1. Field of the Invention
The present invention relates to layered piezoelectric elements used in, for example, piezoelectric pumps and to piezoelectric pumps. More particularly, the present invention relates to a layered piezoelectric element in which a central portion is bent and displaced in a direction opposite to the direction in which peripheral portions surrounding the central portion are bent and displaced and to a piezoelectric pump including such a layered piezoelectric element.
2. Description of the Related Art
Piezoelectric pumps that use piezoelectric elements to discharge liquids, etc., are known. A typical piezoelectric pump includes a pump main body including a pump chamber and a piezoelectric element that is fixed to the pump main body so as to close an opening of the pump chamber. When a voltage is applied to bend and displace the piezoelectric element, the displacement of the piezoelectric element causes the volume of the pump chamber to be varied. As a result, the liquid is led to the pump chamber or is discharged from the pump chamber.
In order to achieve a larger amount of discharge, the center portion of the piezoelectric element is required to be greatly displaced.
In the above situation, Japanese Unexamined Patent Application Publication No. 3-54383 discloses a piezoelectric pump using a piezoelectric element shown in
One end of an alternating-current power supply 1009 is electrically connected to the metal plate 1004 serving as a common electrode. The other end of the alternating-current power supply 1009 is electrically connected to the peripheral electrodes 1006 and 1008 via a controller 1010 and is electrically connected to the central electrodes 1005 and 1007 via an inverter 1011.
The first and second piezoelectric bodies 1002 and 1003 are wholly polarized in the same thickness direction, as shown by arrows P.
A voltage applied to the central electrodes 1005 and 1007 is out of phase with a voltage applied to the peripheral electrodes 1006 and 1008 by 180 degrees.
Accordingly, the direction of an electric field E applied to the central portion is opposite to the directions of the electric fields E applied to the peripheral portions in each of the piezoelectric bodies 1002 and 1003. Accordingly, if an expansion displacement occurs in the central portion of the piezoelectric body 1002 as shown in
Consequently, great displacement can be achieved in the central portion in the piezoelectric element 1001.
However, since the voltage applied to the central electrode 1005 formed on the external surface of the first piezoelectric body 1002 is different from the voltage applied to the peripheral electrodes 1006 formed thereon, a short circuit due to migration can occur between the central electrode 1005 and the peripheral electrodes 1006. Similarly, a short circuit can occur between the central electrode 1007 and the peripheral electrodes 1008 also on the bottom surface of the second piezoelectric body 1003.
In addition, a drive circuit becomes complicated because it is necessary to provide the complicated wiring and further to provide the inverter 1011, as shown in
In contrast, WO Publication 2008/007634 discloses a piezoelectric pump using a piezoelectric element shown in
A central electrode 1106 and peripheral electrodes 1107 are formed on the top surface of the layered piezoelectric ceramic body 1105. A central electrode 1108 and peripheral electrodes 1109 are formed on the bottom surface of the layered piezoelectric ceramic body 1105. The central portion is polarized in a direction from the top surface of the layered piezoelectric ceramic body 1105 to the bottom surface thereof, as shown by arrows P in
In driving, a first voltage is applied to the central electrode 1106 and the peripheral electrodes 1109, a second voltage is applied to the central electrode 1108 and the peripheral electrodes 1107, and a third voltage having a magnitude between the magnitude of the first voltage and that of the second voltage is applied to the electrode 1104. In other words, the first voltage > the third voltage > the second voltage.
Accordingly, also in the piezoelectric element 1101, if the first piezoelectric layer 1102 is subjected to the expansion displacement, the central portion of the second piezoelectric layer 1103 is subjected to the contraction displacement and the peripheral portions are displaced in a direction opposite to the displacement direction of the central portion in the first and second piezoelectric layers. Consequently, it is possible to increase the amount of displacement in the central portion also in the piezoelectric element 1101.
As described above, although the piezoelectric element used in the piezoelectric pump is strongly required to increase the amount of displacement in the central portion, it is not possible to sufficiently meet such a requirement with the piezoelectric element 1001 described in Japanese Unexamined Patent Application Publication No. 3-54383.
In the piezoelectric element 1001, the central portion of the first piezoelectric body 1002 is displaced in a direction opposite to the displacement direction of the central portion of the second piezoelectric body 1003, as described above. However, the electric field is applied to either of the piezoelectric bodies in a direction opposite to the polarization direction in the driving. For example, in the state shown in
In contrast, in the piezoelectric element 1101 shown in
To overcome the problems described above, preferred embodiments of the present invention provide a piezoelectric element which is capable of increasing the driving voltage to achieve a larger amount of displacement and in which migration between electrodes hardly occurs and also provide a piezoelectric pump including such a piezoelectric element.
According to a preferred embodiment of the present invention, a layered piezoelectric element includes a layered piezoelectric body including a first piezoelectric layer, a second piezoelectric layer, and a third piezoelectric layer layered between the first and second piezoelectric layers; first and second excitation electrodes that are opposed to each other with the first piezoelectric layer of the piezoelectric body sandwiched therebetween and that are positioned in a central area when the first piezoelectric layer is viewed in plan; and third and fourth excitation electrodes that are opposed to each other with the second piezoelectric layer sandwiched therebetween and that are arranged in areas around the area where the first and second excitation electrodes are provided. A portion of the first piezoelectric layer in a first driving area in which the first excitation electrode is overlapped with the second excitation electrode via the first piezoelectric layer is polarized in a thickness direction of the layered piezoelectric body and a portion of the second piezoelectric layer in a second driving area in which the third excitation electrode is overlapped with the fourth excitation electrode via the second piezoelectric layer is polarized in the same direction as in the first driving area.
In a specific aspect of the layered piezoelectric element according to a preferred embodiment of the present invention, a fourth piezoelectric layer is layered outside at least one of the first and second piezoelectric layers in a layering direction. In this case, since at least either of the first and second excitation electrodes and the third and fourth excitation electrodes is covered with the fourth piezoelectric layer, a short circuit between the first and second excitation electrodes and/or a short circuit between the third and fourth excitation electrodes hardly occurs. In addition, since liquid is hardly in contact with the first and second excitation electrodes and/or the third and fourth excitation electrodes, these excitation electrodes are hardly corroded.
In another specific aspect of the layered piezoelectric element according to a preferred embodiment of the present invention, no piezoelectric layer is provided outside the first and second piezoelectric layers, the second excitation electrode is disposed on an external surface of the first piezoelectric layer, and the third excitation electrode is disposed on an external surface of the second piezoelectric layer. As in the above case, it is acceptable not to provide the fourth piezoelectric layer. In this case, the manufacturing process can be simplified and the amount of displacement can be increased because the fourth piezoelectric layer does not exist.
In another specific aspect of the layered piezoelectric element according to a preferred embodiment of the present invention, all of the piezoelectric layers preferably are uniformly polarized in the thickness direction. In this case, the polarization can be easily performed.
In a preferred embodiment of the present invention, in the first and second driving areas, the first and second piezoelectric layers may be polarized in the thickness direction and a portion of the piezoelectric body excluding the first and second driving areas may not be polarized.
In addition, in another specific aspect of the layered piezoelectric element according to a preferred embodiment of the present invention, when viewed in plan, the first and second driving areas are arranged so that an outer margin of the first driving area is in contact with a margin of the second driving area at the side of the first driving area. In this case, it is possible to reduce the size of the layered piezoelectric element.
In a preferred embodiment of the present invention, when viewed in plan, an outer margin of the first driving area may be isolated from a margin of the second driving area at the side of the first driving area and a buffering portion may be arranged between the first and second driving areas. In this case, the presence of the buffering portion can produce a larger amount of displacement.
In the layered piezoelectric element according to a preferred embodiment of the present invention, a pair of second driving areas may be arranged on both sides of the first driving area or the second driving area may be arranged so as to surround the first driving area.
In another specific aspect of the layered piezoelectric element according to a preferred embodiment of the present invention, the first and second excitation electrodes each preferably have a square or rectangular planar shape and the third and fourth excitation electrodes each preferably have a rectangular planar shape, for example. In this case, it is possible to easily and accurately form the excitation electrodes each having a square or rectangular planar shape by printing with conductive paste or the like, for example.
A piezoelectric pump according to a preferred embodiment of the present invention includes a pump main body that includes a pump chamber and a piezoelectric element that is held in the pump main body so as to close the pump chamber and that is bent and displaced in response to a voltage that is applied to vary the volume of the pump chamber. The portion of the piezoelectric element closing the pump chamber includes a central portion and peripheral portions surrounding the central portion. In the piezoelectric pump in which a center portion is bent and displaced in a direction opposite to the direction in which a driving portion is bent and displaced in response to a driving voltage that is applied, the piezoelectric element includes the layered piezoelectric element structured in accordance with a preferred embodiment of the present invention.
In the above-described piezoelectric pump, the layered piezoelectric element can be fixed and held in various manners. Even if the layered piezoelectric element is fixed at the peripheral portions, a larger amount of displacement can be achieved at the central portion. In a specific aspect, the layered piezoelectric element is fixed on one side of the diaphragm, and a plane opposite the plane of the diaphragm at which the layered piezoelectric element is fixed is arranged so as to close the pump chamber. In other words, the unimorph piezoelectric resonator includes the layered piezoelectric element and the diaphragm, thus achieving a much larger amount of displacement. In this case, the piezoelectric element may include the diaphragm and the layered piezoelectric element and may be fixed at a margin of the diaphragm. Alternatively, the piezoelectric element may be fixed at margins of both of the diaphragm and the layered piezoelectric element.
In the layered piezoelectric element according to a preferred embodiment of the present invention, the portions that are driven by a piezoelectric effect when a voltage is applied correspond to the first driving area and the second driving areas, the first driving area is positioned at the central portion and the second driving areas are positioned at the peripheral portions, the first and second driving areas are arranged in the first and second piezoelectric layers, respectively, and both of the driving areas have the same polarization direction and the same direction in which the electric field is applied. Accordingly, it is possible to apply a driving voltage having a magnitude greater than that of a coercive electric field to both of the first and second driving areas. Consequently, even if the layered piezoelectric element is fixed at peripheral portions, it is possible to achieve a larger amount of displacement in a central area.
In addition, since the excitation electrodes connected different voltages do not exist in planes at the same height in the layered piezoelectric element, migration between the electrodes hardly occurs.
Consequently, the use of the layered piezoelectric element according to a preferred embodiment of the present invention allows the amount of discharge in, for example, the piezoelectric pump to be increased and allows the reliability to be improved because a failure due to the migration between the electrodes hardly occurs.
Other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings.
Specific preferred embodiments of the present invention will herein be described with reference to the attached drawings to disclose the present invention.
A piezoelectric pump 1 includes a pump main body 2. The pump main body 2 includes a plate member having a depression on its top surface in the present preferred embodiment. The pump main body 2 is preferably made of a material, such as metal or synthetic resin, for example, having a relatively high rigidity.
A piezoelectric element 3 is arranged so as to close the depression of the pump main body 2. The piezoelectric element 3 has a unimorph structure in which a layered piezoelectric element 5 is fixed on the top surface of a diaphragm 4 defined by a metal plate. The layered piezoelectric element 5 will be described in detail below.
The depression of the pump main body 2 is closed with the piezoelectric element 3 to define a pump chamber 2a.
A margin of the diaphragm 4 is sandwiched between the top surface of the pump main body 2 and a pressure plate 12 to be fixed. Accordingly, the unimorph piezoelectric element 3 is mechanically held along a margin.
If a central portion of the piezoelectric element 3, specifically, a central portion of the layered piezoelectric element 5 is bent and displaced, the volume of the pump chamber 2a is varied. For example, if the central portion of the layered piezoelectric element 5 is displaced so as to protrude downward, the volume of the pump chamber 2a is decreased.
An entry-side valve chest 7 is connected to the pump chamber 2a via a connection channel 6. The entry-side valve chest 7 has an entry-side check valve 8 arranged therein. The entry-side check valve 8 is mounted so as to close an opening 7a provided at an upper portion of the entry-side valve chest 7. In suction of liquid, the entry-side check valve 8 is opened to lead the liquid to the entry-side valve chest 7. The entry-side check valve 8 prevents the liquid in the entry-side valve chest 7 from flowing toward the opening 7a.
At the other side, an exit-side valve chest 10 is connected to the pump chamber 2a via a connection channel 9. An exit-side check valve 11 is arranged under an opening 10a of the exit-side valve chest 10. The exit-side check valve 11 is fixed to the top surface of the diaphragm 4 so as to close an opening 4a provided in the diaphragm 4. The exit-side check valve 11 permits the liquid to move to the upper side of the diaphragm 4 but prevents the liquid from moving toward the connection channel 9 through the opening 4a.
Although the pump chamber 2a preferably has a rectangular planar shape in this preferred embodiment, the pump chamber 2a may have another shape. For example, the pump chamber 2a may have a circular planar shape.
In the piezoelectric pump 1, if the piezoelectric element 3 is bent and displaced, the volume of the pump chamber 2a is varied to cause inflow or discharge of the liquid. For example, if the central portion of the layered piezoelectric element 5 is displaced so as to protrude downward, the volume of the pump chamber 2a is decreased. Returning to an initial state shown in
If the central portion of the layered piezoelectric element 5 is bent and displaced again so as to protrude downward, the volume of the pump chamber 2a is decreased. As a result, the liquid in the pump chamber 2a is moved toward the exit-side valve chest 10 and is discharged from the opening 10a.
In order to increase the amount of discharge of the liquid in the piezoelectric pump 1, the layered piezoelectric element 5 is strongly required to increase its amount of displacement.
A layered piezoelectric element according to a first preferred embodiment of the present invention will now be described with reference to
As shown in
In this layered piezoelectric body, a first piezoelectric layer 21 is layered on a second piezoelectric layer 22 via a third piezoelectric layer 23. A fourth piezoelectric layer 24 is layered underneath the first piezoelectric layer 21. A fourth piezoelectric layer 25 is also layered on the second piezoelectric layer 22.
As shown in an exploded perspective view in
The first and second excitation electrodes 26 and 27 are each positioned in a central area with the layered piezoelectric element 5 viewed in plan. The central area is an area including the center in the plan view and is an area that is positioned on the inner side of the plan view, compared with peripheral portions described below.
In contrast, third excitation electrodes 28 and 29 are provided on the top surface of the third piezoelectric layer 23, that is, under the bottom surface of the second piezoelectric layer 22. The third excitation electrodes 28 and 29 are positioned in the peripheral areas with the layered piezoelectric element 5 viewed in plan. In other words, the third excitation electrodes 28 and 29 are arranged so as not to be overlapped with the first and second excitation electrodes 26 and 27 in the thickness direction.
Fourth excitation electrodes 30 and 31 are arranged so as to be overlapped with the third excitation electrodes 28 and 29, respectively, via the second piezoelectric layer 22.
As shown in
In addition, as shown by arrows P in
The portion of the piezoelectric body excluding the first and second driving areas is preferably not polarized. Accordingly, during the polarization, a polarization voltage is applied between the first and second excitation electrodes 26 and 27, between the third excitation electrode 28 and the fourth excitation electrode 30, and between the third excitation electrode 29 and the fourth excitation electrode 31 for the polarization.
In order to manufacture the layered piezoelectric body, conductive paste is applied on ceramic green sheets primarily made of appropriate piezoelectric ceramic powder to manufacture the ceramic green sheets on which the first, second, third, and fourth excitation electrodes are formed. These ceramic green sheets are layered and a plain ceramic green sheet on which the fourth piezoelectric layer 25 is layered on these ceramic green sheets to attach the layers by pressure in the thickness direction. Then, after the resulting layered body is fired or before the resulting layered body is fired, the first and second terminal electrodes 32 and 33 are formed.
The above ceramic green sheets are produced by sheet forming of ceramic green paste primarily made of appropriate piezoelectric ceramic powder, such as lead zirconate titanate ceramics, for example. The excitation electrodes 26 to 31 are preferably formed by printing with conductive paste, such as Ag or Ag—Pd paste, for example, on the ceramic green sheets and baking of the ceramic green sheets in the firing.
The terminal electrodes 32 and 33 can preferably be formed of appropriate metal, such as Ag, Cu, or Ag—Pd, for example. The terminal electrodes 32 and 33 may be formed by a thin film forming method, such as deposition, plating, or sputtering, for example, instead of the application and baking of the conductive paste.
The above piezoelectric ceramics and the metallic material of which the electrodes are composed are not particularly restricted.
In the layered piezoelectric element 5 of the present preferred embodiment, the central portion is greatly bent and displaced when the layered piezoelectric element 5 is fixed in areas denoted by C in
In other words, as shown in
Accordingly, since the polarization direction P is equal to the direction E in which the electric fields are applied in the first and second driving areas, the displacement occurs so as to cause lateral contraction, as shown in
Inversely, in the second piezoelectric layer 22, the second driving areas, that is, the peripheral portions are subjected to the contraction displacement and the central area sandwiched between the second driving areas is subjected to the expansion displacement. Accordingly, since the peripheral portions are displaced in a direction opposite to the displacement direction of the central portion in both of the first and second piezoelectric layers 21 and 22, greater bending and displacement is produced in the central portion when the layered piezoelectric element 5 is fixed in the peripheral portions denoted by C.
In addition, since the polarization direction P is equal to the direction E in which the electric fields are applied in the layered piezoelectric element 5 of the present preferred embodiment, a voltage having a magnitude greater than that of a coercive electric field can be applied to the layered piezoelectric element 5 to drive the layered piezoelectric element 5, so that a larger amount of displacement can be achieved.
Accordingly, in
The first driving area of the layered piezoelectric element 5 is not necessarily matched with the pump chamber 2a in the planar shape. The pump chamber 2a may have a planar shape larger than that of the first driving area or may be smaller than the planar shape of the first driving area.
In addition, although the margin of the diaphragm 4 is sandwiched between the pressure plate 12 and the pump main body 2 to fix the margin of the diaphragm 4 in the present preferred embodiment, a structure may be adopted in which the margin of the layered piezoelectric element 5 is further fixed with the pressure plate 12 or other suitable structure.
Furthermore, the multiple electrodes at the same height are not connected to different voltages in the layered piezoelectric element 5. For example, the third excitation electrodes 28 and 29 are connected to the same voltage and the fourth excitation electrodes 30 and 31 are connected to the same voltage. Accordingly, migration does not occur between the multiple electrodes formed at the same height. In addition, since the first and second excitation electrodes 26 and 27 are formed at heights different from those of the third and fourth excitation electrode 28 to 31, migration does not occur between the first and second excitation electrodes 26 and 27 and the third and fourth excitation electrodes 28 to 31.
Specifically, since the third piezoelectric layer 23 is arranged between the first piezoelectric layer 21 and the second piezoelectric layer 22, the second excitation electrode 27 is isolated from the third excitation electrodes 28 and 29 in the layering direction, that is, in the thickness direction of the layered piezoelectric element 5. Accordingly, migration does not occur between the third excitation electrodes 28 and 29 and the second excitation electrode 27.
Furthermore, since the first excitation electrode 26 and the fourth excitation electrodes 30 and 31 are covered with the fourth piezoelectric layers 24 and 25, respectively, a short circuit due to contact with liquid is prevented and corrosion of the excitation electrodes is also prevented.
A layered piezoelectric element 41 of the second preferred embodiment is similar to the layered piezoelectric element 5 of the first preferred embodiment except that the entire layered piezoelectric element is subjected to the polarization processing in a direction from the bottom to the top, as shown by an arrow P. In the layered piezoelectric element 5 of the first preferred embodiment, the piezoelectric body is polarized only in the first and second driving areas. Accordingly, in the polarization, the polarization voltage is applied between the first and second excitation electrodes 26 and 27, between the third excitation electrode 28 and the fourth excitation electrode 30, and between the third excitation electrode 29 and the fourth excitation electrode 31 for the polarization. In contrast, the entire layered piezoelectric element is uniformly subjected to the polarization processing in the second preferred embodiment. Accordingly, in the polarization, polarization electrodes are provided on the top surface and the bottom surface after the layered piezoelectric body is manufactured and a voltage is applied between the polarization electrodes for the polarization processing. In this case, the polarization electrodes on the top surface and the bottom surface are removed after the polarization processing. However, the polarization electrodes may not be removed.
Although it is necessary to separately form the polarization electrodes in the second preferred embodiment, the polarization can be easily performed because it is sufficient to uniformly polarize the entire layered piezoelectric body at a stage at which a mother layered piezoelectric body is manufactured.
A printing method or other suitable method is used to form the fourth excitation electrodes 30 and 31 on the top surface of the layered piezoelectric element 51. However, with the printing method, it is difficult to form the fourth excitation electrodes 30 and 31 so as to exactly overlap the lower excitation electrodes 28 and 29. A shift in the printing position can cause the amount of displacement in the second driving areas to be reduced so as to reduce the amount of displacement by contraries.
In contrast, in the first and second preferred embodiments, the multiple ceramic green sheets to which the printing with the conductive paste is subjected are layered so that the first excitation electrode exactly opposes the second excitation electrode and the third excitation electrodes exactly oppose the fourth excitation electrodes. It is easier to increase the accuracy of the layering than to increase the accuracy of the printing positions. Accordingly, according to the present preferred embodiment, it is possible to further reduce the variation in the amount of displacement and to suppress a reduction in the amount of displacement.
In addition, since the first excitation electrode 26 and the fourth excitation electrodes 30 and 31 are externally exposed, a short circuit or corrosion due to adhesion of liquid may occur.
In contrast, such a short circuit or corrosion is prevented in the first and second preferred embodiments. Accordingly, the layered piezoelectric elements 5 and 41 of the first and second preferred embodiments are preferable.
In the layered piezoelectric element 5 of the first preferred embodiment, buffering portions 34 and 35 are arranged between the first driving area and the second driving areas. In other words, a certain distance R is kept between edges of the first and second excitation electrodes 26 and 27 and the opposing edges of the third and fourth excitation electrodes 28 and 30 in a portion where the first and second excitation electrodes 26 and 27 are adjacent to the third and fourth excitation electrodes 28 and 30 in the lateral direction in
Accordingly, the presence of the buffering portions 34 and 35 produces greater bending and displacement in the central area.
However, as in the fifth preferred embodiment shown in
In the manufacturing of each of the layered piezoelectric elements of various preferred embodiments of the present invention, as shown in
In this case, electrodes 91 each defining portions of the terminal electrodes 32 and 33 are formed in advance and, after the division, the remaining electrode portions are formed so as to continue into the electrode 91 on side surfaces of the layered piezoelectric body in order to form the terminal electrodes 32 and 33.
Although the third and fourth excitation electrodes each preferably having a rectangular planar shape, for example, are arranged outside the square first and second excitation electrodes 26 and 27 in the above preferred embodiments, circular first and second excitation electrodes 101 and 102 and ring-shaped peripheral electrodes 103 and 104 may be used as in a modification shown in exploded perspective views in
While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.
Number | Date | Country | Kind |
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2008-107759 | Apr 2008 | JP | national |
Number | Date | Country | |
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Parent | PCT/JP2009/001698 | Apr 2009 | US |
Child | 12796764 | US |