GB1589746A

GB1589746A - Piezoelectric element and a process for the production thereof

Info

Publication number: GB1589746A
Application number: GB35302/77A
Authority: GB
Original assignee: Daikin Industries Ltd
Current assignee: Daikin Industries Ltd
Priority date: 1976-08-25
Filing date: 1977-08-23
Publication date: 1981-05-20
Also published as: JPS5326995A; DE2738220A1; FR2363202A1; FR2363202B1

Description

(54) PIEZOELECTRIC ELEMENT AND A PROCESS FOR THE PRODUCTION THEREOF (71) We, DAIKIN KOGYO CO. LTD., a Japanese Body Corporate, of No. 8, Umeda, Kita-ku, Osaka-shi, Osaka-fu, Japan, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a piezoelectric element made of copolymer of vinylidene fluoride and trifluoroethylene, and to a process for the production thereof.

When a film of a polymeric material is cooled from an elevated temperature in a strong electric field, an electret is formed, i.e. both surfaces of the film being polarized respectively to a positive electrode and a negative electrode, and the resulting polarized film possesses piezoelectric properties. In order to provide this film with a high piezoelectricity such as is satisfactory for practical use, it is necessary first to stretch the film strongly in one direction and then subject the stretched film to polarization. This enhancement of the piezoelectricity by stretching is most marked in a polymeric material of high polarity such as polyvinylidene fluoride, polyvinyl fluoride or polyvinyl chloride. In fact, films made of these polymeric materials are widely used as piezoelectric elements.

These polymeric materials generally possess excellent molding properties and have a high flexibility, and therefore they can be readily shaped into films, which are freely deformable.

Disadvantageously however, the films made from those polymeric materials produce anisotropy on stretching.

Attempts have recently been made to use compositions comprising dielectric ceramics in powder form and polymeric materials possessing good piezoelectric properties for the preparation of piezoelectric elements. While these compositions provide elements having a high piezoelectricity, they are inferior to the polymeric materials themselves in their molding properties. Thus, it is difficult to obtain thin films from them.

We have now found that a copolymer of vinylidene fluoride and trifluoroethylene, within a certain range of proportions, is very suitable for the preparation of a piezoelectric element since the copolymer can be readily molded and the resulting molded product has a high piezoelectric constant even without stretching.

According to the present invention, there is provided a piezoelectric element comprising a copolymer consisting of vinylidene fluoride and trifluoroethylene in a molar ratio of from 54:46 to 10:90, preferably from 54:46 to 35:65, the said copolymer having been subjected to polarization.

It should be particularly noted that the piezoelectric constant of the copolymer is much higher than that of polyvinylidene fluoride, which was previously thought to have the highest piezoelectric constant of the various polymeric materials used for the preparation of piezoelectric elements.

The vinylidene fluoride-trifluoroethylene copolymer of the present invention may be produced by polymerization of vinylidene fluoride and trifluoroethylene according to a per se conventional procedure. Since the reactivity ratios of vinylidene fluoride and trifluoroethylene in the copolymerization, for example, at an ambient temperature are respectively 0.7 and 0.5 and therefore the proportion of these ratios is close to 1, these monomers may be subjected to polymerization in a molar ratio nearly equal to the desired molar ratio of the monomeric units in the copolymer which is to be produced.

A typical example of a process for production of the vinylidene fluoride-trifluoroethylene copolymer is as follows (the relationship between volume and weight corresponding to that between milliliter and gram): 20.5 parts by weight of trifluoroethylene and 17.5 parts by weight of vinylidene fluoride were charged under pressure into a 20() parts by volume pressure resistant reaction vessel, charged with 4() parts by weight of trifluorotrichloroethane and 0.16 part by weight of di-(3,5,6-trichloroperfluorohexanoyl) peroxide and kept below 0 C.

The reaction vessel was immersed in a water tank at 2() C, to initiate the polymerization.

The pressure in the reaction vessel was raised to 20 kg/cm2G at the highest and then lowered to 7.5 kg/cm2U, 12 hours after the initiation. 29 parts by weight of a vinylidene fluoridc-trifluorocthylene copolymer was recovered from the reaction mixture as a white clump, which was pulverized in water using a mixer and dried to give fine granules. By the material balance between the starting monomers and the copolymer produced, and also elementary analysis. the trifluoroethylene content in the copolymer was found to be about 46 molar . The intrinsic viscosity of the copolymer when measured as its dimethylformamide solution was 1.8 (1()() ml/g at 350C). The analysis by a differential scanning calorimeter (manufactured by Rigaku Denki Co., Ltd.) revealed that the crystal melting point was 1620C and the initiating temperature of thermal decomposition was 338 C. The copolymer was readily soluble in acetone and dimethylformamide and could be film-cast from its solution in such solvents.

The vinylidene fluoridc-trifluoroethvlene copolymer of the invention is a thermoplastic resin which is usuallv a translucent solid and which is soluble in many kinds of solvents other than those in which polyvinylidene fluoride is soluble.

The following tahle shows the melting points, the specific gravities and the piezoelectric constants of vinylidene fluoride-trifluoroethylene copolymers having different molar ratios of vinylidene fluoride and trifluoroethylene. compared to those of polyvinylidene fluoride and polytrifluoroethylene.

Molar ratio of Specific Piezoelectric VdF/TrFE Melting point (oC)2) gravity constant (d3l) Powder Film (C/N)4) ()()/ () (PVdf) 176.2 175.X 1.7665 1.09 x ()8/ 2 159.3 159.9 1.7852 1.23 x t)()/ 10 142.1 146.7 1.8206 1.85 x 10-12 85/ 15 149.6 14').4 1.8663 3.33 x 10-12 6()/ 40 155.5 155.2 1.8947 7.50 x 10-12 54/ 46 161.5 16().2 1.8947 6.33 x 10-12 44/ 56 161.6 163.8 233 6.83 x 10-12 36/ 64 167.6 165.7 1.9296 2.43 x 10-12 20/ 8() 175.2 175.') 1.y435 2.8() x 10-13 ()/1()() (PTrFE) 19X 3 199.4 1.9905 4.64 x 10-14 Notes: 1) The copolymers and the homopolymers were prepared in a similar manner to the procedure as hereinabove exemplified. VdF, vinylidene fluoride; TrFE, trifluoroethylene,; PVdF, polyvinylidene fluoride (the melting points described in known literatures being 1(2 - IX2 C); PTrFE. polytrifluoroethylene.

2) The mclting points were measured bv a diffcrential scanning calorimeter. The film was prepared by compression molding the powder at 220"C under a pressure of 100 kg/cm2 to make a sheet ().3 mm thick and stretching the sheet uniaxially to make a film 0.17 mm thick.

3) The piezoelectric constant was determined on the sheet prepared by compression molding the copolymer or the homopolymer at 27() C under a pressure of 100 kg/cm2, followed by polarization as in Example 1. Measurement was made by the use of a piezoelectric constant mcasuring apparatus "Piezotron-T" (manufactured by Toyo Seiki Co., Ltd.). (Piczotron is a Registered Trade Mark).

The vinylidene fluoride-trifluoroetl1ylcnc copolymer of the invention can be shaped into various forms by conventional melt molding techniques (e.g. compression molding, extrusion molding and injection molding). Alternatively, they may be dissolved or dispersed in a liquid medium at room temperature or an elevated temperature, followed by application of the resulting solution or dispersion onto a substrate material to form a film.

As another alternative procedure, it may be powder-coated onto the surface of a substrate material to form a coating layer.

For the preparation of the piezoelectric element of the present invention, the copolymer of vinylidene fluoride and trifluoroethylene is shaped into an appropriate form and then subjected to polarization. For example, the copolymer may be pressed under heating (e.g.

at about 280"C) to form a sheet. The sheet is then contacted with electrodes, and a D.C.

electric field is applied thereto at a temperature higher than room temperature for a certain period of time (e.g. about one hour), followed by lowering the temperature to room temperature. The polarized sheet has a sufficiently high piezoelectric property that it can be used as a piezoelectric element and it does not have any anistropy. When desired, the sheet may be previously stretched and then subjected to polarization as above. In this case, the resultant polarized product may have a higher piezoelectric constant. Instead of being compression molding as above, the copolymer may be dissolved in a suitable solvent, such as acetone or dimethylformamide and cast molded to form a cast film which is then subjected to polarization to give a polarized film.

The D.C. electric field to be applied is desirably from about 8 to 15 KV/mm, preferably from about 10 to 12 KV/mm. The temperature of the polarization is preferably in the range of from 80 to 1300C.

The polarized shaped product of the copolymer of vinylidene fluoride and trifluoroethylene may be used as a piezoelectric element in the electro-mechanical field.

The copolymer of vinylidene fluoride and trifluoroethylene may be admixed or combined with any other organic or inorganic piezoelectric material (e.g. polyvinylidene fluoride) prior to the shaping or the polarization.

WHAT WE CLAIM IS: 1. A piezoelectric element comprising a copolymer consisting of vinylidene fluoride and trifluoroethylene in a molar ratio of from 54:46 to 10:90, the said copolymer having been subjected to polarization.

2. A piezoelectric element as claimed in claim 1 wherein the copolymer consists of vinylidene fluoride and trifluoroethylene in a molar ratio of from 54:46 to 35:65.

3. A piezoelectric element as claimed in claim 1 or claim 2 wherein the polarization has been effected by applying a D.C. electric field to the copolymer at a temperature in the range of from 80 to 1300C, followed by cooling.

4. A piezoelectric element as claimed in claim 3 wherein the D.C. electric field applied to the copolymer is from 8 to 15 KV/mm.

5. A piezoelectric element as claimed in claim 4 wherein the D.C. electric field applied to the copolymer is from 10 to 12 KV/mm.

6. A piezoelectric element as claimed in any one of the preceding claims wherein the copolymer of vinylidene fluoride and trifluoroethylene is combined with another organic or inorganic piezoelectric material.

7. A piezoelectric element as claimed in claim 1 substantially as hereinbefore described with reference to any one of the specific Examples.

8. A process for preparing a piezoelectric element, which comprises polarizing a copolymer consisting of vinylidene fluoride and trifluoroethylene in a molar ratio of from about 54:46 to 10:90.

9. A process as claimed in claim 8 wherein the polarization of the copolymer is effected by applying thereto a D.C. electric field at a temperature of from 80 to 1300C, followed by cooling.

10. A process as claimed in claim 9 wherein the D.C. electric field applied to the copolymer is from 8 to 15 KV/mm.

11. A process as claimed in claim 10 wherein the D.C. electric field applied to the copolymer is from 10 to 12 KV/mm.

12. A process as claimed in any one of the claims 8 to 11 wherein the polymer is in shaped form.

13. A process as claimed in claim 8 substantially as hereinbefore described with reference to any one of the specific Examples.

14. A piezoelectric element whenever prepared by a process as claimed in any one of claims 8 to 13.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. various forms by conventional melt molding techniques (e.g. compression molding, extrusion molding and injection molding). Alternatively, they may be dissolved or dispersed in a liquid medium at room temperature or an elevated temperature, followed by application of the resulting solution or dispersion onto a substrate material to form a film. As another alternative procedure, it may be powder-coated onto the surface of a substrate material to form a coating layer. For the preparation of the piezoelectric element of the present invention, the copolymer of vinylidene fluoride and trifluoroethylene is shaped into an appropriate form and then subjected to polarization. For example, the copolymer may be pressed under heating (e.g. at about 280"C) to form a sheet. The sheet is then contacted with electrodes, and a D.C. electric field is applied thereto at a temperature higher than room temperature for a certain period of time (e.g. about one hour), followed by lowering the temperature to room temperature. The polarized sheet has a sufficiently high piezoelectric property that it can be used as a piezoelectric element and it does not have any anistropy. When desired, the sheet may be previously stretched and then subjected to polarization as above. In this case, the resultant polarized product may have a higher piezoelectric constant. Instead of being compression molding as above, the copolymer may be dissolved in a suitable solvent, such as acetone or dimethylformamide and cast molded to form a cast film which is then subjected to polarization to give a polarized film. The D.C. electric field to be applied is desirably from about 8 to 15 KV/mm, preferably from about 10 to 12 KV/mm. The temperature of the polarization is preferably in the range of from 80 to 1300C. The polarized shaped product of the copolymer of vinylidene fluoride and trifluoroethylene may be used as a piezoelectric element in the electro-mechanical field. The copolymer of vinylidene fluoride and trifluoroethylene may be admixed or combined with any other organic or inorganic piezoelectric material (e.g. polyvinylidene fluoride) prior to the shaping or the polarization. WHAT WE CLAIM IS:

1. A piezoelectric element comprising a copolymer consisting of vinylidene fluoride and trifluoroethylene in a molar ratio of from 54:46 to 10:90, the said copolymer having been subjected to polarization.