JPH05206533A - Method of using piezoelectric element - Google Patents

Abstract

PURPOSE:To increase the displacement level of a piezoelectric element in the voltage impressed time by a method wherein the temperature is controlled within the range from the temperature higher than the temperature whereat the residual polarization of a piezoelectric plate starts to decline to the temperature lower than the Curie temperature of the piezo-electric plate simultaneously impressing a pair of electrodes with the voltage higher than the withstanding voltage of the piezo-electric plate. CONSTITUTION:The title piezoelectric element is used within the high temperature range wherein the application of the piezo-electric element is conventionally prohibited i.e., from the temperature higher than the temperature whereat the residual polarization of a piezoelectric plate starts to decline to the temperature lower than the Curie temperature of the piezo-electric plate. The element can be hardly used at the temperature higher than the Curie temperature whereat the piezoelectric characteristics are to be lost. Besides, the displacement level of the piezoelectric element can not be increased at the temperature lower than the temperature whereat the residual polarization of the piezo-electric plate starts to decline while at the temperature near the Curie temperature, the residual polarization is to notably decline. However, the polarization level higher than that of the same voltage at the conventionally applicable temperature can be caused by impressing a pair of electrodes with the voltage higher than the withstand voltage of the piezoelectric plate thereby enabling the displacement level to be increased. That is, the polarization can be hardly caused by impressing the pair of electrodes with the voltage lower than the withstand voltage.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of using a piezoelectric element used as a piezoelectric actuator.

[0002]

2. Description of the Related Art In recent years, piezoelectric actuators utilizing piezoelectric characteristics have been used in place of actuators utilizing electromagnetic force. For example, PbTiO ₃ —PbZrO ₃ system ceramics (PZT) shows excellent piezoelectric characteristics and is widely used as a piezoelectric actuator.

By the way, PZT is applied at a voltage as low as possible.
In order to maximize the piezoelectric characteristics of piezoelectric materials such as
It is necessary to form a thin plate-shaped piezoelectric plate from the piezoelectric material and apply a voltage to both the front and back surfaces thereof. However, with a thin plate, the displacement is extremely small. Therefore, electrodes are formed on both sides of the piezoelectric plate from silver paste etc. to form piezoelectric elements,
As a piezoelectric laminate in which a plurality of piezoelectric elements are alternately laminated with electrode plates, a predetermined displacement amount is obtained by the total displacement of the piezoelectric elements.

As a measure of the piezoelectric characteristics of the piezoelectric element,
A piezoelectric constant (hereinafter referred to as d constant) corresponding to the amount of displacement per 1 V of voltage is typically used. It is known that the d constant gradually increases as the temperature rises, as shown in FIG. 1, but it greatly decreases near the Curie temperature. Therefore, conventionally, it is common sense to use it at a temperature considerably lower than the Curie temperature, and rather, a method of using it while cooling is proposed in order to prevent generation of cracks due to a difference in thermal expansion.

[0005]

However, the conventional piezoelectric element has a drawback that the displacement amount when a voltage is applied is small. That is, even with a laminated piezoelectric actuator in which a plurality of piezoelectric elements are laminated, the displacement amount is smaller than that of an electromagnetic actuator of the same length, and a mechanical displacement magnifying mechanism using a hydraulic mechanism or a link mechanism is used. I had to do it. Therefore, the number of parts increases and the arrangement space also increases, resulting in an increase in cost for use.

The reason why the displacement of the piezoelectric element is small is explained as follows. That is, the amount of displacement of the piezoelectric material depends on the degree of polarization when a voltage is applied, and the greater the polarization, the greater the amount of displacement. By the way, the relationship between the electric field of the piezoelectric material and the degree of polarization shows a hysteresis curve as shown in FIG. As can be seen from FIG. 4, even if the electric field becomes zero, the polarization does not become zero and shows a constant value. This is called remanent polarization and is indicated by point b in FIG. In a general laminated piezoelectric actuator, each piezoelectric element is marketed in a state where polarization processing is performed and the residual polarization is retained. Therefore, for example, when a positive voltage is applied during use, the maximum value of the polarization becomes the point a in FIG. 4, and after all, when the voltage is applied, only a displacement amount corresponding to the difference between the polarizations of a and b can be obtained.

The present invention has been made in view of the above circumstances, and an object thereof is to increase the amount of displacement of the piezoelectric element when a voltage is applied.

[0008]

However, as a result of diligent research, the present inventor has found that the piezoelectric element, which is conventionally used at a temperature considerably lower than the Curie temperature, is heated to a temperature near the Curie temperature. The present invention has been completed by discovering that when a voltage equal to or higher than the coercive voltage is applied, the d constant greatly decreases, but the displacement amount greatly increases.

That is, a method of using the piezoelectric element of the present invention for solving the above-mentioned problems is a method of using a piezoelectric element including a piezoelectric plate and a pair of electrodes formed on both front and back surfaces of the piezoelectric plate. It is characterized in that the temperature is controlled to a temperature range below the Curie temperature of the piezoelectric plate above the temperature at which polarization begins to decrease, and a voltage above the coercive voltage of the piezoelectric plate is applied to the pair of electrodes.

The piezoelectric element is composed of a piezoelectric plate and a pair of electrodes. The piezoelectric plate may be made of a piezoelectric material such as PZT and may be the same as the conventional one, and the electrodes may be made of silver paste or the like in the conventional manner. That is, as the piezoelectric element used in the present invention, the conventional piezoelectric element can be used as it is. The greatest feature of the present invention is
The piezoelectric element is used at a high temperature which is conventionally unusable, that is, at a temperature above the temperature at which the residual polarization of the piezoelectric plate begins to decrease and below the Curie temperature of the piezoelectric plate. When the operating temperature is higher than the Curie temperature, the piezoelectric characteristics are destroyed and it is difficult to use. Also, if it is used at a temperature lower than the temperature at which the remanent polarization of the piezoelectric plate begins to decrease,
Almost the same as the conventional usage conditions, and no increase in displacement can be obtained. It is known that the temperature at which the remanent polarization of the piezoelectric plate begins to decrease is approximately equal to the temperature at which the d constant begins to decrease, although it depends on the material of the piezoelectric plate. It can also be said to be a temperature at which the capacitance is within 50% of the capacitance at the Curie temperature.

Another feature of the present invention is that a voltage higher than the coercive voltage of the piezoelectric plate is applied for use. Here, the coercive voltage means a voltage at which the polarization of the piezoelectric plate begins to be reversed. Near the Curie temperature, the remanent polarization is significantly reduced.
However, when a voltage equal to or higher than the coercive voltage is applied, polarization is performed again, and moreover, as will be described later, a polarization larger than the polarization amount of the same voltage at the conventional operating temperature is obtained, so that the displacement amount increases. If a voltage lower than the coercive voltage is applied, it is difficult to re-polarize and the amount of displacement cannot be obtained. Since the coercive voltage decreases as the temperature rises, in the present invention used at a high temperature such as near the Curie temperature, the applied voltage can be made smaller than the conventional one as long as the same displacement amount as the conventional one can be obtained. Is.

[0012]

At a temperature near the Curie temperature, the relationship between the electric field and the degree of polarization is as shown in FIG. That is, since the molecular motion becomes active due to the temperature increase, the crystal motion also becomes active, and the remanent polarization (point b) is decreased. Therefore, when no voltage is applied, the amount of displacement depends on the residual polarization and is extremely small. By applying a voltage equal to or higher than the coercive voltage, the unpolarized crystal is repolarized, and a larger polarization (point a) than the conventional one can be obtained due to the influence of thermal expansion and the activity of molecular motion. Therefore, the displacement amount based on the difference between a and b becomes extremely large.

The contradiction that the displacement amount increases while the d constant decreases is explained as follows. That is, the d constant is obtained from the electromechanical coupling constant and the electrostatic capacity, and it is unavoidable that the d constant is measured at a comparatively low voltage due to the relationship of the measuring device. Therefore, in the operating temperature range of the present invention, re-polarization becomes difficult, and the d constant sharply decreases. However, by applying a voltage equal to or higher than the coercive voltage, repolarization became possible. Therefore, if the d-constant can be measured under a voltage applied above the coercive voltage, it should show a high d-constant near the Curie temperature.

[0014]

EXAMPLES The present invention will be specifically described below with reference to examples. (Example) The component ratio of the main component is Pb (Zr _0.52 Ti _0.48 ).
As represented by O ₃ , 50 mol% of lead oxide (PbO),
Powder containing zirconium oxide (ZrO ₂ ) 26 mol% and titanium oxide (TiO ₂ ) 24 mol%, and niobium pentoxide (Nb ₂ O ₅ ) 0.1 to 3 as an accessory component.
The powder added with mol% was mixed with distilled water in a ball mill.
Mix for 8 hours. After dehydration and drying of this, 900 ℃ in air
It was calcined for 1 hour.

The calcined powder was wet mixed again for 48 hours, dehydrated and dried. Then, 1% by weight of PVA (polyvinyl alcohol) was added to the dry powder as a binder, and after granulation, a molding pressure of 1000 kg / cm ³ gave a diameter of 20 mm and a thickness of 0.4 m.
A m-shaped disk was formed and fired at 1250 ° C. for 1 hour to obtain a piezoelectric plate. The Curie temperature of this piezoelectric plate is 235 ° C.

Silver paste was screen-printed on both front and back surfaces of this piezoelectric plate and dried to form a pair of electrodes. Then, in silicon oil at 100 ° C, 50k between both electrodes
A voltage of v / cm was applied and polarization treatment was performed for 30 minutes to obtain a piezoelectric element. The piezoelectric actuator 1 was manufactured by alternately stacking 68 piezoelectric plates and stainless electrode plates. Then, it is arranged in the control device shown in FIG.
The amount of displacement at that time was obtained.

This control device has a piezoelectric actuator 1.
And a cylindrical heater 3 arranged on the outer periphery of the piezoelectric actuator 1, and an electric circuit. Piezoelectric actuator 1
A rectangular voltage is applied by the driving power source 2 to intermittently continuously drive. A temperature sensor 4 is provided in the piezoelectric actuator 1.
Are arranged. The signal from the temperature sensor 4 is input to the heater control circuit 5. The heater control circuit 5 compares the signal from the temperature sensor 4 with a specified value and controls the heater power supply 6 for heating the heater 3. Thereby, the piezoelectric actuator 1 is configured to be heated to a predetermined temperature and kept at that temperature.

Using this controller, while controlling the internal temperature of the piezoelectric actuator 1 to be 230 ± 2 ° C.,
A rectangular voltage of 400 V was applied under no load. In addition, in this piezoelectric element, the coercive voltage at 230 ° C. is about 30V. As a result, the piezoelectric actuator 1 has a maximum of about 75
A displacement amount of μm was obtained. In the driving conditions of the example,
FIG. 1 shows a change in the displacement amount when only the driving temperature is changed. As can be seen from FIG. 1, the displacement amount gradually increases with increasing temperature up to around 195 ° C. where the remanent polarization starts to decrease, but the increase amount is slight. However, when the temperature exceeds the temperature at which the remanent polarization starts to decrease, the amount of displacement sharply increases with the decrease in the remanent polarization up to the Curie temperature. (Comparative Example) Using the same piezoelectric actuator 1 as in Example, a rectangular voltage of 400 V was applied in a room temperature (25 ° C.) atmosphere in an unloaded state. As a result, the piezoelectric actuator 1
A maximum displacement of about 30 μm was obtained. (Evaluation) That is, according to the method of use of the example, the displacement amount increased by a maximum of 2.5 times as compared with the conventional use at room temperature.

[0019]

That is, according to the method of using the piezoelectric element of the present invention, an extremely high displacement amount can be obtained, so that the displacement magnifying mechanism which has been conventionally required is not required, and a compact piezoelectric actuator can be obtained and the cost can be reduced. Can be reduced.

[Brief description of drawings]

FIG. 1 is a graph showing changes in displacement amount corresponding to changes in temperature, residual polarization, and changes in d constant.

FIG. 2 is a block diagram of a control device used in an embodiment of the present invention.

FIG. 3 is a graph showing the relationship between electric field and polarization in the method of use of the present invention.

FIG. 4 is a graph showing the relationship between electric field and polarization in a conventional method of use.

[Explanation of symbols]

1: Piezoelectric actuator 2: Driving power supply
3: Heater 4: Temperature sensor 5: Heater control circuit
6: Heater power supply

Claims (1)

[Claims] 1. A method of using a piezoelectric element comprising a piezoelectric plate and a pair of electrodes formed on both front and back surfaces of the piezoelectric plate, wherein the piezoelectric plate has a temperature above a temperature at which residual polarization of the piezoelectric plate begins to decrease. A method of using a piezoelectric element, which comprises controlling the temperature within a Curie temperature range and applying a voltage equal to or higher than a coercive voltage of the piezoelectric plate to the pair of electrodes.