liquid sensor

Abstract

This invention provides a liquid sensor that can be used even when the concentration of the substance being measured in the liquid is high or when the viscosity of the liquid is high. [Solution] The liquid sensor (1) comprises a piezoelectric element (10) having a polarization-reversing piezoelectric laminate (11) which is made up of four or more piezoelectric layers (111, 112) each having a fundamental wave resonance frequency that is the value obtained by dividing the speed of sound by twice the thickness, such that the direction of the polarization component in a direction parallel to or perpendicular to the plane of the layer alternately reverses, and a resonance frequency measuring unit (20) for measuring the resonance frequency of the piezoelectric element (10).

Description

[Technical Field]

[0001] This invention relates to a liquid sensor used for detecting substances contained in a liquid, measuring the viscosity of a liquid, and the like. [Background technology]

[0002] One type of liquid sensor uses a piezoelectric element, which consists of a piezoelectric film (a film made of piezoelectric material) sandwiched between a pair of electrodes. In such a liquid sensor, when an AC voltage is applied between the electrodes while the liquid is in contact with the piezoelectric element, resonance occurs when the frequency of the AC voltage matches the resonant frequency of the piezoelectric element. The resonant frequency of the piezoelectric element is proportional to the square root of the elastic modulus of the piezoelectric film and also proportional to the square root of the total mass of the piezoelectric element. When minute particles contained in the liquid adhere to the piezoelectric element, the mass of the particles that vibrate together with the piezoelectric element increases, causing the resonant frequency to change (decrease). Therefore, by measuring the resonant frequency of the piezoelectric element, the presence of minute particles contained in the liquid can be detected.

[0003] By utilizing this principle, liquid sensors can be used as biosensors to detect enzymes, antigens, etc. (see, for example, Patent Document 1). In this biosensor, a substance consisting of a biochemical substance that adsorbs a specific enzyme or an antibody that reacts with a specific antigen is attached to the surface of a piezoelectric element, and then the liquid to be measured is brought into contact with the surface of the piezoelectric element. As a result, the enzymes, antibodies, etc. are adsorbed to or react with the substance, which lowers the resonant frequency of the piezoelectric element. By measuring the resonant frequency of the piezoelectric element, enzymes, antibodies, etc. in the liquid can be detected.

[0004] Furthermore, liquid sensors using piezoelectric elements can also be used to measure the viscosity of liquids (see, for example, Patent Document 2). When an AC voltage is applied to a piezoelectric element while it is in contact with a liquid, vibrations generated by the piezoelectric film penetrate into the liquid. In this case, the higher the viscosity of the liquid, the longer the viscous penetration length, which is the distance the vibrations penetrate into the liquid. The resonant frequency of the piezoelectric element can be determined by considering the mass of the piezoelectric element plus the mass of the liquid within the range of the viscous penetration length, and the resonant frequency decreases as the viscous penetration length increases. Thus, by measuring the resonant frequency, the viscosity of the liquid in contact with the piezoelectric element can be determined.

[0005] When a piezoelectric element with a piezoelectric film that generates vibrations in the thickness direction is used as a liquid sensor, longitudinal wave vibrations penetrate the liquid. Since longitudinal waves can propagate over long distances in a liquid, the resonance generated in the piezoelectric element is weakened or disappears altogether. In contrast, if a piezoelectric element with a piezoelectric film made of quartz or the like that can generate "thickness-slip vibrations," which are vibrations perpendicular to the thickness direction, is used, the generated vibrations are transverse waves. As a result, they hardly propagate in the liquid, and the vibrations penetrate only within a small range (for example, on the order of 0.1 μm if the piezoelectric film is made of quartz and the liquid is water) from the boundary between the piezoelectric element and the liquid. Therefore, by using transverse waves, the piezoelectric element can resonate strongly even in a liquid and be used as a liquid sensor. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2003-240694 [Patent Document 2] Japanese Patent Publication No. 2002-323424 [Overview of the project] [Problems that the invention aims to solve]

[0007] For resonance to occur within a piezoelectric element, sound waves propagating perpendicularly within the piezoelectric film must be reflected at the boundary with the outside. However, when the concentration of the substance being measured in the liquid increases, or when the viscosity of the liquid increases, the difference in acoustic impedance between the piezoelectric film and the liquid (outside the piezoelectric film) decreases, and sound wave reflection at the boundary between the two may almost disappear. In such cases, even if a piezoelectric film that exhibits thickness-slip vibration is used, the piezoelectric element will not resonate and will not be able to be used as a liquid sensor.

[0008] The problem that this invention aims to solve is to provide a liquid sensor that can be used even when the concentration of the substance to be measured in the liquid is high or when the viscosity of the liquid is high. [Means for solving the problem]

[0009] The liquid sensor according to the present invention, which was developed to solve the above problems, a) A piezoelectric element comprising a polarization-reversing piezoelectric laminate, in which four or more piezoelectric layers, each made of a piezoelectric material, have approximately equal fundamental wave resonance frequencies, which are the value obtained by dividing the speed of sound by twice the thickness, are stacked such that the direction of the polarization components in directions parallel to or perpendicular to the plane of the layers alternately reverses, b) A resonant frequency measuring unit for measuring the resonant frequency of the piezoelectric element It is equipped with.

[0010] The liquid sensor according to the present invention uses a piezoelectric element comprising a polarization-reversing piezoelectric laminate, in which four or more piezoelectric layers are stacked such that the direction of the polarization components in directions parallel to or perpendicular to the plane of the layers alternately reverses, with each layer having approximately equal fundamental wave resonance frequency v / 2d, which is the value obtained by dividing the speed of sound v by twice the thickness d. When an AC voltage having the same frequency as the fundamental wave resonance frequency v / 2d is applied in the thickness direction of this polarization-reversing piezoelectric laminate, vibrations with a resonance wavelength where the thickness d of one piezoelectric layer is half a wavelength are formed within the polarization-reversing piezoelectric laminate for n / 2 wavelengths (where n is the number of piezoelectric layers). The more waves formed within the polarization-reversing piezoelectric laminate (the total number of waves in the entire polarization-reversing piezoelectric laminate; different from the "wave number," which is the number of waves per unit length), that is, the more layers there are, the easier it is to form a stable resonance state, and the more vibration energy is confined to the entire polarization-reversing piezoelectric laminate, resulting in a higher acoustic impedance.

[0011] When the liquid in the liquid cell comes into contact with the piezoelectric element, the resonant frequency of the piezoelectric element decreases below the fundamental resonant frequency due to the substance being measured in the liquid adhering to the piezoelectric element and the viscosity of the liquid, but the acoustic impedance remains high. Therefore, the liquid sensor according to the present invention can be used even when the concentration of the substance being measured in the liquid is high or the viscosity of the liquid is high, as vibrational energy is less likely to leak into the liquid being measured that is in contact with the piezoelectric element, and the piezoelectric element can be made to resonate strongly.

[0012] A polarization-reversing piezoelectric laminate may generate either transverse or longitudinal waves. Transverse waves are generated if the polarization of the piezoelectric material in each piezoelectric layer is parallel to the in-plane direction of the layer, and longitudinal waves are generated if it is perpendicular to the layer. When transverse waves are generated, vibrations of the piezoelectric element penetrate only within a small range from the boundary with the liquid, similar to conventional liquid sensors. Furthermore, because the acoustic impedance is higher than that of conventional liquid sensors, sound wave reflection is more likely to occur at the boundary between the piezoelectric element and the liquid even when the liquid viscosity is high, allowing the piezoelectric element to resonate. On the other hand, when longitudinal waves are generated, because the acoustic impedance is higher than that of conventional liquid sensors, vibrations are less likely to leak from the piezoelectric element, allowing the piezoelectric element to resonate strongly even when longitudinal waves are generated.

[0013] The detection of the object to be measured and the measurement of the viscosity of the liquid can be performed based on the resonant frequency of the piezoelectric element measured in the resonant frequency measurement unit, similar to conventional liquid sensors. The resonant frequency measurement unit can utilize a network analyzer, frequency counter, or the like, similar to conventional liquid sensors.

[0014] The liquid sensor according to the present invention may further include a liquid cell that stores or passes a liquid to be measured, wherein at least a portion of the piezoelectric element is positioned in contact with the liquid.

[0015] Alternatively, instead of providing such a liquid cell in the liquid sensor, the liquid sensor may be used by inserting the piezoelectric element into a liquid stored outside or flowing outside the liquid sensor according to the present invention.

[0016] In the liquid sensor according to the present invention, a polarization-reversing piezoelectric laminate can preferably be made in which each piezoelectric layer is laminated such that the polarization is inclined from directions perpendicular to the in-plane direction of the layer and from directions parallel to the layer, and the direction of the component parallel to the piezoelectric layer alternately reverses. By using a piezoelectric sensor equipped with such a polarization-reversing piezoelectric laminate, a resonance of transverse waves is formed in the piezoelectric sensor, where the thickness d of one piezoelectric layer is half a wavelength and vibrations in the in-plane direction propagate in the lamination direction. This polarization-reversing piezoelectric laminate can be manufactured by magnetron sputtering, in which sputtered particles made of a piezoelectric material such as ScAlN are incident in a direction inclined with respect to the normal to the surface of the substrate, the substrate is rotated 180° around the normal as an axis, and then sputtered particles are incident in the same way, and this operation is repeated.

[0017] In the liquid sensor according to the present invention, in order to further stabilize the resonance of the piezoelectric element, it is preferable that the polarization-reversing piezoelectric laminate has 30 or more piezoelectric layers stacked on top of each other. [Effects of the Invention]

[0018] According to the present invention, a liquid sensor that can be used even when the concentration of the analyte in the liquid is high or the viscosity of the liquid is high can be obtained.

Brief Description of Drawings

[0019] [Figure 1] Schematic cross-sectional view showing an embodiment of the liquid sensor according to the present invention. [Figure 2] Schematic cross-sectional view showing a piezoelectric element included in the liquid sensor of this embodiment. [Figure 3] Schematic configuration diagram showing a magnetron sputtering apparatus used when manufacturing the piezoelectric element included in the liquid sensor of this embodiment. [Figure 4] Graph showing the results obtained by calculating the real part values of the electrical impedance of the piezoelectric element in the state where liquids with viscosities of 1000 mPa·s and 10000 mPa·s are brought into contact, as well as in the state where air is brought into contact for reference, for the liquid sensors of this embodiment (the number of piezoelectric layers is (a) 30 layers, (b) 4 layers) and the comparative example (the same, (c) 1 layer). [Figure 5] Graph showing the results obtained by calculating the real part values of the electrical impedance of the piezoelectric element in the state where a plurality of types of liquids with different viscosities are brought into contact with the liquid sensor of this embodiment (the number of piezoelectric layers is 30 layers). [Figure 6] Graph showing the difference in resonance frequency due to viscosity obtained from FIG. 5. [Figure 7] Graph showing the results obtained by calculating the real part values of the electrical impedance of the piezoelectric element in the state where a plurality of types of liquids with different viscosities are brought into contact with the liquid sensor of this embodiment (the number of piezoelectric layers is 4 layers). [Figure 8] Schematic cross-sectional views showing piezoelectric elements included in the liquid sensor of a modified example, where the polarization of the piezoelectric layer is (a) parallel to the in-plane direction of the layer and (b) perpendicular to the layer. [Figure 9] Graph showing the results obtained by calculating the change in the real part value of the electrical impedance with frequency in the case where the polarization of the piezoelectric layer is perpendicular to the layer and the number of layers is (a) 30 layers (this example), (b) 4 layers (this example), and (c) 1 layer (comparative example). [Modes for carrying out the invention]

[0020] An embodiment of the liquid sensor according to the present invention will be explained using Figures 1 to 9.

[0021] Figure 1 shows the overall configuration of the liquid sensor 1 of this embodiment. The liquid sensor 1 of this embodiment comprises a piezoelectric element 10, a resonant frequency measuring unit 20, and a liquid cell 30.

[0022] As shown in Figure 2, the piezoelectric element 10 includes a polarization reversal piezoelectric laminate 11, a first electrode 121, and a second electrode 122. The polarization reversal piezoelectric laminate 11 is formed by alternately stacking four or more first piezoelectric layers 111 and second piezoelectric layers 112 made of piezoelectric material. Preferably, ten or more layers of the first piezoelectric layers 111 and second piezoelectric layers 112 are stacked together, and more preferably, thirty or more layers are stacked together.

[0023] Both the first piezoelectric layer 111 and the second piezoelectric layer 112 have polarization P inclined in a direction parallel to the in-plane direction (the transverse direction in Figure 2) and in a direction perpendicular to the layer (the longitudinal direction in Figure 2). The polarization P component in the direction parallel to the in-plane direction is opposite to that of the first piezoelectric layer 111 and the second piezoelectric layer 112 (the former is directed to the right in Figure 2, and the latter to the left in Figure 2). The first piezoelectric layer 111 and the second piezoelectric layer 112 are made of the same material, and sound waves propagate at the same speed of sound v. In this embodiment, since transverse wave resonance occurs within the polarization-reversing piezoelectric laminate 11, the speed of sound v here is the speed of sound for transverse waves (the speed of sound for transverse waves is half the speed of sound for longitudinal waves). Also, the first piezoelectric layer 111 and the second piezoelectric layer 112 have the same thickness d.

[0024] The first electrode 121 and the second electrode 122 are provided so as to sandwich the polarization reversal piezoelectric laminate 11 from both sides in the stacking direction (vertical direction in Figure 2) of the first piezoelectric layer 111 and the second piezoelectric layer 112.

[0025] In this embodiment, the piezoelectric material of the polarization reversal piezoelectric laminate 11 (first piezoelectric layer 111 and second piezoelectric layer 112) is Sc x Al1-x N (scandium aluminum nitride, 0 < x < 1) can be preferably used. Sc x Al 1-x N has a wurtzite crystal structure which is hexagonal. Sc x Al 1-x In addition to AlN (aluminum nitride), ZnO (zinc oxide), Mg 1-x Zn x Piezoelectric materials such as O (magnesium zinc oxide) may be used. Metals such as Au (gold) and Al (aluminum) can be used for the materials of the first electrode 121 and the second electrode 122.

[0026] The piezoelectric element 10 can be manufactured by the following method using the magnetron sputtering apparatus 50 shown in FIG. 3. First, a plate material made of quartz glass, Si, etc. is prepared, and a metal that will become the second electrode 122 is vapor-deposited on the surface of the plate material to produce the substrate to be processed S. Next, the substrate to be processed S is placed on the substrate holder 52 of the magnetron sputtering apparatus 50 with the second electrode 122 on the surface. Also, a target T made of a ScAl alloy is placed on the cathode electrode 51 of the magnetron sputtering apparatus 50. In this state, magnetron sputtering is performed in a gas atmosphere containing nitrogen to generate sputter particles SP. The generated sputter particles SP are incident on the surface of the substrate to be processed S fixed to the substrate holder 52 at an inclined angle, thereby forming one layer of the second piezoelectric layer 112 in which the polarization P is inclined with respect to the in-plane direction of the layer. Next, the substrate to be processed S is rotated 180° around an axis perpendicular thereto, and then the sputter particles SP are incident on the surface of the substrate to be processed S at an inclined angle in the same manner as above. Thereby, one layer of the first piezoelectric layer 111 is formed in which the polarization P is inclined with respect to the in-plane direction of the layer and the in-plane component parallel to the polarization P is 180° different from that of the second piezoelectric layer 112. Hereinafter, by repeating the same operation, four or more layers of the second piezoelectric layer 112 and the first piezoelectric layer 111 are alternately formed. Finally, the first electrode 121 is formed by vapor-depositing a metal on the surface of the uppermost first piezoelectric layer 111, thereby obtaining the piezoelectric element 10.

[0027] The plate material, which is part of the substrate S to be processed and consists of quartz glass, Si, etc., may be removed after the piezoelectric element 10 is fabricated, or it may be left as is. In this embodiment, since the polarization reversal piezoelectric laminate 11 has a high acoustic impedance, even if the plate material is left in place, it is possible to suppress the leakage of vibrations generated in the polarization reversal piezoelectric laminate 11 from the plate material.

[0028] The resonant frequency measurement unit 20 is a network analyzer, a device that can measure acoustic impedance and other parameters while varying the frequency. Acoustic impedance is a complex number, and the frequency at which its real part reaches its maximum value corresponds to the resonant frequency.

[0029] The liquid cell 30 has a flow path 32 from an inlet 31 to an outlet 33 for the liquid to be measured (hereinafter referred to as the "liquid sample"), and a fixing portion 34 for the piezoelectric element 10 provided in the flow path 32. The piezoelectric element 10 is fixed in the fixing portion 34 such that a part of the surface of the piezoelectric element 10 is in contact with the liquid flowing in the flow path 32.

[0030] Next, the operation of the liquid sensor 1 in this embodiment will be described.

[0031] In the piezoelectric element 10, the polarization P of the first piezoelectric layer 111 and the second piezoelectric layer 112 is tilted from a direction parallel to the in-plane direction and a direction perpendicular to the layer, and is opposite to that of the other in the direction parallel to the in-plane direction. Therefore, when an AC input signal (AC voltage) is input between the first electrode 121 and the second electrode 122 from the resonance frequency measuring unit 20, the first piezoelectric layer 111 and the second piezoelectric layer 112 vibrate in opposite directions in the in-plane direction. When the frequency of the input signal matches the resonance frequency of the polarization reversal piezoelectric laminate 11 described below, resonance occurs within the polarization reversal piezoelectric laminate 11 due to mechanical vibration at that frequency.

[0032] When the piezoelectric element 10 is not in contact with the liquid, a half-wavelength resonance is generated in the thickness direction of the first piezoelectric layer 111 and the second piezoelectric layer 112, respectively. Therefore, the resonance wavelength of the polarization-reversing piezoelectric laminate 11 is 2d, which is twice the thickness d of the first and second piezoelectric layers 111 and 112. The resonance frequency is the fundamental resonance frequency v / 2d, which is the value obtained by dividing the speed of sound v of the first and second piezoelectric layers 111 and 112 by twice the thickness d. Consequently, when the frequency of the input signal is changed, resonance occurs in the polarization-reversing piezoelectric laminate 11 when the fundamental resonance frequency v / 2d matches the frequency of the input signal. In this embodiment, the first piezoelectric layer 111 and the second piezoelectric layer 112 are made of the same material, so their speed of sound v is the same and their thickness d is also the same. However, if the first piezoelectric layer 111 and the second piezoelectric layer 112 are made of different materials (have different speeds of sound), the thickness of each layer is set so that v / 2d is the same value.

[0033] When using the liquid sensor 1, a liquid sample is flowed through the flow path 32 of the liquid cell 30, and an input signal is input from the resonance frequency measurement unit 20, changing the frequency of the input signal. When the resonance frequency of the polarization reversal piezoelectric laminate 11, in which the piezoelectric element 10 is in contact with the liquid sample, matches the frequency of the input signal, resonance occurs within the polarization reversal piezoelectric laminate 11 due to mechanical vibration at that frequency. At this time, the resonance frequency decreases from the fundamental resonance frequency v / 2d because the layer of liquid vibrating together with the oscillator (viscous penetration length) becomes thicker as the viscosity of the substance being measured and the liquid sample adhering to the piezoelectric element 10 increases.

[0034] Furthermore, as the concentration of the analyte in the liquid sample increases, the amount of the analyte adhering to the piezoelectric element 10 increases (more precisely, the amount of the analyte newly adhering to the piezoelectric element 10 is greater than the amount detaching from the piezoelectric element 10), thereby reducing the resonance frequency. On the other hand, when the concentration of the analyte in the liquid sample decreases, the amount detaching from the piezoelectric element 10 is greater than the amount of the analyte newly adhering to the piezoelectric element 10, thereby increasing the resonance frequency. Similarly, as the viscosity of the liquid sample increases, the resonance frequency decreases, and as the viscosity decreases, the resonance frequency increases. By measuring such increase / decrease in the resonance frequency by the resonance frequency measurement unit 20, it is possible to detect an increase or decrease in the concentration of the analyte in the liquid sample and an increase or decrease in the viscosity of the liquid sample.

[0035] In the liquid sensor 1 of the present embodiment, since the polarization-inverted piezoelectric laminate 11 in which the first piezoelectric layer 111 and the second piezoelectric layer 112 are alternately laminated in total of four or more layers is used for the piezoelectric element 10, a stable resonance state is likely to be formed and the acoustic impedance becomes high. As a result, even when the concentration of the analyte in the liquid sample is high or the viscosity of the liquid sample is high, it becomes difficult for the vibration energy to leak to the liquid sample contacting the piezoelectric element 10, and the piezoelectric element 10 can be strongly resonated, so that the liquid sensor 1 can be used.

[0036] [[ID=⑧]]The results of confirming the characteristics of the liquid sensor 1 of the present embodiment by calculation are shown below. In this calculation, the materials of the first piezoelectric layer 111 and the second piezoelectric layer 112 were set as Sc x Al 1-x N(x = 0.6), and the calculation was performed assuming that the polarization P is inclined at an angle of 45° with respect to the lamination direction.

[0037] First, using two piezoelectric elements 10, each having (a) 30 layers and (b) 4 layers combined, and with individual piezoelectric layers having a thickness of 1.35 μm, the electrical impedance was calculated under the condition that liquids with viscosities of 1000 mPa·s and 10000 Pa·s were brought into contact with the entire surface of one electrode side of each piezoelectric element 10. For comparison, a Sc in which the polarization P is tilted at an angle of 45° with respect to the stacking direction was also used. x Al 1-x The same calculations were performed for the case of a piezoelectric element having only one piezoelectric layer (c) consisting of N(x=0.6). The calculation results are shown in the graphs of Figures 4(a) to (c). No peak is observed when there is only one piezoelectric layer, whereas peaks are observed when there are 30 and 4 piezoelectric layers. These peaks indicate that resonance is occurring in the piezoelectric element 10. From these results, it can be seen that the comparative example cannot generate resonance in the piezoelectric element, which is a condition for use as a liquid sensor when contacted with a liquid, whereas this embodiment can generate resonance in the piezoelectric element 10.

[0038] Next, for the piezoelectric element 10 with 30 layers used in the calculation in Figure 4(a), the real part of the electrical impedance was calculated for each case where the viscosity of the liquid in contact was 1, 10, 100, 1000, and 10000 (all in units of mPa·s). The calculation results are shown in the graph in Figure 5. From this graph, it can be seen that as the viscosity of the liquid in contact increases, the peak of the real part of the electrical impedance shifts to a lower frequency. The frequencies of these peaks are the resonant frequencies for each viscosity. The relationship between viscosity and resonant frequency obtained from Figure 5 is shown in the graph in Figure 6. From this graph, it can be confirmed that different resonant frequencies are obtained depending on the viscosity of the liquid in contact. In other words, with the liquid sensor 1 using the piezoelectric element 10 of this embodiment, the viscosity of the liquid in contact can be determined from the obtained resonant frequency.

[0039] Next, for the piezoelectric element 10 with four layers used in the calculations in Figure 4(b), the real part of the electrical impedance was calculated for each case where the viscosity of the liquid in contact was 1, 10, 100, 1000, and 10000 (all in units of mPa·s). The calculation results are shown in the graph in Figure 7. From this graph, it can be said that even when the piezoelectric layer has four layers, the piezoelectric element 10 can be made to resonate when in contact with a liquid, and different resonance frequencies can be obtained depending on the viscosity of the liquid in contact. In other words, it can be confirmed that a piezoelectric element 10 with four layers can also be used in the liquid sensor of the present invention.

[0040] The present invention is not limited to the embodiments described above, and various modifications are possible.

[0041] For example, in the above embodiment, a piezoelectric element 10 was used which comprises a first piezoelectric layer 111 and a second piezoelectric layer 112 whose polarization P is inclined with respect to the in-plane direction of the layer. However, a piezoelectric element 10A may be used which comprises a first piezoelectric layer 111A and a second piezoelectric layer 112A whose polarization P is parallel to the in-plane direction of the layer and opposite to each other, as shown in Figure 8(a), or a piezoelectric element 10B which comprises a first piezoelectric layer 111B and a second piezoelectric layer 112B whose polarization P is perpendicular to the layer and opposite to each other, as shown in Figure 8(b). In either case, the total number of layers of the first and second piezoelectric layers is four or more.

[0042] The electrical impedance of the piezoelectric element 10B shown in Figure 8(b) was calculated. Here, calculations were performed for cases where the thickness of each piezoelectric layer is 1.35 μm, and the total number of layers (first piezoelectric layer 111B and second piezoelectric layer 112B) is 30 and 4. For comparison, calculations were also performed for a piezoelectric element with only one piezoelectric layer, where the polarization P is perpendicular to the layer and the thickness is the same as in the examples of 30 and 4 layers. In these calculations, the electrical impedance was determined under the condition that water (viscosity 0.89 mPa·s) was in contact with the entire surface of one electrode side of the piezoelectric element. For reference, similar calculations were performed for the case where water was not in contact (the piezoelectric element was surrounded by air). Figure 9 shows the calculation results for the number of piezoelectric layers (a) 30, (b) 4, and (c) 1. In the comparative example (c), resonance occurs when the piezoelectric element is surrounded by air, but it does not occur when water is in contact. In contrast, in embodiments (a) and (b) of the present invention, resonance occurs even when water is in contact with the element. These results indicate that the acoustic impedance is higher when there are multiple piezoelectric layers than when there is only one piezoelectric layer. Therefore, even with longitudinal waves, which tend to propagate easily through liquids, vibrations are less likely to leak from the piezoelectric element into the liquid, allowing the piezoelectric element to resonate.

[0043] Further variations are described below. In the above embodiment, a liquid cell 30 equipped with a channel 32 through which a liquid sample flows was used, but instead, a liquid cell (liquid container) in which a liquid sample is stored without flowing may be used.

[0044] Alternatively, the liquid sensor may consist of a piezoelectric element 10 and a resonance frequency measuring unit 20, without including the liquid cell as a component of the liquid sensor. In this case, the piezoelectric element 10 is immersed in a liquid sample present outside the liquid cell, or brought into contact with the sample on the liquid surface of the sample, and the resonance frequency of the piezoelectric element 10 is measured using the resonance frequency measuring unit 20 while the piezoelectric element 10 and the sample are in contact.

[0045] In the embodiment described above, a network analyzer was used for the resonance frequency measurement unit 20, but a frequency counter or the like may be used instead. [Explanation of Symbols]

[0046] 1…Liquid sensor 10, 10A, 10B... Piezoelectric elements 11…Polarization reversal piezoelectric laminate 111, 111A, 111B... First piezoelectric layer 112, 112A, 112B... Second piezoelectric layer 121...1st electrode 122…Second electrode 20…Resonance frequency measurement section 30…Liquid Cell 31...Inlet 32…flow channel 33... Outlet 34…Fixed part 50…Magnetron sputtering apparatus 51... Cathode electrode 52... Circuit board holder

Claims

1. a) A piezoelectric element comprising a polarization-reversing piezoelectric laminate, in which four or more piezoelectric layers, each made of a piezoelectric material, have approximately equal fundamental wave resonance frequencies, which are the value obtained by dividing the speed of sound by twice the thickness, are stacked such that the direction of the polarization components in directions parallel to or perpendicular to the plane of the layers alternately reverses, b) A resonant frequency measuring unit for measuring the resonant frequency of the piezoelectric element A liquid sensor equipped with the following features.

2. Furthermore, the liquid sensor according to claim 1, comprising a liquid cell for storing or passing a liquid to be measured, wherein at least a portion of the piezoelectric element is positioned in contact with the liquid.

3. The liquid sensor according to claim 1 or 2, wherein the polarization-reversing piezoelectric laminate is constructed by laminating each piezoelectric layer such that the polarization is tilted from directions perpendicular and parallel to the piezoelectric layer, and the direction of the component parallel to the piezoelectric layer alternately reverses.

4. The liquid sensor according to claim 1 or 2, wherein the polarization reversal piezoelectric laminate is formed by laminating 30 or more piezoelectric layers.

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