PIEZOELECTRIC COMPOSITE MATERIALS HAVING IMPROVED PIEZOELECTRIC PROPERTIES

Abstract

A piezoelectric composite material is based on a polymeric matrix and on piezoelectric inorganic fillers, characterized in that the material additionally comprises at least one ionic liquid of general formula Q+ A−, in which Q+ represents a cation chosen from quaternary ammonium cations and quaternary phosphonium cations and A− represents any anion capable of forming a liquid salt at a temperature of less than 100° C. A device comprising at least one layer based at least one piezoelectric composite material defined above and at least two electrodes positioned on either side of the layer and a tire comprising at least one piezoelectric device defined above are also set forth.

Claims

1.-15. (canceled)

16. A piezoelectric composite material based on a polymeric matrix and on piezoelectric inorganic fillers, wherein the piezoelectric composite material additionally comprises at least one ionic liquid of general formula Q.sup.+A.sup.−, in which Q.sup.+ represents a cation selected from quaternary ammonium cations and quaternary phosphonium cations and A.sup.− represents any anion capable of forming a liquid salt at a temperature of less than 100° C.

17. The piezoelectric composite material according to claim 16, wherein Q.sup.+ is a quaternary ammonium cation selected from the group consisting of imidazolium cations, pyrazolium cations, pyridinium cations, pyrimidinium cations, tetra(C.sub.1-C.sub.6)alkylammonium cations, guanidium cations and pyrrolidium cations.

18. The piezoelectric composite material according to claim 16, wherein Q.sup.+ is an imidazolium cation corresponding to the formula (I): ##STR00004## in which R.sub.1 represents an alkyl group comprising from 1 to 15 carbon atoms, optionally substituted by one or more C.sub.6-C.sub.30 aryl, thiol or hydroxy groups or interrupted by one or more oxygen or sulfur atoms or by one or more NR′ groups, and R.sub.2, R.sub.3, R.sub.4, R.sub.5 and R′, which are identical or different, each represent a hydrogen atom or an alkyl group comprising from 1 to 6 carbon atoms, or a C.sub.6-C.sub.30 aryl group, optionally substituted by one or more C.sub.1-C.sub.4 alkyl groups.

19. The piezoelectric composite material according to claim 18, wherein Q.sup.+ is selected from the group consisting of 1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1-hexyl-m-methylimidazolium, 1-octyl-3-methylimidazolium, 1-decyl-3-methylimidazolium, 1,3-dibutylimidazolium and 1-butyl-2,3-dimethylimidazolium.

20. The piezoelectric composite material according to claim 16, wherein A.sup.− is selected from the group consisting of acetate, trifluoroacetate, propionate, chloride, hydroxide, sulfate, trifluoromethanesulfonate, pentafluoromethanesulfonate and phosphates.

21. The piezoelectric composite material according to claim 16, wherein a content of the at least one ionic liquid is greater than or equal to 0.1 part by weight per hundred parts by weight of polymer.

22. The piezoelectric composite material according to claim 16, wherein the piezoelectric inorganic fillers are selected from piezoelectric ceramics.

23. The piezoelectric composite material according to claim 16, wherein the piezoelectric inorganic fillers are selected from the group consisting of barium titanate, lead titanate, lead zirconate titanate, lead niobate, lithium niobate, potassium niobate and mixtures thereof.

24. The piezoelectric composite material according to claim 16, wherein a content of the piezoelectric inorganic fillers is within a range extending from 5% to 80% by volume, with respect to a total volume of the piezoelectric composite material.

25. The piezoelectric composite material according to claim 16, wherein the polymeric matrix comprises at least one polymer selected from the group consisting of thermoplastic polymers, thermoplastic elastomers, thermosetting polymers, diene elastomers and mixtures thereof.

26. The piezoelectric composite material according to claim 16, wherein the polymeric matrix comprises at least one thermoplastic elastomer and/or one diene elastomer.

27. The piezoelectric composite material according to claim 16, wherein the polymeric matrix comprises at least one diene elastomer selected from the group consisting of natural rubber, ethylene/propylene/diene monomer copolymers, synthetic polyisoprenes, polybutadienes, butadiene copolymers, isoprene copolymers and mixtures thereof.

28. The piezoelectric composite material according to claim 26, further comprising at least one crosslinking system.

29. A device comprising at least one layer based on at least one piezoelectric composite material according to claim 16 and on at least two electrodes positioned on either side of the at least one layer.

30. A tire comprising at least one piezoelectric device according to claim 29.

Description

EXAMPLES

[0190] The aim of the examples presented below is to compare the piezoelectric properties of the piezoelectric composite materials M1, M2 and M3 in accordance with the invention, in comparison with a piezoelectric composite material C not in accordance.

[0191] Unless otherwise mentioned, the contents of the various constituents of the piezoelectric composite materials presented in Table 1 are expressed as parts by weight per 100 parts by weight of polymer. All the composite materials exhibit a content of piezoelectric inorganic fillers of 33% by volume, with respect to the total volume of the composite material.

TABLE-US-00001 TABLE 1 Material C M1 M2 M3 Polymeric matrix (1) 100.00 100.00 100.00 100.00 Piezoelectric inorganic filler (2) 320.10 322.80 326.30 334.60 Ionic liquid (3) (—) 1.00 2.00 5.00 Crosslinking system (4)  0.75 0.75 0.75 0.75 (1) Polymeric matrix: Styrene-butadiene copolymer (SBR), solution-polymerized (S-SBR), non-functional, non-extended. Its microstructure is as follows: 24 mol % of 1,2-polybutadiene units, with respect to the butadiene part, and 26.5% by weight of styrene units, with respect to the total weight of the copolymer. It has a Tg = −48° C. The glass transition temperature, Tg, is measured in a known way by DSC (Differential Scanning Calorimetry) according to Standard ASTM D3418 of 1999. The microstructure of the S-SBR (relative distribution of 1,2-vinyl, trans-1,4- and cis-1,4-butadiene units) and the quantitative determination of the content by weight of styrene in the S-SBR are determined by near-infrared (NIR) spectroscopy. The principle of the method is based on the Beer-Lambert law generalized for a multicomponent system. As the method is indirect, it involves a multivariate calibration [Vilmin, F., Dussap, C. and Coste, N., Applied Spectroscopy, 2006, 60, 619-29] carried out using standard elastomers having a composition determined by .sup.13C NMR. The styrene content and the microstructure are then calculated from the NIR spectrum of an elastomer film approximately 730 μm in thickness. The spectrum is acquired in transmission mode between 4000 and 6200 cm.sup.−4 with a resolution of 2 cm.sup.−1 using a Bruker Tensor 3 7 Fourier-transform near-infrared spectrometer equipped with an InGaAs detector cooled by the Peltier effect. (2) Piezoelectric inorganic filler: BaTiO.sub.3: average diameter of the fillers: 500 nm, density 5.85 g/cm.sup.3, sold by Inframat Advanced Materials. (3) Ionic liquid: ENIM Ac (1-ethyl-3-methylimidazolium acetate), sold by Sigma-Aldrich. Melting point = −20° C., measured as described in the description. (4) Crosslinking system: Luperox 231 (1,1-di(t-butylperoxy)-3,3,5-trimethylcyclohexane supported at 40% by weight on calcium carbonate), sold by Arkema. Process for the preparation of the piezoelectric composite material

[0192] The piezoelectric composite materials are prepared in an 85 cm.sup.3 Polylab internal mixer, filled to 70%, the initial vessel temperature of which is 80° C., the polymeric matrix, the piezoelectric inorganic fillers and the ionic liquid for the composite materials M1, M2 and M3. Thermomechanical working is then carried out at 80 revolutions/min for 3 min until a maximum dropping temperature of 150° C. is reached (non-productive phase). The mixture thus obtained is recovered, it is cooled and then the crosslinking system is added on an external mixer (homofinisher) at a temperature of 25° C., the whole being mixed in 12 cross-passes (productive phase). The materials thus obtained are subsequently calendered in the form of plaques (thickness of 2 to 3 mm) and cured using a press at 150° C. for 20 minutes in a 330 cm.sup.2 mould under 8 tonnes.

[0193] On conclusion of this operation, it is entirely possible to cut out the laminates with a hollow punch or any other cutting means in order to produce a piezoelectric device with its two electrodes having the shape and the size desired.

[0194] The device is subsequently prepared. More specifically, parallelepipeds of 20 mm×80 mm×2 mm (width×length×thickness) (subsequently also referred to as test specimens) are subsequently cutout from the plaques obtained above. To facilitate the polarization and to make possible the measurements, the test specimens are metallized on the two faces having the greatest dimensions. The metallization, in this instance with gold, can be carried out manually with a lacquer or by cathode sputtering or any other known method. In the case in point, a metallizer (Denton Desk V from Denton Vacuum) is used to deposit the two gold electrodes with a current intensity of 40 mA for 25 seconds.

[0195] The device is subsequently placed in a bath of silicone oil (Bluesil FLD 47V5000 sold by Bluestar Silicones) for the polarization stage. An MCP Lab Electronics SPN6000A electrical generator is used to apply an electric field for 10 minutes to both terminals of the test specimen (i.e., connected to the two metallized faces). The polarization is carried out at a temperature of 60° C. Two intensities of direct electric fields are used: 1 V/μm (condition A) and 4 V/μm (condition B). Once polarized, the test specimens are short-circuited to evacuate a maximum of residual charges.

Measurement of the Piezoelectric Coefficient d.SUB.33.:

[0196] The piezoelectric coefficient d.sub.33 makes it possible to determine the deformation capacity of a composite material, this deformation taking place parallel to the axis of polarization.

[0197] The measurement of the electromechanical response of the test specimens is carried out on a dynamic measurement bench. The sample is prestressed with a force of 10 N and then it is subjected to compressive stress with a force of 5 N at a frequency of 1 Hz and at a temperature of 23° C.

[0198] The signal generated by the piezoelectric composite material is recovered at the terminals of the sample by a specific jaws platform, then amplified and measured on an oscilloscope.

[0199] The charge Q (pC picocoulomb) released at each mechanical stressing is deduced from the peak-to-peak voltage read on the oscilloscope. Thus the piezoelectric coefficient d.sub.33 (pC/N (picocoulomb/newton)) can be calculated. The coefficient d.sub.33, known to a person skilled in the art, represents the piezoelectric coefficient measured by application of a stress in the direction parallel to the direction of polarization of the sample. In the case of a parallelepipedal sample, the direction of polarization corresponds to the smallest thickness (direction 3) and the stress is applied along the same thickness (direction 3).

[0200] The following notation can be adopted:

d.sub.33=ΔP3/Δσ3,

with ΔP3 the macroscopic polarization variation in the direction 3 and Δσ3 the stress applied in the direction 3.

[0201] This coefficient is calculated by the following formula:

d33=[Q(pC)×Thickness (m)]/[Force (N)×Length (m)]

in the case where the electrode covers the entire surface of the test specimen.

Results

[0202] Table 2 presents the results of measurement of piezoelectric coefficient d.sub.33 for the three piezoelectric composite materials of the invention in comparison with the control piezoelectric composite material, measured after a polarization according to the condition A or the condition B.

TABLE-US-00002 TABLE 2 Piezoelectric coefficient d.sub.33 C M1 M2 M3 Condition A: 1 V/μm 0.1 0.3 2.0 2.8 Condition B: 4 V/μm 0.7 2.0 6.5 n.m n.m = not measured

[0203] The results of Table 2 show that, for a given polarization condition, for example the condition A, the piezoelectric coefficient d.sub.33 of the composite materials M1, M2 and M3 according to the invention is very significantly improved in comparison with that of the composite material C outside the invention. This coefficient increases by at least a factor of 300% (comparison of the composite material M1 according to the invention with respect to the composite material C outside the invention). The ionic liquid thus makes it possible, surprisingly, to improve the piezoelectric properties of a composite material to which it is added, after polarization under the same conditions of temperature, of time and of electric field intensity as a composite material not comprising ionic liquid.

[0204] The results presented in Table 2 also show that, in order to obtain given piezoelectric properties for a composite material according to the invention (for example a piezoelectric coefficient d.sub.33=0.2), it is possible either to modulate the electric force of the electric field (composite material M1, condition B) or to modulate the amount of ionic liquid added (composite material M2, condition A). The piezoelectric properties of the composite materials of the invention can thus be modulated easily and simply, which represents another advantage of the present invention.