (PSS), sulfonates of polysulfones, polyether ether ketone (PEEK), acrylonitrilebutadiene styrene (ABS), polyether ketone ketone (PEKK), polyphenylene sulfide (PPS), or blends or combinations thereof.

More preferably, the polymer matrix is a polymer selected from vinylidene fluoride (VDF) polymer.

In some embodiments of the invention, the polymer matrix is a vinylidene fluoride (VDF) polymer selected from polyvinylidene fluoride (PVDF), polyvinylidene fluoride-co-tri-fluoroethylene (PVDF-TrFE), poly(vinylidene fluoride-co-hexafluoropropylene)(PVDF-HFP), polyvinylidene fluoride-co-tetrafluoro ethylene (PVDF-TFE), poly(vinylidene fluoride-co-trifluoroethylene-co-chlorofluoroethylene) (PVDF-TrFE-CFE),poly(vinylidene-fluoride-co-trifluoroethylene-co- chlorotrifluoroethylene)(PVDF-TfFE-CTFE) or combinations thereof. Preferably, in some embodiments of the invention, the polymer matrix is a vinylidene fluoride (VDF) polymer selected from poly(vinylidene fluoride-co-trifluoroethylene-co-chlorofluoroethylene) (PVDF-TrFE-CFE), or polyvinylidene fluoride-co-tri-fluoroethylene (PVDF-TrFE).

In some embodiments of the invention, the piezoelectric composite comprises:.

In a preferred aspect of the invention, the invention is directed to a film comprising the piezoelectric composite of the present invention. In yet another aspect of the present invention, the invention is directed to an article of manufacture comprising a film, wherein the film comprises the piezoelectric composite of the present invention.

In some embodiments of the invention, the invention relates to the use of the film comprising the piezoelectric composite of the present invention, for improving the piezoelectric property of the article of manufacture. Non-limiting examples of piezoelectric property includes piezoelectric charge coefficient (d<NUM>), relative permittivity and dielectric loss (tan δ).

In some aspects of the invention, the invention is directed to a method for preparing a film comprising the piezoelectric composite of the present invention, wherein the method comprises:.

In some embodiments of the invention, the precursor film comprises the piezoelectric composite. In some aspects of the invention, the precursor film comprises ≥ <NUM> wt. %, preferably ≥ <NUM> wt. %, preferably <NUM> wt. %, of the piezoelectric composite, with regard to the total weight of the precursor film.

In some embodiments of the invention, the electric polarization treatment is conducted at an electric field ≥ <NUM> KV/mm and ≤ <NUM> KV/mm for a time period ≥ <NUM> minute and ≤ <NUM> minutes and at a temperature ≥ <NUM> and ≤ <NUM>, preferably at a temperature ≥ <NUM> and ≤ <NUM>.

In some embodiments of the invention, the invention, is directed to a film comprising the piezoelectric composite, wherein the film is obtained by a method comprising:.

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. 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.

A solution to some or all of the drawbacks in existing art, resides in a piezoelectric composite as described in this disclosure. Accordingly, the invention, is based, in part, on a piezoelectric composite comprising a polymer matrix containing a plurality of polymer coated piezoelectric filler particles dispersed in the polymer matrix.

Advantageously, the piezoelectric composite or a film comprising such a composite, demonstrates a suitable piezoelectric property having a low dielectric loss (tan δ) while retaining a suitable piezoelectric charge coefficient (d<NUM>). In particular, the invention now enables a skilled artisan to make devices or articles of manufacture comprising such a film, which demonstrate excellent piezoelectric performance even when operating such devices or articles in humid conditions.

The terms "wt. %", "volume %" or "mol. %" refers to a weight, volume, or molar percentage of a component, respectively, based on the total weight, the total volume, or the total moles of the material that includes the component. Any numerical range used throughout this disclosure shall include all values and ranges there between unless specified otherwise. For example, a boiling point range of <NUM> to <NUM> includes all temperatures and ranges between <NUM> and <NUM> including the temperature of <NUM> and <NUM>.

" The words "comprising" (and any form of comprising, such as "comprise" and "comprises"), "having" (and any form of having, such as "have" and "has"), "including" (and any form of including, such as "includes" and "include") or "containing" (and any form of containing, such as "contains" and "contain") are inclusive or open-ended and do not exclude additional, unrecited elements or method steps. The process of the present invention can "comprise", "consist essentially of," or "consist of" particular ingredients, components, compositions, etc., disclosed throughout the disclosure.

The present disclosure describes a piezoelectric composite, which demonstrates low dielectric loss even when such a piezoelectric composite is exposed to humid or high moisture environment. Desirably, the piezoelectric composite retains suitable piezoelectric charge coefficient (d<NUM>) even when exposed to a high moisture environment.

In some aspects of the invention, the invention relates to a piezoelectric composite, comprising: (a) a polymer matrix; and (b) a plurality of coated piezoelectric filler particles, wherein each of the plurality of coated piezoelectric filler particles is dispersed in the polymer matrix, and further wherein each of the plurality of coated piezoelectric filler particles comprises a having at least one polar functional group.

The expression "plurality of coated piezoelectric filler particles" as used throughout this disclosure means <NUM> or more, preferably <NUM> or more, preferably <NUM> or more, preferably <NUM> or more, <NUM> or more, preferably <NUM> or more, preferably <NUM> or more, preferably <NUM> or more of coated piezoelectric filler particles being dispersed throughout the polymer matrix.

The expression "coated piezoelectric filler particles comprises a piezoelectric material having at least a portion of its outer surface coated with a polymeric material" means each of the piezoelectric filler particles is partially or completely coated by a polymeric material or an aggregate of one or more piezoelectric filler material is partially or completely coated by a polymeric material. In some preferred embodiments of the invention, the piezoelectric filler particle is completely coated by a polymeric material such that the particle is encapsulated by the polymeric material. In some preferred embodiments of the invention, the piezoelectric filler particles are coated with the polymeric material prior to dispersing the coated piezoelectric filler particles in the polymer matrix.

A piezoelectric composite having a coated filler particle may for example be visually distinguished from a piezoelectric composite having uncoated filler by evaluating the surface porosity of a piezoelectric composite using Scanning Electron Microscopy (SEM). For example, a piezoelectric composite having uncoated KNLN3 particles may be distinguished from a piezoelectric composite having PDMS coated KNLN3 particles, by evaluating the SEM images of <FIG>.

Similarly, from the SEM images under <FIG>, the distinction between piezoelectric composites having uncoated and coated fillers may be ascertained by a skilled person, where the SEM image of the precursor film/piezoelectric composite of <FIG> indicates reduced surface porosity compared to the precursor film/piezoelectric composite illustrated under <FIG>.

Further, a coated filler particle may be distinguished from an uncoated filler particle using a SEM image (e.g. <FIG>), thereby enabling a skilled person to clearly distinguish between a coated filler particle and an uncoated filler particle. Further from the results provided under Example <NUM> and Example <NUM> in this disclosure, the lower dielectric loss and improved piezoelectric performance of films containing composites having coated filler particles is clearly evidenced over that of fillers having composites with uncoated filler particles.

In some embodiments of the invention, the polymeric material comprises at least one polar functional group selected from hydroxyl group, carboxylate group, ester group, sulfonic acid group, amide group, siloxane group, amino group, sulfhydryl, phosphate, ether group, halogen group, and nitrile group. In some embodiments of the invention, the amide functional group for example may be cyclic amide with a group having -(C=O)-N- being part of a cyclic ring as present in polyvinylpyrolidone (PVP). Without wishing to be bound by any specific theory, the presence of a polar functional group aids in the adhesion of the polymeric material on the surface of the piezoelectric material. In some preferred aspects of the invention, the polymeric material comprises (i) a hydrophilic part which enables the adhesion to the piezoelectric material, and (ii) a hydrophobic part which serves as a moisture barrier coating.

In some embodiments of the invention, the plurality of coated piezoelectric filler particles are present in an amount ≥ <NUM> volume% and ≤ <NUM> volume%, alternatively ≥ <NUM> volume% and ≤ <NUM> volume%, alternatively ≥ <NUM> volume% and ≤ <NUM> volume%, with regard to the total volume of the piezoelectric composite. In some preferred embodiments of the invention, the plurality of coated piezoelectric filler particles are present in an amount of <NUM> volume% with regard to the total volume of the piezoelectric composite.

In some embodiments of the invention, the piezoelectric material is selected from hydroxyapatite, apatite, lithium sulfate monohydrate, sodium bismuth titanate, quartz, tartaric acid, poly(vinylidene difluoride), barium titanate, potassium sodium niobate (KNaNb)O<NUM> (KNN), lead zirconate titanate (PZT), lead niobium titanate (PNT), lead scandium niobium titanate (PSNT), lead metaniobate, lithium doped potassium sodium niobate (KNLN) represented by the formula (K<NUM>Na<NUM>)<NUM>-tLitNbO<NUM>, wherein the variable 't' ranges from greater than <NUM> to less than <NUM> (<NUM> <t<<NUM>), alternatively greater than <NUM> to less than <NUM>.

In some preferred embodiments of the invention, the piezoelectric material is a lead free piezoelectric material selected from hydroxyapatite, apatite, lithium sulfate monohydrate, sodium bismuth titanate, quartz, tartaric acid, poly(vinylidene difluoride), barium titanate, potassium sodium niobate (KNaNb)O<NUM> (KNN), lithium doped potassium sodium niobate (KNLN) represented by the formula (K<NUM>Na<NUM>)<NUM>-tLitNbO<NUM>, wherein the variable 't' ranges from greater than <NUM> to less than <NUM> (<NUM><t<<NUM>), alternatively greater than <NUM> to less than <NUM>.

In some aspects of the invention, the piezoelectric material has a suitable size and morphology for piezoelectric application and can be prepared for example, by a two-step calcination process. For example, the steps for preparing the piezoelectric material includes obtaining a precursor product after a first calcination step involving the calcination of a stoichiometric mixture of precursor metal salts such as carbonate metal salts or a mixture of oxide compounds.

The first calcination step involves calcination at a temperature > <NUM>, preferably > <NUM> and < <NUM>. The first calcination step maybe carried out for at least two hours, alternatively for at least three hours at a heating rate of <NUM> min-<NUM> to obtain the precursor product. In some embodiments of the invention, the precursor product is a ceramic product. The precursor product is subsequently milled using a mortar and pestle or by hand, to form a powder. The powder so obtained may be subjected to a second step of calcination carried out a temperature lower than the first calcination step, for example at a temperature < <NUM>. The second calcination step may be carried out for at least nine hours, preferably at least ten hours at a heating rate of <NUM> min-<NUM>. The precursor metal salts can be sodium carbonate or potassium carbonate or lithium carbonate, while metal oxides can be for example, niobium oxide.

The coated piezoelectric filler particles can be prepared by following the general procedure: (a) providing a coating solution comprising a solubilized polymer or the corresponding monomer dissolved in a solvent; (b) dispersing one or more piezoelectric particle containing the piezoelectric material in the coating solution and forming a coating precursor solution; (c) removing the solvent from the coating precursor solution under conditions suitable for coating the one or more piezoelectric particle with the polymer and forming the coated piezoelectric filler particle. In some aspects of the invention, conditions suitable for coating include conditions of polymerizing the monomer to form a coating around the piezoelectric particle.

In some aspects of the invention, the solvent may be removed using a low pressure evaporation in a rotary evaporator. It is believed that by removing the solvent in this manner ensures intimate contact between the piezoelectric particle and polymer. The selection of solvent would depend largely upon the solubility of the polymer. For example, when polyvinylpyrolidone (PVP) is used as a coating polymer, the solvent used can be water. On the other hand, when polydimethylsiloxane (PDMS) is used as a coating polymer the solvent is cyclohexane.

In some embodiments of the invention, the polymeric material is selected from polydimethylsiloxane (PDMS), polyvinylpyrrolidone (PVP), grafted polyolefins, acrylate and methacrylate polymers, polyethylene glycol, vinyl acetate polymers polycarbonate, vinylidene fluoride (VDF) polymers, cyano polymers, poly(phenylene oxide) or combinations thereof. In some preferred embodiments of the invention, the polymeric material is selected from polydimethylsiloxane (PDMS), or polyvinylpyrrolidone (PVP).

In some embodiments of the invention, the polymeric material is polydimethylsiloxane (PDMS) and the piezoelectric material is a lithium doped potassium sodium niobate (KNLN) represented by the formula (K<NUM>Na<NUM>)<NUM>-tLitNbO<NUM> wherein 't' is <NUM> (KNLN3) or 't' is <NUM> (KNLN6). For the purpose of clarification, the expression, KNLN3 is used to represent the piezoelectric material of formula (K<NUM>Na<NUM>)<NUM>-tLitNbO<NUM> when the value of 't' is <NUM> and the expression, KNLN6 is used to represent the piezoelectric material of the formula (K<NUM>Na<NUM>)<NUM>-tLitNbO<NUM> when the value of 't' is <NUM>.

In some preferred aspects of the invention, the polymer matrix is a vinylidene fluoride (VDF) polymer selected from polyvinylidene fluoride (PVDF), polyvinylidene fluoride-co-tri-fluoroethylene (PVDF-TrFE), poly(vinylidene fluoride-co-hexafluoropropylene)(PVDF-HFP), polyvinylidene fluoride-co-tetrafluoro ethylene (PVDF-TFE), poly(vinylidene fluoride-co-trifluoroethylene-co-chlorofluoroethylene) (PVDF-TrFE-CFE), poly(vinylidene-fluoride-co-trifluoroethylene-co-chlorotrifluoroethylene) (PVDF-TrFE-CTFE) or combinations thereof.

In some embodiments of the invention, the polymer matrix is a polyolefin co-polymer selected from ethylene alpha-olefin copolymers, propylene alpha-olefin copolymers, ethylene-propylene alpha-olefin terpolymers, wherein the copolymers are derived from one or more alpha-olefins moieties selected from <NUM> -butene, <NUM>-methyl-<NUM>-pentene, <NUM>-hexene, <NUM>-octene, <NUM>-decene, <NUM>-dodecene, <NUM>-tetradecene, <NUM>-hexadecene, <NUM>-octadecene and <NUM>-eicosene, or combinations polyolefin thereof. In some preferred embodiments of the invention, the polymer matrix is a polyolefin co- polymer comprising moieties derived from ethylene and <NUM>-octene. In some preferred embodiments of the invention, the polymer matrix is a polyolefin co-polymer comprising moieties derived ethylene and <NUM> - hexene.

According to the invention, the polymer matrix and the polymeric material coating of the piezoelectric material are of different polymer composition.

Preferably the polymeric material is selected from polydimethylsiloxane (PDMS), polyvinylpyrrolidone (PVP), grafted polyolefins, polyethylene glycol, vinyl acetate polymers, cyano polymers, or combinations thereof and preferably the polymer matrix is a polymer selected from vinylidene fluoride (VDF) polymer, polycaprolactone, polyolefin co-polymers, polysiloxane-polycarbonate copolymers, acrylate and/or methacrylate copolymers, polyethylene terephthalate (PET), polycarbonate (PC), polybutylene terephthalate (PBT), poly(<NUM>,<NUM>-cyclohexylidene cyclohexane-<NUM>,<NUM>-dicarboxylate) (PCCD), glycol modified polycyclohexyl terephthalate (PCTG), poly(phenylene oxide) (PPO), polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC), polystyrene (PS), polyethyleneimine or polyetherimide (PEI) and their derivatives, terephthalic acid (TPA) polymers, poly(cyclohexanedimethylene terephthalate) (PCT), polyethylene naphthalate (PEN), polyamide (PA), polysulfone sulfonate (PSS), sulfonates of polysulfones, polyether ether ketone (PEEK), acrylonitrilebutadiene styrene (ABS), polyether ketone ketone (PEKK), polyphenylene sulfide (PPS), or blends or combinations thereof.

Preferably the polymeric material is selected from polydimethylsiloxane (PDMS), or polyvinylpyrrolidone (PVP) and the polymer matrix is a polymer selected from vinylidene fluoride (VDF) polymer.

In a preferred aspect of the invention, the invention is directed to a film comprising the piezoelectric composite of the present invention. The expression "film" as used throughout this disclosure means a film which is piezo active.

In some embodiments of the invention, the invention is directed to a film comprising the piezoelectric composite, wherein the piezoelectric composite comprises:.

Further, without wishing to be bound by any specific theory, it is believed that by processing the composite in the form of a film, the film can be tuned to have suitable mechanical flexibility and thereafter incorporated in various articles of manufacture.

In some embodiments of the invention, the polymer solution may be in the form of a polymer matrix. Preferably, the piezoelectric filler particles are coated prior to introducing the coated filler particles into the polymer solution. Without wishing to be bound by any specific theory, the coating of the filler particles prior to dispersing in the polymer solution, improves the dispersion of the filler particles and imparts improved piezoelectric property to a film comprising piezoelectric composite of the present invention.

Accordingly, in some embodiments of the invention, the invention is directed to a film obtained by a method comprising the steps of:.

Some of the differences in properties between the precursor film and film, is that the film is piezo-active while the precursor film is not piezo-active.

In some embodiments of the invention, the substrate used for casting is a glass plate. Non-limiting examples of organic solvent used for preparing the film include tetrahydrofuran (THF), methyl ethyl ketone (MEK), dimethylsulfoxide (DMSO), dimethylformamide (DMF), N-methyl-<NUM>-pyrrolidone (NMP), <NUM>,<NUM> dichloro benzene, <NUM>,<NUM>,<NUM> trichlorobenzene, cyclehexane, toluene, p-xylene or combinations thereof. In some preferred embodiments of the invention, the organic solvent is a <NUM>/<NUM> volume mixture of tetrahydrofuran (THF) and dimethylformamide (DMF). In some preferred embodiments of the invention, the organic solvent is dimethylformamide (DMF).

In various embodiments of the invention, casting of the precursor piezoelectric composite solution can include spreading the precursor piezoelectric composite solution on a substrate. The expression "casting" as used for the purposes of this invention includes the process of removing the solvent. Non-limiting examples of casting include air casting (e.g., the precursor piezoelectric composite solution can be passed under a series of air flow ducts that controls the evaporation of the organic solvents in a particular set period of time such as <NUM> to <NUM> hours), solvent or emersion casting, (e.g., the precursor piezoelectric composite solution is spread onto a moving belt and run through a bath or liquid in which the liquid within the bath exchanges with the organic solvent). The spreading of the precursor piezoelectric composite solution on the substrate can be carried out using a doctor blade, rolling spreader bar or any such suitable apparatus.

In some aspects of the invention, the precursor piezoelectric composite solution is dried at any temperature ≥ <NUM> and ≤ <NUM>, alternatively ≥ <NUM> and ≤ <NUM>, to remove the organic solvent and forming the precursor film. In some embodiments of the invention, the precursor film is annealed prior to poling. The annealing of the precursor film may be carried out at any temperature ≥ <NUM> and ≤ <NUM> for any time period ≥<NUM> hour and ≤ <NUM> hours, preferably annealing the precursor film at a temperature of <NUM> for any time period ≥ <NUM> hours and ≤ <NUM> hours.

The expression "poling" as used throughout this disclosure means a process of inducing electric polarization in a material to specifically orient the piezoelectric filler particles along the direction of the applied field or any such predetermined orientation. During electric polarization, the coated piezoelectric filler particles can be connected to one another in a linear or a semi-linear manner (e.g., chains of particles). Columns of piezoelectric particles are suitably formed by aligning of more than one chain. By way of example, the film precursor, can be poled at a selected electric field at a specific temperature. The selected temperature may for example, be chosen in accordance with a desired dipole orientation, or a desired polarization strength, or as required for an intended application area. The applied voltage parameter for poling can be selected in various ways. For example, the applied voltage level parameter can be selected as constant, or changing (e.g., ramped) over a period of time. In some embodiments of the invention, poling is performed using corona discharge in a corona polarization instrument having an electrode gap of <NUM> to <NUM>.

In some embodiments of the invention, the electric polarization treatment is conducted at an electric field ≥ <NUM> KV/mm and ≤ <NUM> KV/mm, alternatively ≥ <NUM> KV/mm and ≤ <NUM> KV/mm, or alternatively ≥ <NUM> KV/mm and <NUM> KV/mm, for a time period ≥ <NUM> minute and ≤ <NUM> minutes, alternatively ≥ <NUM> minutes and ≤ <NUM> minutes, or alternatively ≥ <NUM> minutes and ≤ <NUM> minutes, and at any temperature ranging ≥ <NUM> and ≤ <NUM>, or alternatively from ≥ <NUM> and ≤<NUM>.

In some aspects of the invention, the dielectric loss (tanδ) for a film comprising the piezoelectric composite is sufficiently low even when the film is exposed to humid or a high moisture environment. In other words, when the film comprising the piezoelectric composite of the present invention, is used in devices such as actuators or sensors, internal heating of the devices due to dielectric loss is minimized resulting in a more efficient operation of the device.

The dielectric loss may for example be measured by applying alternate current voltage and measuring the phase difference between the voltage waveform and the resulting current waveform, which provides the (tanδ) value or the dielectric loss. Dielectric loss may for example be measured using any suitable techniques such as by using impedance analyzer such as Agilent 4294A or the Novocontrol High Frequency Impedance Analyzer, Model Alpha AT.

In some aspect of the invention, the inventors surprisingly found that a film comprising the piezoelectric composite of the present invention, retained sufficiently high piezoelectric charge coefficient (d<NUM>) along with low dielectric loss even when the film is exposed to humidity.

In some embodiments of the invention, the piezoelectric charge coefficient (d<NUM>) ≥ <NUM> pC/N and ≤ <NUM> pC/N, alternatively from ≥ <NUM> pC/N and ≤ <NUM> pC/N, or alternatively ≥ <NUM> pC/N and ≤ <NUM> pC/N, wherein the piezoelectric charge coefficient (d<NUM>) is measured at a frequency of <NUM>. The piezoelectric charge coefficient (d<NUM>) may be measured by using any known instruments typically used in the industry such as by the PiezoMeter System, model PM300. The dielectric loss, piezoelectric charge coefficient may for example be measured in accordance with the ASTM D <NUM>-<NUM>.

In some embodiments of the invention, the film comprising the piezoelectric composite of the present, invention has a thickness ≥ <NUM> micrometers and ≤ <NUM> micrometers, alternatively a thickness ≥ <NUM> micrometers and ≤ <NUM> micrometers, or alternatively a thickness of ≥ <NUM> micrometers and ≤ <NUM> micrometers.

In various aspects of the invention, the invention is directed to an article of manufacture comprising the film comprising the piezoelectric composite of the present invention. In some embodiments of the invention, the article of manufacture is a component of a touch panel, a human machine interface, an integrated keyboard, or a wearable device.

Specific examples demonstrating some of the embodiments of the invention are included below. The examples are for illustrative purposes only and are not intended to limit the invention. It should be understood that the embodiments and the aspects disclosed herein are not mutually exclusive and such aspects and embodiments can be combined in any way. Those of ordinary skill in the art will readily recognize parameters that can be changed or modified to yield essentially the same results.

Purpose: Example <NUM> illustrates a method of preparation of piezoelectric fillers (KNLN3) having the formula K<NUM>Na<NUM>Li<NUM>NbO<NUM>.

Materials: The table below provides a summary of the materials used for preparing the fillers and the suppliers from whom the raw materials were procured:.

Process for preparation: The following general method was practiced for preparing the (KNLN3) piezoelectric ceramic filler: stoichiometric proportions of metal compound precursors of K<NUM>CO<NUM>, Na<NUM>CO<NUM>, Li<NUM>CO<NUM>, and Nb<NUM>O<NUM> powders were mixed in a cyclohexane medium using polypropylene lined mixer and using zirconia balls for <NUM> hours for homogenization. The resulting suspension so obtained was filtered and dried in a hot air oven for <NUM> hours to form the metal salt mixture composition. The metal salt mixture composition was subjected to a first and second calcination step to obtain the piezoelectric filler.

First calcination step: The calcination of the dried metal salt mixture composition was performed in a closed alumina crucible at a first calcination temperature by heating the samples at a rate of <NUM>° C/min until the first calcination temperature was reached at <NUM>. Thereafter, the temperature was maintained at the first calcination temperature for <NUM> hours and subsequently cooled to ambient temperature. After the first calcination, the powder was ball milled for <NUM> hours to refine the particle size.

Second calcination step: After ball milling, samples were heated to a second calcination temperature at a rate of <NUM>/min until the second calcination temperature was reached at <NUM> and thereafter maintained for <NUM> hours, and then cooled to ambient temperature. After calcination, the powders were ultrasonicated for <NUM> hour in a cyclohexane medium, dried at <NUM> for <NUM> hours and stored in an air ventilated drying oven to avoid moisture absorption.

Purpose: Example <NUM> illustrates the performance of a film comprising piezoelectric composite having <NUM> volume% of polyvinyl-pyrrollidone (PVP) coated KNLN3 as piezoelectric filler particles which are dispersed in a PVDF-TrFE-CFE polymer matrix. The KNLN3 particles are represented by the formula (K<NUM>Na<NUM>)<NUM>-tLitNbO<NUM> at 't' is <NUM> (K<NUM>Na<NUM>Li<NUM>NbO<NUM>) and are prepared in accordance with the method described under Example <NUM>.

Material used: The following are the details of the material used:.

Method for preparing the coated PVP piezoelectric filler particles: About <NUM> of KNLN3 was suspended in <NUM> of a <NUM> aqueous solution containing polyvinylpyrrolidone (PVP) and thereafter stirred for <NUM> hours. The particles were then isolated by centrifuging for <NUM> minutes at <NUM> rpm and the supernatant was decanted. The piezoelectric filler particles so obtained were subsequently dried overnight at <NUM> to obtain the PVP coated filler particles.

Method for preparing a film containing the piezoelectric composite having PVP coated piezoelectric particles: The general method used for preparing the film containing the piezoelectric composite was as follows: (a) dissolving a polymer in an organic solvent and forming a polymer solution; (b) adding a plurality of PVP coated piezoelectric filler particles to the polymer solution and forming a precursor piezoelectric composite solution; (c) thereafter, casting the precursor piezoelectric composite solution on a substrate and forming a precursor film; and (d) subjecting the precursor film to an electric polarization treatment and forming the film. For the purpose of the present example, the precursor film was annealed prior to the electric polarization treatment.

In particular to produce the piezoelectric composite, a <NUM> wt. % solution of poly(vinylidene fluoride-co-trifluoroethylene-co-chlorofluoroethylene) (PVDF-TrFE-CFE) (obtained from Piezotech Arkema) in dimethylformamide (DMF), was prepared (polymer solution). The PVP coated KNLN3 particles were mixed into this polymer solution using a Hauschild DAC <NUM> FVZ planetary speed mixer at <NUM> rpm for <NUM> minutes to obtain the precursor piezoelectric composite solution. Thereafter, precursor piezoelectric composite solution was subjected to degassing, for several minutes in a vacuum pot, and thereafter the precursor piezoelectric composite solution was casted on a glass plate using a doctor blade technique at a cast height of <NUM> to form a precursor film.

The precursor film was dried in a vacuum oven at <NUM> for <NUM> hour before annealing at <NUM> for <NUM> hours. In order to make the precursor film piezoactive, poling was carried out in a heated silicone oil bath at <NUM> for <NUM> minutes at a field of <NUM> kV/mm. Once poling was complete the field was switched off and the precursor film was removed from the bath and thereafter thermally quenched to obtain a film containing the PVP coated KNLN3 filler particles. Once annealing was finished a number of discs (<NUM> diameter) were cut from the composite for electrical testing. Gold electrodes were sputtered onto the composite discs using a Quorum Q300T sputter coater.

The electrical properties of the discs were measured using an Agilent 4263B LCR meter. Broadband dielectric spectroscopy was performed using a Single-Unit Dielectric Alpha Analyzer from Novocontrol. The piezoelectric properties were measured using a PM300 Berlin court-type piezometer from Piezotest with a static force of <NUM> N and a dynamic force of <NUM> N peak to peak with sinusoidal excitation at <NUM>. Electrical measurements were performed using a Novocontrol Alpha Dielectric Analyzer in the frequency range from <NUM> to <NUM> at room temperature with a fixed potential difference of <NUM> V.

Evaluation of piezoelectric property of a film having PVP coated piezoelectric fillers: For evaluating the efficacy of the coating on the piezoelectric filler particles against moisture, the dielectric loss (tanδ) was evaluated using broadband dielectric spectroscopy. Alternate current having a frequency within the frequency range of <NUM>-<NUM><NUM> Hz was thereafter used for the evaluation.

The piezoelectric film samples were analyzed for their piezoelectric property under different relative humidity conditions. Three different film samples were derived from the film obtained from the practice of Example <NUM>. Subsequently, each of the film samples were aged for two days under a specific relative humidity (Rh) condition (<NUM>%, <NUM>%, <NUM>%). For exposure at <NUM>% relative humidity (Rh), the film sample was stored in a chamber under humid conditions for two days. For exposure at <NUM>% humidity, the film sample was stored for two days and thereafter exposed to ambient air atmosphere. For exposure at <NUM>% humidity, a film sample was stored in a vacuum pot in presence of a desiccating silica gel for two days.

As an experiment control, the dielectric loss (tanδ) was determined for a film prepared using uncoated KNLN3 filler particles dispersed in a PVDF-TrFE-CFE polymer matrix. The effect of humidity on dielectric loss was determined and the results of the test are provided below in Table <NUM>:.

In addition to the above tests, the piezoelectric charge coefficient (d<NUM>), dielectric loss (tanδ) and relative permittivity was measured at <NUM> at relative humidity (Rh) ~<NUM>% and the results are provided in the table below:.

Results: From Table <NUM>, it is evident that the piezoelectric filler particles on being coated with PVP results in lower dielectric loss even at high relative humidity (Rh) environment of <NUM>%. For example, at a frequency of <NUM>, the dielectric loss (tan(δ)) of a film exposed to <NUM>% relative humidity (Rh=<NUM>%) and having PVP coated fillers had nearly <NUM> % lower dielectric loss (tan(δ)) compared to a film having uncoated filler particles. Similarly at a frequency of <NUM>, the dielectric loss (tan(δ)) of a film having PVP coated fillers was more than <NUM>% lower compared to a film having uncoated filler particles.

From Table <NUM>, it is evident that the film having PVP coated piezoelectric fillers, has lower dielectric loss (Tan(δ) of about <NUM>% lower) compared to a film having uncoated filler particles while improving piezoelectric charge coefficient (d<NUM>). It is suspected that the reason why dielectric loss is reduced significantly with PVP coating is due to the improvement in the dispersion of the filler particles in the polymer matrix, which is also evident from the SEM images. Further, although relative permittivity increases marginally for the film sample containing coated filler particles, it is expected that such marginal increase in relative permittivity will not adversely affect piezoelectric constant 'g', indicating that sensing performance of the film using piezoelectricity, is retained.

From <FIG>, the incorporation of the PVP coat on the KNLN particle may be further concluded from the Fourier Transformed Infrared spectrograph (FT-IR). The spectrograph indicates characteristic signals for the <NUM>) uncoated KNLN3 powder, <NUM>) the isolated PVP coating, and <NUM>) of the KNLN3 powder after functionalization with PVP. The appearance of characteristic spectra shifts at <NUM>-<NUM> and <NUM>-<NUM> for C-H and C-N stretching respectively, confirming the presence of PVP on the surface of the KNLN3 particles.

In fact for PVP, it is believed that the presence of polar C-N and C=O bonds enable improved adhesion with the highly polar and hydrophilic ceramic surface of the KNLN3 particles, while the non-polar alkyl chain results in improved compatibilization with the polymer matrix.

<FIG> illustrates a SEM image where a composite prepared using PVP coated filler particles demonstrates improved microstructure morphology indicating improved compatibilization between the PVP coated filler particles and the polymer matrix, as compared to a composite having uncoated filler particles as shown in <FIG> may represent a typical piezoelectric composite material in which the piezoelectric filler particles are dispersed in a matrix without the fillers having a surface coating and is therefore outside the scope of the invention.

Purpose: Example <NUM> illustrates the performance of a piezoelectric composite having <NUM> volume% of polydimethylsiloxane (PDMS) coated KNLN3 as the piezoelectric filler particles dispersed in a polyvinylidene flttoride-co-tri-fluoroethylene polymer matrix. The KNLN3 filler particles are represented by the formula (K<NUM>Na<NUM>)<NUM>-tLitNbO<NUM> where 't' is <NUM> (K<NUM>Na<NUM>Li<NUM>NbO<NUM>) and are prepared in accordance with the method described under Example <NUM>.

Method for preparing the PDMS coated piezoelectric filler particles: The piezoelectric particles were obtained in the manner as described in Example <NUM>. Thereafter, the particles were coated with polydimethylsiloxane polymer using the following steps: <NUM> of dimethyl siloxane (PDMS monomer), was dissolved in <NUM> of cyclohexane to form a solution. About <NUM> of KNLN3 particles were added to this solution and stirred with a magnetic stirrer for <NUM> hours before the solvent was removed by low pressure evaporation in a rotary evaporator to obtain polydimethylsiloxane (PDMS) coated piezoelectric filler particles.

Method for preparing a film containing PDMS coated piezoelectric particles: The method of preparing the film was identical to that described in Example <NUM>, except that PDMS coated filler particles were used.

Evaluation of piezoelectric property using PDMS coated piezoelectric fillers: The film was evaluated for piezoelectric performance. The piezoelectric charge coefficient (d<NUM>), dielectric loss (tanδ) and relative permittivity were measured at <NUM> (<NUM>) and the results are provided in the table below:.

Result: From Table <NUM>, it is evident that the film prepared using PDMS coated KNLN3 filler particles had nearly <NUM>% lower dielectric loss while retaining similar levels of piezoelectric charge coefficient (d<NUM>). It may be concluded that the film of Example <NUM> demonstrates desirable level of piezoelectric performance. Advantageously, relative permittivity (εr or dielectric constant) reduces significantly when using PDMS coated filler particles, indicating a possible increase in the value of piezoelectric voltage constant (g) and rendering such film to be suitable for use in a sensing device.

<FIG> illustrates SEM images of an uncoated KNLN3 particles. <FIG> illustrates a PDMS coated KNLN3 particles. <FIG> illustrates precursor film cross section comprising uncoated KNLN3 particles and <FIG> illustrates precursor film cross section comprising PDMS coated KNLN3 particles.

<FIG> may represent a typical piezoelectric composite material in which the piezoelectric filler particles are dispersed in a matrix without the fillers having a surface coating and is therefore outside the scope of the invention.

Comparing the images in <FIG> the successful coating of PDMS is evident from the surface morphology, which is clearly visible in image <FIG>. Further, from <FIG>, it is evident that the surface porosity is considerably reduced indicating improved adhesion of the filler particles to the polymer matrix.

Purpose: Example <NUM> illustrates the performance of a piezoelectric composite having <NUM> volume% of <NUM>-amino(propyl)-methyl-diethoxysilane (ADS) functionalized KNLN3 as piezoelectric filler particles. The KNLN3 particles are represented by the formula (K<NUM>Na<NUM>)<NUM>-tLitNbO<NUM> at 't' is <NUM>(K<NUM>. 485Na<NUM>Li<NUM>NbO<NUM>) and are prepared in accordance with the method described under Example <NUM>. The polymer matrix used was poly(vinylidene fluoride-co-trifluoroethylene-co-chlorofluoroethylene) (PVDF-TrFE-CFE).

The functionalization of the piezoelectric filler particles with ADS was carried out similar to the manner in which PVP was coated on the filler particles except ADS merely surface functionalized the filler particles and did not form an encapsulating polymer coating.

Evaluation of piezoelectric property using ADS functionalized piezoelectric fillers: The effect of surface functionalization on the property of dielectric loss was evaluated in the manner described in Example <NUM>. The results are shown in Table <NUM> below:.

Result: From Table <NUM>, it is evident that films prepared using ADS surface functionalized filler particles demonstrated lower dielectric loss than that of films prepared using the uncoated filler particles.

On the other hand, comparing the results provided under Table <NUM> with that of Table <NUM>, it is evident that films prepared using PVP coated KNLN3 filler particles demonstrated lower dielectric loss compared to films prepared using ADS functionalized filler particles. For example, at a frequency of <NUM> and exposed at <NUM>% relative humidity (Rh), the film having PVP coated filler particles had a <NUM>% lower dielectric loss compared to a film using ADS functionalized filler particles (Table <NUM>). Similarly, at <NUM> and exposed at <NUM>% relative humidity (Rh), the film having PVP coated filler particles had a <NUM>% lower dielectric loss compared to a film using ADS functionalized filler particles.

From these data it is clear that films having ADS functionalized filler particles perform better than films having uncoated KNLN3 filler particles in most conditions. However, they do not perform as well as the films comprising PVP coated filler piezoelectric particles. It is suspected that the reason that ADS functionalized filler particles do not impart similar levels of performance that of a film having PVP coated filler particles (Example <NUM>) does, is because ADS does not form an encapsulating polymer coating around the filler particles. On the other hand, PVP coating is formed by a relatively simple polymerization process using methods such as solution casting. Therefore, it is believed that PVP forms a more complete shell around the KNLN3 filler particles.

Purpose: Example <NUM> illustrates the performance of a piezoelectric composite having <NUM> volume% of (<NUM>-mercaptopropyl)methyldimethoxysilane (MPMDS) functionalized KNLN3 as piezoelectric filler particles. The KNLN3 particles are represented by the formula (K<NUM>Na<NUM>)<NUM>-tLitNbO<NUM> at 't' is <NUM>(K<NUM>Na<NUM>Li<NUM>NbO<NUM>) and are prepared in accordance with the method described under Example <NUM>.

The functionalization of the piezoelectric filler particles with MPMDS was carried out similar to the manner in which PVP was coated on the filler particles except MPMDS merely surface functionalized the filler particles and did not form an encapsulating polymer coating around the filler particle. The polymer matrix used was poly(vinylidene fluoride-co-trifluoroethylene-co-chlorofluoroethylene) (PVDF-TrFE-CFE).

Evaluation of piezoelectric property using MPMDS functionalized piezoelectric fillers: The effect on surface functionalization on the property of dielectric loss was evaluated in the manner described in Example <NUM>. The results are shown in Table <NUM> below:.

Result: From Table <NUM>, it is evident that films containing MPMDS functionalized filler particles perform marginally better than films having uncoated KNLN filler particles in dry atmosphere (<NUM>% relative humidity). In all other conditions of humidity, it is evident that surface functionalization of the filler particles with MPMDS, results in the deterioration of the dielectric losses of the film. When compared to the performance with a film having PVP coated filler particles, the film having PVP coated filler particles had a significantly lower dielectric loss. For example, at a frequency of <NUM> and exposed at <NUM>% relative humidity, the film having PVP coated filler particles had a <NUM>% lower dielectric loss as compared to a film having MPMDS functionalized filler particles.

Further, from the SEM image shown in <FIG>, when the KNLN filler particles are coated with MPMDS it is inferred that the adhesion of the polymer to the filler particles are drastically reduced with a visibly poorer microstructure after cracking and a corresponding increase in the dielectric loss.

Claim 1:
A piezoelectric composite, comprising:
a. a polymer matrix; and
b. a plurality of coated piezoelectric filler particles, wherein each of the plurality of coated piezoelectric filler particles is dispersed in the polymer matrix, and further wherein each of the plurality of coated piezoelectric filler particles comprises a piezoelectric material having at least a portion of its outer surface coated with a polymeric material having at least one polar functional group,
characterized in that the polymer matrix and the polymeric material coating of the piezoelectric material are of different polymer composition.