DE2738220A1 - PIEZOELECTRIC ELEMENT AND METHOD OF ITS MANUFACTURING - Google Patents

Description

KRAUS & VVEISERT

DR. WALTER KRAUS DIPLOMA CHEMIST DR.-ING. ANNEKÄTE WEISERT DIPL.-ING. SPECIALIZATION IN CHEMISTRY IRMGARDSTRASSE 1S D-8OOO MÜNCHEN 71 TELEPHONE 089 / 797O77-7g 7O78 TELEX 05-212186 kpatd

TELEGRAM CRAUS PATENT

1612 WK / rm

MIKIN KOGYO COo, LTD. Osaka / Japan

Piezoelectric element and process for its manufacture

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1612 tfK / rm

\ 273822U

B e s c h r e i b u η g

The invention relates to a piezoelectric element made from a copolymer of vinylidene fluoride and trifluoroethylene. The invention also relates to Making this item.

When a film made of a polymeric material is cooled in a strong electric field at an elevated temperature, then an electret is formed, i.e., both surfaces of the film become a positive electrode and a negative electrode Electrode polarizes, and the resulting polarized film has piezoelectric properties. So this film To obtain as high a piezoelectricity as is satisfactory for practical purposes, it is necessary first strongly stretching the film in one direction and then subjecting the stretched film to polarization. One such amplification of the piezoelectricity by stretching is with a polymeric material with higher polarity, such as Polyvinylidene fluoride, polyvinyl fluoride or polyvinyl chloride, more pronounced. Indeed, films are made of these polymeric materials widely air; piezoelectric elements are used. These polymeric materials have Im generally excellent deformation properties and they have high flexibility so that they can be easily deformed into films which are freely deformable. Disadvantageously, however, have films made from these polymers Materials are produced, after stretching or Rekken an anisotropy.

In recent times attempts have been made to produce compounds containing dielectric ceramics in powder form and polymeric materials with high piezoelectric properties included, for the production of piezoelectric elements

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to use. With these masses, elements with high piezoelectricity can be obtained, but they are inferior to the polymeric materials themselves in terms of their deformation properties. Thus, thin films can hardly be obtained with it.

It has now been found that a copolymer of vinylidene fluoride and trifluoroethylene in a certain composition range is quite suitable for making a piezoelectric element, since the copolymer is light can be deformed and the resulting molded product even if it is not stretched exhibits a high piezoelectric constant. In particular, it should be noted that the piezoelectric constant of the copolymer is considerably higher than that of polyvinylidene fluoride, the latter material has heretofore been considered to be the material with the highest piezoelectric constant among various polymeric materials that are used in practice for the manufacture of piezoelectric elements.

The invention therefore relates to a piezoelectric element which is characterized in that it is a copolymer consisting of vinylidene fluoride and trifluoroethylene, in a molar ratio of from about 95: 5 to 10:90, preferably from about 90:10 to 35:65, the copolymer has been subjected to polarization.

The vinylidene fluoride / trifluoroethylene copolymers according to the invention can be polymerized with vinylidene fluoride Trifluoroethylene can be produced by processes known per se. Since the reactivity ratios of vinylidene fluoride and trifluoroethylene in the copolymerization for example 0.7 and 0.5 at ambient temperature

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and, therefore, since the ratio of these ratios is close to 1, these monomers can polymerize in one Molar ratio subjected to nearly the desired molar ratio of the monomer units in the one to be produced Copolymers is the same.

A typical example for the preparation of the vinylidene fluoride / trifluoroethylene copolymer is given below (the relationship between volume and weight corresponds to those between ml and g);

In a pressure-resistant reaction vessel with 200 parts by volume, which has been charged with 40 parts by weight of trifluorotrichloroethane and 0.16 parts by weight of di- (3,5,6-trichloroperfluorohexanoyl) peroxide and which is kept below 0 ⁰ C, 20.5 parts by weight of trifluoroethylene and 17.5 parts by weight of vinylidene fluoride introduced under pressure. The reaction vessel is immersed in a water tank at 20 ^° C., whereby the polymerization is initiated. The pressure in the reaction vessel is increased to a maximum of 20 atmospheres and then decreased to 7.5 atmospheres 12 hours after initiation of the polymerization. 29 parts by weight of a vinylidene fluoride / trifluoroethylene copolymer are obtained as a white lump from the reaction mixture. This is pulverized in water by means of a mixer and dried, whereby fine granules are obtained. From the material balance between the starting monomers and the copolymer produced, as well as from the elemental analysis, it is confirmed that the trifluoroethylene content in the copolymer is about 46 mol%. The intrinsic molar viscosity of the copolymer, measured in its dimethylformamide solution, is 1.8 (100 ml / g at 35 ° C.). Analysis by a differential scanning calorimeter (manufactured by Rigaku Denki Co., Ltd.) shows that the crystal melting point

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162 ° C and the initiation temperature of thermal decomposition is 338 ° C. The copolymer is easily soluble in acetone and dimethylformamide and it can be extracted from its solution be film cast in such solvents.

The vinylidene fluoride / trifluoroethylene copolymer of the present invention is a thermoplastic resin commonly known as translucent solid is present. It is soluble in many types of solvents, including polyvinylidene fluoride is soluble.

In the table below are the melting points, specific gravity and piezoelectric constants of Vinylidene fluoride / trifluoroethylene copolymers with different molar ratios of vinylidene fluoride and trifluoroethylene compared to the corresponding values of polyvinylidene fluoride and polytrifluoroethylene compiled

Molar ratio of
VdF / TrFE ^{1 year}

Melting point ( ⁰ C) powder film

specific weight

piezoelectric constant

^(d 31> λ (C? N) 3)

O (PVdf)

100/98/90/10 85/15 60/40
54/46
44/56
36/64
20/80 0/100 (PTrFE)

176.2

159.3 142.1 149.6 155.5 161.5 161.6 167.6 175.2 198.3

175.8 159.9 146 * 7 149.4 155.2 160.2 163.8 165.7 175.9 199.4

1.7665 1.7852 1.8206 1.8663 1.8947 1.8947 1.9233 1.9296 1.9435 1.9905

1.09 x 1.23 x 1.85 x 3.33 x 7.50 χ

6.33 x 6.83 x 2.43 χ 2.80 χ 4.64 χ

10 10 "10 10

, -13

-12

-12

10 "

10

10

10

10 "

-12

-13

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Footnotes: 1) The copolymers and the homopolymers were in in a similar manner as indicated above. VdF - vinylidene fluoride; TrFE = trifluoroethylene; PVdF = polyvinylidene fluoride (the melting points given in the literature are 162 to 182 ° C); PTrFE = polytrifluoroethylene.

2) The melting points were measured by a differential scanning calorimeter. The film was produced by compression, in that the powder was ^{deformed at 220 °} C. and a pressure of 100 kg / cm to form a film with a thickness of 0.3 mm and the film was uniaxially stretched to form a film with a thickness of 0.17 mm /. was stretched.

3) The piezoelectric constant was determined on a film prepared by compression molding the copolymer or the homopolymer at 270 ° C and under a pressure of 100 kg / cm and then polarizing it as in Example 1. The measurements were made with a piezoelectric constant ^{measuring device M} Piezotron-T ⁿ (manufactured by Toyo Seiki Co., Ltd.).

The vinylidene fluoride / trifluoroethylene copolymer of the present invention can be formed by conventional melt molding techniques (e.g. compression deformation, extrusion deformation, injection deformation) can be deformed into different shapes. Alternatively, it can be in a liquid medium at room temperature or elevated temperature are dissolved or dispersed, whereupon the resulting solution or dispersion on a Substrate material is applied to a film. According to a further alternative possible way, it can be applied to the surface

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of a substrate material as powder, thereby obtaining a coating layer.

To produce the piezoelectric element of the invention, the copolymer of vinylidene fluoride and trifluoroethylene is molded into a suitable shape and then subjected to polarization. For example, the copolymer can be pressed with heating (for example to about 280 ^° C.) to form a filia or a plate or a sheet. The film is contacted with electrodes. An electric direct current field is applied at a temperature above room temperature for a certain period of time (eg about 1 hour), after which the temperature is lowered to room temperature. The film or plate polarized in this way has a sufficiently high piezoelectricity that it can be used in practice as a piezoelectric element. It has no anisotropy. If desired, the film or the plate can be stretched or stretched beforehand and then subjected to polarization, as described above. In such a case, the resulting polarized product may show a higher piezoelectric constant. Instead of performing compression deformation as described above, the copolymer can also be dissolved in a suitable solvent such as acetone or dimethylformamide and formed into a thin film molding, which is then subjected to polarization, whereby a polarized film is obtained.

The DC electric field applied during polarization can be lower than the potential generated by the Molded product from the copolymer is tolerated. A higher electric field is better. For practical purposes it is appropriate to about 8 to 15 KV / mm, preferably about

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10 to 12 KV / mm, to be used. The temperature at the polarization can usually be 80 to 130 ° C.

The polarized molded product made of the vinylidene fluoride copolymer and trifluoroethylene is suitable as a piezoelectric element for electromechanical applications.

The copolymer of vinylidene fluoride and trifluoroethylene can furthermore with any other organic or inorganic piezoelectric materials (e.g. polyvinylidene fluoride) are mixed or mixed prior to deformation or polarization be combined.

The invention is illustrated in the examples. example 1

Powdered vinylidene fluoride / trifluoroethylene copolymers (Trifluoräthylengehalt 40 mol%) prepared by suspension polymerization is applied to a flat shape, which is brought into a heating press, which is held at 270 ⁰ C. After maintaining this temperature for 30 minutes, a pressure of 100 kg / cm is applied for 10 minutes. The mold is rapidly cooled with water and the molded product is taken out of the mold, whereby a film with a thickness of 0.3 is now obtained. The film is metallized with aluminum on both surfaces to which aluminum foils with a width of 1 mm are connected by means of an electroconductive paint Dotite (manufactured by Fujikura Chemical Co., Ltd.). The film is heated to 100 ° C. for 30 minutes while applying a direct current electric field of 10 KV / mm, and held at this temperature for 2 hours. It is then cooled to room temperature. Of the

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rf

film thus polarized has a piezoelectric constant Cd ^ ₁ ) of 7.5 × 10 C / N as measured by a piezoelectric constant measuring device "Piezotron-T" (manufactured by Toyo Seiki Co., Ltd.).

Example 2

The same copolymer as in Example 1 is dissolved in methyl ethyl ketone and the resulting solution is up applied to the surface of an aluminum plate to make a cast film 90 thick * The film is described in the same way as in Example 1, polarized. The polarized film shows a piezoelectric constant (d, ^) of 21.5 χ 10 C / N.

Example 3

The same copolymers as in Example 1 are compression-molded as in Example 1, whereby a film with a thickness of 0.3 mm is produced. The film is uniaxially stretched at a stretching ratio of about 2 at room temperature, thereby obtaining a film having a thickness of 0.17 mm. The stretched film is polarized as in Example 1. The polarized film shows a piezoelectric constant Cd ^ ₁ ) of 19.2 χ 10 " ¹² C / N.

Comparative example 1

Polyvinylidene fluoride is compression molded as in Example 1, but working at 220 ° C, whereby a film with a thickness of 0.3 mm is obtained. The film is polarized in the same manner as in Example 1. The polarized film shows a piezoelectric constant (d, ^) of 1.09 x 10 ~ ¹³ C / N.

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Claims (6)

PATENT LAWYERS DR. WALTER KRAUS DIPLOMA CHEMIST - DR.-ING. ANNEKÄTE WEISERT DIPL-ING. SPECIALIZATION CHEMISTRY IRMGARDSTRASSE 15 · D-8OOO MUNICH 71 · TELEPHONE 089 / 797077-79 70 78 ■ TELEX O5-212156 kpat d TELEGRAM CRAUS PATENT 1612 WK / rm P a t e η t a η s ρ r ü c h e \ J Piezoelectric element, characterized in that it consists of a copolymer Vinylidene fluoride and trifluoroethylene in a molar ratio of about 95: 5 to 10:90, the copolymer having been subjected to polarization. 2. Piezoelectric element according to claim 1, characterized in that the copolymer of vinylidene fluoride and trifluoroethylene in a molar ratio of about 90:10 to 35:65. 3. Piezoelectric element according to claim 1, characterized in that the polarization is in in such a way that a direct current electric field is applied to the copolymer at a temperature of about 80% up to 130 ° C and then cools down. 4. Piezoelectric element according to claim 3, characterized in that the electric direct current field has about 8 to 15 KV / mm. 5. A method for producing a piezoelectric element, characterized in that one 809810/0780 a copolymer consisting of vinylidene fluoride and trifluoroethylene in a Hol ratio of about 95: 5 to 10:90, polarized by applying an electric direct current field to this at a temperature of about 80 to 130 ⁰ C and then cooling. 6. The method according to claim 5, characterized in that the copolymer is in a shaped form is present. 809810/0780