PIEZOELECTRIC POLYMER BLEND AND COMPOSITE COMPOSITIONS INCLUDING LITHIUM-DOPED POTASSIUM SODIUM NIOBATE

Abstract

A polymer composite exhibiting piezoelectric properties can be formed for flexible and/or thin film applications, in which the polymer composite includes a polymer matrix and a piezoelectric ceramic filler embedded in the polymer matrix. The polymer matrix may include 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 approximately 20. The piezoelectric ceramic filler can be lithium doped potassium sodium niobite (KNLN), and be approximately 40-70% by volume of the polymer composite. The remaining 30-60% by volume may be the polymer matrix, which may itself be approximately 5-20% by weight second polymer and 80-95% fluorinated polymer.

Claims

1. A polymer composite, comprising: a polymer matrix comprising at least two polymers: a first polymer comprising PVDF-TrFE-CFE, and a second polymer comprising an acrylic polymer of 5-15 wt % of the polymer matrix; and a piezoelectric ceramic filler embedded in the polymer matrix, the piezoelectric ceramic filler comprising lithium doped potassium sodium niobate (KNLN).

2. The polymer composite of claim 1, in which the piezoelectric ceramic filler comprises approximately 40-70% by volume of the polymer composite.

3. The polymer composite of claim 1, in which the second polymer is compatible with the first polymer.

4. The polymer composite of claim 1, in which the polymer matrix of the polymer composite comprises approximately ten percent by weight or more of the second polymer.

5. The polymer composite of claim 1, in which the second polymer has a dielectric constant of less than approximately 20.

6. The polymer composite of claim 1, further comprising a flexible substrate attached to the polymer composite, and in which the polymer composite is a mechanically-flexible thin film.

7. The polymeric composite of claim 1, wherein the polymer composite has a thickness between 50 and 200 microns.

8. The polymer composite of claim 1, in which the second polymer comprises at least one of poly(methyl methacrylate) (PMMA), poly(butyl acrylate) (PBA), poly(hydroxy ethyl methacrylate) (PHEMA), or Acrylonitrile-styrene-acrylate (ASA).

9. polymer composite of claim 1, in which the polymer composite is characterized by a piezoelectric strain constant of between approximately 30 and approximately 70 pC/N, and by a piezoelectric voltage constant between approximately 100 and approximately 300 mV-m/N.

10. A method of manufacturing a thin film using the polymer composite as claimed in claim 1.

11. The method of claim 10, in which the method comprises: dissolving the first polymer into a solution of the second polymer in a solvent to form a two-polymer solution, wherein the solvent is characterized by a dielectric constant of at least 20 and a boiling point of at least 80 degrees C.; adding the piezoelectric ceramic filler to the two-polymer solution to form a dispersion or suspension; forming the polymer composite thin film by casting and drying the solvent; and subjecting the polymer composite to an electric polarization.

12. The method of claim 11, in which dissolving the first polymer into the solution of the second polymer produces a two-polymer solution comprising 5 to 20 wt./vol % of polymer, preferably 10 wt./vol. % to 12 wt./vol. % of polymer.

13. The method of claim 12, further comprising annealing the polymer composite in an inert atmosphere.

14. A piezoelectric sensor comprising the polymer composite of claim 1, in which the piezoelectric sensor is configured to generate an analog signal proportional to an amount of deflection applied to the piezoelectric sensor by a user, in which the piezoelectric sensor is integrated in a mobile device.

15. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] For a more complete understanding, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

[0025] FIG. 1 shows a method of manufacturing a piezoelectric polymer composite according to some embodiments of the disclosure.

[0026] FIG. 2 shows DSC thermograms of piezoelectric polymer blends with a 90:10 weight ratio of PVDF-TrFE-CFE:Polymer 2 according to some embodiments of the disclosure.

[0027] FIG. 3 shows DSC thermograms of piezoelectric polymer blends with varying weight ratios of PVDF-TrFE-CFE:ASA according to some embodiments of the disclosure.

[0028] FIG. 4 shows graphs for stress-strain measurements of piezoelectric polymer blends having a 90:10 weight ratio of PVDF-TrFE-CFE:Polymer2 according to some embodiments of the disclosure.

DETAILED DESCRIPTION OF THE INVENTION

[0029] A method of manufacturing a piezoelectric polymer composite is shown in FIG. 1. The method 100 begins at block 102 with dissolving the two polymers in a solvent to form a two-polymer solution, which may form a two-polymer solution having a concentration of polymer to solvent of at least approximately 12%, such as Rained by means 12 g PVDF-TrFE-CFE resin dissolved in 100 mL solvent. The polymer concentration in the solution used for making piezocomposites may be 5-20% (wt/v), or more preferably 8-15%, or more preferably 10-13% (wt/v). A composite with less than 5% (wt/v) may result in a non-continuous film. Concentrations above 20% (wt/v) may result in difficulty creating high filler loading. In some embodiments, less than 5% (wt/v) may be possible with different manufacturing techniques to still obtain desirable piezoelectric and other properties. Then, at block 104, the method 100 continues with optionally adding piezoelectric ceramic filler to the two-polymer solution to form a dispersion. In some embodiments, such as a polymer blend manufactured as a piezoelectric material, no piezoelectric ceramic filler is added. At block 106, the polymer composite thin film may be formed by casting on a substrate and drying the solvent. At block 108, the cast piezocomposite film is subjected to an electric polarization. The method 100 may be performed without exceeding a temperature of about 120 degrees C., such that the manufacturing process can be used for the manufacturing of the piezoelectric composites on flexible substrates. The temperature may be selected as 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 on the substrate, the piezoelectric composite is annealed in an inert atmosphere (such as nitrogen gas). Although embodiments of the present invention, have been described with reference to blocks of FIG. 1, it should be appreciated that operation of the present invention is not limited to the particular blocks and/or the particular order of the blocks illustrated in FIG. 1. Accordingly, embodiments of the invention may provide functionality as described herein using various blocks in a sequence different than that of FIG. 1.

[0030] 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 microns, 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 microns.

[0031] Properties of the piezoelectric composite include electrical and mechanical properties. Non-limiting examples of electrical properties can include piezoelectric constant, dielectric constant, and the like. The d.sub.33 of the piezoelectric composite be at least, equal to, or between 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. By way of example, the piezoelectric composite can have a dielectric constant that is 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 from 90 to 210. The lead-free piezoelectric composite can have a storage modulus can range from 100 to 325 MPa, or at least, equal to, or between any two of 100, 125, 150, 175, 200, 225, 250, 275, 300, and 325 MPa. Storage modulus can be measured according to ISO 6721 at room temperature and a 1 Hz strain of 0.2%. The lead-free piezoelectric composite can have an elongation break of 30 to 500% under uniaxial loading at room temperature (e.g., 25 to 35° C.). Elongation break can be measured using standard dynamic mechanical analyzer.

[0032] A polymer composite exhibiting piezoelectric properties may be formed for film, sheet, or coating film applications, in which the polymer composite includes a polymer matrix and a piezoelectric ceramic filler embedded in the polymer matrix. Although film examples are described in certain embodiments, the materials described may be used or adapted for sheets or coatings. The polymer matrix may include 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 approximately 20. The piezoelectric ceramic filler can be lithium doped potassium sodium niobate (KNLN), and be approximately 40-70% by volume of the polymer composite. The remaining 30-60% by volume may be the polymer matrix, which may itself be approximately 2-10% by weight second polymer and 90-98% fluorinated polymer. The resulting piezoelectric properties of the piezoelectric composite may be a piezoelectric strain constant of between approximately 30 and approximately 70 pC/N; and a piezoelectric voltage constant between approximately 100 and approximately 300 mV-m/N. Example embodiments using KNLN ceramic filler, PVDF-TrFE-CFE as polymer 1, DMAc as solvent, and 40 volume percent KNLN loading are shown in Table 1. Also shown in some examples Table 1 are polymer blends with no KNLN particles.

TABLE-US-00001 TABLE 1 Piezoelectric composite and blend compositions (e.g., films or coatings) using PVDF-TrFE-CFE as the first polymer in the polymer matrix, some loaded with KNLN piezoelectric filler. Polymer KNLN Melting Young's Strain Example Polymer 2 weight loading temp(s) ΔH d.sub.33 modulus at break no. 2 (%) (vol %) (° C.) (J/g) (pC/N) (Mpa) (%) 1 ASA 10 40 23.5 36 74 2 PMMA 10 40 29 79 2 3 PPO 10 40 33.6 32 135 4 ASA 5 — 128 14 <1 5 ASA 10 — 106, 156 17, 25 14.3 62 128 6 ASA 20 — 128 12 <1 7 PMMA 10 — 118, 150 1.97, 10.7 6.9 202 98 8 PPO 10 — 121 12 <1

[0033] Examples 1-3 were prepared by dissolving Polymer 2 in DMAc. PVDF-TrFE-CFE was then added and stirred until it dissolved completely. The concentration of the solution was ˜12% (w/v). Subsequently, the desired amount of KNLN was added slowly to the solution, under stirring at 200-250 rpm using a magnetic stirrer. After stirring for 30 min, the mixture was casted into a thin film using doctor blade on to a glass plate, followed by drying in open air for 24 h. After drying, the films were peeled from the glass plate and annealed in a nitrogen environment. The compositions of the piezoelectric polymer composites prepared following the procedure mentioned above are provided in Table 1.

[0034] Examples 4-8 were prepared by dissolving Polymer 2 in DMAc. PVDF-TrFE-CFE was then added and stirred until it dissolved completely. The concentration of the solution was ˜12% (w/v). The solution was casted into a thin film using doctor blade onto a glass plate, followed by drying in open air for 24 h. After drying, the films were peeled from the glass plate and annealed in a nitrogen environment. The compositions of the piezoelectric polymer blends prepared following the procedure mentioned above are provided in Table 1.

[0035] The piezoelectric characteristics vary between the examples of Table 1. Comparison of Examples 5, 6, 7 and 8 shows that Example 5 and 7 demonstrate piezoelectricity upon poling, with the d.sub.33 of Example 5 being higher than that of Example 7.

[0036] FIG. 2 shows DSC thermograms of piezoelectric polymer blends with a 90:10 weight ratio of PVDF-TrFE-CFE:Polymer2 according to some embodiments of the disclosure. Neat PVDF-TrFE-CFE shows a single transition at 125° C., while two melting transitions are observed in Examples 5 and 7. There is a significant improvement of crystallinity at these compositions. On the other hand, the melting transition of PVDF-TrFE-CFE remains almost intact in the blend containing PPO (such as Example 8) while only broadening is observed.

[0037] FIG. 3 shows DSC thermograms of piezoelectric polymer blends with varying weight ratios of PVDF-TrFE-CFE:ASA according to some embodiments of the disclosure. FIG. 3 shows the effect of PVDF-TrFE-CFE/ASA blend ratio on the transition temperature. A shift in the melting transitions is evident at a 90/10 PVDF-TRFE-CFE/ASA blend, while in 95/5 and 80/20 blends there is approximately no change in the transition temperature.

[0038] FIG. 4 shows graphs for stress-strain measurements of piezoelectric polymer blends having a 90:10 weight ratio of PVDF-TrFE-CFE:Polymer2 according to some embodiments of the disclosure. The modulus of Example 7 is higher than Example 5.

[0039] The shift of melting transitions and improvement of crystallinity in PVDF-TrFE-CFE/acrylate polymer blends at 90/10 blend ratio indicates excellent compatibility between the two components of the blends. The induced crystallization increases piezoelectricity in the blend composition. The increased crystallinity and the high modulus of PMMA led to an improvement in modulus of PVDF-TrFE-CFE/PMMA blend. Comparison of Examples 1, 2, and 3 shows that the presence of acrylate polymer as third component lowers d.sub.33 slightly, however it leads to improvement in modulus, specifically with PMMA (Table 1), which is due to the induced crystallization in PVDF-TrFE-CFE by acrylate polymers.

[0040] Although embodiments of the present application and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the embodiments as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the above disclosure, 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 may be utilized. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.