A lead-free three-component piezoelectric polymer composite

Abstract

A moldable polymer composite material with piezoelectric properties for flexible and / or thin-film applications, wherein the polymer composite material comprises a polymer matrix and a piezoelectric ceramic filler embedded in the polymer matrix. The polymer matrix may comprise at least two polymers: a first polymer and a second polymer. The first polymer may be a fluorinated polymer, and the second polymer may be compatible with the first polymer and have a dielectric constant less than about 20. The piezoelectric ceramic filler may be a lead-free ceramic filler such as barium titanate, and may comprise about 40-70 vol% of the polymer composite material. The remaining 30-60 vol% may comprise the polymer matrix, which itself may comprise about 5-20 wt% of the second polymer and 80-95 wt% of the fluorinated polymer.

Description

[0001] Cross-referencing related applications

[0002] none. Technical Field

[0003] This invention relates primarily to piezoelectric materials. More specifically, this invention relates to polymer materials possessing piezoelectric properties. Background Technology

[0004] Piezoelectric materials are materials in which there is a definite relationship between the electric charge accumulated in the material and the mechanical stress applied to it. Some traditional piezoelectric materials are inorganic ceramics (such as piezoelectric ceramics), which are heavy and brittle. These inorganic ceramics require high-temperature processing exceeding 500°C. Other traditional piezoelectric materials are piezoelectric polymers, such as PVDF and PVDF-TrFE copolymers. Compared to piezoelectric ceramics, these piezoelectric polymers have a very low piezoelectric charge / strain constant d. The piezoelectric charge / strain constant d is an important parameter for evaluating the driving ability of piezoelectric materials under the action of an electric field, while the piezoelectric voltage constant g is used to measure the inductive ability of piezoelectric materials. Inorganic piezoelectric ceramics have high d(d 33 >150pC / N), but the high dielectric constant limits its g value (g 33 <50mV.m / N). In contrast, piezoelectric polymers have lower d(d 33 ≈13-28pC / N) and higher g(g 33 ≈200mV-m / N). The d 33 The value refers to the induced polarization per unit stress applied in direction 3 (parallel to the direction in which the ceramic element is polarized). The g... 33 The value refers to the induced electric field per unit stress applied in direction 3 (parallel to the direction in which the ceramic element is polarized). A high d value is required. 33 and g 33 Materials with a value of d. For materials identified as piezoelectric materials, d 33 The value should be greater than approximately 1 pC / N. Summary of the Invention

[0005] High-performance piezoelectric materials are ideal for sensors, actuators, and energy harvesters in healthcare and biomedical applications, as well as wearable electronic devices. These and other applications require piezoelectric materials with mechanical flexibility. Embodiments of this invention provide piezoelectric composite films made using lead-free piezoelectric ceramic fillers. The piezoelectric polymer composite material offers one or more of the following: mechanically flexible piezoelectric properties, film-forming ability, unsupported film and substrate-supported film, a simple method for preparing the piezoelectric polymer composite material, and / or a low-temperature processing technology.

[0006] An example of a polymer composite material includes a polymer matrix of at least two polymers and a piezoelectric ceramic filler embedded in the polymer matrix. The polymer matrix may include a fluorinated polymer and a second polymer with a dielectric constant less than about 20. The properties of the second polymer affect the dielectric constant of the matrix and, in turn, the dispersion of the filler in the matrix, which in turn affects the dielectric constant of the piezoelectric polymer composite material. In addition to having a dielectric constant less than 20, the second polymer is also compatible with the first polymer. The compatibility of the polymers can be determined based on the transition temperature (T0) of the polymer blend relative to the pure polymer. m or T g The compatibility is determined by the migration of the piezoelectric ceramic filler. When the migration exceeds 2°C, the polymer may be compatible. Examples of second polymers compatible with PVDF-TrFE-CFE (when PVDF-TrFE-CFE is a fluorinated polymer) in the polymer matrix include PVDF-TrFE (with a dielectric constant of approximately 8-10), PC (with a dielectric constant of approximately 3), and PPO (with a dielectric constant of approximately 2.5), all of which have dielectric constants less than 20. In some embodiments, the loading of the piezoelectric ceramic filler in the polymer matrix can be approximately 40-70 vol%. For PVDF-TrFE-CFE-based polymer matrices, loadings of the piezoelectric ceramic filler outside this range result in mechanical brittleness. In some embodiments, the loading of the second polymer in the polymer matrix can be 5-20 wt%. For PVDF-TrFE-CFE-based polymer matrices, loadings of the second polymer outside this weight range degrade the piezoelectric properties.

[0007] According to various aspects of the present invention, the preparation of piezoelectric polymer composites may include dissolving two polymers of a polymer matrix in a solvent to form a dual polymer solution, adding a piezoelectric ceramic filler to the dual polymer solution to form a suspension, and forming a polymer composite film by casting onto a substrate and drying the solvent. During processing, the piezoelectric polymer composite may be subjected to polarization treatment. In some embodiments, the composite film is annealed and / or corona-polarized. Furthermore, in some embodiments, melt processing techniques may be used to prepare the piezoelectric composite film; the piezoelectric composite film may be customized as (0-3) (random), (1-3) (structured) piezoelectric composites; and / or other in-situ and out-of-situ techniques may be used to prepare the piezoelectric composite. (0-3) piezoelectric polymer composites refer to the connectivity between the ceramic and polymer components. The first number corresponds to the connectivity in the ceramic component, and the second number corresponds to the connectivity in the polymer. The composite material exhibits (0-3) connectivity when the piezoelectric ceramic particles are surrounded by a three-dimensionally connected polymer phase. Compared to (0-3) piezoelectric composites, some (1-3) piezoelectric composites may have higher dp. 33 value.

[0008] According to various aspects of the present invention, polymer composite materials can be deposited as thin films on a substrate to form piezoelectric devices. In some embodiments, the polymer composite material is a mechanically flexible thin film formed on a flexible substrate as part of the fabrication of a flexible electronic device. Elongation at break (percentage) values ​​can be used to represent the mechanical flexibility of the material. In some embodiments, the elongation at break (percentage) of the piezoelectric composite film is greater than 25%. Such electronic devices may include piezoelectric sensors configured to generate analog signals proportional to the amount of deflection applied to the piezoelectric sensor by a user. The piezoelectric sensor may be integrated into a mobile device for receiving user input to control the mobile device. In one embodiment of a wearable medical device, mechanical vibrations of body organs, such as a heartbeat, can be converted into electrical signals and transmitted to a smartphone or other computing device via Bluetooth, Wi-Fi, or other signals. In another example of the use of piezoelectric materials, the material can be used as a converter to transform output signals, for example, configuring the piezoelectric material as a speaker or buzzer to convert the signal into audible sound. In another example of the use of piezoelectric materials, the material may be transparent and incorporated into an electronic display screen to form a touch screen device that can display information to a user and receive feedback from the user on the displayed information by tapping the screen.

[0009] According to some embodiments of the present invention, an example of a piezoelectric material comprises a three-component piezoelectric composite material containing polymer 1 / polymer 2 / piezoelectric filler, wherein polymer 1 is PVDF-TrFE-CFE, polymer 2 is a low-dielectric polymer (dielectric constant <20) compatible with PVDF-TrFE-CFE, and the piezoelectric filler is barium titanate. This material, as well as other materials of the present invention, can be prepared using solvents with a dielectric constant >20 and a boiling point ≥80°C, wherein the use of the solvent can provide high dielectric constant. 33 and g 33 Composite piezoelectric composite films. High-boiling-point and high-dielectric-point solvents facilitate filler dispersion in the matrix, thereby improving the piezoelectric properties of the film. In the composite, the loading of piezoelectric filler can be greater than 40 vol%. The desired high dg of the formed piezoelectric composite film... 33 and g 33 The value can be d 33 ≈30-70pC / N and g 33 ≈100-300mV-m / N. According to some embodiments of the invention, another example of a piezoelectric material includes a three-component piezoelectric composite material containing polymer 1 / polymer 2 / piezoelectric filler, wherein polymer 1 is PVDF-TrFE-CFE, polymer 2 is a low-dielectric polymer (dielectric constant <20) compatible with PVDF-TrFE-CFE, and the piezoelectric filler is lithium-doped potassium sodium niobate (KNLN).

[0010] According to embodiments of the invention, piezoelectric composite materials and devices incorporating such piezoelectric composite materials can provide cost benefits compared to commercial piezoelectric polymers. Furthermore, lead-free piezoelectric composite materials are solution-processable and less expensive than commercial piezoelectric polymers. Additionally, larger unsupported piezoelectric polymer films can be fabricated using the preparation methods described herein. Moreover, the piezoelectric composite film described herein can be made mechanically flexible because the method allows for the formation of a piezoelectric layer of the desired thickness at low temperatures of approximately 110°C or not exceeding 150°C. In some disclosed preparation techniques, piezoelectric composite materials can be deposited onto a substrate and polarized more effectively than through contact polarization. In some embodiments, depending on application requirements, the desired metal electrode pattern can be patterned on the polarized active layer.

[0011] Examples of piezoelectric fillers include any lead-free ceramic or single-crystal material. Non-limiting examples of piezoelectric materials include inorganic compounds of the perovskite series. Non-limiting examples of piezoelectric ceramics having a perovskite structure include barium titanate (BaTiO3), hydroxyapatite, apatite, lithium sulfate monohydrate, potassium sodium niobate, sodium bismuth titanate, quartz, organic materials (e.g., tartaric acid, polyvinylidene fluoride fiber), or combinations thereof. In a preferred embodiment, the piezoelectric additive is BaTiO3. The particle size of lead-free piezoelectric particles can be 200-1000nm or 250-350nm, or at least equal to or between any two of the following: 200nm, 225nm, 250nm, 275nm, 300nm, 325nm, 350nm, 375nm, 400nm, 425nm, 450nm, 475nm, 500nm, 525nm, 550nm, 575nm, 600nm, 625nm, 650nm, 675nm, 700nm, 725nm, 750nm, 775nm, 800nm, 825nm, 850nm, 875nm, 900nm, 925nm, 950nm, 975nm, and 1000nm.

[0012] Examples of polymer materials include thermosetting polymers, copolymers and / or monomers, thermoplastic polymers, copolymers and / or monomers, or thermosetting / thermoplastic polymers or polymer blends. Thermosetting polymers are plastic before heating and can be molded. The matrix can be made from a composition containing a thermoplastic polymer, and may also include other non-thermoplastic polymers, additives, etc., that can be added to the composition. Thermosetting polymer matrices are cured or crosslinked and tend to lose their ability to become flexible or plastic upon increasing temperature. Non-limiting examples of thermosetting polymers used to prepare polymer films include epoxy resins, epoxy vinyl esters, alkyd resins, amino polymers (e.g., polyurethane, urea-formaldehyde), diallyl phthalate, phenolic polymers, polyesters, unsaturated polyester resins, dicyclopentadiene, polyimides, silicone polymers, cyanate esters of polycyanate, thermosetting polyacrylic resins, phenolic plastics, thermosetting plastics, benzoxazoles, or copolymers or blends thereof.

[0013] Thermoplastic polymer matrices can become flexible or malleable above a certain temperature and can cure below a certain temperature. The polymer matrix of the composite material may include the thermoplastic or thermosetting polymers discussed in this application, their copolymers, and blends. Non-limiting examples of the thermoplastic polymers include polyvinylidene fluoride (PVDF), PVDF-based polymers, PVDF copolymers (poly(vinylidene fluoride-trifluoroethylene)(PVDF-TrFE)), PVDF trimers (poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene)(PVDF-TrFE-CFE), poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene)(PVDF-TrFE-CTFE)), odd-number nylon, cyanopolymers, polyethylene terephthalate (PET), polycarbonate (PC) series polymers, polybutylene terephthalate (PBT), poly(1,4-cyclohexylcyclohexane-1,4-dicarboxylate) (PCCD), and glycol-modified polycyclohexane terephthalate (…). Poly(phenylene oxide) (PCTG), polyphenylene oxide (PPO), polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC), polystyrene (PS), polymethyl methacrylate (PMMA), polyethyleneimine or polyetherimide (PEI) and its derivatives, thermoplastic elastomers (TPE), terephthalic acid (TPA) elastomers, poly(cyclohexanedimethyl terephthalate) (PCT), polyethylene naphthalate (PEN), polyamide (PA), sulfonated polysulfone (PSS), sulfonates of polysulfone, polyetheretherketone (PEEK), polyetherketone (PEKK), acrylonitrile-butadiene-styrene (ABS), acrylonitrile-styrene-acrylate (ASA) polymers, polyphenylene sulfide (PPS), and copolymers or blends thereof. Furthermore, other thermoplastic polymers known to those skilled in the art, as well as those subsequently developed, may also be used in this invention. The thermoplastic polymers may be included in compositions comprising the polymers and additives. Non-limiting examples of the additives include coupling agents, antioxidants, heat stabilizers, flow modifiers, colorants, and any combination thereof. In a preferred embodiment, a polyvinylidene fluoride (PVDF) polymer, its copolymers, or trimers are used. The trimer may be poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (PVDF-TrFE-CFE).

[0014] Examples of solvents used in the production of lead-free piezoelectric composites can be any solvent with a dielectric constant ≥20 and a boiling point ≥80°C. Non-limiting examples of the solvent include dimethylacetamide (DMAc), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), or combinations thereof. In some embodiments, the solvent is DMAc. The dielectric constant of the solvent can be at least, equal to, or greater than 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, or 80. The boiling point of the solvent can be at least, equal to, or higher 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, or 210°C. Various combinations of dielectric constant and boiling point are acceptable as long as both of the above criteria are met.

[0015] The terms “wt%”, “vol%”, or “mol%” refer to the weight, volume, or molar percentage of a component, based on the total weight, volume, or moles of the material comprising that component. In a non-limiting example, 10 moles of a component in 100 moles of material is considered 10 mol% of the component.

[0016] The term “basically” and its variations are defined as including ranges of 10%, 5%, 1%, or 0.5%.

[0017] The terms “suppress” or “reduce” or “prevent” or “avoid” or any variation thereof, when used in the claims and / or specification, include any measurable reduction or complete suppression to achieve the desired result.

[0018] The term "effective," when used in the claims and / or description, means sufficient to achieve the desired, anticipated, or intended result.

[0019] When used with the terms “comprising,” “including,” “containing,” or “having” in the claims or description, the indefinite article can mean “one,” but it also has the same meaning as “one or more,” “at least one,” and “one or more.”

[0020] The terms “including” (and any variations thereof), “having” (and any variations thereof), “comprising” (and any variations thereof), or “containing” (and any variations thereof) are inclusive or open-ended and do not exclude additional, uncited elements or method steps.

[0021] The method of the present invention may "comprise" specific ingredients, components, compositions, etc. disclosed throughout the specification, substantially consist of or be composed of them.

[0022] Other objects, features, and advantages of the present invention will become apparent from the following drawings, detailed description, and embodiments. However, it should be understood that while these drawings, detailed descriptions, and embodiments illustrate specific embodiments of the invention, they are illustrative only and not restrictive. Furthermore, it should be understood that variations and modifications within the spirit and scope of the invention will become apparent to those skilled in the art due to this detailed description. In further embodiments, features of a particular embodiment may be combined with features of other embodiments. For example, a feature of one embodiment may be combined with features of any other embodiment. In further embodiments, additional features may be added to the specific embodiments described herein. Attached Figure Description

[0023] For a more comprehensive understanding, please now refer to the following description in conjunction with the accompanying drawings, in which:

[0024] Figure 1 A method for preparing piezoelectric polymer composite materials according to some embodiments of the present invention is provided.

[0025] Figure 2 DSC thermograms of blends of PVDF-TrFE-CFE with various second polymer examples and pure PVDF-TrFE-CFE according to some embodiments of the present invention are provided.

[0026] Figure 3A -D provides a top view of a three-component piezoelectric polymer composite material containing 10 wt% PPO in a matrix according to some embodiments of the present invention. Figure 3A and Figure 3C ) and bottom ( Figure 3B and Figure 3D Scanning electron microscope images of the surface. Figure 3A The thin film in -B was prepared using MEK solvent. Figure 3C Thin films in -D were prepared using DMAc solvent.

[0027] Figure 4 The dielectric constant versus frequency of PVDF-TrFE-CFE / BT and a three-component piezoelectric polymer composite material according to some embodiments of the present invention are given. The three-component piezoelectric polymer composite material has a BT loading of 60 vol% and contains 10 wt% of a second polymer, namely PVDF, PVDF-TrFE, PC and PPO, in the matrix. The composite material is prepared using DMAc.

[0028] Figure 5The invention provides graphs showing the dielectric constant versus frequency for piezoelectric polymer composites with and without PPO in the matrix, for composites prepared using MEK, according to some embodiments of the invention.

[0029] Figure 6 The dielectric constant versus frequency of a three-component piezoelectric polymer composite material prepared using MEK and DMAc according to some embodiments of the present invention is shown. Detailed Implementation

[0030] A polymer composite material with piezoelectric properties, moldable for thin film, sheet, or coating applications, wherein the polymer composite material comprises a polymer matrix and a piezoelectric ceramic filler embedded in the polymer matrix. Although examples of thin films are described in some embodiments, the described material can also be used or is suitable for sheets or coatings. The polymer matrix may comprise at least two polymers: a first polymer and a second polymer. The first polymer may be a fluorinated polymer, and the second polymer may be compatible with the first polymer and have a dielectric constant of less than about 20. The piezoelectric ceramic filler may be a lead-free ceramic filler such as barium titanate, and approximately 40-70 vol% of the polymer composite material.

[0031] Figure 1A method for preparing piezoelectric polymer composites is provided. Method 100 begins at block 102, wherein two polymers are dissolved in a solvent to form a dual-polymer solution with a polymer concentration of at least about 12% relative to the solvent, for example, by dissolving 12 g of PVDF-TrFE-CFE resin in 100 mL of solvent. The polymer concentration in the solution used to prepare the piezoelectric composite can be 5-20% (wt / v), or more preferably 8-15% (wt / v), or even more preferably 10-13% (wt / v). Composites with polymer concentrations below 5% (wt / v) may form discontinuous films. Polymer concentrations exceeding 20% ​​(wt / v) may result in difficulty in achieving high filler loadings. In some embodiments, when the polymer concentration is below 5% (wt / v), the desired piezoelectric and other properties can be obtained through different preparation techniques. Then, at block 104, method 100 continues, optionally adding a piezoelectric ceramic filler to the dual-polymer solution to form a dispersion. In some embodiments, the polymer blend prepared, for example, as a piezoelectric material, does not include the piezoelectric ceramic filler. At block diagram 106, a polymer composite film can be formed by casting onto a substrate and drying the solvent. At block diagram 108, the cast piezoelectric composite film is polarized. Method 100 can be performed at a temperature not exceeding about 120°C, thus this preparation process can be used to prepare piezoelectric composites on flexible substrates. The temperature can be selected to be approximately the Curie temperature of the barium titanate piezoelectric ceramic filler and approximately the melting temperature of the PVDF-TrFE-CFE polymer matrix. In some embodiments, after casting the film onto the substrate, the piezoelectric composite is annealed in an inert atmosphere (such as nitrogen). Although referenced... Figure 1 The block diagram illustrates an embodiment of the invention, but it should be understood that the operation of the invention is not limited to this. Figure 1 The specific block diagrams and / or the specific block diagram order shown. Therefore, embodiments of the present invention can use different... Figure 1 The various block diagrams are arranged in sequence to provide the functionality described in this article.

[0032] As described below, several piezoelectric composite films were prepared and characterized based on a semi-crystalline trimer, namely poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene)—PVDF-TrFE-CFE—as the first polymer in the polymer matrix. Four different polymers were tested as the second polymer to prepare three-component piezoelectric composite compositions. Examples of the material composition of the piezoelectric composite films according to different embodiments of the present invention are shown in Table 1.

[0033] To collect the data in Table 1, the second polymer was dissolved in a solvent, and then PVDF-TrFE-CFE resin powder was added. The concentration of the solution was approximately 12 wt% / vol%. Then, using a magnetic stirrer at 200-250 rpm, the required amount of barium titanate (BT) was slowly added. After stirring for 30 minutes, the mixture was cast into a film on a substrate using a scraper and then dried in the open air. The drying time was adjusted according to the solvent used to prepare the composite material. After drying, the film was peeled off from a glass plate and annealed under nitrogen. Table 1 shows the composition of the polymer composite material prepared according to the above procedure and the solvent used to prepare the composite material.

[0034] Table 1. Compositions of piezoelectric polymer composites (e.g., films or coatings) using PVDF-TrFE-CFE as the first polymer in the polymer matrix.

[0035]

[0036] Table 2 describes various aspects of the second polymer used in the polymer matrix according to some embodiments of the present invention. In each sample, the dielectric constant of the second polymer is less than 20.

[0037] Table 2 Physical properties of the second polymer sample in the polymer matrix

[0038] Polymer 2 Dielectric constant Melting temperature (°C) Glass transition temperature (°C) PVDF 8-12 169.5 - PVDF-TrFE 9-12 151 - PC ～3 - 151 PPO 2-3 - 135

[0039] The compatibility of the second polymer with PVDF-TrFE-CFE was evaluated based on the melt temperature shift of the second polymer when blended with PVDF-TrFE-CFE compared to the melt temperature of the pure polymer. Figure 2 DSC thermograms of blends of PVDF-TrFE-CFE with various second polymer examples and pure PVDF-TrFE-CFE according to some embodiments of the present invention are given. Figure 2 As shown, in the blend, the melt transition temperatures of PVDF-TrFE and PVDF are 142 (ΔT) and 142 (ΔT) respectively. m ~9℃) and 168.5℃ (ΔT) m (~1℃). Unlike PVDF and PVDF-TrFE, PPO is a completely amorphous polymer, while PVDF-TrFE-CFE is semi-crystalline, and the melting peak in the DSC thermogram is generated only by PVDF-TrFE-CFE. Compared with the pure trimer, the melting point of PVDF-TrFE-CFE blended with PPO is broadened, and the enthalpy of melting decreases from 17 J / g to 11 J / g. According to these criteria, the compatibility of PVDF, PVDF-TrFE, and PPO with PVDF-TrFE-CFE is incompatible, partially compatible, and compatible, respectively.

[0040] Two different solvents, MEK and DMAc (with dielectric constants of 18.5 and 47, and boiling points of 80°C and 189°C, respectively), were selected to prepare piezoelectric polymer composite films. Figure 3A -D provides a top view of a three-component piezoelectric polymer composite material containing 10 wt% PPO in a matrix according to some embodiments of the present invention. Figure 3A and Figure 3C ) and bottom ( Figure 3B and Figure 3D Scanning electron micrographs of the surface of the composite material, showing that the matrix contains PPO as polymer 2, barium titanate loading is 40 vol%, and it is prepared using MEK and DMAc. Figure 3A The thin films in -B were prepared using MEK solvent, and Figure 3C The films in -D were prepared using DMAc solvent. SEM micrographs show that the barium titanate particles in the composite material form aggregates with nearly spherical shapes. In Example 7, due to the rapid evaporation of the low-boiling-point solvent MEK, the filler particles protrude from the top surface of the film, while on the bottom surface, the polymer forms an insulating layer on the filler particles. In Example 8, a depression appears on the top surface, formed by the settling of high-density filler particles before the high-boiling-point solvent evaporates. Interconnected spherical filler particles are visible on the bottom surface, forming a polymer network structure. Solvents with different dielectric constants and boiling points affect the dispersibility of the filler in the polymer composite material. Table 3 shows the piezoelectric strain constant d of different low-dielectric polymers on the piezoelectric polymer composite material. 33 piezoelectric voltage constant g 33 And the effect of dielectric constant.

[0041] Table 3. Effect of the second polymer in the polymer matrix on the piezoelectric and dielectric properties of piezoelectric composite compositions (such as films or coatings).

[0042] Example No. <![CDATA[d 33 (pC / N)]]> <![CDATA[ε r ]]> <![CDATA[g 33 (mV.m / N)]]> 1 49.2 57.2 97.1 2 41.9 47.1 100.4 3 42.2 22 216.7 4 2.7 - - 5 37.5 15.9 231.7 6 6.1 - - 7 19.3 69.6 31.4 8 24 14.7 184 PVDF-TrFE-CFE / BT(60) 50 55.1 102.5

[0043] A comparison was made between PVDF-TrFE-CFE / PVDF-based piezoelectric composites and composites using only PVDF-TrFE-CFE as the polymer matrix. The results showed that PVDF has a significant effect on the piezoelectric properties of d... 33 and ε r The impact is not significant (as shown in Table 3 and...) Figure 4 (As shown), this is because PVDF is incompatible with PVDF-TrFE-CFE. Figure 4The dielectric constant versus frequency of PVDF-TrFE-CFE / BT and a three-component piezoelectric polymer composite material according to some embodiments of the present invention are given. The three-component piezoelectric polymer composite material has a 60 vol% BT loading and contains 10 wt% of a second polymer, namely PVDF, PVDF-TrFE, PC, and PPO, in the matrix. The composite material is prepared using DMAc.

[0044] Comparison of films containing only PVDF-TrFE-CFE as the polymer matrix with other polymer matrices (PVDF-TrFE-CFE / PVDF-TrFE, PVDF-TrFE-CFE / PC, and PVDF-TrFE-CFE / PPO) shows that PVDF-TrFE, PC, and PPO as the second polymer lead to d 33 and ε r Reduce (e.g.) Figure 4 As shown). Figure 5 As shown, the piezoelectric polymer composite material with a 40 vol% barium titanate loading prepared using MEK as a solvent further supports this conclusion. Figure 5 According to some embodiments of the present invention, graphs showing the dielectric constant versus frequency of piezoelectric polymer composites with and without PPO in the matrix for composites prepared using MEK are provided. The effect of using PPO as the second polymer matrix may be significant compared to composites with PVDF-TrFE-CFE as the polymer matrix. 33 and ε r The reduction can be attributed to the compatibility between PVDF-TrFE-CFE and the second polymer with a dielectric constant of less than about 20.

[0045] Figure 6 The effects of two different solvents on the dielectric constant of a three-component piezoelectric composite material according to some embodiments of the present invention are presented. Comparing Examples 7 and 8 prepared using MEK and DMAc respectively, it is shown that a high-boiling-point solvent with a high dielectric constant can significantly reduce the dielectric constant of the composite material compared to a low-boiling-point solvent with a low dielectric constant. A lower dielectric constant in the polymer composite material increases g. 33PVDF-TrFE, PC, and PPO are just three examples of second polymers with a dielectric constant below 20 that are compatible with PVDF-TrFE-CFE. Other polymers are compatible with PVDF-TrFE-CFE and can be used in the polymer matrix of piezoelectric composite films to achieve the desired piezoelectric properties. Furthermore, polymers other than PVDF-TrFE-CFE can be used as the first polymer, along with a second polymer with a dielectric constant below 20 that is compatible with the first polymer. Moreover, although the ratio of the first polymer to the second polymer is 90% / 10% in the films of each embodiment, other ratios can be used to achieve different properties of the piezoelectric composite films.

[0046] When forming piezoelectric composites, the solvent used to bond the first and second polymers should have a dielectric constant greater than 20 and a boiling point ≥80°C to obtain the desired d 33 and g 33 The boiling point and desired mechanical properties are also considered. In some embodiments, the boiling point can be significantly higher than 1000°C, such as around 1800°C or higher, while having a dielectric constant greater than 20. Although the solvent mixing process for preparing examples of piezoelectric composite films is described here, other preparation processes such as standard melt mixing can also be used to give these same blends the desired piezoelectric properties.

[0047] In an example of a solvent mixing process for a lead-free three-component PVDF-TrFE-CFE-based piezoelectric composite, the second polymer is first dissolved in a solvent. As mentioned above, the second polymer should be selected to be compatible with PVDF-TrFE-CFE and have a dielectric constant less than 20. The solvent should be selected as described above, with a dielectric constant greater than 20 and a boiling point ≥80°C. Then, the first polymer, PVDF-TrFE-CFE, is added and stirred until completely dissolved. In the above embodiment, the solution concentration is approximately 12 wt% / vol%. Then, barium titanate (BT) is slowly added to the solution while stirring. After stirring for approximately 30-45 minutes to ensure substantially complete mixing, the mixture is cast into a film on a substrate using a scraper and then dried in the open air. After drying, the film is peeled off the substrate and annealed under nitrogen.

[0048] Piezoelectric composite materials can be prepared by solution casting or molding methods. A solution of polymer is obtained. The solution may include a solvent and a polymer. The solution may include at least, equal to, or between any two of 1.5 wt / vol%, 5 wt / vol%, 10 wt / vol%, 15 wt / vol%, and 20 wt / vol%. In some embodiments, the solution includes 10-12 wt% PVDF or a blend of PVDF-TRFE-CFE or both. In some embodiments, no compatibility modifier is used to prepare the lead-free polymer composite material of the present invention.

[0049] The piezoelectric additive can be dispersed or suspended in a polymer solution. The piezoelectric additive can be a plurality of (e.g., two or more, suitably five or more, ten or more, fifty or more, one hundred or more, five hundred or more, one thousand or more, etc.) 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 a matrix to achieve 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 producing the piezoelectric composite material of the present invention. The following description refers to dispersions, but it also applies to suspensions. In one embodiment, the dispersion comprises PVDF, PVDF-TRFE, PVDF-TRFE-CFE, or blends thereof, and barium titanate. In some embodiments, the dispersion may be molded or cast. Molding or casting may include mechanical or physical processes to transform the dispersion into the desired shape. Molding may also include simply placing the dispersion into a desired container or reservoir to maintain it in a certain shape or form. It should be noted that the molded shape is not necessarily the final shape, as additional processing (e.g., machining, molding, etc.) can be performed on the finally cured composite material. The dispersion molding process used in the methods described herein primarily aims to give the dispersion a certain initial structure before further processing. No rigidity or specific shape is required.

[0051] The casting process can involve pouring the dispersion onto a casting surface. Non-limiting examples of casting include air casting (e.g., passing the dispersion through a series of airflow channels over a specific set time period, such as 24-48 hours, to control solvent evaporation), solvent casting, or immersion casting, such as spreading the dispersion on a moving belt and passing it through a bath or liquid in which the liquid exchanges with the solvent. Spreading the dispersion onto the casting surface can be achieved using a scraper, a rolling spreader, or any flat extrusion die configuration.

[0052] During casting or molding, the solvent can be removed, thereby retaining the dispersion on the substrate or in the mold. Solvent removal can be assisted by heating. For example, the material can be heated to a temperature at least, equal to, or between any two of 50°C, 55°C, 60°C, 65°C, 70°C, 75°C, and 80°C. The resulting molded polymer composite material can be annealed at a temperature at least, equal to, or between any two of 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, 115°C, and 120°C for the required time period (e.g., 5, 10, 15, 20, 25 hours or any range or value between these). The molded 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 lead-free piezoelectric polymer precursor composites can remove solvent at least, equal to, or between any two of 50 wt%, 55 wt%, 60 wt%, 65 wt%, 70 wt%, 75 wt%, 80 wt%, 85 wt%, 90 wt%, 95 wt%, and 100 wt%.

[0053] After annealing, the molded polymer composite material can be placed under certain conditions to induce polarization of the lead-free piezoelectric additives (e.g., multiple particles) in the polymer composite material. During polarization, the piezoelectric particles can be linearly or semi-linearly interconnected (e.g., particle clusters). Piezoelectric particle columns can be suitably formed by stacking or arranging more than one particle cluster. In a non-limiting example, the molded polymer composite material can be polarized. For example, the polymer composite material can be polarized with a selected electric field at room temperature (e.g., after the composite material has cooled), or with a selected electric field at a selected temperature, at least one of which is selected based on the desired dipole orientation, the desired polarization intensity, or the characteristics of the article.

[0054] The polarization temperature can be selected based on the desired dipole orientation and / or desired polarization intensity, or based on the desired stress state of the finished actuator. For example, polarization can be performed within a selected cooling temperature range, by a selected heating temperature, or by a selected range of heating and cooling temperatures. In some examples, the polarization can be performed within a temperature “range” (e.g., a selected range) rather than a specific constant temperature. In some embodiments, the polarization can be performed at a temperature at least equal to or between any two of 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, 115°C, and 120°C. The applied voltage level parameter for polarization can be selected in various ways. For example, the applied voltage level parameter can be selected to be constant or varied over a period of time (e.g., gradually increasing). In some embodiments, polarization is performed using corona discharge with an electrode gap of 0.5–1.5 cm or about 1 cm for the required time (e.g., about 1 hour).

[0055] For electrical measurements involving the above samples, circular gold electrodes were deposited on both sides of the composite film. The dielectric constant and loss tangent were measured at room temperature in the frequency range of 100 Hz–1 MHz. Corona polarization was performed on the cast piezoelectric composite film to test the piezoelectric response. Corona polarization of the piezoelectric composite was performed under the following detailed conditions: the electrode was maintained at a high voltage (typically 10 kV), the polarization temperature was 110 °C, the electrode gap was 1 cm, and the polarization time was 1 hour. The polarized film was held at room temperature for 48 hours, and then the piezoelectric strain constant (dp) of the polarized film was measured at ambient temperature at a frequency of 110 Hz, a clamping force of 10 N, and an oscillating force of 0.25 N. 33 ), thus obtaining the values ​​shown in Table 3 above.

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

[0057] In some embodiments, the piezoelectric composite material can take on any shape or form.

[0058] In some embodiments, the piezoelectric composite material is a film or sheet. In some embodiments, the thickness of the film or sheet is 50-200 μm or at least, equal to, or between any two of 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, and 200 μm.

[0059] The properties of the piezoelectric composite material include electrical and mechanical properties. Non-limiting examples of the electrical properties may include piezoelectric constant, dielectric constant, etc. The d of the piezoelectric composite material... 33The dielectric constant is at least, equal to, or between any two of 40 pC / N, 45 pC / N, 50 pC / N, 55 pC / N, 56 pC / N, 57 pC / N, 58 pC / N, 59 pC / N, 60 pC / N, 61 pC / N, 62 pC / N, 63 pC / N, 64 pC / N, 65 pC / N, 66 pC / N, 67 pC / N, 68 pC / N, 69 pC / N, and 70 pC / N. For example, the dielectric constant of the piezoelectric composite material can be less than, equal to, or between any two of 120, 115, 110, 105, 100, 95, 90, 85, 80, 75, 70, 65, 60, 55, 50, 45, 40, and 35. In some embodiments, the dielectric constant is 90-210. The storage modulus of the lead-free piezoelectric composite material can be 100-325 MPa, or at least equal to or between any two of 100, 125, 150, 175, 200, 225, 250, 275, 300, and 325 MPa. The storage modulus can be measured according to ISO 6721 at room temperature and 0.2% strain at 1 Hz. Under uniaxial loading at room temperature (e.g., 25-35°C), the elongation at break of the lead-free piezoelectric composite material can be 30-500%. The elongation at break can be measured using a standard dynamic mechanical analyzer.

[0060] Although the embodiments and advantages of this application have been described in detail, it should be understood that various changes, substitutions, and modifications can be made herein without departing from the spirit and scope of the embodiments as defined by the appended claims. Furthermore, the scope of this application is not limited to the specific embodiments of the processes, machines, preparations, combinations of substances, means, methods, and steps described in the specification. As will readily be understood by those skilled in the art from the foregoing disclosure, processes, machines, preparations, combinations of substances, means, methods, or steps that are currently existing or will be developed thereafter can be utilized to perform substantially the same function or achieve substantially the same results as the corresponding embodiments described herein. Therefore, the appended claims are intended to include all such processes, machines, preparations, combinations of substances, means, methods, or steps within their scope.

Claims

1. A lead-free piezoelectric polymer composite material, comprising: A polymer matrix comprising at least two polymers: a first polymer comprising a fluorinated polymer, and a second polymer compatible with the first polymer and having a dielectric constant less than 20, wherein the polymer matrix comprises 5-20 wt% of the second polymer; and Lead-free piezoelectric ceramic filler embedded in a polymer matrix, wherein the piezoelectric ceramic filler accounts for 40-70 vol% of the polymer composite material.

2. The polymer composite material according to claim 1, wherein the first polymer comprises PVDF-TrFE-CFE.

3. The polymer composite material according to claim 1 or 2, further comprising a flexible substrate connected to the polymer composite material, wherein the polymer composite material is a mechanically flexible film.

4. The polymer composite material according to claim 3, wherein the thickness of the mechanically flexible film is 50-200 μm.

5. The polymer composite material according to claim 1 or 2, wherein the piezoelectric ceramic filler is barium titanate particles with a particle size of 250-350 nm.

6. The polymer composite material according to claim 1 or 2, wherein the polymer composite material has the following piezoelectric properties: a piezoelectric strain constant of 30-70 pC / N and a piezoelectric voltage constant of 100-300 mV·m / N.

7. A method for preparing a thin film using the polymer composite material according to any one of claims 1-6, wherein the method comprises: A first polymer is dissolved in a solution of a second polymer in a solvent to form a dipolymer solution, wherein the solvent has a dielectric constant of at least 20 and a boiling point of at least 80°C. Lead-free piezoelectric ceramic filler is added to the dual polymer solution to form a dispersion or suspension; Polymer composite films are formed by casting and solvent drying; and The polymer composite film is subjected to electrodeization treatment.

8. The method of claim 7, wherein the bipolymer solution formed by dissolving the first polymer in a solution of the second polymer comprises 5-20 wt / vol% of polymer.

9. The method of claim 8, wherein the bipolymer solution formed by dissolving the first polymer in a solution of the second polymer comprises 10-12 wt / vol% of the polymer.

10. The method according to any one of claims 7-9, further comprising annealing the polymer composite film in an inert atmosphere.

11. A piezoelectric sensor comprising a polymer composite film prepared according to any one of claims 7-10, wherein the piezoelectric sensor is configured to generate an analog signal proportional to an amount of deflection applied to the piezoelectric sensor by a user, wherein the piezoelectric sensor is integrated in a mobile device.

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