JPH0360082A - Stabilizing method for thermal expansion characteristic of piezoelectric element - Google Patents

Abstract

PURPOSE:To stabilize the size of a remaining polarization and to reduce an irregularity in thermal expansion characteristic of a piezoelectric element by conducting a polarizing method for performing once or repeating more than once of one cycle of voltage rising, keeping and voltage falling. CONSTITUTION:A piezoelectric element 11 is formed by alternately laminating piezoelectric ceramic layers 1 and inner electrode layers 2 in a structure in which the four side faces of the layer 2 are exposed. The opposed side faces of one set are alternately covered at every other layer with glass layers 3 to be electrically insulated. The side faces are further covered with silver paste to form outer electrode layers 4. As a result, the layers 1 are connected electrically in parallel and mechanically in series. The elements 11 are employed, for example, by polarizing at an ambient temperature, an element applying voltage is raised to 150V in 3 seconds, kept for 10 seconds, and fallen as one cycle, this quasi-step voltages are generated repeatedly six times. Thus, an irregularity in the thermal expansion coefficients of the elements can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for stabilizing the thermal expansion characteristics of a piezoelectric element used for precise position control or as a drive actuator when moving an object.

[Conventional technology]

Conventionally, piezoelectric elements have been applied to 7-cut units for precision positioning, such as those used for minute positioning of X-Y stages, minute control of the angle of polarizing mirrors in laser devices, and the production of electrical relays. There is an example.

These devices utilize strain caused by the inverse piezoelectric effect of a piezoelectric element directly, or mechanically expand this strain to a desired size using a lever principle.

In order to generate strain due to the inverse piezoelectric effect of a piezoelectric element, it is necessary to perform a so-called polarization operation in which a voltage is applied in advance to give spontaneous polarization to the element. This polarization operation causes residual polarization in the element even after the voltage is removed.

Generally, the length of a piezoelectric element changes at a rate of several PPM/V with respect to the applied voltage (here, the element length refers to the element dimension in the direction in which strain occurs parallel to the polarization direction). On the other hand, the temperature dependence of the element length is a big problem because it changes at a rate of several PPM/° C. with respect to temperature. The problem becomes even more serious when piezoelectric strain is mechanically expanded and utilized.

It is known that the temperature dependence of the element length of a piezoelectric element is a phenomenon that occurs because the above-mentioned residual polarization disappears with temperature.

[Problem to be solved by the invention]

The actuator using the conventional piezoelectric element described above is
It has the following drawbacks.

For example, if we consider the case where the element temperature is increased by controlling the ambient temperature, the element length will shrink. This is because residual polarization disappears due to thermal disturbance. That is, the magnitude of the reduction in element length is determined depending on the magnitude of residual polarization of the piezoelectric element.

On the other hand, the magnitude of residual polarization decreases over time even when left at room temperature because the free energy tends to decrease due to thermodynamic relaxation effects. As a result, the magnitude of residual polarization changes depending on the above-mentioned relaxation effect or the degree of thermal history.

That is, the thermal expansion coefficient of the piezoelectric element varies in accordance with the variation in the magnitude of its residual polarization. Furthermore, if the piezoelectric strain is mechanically amplified, the absolute value of this variation will also be amplified, as a matter of course.

An object of the present invention is to provide a method for stabilizing the thermal expansion characteristics of a piezoelectric element, which can stabilize the magnitude of residual polarization and reduce variations in the thermal expansion characteristics of the piezoelectric element.

[Means to solve the problem]

The method for stabilizing the thermal expansion characteristics of the piezoelectric element of the present invention is to use a polarization method in which voltage step-up, hold, and step-down are repeated one or more times as one cycle, or the above-mentioned voltage application cycle. In addition, the element temperature increases,
It is constructed by performing a polarization method that also includes keeping and cooling.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIG. 4 shows the structure of a laminated piezoelectric ceramic element 11 (hereinafter abbreviated as element) in which the effect of the thermal expansion property stabilization method according to the present invention was confirmed. Moreover, FIG. 5 is an enlarged view of the longitudinal section of FIG. 4. The piezoelectric ceramic layer 1 is made of lead zirconate titanate (PZT) and has a thickness of approximately 110 mm per layer.
It is μm.

The internal electrode layer 2 is made of a silver-palladium alloy and has a thickness of 1
Approximately 5 μm per layer.

This element has the piezoelectric ceramic layer 1 and the internal electrode layer 2 stacked alternately, and has a structure in which four sides of the internal electrode layer 2 are exposed. On one set of sides facing this,
As shown in FIG.
Cover and electrically insulate. Silver paste is further applied to this side surface to form an external electrode layer 4. As a result, the piezoceramic layers 1 are connected electrically in parallel and mechanically in series. The size of this element is 2 x 3 cabinets with a height of 10 mm.
It is mm. The coefficient of thermal expansion of this element is from -55℃ to 160℃.
The temperature was investigated up to ℃. The Curie point of this piezoelectric ceramic is approximately 1
The temperature is 45°C. As is well known, the piezoelectric effect does not occur above the Curie point. Furthermore, -55°C is the lower limit of the guaranteed usage range for electrical parts.

Therefore, the above temperature range covers the practical use range of this device.

As a measurement level, an unpolarized element was polarized in four ways to artificially change the magnitude of residual polarization.

The polarization method is shown in Table 1.

Table 1 Coefficient of Thermal Expansion Table 1 also lists the coefficient of thermal expansion (-1
The average temperature from 5°C to 100°C) is also shown. All polarizations were performed at room temperature. Level No. (■) in Table 1 is the polarization method according to the present invention. FIG. 1 shows the polarization method, with voltage on the vertical axis and time on the horizontal axis. According to FIG. 1, the voltage applied to the element is raised to 150 V in 3 seconds, held for 10 seconds, and a pseudo-step voltage is generated six times, with one cycle of dropping the voltage. The number of repetitions was determined based on the results shown in Figure 2. FIG. 2 shows the characteristics between the electromechanical coupling coefficient Rs s which is correlated with the number of polarization cycles and the magnitude of residual polarization. Here, using the resonance frequency Fit and anti-resonance frequency FA of the element, R is calculated according to equation (1) in FIG.
I asked for 33. According to this, the electromechanical coupling coefficient R33 is constant at about 70% for 6 cycles or more. Therefore, this was regarded as the saturation value at room temperature, and the repetition cycle was determined to be 6 times.

Figure 3 shows that the ambient temperature is controlled immediately after polarization using methods (II) and (Iv) in Table 1, and the element temperature is lowered to -55.
These are the results of thermal expansion measurements when the temperature was lowered to 160°C and then raised to 160°C. The reason why the length of the element using the polarization method shown in (■) decreases is mainly due to the disappearance of residual polarization. Therefore, in the case of an unpolarized element, the element length hardly changes. That is, if the magnitude of residual polarization differs, the coefficient of thermal expansion will apparently differ. In other words, it can be said that the coefficient of thermal expansion will vary unless a certain level of residual polarization is given during polarization. According to the results in Table 1, the coefficient of thermal expansion is 10. 305×
It varies from 10-'/'C to -7,385X 10-'/'C. The magnitude of remanent polarization that can be effectively standardized most easily is the saturated state or a state close to it. The results of measuring the thermal expansion coefficient (average from -15°C to 100°C) after several levels of polarization using the polarization method according to the present invention in (11) in Table 1 are shown. However, polarization was performed at room temperature.

Table 2 Coefficient of Thermal Expansion In Table 2, "Polarized" indicates that polarization according to the present invention was applied.

From Table 2, average value U+ = 7.380X10-'/℃ standard deviation σ
. -+=0.008X10-'/℃ Meanwhile, Table 2 (1)
When using an element with the same history as , and applying a polarization method of applying C, 150V for 30 seconds, the average value (tx = 5.841
10-'/'C standard deviation σ-1= 0.481 X
10-'/'C, and the variation was reduced by about two orders of magnitude. It goes without saying that if the piezoelectric ceramic material and polarization temperature are different, the appropriate number of polarization cycles will be different. Further, the functional form of voltage with respect to time (step-up, step-down form) may be arbitrary.

FIG. 6 shows a polarization method according to another embodiment of the present invention.

In this example, a step was added in which the device temperature was raised to around 145° C., which is the Keeley point, and after being kept for 30 minutes, the device was slowly cooled to room temperature. During this period, the element temperature rises until it cools down to room temperature.
Continue to apply C, 150V to the element. This polarization method was applied to an element having a history before polarization similar to that of the first example shown in Table 2, and the coefficient of thermal expansion (average from -15°C to 100°C) was investigated. Table 3 shows the results.

Table 3 Coefficient of Thermal Expansion In Table 3, "Polarized" indicates that the polarization according to the present invention has been applied.As a result, the average value "s" is 9.128 X10-'
/'C standard deviation σm-1=o. 003×10-'/°C. According to this method, the residual polarization increases to the maximum and the coefficient of thermal expansion increases, but the variation becomes smaller than in the first embodiment. Next, a third example will be described. The same laminated piezoelectric ceramic element 11 as shown in Table 2 of the first embodiment is assembled into the displacement expansion structure shown in FIG.
(Same as Example): The thermal expansion characteristics of the output end 16 were investigated using two polarization methods: applying D, C, 150V for 30 seconds and performing 6 cycles of increasing, keeping, and decreasing the voltage. Hinge 12 in FIG. Base 13. Arm 14 is made of 42% NiJS-Fe alloy and leaf spring 15. S-304 was used. Hinge 12. Base 13. The arm 14 was created by wire electrical discharge machining. The plate spring 15 was press-formed from a plate processed by wire electrical discharge machining, as shown in FIG. here 42
The coefficient of thermal expansion of %Ni-Fe alloy is 4.4 X 10-'
/'C15US 304's is 17.3 x 10-
@/°C. In the same polarization method as in the first embodiment of the present invention (see 4 in Table 2), there is an average temperature dependence of 0.6 μm/'C, that is, a temperature drift of the output end position, and its standard deviation is 0.01 p. m/'C, whereas in the polarization method of applying C, 150V for 30 seconds, the average was 1.2μ
m/'C, and the standard deviation was 0.6 μm/'C. Therefore, the variation in thermal expansion characteristics of the output end 16 can be reduced by about one order of magnitude.

〔Effect of the invention〕

As explained above, the present invention employs a method of polarizing a piezoelectric element by (1) repeating the cycle of increasing, keeping, and decreasing the voltage one or more times; (2) increasing the element temperature in conjunction with (1); By either keeping the temperature or decreasing the temperature, it is possible to reduce variations in the coefficient of thermal expansion of the piezoelectric element.

Furthermore, as a result, even when a piezoelectric element is incorporated into a displacement magnification mechanism, variations in temperature drift in the output end position can be reduced.

[Brief explanation of drawings]

FIG. 1 is a voltage-time correlation diagram showing the polarization method of an embodiment of the present invention, FIG. 2 is a characteristic diagram between the number of polarization cycles and the electromechanical coupling coefficient according to the polarization method of the first embodiment of the present invention, and FIG. Figure 3 is a diagram of thermal expansion characteristics in the case of polarization and non-polarization according to the first embodiment of the present invention, Figure 4 is a perspective view of a laminated piezoelectric ceramic element to which the polarization method of the present invention is applied, and Figure 5 is a perspective view of a device. The figure is an enlarged longitudinal cross-sectional view of FIG. 4, FIG. 6 is a correlation diagram of voltage and time, temperature and time, showing the polarization method according to another embodiment of the present invention, and FIG. 7 is a diagram showing the polarization method according to the present invention. It is a perspective view of the applied piezoelectric actuator with a displacement magnifying mechanism. DESCRIPTION OF SYMBOLS 1...Piezoelectric ceramic layer, 2...Internal electrode layer, 3...Glass layer, 4...External electrode layer, 11... Laminated piezoelectric ceramic element, 1
2...Hinge, 13...Base, 14
...Arm, 15...Plate fly, 16.
...Output end, 20...Piezoelectric actuator with displacement magnification mechanism.

Claims (2)

[Claims] (1) A method for stabilizing the thermal expansion characteristics of a piezoelectric element, which is characterized by employing a polarization method in which voltage step-up, voltage hold, and voltage step-down are repeated one or more times as one cycle. (2) A method for stabilizing thermal expansion characteristics of a piezoelectric element according to claim (1), characterized in that the temperature of the element is increased, maintained, and lowered at the same time.