JP4883768B2 - Piezoelectric sensor - Google Patents

Description

  The present invention relates to a piezoelectric sensor that detects a physical quantity such as acoustic emission, vibration, and acceleration with a piezoelectric ceramic thin film element in a high-temperature environment such as an internal combustion engine or a plant such as a nuclear power plant. is there.

  In order to detect abnormalities in structures that generate high-temperature atmosphere such as piping and valves in a plant such as a nuclear power plant or an engine of an internal combustion engine, sensors are installed inside the structure. For example, acoustic emission sensors that detect acoustic emissions, which are elastic waves generated when cracks or cracks occur in structures, and piezoelectric vibration sensors that detect abnormal vibration and acceleration information are used. Various types such as a compression type, a cantilever type, a diaphragm type, and a shear type are known.

  Among these, as described in Patent Document 1 (as shown in FIG. 1), the compression type thin film type piezoelectric sensor sequentially includes a pedestal, a pedestal side electrode, a piezoelectric body, a load body side electrode, and a load body. It consists of the laminated body which laminated | stacked, and attaches the lower surface of the base to a structure, and is used. When vibration occurs in the structure, the vibration is transmitted to the pedestal side of the sensor. Then, the vibration transmitted to the pedestal side of the sensor is transmitted to the piezoelectric body, and a compressive stress or a tensile stress proportional to the vibration acceleration is generated in the piezoelectric body. An electric signal (charge) proportional to the stress is generated on both sides of the piezoelectric body, and the two electrodes disposed on both sides of the piezoelectric body take out the charge. By measuring the extracted electric charge, the magnitude and acceleration of the vibration of the structure can be detected.

  Conventionally, as a piezoelectric body used in such a piezoelectric sensor, various types as shown in Table 1 have been used.

  Patent Documents 1 and 2 disclose examples of a piezoelectric sensor using lead zirconate titanate (PZT) or polyvinylidene fluoride (PVDF) as a piezoelectric body. However, these piezoelectric bodies have a low Curie temperature, and their application limit temperature is about 300 ° C. at the maximum. On the other hand, the inside of the structure has a high combustion temperature (around 500 ° C.). For this reason, the piezoelectric body of the piezoelectric sensor installed in the inside becomes a considerably high temperature (around 400 ° C.) and reaches the Curie temperature. When the piezoelectric body becomes a high temperature exceeding the Curie temperature, the piezoelectric characteristics are deteriorated and cannot be used. Therefore, when using a piezoelectric body having a low Curie temperature as described above, it is necessary to separately provide cooling means for maintaining the piezoelectric body at an appropriate temperature, as described in Patent Document 3. However, the provision of the cooling means causes a problem that the structure of the piezoelectric sensor becomes complicated and the cost increases.

In order to solve the above problem, a method of using a piezoelectric body having a high Curie temperature in a high temperature environment is conceivable. Patent Document 4 discloses a method of using lithium niobate (LiNbO ₃ ) having a high Curie temperature. As shown in Table 1, the lithium niobate has a Curie temperature of about 1210 ° C., and can be used in a high temperature environment without using a cooling means. However, lithium niobate is inferior in workability, is difficult to be thinned, and has a problem that piezoelectric characteristics cannot be obtained unless it is in a single crystal state. In addition, there is a problem that production and processing are difficult and costly.

  In addition, when quartz is used for the piezoelectric body, there is a problem that it is difficult to reduce the thickness as described above.

An example of a method for solving these problems is disclosed in Patent Document 5. The piezoelectric sensor of Patent Document 5 uses a piezoelectric body having no Curie temperature, for example, aluminum nitride (AlN) or zinc oxide (ZnO). Specifically, the piezoelectric sensor is formed by laminating a base layer, a piezoelectric thin film layer (aluminum nitride), and an upper electrode in this order on a metal diaphragm, and an upper portion that extracts charges generated from the piezoelectric thin film layer. A signal output rod is fixed to the electrode by crimping. The pressure received by the metal diaphragm from the outside is transmitted to the piezoelectric thin film layer, and the piezoelectric thin film layer generates an electric charge according to the pressure. The pressure is detected by measuring the electric charge through a signal output rod. Thereby, a low-priced piezoelectric sensor that is small and excellent in heat resistance can be realized.
Japanese Patent Laid-Open No. 6-148011 (release date May 27, 1994) Japanese Patent Laid-Open No. 10-206399 (publication date: August 7, 1998) Japanese Patent Laid-Open No. 5-203665 (Released on August 10, 1993) Japanese Patent Laid-Open No. 5-34230 (publication date February 9, 1993) JP 2004-156991 A (publication date June 3, 2004)

  However, in the above-described conventional configuration, aluminum nitride having a small piezoelectric constant d33 as shown in Table 1 is used as the piezoelectric body of the piezoelectric element, so that it is not possible to generate sufficient charges according to external pressure. . In addition, since the piezoelectric element is tightly fixed to the signal output rod, the deformation amount of the piezoelectric element due to external pressure is limited. Piezoelectric elements are deformed by pressure from the outside and generate electric charges according to the stress generated inside them. Therefore, if the amount of deformation of the piezoelectric element is small, electric charges according to external pressure can be generated accurately. I can't. Therefore, a piezoelectric sensor using such a piezoelectric element has a problem that sensitivity is low. In particular, since a sensor for detecting an abnormality of a structure needs to detect even a slight abnormality, its sensitivity is an important problem. In addition, other piezoelectric bodies having a large piezoelectric constant d33 shown in Table 1 are not suitable for use in a high temperature environment because the above-described problems occur.

  Here, the piezoelectric constant d33 is a value representing the amount of charge generated from the piezoelectric body with respect to the pressure applied to the piezoelectric body. That is, if the value of the piezoelectric constant d33 is large, the amount of generated charge is large. As the amount of generated charge increases, even if the applied pressure is small, it is possible to generate charges corresponding to the pressure. Therefore, the large amount of charge affects the sensitivity of the piezoelectric element.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a piezoelectric sensor that can obtain high sensitivity using a piezoelectric element having low sensitivity.

  In order to solve the above-described problem, a piezoelectric sensor according to the present invention includes a piezoelectric element that receives a pressure on a first surface and generates a charge by the pressure. A space is formed in a region where the opposite second surface faces.

  When the piezoelectric element is deformed by receiving a force from the outside, a compressive stress and a tensile stress are generated inside the piezoelectric element, and a charge corresponding to the stress is generated. Therefore, if the amount of deformation is large, the stress generated in the inside becomes large and more charges are generated.

  According to the above configuration, since the space is formed in the region where the second surface opposite to the first surface of the piezoelectric element faces, the piezoelectric element that has received the pressure moves in the direction of pressure, that is, the direction of the space. , And can bend according to the pressure. Thus, by forming the space on the surface opposite to the surface receiving the pressure, the piezoelectric element can be bent in the direction of the space without being obstructed. Thereby, even if the pressure generated outside is slight, the piezoelectric element can be bent, so that the piezoelectric element can accurately generate electric charges according to the pressure. Therefore, a highly sensitive piezoelectric sensor can be obtained even when a piezoelectric element with low sensitivity is used.

  In the piezoelectric sensor according to the present invention, a conductive member is further provided on the second surface of the piezoelectric element, and a connection surface where the conductive member is electrically connected to the piezoelectric element is the piezoelectric element. It is preferable that the second surface is in contact with only the outer peripheral region.

  According to the above configuration, the conductive member is in contact with only the outer peripheral region of the surface that generates the charge opposite to the surface receiving the pressure of the piezoelectric element. A space is formed. Therefore, the piezoelectric element that has received the pressure can bend in the direction of the pressure, that is, in the direction of the space, and generates an electric charge according to the stress generated therein. The pressure applied to the piezoelectric element can be detected by the conductive member taking out the electric charge generated from the piezoelectric element. Thus, since the piezoelectric element can bend in the direction of the space without being obstructed by the conductive member that extracts the electric charge, the electric charge corresponding to the pressure can be generated accurately. Therefore, a highly sensitive piezoelectric sensor can be obtained even when a piezoelectric element with low sensitivity is used.

  In the piezoelectric sensor according to the present invention, the conductive member is a conductive rod-shaped member, and one end surface in the axial direction serves as the connection surface, and a recess is formed at a position near the center of the connection surface. preferable.

  According to said structure, the recessed part is formed in the position near the center part of the connection surface of a conductive member and a piezoelectric element in a conductive member. As a result, a space is formed on the surface opposite to the surface receiving the pressure of the piezoelectric element. Therefore, when the piezoelectric element receives pressure from the outside, the piezoelectric element can be deflected in the direction of pressure, that is, in the inner direction of the concave portion of the conductive member without being obstructed by the conductive member. And the stress according to the bending of a piezoelectric element generate | occur | produces inside a piezoelectric element, and the electric charge according to the stress generate | occur | produces. As described above, the piezoelectric element can be bent by forming the concave portion in the conductive member.

  The piezoelectric sensor according to the present invention further includes a housing for mounting the piezoelectric element, wherein an opening formed in a front end wall of the housing is located on the first surface, and the second surface It is preferable that the piezoelectric element is fixed between the front end wall and the rod-shaped member by the rod-shaped member disposed on the surface.

  According to said structure, the opening part is formed in the front-end wall of the housing | casing for mounting | wearing with a piezoelectric element, The opening part is located in the said 1st surface. And the piezoelectric element is being fixed between the said front end wall and the rod-shaped member with the rod-shaped member arrange | positioned at the said 2nd surface.

  Accordingly, the piezoelectric element is fixed between the front end wall of the housing in which the opening is formed and the rod-like member in a state where the pressure receiving surface (first surface) is exposed from the opening. Therefore, the piezoelectric element can receive a pressure from the outside and bend only in the direction of the pressure.

  In the piezoelectric sensor according to the present invention, the piezoelectric element includes an upper electrode that extracts charges generated from the piezoelectric element, and the upper electrode has a compressive stress generated in the piezoelectric element due to the action of the pressure. It is preferable to be provided so as to take out electric charges generated by tensile stress.

  The piezoelectric element is bent to generate a compressive stress and a tensile stress therein. Specifically, when pressure is applied to the piezoelectric element and bending occurs, the vicinity of the center of the piezoelectric element is pulled toward both ends. Thereby, tensile stress is generated near the center of the piezoelectric element. On the other hand, the vicinity of both ends of the piezoelectric element is compressed in the contraction direction. Thereby, compressive stress is generated near both ends of the piezoelectric element.

  By the way, the charge generated by the compressive stress in the piezoelectric element and the charge generated by the tensile stress have different polarities. For this reason, if charges generated by both stresses are taken out by the electrode layer, they are canceled out and sufficient charges cannot be taken out.

  According to said structure, the upper electrode is provided so that the electric charge which generate | occur | produces with the compressive stress or tensile stress of a piezoelectric element may be taken out.

  Thereby, for example, the upper electrode can be taken out from the piezoelectric element without canceling out the charge generated by the compressive stress with the charge generated by the tensile stress. Therefore, the upper electrode can extract a sufficient amount of charge from the piezoelectric element. Thereby, even if the pressure applied to the piezoelectric element is small, more charges can be taken out from the piezoelectric element, so that even a slight pressure generated outside can be detected.

  In the piezoelectric sensor according to the present invention, the piezoelectric element includes a piezoelectric thin film layer that generates a charge under pressure, and an upper electrode that extracts the charge generated from the piezoelectric thin film layer, and the upper electrode is the piezoelectric thin film. It is preferable that only the area | region of the outer peripheral side in the said 2nd surface of a layer is in contact.

  According to the above configuration, the upper electrode is in contact with only the outer peripheral side region of the surface that generates the charge opposite to the surface receiving the pressure of the piezoelectric thin film layer, so that the charge is generated on the piezoelectric thin film layer side. A space is formed. Therefore, the piezoelectric thin film layer that has received the pressure can bend in the direction of the pressure, that is, the direction of the space, and generates an electric charge according to the stress generated in the inside. The pressure applied to the piezoelectric element can be detected by the upper electrode taking out the charge generated from the piezoelectric thin film layer.

  The piezoelectric sensor according to the present invention includes a housing for mounting the piezoelectric element, and the piezoelectric element includes a piezoelectric thin film layer that generates a charge under pressure, and the piezoelectric thin film layer includes the piezoelectric thin film layer. It is preferable that a portion of the front end wall where the piezoelectric thin film layer is formed is processed so as to function as a diaphragm. Specifically, the thickness of the front end wall on which the piezoelectric thin film layer is formed is preferably 0.005 mm or more and 10 mm or less.

  According to said structure, the piezoelectric thin film layer which generate | occur | produces an electric charge according to a pressure is directly formed in the front-end wall of a housing | casing. And the front end wall in which the piezoelectric thin film layer is formed is processed so that it may function as a diaphragm. Here, the diaphragm refers to a film-like body that deforms according to an applied pressure.

  Thereby, the front end wall which receives pressure can bend in the direction of pressure, and the piezoelectric thin film layer can also be bent accompanying this bend. As described above, since the piezoelectric thin film layer can be bent even if it is directly formed on the housing, the structure of the piezoelectric element can be simplified. Therefore, a highly sensitive piezoelectric sensor that is small in size and low in cost can be obtained.

  The piezoelectric sensor according to the present invention includes a housing for mounting the piezoelectric element. The piezoelectric element is positioned on the first surface side and on the second surface side. A piezoelectric thin film layer that generates a charge under pressure, and an upper electrode that extracts the charge generated from the piezoelectric thin film layer, the housing has an opening, and the pressure transmission member includes The piezoelectric thin film layer and the upper electrode are accommodated in the housing, and only the outer peripheral side region of the pressure transmission member is provided in the opening so as to be in contact with the region around the opening. preferable.

  According to said structure, the pressure transmission member which receives a pressure is provided in the said opening part so that the area | region around the opening part in the housing | casing which accommodates a piezoelectric thin film layer and an upper electrode may be contact | connected.

  As a result, the casing and the pressure transmission member do not contact each other in the region on the central side other than the region on the outer peripheral side. That is, a space is formed in the inner direction of the casing, that is, on the side where electric charges are generated in the piezoelectric thin film layer. Therefore, the pressure transmission member that has received the pressure can bend in the direction of the pressure, that is, the direction of the space, and the piezoelectric thin film layer can be bent along with the bending. Thus, by forming the space on the surface opposite to the surface receiving the pressure, the piezoelectric element can be bent in the direction of the space without being obstructed.

  In the piezoelectric sensor according to the present invention, it is preferable that a conductive wire is connected to the upper electrode.

  According to said structure, since the conducting wire is connected to the upper electrode, the electric charge generated from the piezoelectric thin film layer is taken out by the conducting wire. And the pressure applied to the pressure transmission means can be detected by detecting the taken-out electric charge.

  In the piezoelectric sensor according to the present invention, the upper electrode is an annular member, and one surface serves as a connection surface connected to the piezoelectric thin film layer, and this connection surface is a region on the outer peripheral side of the second surface of the piezoelectric thin film layer. It is preferable that it touches only.

  According to said structure, since the annular | circular shaped upper electrode is in contact with only the area | region of the outer peripheral side in the 2nd surface of a piezoelectric thin film layer, the hollow space is formed in the inside of an upper electrode. Therefore, the piezoelectric thin film layer can be bent in the spatial direction of the upper electrode.

  In the piezoelectric sensor according to the present invention, the piezoelectric element includes an upper electrode that extracts electric charges generated from the piezoelectric element, and the upper electrode is caused by a compressive stress generated in the piezoelectric element by the action of the pressure. It is preferable that the electric charges generated and the electric charges generated by the tensile stress are individually taken out.

  The piezoelectric element is subjected to bending caused by external pressure, and compressive stress and tensile stress are generated inside the piezoelectric element. However, the charge generated by the compressive stress in the piezoelectric element and the charge generated by the tensile stress have different polarities. Therefore, if the charges generated by both stresses are taken out by the electrode layer, they are canceled out and sufficient charges are taken out. I can't.

  On the other hand, according to said structure, the upper electrode is provided so that the electric charge generated by a compressive stress and the electric charge generated by a tensile stress may each be taken out separately.

  Thereby, since the electric charge generated by the compressive stress and the electric charge generated by the tensile stress can be taken out separately, a sufficient amount of electric charges can be detected by adding both together. In addition, in order to obtain the amplified charge (output) by adding both, it is possible to invert the polarity of one charge. Thereby, even if the pressure applied to the piezoelectric element is small, more charges can be taken out from the entire piezoelectric element, so that even a slight pressure generated outside can be detected. That is, the sensitivity of the piezoelectric sensor can be further improved.

  In the piezoelectric sensor according to the present invention, the piezoelectric element includes a piezoelectric thin film layer that generates an electric charge upon receiving pressure, and an upper electrode that extracts an electric charge generated from the piezoelectric thin film layer, and the upper electrode includes the piezoelectric element. It is preferable that the second surface of the thin film layer is individually provided in the outer peripheral region and the central region.

  Here, the piezoelectric element is subjected to bending caused by pressure from the outside, and compressive stress is generated in a region on the outer peripheral side of the second surface, and in a region near the center portion of the second surface. Tensile stress is generated.

  According to the above configuration, the upper electrode is individually provided in the outer peripheral side region and the central region on the surface that generates charges opposite to the surface receiving the pressure of the piezoelectric thin film layer. It is possible to individually take out the charge generated by the above and the charge generated by the tensile stress.

  Therefore, a sufficient amount of charges obtained by adding the charges generated by compressive stress and tensile stress can be detected by a method of reversing the polarity of one of the charges. Thereby, even if the pressure applied to the piezoelectric element is small, more charges can be taken out from the piezoelectric element, so that even a slight pressure generated outside can be detected. That is, the sensitivity of the piezoelectric sensor can be further improved.

  As described above, the piezoelectric sensor according to the present invention includes a piezoelectric sensor that includes a piezoelectric element that receives pressure on the first surface and generates electric charge by the pressure, and is opposite to the first surface. In this configuration, a space is formed in a region facing the surface.

  Accordingly, the piezoelectric element can be bent into the space according to the pressure generated outside, and the electric charge according to the bending can be generated accurately. Therefore, it is possible to provide a piezoelectric sensor that can obtain high sensitivity by using a piezoelectric element with low sensitivity.

  The following describes one embodiment of the present invention with reference to FIGS. In the present embodiment, a case where a piezoelectric sensor is applied to an internal combustion engine will be described as an example.

  FIG. 1 is a cross-sectional view showing a schematic configuration of a piezoelectric sensor according to the present embodiment.

  The piezoelectric sensor 1 detects an abnormality in the cylinder by detecting a pressure generated from the internal combustion engine cylinder. The piezoelectric sensor 1 includes a signal transmission unit 2 and a piezoelectric element 10.

  The piezoelectric element 10 receives a pressure generated in the cylinder and generates a charge corresponding to the pressure. Details of the piezoelectric element 10 will be described later.

  The signal transmission unit 2 includes a housing 20, a signal output rod (conductive member, electrode layer) 30, and an electrically insulating ring column 40. The signal transmission unit 2 transmits the electric charge generated from the piezoelectric element 10 as a detection signal to a signal carrying cable (not shown) via the signal output rod 30.

  The housing 20 includes a front housing 22 and a rear housing 21 in front and rear in the axial direction, and a shaft hole 50 penetrating from the front end to the rear end is formed therein. In addition, since the housing | casing 20 is equipped with the internal combustion engine used as high temperature, it is preferable to be comprised with the heat resistant metal.

  The rear housing 21 includes a large-diameter portion 21a and a small-diameter portion 21b in front and rear in the axial direction, and the large-diameter portion 21a has an inner diameter and an outer diameter larger than those of the small-diameter portion 21b. A connector for attaching a signal carrying cable (not shown) is inserted into the large diameter portion 21a.

  The front housing 22 includes a large-diameter portion 22a having a large outer diameter and a small-diameter portion 22b having a small outer diameter at the front and rear in the axial direction. A large-diameter hole 51 that constitutes a part of the shaft hole 50 is formed inside the front housing 22, and the small-diameter portion 21b of the rear housing 21 is screwed into the large-diameter hole 51 of the large-diameter portion 22a, for example. Are fitted.

  An opening 53 having a smaller diameter than the large-diameter hole 51 is formed in the front end wall 22d of the small-diameter portion 22b in order to take in pressure from the outside. Further, a step 22c for mounting the piezoelectric element 10 is formed around the opening 53 in the inner surface of the front end wall 22d.

  The shaft hole 50 is a connecting hole of a large diameter hole 51 having a large diameter on the small diameter portion 22b side and a small diameter hole 52 having a small diameter on the small diameter portion 21b side. A cylindrical electrical insulating ring column 40 having an outer diameter slightly smaller than the diameter of the large diameter hole 51 is inserted into the large diameter hole 51, and the through hole of the electrical insulating ring column 40 is made of metal. The signal output rod 30 is installed.

  The signal output bar 30 is in the shape of a round bar, and has an outer diameter on the front end side that is larger than that of other parts, and is provided with a step part 31. A recess 32 is formed in the front end surface of the signal output rod 30. Therefore, the front end surface of the signal output rod 30 is a ring-shaped annular surface. In addition, the shape of the recessed part 32 is not specifically limited, For example, cylindrical shape, prismatic shape, cone shape, pyramid shape etc. can be selected suitably.

  Further, the rear end portion of the signal output rod 30 passes through the small diameter hole 52 in the small diameter portion 21b of the rear housing 21 and reaches the inside of the large diameter portion 21a, and is connected to a signal carrying cable (not shown). Further, a metal electrode 33 is provided on the front end surface of the signal output rod 30, and the electrode 33 is configured to be able to contact the piezoelectric element 10 at the step portion 22 c of the small diameter portion 22 b of the front housing 22.

  The opening 53 is located inside the cylinder, and pressure generated inside the cylinder enters from the opening 53 and reaches the piezoelectric element 10. Further, the signal output rod 30 is in contact only with the piezoelectric element 10 and the electrically insulating ring column 40 in the shaft hole 50 and is electrically insulated from the housing 20.

  The piezoelectric sensor 1 is configured as described above. Here, a method for assembling the piezoelectric sensor 1 will be described below.

  First, the piezoelectric element 10 is placed on the step 22 c of the front housing 22. At this time, the front end surface (first surface) of the piezoelectric element 10 is exposed from the opening 53 of the front housing 22. Next, the signal output rod 30 is inserted into the front housing 22 and is in contact with and fixed to the rear end surface (second surface) of the piezoelectric element 10. Next, the electrically insulating ring column 40 is inserted into the front housing 22 with the signal output rod 30 inserted therein, and is in contact with and fixed to the step portion 31 at the front end of the signal output rod 30. Thereafter, the small diameter portion 21 b of the rear housing 21 is inserted into the large diameter portion 22 a of the front housing 22, and the front end wall of the rear housing 21 pushes the rear end wall of the electrical insulating ring column 40. Then, the piezoelectric element 10, the signal output rod 30, and the electrically insulating ring column 40 are fixed in a state where they are crimped inside the front housing 22. The piezoelectric sensor 1 can be formed by the above method.

  Next, the piezoelectric element 10 will be described in detail with reference to FIG. FIG. 2 is a cross-sectional view showing the piezoelectric element 10 according to the present embodiment.

  The piezoelectric element 10 is formed by forming a base layer 12, a piezoelectric thin film layer 13, and an upper electrode 14 in this order on a metal diaphragm (pressure transmission member) 11.

  For each film formation, a physical vapor deposition method (PVD) method, that is, a method of forming a thin film by evaporating a substance by a physical method and condensing it on a member to be formed can be used. Specifically, for example, vacuum deposition methods such as resistance heating deposition or electron beam heating deposition, various sputtering methods such as DC sputtering, high frequency sputtering, RF plasma assisted sputtering, magnetron sputtering, ECR sputtering, or ion beam sputtering, and high frequency ion plating. These include various ion plating methods such as a plating method, activated vapor deposition or arc ion plating, molecular beam epitaxy, laser application, ion cluster beam vapor deposition, and ion beam vapor deposition.

  The metal diaphragm 11 is in contact with the space in the cylinder in which pressure is generated, and receives the pressure generated in the cylinder that has entered from the opening 53. The metal diaphragm 11 transmits the pressure to the piezoelectric thin film layer 13 through the base layer 12. The metal diaphragm 11 also has a function as a substrate that supports the piezoelectric element 10. In addition, a diaphragm means the film-like body which deform | transforms according to the applied pressure. Therefore, the metal diaphragm 11 is bent by receiving pressure from the outside.

  In addition, since the metal diaphragm 11 is installed in the internal combustion engine cylinder which becomes high temperature, it needs to have heat resistance. Therefore, it is preferable to use a heat resistant metal material equivalent to, for example, Inconel or SUS630 for the metal diaphragm 11. Further, the surface of the metal diaphragm 11 on the side on which the piezoelectric thin film layer 13 is formed is ground by a polishing method or a chemical method in order to prevent the piezoelectric thin film layer 13 from cracking or peeling and to enhance the orientation of the crystal axis. It is preferable to carry out mirror surface processing.

  The underlayer 12 is a buffer layer between the metal diaphragm 11 and the piezoelectric thin film layer 13 formed on the underlayer 12. The underlayer 12 has a role of improving the polar orientation of the piezoelectric thin film layer 13, the orientation of the crystal axis, and the wettability with the metal diaphragm 11. Further, the base layer 12 has a function as a lower electrode.

  As the material of the underlayer 12, TiN, MoSi2, Si3N4, Cr, Fe, Mg, Mo, Nb, Ta, Ti, Zn, Zr, W, Pt, Al, Ni, Cu, Pd, Rh, Ir, Ru, Au, or Ag can be used, and a single layer or a multilayer of two or more layers using a plurality of materials can be used.

  The piezoelectric thin film layer 13 has piezoelectric characteristics, and generates electric charges when it receives pressure from the outside. Specifically, the piezoelectric thin film layer 13 is bent along with the bending generated in the metal diaphragm 11. A compressive stress and a tensile stress are generated inside the piezoelectric thin film layer 13 according to the bending of the piezoelectric thin film layer 13. Electric charges are generated from the piezoelectric thin film layer 13 by the action of the internal stress. As described above, the piezoelectric thin film layer 13 receives the pressure and generates a charge corresponding to the pressure. The piezoelectric thin film layer 13 is preferably formed by sputtering aluminum nitride (AlN) or zinc oxide (ZnO).

  The upper electrode 14 takes out the electric charge generated from the piezoelectric thin film layer 13 by the pressure received by the metal diaphragm 11 together with the base layer 12. The extracted electric charge is transmitted as a detection signal to a signal carrying cable (not shown) through a signal output bar 30 in contact with the upper electrode 14. The material of the upper electrode 14 can be the same material as that of the underlayer 12, but need not be the same, and may be appropriately selected depending on the compatibility with the piezoelectric thin film layer 13 and the electrode 33. The structure may be a single layer or multiple layers.

  Next, the configuration of the piezoelectric element 10 and the signal output rod 30 will be described with reference to FIG.

  As described above, since the recess 32 is formed at the front end portion of the signal output rod 30 that contacts the upper electrode 14 of the piezoelectric element 10, there is a space between the piezoelectric element 10 and the signal output rod 30. It is formed. Thereby, the piezoelectric element 10 can bend in the direction of the space without being obstructed by the signal output rod 30 when receiving pressure from the outside.

  In the present embodiment, the shape of the recess 32 in the signal output rod 30 is a ring-shaped annular surface as shown in FIGS. 4 (a) and 4 (b), but is not limited to this. is not. Another shape may be a hemispherical shape as shown in FIG. Moreover, as shown in FIG.4 (d), the cross section of the recessed part 32 in the signal output rod 30 may be square shape. Moreover, it is preferable that the depth of the recessed part 32 is 0.01 mm or more and 10 mm or less.

  In the present embodiment, the signal output rod 30 is a rod-shaped member having a recess 32, but may be an annular electrode layer. Thereby, since the hollow space is formed in the inside of the electrode layer, the piezoelectric element 10 can be bent in the space direction of the electrode layer.

  Next, the operation of the piezoelectric sensor 1 will be described with reference to FIG.

  First, the metal diaphragm 11 receives pressure generated by vibration or the like inside the internal combustion engine cylinder. The metal diaphragm 11 that has received the pressure is bent according to the pressure in the direction in which the pressure is applied, that is, in the spatial direction of the recess 32 of the signal output rod 30. Along with the bending of the metal diaphragm 11, the underlying layer 12 and the piezoelectric thin film layer 13 are similarly bent in the inner direction of the recess 32. When the piezoelectric thin film layer 13 is bent, stress is generated inside the piezoelectric thin film layer 13. Then, charges corresponding to the stress generated inside the piezoelectric thin film layer 13 are generated from the piezoelectric thin film layer 13. Next, the base layer 12 and the upper electrode 14 disposed on both surfaces of the piezoelectric thin film layer 13 take out the charges generated in the piezoelectric thin film layer 13. The extracted charge is transmitted to the electrode 33 and transmitted as a detection signal to a signal carrying cable (not shown) via the signal output bar 30. By measuring this detection signal, the pressure generated in the internal combustion engine cylinder can be detected. As described above, the piezoelectric sensor 1 according to the present embodiment is configured to detect the pressure generated outside using the bending effect of the piezoelectric element 10.

  As described above, the piezoelectric element 10 according to the present embodiment can be bent in the spatial direction of the recess 32 without being obstructed by the signal output rod 30. Thereby, even if the pressure generated outside is slight, the piezoelectric element 10 can be bent, and the pressure can be detected based on the electric charge generated according to the bending. Therefore, even when a piezoelectric element with low sensitivity is used, a highly sensitive piezoelectric sensor can be obtained by utilizing the bending effect of the piezoelectric element.

  Here, the structure which takes out the electric charge which generate | occur | produces from the piezoelectric thin film layer 13 using a conducting wire instead of the signal output rod 30 is demonstrated. FIG. 5 is a cross-sectional view showing a schematic configuration of the piezoelectric sensor 1 that takes out the electric charge generated from the piezoelectric thin film layer 13 by a conducting wire.

  The housing 20 in this configuration is preferably a cylindrical member with only one end face released. The one end face is connected to the metal diaphragm 11. Therefore, a sealed space is formed inside the housing 20. Further, the piezoelectric thin film layer 13 and the upper electrode 14 are formed in this order on the surface of the metal diaphragm 11 on the space side, and a conductive wire 60 is electrically connected to the upper electrode 14.

  Thereby, the piezoelectric element 10 bends in the direction of the space by receiving pressure from the outside, and generates electric charges. Then, this electric charge is taken out by the upper electrode 14 and transmitted as a detection signal to the signal carrying cable through the conducting wire. Thus, by forming a space between the housing 20 and the piezoelectric element 10, the piezoelectric element 10 can bend in the direction opposite to the surface receiving pressure, that is, in the direction of the space. Therefore, a sufficient charge can be obtained by utilizing the bending effect of the piezoelectric element 10.

  Here, the relationship between the thickness of the metal diaphragm 11 and the amount of charge generated from the piezoelectric thin film layer 13 will be described below.

  For example, when the thickness of the metal diaphragm 11 is reduced, the amount of bending is increased. Therefore, more charges are generated from the piezoelectric thin film layer 13. Further, when the thickness of the metal diaphragm 11 is increased, the amount of bending is reduced. Therefore, the amount of charge generated from the piezoelectric thin film layer 13 is reduced. Thus, by changing the thickness of the metal diaphragm 11, the amount of charge generated from the piezoelectric thin film layer 13 can be changed. That is, the sensitivity of the piezoelectric sensor 1 can be changed by arbitrarily setting the thickness of the metal diaphragm 11.

  Here, the upper electrode 14 may be formed so as to take out electric charges generated by compressive stress or tensile stress generated in the piezoelectric thin film layer 13.

  In the present embodiment, a configuration in which the upper electrode 14 takes out electric charges generated by compressive stress generated in the piezoelectric thin film layer 13 will be described. FIG. 6 is a cross-sectional view of the piezoelectric element 10 in which the upper electrode 14 is formed only in the outer peripheral side region of the piezoelectric thin film layer 13.

  The piezoelectric thin film layer 13 generates a bend accompanying the bend generated in the metal diaphragm 11, thereby generating a compressive stress and a tensile stress therein. Specifically, as shown in FIG. 7, the central vicinity 10 a of the piezoelectric thin film layer 13 is pulled toward both ends by the bending of the metal diaphragm 11. As a result, a tensile stress is generated near the center 10 a of the piezoelectric thin film layer 13. Further, the vicinity 10b of both ends of the piezoelectric thin film layer 13 is compressed in the contraction direction. Thereby, compressive stress is generated in the vicinity 10 b of both ends of the piezoelectric thin film layer 13.

  By the way, the charge generated by the compressive stress and the charge generated by the tensile stress have different polarities. Therefore, if charges generated by both stresses are taken out by the upper electrode 14, they are canceled out and sufficient charges cannot be taken out. Therefore, the upper electrode 14 is formed only in a range where the compressive stress of the piezoelectric thin film layer 13 is generated.

  According to the above configuration, the upper electrode 14 can take out only the electric charge generated by the compressive stress from the piezoelectric thin film layer 13 without canceling out the electric charge generated by the tensile stress. Therefore, the upper electrode 14 can extract a sufficient amount of charge from the piezoelectric thin film layer 13. As a result, even if the external pressure transmitted to the metal diaphragm 11 is small, more electric charge can be taken out from the piezoelectric thin film layer 13, so that the slight pressure generated inside the cylinder of the internal combustion engine can be detected. Can do. That is, the piezoelectric sensor 1 with further improved detection sensitivity can be realized.

  Further, the upper electrode 14 may be provided so as to individually take out the charge generated by the compressive stress generated in the piezoelectric thin film layer 13 due to the pressure and the charge generated by the tensile stress. Specifically, as shown in FIGS. 8A and 8B, the upper electrode 14 is individually provided in a region on the outer peripheral side and a region near the center of the piezoelectric thin film layer 13, respectively. It is. 8A and 8B, the electrode provided in the outer peripheral region is illustrated as the external electrode 14b, and the electrode provided in the region closer to the center is illustrated as the central electrode 14a. In order to take out the electric charge generated from the piezoelectric thin film layer 13, an underlayer 12 serving as a lower electrode may be provided on the lower surface of the piezoelectric thin film layer 13, and a metal diaphragm as shown in FIGS. You may provide the electrode 14c in the arbitrary positions of 11. Electrical wiring (not shown) is connected to each of the electrodes 14a, 14b, and 14c.

  As a result, the charge generated by the compressive stress acting on the outer peripheral side region of the piezoelectric thin film layer 13 is centered on the charge generated by the external electrode 14b and the tensile stress acting on the region near the center of the piezoelectric thin film layer 13. The electrodes 14a can be taken out individually.

  Therefore, a sufficient amount of charges can be detected by adding the charges taken out individually. In addition, in order to obtain the amplified charge (output) by adding both, it is possible to invert the polarity of one charge. Thereby, the sensitivity of the piezoelectric sensor 1 can be further improved.

  In order to form the upper electrode 14 in a range where the compressive stress of the piezoelectric thin film layer 13 is generated, the upper electrode 14 may be formed in an annular shape, or may be divided into two or more. good.

  In the above configuration, since the upper electrodes 14 are provided at both ends of the piezoelectric thin film layer 13, a space is formed between the upper electrodes 14. Thereby, the piezoelectric thin film layer 13 can bend in the direction of the signal output rod 30 when receiving pressure. Accordingly, electric charges can be generated by utilizing the bending of the piezoelectric thin film layer 13 without providing the recess 32 in the signal output rod 30.

  Furthermore, the upper electrode 14 may be formed only in a range where the tensile stress of the piezoelectric thin film layer 13 is generated. Thereby, the upper electrode 14 can take out only the electric charge generated from the piezoelectric thin film layer 13 by the tensile stress. Thereby, the same effect as the case where only the electric charge generated by the compressive stress is taken out can be obtained.

  Further, as shown in FIGS. 9A and 9B, the piezoelectric thin film layer 13 is divided into ranges where compressive stress and tensile stress are generated and formed on the metal diaphragm 11, and the divided piezoelectric thin film is formed. The center electrode 14 a and the external electrode 14 b may be formed corresponding to the layer 13. And it is set as the structure which reverses the polarity of any one of the electric charge which generate | occur | produces by the compressive stress, or the electric charge which generate | occur | produces by tensile stress. As a result, it is possible to take out a charge obtained by adding the charge generated by the compressive stress and the charge generated by the tensile stress. Therefore, since the charge that can be taken out from the piezoelectric thin film layer 13 can be further increased, the piezoelectric sensor 1 with higher sensitivity can be realized.

  In the present embodiment, the upper electrode 14 is formed on the piezoelectric thin film layer 13. However, the upper electrode 14 is not particularly limited, and the upper electrode 14 and the piezoelectric thin film layer 13 are connected by a signal line. Also good. Thereby, the electric charge generated by the compressive stress or tensile stress generated in the piezoelectric thin film layer 13 can be detected via the signal line.

  Here, as shown in FIG. 10, the piezoelectric thin film layer 13 may be formed only in a range where the compressive stress is generated in the metal diaphragm 11. As a result, only the compressive stress acts on the piezoelectric thin film layer 13, so that the upper electrode 14 can extract only the electric charge generated by the compressive stress from the piezoelectric thin film layer 13. Therefore, as described above, the upper electrode 14 can extract more charges from the piezoelectric thin film layer 13 even if the external pressure transmitted to the metal diaphragm 11 is small. That is, a highly sensitive piezoelectric sensor 1 can be realized. In addition, the shape of the piezoelectric thin film layer 13 may be formed in an annular shape, or may be divided into two or more. Alternatively, the piezoelectric thin film layer 13 may be formed only in a range where tensile stress is generated.

  Here, as shown in FIGS. 11 and 12, the piezoelectric thin film layer 13 is directly formed on the front end wall 22d of the front housing 22, and the portion of the front end wall 22d where the piezoelectric thin film layer 13 is formed is subjected to diaphragm processing. It is good also as composition to do. As a result, the piezoelectric thin film layer 13 can be bent toward the inside of the recess 32 formed in the signal output rod 30 in association with the bending of the front end wall 22 d that receives external pressure. Electric charges are generated according to the bending of the piezoelectric thin film layer 13. Thus, since the housing | casing 20 for mounting | wearing with the piezoelectric element 10 can serve as the function of the metal diaphragm 11, the metal diaphragm 11 becomes unnecessary. Therefore, since the structure of the piezoelectric element 10 can be simplified, the piezoelectric sensor can be reduced in size and the cost can be reduced.

  Examples of the diaphragm processing method include machining, chemical etching, electric discharge machining, and laser machining. And it is preferable that the thickness of the part by which the piezoelectric thin film layer 13 in the front end wall 22d is formed by the said process shall be 0.005 mm or more and 10 mm or less.

  Further, as shown in FIG. 13, a configuration is provided in which a recess 22e is provided in the front end wall 22d of the front casing 22 on which the piezoelectric thin film layer 13 is formed, and the upper electrode 14 is provided only above the edge portion 22f in the recess 22e. It is also good. As a result, when the pressure receiving portion 22g formed on the upper surface in the concave portion 22e receives pressure and is bent, a compressive stress is generated in the piezoelectric thin film layer 13 located above the edge portion 22f to generate charges. . And the upper electrode 14 can take out the electric charge which generate | occur | produced by this compressive stress. Thus, sufficient charges can be taken out by utilizing the bending of the housing 20 without using the metal diaphragm 11. Therefore, a highly sensitive piezoelectric sensor 1 can be realized with a simple configuration.

  As described above, since a space is formed between the upper electrodes 14, the piezoelectric thin film layer 13 can bend in the direction of the signal output rod 30 when receiving pressure. Therefore, the signal output rod 30 may not be provided with the recess 32.

  Further, the front end wall 22d on which the piezoelectric thin film layer 13 is formed may be formed of a semiconductor such as silicon. By using silicon, mass production becomes possible and an inexpensive and small piezoelectric sensor 1 can be realized. FIG. 14 is a cross-sectional view showing a schematic configuration of the piezoelectric sensor 1 when the front end wall 22d shown in FIG. 13 is formed of silicon.

  Here, in the present embodiment, the piezoelectric thin film layer 13 is formed on the base layer 12, but the piezoelectric thin film layer 13 may be formed directly on the metal diaphragm 11. Note that when the base layer 12 is provided as in the present embodiment, the diaphragm is not limited to a metal, and for example, a semiconductor such as a polymer, a metal oxide, a metal nitride, or silicon is used. Also good.

  The material of the piezoelectric thin film layer 13 used in this embodiment is preferably aluminum nitride (AIN). The reason for this is that aluminum nitride has stable piezoelectric characteristics even at high temperatures, and has no influence on the environment because it does not use heavy metals such as lead. The piezoelectric thin film layer 13 may be made of other materials as long as the piezoelectric material does not have a Curie temperature. The “piezoelectric material having no Curie temperature” is a material that has piezoelectric characteristics and does not cause polar dislocation as the temperature rises, and includes, for example, a substance having a crystal structure of a wurtzite structure. Specific examples of the substance having a wurtzite crystal structure include aluminum nitride (AlN) and zinc oxide (ZnO).

  Such a piezoelectric material does not lose its piezoelectric properties until the crystal melts or sublimes. Further, a substance having a wurtzite crystal structure has piezoelectric properties because there is no symmetry in the crystal, and since it is not a ferroelectric material, there is no Curie temperature. Therefore, the piezoelectric element made of the above-described piezoelectric material has excellent heat resistance and does not deteriorate the piezoelectric characteristics. Even if it is exposed to a high temperature close to 500 ° C. like the inside of a cylinder of an internal combustion engine, the piezoelectric element There is no loss of functionality. Therefore, it is possible to use the internal combustion engine cylinder without using a piezoelectric element cooling device. As described above, the piezoelectric element made of the above-described piezoelectric material has excellent heat resistance and does not deteriorate the piezoelectric characteristics even at high temperatures. Moreover, it is excellent in workability and suitable for reducing the thickness.

  Here, a method for forming a thin film of a piezoelectric material having no Curie temperature can be realized by using a known technique. Specifically, an insulating substrate made of a sintered body of oxide-based, carbon-based, nitrogen-based or boride-based ceramics or quartz glass, or a conductive substrate made of a heat-resistant metal material such as Inconel or SUS630. A thin film can be formed by growing a piezoelectric ceramic on a single crystal while controlling the polarity.

  In addition, the thickness of the piezoelectric thin film layer 13 of the piezoelectric sensor 1 of the present invention is preferably in the range of 0.1 μm to 100 μm. Further, it is more preferably 0.5 μm or more and 20 μm or less, and further preferably 1 μm or more and 10 μm or less. If the thickness of the piezoelectric thin film layer 13 is less than 0.1 μm, a short circuit is likely to occur between the base layer 12 and the upper electrode 14, and if it is greater than 100 μm, the film formation time becomes long.

  The piezoelectric thin film layer 13 preferably has a dipole orientation degree of 75% or more, and more preferably 90% or more, in order to maintain good piezoelectric characteristics. The degree of dipole orientation is the ratio of the crystal column forming the electric dipole with the positive or negative polarity of the thin film surface in the same direction. If the polarity direction of the crystal column is completely random, the piezoelectric properties of the crystal columns cancel each other, and the piezoelectric property disappears in the entire thin film. When the dipole orientation degree of the piezoelectric thin film layer 13 is less than 75%, the apparent piezoelectric constant d33 becomes less than half that when the dipole orientation degree is 100%, and the piezoelectric characteristics of the piezoelectric thin film layer 13 deteriorate, This is because the pressure cannot be detected well. If the degree of dipole orientation is 75% or more, sufficient piezoelectric characteristics can be secured.

  In order to set the dipole orientation to 75% or more, it is necessary to easily align the first atoms when the crystal column grows, and the material of the underlayer 12 is a metal having the same component as the material of the piezoelectric thin film layer 13. For example, when AlN is used for the piezoelectric thin film layer 13, the base layer 12 is preferably made of Al, and when ZnO is used for the piezoelectric thin film layer 13, the base layer 12 is preferably made of Zn. When the underlayer 12 is a multilayer, it is preferable that the uppermost layer, that is, the layer in contact with the piezoelectric thin film layer 13 is made of a metal having the same component as the material of the piezoelectric thin film layer 13.

  In this embodiment, the metal diaphragm 11 is used as the pressure transmission member. However, even if the pressure is transmitted from the metal diaphragm 11 to the piezoelectric thin film layer 13 through another member such as a pressure transmission rod. In addition, the piezoelectric sensor 1 having excellent heat resistance and no cooling means can be realized.

  Further, the pressure transmission member is not limited to the metal diaphragm 11 and may be ceramics such as a glass substrate, for example. Specific ceramics include oxide ceramics such as aluminum oxide, magnesium oxide, zirconia, titanium oxide, yttrium oxide, and silicon oxide, and nitride ceramics such as aluminum nitride, silicon nitride, boron nitride, and gallium nitride. And carbide ceramics such as silicon carbide and tungsten carbide, and silicide ceramics such as molybdenum disilicide. Thereby, compared with a metal, corrosion resistance, heat resistance, pressure resistance, etc. improve. In addition, the experimental result which investigated the characteristic of the piezoelectric sensor when a glass substrate is used is mentioned later.

  For the piezoelectric sensor 1 according to the present embodiment, an experiment was performed to measure the amount of charge generated from the piezoelectric element 10 with respect to the pressure applied to the piezoelectric element 10.

  FIG. 15 shows the measurement system used in this experiment. Specifically, the piezoelectric element 10 is covered with a cylindrical glass tube, and a sealed space equal to the atmospheric pressure is created inside. Then, the upper electrode 14 of the piezoelectric element 10 and the amplifier are connected by a low noise coaxial cable, and the amplifier is connected to an oscilloscope. In this experiment, the upper electrode 14 having a thickness of 200 nm and the aluminum nitride (piezoelectric thin film layer 13) having a thickness of 1 μm were formed on the metal diaphragm 11 having a thickness of 0.2 mm by the sputtering method. Two types of piezoelectric elements 10 of type and B type in which the aluminum nitride film was formed on the metal diaphragm 11 were used.

  In the above measurement system, the atmospheric pressure in the sealed space is drawn at once and suddenly depressurized. At this time, the metal diaphragm 11 of the piezoelectric element 10 receives a force in the direction of the sealed space due to the sudden pressure reduction, and is bent. And the piezoelectric thin film layer 13 bends along with the bending of the metal diaphragm 11, and stress is generated in the inside thereof. Electric charges are generated from the piezoelectric thin film layer 13 due to the stress generated in the piezoelectric thin film layer 13. The charges generated at this time can be taken out by the upper electrode 14 and measured with an oscilloscope.

  FIG. 16 is a waveform diagram obtained by measuring the amount of charge taken out from the piezoelectric element 10 when the sealed space is rapidly depressurized. FIG. 17 is a graph showing the relationship between the force applied to the piezoelectric element 10 and the charge amount obtained from the signal amplitude shown in the waveform diagram of FIG. 16. FIG. 18 shows the relationship between the force and the charge amount. It is the graph which showed the relationship between the pressure and output voltage which converted.

  From the results of FIG. 17, it was found that the force applied to the metal diaphragm 11 and the amount of charge taken out from the upper electrode 14 are in a proportional relationship. The slope of the straight line indicating the proportional relationship indicates the apparent piezoelectric constant d33 (pC / N), that is, the piezoelectric constant of the piezoelectric element 10. Here, in the conventional configuration in which the bending effect of the piezoelectric element 10 is not used, the piezoelectric constant d33 is d33 = 2. On the other hand, when the bending effect of the piezoelectric element 10 in the present embodiment is used, the piezoelectric constant d33 of the piezoelectric element 10 is d33 = 30.808 for the A type and d33 = 222 for the B type. 305. Thus, it has been found that the value of the piezoelectric constant d33 of the piezoelectric element 10 is greatly increased by utilizing the bending effect of the piezoelectric element 10.

  Next, the configuration shown in FIGS. 8A and 8B, that is, the external electrode 14 b is separately provided in the outer peripheral region on the upper surface of the piezoelectric thin film layer 13, and the central electrode 14 a is separately provided in the region closer to the center. In the configuration in which the electrode 14c is provided at any position, an experiment was performed to measure the voltage output from the piezoelectric element 10 (piezoelectric thin film layer 13) with respect to the pressure applied to the piezoelectric element 10.

  FIG. 19 shows the measurement system used in this experiment. This measurement system is equivalent to the measurement system shown in FIG. 15 described above, and a 1 μm thick aluminum nitride film (piezoelectric thin film layer 13) is formed on a metal diaphragm 11 of Inconel 601 having a thickness of 0.2 mm. A piezoelectric element provided with a central electrode 14a and an external electrode 14b having a thickness of 0.05 to 0.3 μm is used. An electrode 14c having a thickness of 0.05 to 0.3 μm is provided at one end of the metal diaphragm. The external electrode 14b and the electrode 14c are connected to the amplifier 1 via a low noise coaxial cable, and the center electrode 14a and the electrode 14c are connected to the amplifier 2 via a low noise coaxial cable. Connect with.

  FIG. 20 is a graph showing a waveform of pressure output from the piezoelectric thin film layer 13 when pressure is applied to the piezoelectric element 10 in the measurement system. In the measurement system, the pressure is originally applied from the direction opposite to the direction applied to the piezoelectric element 10 so that the output characteristics of the piezoelectric element 10 can be easily obtained.

  As shown in FIG. 20, it was found that the output signal can be detected simultaneously from the external electrode 14b connected to the amplifier 1 and the central electrode 14a connected to the amplifier 2. Thus, it was found that even when a plurality of electrodes are used, the output signals can be detected simultaneously in each. As a result, the output signals individually detected can be added together by matching the polarities of the output signals. Therefore, even if the pressure applied from the outside is small, a larger output can be obtained, so that the sensitivity of the piezoelectric sensor can be improved.

  Next, an experiment for examining temperature characteristics and frequency characteristics in the piezoelectric sensor 1 according to the present embodiment was performed.

  FIG. 21 is a graph obtained by measuring the change in piezoelectric response to the temperature change of the piezoelectric sensor 1. The piezoelectric response can be considered by measuring the amount of charge per Newton (N), that is, the piezoelectric constant. As shown in FIG. 21, it was found that the piezoelectric sensor 1 can maintain stable piezoelectric characteristics up to 600 ° C. Thereby, according to the piezoelectric sensor 1 in the present embodiment, it is possible to perform stable measurement in a high temperature environment of about 500 ° C. inside the structure or the like.

  FIG. 22 is a graph showing frequency characteristics in the piezoelectric sensor 1. As shown in FIG. 22, it was found that measurement was possible from 0.3 Hz to 100 Hz.

  Next, in the piezoelectric sensor 1 according to the present embodiment, the amount of electric charge generated from the piezoelectric element 10 (piezoelectric thin film layer 13) with respect to the pressure applied to the piezoelectric element 10 when the glass substrate 11a is used as the pressure transmission member. Experiments to measure were performed.

  FIG. 23A is a plan view showing a schematic configuration of the piezoelectric element 10 when the glass substrate 11a is used as the pressure transmission member in the piezoelectric sensor 1, and FIG. 23B is a measurement used in this experiment. This shows the system. In this measurement system, an aluminum nitride (piezoelectric thin film layer 13) having a thickness of 1 μm is formed on a glass substrate 11a having a thickness of 1 mm, and an upper electrode 14 having a thickness of 0.05 to 0.3 μm is provided thereon. Piezoelectric elements are used. A lower electrode 12 is provided between the glass substrate 11 a and the piezoelectric thin film layer 13. Then, the lower electrode 12 and the upper electrode 14 are connected to the amplifier by a low noise coaxial cable, and the amplifier is connected to an oscilloscope.

  FIG. 24 is a graph showing the amount of charge generated from the piezoelectric thin film layer 13 when pressure is applied to the piezoelectric element 10 in the measurement system. From the results shown in the figure, it was found that the force applied to the glass substrate 11a and the amount of charge taken out from the piezoelectric thin film layer 13 are in a proportional relationship. When the piezoelectric response, that is, the piezoelectric constant d33 (pC / N) is calculated from the slope of the straight line indicating the proportional relationship, d33 = 75.4 is obtained. Here, in the conventional configuration using aluminum nitride, d33 = 2 as described above. Thus, it has been found that even when a glass substrate is used as the pressure transmission member, the value of the piezoelectric constant d33 of the piezoelectric element 10 is significantly increased by utilizing the bending effect of the piezoelectric element 10.

  The piezoelectric sensor of the present invention can detect vibrations, pressures, etc. of an object to be measured in a high-temperature environment, and therefore can be applied to pressure fluctuation measurement of high-temperature and high-pressure fluid in pipes and tanks in plants such as nuclear power plants. it can.

1 is a cross-sectional view illustrating a schematic configuration of a piezoelectric sensor according to an embodiment of the present invention. It is sectional drawing which shows the piezoelectric element in the piezoelectric sensor shown in FIG. It is sectional drawing which shows the relationship between the piezoelectric element in the piezoelectric sensor shown in FIG. 1, and a signal output rod. 4A is a longitudinal sectional view of a signal output rod having a cylindrical recess, FIG. 4B is a transverse sectional view of the signal output rod having a cylindrical recess, and FIG. FIG. 4D is a longitudinal sectional view of a signal output rod having a hemispherical recess, and FIG. 4D is a transverse sectional view of the signal output rod having a prismatic recess. It is sectional drawing which shows schematic structure of the piezoelectric sensor which takes out the electric charge which generate | occur | produces from a piezoelectric element using conducting wire. FIG. 2 is a cross-sectional view of a piezoelectric element in which an upper electrode is formed only in a region on the outer peripheral side of a piezoelectric thin film layer in the piezoelectric sensor shown in FIG. 1. FIG. 2 is a cross-sectional view of the piezoelectric sensor shown in FIG. 1 when bending occurs in the piezoelectric element. FIG. 8A is a plan view of the piezoelectric element in the case where the upper electrode is individually provided in the outer peripheral side region and the central portion of the piezoelectric thin film layer in the piezoelectric sensor shown in FIG. b) is an aa cross-sectional view of the piezoelectric element. 9A is a cross-sectional view of a piezoelectric element formed by dividing a piezoelectric thin film layer and an upper electrode in the piezoelectric sensor shown in FIG. 1, and FIG. 9B is a plan view of the upper electrode. . FIG. 2 is a cross-sectional view of a piezoelectric element in which a piezoelectric thin film layer is formed only in a region on the outer peripheral side in the piezoelectric sensor shown in FIG. 1. It is sectional drawing which shows schematic structure of the piezoelectric sensor at the time of directly forming into a front end wall of a housing | casing the piezoelectric thin film layer shown in FIG. It is sectional drawing to which the housing | casing at the time of forming the piezoelectric thin film layer shown in FIG. 1 directly on the front end wall of a housing | casing was expanded. It is sectional drawing which shows schematic structure of the piezoelectric sensor in the case of providing a recessed part in the front end wall of the housing | casing which formed the piezoelectric thin film layer shown in FIG. It is sectional drawing which shows schematic structure of the piezoelectric sensor at the time of forming the front end wall which has a recessed part shown in FIG. 13 with silicon. It is sectional drawing which shows the measurement system for measuring the electric charge generated from a piezoelectric element with respect to the pressure applied to a piezoelectric element about the piezoelectric sensor concerning this embodiment. It is a wave form diagram which shows the result of having measured the electric charge taken out from the piezoelectric element in the said measurement system with the oscilloscope. It is the graph which showed the relationship between the force applied to a piezoelectric element in the said measurement system, and the electric charge amount calculated | required from the signal amplitude shown in the waveform diagram measured with the oscilloscope. It is the graph which showed the relationship between the pressure converted from the force applied to the piezoelectric element in the said measurement system, and the electric charge amount calculated | required from the signal amplitude shown in the waveform figure measured with the oscilloscope, and output voltage. It is sectional drawing which shows the measurement system for measuring the electric charge generated from the said piezoelectric element with respect to the pressure applied to the piezoelectric element shown to Fig.8 (a) and FIG.8 (b). 20 is a graph showing a waveform of a voltage output from the piezoelectric element when pressure is applied to the piezoelectric element in the measurement system shown in FIG. It is the graph which measured the change of the piezoelectric response with respect to the temperature change of the piezoelectric sensor concerning this embodiment. It is a graph which shows the frequency characteristic in the piezoelectric sensor concerning this embodiment. FIG. 23A is a plan view showing a schematic configuration of a piezoelectric element when a glass substrate is used as a pressure transmission member in the piezoelectric sensor according to the present embodiment, and FIG. It is sectional drawing which shows the measurement system for measuring the electric charge generated from the said piezoelectric element with respect to the pressure applied to a. It is a graph which shows the electric charge amount which generate | occur | produces from a piezoelectric thin film layer when a pressure is applied to a piezoelectric element in the measurement system shown in FIG.23 (b).

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Piezoelectric sensor 2 Signal transmission part 10 Piezoelectric element 10a Near center 10b Near both ends 11 Metal diaphragm (pressure transmission member)
12 Underlayer 13 Piezoelectric thin film layer 14 Upper electrode 20 Housing 21 Rear housing 22 Front housing 22d Front end wall 30 Signal output bar (conductive member, electrode layer)
32 Concave portion 40 Electrically insulating ring column 53 Opening portion 60 Conductor

Claims (3)

In a piezoelectric sensor having a piezoelectric element that receives pressure on the first surface and generates electric charge by the pressure,
The piezoelectric element includes a piezoelectric thin film layer that generates an electric charge under pressure and an upper electrode that extracts electric charge generated from the piezoelectric thin film layer,
The upper electrode is in contact only with the outer peripheral region of the second surface opposite to the first surface of the piezoelectric thin film layer,
A conductive rod-shaped member provided on the second surface of the piezoelectric element and having no recess on one end surface in the axial direction is in contact with the upper electrode at the end surface. A piezoelectric sensor, wherein a member and the piezoelectric element are electrically connected, and a space is formed between the second surface and the conductive member . Before SL upper electrode, the piezoelectric sensor according to claim 1, wherein the pressure is provided to retrieve to thus generated charges to compressive stress generated in the interior of the piezoelectric element action. A housing for mounting the piezoelectric element ;
Before SL piezoelectric thin film layer is formed on the front end wall of the housing, in claim 1, wherein the portion of the piezoelectric thin-film layer is formed is processed so as to function as a diaphragm in the front end wall The piezoelectric sensor as described.