JP2004319966A - Piezoelectric actuator and piezoelectric ceramic - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a piezoelectric actuator and a piezoelectric ceramic which hardly have characteristic variation due to thermal hysteresis. <P>SOLUTION: THe piezoelectric actuator is constituted by laminating internal layers and internal electrode layers alternately. Here, electric capacity under electric field intensity of ≥1.0 kV/mm at temperature T1 is denoted as C1, and electric capacity under electric field intensity of ≥1.0 kV/mm at T1 right after thermal hysteresis of temperature T2 which is ≥100°C different from T1 is given is denoted as C2; and the rate of variation in electric capacity from C1 to C2 is ≤5.5% and dielectric loss is ≤8.2%. The piezoelectric layer is formed of a piezoelectric ceramic obtained by adding assistant oxide to a piezoelectric material represented as a chemical formula PbZrO<SB>3</SB>-PbTiO<SB>3</SB>-Pb(Y<SB>1/2</SB>Nb<SB>1/2</SB>)O<SB>3</SB>-Pb(Mn<SB>1-z</SB>W<SB>z</SB>)O<SB>3</SB>having part of Pb substituted for by one or more kinds selected out of Ba, Ca, and Sr and satisfies 0.25≤z≤0.40 and baking them. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a piezoelectric actuator that can be used as a drive source of an injector for a fuel injection device, and a piezoelectric ceramic that can be used for the piezoelectric actuator.

In some cases, a piezoelectric actuator is used as a drive source in an injector for a fuel injection device used in an internal combustion engine such as a vehicle engine. The piezoelectric actuator is composed of a laminated piezoelectric element in which piezoelectric layers composed of piezoelectric ceramics and internal electrode layers are alternately laminated.
Further, as the piezoelectric ceramic, a piezoelectric ceramic made of lead zirconate titanate or the like is often used.

JP-A-8-183660

  By the way, in the vicinity of the injector in the use environment described above, the temperature of the environment changes rapidly, and the characteristics of the injector change due to the heat history, which may cause a problem in controlling the fuel injection amount. This problem is caused by the fact that the characteristics of the piezoelectric actuator fluctuate due to the thermal history.

  The present invention has been made in view of such a conventional problem, and has as its object to provide a piezoelectric actuator and a piezoelectric ceramic having stable characteristics even when subjected to a heat history.

A first invention is a piezoelectric actuator in which a piezoelectric layer composed of a piezoelectric ceramic and an internal electrode layer are alternately laminated,
Arbitrary temperature: The electric capacity under an electric field strength of 1.0 kV / mm or more at T1 is defined as C1, and 1.0 kV at T1 immediately after receiving a thermal history at a temperature T2 having a difference of 100 ° C. or more from T1. Wherein the rate of change from C1 to C2 is 5.5% or less, where C2 is the capacitance under an electric field strength of / mm or more (provided that -40 ° C ≦ T1 <T2 ≦ 200 ° C.) (Claim 1).

The operation and effect according to the first invention will be described.
In the piezoelectric actuator according to the first aspect of the invention, the change rate of the electric capacity under an electric field strength of 1.0 kV / mm or more before and after the thermal history given in the above temperature range is 5.5% or less.
The piezoelectric actuator according to the first invention has a stable electric capacity.
When the electric capacity is stabilized, the variation in the transient characteristic of displacement is reduced, and the extension speed of the piezoelectric layer when energy is applied, that is, the extension speed of the piezoelectric actuator becomes substantially constant.

  Further, the second invention has a relative permittivity at an arbitrary temperature: T1 under an electric field strength of 1.0 kV / mm or more as ε1, and receives a thermal history at a temperature T2 having a difference of 100 ° C. or more from T1. The relative dielectric constant under an electric field strength of 1.0 kV / mm or more at T1 immediately after the temperature is ε2, and the rate of change from ε1 to ε2 is 5.5% or less. However, -40 ° C ≦ T1 <T2 ≦ 200 ° C.).

  The piezoelectric ceramic according to the second invention has a relative dielectric constant change rate of 5.5% or less under an electric field strength of 1.0 kV / mm or more before and after the thermal history given in the above temperature range. A laminated piezoelectric element having a piezoelectric layer made of porcelain has a stable electric capacity, and the stable electric capacity reduces variations in the transient characteristics of displacement. Therefore, the elongation speed of the piezoelectric layer when energy is applied, that is, the elongation speed of the piezoelectric layer becomes substantially constant.

  As described above, according to the first and second inventions, it is possible to provide a piezoelectric actuator and a piezoelectric ceramic having stable characteristics even when subjected to a thermal history.

The electric field strength at the time of driving according to the first invention is a range of electric field strength applied when the piezoelectric actuator is driven in a normal use environment, and the temperature range is a temperature range applied to a normal use environment of the piezoelectric actuator.
Therefore, if the rate of change of the electric capacity before and after the thermal history is 5.5% or less in the above-mentioned electric field strength range and temperature range, the characteristic change due to the thermal history hardly occurs when the piezoelectric actuator is actually used.
If the rate of change of the electric capacity is greater than 5.5%, the change in the transient characteristic of displacement becomes large, and the effect according to the present invention may be lost.
It is most preferable that the rate of change in electric capacity before and after the heat history becomes 0%.

The electric capacity before and after the heat history will be described.
While driving the piezoelectric actuator at a predetermined drive voltage (electric field strength), the piezoelectric actuator was heated from the temperature T1 to the temperature T2 and returned to the temperature T1 again.
Assuming that the electric capacity of the piezoelectric actuator before heating is C1, the electric capacity of the piezoelectric actuator after heating, cooling, and returning to the temperature T1 again is C2, the value of 100 × (C2−C1) / C1 is the first value. The piezoelectric actuator according to the invention of (1) is 5.5% or less.

In the first (and second) invention, the range of the temperature T1 to the temperature T2 to the temperature T1 is selected from −40 ° C. to 200 ° C., and the electric capacitance is measured while driving the piezoelectric actuator. The strength is set to 1.5 kV / mm or more.
The detailed measurement method is described in Example 1.

More preferably, the rate of change is 5% or less (claim 2).
As a result, the change in the transient characteristic of the displacement is reduced, and the effect of the present invention can be further enhanced.
More preferably, the rate of change is 4.8% or less (claim 3).
As a result, the change in the transient characteristic of the displacement is reduced, and the effect of the present invention can be further enhanced.

Preferably, the piezoelectric actuator has a dielectric loss of 8.2% or less (claim 4).
When the dielectric loss is low, self-heating of the piezoelectric actuator can be suppressed. If the dielectric loss is larger than 8.2%, the amount of self-heating when driving the piezoelectric actuator is increased, and the durability of the piezoelectric actuator may be reduced.

Preferably, the dielectric loss is 8% or less (claim 5).
As a result, the amount of self-heating when driving the piezoelectric actuator becomes smaller, and the effect of the present invention can be further increased.
Further, the dielectric loss is preferably 0.5% or more (claim 6).
When the dielectric loss is less than 0.5%, the displacement performance, which is the basic performance of the piezoelectric actuator, may be greatly reduced in piezoelectric ceramics (such as a lead zirconate titanate-based material) generally used for a piezoelectric layer. is there.

Next, in order to obtain a piezoelectric actuator in which the rate of change in electric capacitance before and after the heat history is 5.5% or less, a chemical formula of PbZrO ₃ − in which a part of Pb is replaced by one or more selected from Ba, Ca, and Sr. _{PbTiO 3 -Pb (Y 1/2 Nb 1/2} ) O 3 -Pb (Mn 1-z W z) piezoelectric material a O ₃ system is 0.25 ≦ z ≦ 0.40 with respect to, Pb oxide It is preferable to form a piezoelectric layer from a piezoelectric ceramic which is fired by adding an auxiliary oxide containing both an oxide and a W oxide (claim 7).

Piezoelectric ceramic according to claim 7, perovskite type having a composition according to Formula _{PbZrO 3 -PbTiO 3 -Pb (Y 1/2} Nb 1/2) O 3 -Pb (Mn 1-z W z) O 3 It is a material in which a part of Pb is replaced with another element (Ba, Ca, Sr) in the compound having the structure.
By replacing Pb with Ba, Ca, and Sr, a large strain is introduced into the perovskite crystal lattice, and a larger piezoelectric effect can be exhibited. Therefore, the piezoelectric actuator including the piezoelectric layer made of the piezoelectric ceramic according to claim 7 can exhibit a large displacement.

In the piezoelectric ceramic according to claim 7, the ratio of W as the donor element and Mn as the acceptor element is set in an appropriate range. Thereby, the generation amount of oxygen defects in the crystal lattice in the piezoelectric material can be reduced, and the change with time in the electric capacity can be suppressed.
Further, by using the auxiliary oxide containing both the Pb oxide and the W oxide according to claim 7, the firing temperature of the piezoelectric material is lowered, and the lower-cost electrode material (Cu, Ag, Pd content is reduced). A piezoelectric actuator can be manufactured using 10 wt% or less of an Ag-Pd alloy. Therefore, a low-cost piezoelectric actuator can be obtained.
When z <0.25, there is a possibility that the change over time in the electric capacity of the piezoelectric actuator becomes large. If z> 0.4, the displacement performance of the piezoelectric actuator may be reduced.

The amount of the auxiliary oxide added to the piezoelectric material is preferably 0.05 to 5 wt% (outer wt%) based on 100 wt% of the piezoelectric material.
If the content is less than 0.05 wt%, the effect of lowering the sintering temperature may be difficult to obtain. If it is more than 5 wt%, the displacement performance of the piezoelectric actuator may be reduced. This is because auxiliary oxides that do not contribute to displacement increase in the piezoelectric material.

Here, the auxiliary oxide containing both Pb oxide and W oxide is a mixture containing Pb oxide and W oxide, or a composite oxide of Pb and W.
Specifically, an auxiliary oxide composed of a Pb oxide and a W oxide, an auxiliary oxide composed of a composite oxide of Pb and W, and an auxiliary composed of a composite oxide of a Pb oxide and Pb and W An oxide, an auxiliary oxide composed of a W oxide and a composite oxide of Pb and W, and an auxiliary oxide composed of a Pb oxide, a W oxide and a composite oxide of Pb and W according to claim 7 It corresponds to an auxiliary oxide.

Further, the piezoelectric actuator can be used as a drive source of an injector for a fuel injection device.
The piezoelectric actuator according to the first invention has a stable electric capacity under a specific electric field strength with respect to the thermal history at the above-described specific range of temperature.
Therefore, variation in the transient characteristics of displacement is reduced, and the extension speed of the piezoelectric layer when a voltage is applied, that is, the extension speed of the piezoelectric actuator becomes substantially constant.
By using such a piezoelectric actuator as a drive source of an injector for a fuel injection device in an internal combustion engine such as a vehicle engine, the variation in fuel injection timing and the variation in fuel injection amount are reduced, and the combustion state control is reduced. Can be performed more precisely.

  In other words, as compared with conventional positioning applications, the use of the piezoelectric actuator is more severe in the case of an internal combustion engine such as an engine for a vehicle, and is used from normal temperature before starting (low temperature of -40 ° C. in some cases). It will be exposed to a heat history of medium high temperature (in some cases, about 200 ° C.) and return to normal temperature after the engine stops.

Here, a description will be given of a point that a change in the electric capacity of the piezoelectric actuator leads to variation in the fuel injection amount,
FIG. 9 is a diagram illustrating a relationship between a change in time and a displacement when constant energy is injected into two piezoelectric actuators having different electric capacities. The electric capacity of the piezoelectric actuator A is Ca, the electric capacity of the piezoelectric actuator B is Cb, and Cb> Ca.
As is clear from the figure, the time required to reach the displacement x is shorter when the capacitance is smaller than when the displacement is larger. That is, if the time required for the piezoelectric actuator A (capacitance Ca) to reach the displacement x is Ta and the time required for the piezoelectric actuator B (capacitance Cb) to reach the displacement x is Tb, then Ta <Tb.
The difference in electric capacity changes the charge injection speed, and as a result, the transient characteristics leading to displacement will differ.

Assuming that the piezoelectric actuator of the same individual receives the thermal history and the electric capacity increases from Ca to Cb, the time required to reach the predetermined displacement x changes before and after the thermal history. Assuming that the displacement x is a displacement amount required for the piezoelectric actuator to open and close the fuel injection valve in the fuel injection device, the injection timing changes each time the electric capacity changes.
Therefore, the piezoelectric actuator according to the first invention can maintain an excellent and stable electric capacity when it receives a thermal history as described above, so that the piezoelectric actuator according to the first invention can be used for a fuel injection device. By using it as a drive source for the injector, variation in the injection amount of the fuel injection device is suppressed, and highly accurate control is possible.
Therefore, excellent effects such as improvement of engine performance such as improvement of fuel efficiency and reduction of exhaust gas can be obtained.

In the piezoelectric ceramic according to the second aspect of the present invention, the change rate of the relative dielectric constant before and after the heat history in a predetermined electric field intensity range and a temperature range is 5% or less. The electric field intensity range is an electric field intensity range applied when the piezoelectric actuator is driven in a normal use environment, and the temperature range is a temperature range applied to the piezoelectric actuator in a normal use environment.
Therefore, a piezoelectric actuator having a piezoelectric layer composed of a piezoelectric ceramic having a relative dielectric constant change rate of 5% or less before and after the thermal history in the electric field intensity range and the temperature range hardly causes a characteristic change due to the thermal history in actual use. .

  If the change rate of the relative dielectric constant is greater than 5.5%, in the piezoelectric actuator having the piezoelectric layer composed of the piezoelectric ceramic, the change in the transient characteristic of displacement becomes large, and the effect according to the present invention may be lost. . It is most preferable that the relative dielectric constant change rate is 0% before and after the heat history.

More preferably, the rate of change is 5% or less (claim 10).
As a result, the change in the transient characteristic of the displacement is reduced, and the effect of the present invention can be further enhanced.
More preferably, the rate of change is 4.8% or less (claim 11).
As a result, the change in the transient characteristic of the displacement is reduced, and the effect of the present invention can be further enhanced.

In the piezoelectric ceramic according to the second invention, the dielectric loss is preferably 8.2% or less (claim 12).
When the dielectric loss is low, self-heating can be suppressed during driving in the piezoelectric actuator having the piezoelectric layer made of the piezoelectric ceramic. If the dielectric loss is larger than 8.2%, the amount of self-heating when driving the piezoelectric actuator is increased, and the durability of the piezoelectric actuator may be reduced.

Further, the dielectric loss is preferably 8% or less (claim 13).
As a result, the amount of self-heating when driving the piezoelectric actuator becomes smaller, and the effect of the present invention can be further increased.
Preferably, the dielectric loss is 0.5% or more.
When the dielectric loss is less than 0.5%, the displacement performance, which is the basic performance of the piezoelectric actuator, may be greatly reduced in piezoelectric ceramics (such as a lead zirconate titanate-based material) generally used for a piezoelectric layer. is there.

In order to obtain a piezoelectric ceramic having a relative dielectric constant change rate of 5.5% or less before and after the heat history, a chemical formula of PbZrO ₃ − in which a part of Pb is replaced with one or more selected from Ba, Ca, and Sr. _{PbTiO 3 -Pb (Y 1/2 Nb 1/2} ) O 3 -Pb (Mn 1-z W z) piezoelectric material a O ₃ system is 0.25 ≦ z ≦ 0.40 with respect to, Pb oxide It is preferable to use a material obtained by adding and sintering an auxiliary oxide containing both a material and a W oxide (claim 15).

The piezoelectric porcelain in Chemical Formula _{PbZrO 3 -PbTiO 3 -Pb (Y 1/2} Nb 1/2) O 3 -Pb (Mn 1-z W z) compounds of the perovskite structure having such composition O ₃ , Pb is a compound in which a part of Pb is replaced by another element (Ba, Ca, Sr).
By this substitution, a large strain is introduced into the perovskite crystal lattice, and a larger piezoelectric effect can be exhibited. Therefore, the piezoelectric actuator including the piezoelectric layer made of the piezoelectric ceramic can exhibit a large displacement.

In addition, since the ratio of the donor element W and the acceptor element Mn is set in an appropriate range, the generation amount of oxygen defects in the crystal lattice in the piezoelectric material is reduced, and the change in the relative dielectric constant with time is suppressed. can do.
In addition, by using the auxiliary oxide, the firing temperature of the piezoelectric material is reduced, and the piezoelectric actuator is formed using a less expensive electrode material (eg, an Ag-Pd alloy having a Cu, Ag, or Pd content of 10 wt% or less). Can be made. Therefore, a piezoelectric material suitable for a piezoelectric actuator which is inexpensive can be obtained.
When z <0.25, there is a possibility that the change with time of the relative dielectric constant becomes large. When z> 0.4, the displacement performance may be reduced.

The amount of the auxiliary oxide added to the piezoelectric material is preferably 0.05 to 5 wt% (outer wt%) based on 100 wt% of the piezoelectric material.
If the content is less than 0.05 wt%, the effect of lowering the sintering temperature may be difficult to obtain. If the content is more than 5 wt%, the displacement performance of the piezoelectric ceramic may decrease. This is because auxiliary oxides that do not contribute to displacement increase in the piezoelectric material.
Here, the auxiliary oxide containing both Pb oxide and W oxide is a mixture containing Pb oxide and W oxide, or a composite oxide of Pb and W.
Specifically, an auxiliary oxide composed of a Pb oxide and a W oxide, an auxiliary oxide composed of a composite oxide of Pb and W, and an auxiliary composed of a composite oxide of a Pb oxide and Pb and W An oxide, an auxiliary oxide composed of a W oxide and a composite oxide of Pb and W, and an auxiliary oxide composed of a Pb oxide, a W oxide and a composite oxide of Pb and W according to claim 7 It corresponds to an auxiliary oxide.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(Example 1)
The piezoelectric actuator according to the present example is composed of a laminated piezoelectric element in which piezoelectric layers composed of piezoelectric ceramics and internal electrode layers are alternately laminated.
An electric capacity at an arbitrary temperature: T1 under an electric field strength of 1.0 kV / mm or more is defined as C1, and 1 at T1 immediately after receiving a thermal history of a temperature T2 having a difference of 100 ° C. or more from T1. Assuming that the capacitance under an electric field strength of 0.0 kV / mm or more is C2, the rate of change from C1 to C2 is 5.5% or less (provided that -40 ° C ≦ T1 <T2 ≦ 200 ° C). Further, the dielectric loss is 8.2% or less.

Moreover, the piezoelectric layer, a portion of the Pb Ba, Ca, Formula was replaced with at least one member selected from _{Sr PbZrO 3 -PbTiO 3 -Pb (Y} 1/2 Nb 1/2) O 3 -Pb (Mn _1-z W _z ) Piezoelectric material obtained by adding an auxiliary oxide containing both Pb oxide and W oxide to a piezoelectric material based on O ₃ and satisfying 0.25 ≦ z ≦ 0.40, followed by firing. Consists of porcelain.
And the piezoelectric actuator of this example can be used as a drive source of an injector for a fuel injection device.

Hereinafter, the piezoelectric actuator of this example will be described in detail.
As shown in FIG. 1, the piezoelectric actuator 1 of the present embodiment includes a unit laminated body 11 formed by joining 20 piezoelectric element units 15.
Each piezoelectric element unit 15 is formed by alternately laminating a piezoelectric layer 151 made of the above-described piezoelectric ceramic and internal electrode layers 153 and 154 made of Ag and Pd. Although not shown in detail in the drawings for the sake of drawing convenience, the number of stacked piezoelectric layers 151 as piezoelectric elements in each piezoelectric element unit 15 is 20.

  The internal electrode layers 153 and 154 are formed so as to reach only one side surface of the piezoelectric layer 151 as shown in FIGS. External electrodes 5 and 6 made of Ag and connected to external power supplies having different potentials are provided on the side surface of the piezoelectric element unit 15 so as to sandwich the piezoelectric element unit 15. The internal electrode layers 153 and 154 are electrically connected alternately in the stacking direction to the external electrodes 5 and 6 on the side surfaces of the piezoelectric element unit 15. Therefore, in the piezoelectric actuator 1, two adjacent internal electrode layers 153 and 154 are connected to electrodes having different potentials.

  As shown in FIG. 1, the piezoelectric actuator 1 of this example has a connection member 4 made of alumina having a Young's modulus larger than that of the piezoelectric element unit 15 and laminated at the end of the unit laminated body 11 in the laminating direction. Become. The buffer unit 2 having a smaller displacement than the piezoelectric element unit 15 and the dummy unit 3 having a smaller Young's modulus than the connecting member 4 are interposed between the unit laminated body 11 and the connecting member 4. The buffer unit 2 and the dummy unit 3 are made of the same piezoelectric ceramic as the piezoelectric layer 151 of the piezoelectric element unit 15.

  As shown in FIG. 1, the buffer unit 2 includes a piezoelectric layer 251 and internal electrode layers 253 and 254. The thickness of the piezoelectric layer 251 in the buffer unit 2 is substantially equal to the thickness of the piezoelectric layer 151 in the piezoelectric element unit 15 on the unit laminated body 11 side, and is larger than the thickness of the piezoelectric layer 151 on the connection member 4 side. The external electrodes 5 and 6 are connected to the internal electrode layers 253 and 254, respectively, with the side surfaces of the buffer unit 2 interposed therebetween, as in the piezoelectric element unit 15.

  Further, the dummy unit 3 is configured by laminating piezoelectric layers similar to the piezoelectric layer 151 and the piezoelectric layer 251, but has no internal electrode layer, and does not sandwich the side surface between the external electrodes 5 and 6. Therefore, it is a place that does not move when the piezoelectric actuator 1 is driven.

Next, a method for manufacturing the piezoelectric actuator 1 of the present example will be described.
The piezoelectric actuator 1 of this example can be manufactured by a widely used green sheet method. This green sheet is prepared as follows.

That is, it is weighed by a known method so as to obtain a composition of a piezoelectric ceramic for obtaining a powder such as lead oxide, zirconium oxide, titanium oxide, niobium oxide, or strontium carbonate, which is a main raw material of the piezoelectric ceramic. At this time, the lead is weighed so as to be rich in 1 to 2 wt% in consideration of the evaporation of the lead. After wet mixing these raw materials, they are calcined at 750 to 950 ° C. to form calcined powder.
Also, powders of lead oxide and tungsten oxide, which are the raw materials of the auxiliary oxide, are weighed so as to have the composition of the auxiliary oxide to be obtained, wet-mixed, and then calcined at 500 ° C. The calcined powder is wet-pulverized and then dried and used as an auxiliary oxide.

  Next, pure water is added to the calcined powder to form a slurry, which is wet-pulverized by a medium stirring mill. After the pulverized product is dried and degreased, a solvent, an auxiliary oxide, a binder, a plasticizer, a dispersant, and the like are added and mixed by a ball mill. Thereafter, the slurry is subjected to vacuum defoaming and viscosity adjustment while being stirred by a stirrer in a vacuum device.

  Next, the slurry is formed into a green sheet having a predetermined thickness by a doctor blade device. The green sheet is punched by a press or cut by a cutter to form a rectangular body having a predetermined size.

Next, a pattern is screen-printed on one surface of the green sheet after molding using a paste of silver and palladium having a ratio of, for example, silver / palladium = 7/3 (hereinafter, referred to as an Ag / Pd paste).
On the surface of the green sheet, a slightly smaller pattern was formed on almost the entire surface by the Ag / Pd paste (see FIG. 2). This is the printed portion for the internal electrode layer 153 (154). A printed portion for the internal electrode layer 153 (154) is not formed on one side of the opposite side of the surface of the green sheet, and becomes an electrode non-formed portion after firing. That is, the internal electrode layer 153 (154) does not reach one end of the opposite side of the green sheet (the portion corresponding to the side surface of the piezoelectric actuator), and the internal electrode layer 153 (154) 154) was formed.

A predetermined number of the green sheets on which the printed portions for the internal electrode layers 153 are formed are prepared based on the required specifications of the displacement of the unit laminate 11 and the buffer unit 2. Also, a required number of green sheets on which the internal electrode layers 153 (154) are not printed are prepared.
Next, these green sheets were laminated so that the unit piezoelectric element 15 according to FIG. 1 was obtained.
In this way, 21 green sheets on which the printed portions for the internal electrode layers 153 (154) are formed are superimposed, and the green sheets on which the printed portions are not formed are superimposed on top of each other. A laminate composed of green sheets was prepared.

  Next, after thermocompression bonding of the laminate, the laminate was degreased in an electric furnace at a temperature of 400 to 700 ° C, fired at a temperature of 900 to 1100 ° C, and then ground into a desired shape. As a result, the green sheet became the piezoelectric layer 151, and the piezoelectric element unit 15 in which the piezoelectric layers 151 and the internal electrode layers 153 and 154 were alternately laminated was obtained. The piezoelectric element unit 15 has 20 active piezoelectric layers as piezoelectric elements. Further, 20 piezoelectric element units 15 were produced in the same manner as described above.

Next, a laminate for the buffer unit 2 was produced by laminating 19 green sheets on which printed portions for the internal electrode layers were formed in the same manner as that used for producing the piezoelectric element unit 15. In the laminated body for the buffer unit 2, a green sheet on which no internal electrode layer is formed is interposed between two layers on the connection member 4 side after the piezoelectric actuator 1 is assembled, and The thickness was set to be twice the thickness of the piezoelectric layer 153 (154) of the piezoelectric element unit.
Further, on the outermost side on the connection member 4 side, a green sheet on which no internal electrode layer is formed is further stacked.
Subsequently, the laminate for the buffer unit was thermocompression-bonded, degreased, and then fired in the same manner as described above to produce a buffer unit.

Next, 20 green sheets on which the internal electrode layers were not formed were stacked to produce a laminate for a dummy unit. Then, the laminate for the dummy unit was thermocompression-bonded, degreased, and fired in the same manner as described above to obtain the dummy unit 3.
The connecting member 4 was manufactured by processing an alumina sintered body block into a desired shape.

Next, the piezoelectric element unit 15, the buffer unit 2, the dummy unit 3, and the connecting member 4 manufactured as described above were stacked, and the piezoelectric actuator 1 was manufactured as follows.
Specifically, first, the external electrodes 5 and 6 made of Ag were formed by baking so as to sandwich the side surfaces of the piezoelectric element unit 15 and the buffer unit 2.

The external electrode 5 is formed at a position where the internal electrode layer 153 or the internal electrode layer 253 of one of the electrodes is exposed in the piezoelectric element unit 15 and the buffer unit 2, and the internal electrode layer 153 or the internal electrode layer 253 is formed. Conduct continuity.
The external electrode 6 is formed at a position where the internal electrode layer 154 or the internal electrode layer 254 of the other pole is exposed, and the internal electrode layer 154 or the internal electrode layer 254 is conducted.
Thereafter, a direct current voltage was applied from the external electrodes 5 and 6 to the internal electrode layers 153 and 154 of the piezoelectric element unit 15 on which the external electrodes 5 and 6 were formed and the internal electrode layers 253 and 254 of the buffer unit 2 to polarize.

Next, as shown in FIG. 1, the 20 piezoelectric element units 15 polarized as described above are stacked on the joint surface 155, the buffer unit 2 is stacked on both ends thereof, and the dummy unit 3 is further formed on both ends thereof. Further, connection members 4 were stacked on both ends.
Thus, the piezoelectric actuator 1 was manufactured as shown in FIGS.
The evaluation of the piezoelectric actuator 1 will be described in a second embodiment.

(Example 2)
In this example, a piezoelectric actuator as shown in Example 1 was manufactured using the piezoelectric materials according to Samples 1 to 8, and the performance thereof was evaluated.
The piezoelectric layer of this example is a piezoelectric ceramic (Pb _0.91 Sr _0.045 Ba _0.045 ) _1.001 (Zr _0.526 Ti _0.474 ) _0.99-x (Y _1/2 Nb _1/2 ) _0.01 (Mn _1-z W _z ) _x O _{In 3.001} , 0.835 PbO.0.165 WO ₃ as an auxiliary oxide is mixed with 0.5 wt% (outer wt%) with respect to 100 wt% of the piezoelectric ceramic. X and Z take values as shown in Table 1.

Next, a method for measuring the capacitance of the piezoelectric actuator will be described.
The measurement circuit 7 according to FIG. 4 is prepared.
In this measurement circuit 7, the capacitance of the piezoelectric actuator 1 is Cp, the capacitance of the capacitor 73 connected in series with the piezoelectric actuator 1 in the measurement circuit 7 is C0, and the voltage applied to the piezoelectric actuator 1 is the input terminal 712 and the ground terminal 711. And the potential difference between the terminal 731 connected to both ends of the capacitor 73 and the ground terminal 732 as Vout.

Further, when a voltage is applied between the terminal 712 and the ground terminal 711, a voltage when the voltage is applied is Vin (ON) and Vout (ON), and when no voltage is applied, the voltage is Vin (OFF) and Vout (OFF). The following relationship is established between these capacitances and potential differences.
Cp = C0 × {Vout (ON) -Vout (OFF)} / [Vin (ON)-{Vout (ON) -Vout (OFF)}]

  Then, the piezoelectric actuator 1 is set in a constant temperature bath with the piezoelectric actuator 1 connected to the measurement circuit 7 and maintained at a temperature of 25 ° C., and a voltage of 150 V (= Vin (ON)) at which the electric field strength becomes 1.8 kV / mm is applied. Actuator 1 was driven. The electric capacity was measured in this state.

  Next, the temperature was raised to 160 ° C. for 30 minutes while the piezoelectric actuator 1 was short-circuited, and the temperature was returned to 25 ° C. to give a thermal history. Thereafter, the piezoelectric actuator 1 was driven at a voltage of 150 V (= Vin (ON)) in the same manner as before the heat history. The electric capacity was measured in this state.

  The electric capacity before and after the heat history obtained from the above measurement is [{electric capacity after heat history (25 ° C.) − Electric capacity before heat history (25 ° C.)} / Electric capacity before heat history (25 ° C.)] The change rate of the electric capacity was obtained by substituting into an equation of × 100. The results are shown in Table 1.

Next, dielectric loss was determined from the PE hysteresis (charge-voltage hysteresis) of the piezoelectric actuator 1, and the relationship with the temperature rise of the piezoelectric actuator due to self-heating was measured.
First, the PE hysteresis was determined as follows.
A measurement jig 8 as shown in FIG. 5 was prepared.
The measuring jig 8 has two guide poles 82 erected on a pedestal 81. A bridge portion 83 is provided on the guide pole 82, and a pressing portion 86 having a pressing plate 841 is provided at the center of the bridge portion 83. The piezoelectric actuator 1 is disposed so as to be sandwiched between the pressing plate 841 and the pedestal 81. The pressing plate 841 applies a preload (preset load) of 10 MPa by a disc spring 85 disposed between the pressing plate 841 and the bridge portion 83.

In this state, an electric field strength of 0 to 1.5 kV / mm, a frequency of 100 Hz, and a sine wave voltage are applied to the piezoelectric actuator 1, and a hysteresis as shown in FIG. 6 is obtained. In the figure, the horizontal axis indicates E: electric field (V / mm), and the vertical axis indicates P: integrated value of electric current (charge density) (C / mm ² ). Then, the locus of P was plotted when the voltage was gradually increased in the same direction as the polarization direction and then gradually decreased. As shown in the figure, the area of the portion surrounded by the obtained hysteresis curve is defined as S1.

Next, as shown in FIG. 7, an area S2 of a line connecting the origin O and the highest point Q of P, a line hanging down from the above Q on the horizontal axis, and a triangle surrounded by the horizontal axis is obtained. The dielectric loss can be calculated by using these values. That is, the dielectric loss (%) can be calculated by S1 / S2 / (2π) × 100.
The dielectric loss obtained in this way makes it possible to estimate the dielectric loss that occurs when the piezoelectric actuator is actually used, as compared with the dielectric loss measured at 1 V by an impedance analyzer that is normally used. .
Table 1 shows the dielectric loss obtained by this method.

Next, measurement of the transient characteristics of the displacement of the piezoelectric actuator will be described.
There is a correlation between the responsiveness of a piezoelectric actuator and the responsiveness of an injector employing the piezoelectric actuator as a drive source.

If the variation in the transient characteristics of the displacement of the piezoelectric actuator is reduced, the extension speed of the piezoelectric actuator, that is, the extension speed of the piezoelectric layer when a voltage is applied becomes substantially constant. If the elongation speed of the piezoelectric actuator is constant, the fuel injection timing in the injector will be constant without being advanced or delayed, so it is necessary to prevent variation in the injection amount and control the combustion state more strictly. Can be.
From the above viewpoints, the change (variation) in the transient characteristics of the piezoelectric actuator composed of the piezoelectric ceramic applied to each sample before and after the heat history of 160 ° C. was evaluated at a temperature of 25 ° C. and an injection energy of 80 mJ, and is shown in Table 1.

  A piezoelectric actuator is set on a jig 8 as shown in FIG. 5, and 10 MPa is applied as a preload (preset load). The piezoelectric actuator is expanded and contracted by connecting the control circuit of the piezoelectric actuator and repeating charging and discharging at 5 Hz. A capacitance type gap sensor (not shown) was arranged above the head 86, and the behavior of the piezoelectric actuator during the charging process (the behavior of the displacement amount with respect to time) was measured with a digital oscilloscope.

  In Table 1, when the transient characteristic is x, the transient characteristic changes greatly before and after the heat history, and when used for an injector for fuel injection, there is a possibility that the injection amount may vary. Indicates that there is a slight change in the transient characteristics before and after the heat history, and when used for an injector for fuel injection, the injection amount may vary slightly, but it is not so much as to cause a problem. This shows a state in which there is almost no change in the transient characteristics before and after the heat history, and there is no possibility that the injection amount will vary when used in an injector for fuel injection.

  By the way, it is clear from Table 1 that the change rate of the electric capacity of the samples 1 and 2 greatly exceeds 5.5%. The variation in the transient characteristics of the displacement of the piezoelectric actuator composed of these two samples was as large as x. Further, since the change rate of the capacitance of the sample 3 is 5.5%, the variation of the transient characteristic of the displacement becomes ○, and the change rate of the capacitance of the samples 4 to 8 is 4.8% or less, and the transient of the displacement is less. The variation in the characteristics was as small as ◎.

Furthermore, as is clear from Table 1, the smaller the rate of change in the capacitance of the piezoelectric actuator, the greater the dielectric loss.
Then, with respect to Samples 5 to 8 in which the rate of change in electric capacity was 1% or less, 80 mJ of energy was applied at 120 ° C., and the sample was driven at 200 Hz.
According to Table 2, the temperature of the piezoelectric actuator according to the sample 5 having the smallest dielectric loss was the lowest, and the durability was excellent.

Here, durability refers to a state in which the displacement performance and mechanical strength of the piezoelectric actuator have deteriorated due to self-heating, indicating that the displacement performance and mechanical strength have decreased. Is not as low as the occurrence of). The state was evaluated as ○, and the state where there was no decrease in displacement performance or mechanical strength was evaluated as ◎.
In Sample 8 having a dielectric loss of more than 8.2%, the temperature of the piezoelectric actuator exceeded 150 ° C. due to self-heating, causing problems such as a decrease in displacement performance, mechanical strength, and a change in electric capacity.
Sample 7 had a dielectric loss of 8.2%, and showed signs of reduction in displacement performance and mechanical strength. Samples 5 and 6 had a dielectric loss of 7.8% or less and little change in displacement performance and mechanical strength.

  Thus, the electric capacity under an electric field strength of 1.0 kV / mm or more at an arbitrary temperature: T1 is defined as C1, and T1 immediately after receiving a thermal history of a temperature T2 having a difference of 100 ° C. or more from T1. When the capacitance under an electric field strength of 1.0 kV / mm or more is C2, the rate of change of the capacitance from C1 to C2 is 5.5% or less (provided that -40 ° C ≦ T1 <T2 ≦ 200 ° C). ), The piezoelectric actuator can obtain a stable electric capacitance. Further, by setting the content to 4.8% or less, a more excellent piezoelectric actuator can be obtained.

When the electric capacitance is stabilized, the variation in the transient characteristics of displacement is reduced, and therefore, the extension speed of the piezoelectric layer when a voltage is applied, that is, the extension speed of the piezoelectric actuator becomes substantially constant.
Furthermore, if the dielectric loss can be reduced to 8.2% or less, the temperature rise of the piezoelectric actuator can be suppressed, self-heating can be prevented, and a piezoelectric actuator having excellent durability can be obtained. Further, by setting the dielectric loss to 7.8% or less, a more excellent piezoelectric actuator can be obtained.
As described above, according to the present embodiment, it is possible to obtain a piezoelectric actuator having stable characteristics even when subjected to a thermal history.

(Example 3)
In this embodiment, an injector incorporating the piezoelectric actuator according to the first embodiment as a drive source will be described.
As shown in FIG. 8, the injector 9 is applied to a common rail injection system of a diesel engine. As shown in the drawing, the injector 9 has an upper housing 92 in which the piezoelectric actuator 1 as a driving unit is housed, and a lower housing 93 fixed to a lower end thereof and having an injection nozzle 94 formed therein. ing.

  The upper housing 92 has a substantially cylindrical shape, and the piezoelectric actuator 1 is inserted and fixed in a vertical hole 921 which is eccentric with respect to the central axis. A high-pressure fuel passage 922 is provided in parallel to the side of the vertical hole 921, and the upper end thereof communicates with an external common rail (not shown) via a fuel introduction pipe 923 projecting upward from the upper housing 92. .

  A fuel outlet pipe 925 communicating with the drain passage 924 protrudes from an upper portion of the upper housing 92, and fuel flowing out of the fuel outlet pipe 925 is returned to a fuel tank (not shown). The drain passage 924 passes through a gap 90 between the vertical hole 921 and the drive unit (piezoelectric actuator) 1, and further has a three-way valve (described later) formed by a passage (not shown) extending downward in the upper and lower housings 92 and 93 from the gap 90. It communicates with 951.

  The injection nozzle portion 94 includes a nozzle needle 941 that slides vertically within the piston body 931, and an injection hole 943 that opens and closes the nozzle needle 941 and injects high-pressure fuel supplied from a fuel reservoir 942 to each cylinder of the engine. Have. The fuel reservoir 942 is provided around an intermediate portion of the nozzle needle 941, and a lower end of the high-pressure fuel passage 922 is opened here. The nozzle needle 941 receives the fuel pressure in the valve opening direction from the fuel reservoir 942 and receives the fuel pressure in the valve closing direction from the back pressure chamber 944 provided on the upper end surface. When the nozzle needle 941 descends, the nozzle needle 941 is lifted, the injection hole 943 is opened, and fuel injection is performed.

  The pressure in the back pressure chamber 944 is increased or decreased by a three-way valve 951. The three-way valve 951 is configured to selectively communicate with the back pressure chamber 944 and the high-pressure fuel passage 922 or the drain passage 924. Here, a ball-shaped valve element that opens and closes a port communicating with the high-pressure fuel passage 922 or the drain passage 924 is provided. The valve body is driven by the driving unit 1 via a large-diameter piston 952, a hydraulic chamber 953, and a small-diameter piston 954 disposed below the driving unit 1.

  By providing such an injector with a piezoelectric actuator having a piezoelectric layer composed of piezoelectric ceramics according to the samples 4 to 8 of the second embodiment, it is possible to reduce variation in fuel injection timing, reduce variation in fuel injection amount, and reduce combustion. State control can be performed more precisely.

FIG. 3 is an explanatory diagram illustrating a piezoelectric actuator according to the first embodiment. FIG. 3 is an explanatory view showing a piezoelectric layer and an internal electrode layer in the first embodiment. FIG. 3 is an explanatory view showing a piezoelectric layer and an internal electrode layer in the first embodiment. FIG. 9 is an explanatory diagram of a measurement circuit that measures the electric capacitance of the piezoelectric actuator during driving according to the second embodiment. FIG. 11 is an explanatory diagram of a measuring tool for measuring the PE hysteresis and the transient characteristics of displacement in the piezoelectric actuator according to the second embodiment. FIG. 9 is a diagram showing the PE hysteresis and the area S1 of the piezoelectric actuator in the second embodiment. FIG. 10 is a diagram showing the PE hysteresis and the area S2 of the piezoelectric actuator in the second embodiment. FIG. 9 is an explanatory diagram illustrating a structure of an injector according to a third embodiment. FIG. 7 is a diagram showing times Ta and Tb until the displacement x is reached in piezoelectric actuators having different electric capacities.

Explanation of reference numerals

1 piezoelectric actuator 151 piezoelectric layer 153, 154 internal electrode layer

Claims (15)

A piezoelectric actuator formed by alternately laminating piezoelectric layers made of piezoelectric ceramics and internal electrode layers,
Arbitrary temperature: The electric capacity under an electric field strength of 1.0 kV / mm or more at T1 is defined as C1, and 1.0 kV at T1 immediately after receiving a thermal history at a temperature T2 having a difference of 100 ° C. or more from T1. Wherein the rate of change from C1 to C2 is 5.5% or less, where C2 is the capacitance under an electric field strength of / mm or more (provided that -40 ° C ≦ T1 <T2 ≦ 200 ° C).   2. The piezoelectric actuator according to claim 1, wherein the rate of change is 5% or less.   2. The piezoelectric actuator according to claim 1, wherein the rate of change is 4.8% or less.   4. The piezoelectric actuator according to claim 1, wherein the dielectric loss is 8.2% or less.   The piezoelectric actuator according to any one of claims 1 to 3, wherein the dielectric loss is 8% or less.   The piezoelectric actuator according to any one of claims 1 to 3, wherein the dielectric loss is 0.5 to 4.0%. In any one of claims 1 to 6, the piezoelectric layer, a portion of the Pb Ba, Ca, Formula was replaced with at least one member selected from _{Sr PbZrO 3 -PbTiO 3 -Pb (Y} 1/2 Nb _1/2 ) O ₃ -Pb (Mn ₁ -z W _z ) O ₃ -based piezoelectric material with 0.25 ≦ z ≦ 0.40, containing both Pb oxide and W oxide A piezoelectric actuator comprising a piezoelectric ceramic which is fired by adding an agent oxide.   The piezoelectric actuator according to any one of claims 1 to 7, wherein the piezoelectric actuator is used as a drive source of an injector for a fuel injection device.   Arbitrary temperature: The relative dielectric constant under an electric field strength of 1.0 kV / mm or more at T1 is ε1, and at T1 immediately after receiving a thermal history at a temperature T2 having a difference of 100 ° C. or more from T1. When the relative dielectric constant under an electric field strength of 0 kV / mm or more is ε2, the rate of change from ε1 to ε2 is 5.5% or less, wherein a piezoelectric ceramic (where −40 ° C. ≦ T1 < T2 ≦ 200 ° C.).   The piezoelectric ceramic according to claim 9, wherein the rate of change is 5% or less.   The piezoelectric ceramic according to claim 9, wherein the rate of change is 4.8% or less.   The piezoelectric ceramic according to any one of claims 9 to 11, wherein the dielectric loss is 8.2% or less.   The piezoelectric ceramic according to any one of claims 9 to 11, wherein the dielectric loss is 8% or less.   The piezoelectric ceramic according to any one of claims 9 to 11, wherein the dielectric loss is 0.5 to 4.0%. The chemical formula PbZrO ₃ -PbTiO ₃ -Pb (Y _1/2 Nb _1/2 ) O according to any one of claims 9 to 14, wherein Pb is partially substituted with at least one selected from Ba, Ca and Sr. Auxiliary oxide containing both Pb oxide and W oxide is added to a piezoelectric material based on _3- Pb (Mn _1-z W _z ) O ₃ where 0.25 ≦ z ≦ 0.40. A piezoelectric ceramic characterized by being fired.

Images