JP4997942B2 - Piezoelectric element - Google Patents

Description

  The present invention relates to a piezoelectric element used as a piezoelectric actuator or the like.

As a piezoelectric element used as a piezoelectric actuator or the like, for example, as described in Patent Document 1, a piezoelectric element formed by a piezoelectric ceramic composition mainly composed of Pb, Zr and Ti is widely known. Yes.
Japanese Patent Laid-Open No. 10-7461

  By the way, in recent years, development of a piezoelectric actuator for controlling fuel injection in an internal combustion engine such as a diesel engine has been actively performed. In this type of piezoelectric actuator, it is necessary to apply a larger electric field than a normal piezoelectric actuator in order to obtain sufficient displacement. However, when the piezoelectric body is formed from the above-described conventional piezoelectric ceramic composition, there is a problem that even if the piezoelectric actuator is driven by a large electric field, a desired displacement cannot be obtained and the piezoelectric characteristics deteriorate. .

  The objective of this invention is providing the piezoelectric element which can make the electric field dependence with respect to a displacement low.

  As a result of intensive studies on the electric field dependency on the displacement of the piezoelectric element, the present inventors have found that a piezoelectric ceramic mainly composed of Pb, Zr and Ti contains P and K in appropriate amounts. As a result, it was found that the piezoelectric characteristics hardly deteriorate even when the piezoelectric element is driven by a large electric field, and the present invention has been completed.

That is, the present invention is a piezoelectric element including a piezoelectric body formed of a piezoelectric ceramic mainly composed of Pb, Zr and Ti, the piezoelectric ceramic including P and K, and the inclusion of P in the piezoelectric ceramic. the amount _is in terms of P 2 _{O 5, 0 <a} P 2 _{O 5} ≦ 250ppm, the content of K in the piezoelectric ceramic, in terms of _{K 2} O, is 30ppm ≦ _{K 2} O ≦ 250ppm It is characterized by this.

Thus, the piezoelectric ceramic forming the piezoelectric body includes P and K in addition to Pb, Zr and Ti as main components. At this time, if the content of P in the piezoelectric ceramic is larger than necessary, when the element is baked in the manufacturing process of the piezoelectric element, P precipitates at the grain boundary of the piezoelectric ceramic and inhibits the grain growth. The particle diameter of the porcelain is reduced, and the number of grain boundaries per unit thickness in the piezoelectric ceramic is increased. For this reason, when a large electric field is applied to the piezoelectric element, a heat loss is generated at the grain boundary of the piezoelectric ceramic, so that a desired displacement cannot be obtained. Therefore, the content of P in the piezoelectric ceramic is limited to 250 ppm or less in terms of P ₂ O ₅ . In addition, when the proportion balance of the contents of P and K in the piezoelectric ceramic is poor, P and K are likely to precipitate at the grain boundaries of the piezoelectric ceramic when firing the element, so that a large electric field was applied to the piezoelectric element. Sometimes, as described above, a desired displacement may not be obtained due to heat loss generated at the grain boundary of the piezoelectric ceramic. Therefore, the content of K in the piezoelectric ceramic, and a 30ppm ≦ _{K 2} O ≦ 250ppm in terms of _{K 2} O. The content of K is derived as a numerical range in which the ratio balance is favorable with respect to the content of P, and has been derived by examination and evaluation by the present inventors.

  As described above, in the present invention, by defining the contents of P and K in the piezoelectric ceramic as described above, it becomes difficult for P and K to precipitate at the grain boundaries of the piezoelectric ceramic when the element is fired. Since grain growth is promoted, the particle diameter of the piezoelectric ceramic is increased, and the number of grain boundaries per unit thickness in the piezoelectric ceramic is reduced. For this reason, it is thought that the heat loss which arises in a piezoelectric ceramic when a big electric field is applied to a piezoelectric element is suppressed. Thereby, even if the piezoelectric element is driven by a large electric field, a desired displacement can be obtained.

  Preferably, the average particle diameter of the piezoelectric ceramic is 2.5 μm or more. Thereby, since the number of grain boundaries per unit thickness in the piezoelectric ceramic is sufficiently reduced, a desired displacement can be reliably obtained even when the piezoelectric element is driven with a large electric field.

  Preferably, the piezoelectric ceramic further contains Zn. In this case, firing can be performed at a lower temperature than when Zn is not contained in the piezoelectric ceramic. By the way, when the firing temperature is low (for example, 1000 ° C. or lower), the firing energy is low. Therefore, when the content of P in the piezoelectric ceramic is large as described above, the grain growth of the piezoelectric ceramic tends to be hindered. Therefore, when Zn is contained in the piezoelectric ceramic, it is particularly effective to contain P and K in the piezoelectric ceramic by an amount within the above range.

  According to the present invention, the electric field dependency on the displacement of the piezoelectric element can be reduced. As a result, a high-performance piezoelectric element can be provided.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of a piezoelectric element according to the invention will be described in detail with reference to the drawings.

  FIG. 1 is a front view showing an embodiment of a piezoelectric element according to the present invention. In the figure, a piezoelectric element 1 of the present embodiment is a laminated piezoelectric element that is applied as a piezoelectric actuator to a fuel injection device of an internal combustion engine mounted on an automobile, for example.

  The piezoelectric element 1 includes a rectangular parallelepiped laminated body 2. The laminated body 2 is disposed so as to sandwich the active layers 5 with the internal electrodes 4A and 4B alternately interposed between the piezoelectric bodies 3 and the active layer 5 from above and below, and a plurality of piezoelectric bodies 3 are laminated. And an upper lid layer 6 and a lower lid layer 7.

The piezoelectric body 3 is formed of a piezoelectric ceramic having a perovskite structure mainly composed of lead zirconate titanate (PZT). Examples of the piezoelectric ceramic include those having the following composition.
(Pb0.97 Sr0.02) [(Zn _1/3 Nb _2/3 ) 0.1 Ti0.435 Zr0.465] O ₃

The piezoelectric ceramic (PZT) contains P and K. The content of P in the piezoelectric _ceramic, in terms of _{P 2 O 5, 0 <P} 2 O 5 ≦ 250ppm, preferably _{0 <P 2 O 5 ≦ 200ppm} . P ₂ O ₅ has a value larger than 0 because it cannot completely prevent mixing from the raw material. The content of K in the piezoelectric ceramic, in terms of _{K 2 O, 30ppm ≦ K 2} O ≦ 250ppm, preferably 30ppm ≦ _{K 2} O ≦ 200ppm.

  The piezoelectric ceramic preferably contains Zn in an amount of 0.4 to 1.7% by weight in terms of ZnO. Thus, the piezoelectric ceramic can be fired at a low temperature of 1000 ° C. or less without deteriorating the characteristics of PZT. At this time, it is more preferable that Me (Nb, Sb, Ta) is contained at a ratio of Me = 2 with respect to Zn = 1. Further, the piezoelectric ceramic may contain W, Ni, Fe, Co, Cu or the like for improving the piezoelectric characteristics.

  The average particle diameter of the piezoelectric ceramic forming the piezoelectric body 3 is preferably 2.5 μm or more, and particularly preferably 2.8 μm or more. In addition, the shape of the particle | grains contained in a piezoelectric ceramic is not restricted to circular or an ellipse, but is various. Therefore, the particle diameter here is the outer diameter of a virtual circle (particle virtual circle) that surrounds the particle and is in contact with at least a part of the particle.

  The internal electrodes 4A and 4B are formed of a conductive material mainly composed of Ag and Pd as an electrode material suitable for an element using PZT.

  The laminate 2 has side surfaces 2a and 2b that face each other. The internal electrode 4A is formed so as to be exposed to the side surface 2a from the inner side than the side surface 2b of the multilayer body 2, and the internal electrode 4B is formed to be exposed to the side surface 2b from the inner side than the side surface 2a of the multilayer body 2. Yes. As a result, the internal electrodes 4A and 4B overlap each other in the stacking direction of the stacked body 2 in the region excluding the side surfaces 2a and 2 side ends of the stacked body 2. The portion sandwiched between the internal electrodes 4A and 4B in the piezoelectric body 3 becomes a piezoelectric active portion that expands and contracts (displaces) in the stacking direction of the stacked body 2 when a voltage is applied to the internal electrodes 4A and 4B (described later).

  The laminated body 2 has a structure in which 300 layers of piezoelectric bodies 3 each having a length of 10 mm, a width of 10 mm, and a thickness of about 85 μm are laminated. The thickness of the internal electrodes 4A and 4B is, for example, about 2 μm.

  An external electrode 8A electrically connected to each internal electrode 4A is provided on the side surface 2a of the multilayer body 2, and an external electrode 8B electrically connected to each internal electrode 4B is provided on the side surface 2b of the multilayer body 2. Is provided. The external electrodes 8 </ b> A and 8 </ b> B extend in the stacking direction of the stacked body 2. The external electrodes 8A and 8B are made of a conductive material containing, for example, one of Ag, Au, and Cu as a main component.

Next, a method for manufacturing such a piezoelectric element 1 will be described. First, powders of PbO, TiO ₂ , ZrO ₂ , ZnO, Nb ₂ O ₅ and SrCO ₃ are prepared as starting materials for the piezoelectric ceramic, and these starting materials are weighed so as to have the above composition. As the content of P and K in the piezoelectric ceramic is in the above range, the addition of P ₂ O ₅ and K ₂ CO ₃ as a starting material, subjected to wet mixing by a ball mill.

Incidentally, in the TiO ₂ P, or in advance to contain K, it may be previously allowed to contain P in ZrO _2. By containing P in TiO _2, the crystal structure of the TiO ₂ will be stabilized and inexpensive anatase can low-temperature firing.

  Subsequently, the mixed powder is temporarily fired in air. At this time, since the mixed powder contains a predetermined amount of K, the mixed powder can be pre-fired at a temperature as low as 900 ° C. And the piezoelectric ceramic powder which has PZT as a main component is obtained by wet-grinding the pre-baked mixed powder with a ball mill.

  Subsequently, a piezoelectric ceramic paste in which an organic binder, an organic solvent, and the like are mixed with the piezoelectric ceramic powder is produced. And a piezoelectric ceramic paste is apply | coated on PET film by a doctor blade method, and the green sheet used as said piezoelectric material 3 is obtained by carrying out sheet shaping | molding by predetermined thickness.

  Subsequently, a conductive paste is prepared by mixing a metal material configured with a ratio of Ag: Pd = 85: 15, an organic binder, an organic solvent, and the like, and the conductive paste is printed on the green sheet by a screen printing method. Then, an electrode pattern to be the internal electrodes 4A and 4B is formed.

  Subsequently, a predetermined number of green sheets on which electrode patterns are formed and green sheets on which electrode patterns are not formed are stacked in a predetermined order. And a green laminated body is obtained by pressing the green sheet of each layer with the pressure of 100 Mpa, applying a heat | fever about 60 degreeC with respect to each green sheet after lamination | stacking.

  Thereafter, the green laminated body is placed on a setter, and degreasing (debinding) of the green laminated body is performed at a temperature of about 400 ° C. for about 10 hours in an air atmosphere. Then, the green laminate is put in a sealed mortar together with a setter, and the green laminate is fired (main firing) at a temperature of about 950 ° C. to 1000 ° C. for about 2 hours. Thereby, the said laminated body 2 as a sintered compact is obtained.

  Subsequently, the fired laminate 2 is cut into predetermined dimensions. Then, external electrodes 8A and 8B are formed on the side surfaces 2a and 2b of the laminate 2 by baking or the like, respectively. Finally, for example, under a temperature of 120 ° C., a polarization process is performed by applying a predetermined voltage for about 3 minutes so that the electric field strength becomes 2 to 3 kV / mm. Thus, the multilayer piezoelectric element 1 that functions as a piezoelectric actuator is completed.

  In such a piezoelectric element 1, when a voltage is applied between the external electrodes 8A and 8B, a voltage is applied between the internal electrodes 4A and 4B connected to the external electrodes 8A and 8B, and an electric field is generated between them. The active layer 5 is displaced in the stacking direction (vertical direction) of the stacked body 2.

  By the way, it is desirable that the amount of displacement of such an element increases as the electric field applied between the internal electrodes 4A and 4B increases. However, in order to reduce the content of Pd in the internal electrodes 4A and 4B from the viewpoint of manufacturing cost and the like, when the piezoelectric body 3 is formed of a piezoelectric material that can be sintered at a low temperature as in this embodiment, For example, as shown by a broken line in FIG. 2, even if a desired displacement is obtained with a driving electric field of 1 kV / mm, a phenomenon in which a desired displacement cannot be obtained with a driving electric field of 2 kV / mm may occur. In particular, when a piezoelectric element is used as a piezoelectric actuator for fuel injection control, it is necessary to generate a large amount of displacement, so the above phenomenon becomes a serious problem.

  The following can be considered as reasons why a desired displacement cannot be obtained when the driving electric field is increased. That is, if the amount of P contained in the piezoelectric ceramic containing PZT as a main component is large, P is deposited at the grain boundary of the piezoelectric ceramic when the green laminate is fired. Further, depending on the ratio of the amount of P and K contained in the piezoelectric ceramic, K may be deposited at the grain boundary of the piezoelectric ceramic when the green laminate is fired.

  Here, when the firing temperature of the piezoelectric ceramic is high (for example, 1200 ° C.), the firing energy becomes higher than the energy for maintaining the grain boundary component, which hinders the grain growth (crystal growth) of the piezoelectric ceramic. There is almost no. However, under the condition where the firing temperature of the piezoelectric ceramic is 1000 ° C. or less, the firing energy is lower than the energy for maintaining the grain boundary component, and thus the grain growth of the piezoelectric ceramic is hindered. For this reason, the particle diameter of the piezoelectric body 3 after being sintered is reduced, and the number of grain boundaries per unit thickness in the piezoelectric body 3 is increased. As a result, when a relatively large electric field (voltage) is applied between the internal electrodes 4A and 4B in the completed piezoelectric element 1, the grain boundary of the piezoelectric body 3 generates heat and a heat loss is generated. Displacement cannot be obtained. That is, the electric field (voltage) dependency on the displacement of the piezoelectric element 1 is deteriorated.

On the other hand, in this embodiment, the P content in the piezoelectric ceramic is converted to P ₂ O ₅ so that 0 <P ₂ O ₅ ≦ 250 ppm, and the K content in the piezoelectric ceramic is changed to K ₂ O. In terms of conversion, 30 ppm ≦ K ₂ O ≦ 250 ppm. That is, the amount of P contained in the piezoelectric ceramic is suppressed to a necessary minimum, and the amount of K contained in the piezoelectric ceramic is appropriate for a small amount of P.

  As a result, even if the green laminate is fired at a temperature of 1000 ° C. or less as described above, P and K are less likely to precipitate at the grain boundaries of the piezoelectric ceramic, and the grain growth of the piezoelectric ceramic is promoted. As a result, the particle diameter of the piezoelectric body 3 is increased, and the number of grain boundaries per unit thickness in the piezoelectric body 3 is reduced. As a result, when a relatively large electric field (voltage) is applied between the internal electrodes 4A and 4B in the completed piezoelectric element 1, heat loss caused by heat generation at the grain boundaries of the piezoelectric body 3 is suppressed. The displacement of can be obtained. Therefore, the piezoelectric element 1 having a small electric field (voltage) dependency on the displacement can be obtained.

  In addition, when the piezoelectric element 1 is displaced, cracks may occur in the laminate 2 due to stress concentration. The progress direction of the cracks is influenced by the state of the grain boundary of the piezoelectric body 3. At this time, if the number of grain boundaries per unit thickness in the piezoelectric body 3 increases, cracks tend to extend in a random direction, so that cracks extending in the stacking direction (vertical direction) of the stacked body 2 are likely to occur. When such a vertical crack occurs, the internal electrodes 4A and 4B having different polarities may be short-circuited to cause dielectric breakdown of the piezoelectric element 1.

  In the present embodiment, since the number of grain boundaries per unit thickness in the piezoelectric body 3 is reduced by increasing the particle diameter of the piezoelectric body 3 after firing, the progress of cracks along the grain boundary of the piezoelectric body 3 is suppressed. Thus, cracks extending in the vertical direction are less likely to occur. Thereby, a short circuit between the internal electrodes 4A and 4B is prevented, and the dielectric breakdown of the piezoelectric element 1 can be avoided.

  As described above, according to the piezoelectric element 1 of the present embodiment, even when driven by a large electric field, the piezoelectric characteristics hardly deteriorate, a sufficient amount of displacement can be secured, and insulation by cracks extending in the vertical direction is possible. Destruction can be prevented. Thereby, it becomes possible to obtain the piezoelectric element 1 with high reliability.

  Actually, a piezoelectric element for evaluation was manufactured and evaluated using the same piezoelectric ceramic as in the above-described embodiment.

Specifically, first, the piezoelectric ceramic powder containing P and K containing PZT as a main component from the above PbO, TiO ₂ , ZrO ₂ , ZnO, Nb ₂ O ₅ , SrCO ₃ , P ₂ O ₅ and K ₂ CO _3. The body was made. The piezoelectric ceramic powder was granulated by adding a polyvinyl alcohol-based binder, and this was pressed at a pressure of about 196 MPa to form a predetermined size.

  Subsequently, the molded body was fired at a temperature of 1000 ° C. in an air atmosphere. This baking can be performed in an atmosphere having a higher oxygen partial pressure than in the air or in pure oxygen. The sintered body thus obtained was processed into a disk shape having a thickness of 2 mm and a diameter of 15 mm, and silver was vapor-deposited on both surfaces of the sintered body to provide electrodes. Then, this element was placed in an insulating oil at 120 ° C., and polarization treatment was performed under the conditions of an electric field strength of 3 kV / mm and a voltage application time of 30 minutes, to obtain a piezoelectric element for evaluation.

  At this time, plural types (10 types) of evaluation piezoelectric elements having different P and K contents in the piezoelectric ceramic were produced.

Table 1 shows the contents of P (P ₂ O ₅ ) and K (K ₂ O) in the piezoelectric body, the average particle diameter of the piezoelectric body, and the piezoelectric strain constant d33 for each type of evaluation piezoelectric element. It is a thing.

  At this time, after polishing and etching of the completed piezoelectric element for evaluation, the particles are observed with a scanning electron microscope, and the particle diameter of the virtual particle circle (described above) is obtained by image processing to calculate the average particle diameter. Went. Further, an electric field of 2 kV / mm was applied to the completed piezoelectric element for evaluation, the amount of displacement of the element was determined using a laser Doppler vibrometer, and the piezoelectric strain constant d33 was calculated from the amount of displacement and the electric field.

As summarized in Table 1, Samples 1 to 4 in which the P content in the piezoelectric ceramic was 0 <P ₂ O ₅ ≦ 250 ppm and the K content in the piezoelectric ceramic was 30 ppm ≦ K ₂ O ≦ 250 ppm. For 7 to 9, the average particle diameter of the piezoelectric body is 2.5 μm or more, and the piezoelectric strain constant d33 is 750 pm / V or more. However, for samples 5, 6, and 10 in which the contents of P and K in the piezoelectric ceramic are not within the above range, the piezoelectric strain constant of which the average particle diameter of the piezoelectric body is smaller than 2.5 μm and 750 pm / V or more. It can be seen that d33 is not obtained.

  FIG. 2 is a graph showing the relationship between the electric field strength and the piezoelectric strain constant d33 for Samples 6 and 7. Note that the solid line in the graph represents the characteristics of the sample 7, and the broken line in the graph represents the characteristics of the sample 6.

  As can be seen from FIG. 2, in the sample 6 (comparative example) in which the contents of P and K in the piezoelectric ceramic are not within the above range, a desired displacement (piezoelectric strain constant d33) is obtained as the electric field strength increases. However, in the sample 7 (Example) in which the contents of P and K in the piezoelectric ceramic are within the above range, a desired displacement amount (piezoelectric strain constant d33) is ensured even when the electric field strength increases.

From the above evaluation results, in a piezoelectric ceramic containing PZT as a main component and containing P and K, the P content is 0 <P ₂ O ₅ ≦ 250 ppm, and the K content is 30 ppm ≦ K ₂ O ≦ 250 ppm. As a result, the effectiveness of the present invention that the electric field (voltage) dependence of the displacement amount is improved can be verified.

  The present invention is not limited to the above embodiment. For example, in the above embodiment, Zn is contained in the piezoelectric ceramic that forms the piezoelectric body in order to perform low-temperature firing of the element, but the composition of the piezoelectric ceramic is particularly Zn if PZT is the main component. It is not restricted to the thing containing.

It is a front view which shows one Embodiment of the piezoelectric element concerning this invention. It is a graph showing the relationship between the electric field strength and the piezoelectric strain constant d33 in an arbitrarily selected piezoelectric element for evaluation.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Piezoelectric element, 2 ... Piezoelectric body.

Claims (3)

A piezoelectric element including a piezoelectric body formed of a piezoelectric ceramic mainly composed of Pb, Zr, and Ti,
The piezoelectric ceramic includes P and K,
The content of P in the piezoelectric in _porcelain, in terms of P 2 _{O 5,} is _{0 <P 2 O 5 ≦ 250ppm} ,
The content of K in the piezoelectric ceramic, in terms of _{K 2} O, the piezoelectric element which is a 30ppm ≦ _{K 2} O ≦ 250ppm.   The piezoelectric element according to claim 1, wherein the piezoelectric ceramic has an average particle diameter of 2.5 μm or more. The piezoelectric element according to claim 1, wherein the piezoelectric ceramic further contains Zn.

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