JPS6318869B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a polymer piezoelectric material.
More specifically, a copolymer of vinylidene fluoride (VDF) and trifluoroethylene (TrFE) [P
(VDF-TrFE)] The present invention relates to a method for manufacturing a polymer piezoelectric material that can obtain a polymer piezoelectric material that has a large thickness piezoelectric effect and has a desired thickness piezoelectric effect even at a relatively low poling voltage. It is already known that a polymer piezoelectric material is produced by poling P(VDF-TrFE).
In addition, the present inventors previously investigated the content of the piezoelectric effect of this polymeric piezoelectric material, and found that depending on the composition ratio of VDF and TrFE, the transverse piezoelectric effect (d ₃₁ ) and the thickness piezoelectric effect (Kt or d ₃₃ ) there are separate changes in the magnitude, and the value of _d31 is VDF = 50-55 mol%
It has been found that the Kt value tends to increase when VDF=70 to 80 mol%, while the Kt value tends to increase when VDF=70 to 80 mol%. These relationships are shown in FIG. 1, with the VDF composition ratio (mol %) plotted on the horizontal axis and the values of d ₃₁ and Kt plotted on the vertical axis. In addition, DSC (Differential Scanning) of this copolymer
Calorimetry) or DTA (Differential
In addition to the endothermic curve (temperature Tm℃) that indicates melting, the endothermic curve (temperature T'm℃) that indicates ferroelectric/paraelectric phase transition is also included in the endothermic curve (temperature T'm℃) based on thermal analysis. There is something to show
After heat treatment at a temperature between T′m (°C) and Tm (°C) or during poling treatment, the electromechanical coupling constant Kt exhibiting the thickness piezoelectric effect
It has been found that a polymer piezoelectric material having a value of approximately 0.3 can be obtained in some cases. However, as a result of further consideration, P.
The composition ratio of VDF in (VDF-TrFE) is approximately 80 mol%
, and furthermore, when it exceeds about 83 mol%, the expected thickness piezoelectric effect is exhibited unless or even if the poling voltage is significantly increased.
It has been found that there are cases where the value cannot be obtained. Purpose of the present invention: The present invention has been made with the object of solving the above-mentioned problems in the case where the expected thickness piezoelectric effect cannot be obtained. Structure of the present invention: A molded article made of a copolymer of 65 to 95 mol% of VDF and 35 to 5 mol% of TrFE or a polymer mainly composed of the copolymer is heated under pressure to a temperature T (℃) of ferroelectric.
Temperature showing an endothermic curve indicating paraelectric phase transition
After performing heat treatment at a temperature between T′m (℃) and the temperature Tm (℃) showing an endothermic curve indicating melting, or while performing the heat treatment, a high Method for manufacturing molecular piezoelectric material. T'm and Tm referred to in the present invention are determined from the DTA curve obtained under the applied pressure. The heat treatment under pressure referred to in the present invention can be carried out in a high-pressure cell device that is commonly used when heating and heat-treating molded products under pressure, and the degree of pressure is preferably 1 Kbar or more, more preferably teeth
2Kbar or more. The poling process referred to in the present invention can be performed by a conventionally well-known poling process apparatus and means used for manufacturing piezoelectric bodies. Effects of the present invention: P with a VDF composition ratio of 65 to 95 mol% (VDF-
Using a molded product (TrFE), it is now possible to produce a polymer piezoelectric material that exhibits the same or a much larger thickness piezoelectric effect than with conventional methods, and at a lower poling voltage. In addition, the main object of the present invention is to solve the problem that conventionally, a good thickness piezoelectric effect may not be obtained especially for P (VDF-TrFE) with a VDF composition ratio of about 80 mol% or more. effective. Next, the present invention will be further explained using specific examples and comparative examples. The VDF composition ratio is 74 mol%, 79 mol%, and
82 mol% of each of P(VDF-TrFE),
A 15 to 20 wt % DMF solution was prepared, cast in vacuum, and held in that state for about 24 hours to obtain a film-like molded product having a thickness of 60 to 100 μm. Next, these were dried in an oven at 100°C for 3 to 4 hours,
The solvent was thoroughly removed. A portion of the 79 mol% product was stretched 4 to 5 times at 80° C. to examine the stretching effect. The pressure treatment uses a high pressure cell to achieve a desired pressure P.
After setting the sample under pressure, it was heated to a predetermined temperature, left for a predetermined heat treatment time, and then cooled. The temperature raising/lowering rate at this time was about 5 to 10°C/min. After cooling to room temperature, the pressure P was removed and the sample was taken out from the high pressure cell. Al electrodes were formed on both sides of the obtained sample by vapor deposition, and the poling temperature was 100-120°C.
The polling process was performed at a polling voltage of 250 to 500 kV/cm and a polling time of about 30 minutes. The electromechanical coupling constant Kt for determining the thickness piezoelectric effect is determined by applying a high frequency voltage (1 to 50 MHz) to the sample and measuring the resonance point according to the method described in J.Appl.Phys. 47 , 949 (1976). This was determined by analyzing the electrical admittance in the vicinity. Further, the value of d ₃₁ , which indicates the transverse piezoelectric effect, was determined by measuring the amount of charge generated on the sample surface and the stress on the cross section of the sample at 110 Hz using Vibron- manufactured by Toyo Sokki Co., Ltd. (A) Details of the case of VDF/TrFE=79/21 copolymer: Table 1 shows this sample (all unstretched), the conventional method, that is, the one heat-treated under normal pressure,
1 shows the conditions for the method according to the present invention, that is, the poling treatment performed by heat treatment under pressure, and the measurement results of the piezoelectric effect of the piezoelectric polymer obtained as a result.

【table】

[Table] The temperature in the polling process in Table 1 is
The temperature was 100℃ and the time was 30 minutes. Moreover, C ^D ₃₃ is the elastic modulus in the thickness direction. As shown in Table 1, in the case of samples treated at normal pressure (Nos. 1 and 2), the poling voltage was
At 257kV/cm, Kt is 0.14, which is not very large.
At 376kV/cm, it becomes 0.29. For samples treated at normal pressure, to obtain a Kt value of about 0.3,
It is necessary to set the polling voltage to approximately 400kV/cm or higher. For samples treated at high pressure (Nos.3~
7) If the heat treatment temperature at this time is set between T′m and Tm (Nos. 4, 6,
7) A Kt value of about 0.27 or more can be obtained with a polling voltage of about 250 kV/cm. This voltage of about 250kV/cm is the same as the above-mentioned approximately 400kV/cm, which is required in the case of normal pressure processing with Kt of about 0.3.
It is clear that the value is much lower than that of . On the other hand, comparing sample No. 4 with samples Nos. 3 and 5, even if the pressure treatment pressure is the same, if the heat treatment temperature does not satisfy the condition of being between T'm and Tm, the Kt will increase. It can be seen that the value tends to decrease. Although not shown in Table 1, this tendency was also the same in the case of 3Kbar. Looking at the relationship between heat treatment time under pressure and Kt, the sample
It is clear that there is not much difference in Kt between 30 minutes and 7 minutes for Nos.4 and 6, and to obtain the desired Kt,
It turns out that pressure and temperature are more important than time. In addition, from sample No. 6, high pressure treatment
The value of d ₃₁ is 6.6 pC/N, and it can be seen that this value is almost the same as the value of d ₃₁ in normal pressure treatment.
As a result, pressure treatment has a more significant effect on improving the thickness piezoelectric effect Kt (enabling processing at a lower poling voltage) than on improving the transverse piezoelectric effect _d31 among the piezoelectric effects. I can see that Next, in order to examine the relationship between pressure P and heat treatment temperature T, we take DTA under high pressure, and T′m and Tm
The pressure dependence of was investigated. This result is shown in FIG. 2, with pressure P (Kbar) plotted on the horizontal axis and temperature (° C.) plotted on the vertical axis. From Figure 2, Tm is 40
It can be seen that it increases with P at ~50°C/Kbar. Furthermore, since the rate of increase of T'm due to P is lower than that of Tm, it is recognized that the temperature difference between the two, ΔT=Tm-T'm, increases as the pressurizing pressure increases. Figure 3 is an example showing the relationship between pressurization pressure, heat treatment temperature, and Kt.
The case is shown. The heat treatment time is
30 minutes, poling at 100℃ under 1bar (normal pressure),
It was carried out at 250kV/cm for 30 minutes. The dotted line in FIG. 3 indicates the average Kt value in the case of normal pressure treated products under the same poling conditions. It is clear that if the poling conditions are the same, the high-pressure treated product will yield a product exhibiting a much larger Kt value. In addition, in relation to the heat treatment temperature T, when combined with the results shown in Figure 2, T'm<
It can be seen that setting T<Tm is effective in increasing Kt. This means that heat treatment under high pressure above the phase transition point T′m results in higher crystallization of the molded product than in the normal case at normal pressure.
It is speculated that this is because an increase in crystal grain size, etc. effectively occurs, and the polarization orientation progresses even at a lower poling voltage. In addition, △T=
Since Tm - T'm increases with the pressurizing pressure P, it is also clear that the range of heat treatment conditions expands as P becomes 2Kbar or 3Kbar. Here, the pressurizing pressure P is further increased by 4Kbar and 5Kbar.
From DTA, △T=Tm−
It is estimated that T'm increases and crystallization progresses further. However, when polling
Based on the premise that a withstand voltage of 200 kV/cm or more is required, if the pressure heat treatment is performed at a treatment temperature of approximately 280°C for more than 10 minutes, the thermal decomposition of the sample will be severe, and the withstand voltage of the resulting molded product will drop significantly. However, it is assumed that the above-mentioned predetermined withstand voltage can be satisfied. From this point of view, it can be said that the upper limit of the heat treatment temperature is about 280°C as a condition for obtaining a pollable molded product.
This limit is shown by a dotted line in FIG. Next, Table 2 shows the relationship between high-pressure treatment and Kt of the stretched material when the VDF composition ratio is 79 mol %. Stretching is performed at a temperature of 80° C. and a magnification of 4 to 5 times, and the poling conditions are the same as in the case of unstretched. As in the case of the unstretched product, crystallization of the stretched product progresses by high-pressure crystallization treatment with T′m<T<Tm (C ^D ₃₃ is approximately 11×10 ⁹ N/m ² ),
It can be seen that a Kt of about 0.3 can be obtained with a polling voltage of about 250 kV/cm.

[Table] (B) Details for the case of VDF/TrFE=82/18 copolymer: Figure 4 a and b show the temperature under a pressure of 2 Kbar with the horizontal axis representing the temperature and the vertical axis representing the magnitude of heat absorption and heat generation. This is the DTA of a molded product by solvent casting and a molded product by solvent crystallization. The former shows two peaks, T′m and Tm (the exothermic heat in between is thought to be due to the influence of residual solvent), and the latter shows T′m,
In addition to Tm, there is another peak T″m,
This T″m is increased by increasing the pressurizing pressure P.
It is presumed that it appeared separately from the Figure 5 shows solution (DMF) cast product, Figure 6 shows
The figure shows the relationship between pressurization pressure P and Tm, T′m (T″m) for each molten crystallized product, with pressurization pressure P (Kbar) on the horizontal axis and temperature (℃) on the vertical axis. This is a diagram showing that as the pressurizing pressure P increases, △T = Tm - T'm increases, and in a molten crystallized product, Tm and T'm completely separate at about 2 Kbar or more. Table 3 shows the relationship between heat treatment conditions and piezoelectric effect under normal pressure and high pressure, and Figure 7 shows the heat treatment temperature when the pressure P is 2Kbar, 3Kbar, and the heat treatment time is 30 minutes. This shows the relationship between T and Kt. All samples are unstretched. Poling conditions are 100° C., 30 minutes, and approximately 250 kV/cm.

[Table] The dotted line in FIG. 7 indicates the approximate value of Kt obtained when polling was carried out after normal pressure treatment. A value of this level cannot be said to have a practically effective thickness piezoelectric effect. From Figures 5 and 6, Figure 7, and Table 3, some of the copolymers in this case can be heat treated with T'm < T < Tm only by high pressure treatment, and the unstretched material has a large piezoelectric potential. It can be seen that an effect (Kt of about 0.1 or more) can be obtained. Note that the copolymer in this case also showed severe thermal decomposition when the temperature exceeded 280°C, as in the case of VDF=79 mol% described above. (C) Details for VDF/TrFE=74/26 copolymer: This copolymer can be prepared by heat treatment under normal pressure, by selecting the heat treatment temperature T as T′m<T<Tm, and By selecting the conditions, it is possible to make a polymer piezoelectric material with Kt of 0.3 or more, but the poling voltage must be set at 400 to 500 kV/cm.
Alternatively, it may be necessary to set it higher than that, and if the polling voltage is about 250 kV/cm, Kt may only be about 0.1. Table 4 and FIG. 8, with the heat treatment temperature T (° C.) plotted on the horizontal axis and Kt plotted on the vertical axis, show the relationship between each treatment condition and the piezoelectric effect. As for the pressure-treated product, as in the case of VDF = 79 mol%, as the pressure P increases, △T = Tm - T'm increases, the range of selection of heat treatment conditions is expanded, and the manufacturing conditions can be changed. In addition to making the selection easier, the polling voltage also
Kt at about half the voltage compared to normal pressure treatment.
It can be seen that it is possible to manufacture a polymer piezoelectric material with a value of approximately 0.3. In addition, the pressure heat treatment time is 30
The poling conditions were 100°C and 30 minutes.

[Table] Examples of the method for manufacturing a polymer piezoelectric material according to the present invention have been described above, and the polymer piezoelectric material obtained in this way can be used as an electromechanical transducer or as a piezoelectric material. It can be used as an element that utilizes the pyroelectric effect that it has at the same time, and in particular, those with a Kt value of 0.2 or more, preferably 0.25 or more, and even more preferably 0.3 or more, can be used in place of conventional inorganic piezoelectric materials. Alternatively, you can replace it with a piezoelectric material made of polyvinylidene fluoride, which matches well with living organisms.
As an ultrasonic receiving/transmitting element that improves the resolution of images obtained, it is preferably used as a component of an ultrasonic diagnostic device for a living body.

[Brief explanation of the drawing]

Figure 1 is a graph explaining the relationship between the composition ratio of VDF and the values of Kt and d ₃₁ , and Figure 2 is a graph explaining the relationship between the composition ratio of VDF and the values of Kt and d 31.
Figure 3 is a graph explaining the relationship between pressurizing pressure and T'm and Tm in the case of VDF = 79 mol%, and Figure 4 is a graph showing the relationship between heat treatment temperature and Kt in the case of VDF = 79 mol%. , b are when VDF=82 mol%
Figures 5 and 6 showing the DTA curve, graphs explaining the relationship between pressurizing pressure and T'm and Tm when VDF = 82 mol%, and Figure 7 show the relationship between the pressurizing pressure and T'm and Tm when VDF = 82 mol%. Graph showing the relationship between heat treatment temperature and Kt,
FIG. 8 is a graph showing the relationship between heat treatment temperature and Kt when VDF=74 mol%.

Claims (1)

[Claims] 1 A molded article made of a copolymer of 65 to 95 mol% vinylidene fluoride and 35 to 5 mol% of ethylene trifluoride or a polymer mainly composed of this is heated under pressure at a treatment temperature T (°C). After performing a heat treatment at a temperature selected between the temperature T′m (°C) showing an endothermic curve indicating dielectric-paraelectric phase transition and the temperature Tm (°C) showing an endothermic curve indicating melting, or,
A method for producing a polymer piezoelectric material, which comprises performing a poling treatment while performing the heat treatment.