JPS61102791A - Manufacture of macromolecular piezoelectric body, excellent in thermal stability - Google Patents

Abstract

PURPOSE:To improve piezoelectric property, thermal stability and film forming property, by fusing a copolymer of vinylidene fluoride and tetrafluoroethylene, whose composition of vinylidene fluoride is 72-87mol% at a melting point or higher, slowly cooling the copolymer, crystallizing the copolymer, and performing polling. CONSTITUTION:For example, dimethyl formamide solution of a copolymer of vinylidene fluoride and tetrafluoroethylene, whose composition of vinylidene fluoride is 81mol%, is made to flow and extended on a glass plate. The solvent is vaporizedat a normal temperature under the pressure reduced state. The melting point of the film is Tm=124 deg.C. The film undergoes heat treatment for one hour at 141 deg.C without separating the film from the glass plate. The film is slowly cooled (2 deg.C/min). Aluminum is evaporated on both surfaces of the film. The film is used as an electrode. A triangular voltage is applied, and an inverted current is measured. During this period, polling is performed. The piezoelectric film obtained in this way has an electro-mechanical coupling constant of Kt=0.207 in the direction of the thickness and the melting point of Tm=143 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Industrial Application Field The present invention relates to a method for producing a polymeric piezoelectric material endowed with piezoelectricity and thermal stability.

(b) Conventional technology The copolymer of vinylidene fluoride and tetrafluoroethylene is known to have pressure centrality (for example, Kobunshi Gakkai Shishu, Vol. 28, No. 9, p. 1794, 1979
Year).

In addition, a copolymer of vinylidene fluoride and trifluoroethylene also has piezoelectricity, but this copolymer has a melting point (Tm)
Since the ferroelectric-paraelectric phase transition point (T19') is as follows, in order to obtain practical piezoelectricity, heat treatment is required at a temperature above the ferroelectric-paraelectric phase transition point (Till') and below the melting point (Tm). , it has been found effective to poll.

On the other hand, in the case of a copolymer of vinylidene fluoride and TeI-Rafluoroethylene, an undrawn film formed from a solution or an undrawn film formed by melt extrusion using a quenched thread i'1 has a melting point ( Since there is no clear ferroelectric-paraelectric phase transition point (1"m') below the melting point (Tm)
) If heat treatment is performed below, the degree of crystallinity will not improve and practical piezoelectricity will not be achieved.

Conventionally, in order to improve the degree of crystallinity, methods of adding a stretching step or crystallizing under high pressure before heat treatment have been carried out.

(c) Problems to be Solved by the Invention The above method of adding a stretching step before heat treatment has the disadvantage that it is difficult to control the thickness. Moreover, the film produced in this manner has a low melting point (Tm) and poor thermal stability. Furthermore, the method of crystallization under high pressure requires complicated processing and is inferior in productivity.

Dear Akira Honki, in view of the shortcomings of the prior art, I
The purpose is to provide a method.

(d) Means for Solving the Problems In the present invention, the vinylidene fluoride composition is 72 to . ε
[7] Melting the vinylidene fluoride and tetrafluoroethylene copolymer of 9 () above the melting point (Tm),
From there, it is slowly cooled to crystallize and polled. In this case, slow cooling does not mean cooling at a cooling W rate of 10° C./min or less when passing through the refining temperature region. More preferably, it is 5°C/min or less, more preferably 2°C/min or less.

Further, voltage application for poling may be performed during slow cooling, or may be performed after cooling. In the latter case, the poling temperature may be any temperature that does not exceed the melting point. The resulting piezoelectric film generally has a point a (1 m) higher than that obtained by adding a conventional stretching process. Moreover, the piezoelectric material made by the method of the present invention has an electromechanical coupling constant (K()) in the thickness direction with good thermal stability, and can maintain a practical Kt up to just below the melting point (Tm). .

Good thermal stability of binding constant (K), melting point (TIll)
In order to maintain a piezoelectric material of 1% that can be used for practical purposes right up to the surface, it is necessary to use a copolymer of vinylidene fluoride and tetrafluoroethylene with a composition of 72 to 87 mol% in the film forming method. is unlimited. That is, the present invention can be applied to any method such as cast film formation from a solution, hot press film formation, or extrusion film formation.

The present invention will be explained below using Examples and Comparative Examples.

- Example 1 (vinylidene fluoride composition 72 mol%, unstretched, melt crystallized film) Vinylidene fluoride composition 72 mol% vinylidene fluoride -
A dimethylformamide solution of the tetrafluoroethylene copolymer was poured onto a glass plate, and the solvent was vaporized at room temperature and under reduced pressure. The melting point of this film was 1'm=136°C. Heat this to a glass plate at 180℃ without removing it.
After holding for a certain period of time, the mixture was slowly cooled (2° C./min). Poling was performed using aluminum as electrodes on both sides of this film (poling temperature Tp=100° C., poling voltage E11=900 KV/Cil, poling time tp-30 minutes).

The piezoelectric film of this type is Kt-0,197, 10m
The temperature was -141°C. The thermal stability of this film is as indicated by the symbol ◯ in FIG. In addition, the coupling constant (Kt)
The thermal stability was measured by placing the sample in an oven for 10 minutes and measuring the binding constant (Kt) at air temperature, which was then repeated with the same sample at successively higher temperatures (hereinafter referred to as Kt). All thermal stability measurements were made using this method.) From FIG. 1, it can be seen that the bond constant EQ (Kt) hardly decreases until near the melting point (Tm) and is stable. In Figure 1, the vertical axis represents the binding constant (K
Standardize with t)! It is a ζ thing.

・Example 2 (Nylidene fluoride composition: 81 mol%, unstretched, melt-crystallized film) A dimethylformamide solution of a polynylidene fluoride-tetrafluoroethylene conjugate having a vinylidene fluoride composition of 81 mol% was placed on a glass plate. Second, the solvent was evaporated at room temperature and under reduced pressure. The melting point of this film is Tm=1-24℃
Met. This was heat-treated at 141° C. for 1 hour without being removed from the glass plate, and then slowly cooled (2° C./min). Aluminum is vapor-deposited on both sides of this film and used as an electrode.
Poling was performed while applying a triangular wave voltage and measuring the reversal current (Tl) = to fig, Ep-1145KV/
cm, frequency 0.011-1z, inversion mouth t110 times).

The piezoelectric film thus obtained is Kt-0,207, Tm=
The temperature was 143°C. The thermal stability of this film is as indicated by the symbol △ in FIG.

Measurement of the reversal current at the 10th reversal during poling of this film t7: Song is shown in FIG. Note that this measurement result excludes the current flowing due to the effect of the capacitor of the film itself.

Reversal current half width ΔEc = 41, 8 KV, /am, residual polarization Pr = 7.1 μC/cJ. 8EC is small and the residual molecule 4i (Pr) is small, indicating that it has been effectively polarized by poling.

・Example 3 (vinylidene fluoride composition 82 mol%, unstretched, melt crystallized film) Vinylidene fluoride-tetrafluoroethylene copolymer dimethylform with vinylidene fluoride composition 82 mol%)'
The Mido solution was poured onto a glass plate, and the reduced JIE toC solvent was vaporized at room temperature. FAj of this film! +,',i
1 m = 124°C. This was heated to 180° C. without being removed from the glass plate, and then slowly cooled (2° C./min). Aluminum was vaporized on both sides of this film and used as electrodes, a triangular wave voltage was applied to the film, and poling was performed while measuring the reversal current.
= room temperature, Ep =923KV/cm, frequency 3'10.
()111z, number of reversals 10 times).

This Clqed f + Ti discipline is Kt = 0.189,
Tlll-141℃ and lζ. Measurement results of the reversal current at the 10th reversal during poling of this film (second
A sharp peak similar to the one shown in the figure was obtained.

ΔEc = 66.7KV/cIll, pr-7, 4μC
/d. ΔEC is small and pr is large, indicating that polarization is effectively achieved by polling. The thermal stability of this film is as indicated by the symbol in FIG. The binding constant (Kt) is the melting point (Tm
) It can be seen that it is stable, with almost no decrease until around .

・Example 4 (Vinylidene fluoride composition: 87 mol%, unstretched.

Melt Crystallized Film) A dimethylformamide solution of a vinylidene fluoride-tetrafluoroethylene copolymer having a vinylidene fluoride composition of 87 mol % was flowed onto a glass plate, and the solvent was vaporized at room temperature and under reduced pressure. The melting point of this film was Tm-120°C. This was kept at 180° C. for 1 hour without being removed from the glass plate, and then slowly cooled (2° C./min). Can you put aluminum on both sides of this film? ? Poling was performed using it as two poles (Tp -100°C, Ep -92'l KV
/am.

[p=3Ω min.]

The piezoelectric film thus obtained had Kt = 0.109, Tm
=136°C. The thermal stability of this film is as indicated by the symbols in FIG.

・Comparative Example 1 (Film with vinylidene fluoride composition of 72 mol%, stretched and heat-treated at below Tm) A vinylidene fluoride-tetrafluoro-ethylene copolymer with a vinylidene fluoride composition of 72 mol% was hot pressed.
I11/! After stretching, the film was stretched approximately 4 times at 65°C.

The melting point of this frame was 141°C. This was sandwiched between metal plates and heat treated in an oven at 120°C for 1 hour, followed by slow cooling (2T, 7 minutes). Apply 7 coats of aluminum to both sides of this film, use it as a wire, and apply 1 coat of polling to it.
p = 100 °C, Ell
=532KV/cm.

El) = 30 minutes). The piezoelectric film thus prepared is Kt
= 0.167, 'I'1ll = 141°C. The thermal stability of this film is as indicated by the symbol .

It can be seen from FIG. 1 that, unlike in the case of the present invention, K decreases linearly as the temperature rises and becomes zero near the melting point.

・Comparative example 2 (vinylidene fluoride composition 81 mol% unstretched, melting point (Tm
) Film heat-treated below) A film produced in the same manner as in Example 2 was heat-treated at 115° C. for 1 hour without being removed from the glass plate, and then slowly cooled (2° C./min). Aluminum was vapor-deposited on both sides of this film and used as electrodes, and poling was performed while measuring the reversal current in the same manner as in Example 2 (Tp-vi, Ep = 1125 KV/am, frequency 0.0IH7, number of reversals 10 times). ). This film had a rating of K[-〇. When the reversal current was measured at the 10th reversal during poling of this film, no very broad and clear peak was observed.

The residual polarization was Pr=2.5 μC/art.

・Comparative Example 3 (Vinyl fluoride composition 81 mol 96, unstretched, melted 8
(1, Rapid cooling -] C column) In the same manner as in Example 1 and 12, the glass plate h1 was removed from the film, and after heat treatment at 142°C for 1 hour,
It was rapidly cooled by placing it in ice water at 0°C. Aluminum on both sides of this film with r'4 rfL? [Used as t, while measuring the reversal current as in Example 2,
1 (Tp = air temperature, Ep = 1049 KVCm, frequency 2h0.OI Hz, number of inversions 10 times).

This novelum was Kt =O. The measurement result of the reversal current at the 10th reversal of the poling Ik1 of this film was very broad and no clear peak was observed.

The residual polarization was Pr=3.3 μC/c+f.

・Comparative Example 4 (Vinylidene fluoride composition: 82 mol%, film without stretching or heat treatment) After forming a film of vinylidene fluoride-tetrafluoroethylene copolymer with a vinylidene fluoride composition of 82 mol% using a hot press, the film was heated at 55°C. It was stretched approximately 6 times. The melting point of this film was Tm=132°C. Poling was performed on both sides of this film using aluminum as electrodes (1'p-90°C, Ep = 71
4 KV/cm, tl) = 1 hour).

The piezoelectric film thus obtained had Kt = 0.150, Tn
+=136°C. The thermal stability of this film is first
This is indicated by the symbol shin in the figure. K[ decreased almost linearly and reached zero near the melting point.

・Comparative Example 5 (vinylidene fluoride composition 99 mol%, unstretched, melt crystallized film) Vinylidene fluoride composition 99 mol% vinylidene fluoride -
A dimethylformamide solution of the tetrafluoroethylene copolymer was applied to a glass plate using Ffij M, and the solvent was evaporated under reduced pressure. The melting point of this film is Tm=1
The temperature was 45°C. 180 minutes without removing this from the glass plate.
℃, and then slowly cooled (2℃/min). Aluminum was vapor-deposited on both sides of this film and used as an electrode, and the poling temperature was 11.
V/cm, tp=10 minutes).

Thus 1! The crushed piezoelectric film had a Kt of 0.064, and its piezoelectricity was small.

(e) Effects of the invention In Examples 1 to 3, the temperature was once raised to above the melting point (Tm) to melt, and then slowly cooled to crystallize, followed by poling. The bonding constant vi(
This shows that a piezoelectric film with good thermal stability, which maintains a practical Kt value just below the melting point (1q), can be produced.

Comparative Example 2 shows that it is essential to melt the Kidori Nakagushi -P, and Comparative Example 3 shows that it is essential to further melt and then slowly cool it.

Note that Δ[: shown in Example 2.3 and Comparative Example 2.3
Regarding C and Pr, △EC is small and Pr is human.
■1 We have found that there is a correlation that the combination constant (let) is large. A comparison of these 11t1 shows that the method of the present invention, in which the temperature is once raised above the melting point (Tm) to melt it, and then slowly cooled to avoid crystallization and poling, is effective.

The thermal stability of the vinylidene fluoride-tetrafluoroethylene copolymer of Example 4 (1 g, 87 moles, 96
It can be said that this shows that the method of the present invention, in which the temperature is increased above m) to avoid melting and then slowly cooled to avoid crystallization, is effective in improving the thermal stability of a copolymer having this composition ratio.

The vinylidene fluoride-tetrafluoroethylene conjugate of Comparative Example 5, which has a vinylidene fluoride composition of 99T-nyl 9a, has an even smaller vinylidene fluoride composition, indicating that the effects of the present invention are not exhibited.

In general, materials imparted with piezoelectricity by poling also exhibit pyroelectricity, but the polymer piezoelectric material according to the present invention also exhibits pyroelectricity and is less thermally stable than conventional materials. This makes it suitable for use under high temperatures, and there is little deterioration.

Therefore, the polymeric H electric material produced by the method according to the present invention is suitable for industrial measurement using piezoelectricity and pyroelectricity.

In addition, many polymer piezoelectric materials with human-like piezoelectricity have high crystallinity and poor workability, but the polymer piezoelectric material produced by the method according to the present invention is flexible and
I am rich/υ in t't. Therefore, it is also suitable for applications that require flexibility, such as Chicove and thin films.

[Brief explanation of the drawing]

Figure 1 shows the thermal stability of the electrical-mechanical coupling constant (Kt >
FIG. 2 is a graph showing the reversal current during poling.

Claims (1)

[Claims] A copolymer of vinylidene fluoride and tetrafluoroethylene with a vinylidene fluoride composition of 72 to 87 mol% is heated to
A method for producing a polymer piezoelectric material with excellent thermal stability, which comprises slow cooling from a temperature below Tm) to crystallization and poling.