JPS61177785A - Bimorph element - Google Patents

Abstract

PURPOSE:To generate little internal stress by thermal expansion even under high temperatures, and to make an electrostriction plate difficult to break even under large displacements, by a method wherein the coefficient of thermal expansion of a shim plate is set at 10<-7>-10<-5>/ deg.C which is approximate to that of the electrostriction plate. CONSTITUTION:The shim plate is made of a metal whose coefficient of thermal expansion is 10<-7>-10<-5>/ deg.C. For example, the title element is produced by sandwiching a shim plate 11 made of 36% Ni-Fe with electrostriction plates 12, 13 made of PZT, and their directions are of the same as shown by arrows A, B. Besides, the shim plate 11 serves as the central electrode. Both surfaces of the electrostriction plates 12, 13 are provided with electrodes 501-504 made of Au, Ag, Ni, etc. by baking, respectively. The electrodes 502 and 503 on the shim plate 11 side are not applied to the end on the fixed part 505 side. leads 507-509 are connected to the electrode 501, shim plate 11, and electrode 504, respectively, on the fixed part 505 side. The plates 12, 13 are adhered to the plate 11 after polarization. Since the coefficient of thermal expansion of the shim plate is approximate to that of the electrostriction plate, the internal stress due to thermal expansion hardly generates even under high temperatures.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a bimorph element formed by bonding at least a pair of ceramic electrostrictive plates via a shim plate that also serves as a center electrode, and particularly relates to a bimorph element formed by bonding at least a pair of ceramic electrostrictive plates via a shim plate that also serves as a center electrode. The present invention relates to a bimorph element suitable for an amplitude-operated piezoelectric actuator.

[Conventional technology]

Bimorph devices operate at high speed by applying voltage.
Its power consumption is also low. For this reason, actuators using bimorph elements in their driving parts are attracting attention as actuators that can replace electromagnets, especially in communication terminal equipment that is desired to be smaller and lower in power consumption.

However, this type of conventional piezoelectric actuator has a small amount of displacement and small generated force, so its field of application is limited. In order to operate a bimorph element with large force and force, a drive circuit that can apply a high voltage while preventing deterioration of polarization and an element structure suitable for large-amplitude operation are required.

Regarding the former, the patent application filed in 1982- related to the invention of the present inventor
There is an unbalanced voltage application polarity switching circuit described in ``Electrostrictive Vibrator 43881 r Electrostrictive Vibrator'', and it has become possible to obtain two to three times the amount of displacement and generated force than before.

[Problem that the invention seeks to solve]

However, it has become clear that when such a large-amplitude operation is performed at high temperatures, a new problem arises in the conventional bimorph element: the electrostrictive plate breaks.

[Means for solving problems]

The bimorph element of the present invention was made in view of the above problems, and the thermal expansion coefficient of the shim plate is 101 to 10-'.
/ 'C metal.

[Effect]

Since the coefficient of thermal expansion of the shim plate is close to that of the electrostrictive plate, almost no internal stress is generated due to thermal expansion even at high temperatures, and therefore the electrostrictive plate is difficult to break even if it is subjected to large displacements. .

〔Example〕

Hereinafter, the present invention will be described in detail along with examples.

Table 1 is a table showing the linear expansion coefficient and magnetic properties of various materials.

Table 1 The inventor of the present application has determined that the cause of the phenomenon in which the electrostrictive plate breaks when a bimorph element is operated with large amplitude at high temperatures is as follows.
It is assumed that this is due to the difference in thermal expansion coefficient between the electrostrictive plate (PZT) and the shim plate.

Therefore, since brass, nickel silver, phosphor bronze, etc. are mainly used for the shim plates of conventional bimorph elements due to their good spring properties, the following experiment was conducted.

Electrostrictive plate 201 made of lead zirconate titanate (PZT)
and brass plate 202 were bonded together using a 100° C. curing adhesive, the temperature was returned to room temperature, and changes in shape were observed.

Note that the shape of the electrostrictive plate 201 and the brass plate 202 (length
Width x Thickness) are 45X12X0.15mm+ and 45X12XO,05mm, respectively.

The resulting two plates are shown in cross-section in FIG.

In other words, it was largely curved so that the electrostrictive plate 201 side was inward. In order to return this to a flat plate shape, it was necessary to apply a force of about F=15 g to the tip with one end fixed. Similar results were obtained when two plates were cured at room temperature and left at elevated temperatures. However, in this case, it is curved so that the brass plate side is on the inside.

Bimorph elements are made of two electrostrictive plates glued together in a sandwich shape via a shim plate, so they will not bend even if left at high temperatures after being glued together at room temperature. The experimental results revealed that an internal stress equivalent to a tip load of approximately 15 g was generated between the shim plate and the electrostrictive plate. Therefore, it has become clear that when operating at high temperatures, in addition to the strain caused by the application of voltage, strain is caused by the thermal expansion of the shim plate.

When measuring the thermal expansion coefficient of piezoelectric ceramics such as PZT, lead titanate, lead niobate, barium titanate, and sodium ptadium nitebate, it is approximately 15 to 50xlO-'/°C, as shown in Table 1. be. Therefore,
The inventor of the present application has developed Ni-Fe system or Ni-F as a shim plate.
We focused on e-Co alloys.

FIG. 3 shows the relationship between the composition of the Ni--Fe alloy and the coefficient of thermal expansion α. As is clear from the figure, Ni
When the Ni content of the -Fe alloy is 30 to 55% by weight, α becomes 100 Xl0-'/°C or less, making it close to an electrostrictive plate. Typical examples include 36χNi-Fe where a is the same as or slightly smaller than the electrostrictive plate, and 45 where α is slightly larger than the electrostrictive plate.
Experiments similar to those described above were conducted using χNi-Fe.

FIG. 4 shows a 36χNi-Fe plate 40H45X12X0.
05 mm) and the same electrostrictive plate 20 used in the above experiment.
1 and 2 are bonded together with a 100° C. curing adhesive and the temperature is returned to room temperature. As can be seen from the figure, there is no deformation like when brass is used. In addition, it was found that when 45XNi-Fe was used, there was some curvature, but the degree of curvature was significantly smaller than in the case of brass.

FIG. 1 is a block diagram showing a piezoelectric actuator in which a bimorph element according to an embodiment of the present invention is used as a driver in a drive circuit capable of large-amplitude operation.

The bimorph element 1 has a shim plate 11 made of 36xNi-Re sandwiched between electrostrictive plates 12 and 13 made of PZT, and the polarization directions are the same as shown by arrows A and B. Note that the shim plate 11 also serves as a center electrode.

FIG. 5 is a perspective view showing the structure of this bimorph element 1 in more detail. Electrodes 501 to 504 made of Au, Ag, Ni, etc. are provided on both sides of the electrostrictive plates 12 and 13, respectively.
is formed by baking. Electrode 50 of 11 shim plates
2 and 503 are not provided at the end on the fixed portion 505 side. The lead wires 507 to 509 are connected to the fixed part 5, respectively.
On the 05 side, the electrode 501. The shim plate 11° is connected to the electrode 504. The electrostrictive plates 12 and 13 are bonded to the shim plate 11 after polarization.

In FIG. 1, the drive circuit includes an unbalanced voltage application drive input circuit 2. Polarity switching circuit 3. Constant current circuit 4. Constant voltage circuit 5
．． It is composed of a control circuit 6 for polarity switching and a logic voltage power supply 7.

The drive input circuit 2 consists of Zener diodes 21 and 22, and applies a constant voltage Vc in the same direction as the polarization direction to one electrostrictive plate of the bimorph element 1, and applies a constant voltage Vc in the same direction as the polarization direction to the other electrostrictive plate. A voltage obtained by subtracting the operating voltage of the Zener diode 21 or 22 from the constant voltage Vc is applied.

The polarity switching circuit 3 is constructed by connecting Darlington-connected transistor circuits 31 to 34 in a bridge configuration.
The polarity of the output voltage is switched according to the output signal of the control circuit 6.

The constant current circuit 4 is made up of a CRD 41.

The constant voltage circuit 5 is an RC circuit using a pnp (pnp) transistor.
This is a C-type step-up constant voltage circuit, and although its internal configuration is omitted, it boosts the power supply voltage Ec (approximately 5 V) of the logic voltage power supply 7 and outputs a constant voltage Vc. The constant voltage Vc is the power supply voltage Ec and an internal Zener diode ZD.

Operating voltage V2. ．． The value obtained by adding , that is, (Ec+V
zoo).

The control circuit 6 performs on/off control of the Darlington transistor circuits 31 to 34 of the polarity switching circuit 3 by switching the switch 61, and when the switch 1 is in the state shown in the figure, the transistor circuits 31 and 32 are turned on and off. cut off,
Transistor circuits 33 and 34 are rendered conductive. When the switch 61 is turned in the opposite direction, the transistor circuit 33
and 34, and transistor circuits 31 and 32 are cut off.
conducts.

Next, the operation of the piezoelectric actuator configured as described above will be briefly explained.

Now, when the switch 61 is in the state shown in the figure, the transistor circuits 31 and 32 are cut off and the transistor circuits 33 and 34 are made conductive, so that the terminal A of the drive input circuit 2 is at the ground level and the terminal B is at the potential Vc. becomes.

Therefore, the output voltage Vc of the constant voltage circuit 5 is applied to the electrostrictive plate 13 in the polarization direction, and vCVZ! is applied to the electrostrictive plate 12 in the reverse polarization direction. +1 is applied, and the tip of the bimorph element 1 is displaced downward.

When the switch 61 is set to the ground level, the transistor circuits 33 and 34 are cut off and the transistor circuits 31 and 32 are made conductive, so that the terminal B of the drive input circuit 2 is set to the ground level and the terminal A is set to the potential Vc.

Therefore, the output voltage Vc of the constant voltage circuit 5 is applied to the electrostrictive plate 12 in the polarization direction, Vc-V2° is applied to the electrostrictive plate 13 in the reverse polarization direction, and the tip of the bimorph element 1 is directed upward. Displace.

An electrostrictive plate depolarizes when a polarization deterioration voltage Vd or more is applied in the reverse polarization direction, but according to this drive circuit, the polarization deterioration voltage Vd is reduced in the polarization direction without applying a polarization deterioration voltage Vd or more in the reverse polarization direction. Since the above high voltage (Vc) can be applied, by switching the switch 61, the bimorph element 1 can be caused to vibrate greatly without deteriorating. Furthermore, according to this drive circuit, even if the applied voltage in the reverse polarization direction is greater than the polarization deterioration voltage Vd and causes depolarization in the electrostrictive plate, as long as the voltage is at a level that does not cause polarization breakdown, the polarization The depolarization is restored by applying a high voltage Vc in the direction, and it is possible to vibrate even more greatly.

FIG. 6 is a diagram showing the displacement characteristics of the bimorph element when a 2 Hz continuous operation test was conducted at a temperature of 70° C. using the drive circuit shown in FIG. 1. Characteristic A.

B is related to the bimorph element according to the present invention, and characteristic A is the one using 36χNi-Fe for the shim plate,
Characteristic B uses 45χNi-Pe. Also,
Characteristics C and D relate to a conventional bimorph element using brass for the shim plate.

Further, Table 2 shows the rupture characteristics of the bimorph element.

Table 2: The evaluation values in this table use the number of experiments as the denominator.
The number of breaks is expressed as the number of molecules.

The bimorph element using 36χNi-Fe for the shim plate is
It can be seen that there is no breakage and the amount of displacement is greatly superior. In addition, when 45χNi-Fe was used for the shim plate, some breakage was observed when high voltage was applied, but it was clearly superior to the conventional bimorph element using brass for the shim plate. I understand.

As a result of experiments on Ni-Fe with other composition ratios,
The piezoelectric ceramic electrostrictive plate has a thermal expansion coefficient α of 100
It has become clear that the shim plate with a temperature of 10-'/'C or less has better fracture resistance than the conventional shim plate.

Such characteristics can also be obtained using a Ni-Co-Fe alloy. FIG. 7 is a characteristic diagram showing the relationship between the composition ratio and thermal expansion coefficient α of the Ni-Co-Fe alloy. For example, A
The dots indicate 29χNi-17χCo-54χFe. In the same figure, the part surrounded by the slope line is α<100 xlo-'
/'c or less, and the alloy indicated by point A is included in this region. It was found that the bimorph element using the alloy contained in the area surrounded by the inclined line as a shim plate is also excellent in terms of breakage resistance.

Note that when the coefficient of thermal expansion of the shim plate is larger than that of the electrostrictive plate, a tensile force is generated in the electrostrictive plate at high temperature, and when it is smaller, a compressive force is generated. Since tensile force is the main influence on rupture, it is appropriate for the thermal expansion coefficient of the shim plate to be equal to or slightly smaller than that of the electrostrictive plate. Good rupture resistance can be obtained even at a temperature of '/'C.

N used in the shim plate of the bimorph device according to the present invention
i-Fe alloys and Ni-Co-Fe alloys have not been considered for conventional shim plates due to their low springiness, but as a result of the above experiments, they have improved fracture resistance by lowering the coefficient of thermal expansion than their springiness. It has become clear that it is more important to deform the shim plate, and it has also been confirmed that bimorph elements that deform by less than 0.1mmmm, at most 1mm per 10mm of length, do not require large springiness in their shim plates. It was done. Note that, if necessary, the springiness can be improved by applying cold working to the shim plate.

In addition, since Ni-Fe and Ni-Co-Fe are magnetic materials as shown in Table 1, in order to generate a strong acting force at the displacement reaching point, as shown in Figure 8 (B), near the displacement reaching point It is conceivable to arrange magnets 801 and 802 at

The solid line in FIG. 8(A) shows the displacement/force generation characteristics when a voltage of opposite polarity is applied to the bimorph element 1 in the state shown in FIG. 8(B), and the magnet 80 shown by the dashed line
It is expressed as the sum of the attraction force of 1.802 and the force generated only by the bimorph element 1 shown by the broken line.

It can be seen from this characteristic diagram that a large acting force can be obtained at the reaching point 803.

FIG. 9 is a partially enlarged sectional view showing the bonded portion between the electrostrictive plate 12 and the shim plate 11 of the bimorph element 1 according to this embodiment. An adhesive 903 is applied between the electrode 502 and the shim plate 11.
is interposed, but the electrode 50 is partially connected to the contact portion 902.
2 and the shim plate 11 are in direct contact with each other, and the adhesive 90
Add conductive powder such as carbon to 3 to increase the solid resistance.
Since the resistance is IOΩ, the electrode 502 and the shim plate 11 are substantially short-circuited. Therefore, the electrode 50
1 and the shim plate 11, all the voltage applied between the electrostrictive plate 1
2, the shim plate 11 can function as a central electrode.

Furthermore, if the adhesive is held in a large displacement state at high temperatures, the adhesive may loosen and the contact portion 902 may separate as shown in FIG.
Since conductive powder such as carbon is mixed in 03, even in such a case, most of the voltage applied between the electrode 501 and the shim plate 11 is applied to the electrostrictive plate 12, and the amount of displacement does not decrease.

Incidentally, an epoxy adhesive was used to bond the shim plate and electrostrictive plate of a conventional bimorph element. The volume resistivity of the epoxy adhesive is 1013 to 101 SΩ. When the thickness of the adhesive layer is 1 to 10 μm, the resistance layer becomes 10 9 Ω or more. Since this value is comparable to or higher than the resistance value of the electrostrictive plate, the applied voltage is mainly applied to the adhesive layer. For this reason, a phenomenon often occurred in which a predetermined voltage was not applied to the electrostrictive plate and the amount of displacement decreased.However, in the bimorph element according to this embodiment, the specific resistance of the adhesive is set to 1010Ω. In a layer of ~10 μm, the resistance becomes 10 7 Ω or less, and most of the applied voltage is applied to the electrostrictive plate.

However, if the specific resistance of the adhesive is too low, leakage occurs due to the adhesive 901 slightly attached to the end of the bimorph element 1, and no voltage is applied to the electrostrictive plate. Therefore, it is desirable that the specific resistance is 10''Ω.

Generally, there is a concern that the adhesion force will be reduced if carbon or the like is mixed into the adhesive, but in this example, the content of the conductive powder is only a very small amount, so the adhesion force is not affected.

〔Effect of the invention〕

As explained above, according to the bimorph element of the present invention, the thermal expansion coefficient of the shim plate is set to 10-7 to 10-'/°C, which approximates the thermal expansion coefficient of the electrostrictive plate. Almost no internal stress is generated due to expansion, and the electrostrictive plate is less likely to break even if it is largely displaced.

Therefore, the reliability of the bimorph element can be significantly improved.

[Brief explanation of drawings]

Figure 1 is a block diagram of a piezoelectric actuator using a bimorph element, which is an embodiment of the present invention, and Figure 2 is used to confirm the stress generated in the bonded body of the brass plate and electrostrictive plate based on the environmental temperature. Figure 3 is a characteristic diagram showing the relationship between Ni content and thermal expansion coefficient in Ni-Fe alloys.
Figure 4 is a cross-sectional view for confirming the stress generated in the bonded body of the Ni-Fe plate and electrostrictive plate based on the environmental temperature, and Figure 5 is a perspective view showing the configuration of the bimorph element in Figure 1. , 6th
The figure shows the drive circuit shown in Figure 1 being used to drive 2
Figure 7 is a displacement characteristic diagram of various bimorph elements during a continuous operation test at Hz. Figure 7 is a characteristic diagram showing the relationship between the composition ratio of Ni-Co-Fe alloy and the thermal expansion coefficient α. A)
is a characteristic diagram showing the relationship between displacement and generated force of the bimorph element shown in the same figure (B), the same figure (B) is a cross-sectional view showing the bimorph element with a magnet arranged at the tip, and Figures 9 and 1θ are Both are partially enlarged cross-sectional views of the bimorph device shown in FIG. 1. 1... Bimorph element, 11... Shim plate, 12.1
3... Electrostrictive plate, 501-504... Electrode. Fig. 2 Fig. 3 N14Kl (ttJ,) Fig. 4 Fig. 6 Self-made: open (H) Fig. 7 Fig. 8 (A) (B)

Claims (5)

[Claims] (1) In a bimorph device in which electrostrictive plates are bonded to both sides of a shim plate that also serves as a central electrode, and electrodes are provided on the outer surface of the electrostrictive plate, the shim plate has a thermal expansion coefficient of 10^-^7 to 10^. −
A bimorph element characterized by being made of metal with a temperature of ＾5/℃. (2) The shim plate is Ni-F with a Ni content of 30 to 55% by weight
The bimorph element according to claim 1, which is an e-based alloy. (3) The bimorph element according to claim 1, wherein the shim plate is made of a Ni-Co-Fe alloy. (4) The shim plate and electrostrictive plate have a resistance of 10^6 to 10^1^3Ω
The bimorph element according to claim 1, wherein the bimorph element is bonded with an adhesive having a volume resistivity of cm. (5) The bimorph element according to claim 4, wherein the adhesive is a resin mixed with carbon particles.