Piezoelectric element, piezoelectric actuator, and mass flow controller

Abstract

This piezoelectric element is provided with a layered body and a conductor layer. In the layered body, a plurality of piezoelectric bodies and a plurality of internal electrodes are stacked. The conductor layer is connected to the internal electrodes and is positioned along the stacking direction of the layered body. An oxide layer including bismuth oxide is positioned in the vicinity of the interface between a piezoelectric body and the conductor layer.

Description

Piezoelectric element, piezoelectric actuator and mass flow controller

[0001] The present disclosure relates to a piezoelectric element, a piezoelectric actuator, and a mass flow controller.

[0002] Conventionally, a piezoelectric actuator has been known that includes piezoelectric elements stacked in a columnar shape and a metal case that houses the piezoelectric elements so that both ends of the piezoelectric elements abut against an inner wall (see, for example, Patent Document 1).

[0003] In this piezoelectric actuator, for example, a conductor layer that is electrically connected to an internal electrode is provided on the side surface of a pillar-shaped piezoelectric element, and a drive voltage is applied to the piezoelectric element via this conductor layer.

[0004] Japanese Patent Application Laid-Open No. 2009-043516

[0005] The piezoelectric element according to the present disclosure includes a laminate and a conductor layer. The laminate is formed by stacking a plurality of piezoelectric bodies and internal electrodes. The conductor layer is connected to the internal electrodes and is positioned along the stacking direction of the laminate. An oxide layer containing bismuth oxide is positioned near the interface between the piezoelectric body and the conductor layer.

[0006] FIG. 1 is a perspective view showing the overall configuration of a piezoelectric actuator according to an embodiment. FIG. 2 is a cross-sectional view taken along line A-A in FIG. 1. FIG. 3 is a perspective view showing the internal structure of a piezoelectric actuator according to an embodiment. FIG. 4 is an enlarged cross-sectional view showing the configuration of a piezoelectric element according to an embodiment. FIG. 5 is an enlarged cross-sectional view showing the configuration of a piezoelectric element according to another embodiment 1. FIG. 6 is an enlarged cross-sectional view showing the configuration of a piezoelectric element according to another embodiment 2. FIG. 7 is an enlarged cross-sectional view showing the configuration of a piezoelectric element according to another embodiment 3. FIG. 8 is an enlarged cross-sectional view showing the configuration of a piezoelectric element according to another embodiment 4. FIG. 9 is an enlarged cross-sectional view showing the configuration of a piezoelectric element according to another embodiment 5. FIG. 10 is a block diagram showing the configuration of a mass flow controller according to an embodiment.

[0007] Hereinafter, embodiments for carrying out a piezoelectric element, a piezoelectric actuator, and a mass flow controller according to the present disclosure (hereinafter referred to as "embodiments") will be described in detail with reference to the drawings. Note that the present disclosure is not limited to these embodiments.

[0008] Furthermore, the embodiments can be combined as appropriate within the scope of not causing any contradiction in the processing content. Furthermore, the same components in the following embodiments are denoted by the same reference numerals, and redundant explanations will be omitted.

[0009] It should also be noted that the drawings are schematic and that the dimensional relationships and ratios of elements may differ from reality. Furthermore, the drawings may contain parts whose dimensional relationships and ratios differ from one another.

[0010] 2. Description of the Related Art Conventionally, a piezoelectric actuator has been known that includes piezoelectric elements stacked in a columnar shape and a metal case that houses the piezoelectric elements so that both ends of the piezoelectric elements abut against an inner wall.

[0011] In this piezoelectric actuator, for example, a conductor layer that is electrically connected to an internal electrode is provided on the side surface of a pillar-shaped piezoelectric element, and a drive voltage is applied to the piezoelectric element via this conductor layer.

[0012] However, in the conventional technology, when a piezoelectric actuator is driven in a high-temperature environment, oxygen vacancies formed in the piezoelectric crystals segregate near the conductor layer on the negative electrode side, which can reduce the dielectric strength of the piezoelectric element, potentially reducing the durability of the piezoelectric element.

[0013] Therefore, there is a need for a technology that can solve the above problems and improve the durability of piezoelectric elements.

[0014] <Overall Configuration of Piezoelectric Actuator> First, the overall configuration of a piezoelectric actuator 1 according to an embodiment will be described with reference to Figures 1 to 3. Figure 1 is a perspective view showing the overall configuration of the piezoelectric actuator 1 according to an embodiment, and Figure 2 is a cross-sectional view taken along line A-A in Figure 1. Also, Figure 3 is a perspective view showing the internal structure of the piezoelectric actuator 1 according to an embodiment.

[0015] 1 to 3, a piezoelectric actuator 1 according to the embodiment includes a piezoelectric element 10, a pair of electrode plates 20, a pair of lead terminals 30, and a case 40. The pair of electrode plates 20 includes electrode plate 20A and electrode plate 20B, and the pair of lead terminals 30 includes lead terminal 30A and lead terminal 30B.

[0016] 3, the piezoelectric element 10 has a columnar shape. The piezoelectric element 10 is, for example, a square pillar (rectangular parallelepiped) with a length of 0.5 mm to 10 mm, a width of 0.5 mm to 10 mm, and a height of 1 mm to 100 mm. Note that the shape of the piezoelectric element 10 is not limited to a square pillar, and may be a hexagonal pillar, an octagonal pillar, a cylindrical pillar, or the like.

[0017] 2, the piezoelectric element 10 includes a piezoelectric body 11, an internal electrode 12, a planned fracture layer 13, and a pair of conductor layers 14. The pair of conductor layers 14 includes a conductor layer 14A and a conductor layer 14B.

[0018] The piezoelectric element 10 is configured by laminating a plurality of piezoelectric bodies 11, internal electrodes 12, and planned fracture layers 13 in a predetermined order along a lamination direction D. In the present disclosure, the lamination direction D of the piezoelectric element 10 coincides with the longitudinal direction of the piezoelectric element 10.

[0019] The piezoelectric body 11 is made of a piezoelectric material having piezoelectric properties, such as piezoelectric ceramics. Examples of the material of such piezoelectric ceramics include lead zirconate titanate (PbZrO ３ -PbTiO ３ , hereinafter also referred to as PZT) or potassium sodium niobate ((K,Na)NbO ３ , hereinafter also referred to as KNN), a perovskite oxide made of lithium niobate (LiNbO ３ ) or lithium tantalate (LiTaO ３ ) etc.

[0020] The average particle size of such piezoelectric ceramics is, for example, 1.6 μm to 2.8 μm, and the thickness of one layer of piezoelectric body 11 is, for example, 3 μm to 250 μm.

[0021] The internal electrode 12 is made of a conductive material and includes a plurality of first electrodes 12a and a plurality of second electrodes 12b. The first electrodes 12a are electrically connected to a conductor layer 14A. The conductor layer 14A is located on one side surface 10a of the piezoelectric element 10 along the stacking direction D. A predetermined positive voltage is applied to the first electrodes 12a, for example, via the conductor layer 14A.

[0022] The second electrode 12b is electrically connected to the conductor layer 14B. The conductor layer 14B is located along the stacking direction D on the side surface 10b of the piezoelectric element 10 opposite to the side surface 10a. A predetermined negative voltage (or ground voltage), for example, is applied to the second electrode 12b via the conductor layer 14B.

[0023] 2, inside the piezoelectric element 10, the first electrode 12a, the second electrode 12b, and the piezoelectric body 11 are laminated so that the piezoelectric body 11 is located between the first electrode 12a and the second electrode 12b. This allows the piezoelectric element 10 to apply a drive voltage to the piezoelectric body 11 by the first electrode 12a and the second electrode 12b.

[0024] The piezoelectric element 10 according to the embodiment is composed of an active portion formed by alternately stacking piezoelectric bodies 11 and internal electrodes 12, and an inactive portion located on both ends of the active portion in the stacking direction D and having the piezoelectric bodies 11.

[0025] The active portion is a portion that expands or contracts (hereinafter also referred to as expansion and contraction) in the stacking direction D when a drive voltage is applied to the piezoelectric element 10 from the outside. On the other hand, the inactive portion is a portion that does not expand or contract even when a drive voltage is applied to the piezoelectric element 10 from the outside.

[0026] In addition, in the present disclosure, the end of the case 40 on the base 41 side is the base end 10 e of the piezoelectric element 10 , and the end of the case 40 on the lid 43 side is the tip end 10 f of the piezoelectric element 10 .

[0027] In the piezoelectric actuator 1 according to the embodiment, the base end 10e of the piezoelectric element 10 (i.e., the base body 41) is fixed, and the tip end 10f of the piezoelectric element 10 (i.e., the lid body 43) is displaced along the stacking direction D.

[0028] The material of the internal electrodes 12 is, for example, a metal whose main component is silver, silver-palladium, silver-platinum, or copper. The internal electrodes 12 can be formed, for example, by co-firing with the piezoelectric body 11. The thickness of the internal electrodes 12 is, for example, 0.1 μm to 5 μm.

[0029] The planned rupture layer 13 is a layer for alleviating stress caused by driving the piezoelectric element 10. Examples of the planned rupture layer 13 include an extremely porous metal layer that does not function as the internal electrode 12, or an extremely porous piezoelectric layer. Note that the planned rupture layer 13 may be omitted in the piezoelectric element 10 according to the embodiment.

[0030] As described above, the pair of conductor layers 14 includes the conductor layer 14A located on the side surface 10a of the piezoelectric element 10 and the conductor layer 14B located on the side surface 10b of the piezoelectric element 10. The conductor layer 14 is located so as to extend over the entire active portion of the piezoelectric element 10.

[0031] The material of the conductor layer 14 is, for example, a metal containing silver or copper as a main component. For example, a metallized layer made of a sintered body of the above metal and glass can be used for the conductor layer 14. The thickness of the conductor layer 14 is, for example, 5 μm to 500 μm.

[0032] Although not shown in the present disclosure, a coating layer made of an insulating material may be disposed on side surface 10c (see FIG. 3) and side surface 10d (see FIG. 3) located between side surface 10a and side surface 10b of piezoelectric element 10. By disposing such a coating layer on side surfaces 10c and 10d, it is possible to reduce creeping discharge that occurs between the electrodes when a high voltage is applied during operation.

[0033] The insulating material for the coating layer may be, for example, a ceramic material, which can follow the expansion and contraction of the piezoelectric element 10 when the piezoelectric actuator 1 is driven and is deformable by stress so that the coating layer itself does not peel off and cause creeping discharge.

[0034] Specifically, the coating layer is made of partially stabilized zirconia, Ln １－Ｘ Si Ｘ AlO ３＋０．５Ｘ Examples of ceramic materials include: Ln represents at least one selected from Sn, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb, and x is 0.01 to 0.3.

[0035] Alternatively, the coating layer may be a piezoelectric material such as barium titanate, lead zirconate titanate, or potassium sodium niobate, which changes the inter-ionic distance within the crystal lattice to relieve the induced stress.

[0036] The pair of electrode plates 20 includes electrode plate 20A and electrode plate 20B, which are electrically connected to the pair of conductor layers 14. Specifically, electrode plate 20A is electrically connected to conductor layer 14A, and electrode plate 20B is electrically connected to conductor layer 14B.

[0037] 3, the electrode plate 20 is positioned along the stacking direction D of the laminate as a whole, and locally meanders so as to intersect with the stacking direction D. At least a portion of the portion of the electrode plate 20 that intersects with the stacking direction D is joined to the conductor layer 14 via a conductive bonding material 21.

[0038] The bonding material 21 may be, for example, an epoxy resin or polyimide resin containing metal powder having high conductivity, such as Ag powder or Cu powder.

[0039] The material of the electrode plate 20 is, for example, a metal such as copper, iron, stainless steel, or phosphor bronze. The width of the electrode plate 20 is, for example, 0.5 (mm) to 10 (mm), and the thickness of the electrode plate 20 is, for example, 0.01 (mm) to 1.0 (mm). The surface of the electrode plate 20 may be plated with a plating film such as tin plating or silver plating to improve electrical conductivity and thermal conductivity.

[0040] 1 and 2, the pair of lead terminals 30 includes lead terminal 30A and lead terminal 30B. The pair of lead terminals 30 are electrically connected to the pair of electrode plates 20, respectively. For example, as shown in FIG. 2, lead terminal 30A is electrically connected to electrode plate 20A via conductive member 31A and conductive adhesive 32A, and lead terminal 30B is electrically connected to electrode plate 20B via conductive member 31B and conductive adhesive 32B.

[0041] As shown in Fig. 2, the case 40 accommodates the piezoelectric element 10 and the electrode plate 20. The case 40 has a base 41, a cylindrical body 42, and a lid 43. The base 41 is columnar (for example, cylindrical) and may have a cylindrical portion 41a that protrudes cylindrically from one end (the upper side in Fig. 2).

[0042] Furthermore, an inner bottom surface 41b located at the bottom of the cylindrical portion 41a contacts the base end 10e of the piezoelectric element 10. The inner bottom surface 41b of the base 41 and the base end 10e of the piezoelectric element 10 may be bonded together with a bonding material (not shown).

[0043] In addition, the base 41 has a pair of through holes 41d that penetrate between the inner bottom surface 41b and the end surface 41c opposite the inner bottom surface 41b, and a pair of lead terminals 30 are inserted into the pair of through holes 41d, respectively.

[0044] An insulating material (such as soft glass) is filled in the gap between the through hole 41d of the base 41 and the lead terminal 30, thereby fixing the lead terminal 30 to the base 41. Furthermore, as shown in FIG. 2 , the lead terminal 30 is inserted through the base 41 from inside the case 40 and protrudes outward from an end surface 41c of the base 41.

[0045] The cylinder 42 has a cylindrical shape (for example, a cylindrical shape). The cylinder 42 also has a bellows shape, and the axial direction of the cylinder coincides with the stacking direction D of the piezoelectric element 10. This allows the cylinder 42 to expand and contract in the axial direction in response to the expansion and contraction of the piezoelectric element 10 in the stacking direction D.

[0046] Furthermore, the cylindrical body 42 has a predetermined spring constant so that it can follow the expansion and contraction of the piezoelectric element 10. The spring constant of the cylindrical body 42 can be adjusted by the thickness of the cylindrical body 42, the shape of the grooves in the cylindrical body 42, the number of grooves in the cylindrical body 42, etc. The thickness of the cylindrical body 42 is, for example, 0.1 (mm) to 0.5 (mm), and the diameter of the cylindrical body 42 is, for example, 5 (mm) to 50 (mm).

[0047] The end 42 a of the cylindrical body 42 on the base body 41 side is welded, for example, with a compressive load applied to the piezoelectric element 10. The cylindrical body 42 may also be welded after being provided with a flange that expands radially outward in a trumpet shape at the end 42 a on the base body 41 side.

[0048] The cylindrical body 42 is formed, for example, by preparing a seamless pipe of a predetermined shape and then processing the seamless pipe into a bellows shape by rolling, isostatic pressing, or the like.

[0049] The lid 43 has a cylindrical shape (for example, a cylindrical shape) with one end closed. The lid 43 is fitted into the end 42 b of the cylindrical body 42, and the inner wall of the end 42 b and the side wall of the lid 43 are fixed together by, for example, welding.

[0050] The inner bottom surface 43a of the lid 43 contacts the tip 10f of the piezoelectric element 10. The inner bottom surface 43a of the lid 43 and the tip 10f of the piezoelectric element 10 may be bonded together with a bonding material (not shown).

[0051] Next, a detailed configuration of a piezoelectric element 10 according to an embodiment will be described with reference to Fig. 4. Fig. 4 is an enlarged cross-sectional view showing the configuration of the piezoelectric element 10 according to the embodiment. Note that Figs. 4 to 9 described below show enlarged cross-sectional views of the interface between the conductor layer 14B, to which a predetermined negative voltage is applied from the outside, and the piezoelectric body 11, and the vicinity thereof, in the piezoelectric element 10.

[0052] 4, in the piezoelectric element 10 according to the embodiment, an oxide layer 15 is located near the interface between the piezoelectric body 11 and the conductor layer 14B. The oxide layer 15 contains bismuth (Bi) oxide. In this case, the oxide layer 15 being located near the interface between the piezoelectric body 11 and the conductor layer 14B means, for example, that the oxide layer 15 is located between the piezoelectric body 11 and the conductor layer 14B.

[0053] Unlike silica glass, the bismuth oxide contained in this oxide layer 15 can change the valence of bismuth depending on the surrounding charge. In the embodiment, even when a predetermined negative voltage is applied to the conductor layer 14B and oxygen vacancies segregate in the piezoelectric body 11 near the conductor layer 14B, the valence of bismuth in the oxide layer 15 changes, thereby maintaining charge neutrality.

[0054] That is, in the embodiment, even when a predetermined negative voltage is applied to the conductor layer 14B, the oxide layer 15 containing bismuth oxide can reduce the segregation of oxygen vacancies in the piezoelectric body 11. Therefore, according to the embodiment, the durability of the piezoelectric element 10 can be improved.

[0055] Furthermore, the valence of bismuth changes more favorably than other elements in the high-temperature environments (for example, about 100°C to 200°C) in which the piezoelectric actuator 1 is required to operate in recent years. Therefore, in the embodiment, by including bismuth oxide in the oxide layer 15, charge neutrality can be maintained favorably even when oxygen vacancies segregate in the piezoelectric body 11.

[0056] 4 shows an example in which the oxide layer 15 is located near one of the pair of conductor layers 14, conductor layer 14B, but the present disclosure is not limited to this example. For example, the oxide layer 15 may be located near the other conductor layer 14A. This can reduce the segregation of oxygen vacancies in the piezoelectric body 11 even when a negative voltage is applied to conductor layer 14A.

[0057] In the present disclosure, oxide layers 15 may be located near both conductor layers 14A and 14B, respectively. This reduces segregation of oxygen vacancies in piezoelectric body 11, even when piezoelectric actuator 1 is used while switching between conductor layer 14A and conductor layer 14B as the conductor layer to which a predetermined negative voltage is applied.

[0058] The oxide layer 15 according to the embodiment may be formed by interdiffusion of, for example, an oxide contained in the piezoelectric body 11 and a bismuth oxide. The oxide contained in the piezoelectric body 11 is, for example, an oxide of an A-site element of a so-called perovskite (mainly lead oxide in the case of PZT, and mainly Na oxide and K oxide in the case of KNN).

[0059] As a method for forming the oxide layer 15 according to the embodiment, for example, first, a bismuth oxide paste (bismuth oxide powder + binder) that will become the oxide layer 15 is printed on the side surface of a laminate that is composed of the piezoelectric body 11, the internal electrode 12, and the planned fracture layer 13.

[0060] Next, a conductor layer paste (metal powder + binder + additives) that will become the conductor layer 14 is printed on the side surface of the laminate. Then, the laminate on which the bismuth oxide paste and the conductor layer paste are printed is fired to form the oxide layer 15 according to the embodiment.

[0061] Another method for forming the oxide layer 15 according to the embodiment is, for example, to first print bismuth oxide paste on the side surfaces of a laminate including the piezoelectric body 11, the internal electrode 12, and the planned fracture layer 13. Then, the laminate on which the bismuth oxide paste has been printed is fired.

[0062] Next, a conductor layer paste that will become the conductor layer 14 is printed on the side surface of the fired laminate. Then, the laminate on which the conductor layer paste has been printed is fired to form the oxide layer 15 according to the embodiment.

[0063] In the embodiment, the bismuth oxide contained in the oxide layer 15 may mainly contain glass. In other words, the bismuth oxide contained in the oxide layer 15 may mainly be composed of amorphous material rather than crystalline material.

[0064] This reduces the formation of grain boundaries between bismuth oxides, thereby reducing the number of conductive paths that short-circuit the inside of the oxide layer 15. Therefore, according to the embodiment, charge neutrality can be maintained between the conductor layer 14 and the piezoelectric body 11.

[0065] In addition, in the embodiment, the bismuth oxide contained in the oxide layer 15 mainly contains glass, and therefore the bismuth oxide is positioned so as to surround the metal crystals that make up the conductor layer 14. Therefore, according to the embodiment, the bismuth oxide contained in the oxide layer 15 improves the effect of preventing a short circuit between the piezoelectric body 11 and the conductor layer 14.

[0066] In addition, in the embodiment, the thickness T1 (see FIG. 4) of the oxide layer 15 at a portion in contact with the internal electrode 12 (for example, the second electrode 12b) may be equal to or less than twice the thickness T2 (see FIG. 4) of a portion away from the internal electrode 12. In other words, in the embodiment, the portion of the oxide layer 15 in contact with the internal electrode 12 does not need to penetrate significantly into the piezoelectric body 11 along the internal electrode 12. The portion where the oxide layer 15 has not penetrated here refers to the portion where the internal electrode 12 is in direct contact with the piezoelectric body 11.

[0067] In this way, the bismuth oxide does not significantly penetrate into the interface between the piezoelectric body 11 and the internal electrode 12, thereby reducing the interference with the exchange of charges between the piezoelectric body 11 and the internal electrode 12. Therefore, according to the embodiment, the response speed of the piezoelectric body 11 can be maintained at a good level.

[0068] Alternative Embodiment 1 Next, various alternative embodiments will be described with reference to Fig. 5 to Fig. 9. Fig. 5 is an enlarged cross-sectional view showing the configuration of a piezoelectric element 10 according to Alternative Embodiment 1, and is an enlarged cross-sectional view showing another example of the configuration of portion H1 shown in Fig. 4. As shown in Fig. 5, in Alternative Embodiment 1, the configuration of oxide layer 15 differs from that of the above-described embodiment.

[0069] Specifically, in another embodiment 1, the oxide layer 15 may have a first region 15a. The first region 15a is a region that contains lead oxide in addition to bismuth oxide. That is, in another embodiment 1, the oxide layer 15 may further contain lead oxide.

[0070] Unlike silica glass, the lead oxide contained in this oxide layer 15 can change the valence of lead depending on the surrounding charge when it is present together with bismuth oxide. In another embodiment 1, even when a predetermined negative voltage is applied to the conductor layer 14B and oxygen vacancies segregate in the piezoelectric body 11 near the conductor layer 14B, the change in the valence of lead in the oxide layer 15 allows charge neutrality to be maintained.

[0071] That is, in another embodiment 1, when a predetermined negative voltage is applied to the conductor layer 14B, the oxide layer 15 further containing lead oxide can reduce the segregation of oxygen vacancies in the piezoelectric body 11. Therefore, according to another embodiment 1, the durability of the piezoelectric element 10 can be improved.

[0072] Furthermore, in another embodiment 1, the oxide layer 15 further contains lead oxide, which can reduce the hardness of the oxide layer 15 compared to when the oxide layer 15 is composed of only bismuth oxide. Therefore, according to another embodiment 1, even if the volume of the interface between the piezoelectric body 11 and the conductor layer 14 changes as the piezoelectric element 10 deforms when driven, peeling of the conductor layer 14 from the piezoelectric body 11 can be reduced.

[0073] 5, the first region 15a may be located near the piezoelectric body 11. This can further reduce the segregation of oxygen vacancies in the piezoelectric body 11 even when a predetermined negative voltage is applied to the conductor layer 14B. Therefore, according to the first embodiment, the durability of the piezoelectric element 10 can be further improved.

[0074] In another embodiment 1, since the first region 15a is located near the piezoelectric body 11, peeling of the conductor layer 14 from the piezoelectric body 11 can be further reduced even when the volume of the interface between the piezoelectric body 11 and the conductor layer 14 changes due to deformation when the piezoelectric element 10 is driven.

[0075] 5, the first regions 15a may be spaced apart and scattered within the oxide layer 15. When a voltage is applied to the piezoelectric element 10 to drive it, the piezoelectric body 11 sandwiched between the internal electrodes 12 expands and contracts, causing stress to concentrate between the conductor layer 14 and the piezoelectric body 11. However, the stress can be alleviated by deformation of the edges of the first regions 15a scattered at the interface between the piezoelectric body 11 and the oxide layer 15.

[0076] Alternative Embodiment 2 Fig. 6 is an enlarged cross-sectional view showing the configuration of a piezoelectric element 10 according to Alternative Embodiment 2, and is an enlarged cross-sectional view showing another example of the configuration of portion H1 shown in Fig. 4. As shown in Fig. 6, in Alternative Embodiment 2, the configuration of the first region 15a differs from that of Alternative Embodiment 1. Specifically, in Alternative Embodiment 2, the first region 15a may be located in a layered form within the oxide layer 15.

[0077] This also makes it possible to reduce the segregation of oxygen vacancies in the piezoelectric body 11 when a predetermined negative voltage is applied to the conductor layer 14B, due to the oxide layer 15 further containing lead oxide. Therefore, according to another embodiment 2, the durability of the piezoelectric element 10 can be improved.

[0078] In another embodiment 2, the oxide layer 15 further contains lead oxide, thereby reducing the hardness of the oxide layer 15. Therefore, according to another embodiment 2, even if the volume of the interface between the piezoelectric body 11 and the conductor layer 14 changes as the piezoelectric element 10 deforms when driven, peeling of the conductor layer 14 from the piezoelectric body 11 can be reduced.

[0079] 6, the layered first region 15a may be located near the piezoelectric body 11. This can further reduce the segregation of oxygen vacancies in the piezoelectric body 11 even when a predetermined negative voltage is applied to the conductor layer 14B. Therefore, according to the second embodiment, the durability of the piezoelectric element 10 can be further improved.

[0080] In another embodiment 2, by positioning the first region 15a in the vicinity of the piezoelectric body 11, peeling of the conductor layer 14 from the piezoelectric body 11 can be further reduced even when the volume of the interface between the piezoelectric body 11 and the conductor layer 14 changes due to deformation when the piezoelectric element 10 is driven.

[0081] In another embodiment 2, the first region 15a may be located in a layered state inside the oxide layer 15. When a voltage is applied to the piezoelectric element 10 and the piezoelectric element 10 is continuously driven for a long period of time, the piezoelectric body 11 sandwiched between the internal electrodes 12 generates heat, which causes segregation of oxygen vacancies. However, the layered first region 15a at the interface between the piezoelectric body 11 and the oxide layer 15 becomes more susceptible to change in valence as the temperature rises, and therefore, the segregation of oxygen vacancies in the piezoelectric body 11 can be reduced.

[0082] <Another Embodiment 3> Fig. 7 is an enlarged cross-sectional view showing the configuration of a piezoelectric element 10 according to another embodiment 3, and is an enlarged cross-sectional view showing another example of the configuration of portion H1 shown in Fig. 4. As shown in Fig. 7, in Another Embodiment 3, the configuration of the first region 15a differs from that of the above-described Another Embodiment 2. Specifically, in Another Embodiment 3, the first region 15a may have a multilayer structure inside the oxide layer 15.

[0083] 7, the first region 15a located near the piezoelectric body 11 may include a first layer 15a1 and a second layer 15a2. Both the first layer 15a1 and the second layer 15a2 contain bismuth oxide and lead oxide. The second layer 15a2 is located closer to the piezoelectric body 11 than the first layer 15a1 and has a higher lead oxide content than the first layer 15a1.

[0084] This also makes it possible to reduce the segregation of oxygen vacancies in the piezoelectric body 11 when a predetermined negative voltage is applied to the conductor layer 14B, due to the oxide layer 15 further containing lead oxide. Therefore, according to another embodiment 3, the durability of the piezoelectric element 10 can be improved.

[0085] In another embodiment 3, the oxide layer 15 further contains lead oxide, thereby reducing the hardness of the oxide layer 15. Therefore, according to another embodiment 3, even if the volume of the interface between the piezoelectric body 11 and the conductor layer 14 changes as the piezoelectric element 10 deforms when driven, peeling of the conductor layer 14 from the piezoelectric body 11 can be reduced.

[0086] Furthermore, in another embodiment 3, the first region 15a is located near the piezoelectric body 11, which further reduces the segregation of oxygen vacancies in the piezoelectric body 11 even when a predetermined negative voltage is applied to the conductor layer 14B. Therefore, according to another embodiment 3, the durability of the piezoelectric element 10 can be further improved.

[0087] In another embodiment 3, by positioning the first region 15a in the vicinity of the piezoelectric body 11, peeling of the conductor layer 14 from the piezoelectric body 11 can be further reduced even when the volume of the interface between the piezoelectric body 11 and the conductor layer 14 changes due to deformation when the piezoelectric element 10 is driven.

[0088] Furthermore, in another embodiment 3, the second layer 15a2, which has a high content of lead oxide, is located near the piezoelectric body 11, which further reduces the segregation of oxygen vacancies in the piezoelectric body 11 even when a predetermined negative voltage is applied to the conductor layer 14B. Therefore, according to another embodiment 3, the durability of the piezoelectric element 10 can be further improved.

[0089] In another embodiment 3, the second layer 15a2, which has a high lead oxide content, is located near the piezoelectric body 11, which further reduces peeling of the conductor layer 14 from the piezoelectric body 11 even when the volume of the interface between the piezoelectric body 11 and the conductor layer 14 changes due to deformation when the piezoelectric element 10 is driven.

[0090] 7 shows an example in which the first region 15a has a two-layer structure, but the present disclosure is not limited to this example, and the first region 15a may have a structure of three or more layers. In this case, the layer located closer to the piezoelectric body 11 may have a higher lead oxide content than the layer farther from the piezoelectric body 11. This can further improve the durability of the piezoelectric element 10.

[0091] Alternative Embodiment 4 Fig. 8 is an enlarged cross-sectional view showing the configuration of a piezoelectric element 10 according to Alternative Embodiment 4. As shown in Fig. 8, Alternative Embodiment 4 differs from the above-described embodiments in the configuration of the conductor layer 14. Specifically, in Alternative Embodiment 4, the conductor layer 14 may have an oxide region 16. The oxide region 16 contains bismuth oxide and is located at a position separated from the oxide layer 15.

[0092] This allows an area inside the conductor layer 14 to be created where stress can be relieved, thereby reducing the likelihood of the conductor layer 14 peeling off from the piezoelectric body 11 even if the volume of the interface between the piezoelectric body 11 and the conductor layer 14 changes as the piezoelectric element 10 deforms when it is driven.

[0093] 8 , the oxide regions 16 may be located scattered within the conductor layer 14. When a voltage is applied to the piezoelectric element 10 to drive it, stress is applied between the conductor layer 14 and the piezoelectric body 11 due to expansion and contraction of the piezoelectric body 11 sandwiched between the internal electrodes 12, and there is a risk that the stress will be concentrated particularly at the ends of the conductor layer 14, causing peeling. However, since the stress is also concentrated at the edges of the oxide regions 16 scattered within the conductor layer 14, the stress concentration at the ends of the conductor layer 14 can be alleviated, and stable driving for a long period of time is possible.

[0094] <Another embodiment 5> Fig. 9 is an enlarged cross-sectional view showing the configuration of a piezoelectric element 10 according to another embodiment 5, and is an enlarged cross-sectional view showing another example of the configuration of portion H2 shown in Fig. 4. As shown in Fig. 9, in another embodiment 5, the piezoelectric body 11 includes a plurality of piezoelectric particles 11a.

[0095] In another embodiment 5, bismuth oxide 17 may be located at the crystal grain boundaries 11b between the plurality of piezoelectric particles 11a. Such bismuth oxide 17 may be, for example, bismuth oxide contained in the oxide layer 15 that has diffused therein.

[0096] In this way, the presence of bismuth oxide 17 at the crystal grain boundaries 11b inside the piezoelectric body 11 allows the bismuth oxide 17 to function as a barrier to the short-circuit path between the piezoelectric body 11 and the conductor layer 14. Therefore, according to another embodiment 5, charge neutrality can be maintained between the conductor layer 14 and the piezoelectric body 11.

[0097] In another method for forming the bismuth oxide 17 according to the fifth embodiment, for example, the side surfaces of the laminate including the piezoelectric body 11, the internal electrodes 12, and the planned fracture layer 13 are first roughened by etching with an etching solution. The etching solution used is, for example, a mixture of sodium cyanide, a nitro compound, and water, or hydrochloric acid diluted with water.

[0098] Next, the etched laminate is washed with water and dried. Then, bismuth oxide paste is printed on the side surface of the dried laminate. Then, the laminate with the bismuth oxide paste printed thereon is fired.

[0099] Next, a conductor layer paste that will become the conductor layer 14 is printed on the side surface of the fired laminate. Then, the laminate on which the conductor layer paste has been printed is fired to form the piezoelectric body 11 according to another embodiment 5.

[0100] <Configuration of Mass Flow Controller> Next, an example of a mass flow controller 100 in which the piezoelectric actuator 1 according to the embodiment is mounted will be described with reference to Fig. 10. Fig. 10 is a block diagram showing the configuration of the mass flow controller 100 according to the embodiment.

[0101] 10, the mass flow controller 100 includes a flow path 101, a flow sensor unit 102, a flow control valve 103, and a control circuit unit 104. A fluid such as a gas flows through the flow path 101. The fluid flows in through an inlet 101a and flows out through an outlet 101b.

[0102] A flow rate sensor unit 102 is connected, for example, in a bypass manner, to a portion of the flow path 101. This flow rate sensor unit 102 is configured to be able to detect the flow rate (mass flow rate) of the fluid flowing through the flow path 101. A flow rate signal detected by the flow rate sensor unit 102 is amplified by an amplifier circuit or the like and transmitted to the control circuit unit 104.

[0103] The flow control valve 103 includes the piezoelectric actuator 1 according to the embodiment, and is configured to be able to control the flow rate of the fluid flowing through the flow path 101. The flow control valve 103 can control the flow rate by, for example, expanding and contracting the piezoelectric actuator 1.

[0104] The control circuit section 104 controls each section. For example, the control circuit section 104 compares the flow rate signal transmitted from the flow rate sensor section 102 with a flow rate signal set in advance by the user or the like.

[0105] Then, a drive signal (drive voltage) that eliminates the difference between the transmitted flow rate signal and a preset flow rate signal is input to the piezoelectric actuator 1 provided in the flow rate control valve 103 .

[0106] The piezoelectric actuator 1 expands and contracts in response to the input drive voltage, and this expansion and contraction controls the opening and closing amount of the flow control valve 103 , thereby controlling the flow rate of the fluid flowing through the flow path 101 .

[0107] As described above, the embodiment includes the piezoelectric element 10 that is highly durable even in high temperature environments, and therefore the mass flow controller 100 can be stable for a long period of time even when used in harsh environments.

[0108] The present disclosure has been described in detail above, but the present disclosure is not limited to the above-described embodiments, and various modifications, improvements, etc. are possible within the scope that does not deviate from the gist of the present disclosure.

[0109] The disclosed embodiments should be considered in all respects as illustrative and not restrictive. Indeed, the above-described embodiments may be embodied in various forms. Furthermore, the above-described embodiments may be omitted, substituted, or modified in various ways without departing from the scope and spirit of the appended claims.

[0110] The present technology can be configured as follows. (1) A piezoelectric element comprising: a laminate including a plurality of piezoelectric bodies and internal electrodes stacked together; and conductor layers connected to the internal electrodes and positioned along the stacking direction of the laminate, wherein an oxide layer containing bismuth oxide is positioned near the interface between the piezoelectric body and the conductor layer. (2) The piezoelectric element according to (1), wherein the bismuth oxide contained in the oxide layer contains glass. (3) The piezoelectric element according to (1) or (2), wherein the oxide layer further contains lead oxide. (4) The piezoelectric element according to (3), wherein the lead oxide contained in the oxide layer is positioned near the interface between the oxide layer and the piezoelectric body. (5) The piezoelectric element according to any one of (1) to (4), wherein the conductor layer has an oxide region containing bismuth oxide. (6) The piezoelectric element according to any one of (1) to (5), wherein the thickness of the oxide layer at a portion in contact with the internal electrode is not more than twice the thickness of a portion away from the internal electrode. (7) The piezoelectric element according to any one of (1) to (6), wherein the piezoelectric body includes a plurality of piezoelectric particles, and bismuth oxide is located at a crystal grain boundary between the plurality of piezoelectric particles. (8) A piezoelectric actuator comprising: the piezoelectric element according to any one of (1) to (7); and a case that houses the piezoelectric element. (9) A mass flow controller comprising: a flow path, a flow sensor unit that detects a flow rate of a fluid flowing in the flow path, a flow control valve that has the piezoelectric element according to any one of (1) to (7), and controls the flow rate of the fluid flowing in the flow path by expanding and contracting the piezoelectric element, and a control circuit unit that controls each unit.

[0111] REFERENCE SIGNS LIST 1 Piezoelectric actuator 10 Piezoelectric element 11 Piezoelectric body 11a Piezoelectric body particle 11b Grain boundary 12 Internal electrode 14, 14A, 14B Conductor layer 15 Oxide layer 15a First region 16 Oxide region 17 Bismuth oxide 40 Case 100 Mass flow controller 101 Flow path 102 Flow rate sensor section 103 Flow rate control valve 104 Control circuit section D Stacking direction

Claims

1. A piezoelectric element comprising: a laminate in which a plurality of piezoelectric bodies and internal electrodes are laminated; and conductor layers connected to the internal electrodes and positioned along the lamination direction of the laminate, wherein an oxide layer containing bismuth oxide is positioned near the interface between the piezoelectric body and the conductor layer.

2. The piezoelectric element according to claim 1, wherein the bismuth oxide contained in the oxide layer includes glass.

3. The piezoelectric element according to claim 1 or 2, wherein the oxide layer further contains lead oxide.

4. The piezoelectric element according to claim 3, wherein the lead oxide contained in the oxide layer is located near the interface between the oxide layer and the piezoelectric body.

5. A piezoelectric element according to any one of claims 1 to 4, wherein the conductor layer has an oxide region containing bismuth oxide.

6. A piezoelectric element according to any one of claims 1 to 5, wherein the thickness of the oxide layer at a portion in contact with the internal electrode is not more than twice the thickness of the portion away from the internal electrode.

7. The piezoelectric element according to any one of claims 1 to 6, wherein the piezoelectric body includes a plurality of piezoelectric grains, and bismuth oxide is located at the crystal grain boundaries between the plurality of piezoelectric grains.

8. A piezoelectric actuator comprising: a piezoelectric element according to any one of claims 1 to 7; and a case for accommodating the piezoelectric element therein.

9. A mass flow controller comprising: a flow path; a flow sensor unit for detecting a flow rate of a fluid flowing within the flow path; a flow control valve having a piezoelectric element as described in any one of claims 1 to 7, the flow control valve controlling the flow rate of the fluid flowing within the flow path by the expansion and contraction of the piezoelectric element; and a control circuit unit for controlling each of the components.

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