JPH0117274B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a polymer piezoelectric material having high performance thickness piezoelectricity. Conventional planar zigzag structure (I
polyvinylidene fluoride (PVDF), which has a film with a type (called a type or β type) that is poled under a high electric field.
is the most well-studied. PVDF molded products obtained by melting at normal pressure or casting from a solution are composed of crystals with a TGTG′ conformation called the mold or α type, but since this is non-piezoelectric, it is usually A piezoelectric film is obtained by converting a molded product consisting of a mold crystal into a mold by uniaxial stretching or roll rolling, and then poling the molded product. However, manufacturing a piezoelectric film that involves such stretching is accompanied by many restrictions. That is, for example, it is difficult to obtain extremely thin films. Furthermore, when combining it with other elements, such as semiconductor elements, it is necessary to bond it after stretching because it cannot be coated on the element. Furthermore, converting PVDF to piezoelectricity usually requires a high electric field of several 100 KV/cm or more, which poses various difficulties, including problems such as dielectric breakdown. The present inventors have already shown that a polymer piezoelectric film is capable of emitting and receiving ultrasonic waves and can be effectively used as an ultrasonic transducer material with low acoustic impedance. Thickness piezoelectricity is used for this purpose, and the related piezoelectric constant is
e ₃₃ , d ₃₃ or the electromechanical coupling coefficient Kt are important. The maximum Kt of PVDF is 0.2, and in this respect, a material with a larger Kt has been desired. Recently, a copolymer P (VDF-TrFE) of vinylidene fluoride (VDF) and trifluoroethylene (TrFE),
In particular, it is known that a copolymer containing 55 mol% of VDF has a relatively large transverse piezoelectric effect d ₃₁ [JP-A-55-127086, JP-A-55-127087, JP-A-Sho 55- Publication No. 127085 or Higashihata et al.
Ferroelectics Vo1.32, P/85-92 (1981)]. _d31
and d ₃₃ , e ₃₃ , and Kt are independent parameters.
We found that the dependence of d ₃₁ and Kt on VDF composition is completely different, that the thickness piezoelectric effect is large when VDF is 65 mol% or more, and that a large Kt can be obtained by heat treatment at a temperature near or above the crystal phase transition point Tm'. (Japanese Unexamined Patent Publication No. 111281/1981). However, P(VDF-TrFE) has the following problems. In other words, as shown in JP-A-56-111281, there was a problem in that when the composition of VDF exceeded 75 mol %, the achievable Kt decreased rapidly. Also, according to experience,
Regarding the piezoelectricity of P(VDF-TrEE), even if they have the same composition ratio, the Kt values of the poled films vary widely, making it difficult to stably manufacture piezoelectric films with uniform performance. It was hot. The aim of the invention is to overcome these difficulties. Furthermore, another object of the present invention is to obtain a non-stretched film with a large Kt using a poling electric field Ep as small as possible (it has traditionally been desirable to apply a poling electric field as high as possible in the production of polymer piezoelectric films). As is known, high electric fields tend to cause dielectric breakdown, so it is desirable to be able to pole with as small an Ep as possible). The present invention for achieving the above object consists of the following configuration. That is, a molded product obtained by removing the solvent from a solution of a copolymer mainly composed of 65 to 95 mol% of vinylidene fluoride and 35 to 5 mol% of trifluoroethylene is heated to a temperature below the melting point (Tm), approximately 1. A method for producing a polymer piezoelectric material, characterized in that poling is performed after heat treatment at a temperature higher than the crystal phase transition point (Tm'), or poling is performed simultaneously with the heat treatment. In the present invention, the thickness and piezoelectricity of P(VDF-TrFE) are
Not only the composition ratio of VDF and TrFE, but also the manufacturing process, thermal history, and P(VDF-
It is based on the discovery that P(VDF-TrFE) is significantly influenced by the microstructure (morphology) of solids. point
After heat treatment at a temperature of Tm′ or higher and lower than the melting point (Tm), poling is performed, or the molded product is roughly
This is a method for producing a polymer piezoelectric material, characterized by poling at a temperature of Tm' or higher and Tm or lower. Here, Tm' appears as an endothermic peak in the temperature-rising DSC curve of the molded product, and is usually several degrees to several tens of degrees lower than Tm. Although it is not fully clear what kind of phase transition this Tm' corresponds to, it is thought to be a ferroelectric-paraelectric phase transition associated with rotation around the molecular axis parallel to the molecular chain. Next, the present invention will be explained in more detail based on examples and drawings. Example 1 VDF:TrFE=79:21 mol% P (VDF/
A solution of 15 parts by weight of TrFE) dissolved in 100 parts by weight of dimethylformamide (DMF) was cast on a glass plate and dried under reduced pressure at room temperature for 24 hours.
Further, after evaporating DMF at 100°C for 3 hours, heat treatment was performed at 140°C for 1 hour under constant length to obtain a film with a thickness of 75 μm. After depositing Al electrodes on both sides, poling was performed at 100° C. under an electric field of 300 KV/cm for 1 hour. The electroacoustic coupling constant and other constants related to thickness piezoelectricity were measured according to the method described in the following literature, and the following results were obtained. Literature: H.Ohigashi, J.Appl.Phys. 47 , 979
(1976) Kt=0.252 ε′ ₃ =6.3 (at 10MHz) ν ₃ =2.35m/s ε″ ₃ =1.0 C ^D ₃₃ =10.5×10 ⁹ N/m ² tan δ=0.16 Q _M =20 *(Note ) ν ₃ : Sound velocity in the thickness direction, C ^D ₃₃ : Elastic modulus in the thickness direction, Q _M : Mechanical Q value, ε′ ₃ , ε″ ₃ : Real and imaginary parts of the complex permittivity, respectively On the other hand, the quasi-static method The transverse piezoelectric constant d ₃₁ of this membrane measured at (110 Hz) is 8.2×10 ⁻¹² C/N. The value of Kt shown above is larger than the maximum value Kt=0.2 obtained with conventional PVDF, and is extremely excellent as an oscillation/reception element for ultrasonic waves, for example.
Note that d ₃₁ is 8.18×10 ^-12 C/N, and d ₃₁ of PVDF
smaller than In other words, d ₃₁ and Kt are not proportional.
Based on current knowledge, it is not possible to infer Kt from the value of _d31 . In addition, when this sample before heat treatment was polled at 130℃ for 1 hour under an electric field of 250KV/cm
Kt=0.215. However, when polling was performed at 120°C for 1 hour, Kt was less than 0.02. [Comparative Example] P (VDF-
TrFE) (79/21) at 260℃ by hot pressing method.
This was quenched in water at 40°C to obtain an unstretched film of 75 μm. After heat treatment was performed under the same conditions as in Example 1, poling was performed by applying an electric field of 467 KV/cm at 100° C. for 1 hour. The performance of the obtained piezoelectric film was as follows. Kt=0.160 ν ₃ =2242m/sec C ^D ₃₃ =9.4×10 ⁹ N/m ² Q _M =18.5 As shown in Example 1, the film formed by casting from a solution is Despite the polling, Kt is large, and the speed of sound, modulus of elasticity,
The mechanical Q value (Q _M ) is also increasing. This indicates that the film obtained from solution according to the present invention has advanced crystallization and a higher degree of orientation of the polar axis in the direction of the electric field at low electric fields. Example 2, 3 Comparative Example 2, 3 P (VDF/TrFE) prepared in the same manner as Example 1
(79/21 mol % ratio) DMF solution cast film (thickness approximately 75 μm) at 140°C (Example 2), 130°C (Example 3), 120°C (Comparative Example 2) and 110°C (Comparative Example 3). ) for 1 hour in a free end state, and then poling was performed at 100° C. for 1 hour.
Table 1 shows the properties of the piezoelectric film obtained. here
Ep is the poling voltage, ν ₃ and C ^D ₃₃ are the sound velocity and elastic modulus in the thickness direction (frequency is approximately 15 MHz) measured by the piezoelectric free resonance method, respectively, and |E|, d ₃₁ are 110 M
These are the Young's modulus in the direction parallel to the membrane and the transverse piezoelectric constant determined in Hz.

[Table] As is clear from Table 1, all samples give similar magnitudes of d ₃₁ by poling, but
Kt strongly depends on the heat treatment temperature Th. Kt is small when heat treated at temperatures below 120℃, but increases rapidly with heat treatment at higher temperatures, and when Th = 140℃, Ep is small.
Nevertheless, it can be seen that a large Kt can be obtained. Note that when this sample was melted and then recrystallized, the peak of the endothermic curve at the phase transition point was 136°C. Example 4 P (VDF-
A solution of 10 parts by weight of TrFE) dissolved in 100 parts by weight of DMF was cast on a glass plate and heated at room temperature for 6 hours, at 60°C for 6 hours, and at 80°C for 6 hours.
The solvent was removed under reduced pressure to obtain a membrane with a thickness of approximately 30 μm.
After depositing Al electrodes on both sides, heat treatment was performed at 140°C for 1 hour, and poling was performed at 100°C for 1 hour under an electric field of 275 KV/cm. The binding coefficient at room temperature was Kt=0.196. Further, ν ₃ =2433 m/s, and d ₃₁ =d ₃₂ =7.5×10 ⁻¹² C/N. In addition, this heat-treated film was prepared at room temperature with Ep=
When polling was performed at 450KV/cm for 1 hour, Kt=0.225 was obtained. The Tm' of this composition P (VDF-TrFE) after melt recrystallization was 123°C. Comparative Example 4 Same P(VDF-TrFE) as used in Example 1
(79/21 mol%) was melted between aluminum plates at 270°C using a hot press, and allowed to cool naturally at room temperature to obtain a film with a thickness of 61 μm. This is heat treated at 140℃ (1
time), and the Kt of the membrane poled by applying an electric field of 741 KV/cm at 100°C was at most 0.241.
The speed of sound ν ₃ was 2318 m/s. In this way, P obtained by melt film formation with natural cooling
(VDF-TrFE) (79/21) requires extremely high poling electric fields to obtain high Kt. The effects of this manufacturing method are as follows. (1) Another method for producing an unstretched piezoelectric film is a melt film forming method. However, in the method of the present invention, the composition ratio,
If the polling temperature and polling time are the same,
Larger Kt can be achieved with lower poling electric field. (2) A large Kt can be obtained even with a VDF component of 75 mol% or more. (3) The film can be formed into the required thickness and shape by applying a solution, spraying, etc., so it is possible to make the film thinner, and it is also possible to form the film on an electrode or IC substrate that has been created in the required shape in advance. Therefore, it is possible to create piezoelectric elements with extremely complex shapes. Furthermore, compared to a stretched film, the film is oriented in substantially the same direction within the film plane, so the film is less prone to cracking and is mechanically stable. (4) Films with consistent Kt values can be manufactured.
The effects of the present invention are as described above, and the reasons for the effects of sections (1) and (2) are considered to be as follows. That is, (i) by forming a film from a solution,
A film is formed that has many amorphous parts, has a disordered crystal structure, or is made of crystals with small grain size. (ii) By heat-treating this at a temperature of approximately Tm' or higher, crystals with increased crystal diameter and fewer defects are grown, and the degree of crystallinity is improved. The film obtained in this way has a smaller electric field (coercive electric field) required to orient the crystal polar axis in the direction of the electric field (ferroelectric polarization orientation), and an increase in crystallinity due to the electric field (domain growth). ) is generated, and it is estimated that large piezoelectricity can be obtained. These can be supported by the following facts. The figure is a DSC curve of the film of Example 2 before heat treatment. The exothermic peak at 127°C was due to crystallization, indicating that this sample contained a large amount of amorphous parts (this crystallization is thought to be induced by molecular rotation, which causes phase transition). The endothermic peak at 122°C is thought to be due to the contribution of the part distributed on the low temperature side of this phase transition). The X-ray diffraction pattern of this sample shows that the 110 and 200 diffraction lines of the crystal are at a diffraction angle of 2θ.
The half width 2Δθ and the diffraction intensity I are shown in Table 2 for the unheated film A, the 140° C. heat-treated film, and the poled film C, respectively.

[Table] From Table 2, it is considered that the heat treatment causes an increase in crystallinity, grain size, or decrease in crystal disorder, and that this further progresses due to poling, possibly through the growth of ferroelectric phase domains. As shown in the examples, the fact that those with a large Kt have a large sound velocity and elastic modulus also supports this. The present invention is effective for P(VDF-TrFE) containing 65 mol% or more and 95 mol% or less of VDF, but 70 mol%
% or more and 82 mol% or less is particularly effective. Solvents used in the present invention include DMF, dimethyl sulfoxide, acetone, methyl ethyl ketone, etc. as long as they can dissolve P(VDF-TrFE).
Can be used generally. Although piezoelectricity has been described in the present invention, the film obtained by the present invention also has a pyroelectric effect.
According to our experience, if the materials are the same, the higher the Kt, the greater the pyroelectric effect. Therefore, it is natural that the film obtained by the present invention can also be used in fields that utilize pyroelectricity. As mentioned above, the preferred unstretched film obtained in the present invention already has a B-type crystal orientation, but if thinning and processability are not so required, the unstretched film is stretched and then subjected to heat treatment. , of course, you can poll. In this case, the stretching further advances the molecular orientation, and the subsequent heat treatment yields a highly selectively oriented film with good crystallinity, making it possible to obtain a piezoelectric material with a lower poling voltage and a larger K _t .

[Brief explanation of drawings]

The figure shows a DSC curve of a film before heat treatment obtained by the method of Example 2 according to the present invention.

Claims (1)

[Claims] 1. A molded product obtained by removing the solvent from a solution of a copolymer mainly composed of 65 to 95 mol% of vinylidene fluoride and 35 to 5 mol% of trifluoroethylene is heated to a temperature below the melting point (Tm) and almost crystalline. Either polling is performed after heat treatment at a temperature above the phase transition point (Tm′), or
Alternatively, a method for manufacturing a polymer piezoelectric material, characterized in that poling is performed simultaneously with the heat treatment.