JP2019216203A - Piezoelectric element, vibration waveform sensor, and vibration waveform sensor module - Google Patents

Abstract

To detect vibration with high sensitivity and reduce environmental load, without using lead-containing ceramics material and monocrystalline material.SOLUTION: In a piezoelectric element 100, a piezoelectric layer 102 and internal electrode layers 104, 106 are alternately layered and a cover layer 108 is layered thereon. As the piezoelectric layer 102, for example, an alkali niobate-based piezoelectric material is used. External electrodes 120, 122 are formed at end faces from which the internal electrode layers 104, 106 are exposed, and high voltages are applied thereto, to perform polarizing the piezoelectric layer 102. The external electrodes 120, 122 are fixed onto an electrical connection part of the substrate 140 via solder or conductive resins 130, 132. Due to the piezoelectric material containing no lead, an electro-mechanical coupling coefficient becomes low in comparison with that of the monocrystalline piezoelectric material. However, by layering many piezoelectric layers, sufficient detection sensitivity for vibration can be obtained and environmental load can also be reduced.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a piezoelectric element, a vibration waveform sensor using the same, and a vibration waveform sensor module. For example, the present invention relates to a piezoelectric element, a vibration waveform sensor, and a vibration waveform sensor suitable for detecting various vibration waveforms such as a pulse. Regarding module improvement.

  In the modern age of the Internet of Things (IoT), the required number of sensors is increasing at an accelerating pace, and there is a need for technologies for manufacturing various sensors at lower cost, environmentally friendly, and in large quantities. For example, Patent Document 1 below discloses a broadband sensor for the purpose of efficiently detecting sound waves, pulse waves, and the like. A piezoelectric element is provided on the surface of an insulating substrate. A cylindrical member having an opening is provided. Conventional vibration waveform sensors for broadband use are large-scale, such as a cantilever structure in order to detect low-frequency vibrations. However, according to this background art, the opening is brought into contact with the body surface and sealed. By forming the cavity, it is possible to detect minute vibrations in a wide band in a compact and low noise manner.

  Patent Literature 2 below uses a bulk acoustic wave of a piezoelectric filter, a piezoelectric resonator, a piezoelectric gyro, or the like by using a piezoelectric single crystal composition having a large electromechanical coupling coefficient or a piezoelectric constant and a small sound speed or a small SAW speed. It is disclosed that a surface acoustic wave device such as a piezoelectric vibrator, a piezoelectric filter, and a sensor can be miniaturized.

International Publication WO2013 / 145352 JP 2006-282433 A

  By the way, as a piezoelectric material having a high electromechanical coupling coefficient k, a piezoelectric single crystal represented by lead zirconate titanate (PZT) or lead titanate (PT) is known and widely used. Therefore, there are concerns about environmental safety. Further, if the piezoelectric single crystal composition is used for the piezoelectric layer, it cannot be said that the manufacturing cost is preferable.

  The present invention focuses on this point, and an object thereof is to perform vibration detection with high sensitivity without using lead-containing ceramics and single crystal materials. Another object is to reduce vibrations and to detect vibrations advantageously in terms of cost.

  The piezoelectric element of the present invention is formed of a plurality of internal electrode layers and a lead-free piezoelectric material, applying a voltage for polarization to the internal electrode layers, and a plurality of piezoelectric layers polarized in the stacking direction, And a cover layer laminated on the front and back sides of the laminate in which the plurality of piezoelectric layers are laminated with the plurality of internal electrode layers interposed therebetween. According to one of the main modes, the piezoelectric element includes external electrodes for external connection, which are formed at both ends in the long side direction of the piezoelectric element and alternately connect the internal electrode layers. .

  The vibration waveform sensor according to the present invention includes a substrate on which the piezoelectric element is mounted so that the polarization direction is perpendicular to the mounting surface.

  A vibration waveform sensor module according to the present invention includes the vibration waveform sensor, and a charge amplifier that outputs a voltage proportional to the amount of charge generated in a piezoelectric element of the vibration waveform sensor. The above and other objects, features and advantages of the present invention will become apparent from the following detailed description and the accompanying drawings.

  According to the present invention, a plurality of lead-free piezoelectric materials are laminated with an internal electrode layer interposed therebetween to constitute a piezoelectric element, and a detection output is obtained by using a charge amplifier. As a result, the same sensitivity can be obtained, the environmental load is reduced, and the temperature conditions during manufacturing and use are eased.

It is a figure which shows the piezoelectric element of Example 1 of this invention, (A) shows a main section, (B) is a figure which shows a lamination | stacking state. (A) shows a graph for comparing the outputs in the example of pulse wave detection according to the above embodiment, and (B) is a graph for comparing the amplification factors of the prior art and the present invention. It is a figure which shows an example of the structure of the vibration waveform sensor of Example 2 of this invention, and the structure of the module.

  Hereinafter, the best mode for carrying out the present invention will be described in detail based on examples.

  First, a first embodiment of the present invention will be described with reference to FIG. 1 and FIG. FIG. 1 shows a piezoelectric element according to the present example. FIG. 1A shows a main cross-sectional structure, and FIG. 1B shows a laminated structure. As shown in these drawings, the piezoelectric element 100 has a structure in which a plurality of piezoelectric layers 102 and internal electrode layers 104 and 106 are alternately laminated, and further, a cover layer or a protective layer 108 is laminated thereon. For example, it has a multilayer structure similar to that of a multilayer ceramic capacitor (MLCC). As the piezoelectric layer 102, a lead-free or lead-free piezoelectric material, for example, an alkali niobate-type piezoelectric material is used, and the thickness is 20 μm. The internal electrode layers 104 and 106 are made of a conductive material such as Ag, Ag-Pd alloy, Ni, Cu, or Ni-Cu alloy, and have a thickness of, for example, 3 μm. As the cover layer 108, an insulating material is used. In this embodiment, the same material as that of the piezoelectric layer 102 is used. The thickness of the cover layer 108 is preferably less than 50 μm in order not to prevent bending due to vibration.

A piezoelectric material represented by the following general formula (1) is preferably used as the alkali niobate-based piezoelectric material, which is an example of a lead-free piezoelectric material.
_{(K 1-w-x Na} w Li x) a (Sb y Ta z Nb 1-y-z) O 3 ···· (1)
Where w, x, y, z, and a are
0 ≦ w ≦ 1,
0.02 <x ≦ 0.1,
0.02 <w + x ≦ 1,
0 ≦ y ≦ 0.1,
0 ≦ z ≦ 0.4,
1 <a ≦ 1.1
It is.

The number of stacked layers may be set as appropriate. For example, the stacked body is manufactured so that the piezoelectric layer 102 has 10 layers. As external dimensions of the piezoelectric element 100, for example, the length of the long side is 3.2 mm, the length (width) of the short side is 1.6 mm, and the thickness is 0.3 mm. As a manufacturing method, a plurality of piezoelectric layers 102 and internal electrode layers 104 and 106 are alternately stacked, a cover layer 108 is further stacked, and then firing is performed. The external electrode 120 is formed on the end surface where the internal electrode layer 104 is exposed, and the external electrode 122 is formed on the end surface where the internal electrode layer 106 is exposed, among the ends in the long side direction of the laminate. That is, the internal electrode layers 104 and 106 are alternately drawn to the end face on the long side, and are connected to the external electrodes 120 and 122 for external connection, respectively.
Next, a high voltage is applied between the external electrodes 120 and 122 to polarize the piezoelectric layer 102 to impart piezoelectricity. The polarization direction of the piezoelectric layer 102 is the laminating direction of the internal electrode layers 104 and 106, that is, the thickness direction.

  In the piezoelectric element 100 obtained as described above, for example, as shown in FIG. 1A, the external electrodes 120 and 122 at both ends in the long side direction are electrically connected to the substrate 140 by soldering or conductive resins 130 and 132. On the mechanical connection. The charge amplifier 50 will be described later.

  According to the present embodiment, since a piezoelectric material containing no lead is used, the electromechanical coupling coefficient is lower than that of a piezoelectric ceramic such as PZT or a single crystal composition. By alternately stacking a plurality of electrode layers, sufficient vibration detection sensitivity can be obtained. FIG. 2A shows the amplitude depending on the material used. The amplitude of alkali niobate (AN) ceramics is as low as one-tenth that of PZT ceramics, but by stacking these, almost the same amplitude is obtained. In addition, alkali niobate ceramics have a higher Curie temperature than PZT ceramics, while PZT ceramics are about 300 ° C., whereas alkali niobate ceramics are 400 ° C. or higher. For this reason, there also exists an advantage that the thermal conditions in a manufacturing process or a use condition come to be eased.

  Next, embodiments of a vibration waveform sensor and a vibration waveform sensor module using the above-described piezoelectric element will be described with reference to FIGS. FIG. 3 shows a basic configuration of the vibration waveform sensor of the present embodiment. FIG. 3A shows a cross section of the vibration waveform sensor 10, and FIG. 3B shows an exploded state. FIG. 3C shows a state viewed from the bottom side. In these drawings, the vibration waveform sensor 10 has a structure in which a piezoelectric element 100 is disposed on the main surface of a substrate 20 and this piezoelectric element 100 is covered with a vibration ring 40 that acts as a vibration introducing body. .

  Among the above-mentioned parts, the substrate 20 is for fixing and supporting the piezoelectric element 100, and for extracting the electrodes and amplifying the signals. On the main surface of the substrate 20, a pair of electrode lands 22 and 23 are provided near the center, and a ground conductor 24 is formed around the pair of electrode lands 22 and 23. The electrode lands 22 and 23 are drawn out by through holes 22A and 23A on the back side of the substrate 20. As shown in FIG. 1A, the external electrodes 120 and 122 of the piezoelectric element 100 are mounted on the electrode lands 22 and 23 by solder or conductive resins 130 and 132, respectively. As described above, the piezoelectric element 100 and the amplifier provided on the back surface side of the substrate 20 are connected by the electrode lands 22 and 23 and the through holes 22A and 23A. An insulating resin may be provided so as to cover the electrode lands 22 and 23, and the piezoelectric element 100 may also be covered with a resin.

  Next, the piezoelectric element 100 is provided with a vibration ring 40 surrounding the piezoelectric element 100, and the vibration ring 40 is electrically connected to the ground conductor 24. The ground conductor 24 is drawn out to the back surface side of the substrate 20 through through holes 24A and 24B (only FIG. 3A is shown). The vibration ring 40 is made of, for example, stainless steel and has conductivity. The vibration ring 40 shares a ground potential with the skin of the human body in contact with the vibration ring 40, introduces microvibration of a living body, for example, the skin, and further has a substrate. It functions as a vibration introducing body that transmits to 20.

  FIG. 4D shows the circuit configuration of the vibration waveform sensor module 200, in which the output of the vibration waveform sensor 10 is input to a charge amplifier (charge amplifier) 50 and amplified. The charge amplifier 50 is mounted on the substrate 20 described above, and a voltage proportional to the electric charge generated in the piezoelectric element 100 is output. The output of the charge amplifier 50 is converted into a digital signal by an A / D converter (not shown) and output.

  In the vibration waveform sensor module 200 as described above, for example, as shown in FIG. 3E, the vibration ring 40 hits the skin BD of the human body at an appropriate position such as a finger of the human body with the medical fixing tape 12 or the like. Is mounted as follows. On the other hand, the pulse wave, which is a volume change caused by the inflow of blood accompanying the pulsation of the heart, is transmitted to the vibration ring 40 of the vibration waveform sensor module 200 as a minute vibration of the skin BD. The vibration of the vibration ring 40 further vibrates the substrate 20 that also functions as a vibration body or a strain generating body, and the minute vibration transmitted from the vibration ring 40 is transmitted to the piezoelectric element 100. Thereby, for example, minute vibration of the skin BD is detected as a voltage signal.

  Incidentally, in this case, in this embodiment, the piezoelectric element 100 has a structure in which many piezoelectric layers 102 are stacked. For this reason, when viewed as a capacitor, the area increases, the amount of accumulated charge also increases, and the output of the charge amplifier 50 that outputs a voltage proportional to the amount of charge also increases.

  FIG. 2B shows a vibration waveform sensor module using a conventional piezoelectric element having one layer of a PZT-based piezoelectric material and a piezoelectric element of the present embodiment in which 10 layers of a lead-free alkaline niobate-based piezoelectric material are laminated. FIG. 7 shows a comparison between pulse wave waveform detection outputs obtained by the vibration waveform sensor module described above. In any case, the shape dimension of the piezoelectric element is 3.2 mm long side × 1.6 mm short side × 0.3 mm thickness. As shown in this graph, the PZT-based conventional structure and this example both have almost similar outputs, and as in this example, lead-free piezoelectric materials with a small electromechanical coupling coefficient were used. However, it can be seen that good waveform detection can be performed by using a laminated structure.

The present invention is not limited to the embodiments described above, and various changes can be made without departing from the gist of the present invention. For example, the following are also included.
(1) The material of each part shown in the said Example is an example, You may use various well-known materials. For example, in the above embodiment, a lead-free alkaline niobate piezoelectric material is used as the piezoelectric layer 102, but various other lead-free piezoelectric materials such as barium titanate may be used. The shape dimensions of the piezoelectric element 100 and the number of stacked piezoelectric layers 102 may be set as appropriate in the same manner.
(2) The pulse wave detection shown in the above embodiment is an example, and may be applied to detection of various vibrations.
(3) Although the charge amplifier 50 is mounted on the substrate 20 in the above embodiment, the charge amplifier 50 may be provided outside the vibration waveform sensor 10.

  According to the present invention, a plurality of non-lead piezoelectric materials are laminated with an internal electrode layer interposed therebetween to constitute a piezoelectric element, and a detection output is obtained using a charge amplifier. The same sensitivity can be obtained as when using a sensor, the environmental load is reduced, and the temperature conditions during manufacturing and use are eased. It is suitable.

10: Vibration waveform sensor 12: Fixed tape 20: Substrate 22, 23: Electrode land 22A, 23A: Through hole 24: Ground conductor 24A, 24B: Through hole 40: Vibration ring 50: Charge amplifier 100: Piezoelectric element 102: Piezoelectric layer 104, 106: Internal electrode layer 108: Cover layer 120, 122: External electrode 130, 132: Conductive resin 140: Substrate 200: Vibration waveform sensor module

Claims (4)

A plurality of internal electrode layers;
Formed of a lead-free piezoelectric material, applying a voltage for polarization to the internal electrode layer, and a plurality of piezoelectric layers polarized in the stacking direction;
A cover layer laminated on the front and back of the laminate in which the plurality of piezoelectric layers are laminated with the plurality of internal electrode layers interposed therebetween;
A piezoelectric element comprising: Formed on both ends of the piezoelectric element in the long side direction, and external electrodes for external connection in which the internal electrode layers are alternately connected;
The piezoelectric element according to claim 1, further comprising: A substrate on which the piezoelectric element according to claim 2 is mounted so that its polarization direction is perpendicular to the mounting surface;
A vibration waveform sensor characterized by comprising: A vibration waveform sensor according to claim 3,
A charge amplifier that outputs a voltage proportional to the amount of charge generated in the piezoelectric element of the vibration waveform sensor;
A vibration waveform sensor module comprising:

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