Piezoelectric element, piezoelectric actuator, and mass flow controller

Abstract

This piezoelectric element comprises a columnar laminate and two conductor layers. In the columnar laminate, internal electrodes and a piezoelectric body having potassium sodium niobate as the main component are alternately laminated. The two conductor layers are positioned on the respective lateral surfaces of the laminate and are connected to the corresponding internal electrodes. The piezoelectric body has a first portion and a second portion. The first portion overlaps the internal electrodes in a planar perspective view in the lamination direction. The second portion is located outside the first portion and does not overlap the internal electrodes in the planar perspective view. The concentration of at least one of potassium and sodium contained in the piezoelectric body is constant in the first portion, decreases stepwise at the interface between the second portion and the first portion, and decreases gradually from the interface to the surface in the second portion.

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 including a piezoelectric element laminated in a columnar shape is known. In this piezoelectric element, for example, a piezoelectric body mainly composed of potassium sodium niobate represented by the general formula (K,Na)NbO ３ is used (see, for example, Patent Document 1).

[0003] International Publication No. 2020 / 230450

[0004] The piezoelectric element of the present disclosure includes a columnar laminate and two conductor layers. In the columnar laminate, a piezoelectric body mainly composed of potassium sodium niobate and internal electrodes are alternately laminated. The two conductor layers are respectively located on the side surfaces of the laminate and are respectively connected to the corresponding internal electrodes. The piezoelectric body has a first portion and a second portion. The first portion overlaps with the internal electrode in a plan view in the stacking direction. The second portion is located outside the first portion and does not overlap with the internal electrode in a plan view. The concentration of at least one of potassium and sodium contained in the piezoelectric body is constant in the first portion, decreases stepwise at the interface between the second portion and the first portion, and gradually decreases from the interface to the surface in the second portion.

[0005] Figure 1 is a perspective view showing the overall configuration of the piezoelectric actuator according to the embodiment. Figure 2 is a cross-sectional view taken along the line II-II shown in Figure 1. Figure 3 is a perspective view showing the internal structure of the piezoelectric actuator according to the embodiment. Figure 4 is a cross-sectional view taken along the line IV-IV shown in Figure 3. Figure 5 is a cross-sectional view taken along the line V-V shown in Figure 4. Figure 6 is a diagram for explaining the configuration of the piezoelectric element according to the embodiment. Figure 7 is a diagram for explaining an example of the manufacturing process of the piezoelectric element according to the embodiment. Figure 8 is a diagram for explaining an example of the manufacturing process of the piezoelectric element according to the embodiment. Figure 9 is a diagram for explaining an example of the manufacturing process of the piezoelectric element according to the embodiment. Figure 10 is a diagram for explaining an example of the manufacturing process of the piezoelectric element according to the embodiment. Figure 11 is a diagram for explaining another example of the manufacturing process of the piezoelectric element according to the embodiment. Figure 12 is a diagram for explaining another example of the manufacturing process of the piezoelectric element according to the embodiment. Figure 13 is a diagram for explaining another example of the manufacturing process of the piezoelectric element according to the embodiment. Figure 14 is a diagram for explaining another example of the manufacturing process of the piezoelectric element according to the embodiment. Figure 15 is an enlarged cross-sectional view showing an example of the crystal structure of the piezoelectric element according to the embodiment. Figure 16 is an enlarged cross-sectional view showing an example of the crystal structure of the piezoelectric element according to the embodiment. Figure 17 is an enlarged cross-sectional view showing another example of the crystal structure of the piezoelectric element according to the embodiment. Figure 18 is an enlarged cross-sectional view showing another example of the crystal structure of the piezoelectric element according to the embodiment. Figure 19 is an enlarged cross-sectional view showing another example of the crystal structure of the piezoelectric element according to the embodiment. Figure 20 is a cross-sectional view showing another example of the configuration of the piezoelectric element according to the embodiment. Figure 21 is a block diagram showing the configuration of the mass flow controller according to the embodiment.

[0006] The embodiments for implementing the piezoelectric element, piezoelectric actuator, and mass flow controller described herein (hereinafter referred to as "Embodiments") will be described in detail below with reference to the drawings. However, these embodiments do not limit the present disclosure.

[0007] Furthermore, each embodiment can be combined as appropriate, provided that the processing content is not inconsistent. Also, the same parts are denoted by the same reference numerals in each of the following embodiments, and redundant explanations are omitted.

[0008] Furthermore, in the drawings referenced below, for the sake of clarity, mutually orthogonal X-axis, Y-axis, and Z-axis directions are sometimes defined, and a Cartesian coordinate system is shown in which the Z-axis direction is the extension / retraction direction of the piezoelectric actuator.

[0009] Furthermore, it should be noted that drawings are schematic representations, and the dimensional relationships and proportions of each element may differ from reality. Moreover, there may be discrepancies in dimensional relationships and proportions between drawings themselves.

[0010] Conventionally, piezoelectric actuators equipped with columnar stacked piezoelectric elements are known. In these piezoelectric elements, for example, the general formula is (K,Na)NbO ３ A piezoelectric material primarily composed of potassium sodium niobate, represented by [formula], is used.

[0011] However, in conventional technology, when a piezoelectric actuator was driven for a long time in a high-temperature environment, potassium and sodium, which are highly volatile, could volatilize from the surface of the piezoelectric element, potentially changing the composition of the piezoelectric material within the element. This could lead to a change in the displacement of the piezoelectric element from its initial value.

[0012] Therefore, there is a need for a technology that can solve the above problems and reduce the change in the displacement of the piezoelectric element over a long period of time.

[0013] <Overall Configuration of the Piezoelectric Actuator> First, the overall configuration of the piezoelectric actuator 1 according to the embodiment will be described with reference to Figures 1 to 5. Figure 1 is a perspective view showing the overall configuration of the piezoelectric actuator 1 according to the embodiment. Figure 2 is a cross-sectional view taken along the line II-II shown in Figure 1. Figure 3 is a perspective view showing the internal structure of the piezoelectric actuator 1 according to the embodiment. Figure 4 is a cross-sectional view taken along the line IV-IV shown in Figure 3. Figure 5 is a cross-sectional view taken along the line V-V shown in Figure 4.

[0014] As shown in Figures 1 to 3, the piezoelectric actuator 1 according to this embodiment comprises 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.

[0015] As shown in Figure 3, the piezoelectric element 10 has a columnar shape. For example, the piezoelectric element 10 is a rectangular prism (cuboid) with dimensions of 0.5 mm to 10 mm in length, 0.5 mm to 10 mm in width, and 1 mm to 100 mm in height. However, the shape of the piezoelectric element 10 is not limited to a rectangular prism; it may also be a hexagonal prism, an octagonal prism, or a cylindrical shape.

[0016] As shown in Figure 2, the piezoelectric element 10 comprises a laminate 11 and a pair of conductive layers 12. The laminate 11 also comprises a piezoelectric element 13, an internal electrode 14, a planned fracture layer 15, and a pair of coating layers 16 (see Figure 3). The piezoelectric element 13 is an example of a first part, and the coating layers 16 are an example of a second part. The pair of conductive layers 12 include conductive layer 12A and conductive layer 12B.

[0017] The laminate 11 is constructed by stacking a plurality of piezoelectric elements 13, internal electrodes 14, and planned fracture layers 15 in a predetermined order along the stacking direction D. In this disclosure, the stacking direction D of the laminate 11 coincides with the longitudinal direction of the piezoelectric element 10.

[0018] The piezoelectric element 13 is composed of piezoelectric ceramics having piezoelectric properties. In this embodiment, the piezoelectric element 13 is potassium sodium niobate ((K,Na)NbO ３ It is composed mainly of ). The thickness of one layer of piezoelectric material 13 is, for example, 3 μm to 250 μm.

[0019] The internal electrode 14 is made of a conductive material and includes a plurality of first electrodes 14a and a plurality of second electrodes 14b. As shown in Figure 4, the first electrodes 14a are electrically connected to a conductor layer 12A. As shown in Figure 3, this conductor layer 12A is located along the lamination direction D on one side surface 11a of the laminate 11. A predetermined positive voltage is applied to the first electrodes 14a, for example, via the conductor layer 12A.

[0020] The second electrode 14b is electrically connected to the conductor layer 12B. This conductor layer 12B is located along the lamination direction D on the side 11b of the laminate 11 opposite to the side 11a. A predetermined negative voltage (or ground voltage) is applied to the second electrode 14b, for example, via the conductor layer 12B.

[0021] As shown in Figure 5, inside the laminate 11, the first electrode 14a, the second electrode 14b, and the piezoelectric element 13 are stacked such that the piezoelectric element 13 is positioned between the first electrode 14a and the second electrode 14b.

[0022] As a result, the piezoelectric element 10 can apply a driving voltage to the piezoelectric body 13 using the first electrode 14a and the second electrode 14b.

[0023] As shown in Figure 3, the piezoelectric element 10 according to this embodiment is composed of an active portion formed by alternately stacking a plurality of piezoelectric bodies 13 and internal electrodes 14, and an inactive portion having a piezoelectric body 13 located at both ends of the stacking direction D in the active portion.

[0024] The active portion is the part of the piezoelectric material sandwiched between electrodes of different polarities that expands or contracts (hereinafter also referred to as "expansion and contraction") in the stacking direction D when a driving voltage is applied to the piezoelectric element 10 from the outside. On the other hand, the inactive portion is the part of the piezoelectric material that is not sandwiched between electrodes of different polarities, and does not expand or contract even when a driving voltage is applied to the piezoelectric element 10 from the outside.

[0025] Furthermore, in this disclosure, the end of the case 40 on the base 41 side is the base end 11e of the laminate 11, and the end of the case 40 on the lid 43 side is the tip end 11f of the laminate 11.

[0026] In the piezoelectric actuator 1 according to this embodiment, the base end portion 11e (i.e., the base body 41) of the laminate 11 is fixed, and the tip portion 11f (i.e., the cover body 43) of the laminate 11 is displaced along the lamination direction D.

[0027] The material of the internal electrode 14 is, for example, a metal mainly composed of silver, silver-palladium, silver-platinum, or copper. The internal electrode 14 can be formed, for example, by co-firing with the piezoelectric body 13. The thickness of the internal electrode 14 is, for example, 0.1 μm to 5 μm.

[0028] The planned fracture layer 15 is a layer for relieving the stress generated by the driving of the piezoelectric element 10. Examples of the planned fracture layer 15 include an extremely porous metal layer that does not function as an internal electrode 14, or an extremely porous piezoelectric layer. In the piezoelectric element 10 according to this embodiment, the planned fracture layer 15 may be omitted.

[0029] As described above, the pair of conductive layers 12 include a conductive layer 12A located on the side surface 11a of the laminate 11 and a conductive layer 12B located on the side surface 11b of the laminate 11. The conductive layers 12 are positioned to extend across the entire active portion of the piezoelectric element 10.

[0030] The material of the conductive layer 12 is, for example, a metal mainly composed of silver or copper. For example, a metallized layer made of a sintered body of the above-mentioned metal and glass can be used for the conductive layer 12. The thickness of the conductive layer 12 is, for example, 5 μm to 500 μm.

[0031] The pair of coating layers 16 are located on side surfaces 11c and 11d of the laminate 11, respectively. Side surfaces 11c and 11d are examples of surfaces, and are located between side surfaces 11a and 11b. In addition, the coating layers 16 cover the first electrode 14a and the second electrode 14b, as shown in Figure 5.

[0032] The coating layer 16 prevents the internal electrode 14 from being exposed from the side surfaces 11c and 11d of the laminate 11. This reduces surface discharge between the two electrodes that occurs when a high voltage is applied during operation.

[0033] The coating layer 16 according to this embodiment is composed of, for example, a piezoelectric material mainly composed of potassium sodium niobate. This allows the coating layer 16 to follow the expansion and contraction of the piezoelectric element 10 when the piezoelectric actuator 1 is driven.

[0034] The pair of electrode plates 20 includes electrode plate 20A and electrode plate 20B, and are electrically connected to a pair of conductor layers 12, respectively. Specifically, electrode plate 20A is electrically connected to conductor layer 12A, and electrode plate 20B is electrically connected to conductor layer 12B.

[0035] As shown in Figure 3, for example, the electrode plate 20 is positioned along the stacking direction D of the laminate 11 as a whole, and locally meanders to intersect the stacking direction D. At least a portion of the electrode plate 20 that intersects the stacking direction D is bonded to the conductor layer 12 via a conductive bonding material 21.

[0036] As such a bonding material 21, for example, an epoxy resin or polyimide resin containing a highly conductive metal powder such as Ag powder or Cu powder can be used.

[0037] The electrode plate 20 is made of 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 coated with a plating film such as tin plating or silver plating to improve electrical and thermal conductivity.

[0038] The pair of lead terminals 30 includes lead terminal 30A and lead terminal 30B, as shown in Figures 1 and 2. The pair of lead terminals 30 are electrically connected to the pair of electrode plates 20, respectively. For example, as shown in Figure 2, lead terminal 30A is electrically connected to electrode plate 20A via a conductive member 31A and a conductive adhesive 32A, and lead terminal 30B is electrically connected to electrode plate 20B via a conductive member 31B and a conductive adhesive 32B.

[0039] As shown in Figure 2, the case 40 houses the piezoelectric element 10 and the electrode plate 20 inside. The case 40 has a base body 41, a cylindrical body 42, and a lid 43. The base body 41 is columnar (for example, cylindrical) and may have a cylindrical portion 41a protruding in a cylindrical shape at one end (the upper side in Figure 2).

[0040] Further, the inner bottom surface 41b located at the bottom of the cylindrical portion 41a contacts the base end portion 11e of the laminate 11. Note that the inner bottom surface 41b of the base body 41 and the base end portion 11e of the laminate 11 may be joined with a joining material not shown in the drawings.

[0041] Further, in the base body 41, a pair of through holes 41d penetrating between the inner bottom surface 41b and the end surface 41c on the opposite side of the inner bottom surface 41b are located, and a pair of lead terminals 30 are respectively inserted into the pair of through holes 41d.

[0042] Then, an insulating material (for example, soft glass or the like) is filled in the gap between the through hole 41d of the base body 41 and the lead terminal 30, whereby the lead terminal 30 is fixed to the base body 41. Further, as shown in FIG. 2, the lead terminal 30 passes through the base body 41 from the inside of the case 40 and protrudes outward from the end surface 41c of the base body 41.

[0043] The cylindrical body 42 has, for example, a cylindrical shape. Further, the cylindrical body 42 has a bellows (bellows) shape, and the axial direction of the cylinder coincides with the stacking direction D of the piezoelectric element 10. Thereby, the cylindrical body 42 can expand and contract in the axial direction following the expansion and contraction of the piezoelectric element 10 in the stacking direction D.

[0044] Further, the cylindrical body 42 has a predetermined spring constant so as to be able to 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 groove shape of the cylindrical body 42, the number of grooves of the cylindrical body 42, and the like. 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.

[0045] Further, the end portion 42a of the cylindrical body 42 on the side of the base body 41 is welded, for example, in a state where a compressive load is applied to the piezoelectric element 10. Further, the cylindrical body 42 may be welded after providing a flange portion that expands in a trumpet shape toward the outside in the radial direction at the end portion 42a on the side of the base body 41.

[0046] The cylindrical body 42 is formed, for example, by preparing a seamless tube having a predetermined shape and then processing the seamless tube into a bellows (bellows) shape by rolling or hydrostatic pressing.

[0047] The lid 43 is cylindrical in shape (for example, cylindrical) with one end closed. The lid 43 is fitted onto the end 42b of the cylindrical body 42, and the inner wall of the end 42b and the side wall of the lid 43 are fixed together, for example, by welding.

[0048] Furthermore, the inner bottom surface 43a of the lid 43 is in contact with the tip portion 10f of the piezoelectric element 10. The inner bottom surface 43a of the lid 43 and the tip portion 10f of the piezoelectric element 10 may be joined together with a bonding material (not shown).

[0049] <Embodiment> Next, the detailed configuration of the piezoelectric element 10 according to the embodiment will be described with reference to Figure 6. Figure 6 is a diagram illustrating the configuration of the piezoelectric element 10 according to the embodiment.

[0050] Figure 6(a) shows the configuration of section R1 shown in Figure 5. Figure 6(b) shows the sodium concentration profile in the piezoelectric element 13 and the coating layer 16 before driving along line L shown in Figure 6(a).

[0051] Furthermore, Figure 6(c) shows the sodium concentration profile in the piezoelectric element 13 and the coating layer 16 after long-term operation along the line L shown in Figure 6(a).

[0052] In this embodiment, as shown in Figure 6(b), the sodium concentration in the piezoelectric element 10 before operation may be constant at concentration C1 in the piezoelectric body 13. Alternatively, the sodium concentration in the piezoelectric element 10 before operation may decrease stepwise from concentration C1 to concentration C2 at the interface 17 between the coating layer 16 and the piezoelectric body 13.

[0053] Furthermore, in the piezoelectric element 10 before operation, the sodium concentration may gradually decrease from concentration C2 to concentration C3 in the coating layer 16 from the interface 17 to the side surface 11c. Although not shown in the figures, a similar concentration profile may also be formed on the side surface 11d (see Figure 5) opposite to side surface 11c.

[0054] When the piezoelectric element 10 is driven, it generates its own heat, and this self-heating occurs particularly significantly at the interface 17 between the piezoelectric body 13 and the coating layer 16. Due to this self-heating and other factors, sodium volatilizes from the side surface 11c of the coating layer 16, causing the sodium near the interface 17 in the coating layer 16 to diffuse towards the side surface 11c.

[0055] In this embodiment, as shown in Figure 6(b), a continuous concentration gradient is provided in advance in the coating layer 16 such that the sodium concentration is lower on the side surface 11c than on the interface 17.

[0056] As a result, sodium volatilizes from the coating layer 16, further increasing this concentration gradient. Consequently, sodium near the interface 17 in the coating layer 16 also preferentially diffuses towards the side surface 11c in proportion to this concentration gradient.

[0057] Furthermore, if a discontinuous (i.e., stepwise) concentration difference of sodium is pre-established at the interface 17 between the piezoelectric element 13 and the coating layer 16, a phenomenon of sodium interdiffusion occurs around the interface 17. The diffusion rate of this interdiffusion phenomenon is then limited by the slower migration speed in order to balance the overall charge.

[0058] In this case, the diffusion of sodium from the coating layer 16 to the piezoelectric element 13 is slower than the diffusion of sodium from the piezoelectric element 13 to the coating layer 16. Therefore, if a discontinuous concentration difference of sodium is created at the interface 17 between the piezoelectric element 13 and the coating layer 16, there is almost no movement of sodium from the coating layer 16 to the piezoelectric element 13.

[0059] As the sodium concentration in the coating layer 16 decreases due to volatilization, the volatilization of sodium gradually decreases, and the inflection point F of the sodium concentration in the coating layer 16 gradually shifts towards the side surface 11c, until finally the volatilization of sodium stops.

[0060] In this disclosure, the inflection point F of the sodium concentration is the point at which the sodium concentration profile switches from being convex downwards to convex upwards.

[0061] Thus, in this embodiment, even after the piezoelectric element 10 has been driven for a long time, the sodium in the piezoelectric body 13, which is the active region, is not affected by volatilization. Therefore, as shown in Figure 6(c), the sodium concentration in the piezoelectric body 13 remains stable for a long time without increasing or decreasing. Accordingly, according to this embodiment, the change in the displacement of the piezoelectric element 10 can be kept small over a long period of time.

[0062] Furthermore, in this embodiment, the sodium in the piezoelectric element 10 has a concentration profile as shown in Figure 6(b), which makes it possible to minimize the change in the displacement of the piezoelectric element 10 over a long period of time, even with a thin coating layer 16.

[0063] Therefore, according to this embodiment, the proportion of the coating layer 16 in the entire piezoelectric element 10 can be reduced, thereby improving the driving force of the piezoelectric element 10. The coating layer 16 in this embodiment may be, for example, 1 μm to 30 μm thick.

[0064] Furthermore, in this embodiment, the sodium concentration difference Ca (= C2 - C3) between the interface 17 and the side surface 11c may be greater than the sodium concentration difference Cb (= C1 - C2) between the interface 17 and the piezoelectric material 13.

[0065] This allows sodium near the interface 17 in the coating layer 16 to diffuse more preferentially toward the side surface 11c than toward the piezoelectric element 13. Therefore, according to this embodiment, the sodium concentration of the piezoelectric element 13 remains stable for a long time, and thus the change in the displacement of the piezoelectric element 10 can be reduced over an even longer period of time.

[0066] In this embodiment, the sodium concentration difference Cb between the interface 17 and the piezoelectric element 13 may be, for example, 1 wt% to 5 wt%.

[0067] Although Figure 6 shows the sodium concentration profile in the piezoelectric element 13 and the coating layer 16, this disclosure is not limited to such an example. For example, in this embodiment, the potassium concentration profile in the piezoelectric element 13 and the coating layer 16 may have the shape shown in Figure 6(b).

[0068] This makes it possible to reduce the concentration change in the piezoelectric element 13, which is the active region, even for potassium, which is as volatile as sodium. Therefore, according to this embodiment, it is possible to reduce the change in the displacement of the piezoelectric element 10 over a long period of time.

[0069] In this disclosure, the sodium and potassium concentration profiles shown in Figure 6(b) can be measured, for example, by ToF-SIMS (Time-of-Flight Secondary Ion Mass Spectrometry) and D-SIMS (Dynamic Secondary Ion Mass Spectrometry).

[0070] <Manufacturing Process> Next, the manufacturing process of the piezoelectric element 10 according to the embodiment will be explained with reference to Figures 7 to 14. Figures 7 to 10 are diagrams illustrating an example of the manufacturing process of the piezoelectric element 10 according to the embodiment. For ease of understanding, Figures 7 to 10 also show the configuration of the R1 section shown in Figure 5.

[0071] In the manufacturing process of the piezoelectric element 10 according to this embodiment, first, a conductive layer paste, which will become the internal electrode 14, is printed on the surface of a ceramic green sheet that will become the piezoelectric body 13. Then, multiple ceramic green sheets with the conductive layer paste printed on them are stacked to form a columnar laminate.

[0072] Next, by firing this laminate at a predetermined temperature, a fired body having a piezoelectric element 13 and a plurality of internal electrodes 14 is obtained.

[0073] As shown in Figure 7, in this fired body, a region is formed near the side surface 13c of the piezoelectric element 13, which corresponds to the side surface 11c (see Figure 3) of the laminate 11 (see Figure 3), where sodium volatilizes during the firing process, resulting in a gradual decrease in sodium concentration.

[0074] Therefore, in this embodiment, following the firing process, a cutting process is performed to remove the region near the side surface 11c where the sodium concentration decreases. As a result, as shown in Figure 8, the sodium concentration from the inside of the piezoelectric body 13 to the side surface 13c is stabilized at concentration C1.

[0075] In the manufacturing process of the piezoelectric element 10 according to this embodiment, a ceramic paste 16A, which will become a coating layer 16 (see Figure 10), is then applied to the side surface 13c of the piezoelectric body 13, as shown in Figure 9. The sodium concentration C2 of this ceramic paste 16A is set to a lower concentration than the sodium concentration C1 of the piezoelectric body 13.

[0076] Next, the sintered body coated with ceramic paste 16A is fired at a predetermined temperature. As a result, the ceramic paste 16A (see Figure 9) becomes the coating layer 16, as shown in Figure 10.

[0077] Furthermore, in the ceramic paste 16A, sodium volatilizes during the firing process, so a concentration gradient is formed in the coating layer 16 such that the sodium concentration is lower on the side surface 11c than on the interface 17. This results in the piezoelectric element 10 according to the embodiment.

[0078] Figures 11 to 14 illustrate another example of the manufacturing process of the piezoelectric element 10 according to the embodiment. For ease of understanding, Figures 11 to 14 also show the configuration of the R1 section shown in Figure 5.

[0079] In the example shown in Figure 11, following the cutting process shown in Figure 8, ceramic paste 16A1 and ceramic paste 16A2, which will form the coating layer 16 (see Figure 10), are applied to the side surface 13c of the piezoelectric element 13. That is, in the example shown in Figure 11, the ceramic paste 16A is applied in two separate layers.

[0080] In the example shown in Figure 11, the sodium concentration C2 of the inner ceramic paste 16A1 is set to a lower concentration than the sodium concentration C1 of the piezoelectric element 13. Furthermore, the sodium concentration C4 of the outer ceramic paste 16A2 is set to a lower concentration than the sodium concentration C2 of the inner ceramic paste 16A1.

[0081] As a result, the subsequent firing process of the ceramic paste 16A more easily creates a concentration gradient in the coating layer 16 such that the sodium concentration is lower on the side surface 11c than on the interface 17, as shown in Figure 10.

[0082] In the example shown in Figure 12, following the cutting process shown in Figure 8, a ceramic paste 16A1, which will become part of the coating layer 16 (see Figure 10), is applied to the side surface 13c of the piezoelectric element 13. The sodium concentration C2 of this ceramic paste 16A1 is set to a lower concentration than the sodium concentration C1 of the piezoelectric element 13.

[0083] Next, the sintered body coated with ceramic paste 16A1 is fired at a predetermined temperature. As a result, the ceramic paste 16A1 (see Figure 12) becomes a coating layer 16a, as shown in Figure 13.

[0084] Furthermore, in the ceramic paste 16A1, sodium volatilizes during the firing process, so a concentration gradient is formed in the coating layer 16a such that the sodium concentration is lower on the surface 16a1 than at the interface 17.

[0085] Next, as shown in Figure 14, a ceramic paste 16A2, which will become part of the coating layer 16 (see Figure 10), is applied to the surface 16a1 of the coating layer 16a. The sodium concentration C4 of this ceramic paste 16A2 is set to a lower concentration than the sodium concentration C2 of the ceramic paste 16A1 (see Figure 12).

[0086] As a result, the subsequent firing process of the ceramic paste 16A2 more easily creates a concentration gradient in the coating layer 16 such that the sodium concentration is lower on the side surface 11c than on the interface 17, as shown in Figure 10.

[0087] In the examples shown in Figures 11 to 14, the ceramic paste that forms the coating layer 16 is applied in two separate layers. However, the ceramic paste that forms the coating layer 16 may be applied in three or more separate layers. This makes it even easier to create a concentration gradient in the coating layer 16 such that the sodium concentration is lower on the side surface 11c than on the interface 17.

[0088] Furthermore, while the examples in Figures 7 to 14 show a method for forming sodium concentration profiles in the piezoelectric element 13 and the coating layer 16, potassium concentration profiles in the piezoelectric element 13 and the coating layer 16 may also be formed using a similar method.

[0089] <Crystal Structure> Next, the crystal structure of the piezoelectric element 10 according to the embodiment will be described with reference to Figures 15 and 16. Figure 15 is an enlarged cross-sectional view showing an example of the crystal structure of the piezoelectric element 10 according to the embodiment, and shows a configuration that is a further enlargement of the R1 portion shown in Figure 5.

[0090] As shown in Figure 15, the average particle size of the coating layer 16 near the interface 17 may be smaller than the average particle size of the piezoelectric element 13 near the interface 17. In other words, near the interface 17, the average particle size of the particles P2 constituting the coating layer 16 may be smaller than the average particle size of the particles P1 constituting the piezoelectric element 13.

[0091] The average particle size may be calculated from SEM (Scanning Electron Microscope) images using the linear intercept method, calculated using software built into a digital microscope (for example, VHX manufactured by Keyence), or calculated from SEM images using image analysis software such as WinROOF (manufactured by Mitani Corporation).

[0092] Sodium and potassium located away from the side surface 11c reach the side surface 11c via the grain boundary phase. Here, because the average grain size of the coating layer 16 is smaller than the average grain size of the piezoelectric material 13, the mesh of the grain boundary phase A2 (see Figure 16) in the coating layer 16 is finer than the mesh of the grain boundary phase A1 (see Figure 16) in the piezoelectric material 13. As a result, the volatilization of sodium and potassium is possible only in the coating layer 16.

[0093] Furthermore, as sodium and potassium volatilize from the side surface 11c, a concentration distribution and gradient of sodium and potassium, as shown in Figure 6(c), is formed inside the coating layer 16, and the inflection point F of the sodium concentration in the coating layer 16 gradually shifts towards the side surface 11c. Then, when an equilibrium state is reached where sodium and potassium can no longer volatilize from the side surface 11c, the volatilization of sodium and potassium stops.

[0094] Therefore, it is possible to reduce the concentration changes of sodium and potassium in the piezoelectric material 13, which is the active region.

[0095] Furthermore, in this embodiment, the average particle size of the coating layer 16 near the side surface 11c may be smaller than the average particle size of the coating layer 16 near the interface 17. In other words, the average particle size of the particles P2 near the side surface 11c may be smaller than the average particle size of the particles P2 near the interface 17.

[0096] Sodium and potassium located away from the side surface 11c reach the side surface 11c via the grain boundary phase. Here, because the average grain size of the coating layer 16 near the side surface 11c is smaller than the average grain size of the coating layer 16 near the interface 17, the network of the grain boundary phase A2 near the side surface 11c is finer than the network of the grain boundary phase A2 near the interface 17. As a result, the volatilization of sodium and potassium is possible only within the coating layer 16.

[0097] Furthermore, as sodium and potassium volatilize from the side surface 11c, a concentration distribution and gradient of sodium and potassium, as shown in Figure 6(c), is formed inside the coating layer 16, and the inflection point F of the sodium concentration in the coating layer 16 gradually shifts towards the side surface 11c. Then, when an equilibrium state is reached where sodium and potassium can no longer volatilize from the side surface 11c, the volatilization of sodium and potassium stops.

[0098] Therefore, it is possible to reduce the concentration changes of sodium and potassium in the piezoelectric material 13, which is the active region.

[0099] Furthermore, in this embodiment, as shown in Figure 15, fine grooves G may be located on the side surface 11c. The presence of grooves G on the side surface 11c can alleviate the stress generated between the piezoelectric element 13 and the coating layer 16.

[0100] Furthermore, the presence of grooves G on the side surface 11c allows for further volatilization of sodium and potassium from the coating layer 16 through these grooves G. As a result, the volatilization of sodium and potassium is possible solely within the coating layer 16.

[0101] Furthermore, as sodium and potassium volatilize from the side surface 11c, a concentration distribution and gradient of sodium and potassium, as shown in Figure 6(c), is formed inside the coating layer 16, and the inflection point F of the sodium concentration in the coating layer 16 gradually shifts towards the side surface 11c. Then, when an equilibrium state is reached where sodium and potassium can no longer volatilize from the side surface 11c, the volatilization of sodium and potassium stops.

[0102] Therefore, it is possible to reduce the concentration changes of sodium and potassium in the piezoelectric material 13, which is the active region.

[0103] Figure 16 is an enlarged cross-sectional view showing an example of the crystal structure of the piezoelectric element 10 according to the embodiment, and shows an enlarged configuration of the R2 portion shown in Figure 15.

[0104] As shown in Figure 16, the area of ​​grain boundary phase A2 per unit area of ​​the coating layer 16 near the interface 17 may be larger than the area of ​​grain boundary phase A1 per unit area of ​​the piezoelectric material 13 near the interface 17.

[0105] In other words, near the interface 17, the area per unit area of ​​the grain boundary phase A2 located between adjacent particles P2 may be larger than the area per unit area of ​​the grain boundary phase A1 located between adjacent particles P1.

[0106] Because the area of ​​grain boundary phase A2 located in the coating layer 16 is larger than the area of ​​grain boundary phase A1 located in the piezoelectric material 13, the amount of sodium and potassium contained in grain boundary phase A2 is greater than that in grain boundary phase A1.

[0107] As sodium and potassium volatilize from the side surface 11c, sodium and potassium are replenished from the particles P2 to the grain boundary phase A2 in the coating layer 16. As a result, the volatilization of sodium and potassium becomes possible only in the coating layer 16.

[0108] Furthermore, as sodium and potassium volatilize from the side surface 11c, a concentration distribution and gradient of sodium and potassium, as shown in Figure 6(c), is formed inside the coating layer 16, and the inflection point F of the sodium concentration in the coating layer 16 gradually shifts towards the side surface 11c. Then, when an equilibrium state is reached where sodium and potassium can no longer volatilize from the side surface 11c, the volatilization of sodium and potassium stops.

[0109] Therefore, it is possible to reduce the concentration changes of sodium and potassium in the piezoelectric material 13, which is the active region.

[0110] Furthermore, in this embodiment, the area of ​​grain boundary phase A2 per unit area of ​​the coating layer 16 near the side surface 11c (see Figure 15) may be larger than the area of ​​grain boundary phase A2 per unit area of ​​the coating layer 16 near the interface 17.

[0111] Because the area of ​​grain boundary phase A2 near side surface 11c is larger than the area of ​​grain boundary phase A2 near interface 17, the amount of sodium and potassium contained in grain boundary phase A2 is greater near side surface 11c.

[0112] As sodium and potassium volatilize from the side surface 11c, sodium and potassium are replenished from the particles P2 into the grain boundary phase A2 near the side surface 11c. As a result, the volatilization of sodium and potassium is possible only within the coating layer 16.

[0113] Furthermore, as sodium and potassium volatilize from the side surface 11c, a concentration distribution and gradient of sodium and potassium, as shown in Figure 6(c), is formed inside the coating layer 16, and the inflection point F of the sodium concentration in the coating layer 16 gradually shifts towards the side surface 11c. Then, when an equilibrium state is reached where sodium and potassium can no longer volatilize from the side surface 11c, the volatilization of sodium and potassium stops.

[0114] Therefore, it is possible to reduce the concentration changes of sodium and potassium in the piezoelectric material 13, which is the active region.

[0115] In this disclosure, the area of ​​grain boundary phase A1 can be calculated, for example, by subtracting the area of ​​particle P1 from the total area of ​​piezoelectric material 13 using image processing. Similarly, the area of ​​grain boundary phase A2 can be calculated, for example, by subtracting the area of ​​particle P2 from the total area of ​​coating layer 16 using image processing.

[0116] Figures 17 to 19 are enlarged cross-sectional views showing another example of the crystal structure of the piezoelectric element 10 according to the embodiment, and correspond to Figure 15 of the embodiment.

[0117] As shown in Figure 17, in this disclosure, manganese (Mn) may be segregated at the interface 17 between the piezoelectric element 13 and the coating layer 16. Manganese has the characteristic of changing its valence state in order to maintain neutrality of the surrounding charge.

[0118] Manganese segregation can be confirmed by measuring with an EPMA (Electron Probe Microanalyzer) when manganese is detected between adjacent crystalline particles of the piezoelectric material 13 and the crystalline particles of the coating layer 16, where manganese is not detected elsewhere, or when more manganese is detected than elsewhere.

[0119] Here, if manganese is present at the interface 17 between the piezoelectric element 13 and the coating layer 16, sodium and potassium will volatilize from the side surface 11c, and when the sodium and potassium of the piezoelectric element 13 attempt to ionize and move to the side surface 11c, the manganese at the interface 17 will change its valency, allowing the sodium and potassium ions to remain in the piezoelectric element 13.

[0120] Therefore, it is possible to reduce the concentration changes of sodium and potassium in the piezoelectric material 13, which is the active region.

[0121] Furthermore, as shown in Figure 18, in this disclosure, lithium (Li) may be segregated at the interface 17 between the piezoelectric element 13 and the coating layer 16. Lithium is more volatile than sodium and potassium. The method for confirming lithium segregation is the same as for manganese.

[0122] Here, if lithium is present at the interface 17 between the piezoelectric element 13 and the coating layer 16, sodium and potassium will volatilize from the side surface 11c, and when the sodium and potassium of the piezoelectric element 13 attempt to ionize and move to the side surface 11c, the lithium at the interface 17 will preferentially volatilize, thus retaining the sodium and potassium ions in the piezoelectric element 13.

[0123] Therefore, it is possible to reduce the concentration changes of sodium and potassium in the piezoelectric material 13, which is the active region.

[0124] Furthermore, as shown in Figure 19, in this disclosure, manganese and lithium may be segregated at the interface 17 between the piezoelectric element 13 and the coating layer 16.

[0125] Here, if manganese and lithium are present at the interface 17 between the piezoelectric element 13 and the coating layer 16, sodium and potassium will volatilize from the side surface 11c. When the sodium and potassium in the piezoelectric element 13 attempt to ionize and move to the side surface 11c, the sodium and potassium ions can be retained in the piezoelectric element 13 due to the effect described above.

[0126] Therefore, the concentration changes of sodium and potassium in the piezoelectric material 13, which is the active region, can be reduced. In the example shown in Figure 19, it is even better if manganese and lithium are separated from each other at the interface 17.

[0127] Figure 20 is a cross-sectional view showing another example of the configuration of the piezoelectric element 10 according to the embodiment, and corresponds to Figure 4 of the embodiment. In the example of Figure 20, the conductive layer 12A is offset to one side of the side surface 11a, and the conductive layer 12B is offset to one side of the side surface 11b.

[0128] Furthermore, in the example shown in Figure 20, the coating layer 16 may be located on side surfaces 11c and 11d, as well as on side surfaces 11a and 11b where the conductor layer 12 is not provided.

[0129] This reduces the volatilization of sodium and potassium contained in the piezoelectric material 13 from sides 11c and 11d, as well as from sides 11a and 11b.

[0130] Therefore, it is possible to reduce the concentration changes of sodium and potassium in the piezoelectric material 13, which is the active region.

[0131] In the embodiments described so far, an example has been shown in which the piezoelectric element 13 and the coating layer 16 are composed of separate components, but this disclosure is not limited to such an example. For example, the piezoelectric element 13 may be positioned to cover the sides 11c and 11d of the laminate 11, and the configuration of the piezoelectric element 13 located on these sides 11c and 11d may be the same as the configuration of the coating layer 16 described so far. This also makes it possible to reduce the change in the amount of displacement of the piezoelectric element 10 over a long period of time.

[0132] <Configuration of Mass Flow Controller> Next, an example of a mass flow controller 100 on which the piezoelectric actuator 1 according to the embodiment is mounted will be described with reference to Figure 21. Figure 21 is a block diagram showing the configuration of the mass flow controller 100 according to the embodiment.

[0133] As shown in Figure 21, the mass flow controller 100 comprises a flow path 101, a flow sensor unit 102, a flow control valve 103, and a control circuit unit 104. A fluid, such as gas, flows through the flow path 101. This fluid flows in through the inlet 101a and out through the outlet 101b.

[0134] A flow sensor unit 102 is connected to a portion of the flow path 101, for example, in a bypass configuration. This flow sensor unit 102 is configured to detect the flow rate (mass flow rate) of the fluid flowing through the flow path 101. The flow rate signal detected by the flow sensor unit 102 is amplified by an amplification circuit and transmitted to the control circuit unit 104.

[0135] The flow control valve 103 is equipped with a piezoelectric actuator 1 according to the embodiment and is configured to control the flow rate of the fluid flowing through the flow path 101. The flow control valve 103 can control the flow rate, for example, by the expansion and contraction of the piezoelectric actuator 1.

[0136] The control circuit unit 104 controls each part. For example, the control circuit unit 104 compares the flow rate signal transmitted from the flow rate sensor unit 102 with a flow rate signal that has been set in advance by the user or the like.

[0137] 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 on the flow control valve 103.

[0138] The piezoelectric actuator 1 expands and contracts in response to the input drive voltage, and this expansion and contraction controls the amount the flow control valve 103 opens and closes, thereby controlling the flow rate of the fluid flowing through the flow path 101.

[0139] As described above, the embodiment includes a piezoelectric element 10 that is highly durable even in high-temperature environments, so the mass flow controller 100 can be made stable for a long time even when used in harsh environments.

[0140] Although the present disclosure has been described in detail above, this disclosure is not limited to the embodiments described above, and various modifications and improvements are possible without departing from the gist of this disclosure.

[0141] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. Indeed, the embodiments described above can be embodied in a variety of forms. Furthermore, the embodiments described above may be omitted, replaced, or modified in various ways without departing from the scope and spirit of the appended claims.

[0142] Furthermore, this technology can take the following configurations: (1) A piezoelectric element comprising: a columnar laminate in which a piezoelectric material mainly composed of potassium sodium niobate and internal electrodes are alternately stacked; and two conductive layers located on the sides of the laminate and connected to the corresponding internal electrodes, wherein the piezoelectric material has: a first portion that overlaps with the internal electrodes in a planar view in the stacking direction; and a second portion located outside the first portion and not overlapping with the internal electrodes in the planar view, wherein the concentration of at least one of potassium and sodium contained in the piezoelectric material is constant in the first portion, decreases in a stepwise manner at the interface between the second portion and the first portion, and gradually decreases from the interface to the surface in the second portion. (2) The piezoelectric element according to (1), wherein the difference in the concentration of at least one of potassium and sodium between the interface and the surface is greater than the difference in the concentration of the same element between the interface and the first portion. (3) The piezoelectric element according to (1) or (2), wherein the average grain size of the second portion near the interface is smaller than the average grain size of the first portion near the interface. (4) The piezoelectric element according to any one of (1) to (3), wherein the area of ​​the grain boundary phase per unit area of ​​the second portion near the interface is larger than the area of ​​the grain boundary phase per unit area of ​​the first portion near the interface. (5) The piezoelectric element according to any one of (1) to (4), wherein the average grain size of the second portion near the surface is smaller than the average grain size of the second portion near the interface. (6) The piezoelectric element according to any one of (1) to (5), wherein the area of ​​the grain boundary phase per unit area of ​​the second portion near the surface is larger than the area of ​​the grain boundary phase per unit area of ​​the second portion near the interface. (7) The piezoelectric element according to any one of (1) to (6), wherein fine grooves are located on the surface. (8) The piezoelectric element according to any one of (1) to (7), wherein manganese is segregated at the interface. (9) A piezoelectric element according to any one of (1) to (7) above, wherein lithium is segregated at the interface.(10) A piezoelectric element according to any one of (1) to (7) above, wherein manganese and lithium are segregated at the interface. (11) A piezoelectric actuator comprising a piezoelectric element according to any one of (1) to (10) above, and a case, wherein the piezoelectric element is housed inside the case. (12) A mass flow controller comprising a flow path, a flow sensor unit for detecting the flow rate of a fluid flowing in the flow path, a flow control valve having a piezoelectric element according to any one of (1) to (10) above, and controlling the flow rate of a fluid flowing in the flow path by the expansion and contraction of the piezoelectric element, and a control circuit unit for controlling each unit.

[0143] 1 Piezoelectric actuator 10 Piezoelectric element 11 Laminate 11a, 11b Sides 11c, 11d Sides (example of surface) 12 Conductor layer 13 Piezoelectric element (example of first part) 14 Internal electrode 16 Coat layer (example of second part) 17 Interface 40 Case 100 Mass flow controller 101 Flow path 102 Flow sensor section 103 Flow control valve 104 Control circuit section A1, A2 Grain boundary phase C1-C3 Concentration Ca, Cb Concentration difference D Lamination direction G Groove

Claims

1. A piezoelectric element comprising: a columnar laminate in which a piezoelectric material mainly composed of potassium sodium niobate and internal electrodes are alternately stacked; and two conductive layers located on the sides of the laminate and connected to the corresponding internal electrodes, wherein the piezoelectric material has a first portion that overlaps with the internal electrodes in a planar view in the stacking direction; and a second portion located outside the first portion and not overlapping with the internal electrodes in the planar view, wherein the concentration of at least one of potassium and sodium contained in the piezoelectric material is constant in the first portion, decreases in a stepwise manner at the interface between the second portion and the first portion, and gradually decreases from the interface to the surface in the second portion.

2. The piezoelectric element according to claim 1, wherein the concentration of at least one of potassium and sodium is such that the difference between the concentration of the same element at the interface and the surface is greater than the difference between the concentration of the same element at the interface and the first portion.

3. The piezoelectric element according to claim 1 or 2, wherein the average particle size of the second portion near the interface is smaller than the average particle size of the first portion near the interface.

4. The piezoelectric element according to any one of claims 1 to 3, wherein the area of ​​the grain boundary phase per unit area of ​​the second portion near the interface is greater than the area of ​​the grain boundary phase per unit area of ​​the first portion near the interface.

5. The piezoelectric element according to any one of claims 1 to 4, wherein the average particle size of the second portion near the surface is smaller than the average particle size of the second portion near the interface.

6. The piezoelectric element according to any one of claims 1 to 5, wherein the area of ​​the grain boundary phase per unit area of ​​the second portion near the surface is greater than the area of ​​the grain boundary phase per unit area of ​​the second portion near the interface.

7. The piezoelectric element according to any one of claims 1 to 6, wherein fine grooves are located on the surface.

8. The piezoelectric element according to any one of claims 1 to 7, wherein manganese is segregated at the interface.

9. The piezoelectric element according to any one of claims 1 to 7, wherein lithium is segregated at the interface.

10. The piezoelectric element according to any one of claims 1 to 7, wherein manganese and lithium are segregated at the interface.

11. A piezoelectric actuator comprising a piezoelectric element according to any one of claims 1 to 10, and a case, wherein the piezoelectric element is housed inside the case.

12. A mass flow controller comprising: a flow path; a flow sensor unit for detecting the flow rate of a fluid flowing in the flow path; a flow control valve having a piezoelectric element according to any one of claims 1 to 10, which controls the flow rate of a fluid flowing in the flow path by the expansion and contraction of the piezoelectric element; and a control circuit unit for controlling each unit.

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