Lead-free piezoelectric composites and methods of making the same

Abstract

Methods of producing a lead-free piezoelectric composite are described. The methods can include adding a lead-free piezoelectric additive to a solution comprising a solvent and a polymer dissolved therein. The solvent can have i) a boiling point > 80 °C at 0.1 MPa and ii) a solubility in water > 0.1 g / g and / or a dielectric constant > 20. The solvent can be removed to form a polymer matrix having the lead-free piezoelectric particles dispersed therein. An electrical treatment of the polymer matrix can form a piezoelectric element. Also described are lead-free piezoelectric composites and devices comprising the lead-free piezoelectric composites.

Description

[0001] Cross-reference to related applications

[0002] This application claims the benefit of priority to Indian Provisional Patent Application No. 201911013259, filed on April 2, 2019, which is incorporated herein by reference in its entirety. Technical Field

[0003] This invention generally relates to a method for preparing lead-free piezoelectric composite materials comprising lead-free piezoelectric particles with an average particle size of 200 nm to 1000 nm dispersed or suspended in a polymer matrix. Specifically, the method includes using a solvent with i) a boiling point ≥80 °C at 0.1 MPa and ii) a solubility in water ≥0.10 g / g and / or a dielectric constant ≥20 to form a dispersion or suspension of lead-free particles, wherein the polymer material is partially or completely dissolved in the solvent, the suspension forms a polymer matrix, and the polymer matrix is ​​then subjected to electrodeization treatment. Background Technology

[0004] Piezoelectric materials can be polymers, ceramics, or single crystals. Ceramics can have a relatively high dielectric constant and good electromechanical coupling coefficient compared to polymers. However, ceramics have high acoustic impedance, resulting in poor acoustic matching with media such as water and human tissue (typically the medium through which signals are transmitted or received). Furthermore, ceramics can exhibit high hardness and brittleness and cannot be shaped on curved surfaces, leading to limited design flexibility in a given transducer. Additionally, the electromechanical resonances of piezoelectric ceramics generate high levels of noise, an undesirable artifact in transducer engineering.

[0005] Single-crystal piezoelectric materials can include crystals of quartz tourmaline and sodium potassium tartrate. Other single crystals can include lead metaniobate (PbNb₂O₆) or relaxor systems, such as Pb(Sc) 1 / 2 Nb 1 / 2 O3-PbTiO3, Pb(In) 1 / 2 Nb 1 / 2 O3-PbTiO3 and Pb(Yb) 1 / 2 Nb 1 / 2 (1-2×)BiScO3×PbTiO3. Similar to ceramics, no single piezoelectric phase (ceramic, crystalline, or polymeric) can provide all the characteristics required for an application; therefore, performance is limited by a trade-off between high piezoelectric activity and low density versus mechanical flexibility.

[0006] Thin, flexible, high-performance piezoelectric materials are in high demand in emerging healthcare and biomedical applications and in sensors, actuators, and energy harvesters for wearable electronics. However, the development of flexible piezoelectric devices is limited due to the heaviness and brittleness of inorganic piezoelectric ceramics. The harsh processing conditions required to obtain excellent piezoelectric properties and to process these materials as thin layers onto a substrate, such as high temperatures (> 500 °C), further limit the choice of substrate materials as flexible substrates that are resistant to high temperatures are needed. On the other hand, piezoelectric polymers such as PVDF and PVDF-TrFE copolymers offer several advantages, including mechanical flexibility, light weight, low temperature, and ease of processing. Despite these advantages over ceramic materials, these materials have lower piezoelectric responses (d33~ 13-28 pC / N) compared to ceramics (d 33 33 of PZT is ~270-400 pC / N) and require higher driving voltages, which pose additional safety and cost issues.

[0007] For these applications, it is desirable to achieve a balance of several conflicting requirements that neither piezoelectric ceramics nor piezoelectric polymers can satisfy. Therefore, piezoelectric composites are an attractive alternative as they can combine the advantages of both materials, i.e. high piezoelectric response and high dielectric constant of ceramics with the mechanical flexibility of polymers. The inherent flexibility or deformability of piezoelectric materials is also an important parameter for device performance, flexibility prevents fatigue and improves the lifetime of the device. The addition of piezoelectric ceramic fillers to polyvinylidene fluoride-based polymers (homo, co, and terpolymers) to form (0-3) composites shows good piezoelectric constants (d 33 33 of ~40-60 pC / N). However, these piezoelectric composites of the prior art are mostly lead-based and also have mechanical brittleness. Due to the high toxicity of lead, environmental issues and biocompatibility integration issues are unavoidable.

[0008] The piezoelectric constants of commercially available lead-free piezoelectric ceramics are lower than those of lead-based piezoelectric ceramic powders. Therefore, it is difficult for lead-free piezoelectric composites to achieve piezoelectric performance comparable to that of lead-based piezoelectric composites (d 33 of PZT is ~270-400 pC / N, while the d 33 33 of barium titanate is ~190 pC / N). Therefore, there is a need to provide lead-free piezoelectric polymer composites that offer excellent piezoelectric performance while retaining the mechanical flexibility of polymers. Various attempts to produce lead-free piezoelectric ceramics have been disclosed. For example, U.S. Patent Application Publication No. 2015 / 0134061 to Friis et al. describes a spinal implant and a method of making the spinal implant, the method comprising dispersing a piezoelectric ceramic in a polymer matrix. The produced composite has a low d 33(pC / N) values. In another example, JP 2016-219804 by Tetsuhiro et al. describes a method of making a lead-free piezoelectric polymer material that includes using an affinity improver such as a surfactant to help disperse the particles in the polymer matrix. The affinity improver can be difficult to remove from the desired polymer matrix and / or is costly.

[0009] Despite the various attempts to produce piezoelectric composites, there remains a need to produce composites with a balance of desired piezoelectric properties and mechanical flexibility. SUMMARY

[0010] A discovery has been made that provides a solution to some of the above problems. The premise of the discovery is to make a flexible piezoelectric material by using i) a solvent with a boiling point > 80 °C at 0.1 MPa and ii) a solubility in water > 0.10 g / g and / or a dielectric constant > 20 to form a dispersion or suspension of lead-free piezoelectric particles, where the polymer is partially or completely dissolved in the solvent. The solvent is removed to form a polymer material with the lead-free piezoelectric particles dispersed therein. The polymer material can be subjected to an electrode polarization treatment to form a piezoelectric polymer material of the invention. The use of a higher boiling point solvent during processing provides the following advantages: 1) the ability to form a flexible piezoelectric material with excellent piezoelectric properties and mechanical flexibility, 2) the ability to form thin films, whether free-standing films or supported films on a substrate, 3) a simple method of making piezoelectric composites, and / or 4) low temperature processability.

[0011] In one aspect of the application, a method of producing a lead-free piezoelectric composite material is described. One method can include a first step (a) of adding lead-free piezoelectric particles having an average particle size of 200 nm to 1000 nm to a solution comprising a polymeric material and a solvent to form a dispersion or suspension, the solvent having i) a boiling point > 80 °C at 0.1 MPa and ii) a solubility in water > 0.1 g / g, preferably 0.15 g / g, more preferably 0.25 g / g and / or a dielectric constant > 20, preferably > 30, more preferably > 55. The polymeric material can be partially or completely dissolved in the solvent and can include a thermoset polymer, a thermoplastic polymer, or a blend thereof, preferably a thermoplastic polymer. Non-limiting examples of thermoplastic polymers include polyvinylidene fluoride (PVDF), a PVDF-based polymer, a PVDF copolymer, a PVDF terpolymer, or a mixture thereof. The PVDF terpolymer can be poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (PVDF-TRFE-CFE). In some aspects, the PVDF-type polymer can exhibit electromechanical properties or ferroelectric properties. The polymeric material can be dissolved in the solvent at a temperature of 15 to 100 °C to produce a solution comprising 5 to 20 wt. / vol.%, preferably 10 wt. / vol.% to 12 wt. / vol.% of the polymer. Non-limiting examples of lead-free piezoelectric particles include barium titanate particles, hydroxyapatite particles, apatite particles, lithium sulfate monohydrate particles, potassium sodium niobate particles, quartz particles, and combinations thereof. In a preferred embodiment, the lead-free piezoelectric particles are barium titanate particles. Non-limiting examples of solvents include methyl ethyl ketone (MEK), dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), or combinations thereof, preferably DMSO. In some cases, the volume percentage of lead-free piezoelectric particles in the composite material can be 15 to 65 vol.%, preferably 20 to 60 vol.%. The particle size of the lead-free piezoelectric particles can be 250 to 350 nm, preferably 300 nm. A second step (b) can include forming a polymer matrix having the lead-free piezoelectric particles dispersed therein. The forming can include (i) casting the dispersion on a substrate to form the polymer matrix, (ii) drying the polymer matrix at 25 to 45 °C, and (iii) annealing the dried polymer matrix at a temperature of 80 to 150 °C for 1 to 50 hours, preferably at 110 °C for 5 to 25 hours. A third step (c) can include subjecting the polymer matrix having the lead-free piezoelectric particles dispersed therein to an electrode polarization treatment. The electrode polarization can include applying a polarization field using a corona discharge. For a piezoelectric composite film, the corona polarization can be performed on the top surface of the film (which is exposed to air during drying of the film) and / or the bottom surface of the film (which is in contact with the substrate during drying of the film).

[0012] In another aspect of the application, a lead-free piezoelectric composite and a lead-free piezoelectric composite precursor are described. The lead-free piezoelectric composite precursor can include a polyvinylidene fluoride (PVDF)-based polymer matrix, lead-free piezoelectric particles having an average particle size of 200 to 1000 nm dispersed in the polymer matrix, and a solvent having i) a boiling point > 80 °C at 0.1 MPa and ii) a solubility in water > 0.1 g / g and / or a dielectric constant > 20. The lead-free piezoelectric composite can include a polyvinylidene fluoride (PVDF)-based polymer matrix and lead-free piezoelectric particles having an average particle size of 200 to 1000 nm dispersed in the polymer matrix. The piezoelectric composite can have a piezoelectric strain constant (d 33 ) of at least 40 pC / N and an elongation at break of 100% to 500% and a storage modulus of 100 to 325 MPa measured using ISO method 6721. The piezoelectric composite can have a piezoelectric strain constant (d 33 ) of at least 40 pC / N and an elongation at break of 100% to 500% and a storage modulus of 100 to 325 MPa measured using ISO method 6721. The elongation at break can be measured at ambient temperature under uniaxial loading using a dynamic mechanical analyzer such as a RDA III analyzer (TA Instruments, USA). The PVDF terpolymer can be poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (PVDF-TRFE-CFE). The lead-free piezoelectric particles can be barium titanate particles having an average particle size of 250 to 350 nm. The composite can be a film or sheet and can have a thickness of 50 to 200 micrometers. Such a piezoelectric polymer composite can be formed in the absence of a compatibilizing agent and / or prepared by the method of the application. In some embodiments, the lead-free piezoelectric polymer composite can include, consist of, or consist essentially of poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (PVDF-TRFE-CFE) and barium titanate particles having an average particle size of 250 to 350 nm.

[0013] In another aspect of the application, a piezoelectric device including the lead-free piezoelectric polymer composite of the application is described. Such a device can be a piezoelectric sensor, a piezoelectric transducer, or a piezoelectric actuator. The device is preferably mechanically flexible.

[0014] Further embodiments of the application are discussed throughout this application. Any embodiment discussed with respect to one aspect of the application is also applicable to other aspects of the application, and vice versa. Each embodiment described herein is understood to be an embodiment of the application applicable to other aspects of the application. It is contemplated that any embodiment discussed herein can be implemented with respect to any method or combination of the application, and vice versa. Moreover, the compositions and kits of the application are useful for implementing the methods of the application.

[0015] The following includes definitions of various terms and phrases used throughout this specification.

[0016] The phrase "compatibility improver" refers to a compound that improves the dispersibility or incorporation of a solid in a polymer. Non-limiting examples of compatibility improvers include surfactants, such as non-ionic surfactants, cationic surfactants, and anionic surfactants.

[0017] The term "about" or "approximately" is defined as nearly or nearly the same as, as understood by one of ordinary skill in the art. In one non-limiting embodiment, the term is defined as within 10%, preferably within 5%, more preferably within 1%, most preferably within 0.5%.

[0018] The term "wt.%" "vol.%" or "mol.%" refers to the weight percent, volume percent, or mole percent of a component, respectively, based on the total weight of material including the component, the total volume of the component, or the total moles of the component. In a non-limiting example, 10 grams of a component in 100 grams of material is 10 wt.% of the component.

[0019] The term "substantially" and variations thereof are defined to include a range of 10%, 5%, 1%, or 0.5%.

[0020] The term "inhibit" or "reduce" or "prevent" or "avoid" or any variation of these terms, when used in the claims and / or the specification, includes any measurable decrease or complete inhibition to achieve the desired result.

[0021] The term "effective," as used in the specification and / or claims, means adequate to accomplish a desired, expected, or intended result.

[0022] The use of the word "a" or "an" when used in the context of the claims or specification means "one," but it is also consistent with the meaning of "one or more," "at least one," and "one or more than one."

[0023] The terms "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.

[0024] The piezoelectric composites of the present invention can "comprise," "consist essentially of," or "consist of" the particular ingredients, components, compositions, etc. disclosed throughout the specification. With respect to the transitional phrase "consisting essentially of," in one non-limiting aspect, the essential and novel features of the piezoelectric composites of the present invention are that they can be lead-free, mechanically flexible, and / or have a piezoelectric strain constant (d 33 ) of at least 40 pC / N.

[0025] 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 limit. Furthermore, changes and modifications within the spirit and scope of the invention will become apparent to the skilled artisan from the detailed description. In further embodiments, features from one embodiment can be combined with features from other embodiments. For example, features from one embodiment can be combined with features from any of the other embodiments. In additional embodiments, additional features can be added to the specific embodiments described herein. BRIEF DESCRIPTION OF DRAWINGS

[0026] The advantages of the present invention can become apparent to those skilled in the art with the help of the following detailed description and accompanying drawings.

[0027] Figures 1A-D Scanning electron micrographs of the top surface of piezoelectric polymer composites prepared using different solvents: a) MEK, b) THF, c) DMSO, d) NMP are shown.

[0028] Figures 2A-D Surface profilers of the top surface of piezoelectric polymer composites prepared using different solvents: a) MEK, b) THF, c) DMSO, d) NMP over a sampling length (5 mm) are shown.

[0029] Figure 3 The effect of piezoelectric ceramic filler loading on the piezoelectric strain constant of comparative lead-based piezoelectric polymer composites and the lead-free piezoelectric polymer composites of the present invention is shown. The bottom surface in contact with the substrate during drying was exposed to corona during polarization.

[0030] Figure 4 The effect of piezoelectric ceramic filler loading on the elongation at break of comparative lead-based piezoelectric polymer composites and the lead-free piezoelectric polymer composites of the present invention is shown.

[0031] Figure 5 The effect of piezoelectric ceramic filler loading on the storage modulus of comparative lead-based piezoelectric polymer composites and the lead-free piezoelectric polymer composites of the present invention is shown.

[0032] Figure 6 Photographic image of a lead-free piezoelectric composite film (Example 3) corona polarized at 60 vol% BT loading according to aspects of the present invention.

[0033] While the application is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings. The drawings can not be to scale. DETAILED DESCRIPTION

[0034] Discoveries have been made that improve the mechanical and electrical properties of lead-free piezoelectric composites. The premise of the discovery is the use of a solvent during the preparation of the polymer matrix. The solvent can have i) a boiling point > 80 °C at 0.1 MPa and ii) a solubility in water > 0.10 g / g and / or a dielectric constant > 20, which allows partial or complete dissolution of the polymer. The dissolution of the polymer allows the lead-free piezoelectric particles to be better dispersed and / or suspended in the polymer matrix formed after the removal of the solvent.

[0035] These and other non-limiting aspects of the present invention are discussed in further detail in the following sections.

[0036] A. Materials

[0037] 1. Piezoelectric Additive

[0038] The piezoelectric additive can be any lead-free ceramic or single crystal material. Non-limiting examples of piezoelectric materials include inorganic compounds of the perovskite group. Non-limiting examples of piezoelectric ceramics having a perovskite structure include barium titanate (BaTiO3), hydroxyapatite, apatite, lithium sulfate monohydrate, potassium sodium niobate, bismuth sodium titanate, quartz, organic materials (e.g., tartaric acid, polyvinylidene fluoride fibers), or combinations thereof. In preferred embodiments, the piezoelectric additive is BaTiO3. The particle size of the lead-free piezoelectric particles can be any of 200 nm to 1000 nm, or 250 nm to 350 nm, or at least 200 nm, 225 nm, 250 nm, 275 nm, 300 nm, 325 nm, 350 nm, 375 nm, 400 nm, 425 nm, 450 nm, 475 nm, 500 nm, 525 nm, 550 nm, 575 nm, 600 nm, 625 nm, 650 nm, 675 nm, 700 nm, 725 nm, 750 nm, 775 nm, 800 nm, 825 nm, 850 nm, 875 nm, 900 nm, 925 nm, 950 nm, 975 nm, and 1000 nm, greater than any of them, equal to any of them, or between any two of them.

[0039] 2. Polymer

[0040] The piezoelectric composite can include a thermoset polymer, copolymer, and / or monomer, a thermoplastic polymer, copolymer, and / or monomer, or a thermoset / thermoplastic polymer or copolymer blend.

[0041] Thermoset polymers are malleable before heating and are capable of forming a mold. The matrix can be made from a composition having a thermoplastic polymer, and can also include other non-thermoplastic polymers, additives, etc. that can be added to the composition. The thermoset polymer matrix cures or crosslinks and tends to lose the ability to become pliable or malleable at elevated temperatures. Non-limiting examples of thermoset polymers used to make the polymer film include epoxy resins, epoxy vinyl esters, alkyd resins, aminopolymers (e.g., polyurethanes, urea-formaldehyde), diallyl phthalate, phenolic polymers, polyesters, unsaturated polyester resins, dicyclopentadiene, polyimides, silicon polymers, cyanate esters of polycyanuric acid ester, thermoset polyacrylic resins, bakelite, duroplast, benzoxazine, copolymers thereof, or blends thereof.

[0042] The thermoplastic polymer matrix can become pliable or malleable above a certain temperature and solidify below that temperature. The polymer matrix of the composite material can include thermoplastic or thermoset polymers discussed throughout this application, copolymers thereof, and mixtures thereof. Non-limiting examples of thermoplastic polymers include polyvinylidene fluoride (PVDF), PVDF-based polymers, PVDF copolymers, PVDF terpolymers, odd-numbered nylons, cyano polymers, polyethylene terephthalate (PET), polycarbonate (PC) family polymers, polybutylene terephthalate (PBT), poly(l,4-cyclohexylidene cyclohexane-l,4-dicarboxylate) (PCCD), poly(cyclohexyl terephthalate glycol) (PCTG), polyphenylene oxide (PPO), polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC), polystyrene (PS), polymethyl methacrylate (PMMA), polyethyleneimine or polyetherimide (PEI) and derivatives thereof, thermoplastic elastomers (TPE), terephthalic acid (TPA) elastomers, poly(cyclohexanedimethylene terephthalate) (PCT), polyethylene naphthalate (PEN), polyamide (PA), polysulfone sulfonate (PSS), sulfonate of polysulfone, polyether ether ketone (PEEK), polyether ketone ketone (PEKK), acrylonitrile butadiene styrene (ABS), polyphenylene sulfide (PPS), copolymers thereof, or blends thereof. In addition to these, other thermoplastic polymers known to those skilled in the art and thermoplastic polymers developed hereinafter can also be used in the context of the present application. The thermoplastic polymer can be included in a composition that includes the polymer and additives. Non-limiting examples of additives include coupling agents, antioxidants, heat stabilizers, flow modifiers, colorants, and the like, or any combination thereof. In a preferred example, a polyvinylidene fluoride (PVDF) polymer, copolymers thereof, or terpolymers thereof is used. The terpolymer can be poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (PVDF-TRFE-CFE).

[0043] 3. Solvent

[0044] The solvent used in the production of the lead-free piezoelectric composite can be any solvent that has i) a boiling point > 80 °C at 0.1 MPa and ii) a solubility in water > 0.1 g / g and / or a dielectric constant > 20. Non-limiting examples of solvents include methyl ethyl ketone (MEK), dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), or combinations thereof. In a preferred embodiment, the solvent is DMSO. The solubility of the solvent in water can be at least, equal to, or greater than 0.1 g / g, 0.15 g / g, 0.20 g / g, 0.25 g / g, 0.3 g / g, 0.35 g / g, 0.4 g / g, 0.45 g / g / , 0.5 g / g, 0.55 g / g, 0.6 g / g, 0.65 g / g, 0.7 g / g, 0.8 g / g, or 0.9 g / g. The boiling point of the solvent at 0.1 MPa can be at least, equal to, or greater than 80 °C, 85 °C, 90 °C, 95 °C, 100 °C, 105 °C, 110 °C, 115 °C, 120 °C, 125 °C, 130 °C, 135 °C, 140 °C, 145 °C, 150 °C, 155 °C, 160 °C, 165 °C, 170 °C, 180 °C, 185 °C, 190 °C, 195 °C, 200 °C, and 210 °C. Various combinations of i) boiling point and ii) solubility and / or dielectric constant can be used as long as the criteria are met. For example, the solvent can have a boiling point of 80 and be highly soluble in water (> 0.5 g / g). In another example, the solvent can have a boiling point > 150 °C and be moderately soluble in water (> 0.1 g / g). In some embodiments, the solvent has a boiling point > 80 °C at 0.1 MPa and a solubility in water > 0.25 g / g, a boiling point > 180 °C at 0.1 MPa and a solubility in water > 0.25 g / g, or a boiling point > 200 °C at 0.1 MPa and a solubility in water > 0.25 g / g. In another example, the solvent can have a boiling point of 80 and a high dielectric constant (> 50). In another example, the solvent can have a boiling point > 150 °C and a moderate dielectric constant (> 20). In some embodiments, the solvent has a boiling point > 80 °C at 0.1 MPa and a dielectric constant > 50, a boiling point > 180 °C at 0.1 MPa and a dielectric constant > 50, or a boiling point > 200 °C at 0.1 MPa and a dielectric constant > 50. Solvents that do not meet the boiling point and solubility, and / or the boiling point and dielectric constant criteria can result in piezoelectric composites with lower (pC / N) values. Table 1 lists the properties of non-limiting solvents used in the present application. 33 (pC / N) values. Table 1 lists the properties of non-limiting solvents used in the present application.

[0045] Table 1

[0046] Properties MEK DMSO NMP Boiling point (°C) 80 189 202 Solubility in water g / 100g 25.6 25.3 10 Dielectric constant 18.5 47 32

[0047] B. Methods of producing piezoelectric composites

[0048] The piezoelectric composite can be manufactured using a solution casting or forming method. A solution of the polymer described in the Material Section can be obtained. The solution can include the solvent described in the Material Section and the polymer described in the Material Section. The solution can include any of 1.5 wt. / vol.%, 5 wt. / vol.%, 10 wt. / vol.%, 15 wt. / vol.%, and 20 wt. / vol.% of the polymer, greater than any of these, equal to any of these, or between any two of these. In some embodiments, the solution includes 10 wt.% to 12 wt.% of PVDF or PVDF-TRFE-CFE or a blend thereof. It is noted that no compatibilizer is used to make the lead-free polymer composites of the present invention.

[0049] The piezoelectric additive can be dispersed or suspended in the polymer solution. The piezoelectric additive can be a plurality (e.g., 2 or more, suitably 5 or more, 10 or more, 50 or more, 100 or more, 500 or more, 1000 or more, etc.) of lead-free piezoelectric particles. The lead-free piezoelectric particles can be dispersed in the solution by any suitable method, including mixing, stirring, folding, or otherwise integrating the lead-free piezoelectric particles into the matrix, thereby creating a uniform dispersion or suspension of the particles in the matrix. In some embodiments, the solution is added to the piezoelectric additive.

[0050] The dispersion or suspension can be subjected to conditions suitable for forming the piezoelectric composite of the present invention. The following description references a dispersion, but it also applies to a suspension. In one case, the dispersion includes PVDF, PVDF-PVDF, or PVDF-TRFE-CFE or a blend thereof, and barium titanate. In some embodiments, the dispersion can be formed or cast. Forming or casting can include a mechanical or physical process to change into a desired form. Forming can also include simply placing the dispersion into a desired container or receptacle, thereby providing it with a retained shape or form. It is noted that the formed form is not necessarily the final form, as additional processing (e.g., machining, forming, etc.) can be done on the final cured composite. The role of forming the dispersion for use in the methods described herein is primarily to impart some initial structure to the dispersion prior to further processing. No rigidity or specific shape is required.

[0051] Casting can be pouring the dispersion onto a casting surface. Non-limiting examples of casting include air casting (e.g., the dispersion is passed under a series of air flow conduits that control the evaporation of solvent over a specific set time period, such as 24 to 48 hours), solvent casting, or dip casting (e.g., the dispersion is spread on a conveyor belt and flowed through a bath or liquid, where the liquid in the bath exchanges with the solvent). Spreading of the dispersion on the casting surface can be accomplished with a doctor blade, a rolling spreader bar, or any of several flat plate extrusion die configurations.

[0052] During casting or shaping, the solvent can be removed, leaving the dispersion in the substrate or mold. Heating can be used to aid in the removal of the solvent. For example, the shaped material can be heated at a temperature of at least any of 50 °C, 55 °C, 60 °C, 65 °C, 70 °C, 75 °C, and 80 °C, greater than any of these, equal to any of these, or between any two of these. The resulting shaped polymer composite can be annealed at a temperature of at least any of 80 °C, 85 °C, 90 °C, 95 °C, 100 °C, 105 °C, 110 °C, 115 °C, and 120 °C, greater than any of these, equal to any of these, or between any two of these for a desired amount of time (e.g., 5, 10, 15, 20, 25 hours, or any range or value therebetween). The shaped material can be a film, sheet, or the like. Some or all of the solvent can be removed during heating. For example, heating and / or annealing the lead-free piezoelectric polymer precursor composite can remove at least any of 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, and 100 wt%, greater than any of these, equal to any of these, or between any two of these.

[0053] After annealing, the shaped polymer composite can be subjected to conditions that induce an electric polarization in the lead-free piezoelectric additives (e.g., the plurality of particles) in the polymer composite. During the electric polarization, the piezoelectric particles can be connected to one another in a linear or semi-linear fashion (e.g., chains of particles). The columns of piezoelectric particles are suitably formed by stacking or aligning one or more chains. In one non-limiting example, the shaped polymer composite can be polarized. For example, the polymer composite can be polarized at room temperature (e.g., after the composite has cooled) with a selected electric field, or at a selected temperature with a selected electric field, at least one of the selected electric field and the selected temperature being selected based on a desired dipole orientation, a desired polarization strength, or a property of the article.

[0054] The temperature for performing the poling can be according to a desired dipole orientation and / or a desired poling strength, or according to a desired stress state of the final actuator. For example, poling can be performed at a selected cooling temperature range, by a selected heating temperature, or by a selected heating temperature and cooling temperature range. In some cases, poling can occur at a "range" of temperatures (e.g., a selected range) rather than a specific constant temperature. In some embodiments, poling can be performed at a temperature of at least any of 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, 115°C, and 120°C, greater than any of these, equal to any of these, or between any two of these. The applied voltage level parameter for poling can be selected in a variety of ways. For example, the applied voltage level parameter can be selected to be constant or varying (e.g., ramped) over a period of time. In some embodiments, poling is performed using a corona discharge using an electrode gap of 0.5 to 1.5 cm or about 1 cm for a desired amount of time (e.g., about 1 hour).

[0055] C. Piezoelectric Composites

[0056] A piezoelectric composite can include a polymer and a lead-free piezoelectric additive. The piezoelectric composite can include at least any of 1, 10, 20, 30, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, and 99 wt.%, greater than any of these, equal to any of these, or between any two of these, of a polymer matrix. The amount of lead-free piezoelectric additive present in the polymer matrix can be at least any of 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, and 65 vol.%, greater than any of these, equal to any of these, or between any two of these. In some embodiments, the piezoelectric composite includes PVDF-TRFE-CFE and 20 vol.% to 60 vol.% of barium titanate particles having an average particle size of 250 to 350 nm. In some embodiments, the piezoelectric composite includes, consists of, or consists essentially of PVDF-TRFE-CFE and 20 vol.% to 60 vol.% of barium titanate particles having an average particle size of 250 to 350 nm. In some embodiments, the piezoelectric composite can have less than 0.1 wt.% of solvent or 0 to 0.1 wt.% of solvent.

[0057] In some embodiments, the piezoelectric composite can have any shape or form. In some embodiments, the piezoelectric composite is a film or sheet. In some embodiments, the film or sheet has a thickness dimension of 50 to 200 micrometers, or at least any one of 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, and 200 micrometers, greater than any of them, equal to any of them, or between any two of them.

[0058] Properties of the piezoelectric composite include electrical properties and mechanical properties. Non-limiting examples of electrical properties can include piezoelectric constant, dielectric constant, etc. The piezoelectric composite can have a d 33 At least, equal to 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, and 70 pC / N, or between any of them. For example, the piezoelectric composite can have a dielectric constant less than any one of 120, 115, 110, 105, 100, 95, 90, 85, 80, 75, 70, 65, 60, 55, 50, 45, 40, and 35, equal to any of them, or between any two of them. In some embodiments, the dielectric constant is 90 to 210. The piezoelectric composite can have a storage modulus in the range of 100 to 325 MPa, or at least any one of 100, 125, 150, 175, 200, 225, 250, 275, 300, and 325 MPa, greater than any of them, equal to any of them, or between any two of them. The storage modulus can be measured according to ISO 6721 at room temperature and 0.2% strain at 1 Hz. The piezoelectric composite can have an elongation at break of 30 to 500% at room temperature (e.g., 25 to 35 °C) under uniaxial loading. The elongation at break can be measured using a standard dynamic mechanical analyzer such as a RDA III analyzer (TA Instruments, USA).

[0059] D. Device

[0060] A piezoelectric device can be included in a device. In preferred cases, the device is flexible. In some particular cases, the piezoelectric material of the present invention can be used in articles having a curved surface, a flexible surface, a deformable surface, etc. Non-limiting examples of such articles include piezoelectric sensors, piezoelectric transducers, piezoelectric actuators. These components can be used in haptics sensitive devices, electronic devices (e.g. smartphones, tablets, computers, etc.), virtual reality devices, augmented reality devices, fixed devices requiring flexibility such as adjustable mounted wireless earphones and / or earbuds, communication headsets with curvature, medical batches, flexible identification cards, flexible sporting goods, packaging materials, medical devices and / or applications where the presence of a bendable material simplifies the end product design, engineering and / or mass production.

[0061] EMBODIMENTS

[0062] The present invention will be described in greater detail by way of specific embodiments. The following Examples are offered for illustrative purposes, and are not intended to limit the present invention in any manner. One of ordinary skill in the art will readily recognize a variety of noncritical parameters that can be changed or modified to yield essentially the same results.

[0063] Examples 1-8

[0064] (Preparation of Lead-Free Piezoelectric Polymer Composites)

[0065] PVDF-TrFE-CFE resin powder (about 2 g) was dissolved in a solvent (16 mL) in a round bottom flask equipped with a condenser. The desired amount of barium titanate (BT) was then added slowly under stirring using a magnetic stirrer at 200-250 rpm. After stirring for 30 minutes, the mixture was cast into a thin film on a substrate using a doctor blade and then dried in open air. The drying time was adjusted according to the solvent used to prepare the composite. After drying, the thin film was peeled off from the glass plate and annealed under nitrogen. Table 2 provides the composition of the piezoelectric composites prepared following the procedure described above and the solvent used to prepare the composites.

[0066] Comparative Examples 1-4

[0067] (Preparation of Lead-Based Piezoelectric Polymer Composites)

[0068] Lead-based piezoelectric composites were prepared following the procedure of Examples 1-8 described above using PZT instead of BT. Table 2 provides the composition and the solvent used to prepare the piezoelectric composites.

[0069] Table 2

[0070]

[0071]

[0072] Example 9

[0073] (Piezoelectric response of piezoelectric polymer composites)

[0074] The piezoelectric composite films in Table 2 (size 3 cm x 3 cm) were subjected to corona poling to exhibit piezoelectric response. Corona poling of the piezoelectric composites (top surface exposed to air during film drying and / or bottom surface in contact with the substrate during film drying) was performed under the conditions detailed below. The needle was held at a high voltage (typically 10 KV). The poling temperature was 110 °C for the BT-based composites (Examples 1-8) and 115 °C for the PZT-based composites (Comparative Examples 1-4). The electrode gap was 1 cm and the poling time was 1 h. The samples were cooled to room temperature under the same applied voltage. The poled films were kept for 48 h before measuring the piezoelectric strain constant (d 33 ) of the poled films using a Berlin court type d33 meter (PM300, Piezo Test, UK) at ambient temperature with a frequency of 110 Hz, a clamping force of 10 N and an oscillation force of 0.25 N.

[0075] Example 10

[0076] (Mechanical properties of piezoelectric polymer composites)

[0077] The storage modulus (or elastic modulus) of each piezoelectric polymer composite was obtained by performing dynamic mechanical analysis of RDA III as a function of time. Tensile testing under uniaxial loading was performed at ambient temperature.

[0078] As shown in Table 2, four different types of solvents were used to prepare the piezoelectric polymer composites. Table 3 lists the relevant physical properties of the solvents. Fourier transform infrared (FT-IR) spectroscopy confirmed complete removal of the solvents from the composite films. The surface morphology and phase separation of the composite films prepared using different solvents were characterized by scanning electron microscopy (SEM), as shown in Figure 1. Figure 2 shows the surface profilometer plot of the top surface of the composite films over a sampling length of 5 mm. Table 4 lists the solvent-dependent piezoelectric properties (d 33 ) and average surface roughness (Ra) of the piezoelectric polymer composites. The composite films prepared using DMSO (Example 5) and NMP (Example 6) had smoother surfaces and showed higher d 33 .

[0079] It can also be seen in the SEM micrographs (Figure 1) that the barium titanate particles in the composite formed aggregates that were nearly spherical in shape. In Examples 3 and 4, filler aggregates were found (Figures 1(a) and (b)), while in Example 6, indentations were seen at the top surface (Figure 1(d)). In Example 5, spherical interconnected filler particles covered with a thin polymer layer were seen (Figure 1(c)). It is believed that solvent evaporation and high-density ceramic filler particle sedimentation occurred in parallel when the composite film was subjected to drying after the solution casting process. The solubility of the solvent in water can further affect the morphology. When the solvent has a low boiling point (<80 °C) and a low dielectric constant (<20), it evaporates before the filler particles sediment, thus most of the aggregates appear at the top surface. This results in a higher surface roughness and a lower d 33 On the other hand, when the solvent has a high boiling point (>80 °C) and a high dielectric constant (>20), the filler particles start to sediment before the solvent evaporates completely. As the filler particles move away from the top surface, indentations are seen, resulting in a smoother surface and a higher d 33 .

[0080] The particle size of the piezoelectric ceramic filler had a major influence on the piezoelectric strain constant in the lead-free piezoelectric composite, the results of which are summarized in Table 5. Barium titanate powder with an average particle size of 300 nm resulted in a significant improvement in d 33 . Tables 6A and B show the effect of annealing time on the piezoelectric strain constant of the lead-free piezoelectric composite prepared using solvents with a high boiling point (DMSO and NMP). The reduction in the piezoelectric strain constant was less than 10% after annealing at 110 °C for 72 h. Figure 3 The piezoelectric strain constant of the lead-free piezoelectric polymer composite and the lead-based piezoelectric polymer composite is shown as a function of filler loading. The bottom surface of the composite film, which was in contact with the substrate during the drying process, was exposed to corona poling. For both piezoelectric composites, d 33 increased significantly as the filler loading increased to 40 vol%, after which there was little effect. Figure 4 and 5 illustrate the effect of filler loading on the elongation at break and the storage modulus, respectively. From the results, it was determined that the elongation at break decreased significantly when the filler loading in both types of piezoelectric composite exceeded 40 vol%. From the storage modulus data of the lead-free piezoelectric polymer composite, it was observed that the storage modulus decreased as the filler loading increased. In contrast, the storage modulus of the lead-based piezoelectric polymer composite increased sharply initially and then decreased. The piezoelectric composite film remained mechanically flexible at high loadings, a representative example of which is shown in Figure 6 .

[0081] Thus, the present application provides the critical state of lead-free piezoelectric composites on standards based on particle attributes and / or solvent attributes that can be effectively translated to select specific polymer-piezoelectric filler composites for providing the best combination of mechanical properties and d 33 values. The specific particle size and / or solvent of the specific polymer-piezoelectric filler composites that provide the best combination of mechanical properties and d

[0082] Table 3

[0083]

[0084] Table 4

[0085]

[0086] Table 5

[0087]

[0088] Table 6A

[0089]

[0090]

[0091] Table 6B

[0092]

[0093] While embodiments of the application have been described and illustrated in detail, it should be understood that various changes, substitutions and alterations can be made by those skilled in the art without departing from the spirit and scope of the embodiments defined by the appended claims. Moreover, the scope of the application is not intended to be limited to the particular embodiments described in the specification. As one of ordinary skill in the art will readily understand from the disclosure above, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein can be utilized. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.

Claims

1. A method for producing lead-free piezoelectric composite materials, the method comprising: (a) Lead-free piezoelectric particles with an average particle size of 200 nm to 1000 nm are added to a solution containing a PVDF terpolymer and a solvent to form a dispersion or suspension, wherein the solvent has a boiling point ≥80 °C at 0.1 MPa and a solubility in water ≥0.1 g / g. (b) Forming a polymer matrix in which lead-free piezoelectric particles are dispersed; and (c) Electrodeification treatment is applied to the polymer matrix in which lead-free piezoelectric particles are dispersed to form a piezoelectric strain constant d. 33 A piezoelectric composite material with a strength of at least 40 pC / N and an elongation at break of 100 to 500%.

2. The method according to claim 1, wherein the lead-free piezoelectric particles comprise barium titanate particles, hydroxyapatite particles, apatite particles, lithium sulfate monohydrate particles, sodium potassium niobate particles, quartz particles, or combinations thereof.

3. The method according to claim 2, wherein the lead-free piezoelectric particles are barium titanate particles.

4. The method according to any one of claims 1 to 3, wherein the solvent is methyl ethyl ketone (MEK), dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), or a combination thereof.

5. The method according to claim 4, wherein the solvent is DMSO.

6. The method according to claim 1, wherein the volume percentage of piezoelectric particles in the composite material is 15 to 65 vol.%.

7. The method of claim 6, wherein the volume percentage of piezoelectric particles in the composite material is 20 to 60 vol.%.

8. The method according to claim 6, wherein the piezoelectric particles have a particle size of 250 to 350 nm.

9. The method according to claim 8, wherein the piezoelectric particles have a particle size of 300 nm.

10. The method of claim 1, wherein step (a) comprises dissolving the PVDF terpolymer in a solvent at a temperature of 15 to 100°C to produce a solution containing 5 to 20 wt. / vol% of the PVDF terpolymer.

11. The method of claim 1, wherein step (a) comprises dissolving the PVDF terpolymer in a solvent at a temperature of 15 to 100°C to produce a solution containing 10 wt. / vol% to 12 wt. / vol% of the PVDF terpolymer.

12. The method of claim 1, wherein forming the polymer matrix comprises: (i) Casting the dispersion onto a substrate to form a polymer matrix; (ii) Dry the polymer matrix at 25 to 45°C, and (iii) Anneal the dried polymer matrix at a temperature of 80 to 150°C for 1 to 50 hours.

13. The method of claim 1, wherein forming the polymer matrix comprises: (i) Casting the dispersion onto a substrate to form a polymer matrix; (ii) Dry the polymer matrix at 25 to 45°C, and (iii) Anneal the dried polymer matrix at 110°C for 5 to 25 hours.

14. The method of claim 1, wherein inducing polarization comprises applying a polarization field using corona discharge.

15. The method according to claim 1, wherein the PVDF terpolymer is poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (PVDF-TRFE-CFE).

16. A lead-free piezoelectric polymer composite material comprising a polyvinylidene fluoride (PVDF)-based polymer matrix and lead-free piezoelectric particles with an average particle size of 200 to 1000 nm dispersed in the polymer matrix, wherein the piezoelectric strain constant d of the piezoelectric composite material is... 33 It has a strength of at least 40 pC / N and an elongation at break of 30 to 500%.

17. The lead-free piezoelectric polymer composite material according to claim 16, wherein the PVDF-based polymer matrix is ​​poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (PVDF-TRFE-CFE), and the lead-free piezoelectric particles are barium titanate particles with an average particle size of 250 to 350 nm.

18. A piezoelectric device comprising any one of the lead-free piezoelectric polymer composite materials of claim 16 or 17.

19. The piezoelectric device according to claim 18, wherein the piezoelectric device is a piezoelectric sensor, a piezoelectric transducer, or a piezoelectric actuator.

20. The piezoelectric device according to claim 18 or 19, wherein the piezoelectric device is mechanically flexible.

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